science

List of seminarsorganized by the group

[1] 31th of October 2024,[SLIDES]
Roman Josué Armenta Rico,
Real Problem Approaches to Quantum Many-Body Systems Using Machine Learning and Neural Networks,
Instituto de Física, Universidad Nacional Autónoma de México

open abstract
In this talk, we will explore real-world examples of quantum many-body systems, delving into the complexities that arise in both first and second quantization formalisms. We will demonstrate how state-of-the-art neural network ansätze, combined with advanced machine learning techniques, offer powerful tools for addressing these challenges. By presenting concrete case studies, we will illustrate how machine learning not only accelerates computational efficiency but also provides novel insights into the underlying physics of quantum systems.

[2] 29th of October 2024,[SLIDES]
Diego Peña Angeles,
An Introduction to Machine Learning for Solving Quantum Many-Body Problems,
Instituto de Física, Universidad Nacional Autónoma de México

open abstract
Solving the electronic Schrödinger equation for many-body quantum systems remains a central challenge in quantum physics due to the complexity of interacting particles. Traditional methods struggle to efficiently explore the Hilbert space, especially as system size and dimensionality increase. In response to this, neural networks have emerged as powerful numerical ansätze for representing wavefunctions, capable of capturing complex correlations in quantum states. By employing variational Monte Carlo techniques, these neural networks enable the exploration of a broad range of functional forms for ground-state solutions, offering greater flexibility and scalability than conventional methods.

[3] 11th of July 2024,[SLIDES]
Felipe Isaule Rodríguez,
Bound impurities in a one-dimensional Bose lattice gas ,
Instituto de Física, Pontificia Universidad Católica de Chile

open abstract
The progress in realising ultracold atomic mixtures has greatly revitalised the interest in studying impurities immersed in quantum mediums [1]. Of particular interest is the problem of two correlated mobile impurities, as these can form bound particles usually referred to as bipolarons [2]. In this direction, and motivated by studies of bipolarons in BECs [3], the study of bipolaron-like physics in optical lattices has appeared as a new platform to study highly imbalanced mixtures [4-6]. In our recent work [7], we study two mobile bosonic impurities immersed in a one-dimensional optical lattice and interacting with a bosonic bath. We examine the formation of bound dimers of impurities across superfluid- and Mott-like bosonic baths. We also study the dynamics after a quench of the interactions. We reveal that after large interaction quenches from strong to weak interactions the system can show large oscillations over time with revivals of the dimer states. Moreover, we find that the periods depend strongly on the interaction quenches, resulting in distinct regions of oscillations. [1] C. Baroni, G. Lamporesi, and M. Zaccanti, arxiv:2405.14562 (2024). [2] A. S. Alexandrov and N. F. Mott, Rep. Prog. Phys. 57, 1197 (1994). [3] A. Camacho-Guardian, L. Peña Ardila, T. Pohl, and G. Bruun, PRL 121, 013401 (2018). [4] S. Dutta and E. J. Mueller, PRA 88, 053601 (2013). [5] M. Pasek and G. Orso, PRB 100, 245419 (2019). [6] S. Ding, G. A. Domínguez-Castro, A Julku, A Camacho-Guardian, and G. M. Bruun, SciPost Phys. 14, 143 (2023). [7] F. Isaule, A. Rojo-Francàs, and B. Juliá-Díaz, arXiv:2402.03070 (2024).

[4] 26th of June 2024,[SLIDES]
Pavel Ryzhakov, Alireza Hashemi, Angela Ares,
Insights into Classical micro Droplets: Experiments, Simulation, and AI”,
International Centre for Numerical Methods in Engineering/Technical University of Catalonia

open abstract
This presentation aims to provide a comprehensive overview of the physics of classical microdroplets, covering fundamentals such as capillarity, surface tension, wetting phenomena, and droplet interactions. We will introduce our interdisciplinary approach to integrate experimental methodologies, Computational Fluid Dynamics (CFD) simulations, and Artificial Intelligence (AI), which can yield profound insights and advancements in the study of classical microdroplets.

[5] 12th of June 2024,[SLIDES]
Juliette Huynh,
Critical velocity for superfluidity in reduced dimensions: From matter to light quantum fluids,
Université Côte d’Azur, CNRS, Institut de Physique de Nice

open abstract
Superfluidity is the ability of a quantum fluid to flow past a localized obstacle without any loss of kinetic energy. This spectacular phenomenon occurs below a specific flow speed, the so-called critical velocity for superfluidity. In this work, I will present recent results [1,2] about the critical velocity for superfluidity in the one- and two-dimensional mean-field regime. The quantum fluid we consider is quite generic, characterized by an arbitrary local nonlinearity increasing with the fluid density and eventually by linear particle losses. The study of an arbitrary localized obstacle yields results that go beyond Landau’s criterion for superfluidity. We obtain exact, nonperturbative expressions both in the wide- and narrow-obstacle limits for 1D systems, and for wide obstacles in 2D systems, which we then numerically interpolate using an original relaxation algorithm for the stationary problem. Losses, if present, are treated within an adiabatic approach of the dynamics giving results in excellent agreement with full numerics. This makes it possible to get closer to recent experiments with atomic Bose-Einstein condensates and superfluids of light in propagating geometry. Our analytical findings are supported with numerical simulations and in-progress experiments at LKB-Jussieu. [1] J. Huynh, M. Albert, and P.-É. Larré, Critical velocity for superfluidity in the one-dimensional mean-field regime: From matter to light quantum fluids, Phys. Rev. A 105, 023305 (2022) [2] J. Huynh, F. Hébert, M. Albert and P.-É. Larré, Critical velocity of a two-dimensional superflow past a potential barrier of arbitrary penetrability, Phys. Rev. A 109, 013317 (2024)

[6] 16th of May 2024,[SLIDES]
Emanuele Costa,
Deep learning density functional theory for simulating quantum many-body systems,
Institut de Ciències del Cosmos, Universitat de Barcelona

open abstract
Classical simulations of quantum many-body systems are essential for both studying and understanding quantum technologies. In recent years, with the improvement and the development of several methods such as tensor networks and neural quantum states, classical simulations can reach outstanding results in both efficiency and accuracy. In this framework, we present a novel approach based on deep learning density functionals for quantum many-body systems. density functional theory can bypass the wavefunction approach for solving the many-body problem increasing the speed up and with good accuracies. By using deep learning, density functional can be easily provided without approximations and with adaptive functional forms. Here, we present the idea based on the deep learning density functional theory applied to continuous systems, with its limits and novel strategies to overcome them. We also show how the theory and the deep learning method can be extended to spin systems. Finally, we show how it is possible to extend the density functional for time dependent spin models and the application of DL-density functionals into that framework.

[7] 25th of April 2024,[SLIDES]
Cesar Cabrera,
Tracking the confinement-induced hybridization of the Higgs mode in a strongly interacting superfluid,
Institut für Quantenphysik, Universität Hamburg

open abstract
Superconductivity in quantum-confined systems has attracted significant attention due to the implications for both fundamental science and technological development. Confinement modifies the dimensionality of the system, density of states, and critical temperature of superconductors, as the coherence length and the superconducting gap approaches the natural scales of length and energy imposed by the system size. A deep understanding of how confinement influences the superconducting pairing properties and its order parameter dynamics out of equilibrium is a theoretical challenge [1,2,3]. In my talk, I will show how strong confinement in a neutral fermionic superfluid affects the order parameter dynamics along the BEC-BCS crossover. By performing trap modulation spectroscopy in a highly oblate Fermi gas, we reveal a well-defined excitation of the order parameter throughout the entire crossover. On the BCS regime, the resonance position follows twice the pairing gap ∆, signaling a stable Higgs mode. Approaching the strongly correlated regime the mode energy drops below the pairing gap and finally approaches twice the trap frequency on the BEC regime. We interpret our results as hybridization and transition of the Higgs mode into the lowest excitation connected with the trap corresponding to a spatial coherent excitation of the order parameter. This excitation vanishes when approaching the critical temperature of the superfluid, where the order parameter is expected to disappear. The experimental evidence together with an effective field theory deepens our understanding of the stability of the Higgs mode in the absence of particle-hole symmetry and its interplay with confinement. [1] C. M. Varma, J. Low Temp. Phys. 126, 901 (2002). [2] D. Podolsky, et al., Phys. Rev. B Condens. Matter 84, 174522 (2011) [3] D. Pekker and C. M. Varma, Annu. Rev. Condens. Matter Phys. 6, 269 (2015).

[8] 10th of April 2024,[SLIDES]
Ivan Morera,
Kinetic magnetism and magnetic polarons in cold atom systems,
ETH

open abstract
Kinetic frustration is opening a new paradigm in cold atomic systems and two-dimensional moiré materials, since it induces nontrivial magnetic and spin-charge correlations at temperature scales of the order of the tunneling force, giving rise to so-called kinetic magnetism [1,2,3]. This phenomenon appears in the strongly interacting regime of doped Fermi-Hubbard Hamiltonians in non-bipartite lattices, such as the two-dimensional triangular lattice. In this talk I will discuss how kinetic magnetism and magnetic polarons emerge in these systems [4] and how recent experiments with cold atoms using quantum gas microscopes have been able to observe them experimentally [5,6]. [1]I. Morera, A. Bohrdt, W. W. Ho, E. Demler. arXiv:2106.09600. To be published in Phys. Rev. Res. [2] Ivan Morera, Annabelle Bohrdt, Wen Wei Ho, Eugene Demler. Physical Review Research 5 (2), L022048. [3] Livio Ciorciaro, T Smoleński, Ivan Morera, Natasha Kiper, Sarah Hiestand, Martin Kroner, Yang Zhang, Kenji Watanabe, Takashi Taniguchi, Eugene Demler, A İmamoğlu. Nature 623 (7987), 509-513. [4] Ivan Morera, Eugene Demler. arXiv:2402.14074 [5] Martin Lebrat, Muqing Xu, Lev Haldar Kendrick, Anant Kale, Youqi Gang, Pranav Seetharaman, Ivan Morera, Ehsan Khatami, Eugene Demler, Markus Greiner. arXiv:2308.12269. To be published in Nature. [6] Max L Prichard, Benjamin M Spar, Ivan Morera, Eugene Demler, Zoe Z Yan, Waseem S Bakr. arXiv:2308.12951. To be published in Nature.

[9] 6th of March 2024,[SLIDES]
Marek Tylutki ,
Mixtures of Superfluid Quantum Gases,
Warsaw University of Technology

open abstract
I will present my research on mixtures of superfluid ultracold atomic gases. I will focus on a spin-balanced and spin-imbalanced superfluid Fermi gas across the BCS-BEC crossover, one-dimensional binary mixtures of Bose-Einstein condensates and Bose-Fermi superfluid mixtures. In particular, I will discuss the dissipation mechanism in Josephson junctions, the dynamics of vortices and the formation of quantum droplets. [1] Phys. Rev. Lett. 130, 023003 (2023)

[10] 29th of February 2024,[SLIDES]
Krzysztof Pawłowski,
Super-Tonks-Girardeau quench in the extended Bose-Hubbard model,
Centrum Fizyki Teoretycznej PAN

open abstract
My talk is related to 1D bosons with strong interaction and the super-Tonks-Girardeau effect. By incorporating both an optical lattice and nonlocal interactions (specifically nearest neighbour), we discover a previously unexplored phenomenon: the disruption of the state during the quench, but within a specific range of interactions. Our study employs the extended Bose-Hubbard model across various system sizes, starting with analytical results for two atoms and progressing to few-body systems using exact diagonalisation, density matrix renormalisation group and time-dependent variational principle methods. Finally, we use a numerical implementation of the local density approximation for a macroscopic number of atoms. Consistently, our findings unveil a region where the initially self-bounded structure, after a quench -- evaporates. We use this phenomenon to characterize the phase diagram of the extended Bose-Hubbard model in the strong interaction limit. [1] Krzysztof Pawłowski et al. Phys. Rev. A 108, 043304 (2023)

[11] 20th of September 2023,[SLIDES]
Karen Kheruntsyan ,
An interaction-driven heat engine enabled by atom-atom correlations,
University of Queensland

open abstract
Quantum many-body heat engines have been the subject of growing interest in the emerging field of quantum thermodynamics. Such engines can utilize uniquely quantum many-body effects to enhance the performance of classical engine cycles, implying a quantum advantage. In this talk I will introduce and discuss the performance of a many-body Otto engine operating under a sudden interaction quench protocol, with a one-dimensional (1D) Bose gas as a working fluid. I will show how the very operation of this engine is enabled by atom-atom pair correlations in the gas. These correlations are different from those in a classical ideal gas and are the result of an interplay between quantum statistics, inter-particle interactions, and thermal fluctuations; extracting net positive work from the system without such correlations would not be possible. I will also show how the performance of the many-body Otto engine can be further enhanced by allowing particle exchange between the system and the thermal reservoirs, in addition to heat exchange. The performance of this Otto engine can be evaluated using approximate analytic or exact thermodynamic Bethe ansatz results available for the Lieb-Liniger model that describes the 1D Bose gas working fluid, but the broad conclusions that we attain here are not limited to this particular model.

[12] 5th of September 2023,[SLIDES]
Gabriele Spada,
Attractive solution of binary Bose mixtures: Liquid-vapor coexistence and critical point,
Università di Trento

open abstract
He will present a novel study of the thermodynamic behavior of attractive binary Bose mixtures using exact path-integral Monte-Carlo methods. The focus is on the regime of interspecies interactions where the ground state is in a self-bound liquid phase, stabilized by beyond mean-field effects. Calculating the isothermal curves in the pressure vs density plane, they established the extent of the coexistence region between liquid and vapor and they determined the critical point where the line of first-order transition ends. Notably, within the coexistence region, Bose-Einstein condensation occurs in a discontinuous way that could be observed in experiments of mixtures in traps, as the density jumps from the normal gas to the superfluid liquid phase.

[13] 6th of July 2023,[SLIDES]
Raúl Bombín,
A Quantum Monte Carlo based density functional for quantum Dysprosium droplets,
Universitat Politècnica de Catalunya

open abstract
Due to the long-range and anisotropic nature of the dipolar interaction, ultracold dipolar systems offer a unique platform in which to study many-body physics in the quantum degenerate regime. In recent years much attention has been paid to the rich physics that emerge in these systems, such as droplet formation and the existence of supersolid phases. However an accurate characterization of these phenomena is challenging as they are governed by the subtle balance that exists between the importance of quantum correlations and of the inter-particle interaction. From the theoretical point of view this makes it challenge to reproduce with accuracy some observable, such as the critical atom number for droplet formation. In a previous work we showed that Quantum Monte Carlo calculations are able to reproduce with accuracy the critical atom number of dipolar droplets [1]. Here, construct a density functional based on the QMC equation of state that allows to study dipolar systems that is able to reproduce the experimental critical atom number. Thus, our approach outperforms the current state-of-the-art eGPE description, with no additional computational cost. Moreover we discuss how sensitive is the supersolid regime window (in terms of interaction strenght) to the quality of the microscopic calculation. [1] Fabian Böttcher, Matthias Wenzel, Jan-Niklas Schmidt, Mingyang Guo, Tim Langen, Igor Ferrier-Barbut, Tilman Pfau, Raúl Bombín, Joan Sánchez-Baena, Jordi Boronat, and Ferran Mazzanti - Phys. Rev. Research 1, 033088 (2019)

[14] 22nd of June 2023,[SLIDES]
Daniel Pérez Cruz,
Superfluid fraction in disordered quantum fluids ,
Departament de Física, UPC

open abstract
An ultracold dilute gas of bosons can have a fraction of its components in the so-called superfluid state. A superfluid exhibits a series of counter-intuitive properties that are closely related to those of superconductors. As examples we can mention the non-classical rotational moment of inertia, the quantization of vorticity, and the capacity to flow without losses. Determining the superfluid fraction in an actual experiment with ultracold atoms is a non-trivial task. In a series of papers, A. Leggett derived strict upper and lower bounds for such quantity in terms of the local density of particles. Here we study numerically the superfluid fraction of a gas in both optical lattices and disordered (speckle) potentials in one and two spatial dimensions, comparing directly our numerical results to the aforementioned bounds.

[15] 24th of April 2023,[SLIDES]
Arthur Christianen,
The Bose polaron and the Efimov effect,
Max Planck Institute of Quantum Optics, Garching, Germany

open abstract
In ultracold atomic gases, a unique interplay arises between phenomena known from condensed matter, few-body physics and chemistry. A good example is the Bose polaron problem of an atomic impurity in a Bose-Einstein condensate (BEC). On the one hand, the impurity is dressed by excitations from the medium, similar to an electron in a solid. On the other hand, the Efimov effect known from universal few-body physics can cause the impurity to mediate attractive interactions between the bosons from the BEC. As a result, an interesting competition arises between the impurity-mediated attraction and the background repulsion in the BEC. If the attraction dominates, this leads to a local collapse of the BEC onto the impurity and rapid recombination [1,2]. If instead the repulsion dominates, one finds a smooth crossover from a polaron into a small molecule. In our recent work [3], we achieve a unified understanding of these phenomena by comparing two state-of-the-art variational methods, fully including both the boson-impurity and interboson interactions. We show that both the instability and the crossover regime can be studied in realistic experiments, paving the way to an increased understanding of mediated interactions in quantum mixtures of ultracold atoms. [1] A. Christianen, J. I. Cirac, R. Schmidt, Phys. Rev. Lett. 128, 183401 (2022) [2] A. Christianen, J. I. Cirac, R. Schmidt, Phys. Rev. A 105, 053302 (2022) [3] A. Christianen, J. I. Cirac, R. Schmidt, in preparation

[16] 25th of January 2023,[SLIDES]
Andrea Tononi ,
Self-bound fermionic mixtures in 1D,
LPTMS, Université Paris Sud, Francia

open abstract
Self-binding in Bose-Bose mixtures has received lots of theoretical [1] and experimental [2] attention in the recent years, and a few studies discussed also Bose-Fermi droplets [3]. Fermi-Fermi mixtures with interspecies attraction, however, are not expected to display self-bound states, since the fermions of one species should overcome a strong Pauli pressure to bind the fermions of the other. This repulsion is, in fact, the fundamental mechanism that provides stability of Fermi mixtures along the BCS-BEC crossover, whose dimers repel and do not form larger clusters [4]. In our recent work [5], surprisingly, we find that one-dimensional Fermi-Fermi mixtures with sufficiently large mass imbalance can form a self-bound state in the thermodynamic limit. This result elaborates our previous few-body analyses [6], and is based on a mean-field theory in which the heavy fermions are described within the Thomas-Fermi approximation, which is exact in the limit of large mass ratios. Our work sets the basis for future studies of self-bound fermions in higher-dimensional cases, which can shed light on the fundamental behavior of macroscopic fermionic aggregates. References: [1] D. S. Petrov, Quantum Mechanical Stabilization of a Collapsing Bose-Bose Mixture, Phys. Rev. Lett. 115, 155302 (2015). [2] C. R. Cabrera, et al., Quantum liquid droplets in a mixture of Bose-Einstein condensates, Science 359, 301 (2018). [3] D. Rakshit, et al., Self-bound Bose–Fermi liquids in lower dimensions, New J. Phys. 21, 073027 (2019); T. Karpiuk, et al., Bistability of Bose–Fermi mixtures, New J. Phys. 22, 103025 (2020). [4] D. S. Petrov, C. Salomon, and G. V. Shlyapnikov, Weakly bound dimers of fermionic atoms, Phys. Rev. Lett. 93, 090404 (2004). [5] J. Givois, A. Tononi, and D. S. Petrov, Self-binding of one-dimensional fermionic mixtures with zero-range interspecies attraction, arXiv:2207.04742. [6] A. Tononi, J. Givois, and D. S. Petrov, Binding of heavy fermions by a single light atom in one dimension, Phys. Rev. A 106, L011302 (2022).

[17] 16th of December 2022,[SLIDES]
Joachim Brand,
Dynamics of Josephson vortices and vortex molecules in coupled Bose-Einstein condensates,
Dodd-Walls Centre for Photonic and Quantum Technologies and New Zealand Institute for Advanced Study, Massey University, Auckland, New Zealand

open abstract
Linearly coupled binary Bose-Einstein condensates (BECs) offer a rich variety of nonlinear excitations. An elementary low-energy building block is the Josephson vortex. We characterise the families of traveling nonlinear excitations connected to the Josephson and characterise the regimes of stability and positive and negative effective mass. A vortex molecule is an excitation of a two-dimensional binary BEC with linear coupling where vortices in the component condensates are connected by a Josephson vortex, which plays the role of a domain wall of the relative phase. We will discuss the intriguing dynamics of Josephson vortices in the presence of boundary conditions.

[18] 2st of December 2022,[SLIDES]
Genís Lleopart,
Evidence for low-lying correlated gapped states in strained graphene and α-graphyne,
Departament de Ciència de Materials i Química Física & Institut de Química Teòrica i Computacional (IQTC), Universitat de Barcelona

open abstract
Graphene is a 2D gapless Dirac semimetal with ultrahigh charge/spin mobilities. The absence of a semiconducting gap means that currents in graphene cannot be easily modulated, as required in devices (e.g. on/off switching in transistors). Top-down strategies to induce a gap in graphene by introducing defects, distortions, doping, etc. in the 2D material have been largely unsuccessful. A new class of magnetic organic materials called Extended graphenic (XT-graphenic) or Covalent Organic Frameworks (CORFs), which keep the graphene hexagonal symmetry but introduce linkers between α-C [1], have been proposed, showing to be gapless too at their ground-state and bring some light to the opening of its gap by mechanical deformations [2] (compressing and stretching) or applying external electromagnetic fields. Here we focus on the nature of the ground-state of bi-axial strained graphene and α-graphyne. The standard methodology to analyse electronic structure of materials, Density Functional Theory (DFT), have shown to be a useful tool, especially if one uses hybrid density functionals. However, the description of the system depends on the amount of Hartree-Fock Exchange (HFE), which can be tuned [3]. Although nowadays is well known what is the proper way to apply DFT for a rich set of systems (e.g. metals and inorganic systems), due to its hexagonal symmetry and the different electronic solutions in competition (Semimetallic, Antiferromagnetic and Quinoid/Dimerized), CORFs have highlighted some of the deficiencies of DFT and the need of a consistent method in order to reach the physically meaningful description. In addition to DFT calculations, Hubbard-type models will be used to rationalize the nature of low-energy electronic states and its dynamics [4]. Using accurate benchmark results for the latter, we show that the relative energetic stability of electronic states in this correlated 2D system can be accurately captured by DFT calculations using carefully tailored hybrid functionals to extract effective t and U Hubbard parameters. Our tuned DFT approach demonstrates that, while graphene maintains a SEM ground state up to moderate strains, in α-graphyne a low energy correlated Mott-like antiferromagnetic insulating (AFI) state emerges at low strains, which competes with the SEM state [5]. Accurate calculations show that strained graphene possesses correlated gapped states that are not recovered by GGA-based DFT. We show that an electronic analogy exists between this system and α-graphyne and that both are well-described by tuned hybrid (non-GGA) DFT. References [1] J. Adjizian, P. Briddon, B. Humbert, et al. Nat. Commun. 2014, 5, 5842. [2] S. Sorella, K. Seki, O. O. Brovko, T. Shirakawa, S. Miyakoshi, S. Yunoki and E. Tosatti,  Phys. Rev. Lett.  2018, 121, 066402. [3] I. Alcón, F. Viñes, I. de P. R. Moreira and S. T. Bromley, Nat. Commun. 2017, 8, 1957. [4] B. Dong, H. Guo, Z. Liu, T. Yang, P. Tao, S. Tang, R. Saito, and Z. Zhang, Carbon 2018, 131, 223. [5] G. Lleopart, I. de P. R. Moreira and S. T. Bromley, submitted.

[19] 8th of November 2022,[SLIDES]
Francesco Pederiva,
First steps in application of quantum computing to the study of dynamics and structure of many-body systems,
University of Trento, Physics Department, and INFN-TIFPA

open abstract
The recent advancement in quantum technologies has provided the possibility to start experimenting with realistic quantum computing. At present only relatively small systems can be simulated, due to the noise that still affects existing machines. On the other hand, one can start exploring problems that exploit the fundamental feature of such machines, i.e. the possibility of evolving in real time highly correlated states in a time that grows less than exponentially with the dimensionality of the Hilbert space. I will overview two examples that constitute proofs of concept for forthcoming algorithms. The first is an attempt to describe the two-neutron scattering with a simple interaction model that contains a spin-dependent Hamiltonian, and in which quantum computing is used to describe the time evolution of the spin state of the neutrons along the reaction path. The second is an algorithm that turns real time evolution into an imaginary time one by extending the Hilbert space with a "reservoir" and implementing a dynamical cooling process.

[20] 27th of October 2022,[SLIDES]
Juan Sánchez-Baena,
Heating a quantum dipolar fluid into a solid,
Aarhus University, Denmark

open abstract
Raising the temperature of a material enhances the thermal motion of particles. Such an increase in thermal energy commonly leads to the melting of a solid into a fluid and eventually vaporises the liquid into a gaseous phase of matter. Here, we study the finite-temperature physics of dipolar quantum fluids and find surprising deviations from this general phenomenology. In particular, we describe how heating a dipolar superfluid from near-zero temperatures can induce a phase transition to a supersolid state with a broken translational symmetry. The predicted effect agrees with experimental measurements on ultracold dysprosium atoms, which opens the door for exploring the unusual thermodynamics of dipolar quantum fluids. Bibliography: [1] Juan Sánchez-Baena, Claudia Politi, Fabian Maucher, Francesca Ferlaino, Thomas Pohl, "Heating a quantum dipolar fluid into a solid", arXiv: 2209.003355

[21] 27th of October 2022,[SLIDES]
Raúl Bombín,
Laser induced electron-phonon coupling screening for CO adsorbed on Pd(111),
Universidad del País Vasco

open abstract
In the last years it has been proposed to use the internal stretch mode of polar molecules adsorbed metal surfaces to monitor ultrafast processes in pump-probe experiments with femtosecond time resolution. In the majority of systems, the large transient temperatures that are induced in these experiments cause a red-shift on this mode due to the coupling to hot carriers. Conversely, it is commonly accepted that the blue-shift that has been observed in a few systems arises due to the coupling to other phonon modes, i.e. adsorbate frustrated translations or frustrated rotations. Here, we study the CO/Pd(111) system with a robust theoretical framework that includes electron-hole pair excitations and electron-mediated coupling between the vibrational modes. Our results reveal a new mechanism that screens the electron-phonon interaction and originates a blue-shift under current state-of-the-art pump-probe experimental conditions. We show that this has an intimate relation with the presence of an abrupt change in the density of states around the Fermi level.

[22] 4th of October 2022,[SLIDES]
Luca Asteria,
Quantum Gas Magnifier for accessing new regimes with ultracold atoms,
Hamburg University

open abstract
In this contribution we present results on quantum gas magnification as a microscopy tool for ultracold atoms in optical lattices. We developed a matter wave optics protocol which magnifies the atomic density distribution by almost two orders of magnitude [1], allowing to image the magnified cloud in a single-shot and with sub-lattice resolution by using standard optical imaging techniques. This approach works with 3D systems with many particles per lattice site, because it does not have the limitations of a small depth of focus and of light induced collisions. We demonstrate sub-lattice resolution by observing the dynamics within the lattice sites after a lattice potential quench, and we realize single-site addressing using magnetic resonance techniques. We also report on a novel phenomenon that occurs in the regime of many particles per lattice site when introducing a strong force into the system; this makes the correlated tunneling of pairs of atoms the relevant dynamical process and causes the spontaneous formation of a density wave [2]. Quantum gas magnification opens the path for spatially resolved studies of new quantum many-body regimes, like e.g. 3D systems, systems with high occupation numbers, orbital lattices, and lattice geometries with smaller lattice spacing. [1] L. Asteria et al., Nature 599, 571-575 (2021) [2] H. P. Zahn et al., PRX 12, 021014 (2022)

[23] 21st of July 2022,[SLIDES]
Nikolay Yegovtsev,
Strongly interacting impurities in a dilute Bose condensate,
Department of Physics University of Colorado Boulder CO

open abstract
An impurity in a Bose gas is commonly referred to as Bose polaron. For a dilute Bose gas its properties are expected to be universal, that is dependent only on a few parameters characterizing the boson-impurity interactions. When boson-impurity interactions are weak, it has been known for some time that the properties of the polaron depend only on the scattering length of these interactions. In this talk I will examine stronger boson-impurity interactions, keeping their range finite. It will be demonstrated that for attractive interactions between impurity and the bosons up to and including the unitary point of these interactions, all static properties of a Bose polaron in a dilute Bose gas can be calculated in terms of the scattering length and an additional parameter which characterizes the range of the impurity-boson interactions. One can produce explicit expressions for the energy and other properties of polaron for the case when the impurity-boson scattering length is tuned to unitarity. I will also discuss the first order correction in inverse powers of the impurity-boson scattering length around the unitary point. Bibliography [1] Pietro Massignan, Nikolay Yegovtsev, and Victor Gurarie “Universal Aspects of a Strongly Interacting Impurity in a Dilute Bose Condensate” Phys. Rev. Lett. 126, 123403 (2021) [2] Nikolay Yegovtsev, Pietro Massignan, Victor Gurarie “Strongly interacting impurities in a dilute Bose condensate” arXiv:2201.04295

[24] 1st & 5th of July 2022,[SLIDES]
Victor Gurarie,
Quenched BCS-BEC fermionic superfluids,
Department of Physics University of Colorado Boulder CO

open abstract
I will examine the dynamics of the BCS-BEC superfluids which were quenched (the parameters of their Hamiltonians suddenly changed). I will start with talking about the equilibrium behavior of the BCS-BEC superfluids and the BCS-BEC crossover. I will then discuss their dynamics if they are driven out of equilibrium by a sudden quench. If time permits, I will also touch upon our recent work regarding the Loschmidt echo in quenched superfluids. It turns out that the most straightforward way of calculating Loschmidt echo produces its simplified version, the “classical echo”, which deviates quite a bit from the full quantum echo. I will discuss how to approach correctly the calculation of full quantum Loschmidt echo in the fermionic superfluids.

[25] 6th of May 2022,[SLIDES]
Andrea Richaud,
Dynamics of massive point vortices in a binary mixture of Bose-Einstein condensates,
SISSA, Trieste, Italy

open abstract
In quantum matter, vortices are topological excitations characterized by quantized circulation of the velocity field. They are often modelled as funnel-like holes around which the quantum ﬂuid exhibits a swirling ﬂow. In this perspective, vortex cores are nothing more than empty regions where the superﬂuid density goes to zero. In the last few years, this simple view has been challenged and it is now increasingly clear that, in many real systems, vortex cores are not that empty. In these cases, the hole in the superﬂuid is ﬁlled by particles or excitations which thus dress the vortices and provide them with an effective inertial mass. In this talk, I will discuss the dynamics of two-dimensional point vortices of one species that have small cores of a different species. I will show how to derive the relevant Lagrangian itself, based on the time-dependent variational method with a two-component Gross-Pitaevskii (GP) trial function. The resulting Lagrangian resembles that of charged particles in a static electromagnetic field, where the canonical momentum includes an electromagnetic term. I will also show some interesting dynamical regimes. The simplest example is a single vortex within a rigid circular boundary, where a massless vortex can only precess uniformly. In contrast, the presence of a small core mass can lead to small radial oscillations, which are, in turn, clear signatures of the associated inertial effect. The analytical model is then benchmarked against detailed numerical simulations of coupled two-component GP equations with a single vortex and small second-component core. The presence of such radial oscillations is confirmed, implying that this more realistic GP vortex acts as if it has a small massive core. References: [1] A. Richaud, V. Penna, R. Mayol, and M. Guilleumas, Phys. Rev. A 101, 013630 (2020) [2] A. Richaud, V. Penna, and A. L. Fetter, Phys. Rev. A 103, 023311 (2021).

[26] 22 of March 2022,[SLIDES]
Joachim Brand,
Full Configuration Interaction Quantum Monte Carlo for Ultracold Atoms,
Dodd-Walls Centre for Photonic and Quantum Technologies, New Zealand Institute for Advanced Study, Massey University (Auckland, New Zealand)

open abstract
Full configuration interaction quantum Monte Carlo (FCIQMC) is a flavour of projector Monte Carlo and can be thought of as a stochastic extension to exact diagonalisation to find the ground state of an (almost) arbitrary square matrix. I will introduce the basic principles of FCIQMC and touch on recent work we did on population control and on understanding and avoiding the related population control bias. A basic challenge for treating ultracold atoms with short-range interactions is to deal with the wave function cusp, which can be done with lattice renormalisation or with the transcorrelated method, which achieves faster convergence with the size of the basis set. Finally, I will show how a momentum-space representation can be used to calculate the yrast spectrum consisting of the lowest possible energy as a function of the (fixed) total momentum of a mobile spin impurity in a one-dimensional Bose gas. The results give insight into the dynamics of mobile impurities in a non-integrable system and highlight the transition between polaron and depleton physics.

[27] 22 of February 2022,[SLIDES]
Andreas Haller,
Quantum Skyrmion Lattices in Heisenberg Ferromagnets,
Department of Physics and Materials Science, University of Luxembourg

open abstract
Skyrmions are topological magnetic textures which can arise in non-centrosymmetric ferromagnetic materials. In most systems experimentally investigated to date, skyrmions emerge as classical objects. However, the discovery of skyrmions with nanometer length scales has sparked interest in their quantum properties. Here, we simulate the ground states of two-dimensional spin-$1/2$ Heisenberg ferromagnets with Dzyaloshinskii-Moriya interactions and discover a broad region in the zero temperature phase diagram which hosts quantum skyrmion lattices. We argue that the quantum skyrmion lattice phase can be detected experimentally in the magnetization profile via local magnetic polarization measurements as well as in the spin structure factor measurable via neutron scattering experiments. Finally, we explore the resulting quantum skyrmion state, analyze its real space polarization profile and show that it is a non-classical state featuring entanglement between quasiparticle and environment mainly localized near the boundary spins of the skyrmion.

[28] 6th of July 2021,[SLIDES]
Gerard Pascual,
On the quasiparticle nature of the Bose polaron at finite temperature,
Departament de Fisica, Universitat Politècnica de Catalunya

open abstract
The Bose polaron has attracted theoretical and experimental interest because the mobile impurity is surrounded by a bath that undergoes a superfluid-to-normal phase transition. Although many theoretical works have studied this system in its ground state, only few analyze its behavior at finite temperature. We have studied the effect of temperature on a Bose polaron system performing ab-initio Path Integral Monte Carlo simulations. This method is able to approach the critical temperature without losing accuracy, in contrast with perturbative approximations. We have calculated the polaron energy for the repulsive and attractive branches and we have observed an asymmetric behavior between the two branches. When the potential is repulsive, the polaron energy decreases when the temperature increases, and contrariwise for the attractive branch. Our results for the effective mass and the dynamical structure factor of the polaron show unambiguously that its quasi- particle nature disappears close to the critical temperature, in agreement with recent experimental findings. Finally, we have also estimated the fraction of bosons in the condensate as well as the superfluid fraction, and we have concluded that the impurity hinders the condensation of the rest of bosons. [Reference] https://arxiv.org/abs/2105.07738

[29] 6th of July 2021,[SLIDES]
Marc Amorós,
Quantum particles in fractal external potential,
Universitat Politècnica de Catalunya

open abstract
The problem of a quantum particle in presence of an external fractal field is studied by Exact Diagonalization method. Two-dimensional geometry is considered with the external field corresponding to Sierpinski carpet fractal. A similar geometry can be realized in experiments with ultracold atoms and where essentially any arbitrary external field can be projected on the top of a two-dimensional system. Ground-state energy is calculated and is shown to increase exponentially as a function of the fractal iteration number. The excitation spectrum shows a formation of a series of gaps and micro-gaps, thus having an intermediate structure between spectra of periodic (gapped) and continuous (ungapped) systems. Finally, dynamics of spreading of a wave packet is studied and is shown to result in a subdiffusive expansion.

[30] 17th of June, 2021,[SLIDES]
Maxim Olshanii,
Triangular Gross-Pitaevskii breathers and Damski-Chandrasekhar shock waves,
Department of Physics, University of Massachusetts Boston

open abstract
The recently proposed map [arXiv:2011.01415] between the hydrodynamic equations governing the two-dimensional triangular cold-bosonic breathers [Phys. Rev. X 9, 021035 (2019)] and the high-density zero-temperature triangular free-fermionic clouds, both trapped harmonically, perfectly explains the former phenomenon but leaves uninterpreted the nature of the initial (t=0) singularity. This singularity is a density discontinuity that leads, in the bosonic case, to an infinite force at the cloud edge. The map itself becomes invalid at time t=T/4. Here, we first map -- using the scale invariance of the problem -- the trapped motion to an untrapped one. Then we show that in the new representation, the solution [arXiv:2011.01415] becomes, along a ray in the direction normal to one of the three edges of the initial cloud, a freely propagating one-dimensional shock wave of a class proposed by Damski in [Phys. Rev. A 69, 043610 (2004)]. There, for a broad class of initial conditions, the one-dimensional hydrodynamic equations can be mapped to the inviscid Burgers' equation, a nonlinear transport equation. More specifically, under the Damski map, the t=0 singularity of the original problem becomes, verbatim, the initial condition for the wave catastrophe solution found by Chandrasekhar in 1943 [Ballistic Research Laboratory Report No. 423 (1943)]. At t=T/8, our interpretation ceases to exist: at this instance, all three effectively one-dimensional shock waves emanating from each of the three sides of the initial triangle collide at the origin, and the 2D-1D correspondence between the solution of [arXiv:2011.01415] and the Damski-Chandrasekhar shock wave becomes invalid.

[31] 10th of June 2021,[SLIDES]
Dajun Wang,
Observation of the heteronuclear quantum droplet in an ultracold Na and Rb mixture,
Department of Physics, The Chinese University of Hong Kong, Hong Kong SAR, China

open abstract
The beyond-mean-field Lee-Huang-Yang (LHY) correction is ubiquitous in dilute ultracold quantum gases. However, its effects are often elusive due to the typically much larger influence of the mean-field energy. In this work, we study an ultracold mixture of 23Na and 87Rb with tunable attractive interspecies interactions. The LHY effects manifest in the formation of self-bound quantum liquid droplets and the expansion dynamics of the gas-phase sample. A liquid-to-gas phase diagram is obtained by measuring the critical atom numbers below which the self-bound behavior disappears. In stark contrast to trapped gas-phase condensates, the gas-phase mixture formed following the liquid-to-gas phase transition shows an anomalous expansion featuring a larger release energy for increasing mean-field attractions.

[32] 19th of May 2021,[SLIDES]
Mikhail Zvonarev,
Dynamical quantum transition of fast impurities and collective modes,
Univ Paris-Sud, Laboratoire LPTMS, UMR8626, Orsay, F-91405, France CNRS, Orsay, F-91405, France

open abstract
The challenge of understanding the dynamics of a mobile impurity in an interacting quantum many-body medium comes from the necessity of including entanglement between the impurity and excited states of the environment in a wide range of energy scales. In this talk, I investigate the motion of a finite mass impurity injected into (i) a three-dimensional quantum Bose fluid (ii) one-dimensional Fermi gas. For (i) we uncover a transition in the dynamics as the impurity's velocity crosses a critical value which depends on the strength of the interaction between the impurity and bosons as well as the impurity's recoil energy. Surprisingly, effects resembling this transition are also seen for the case (ii). There, we relate characteristic features in the time dynamics of the system with the emergence of a sharp collective mode.

[33] 6th of April 2021,[SLIDES]
Niccolò Traverso Ziani,
Low energy approaches to the one-dimensional Wigner molecule,
Dipartimento di Fisica, Università di Genova, 16146 Genova, Italy CNR spin, 16146 Genova, Italy

open abstract
Due to their charge, electrons interact with each other. When their mutual repulsion overcomes the kinetic energy, they tend to localize and form, in two and three spatial dimensions, a regular lattice called the Wigner crystal [1]. In one dimension, such process is strictly speaking forbidden. However, even in this case, the positions of the electrons can be correlated over lengths that exceed the physical size of the sample under exam. A regular array can then form. Such a state is known as the one-dimensional Wigner molecule and several beautiful experiments report signatures of its formation in suspended semiconducting carbon nanotubes [2,3]. The aims of the talk are summarizing the experimental results concerning the one-dimensional Wigner molecule and discussing some simple, but predictive, low energy effective models. Particular attention will be devoted to the Luttinger liquid [4] and the spin incoherent Luttinger liquid [5] pictures. A brief extension to fractional one-dimensional Wigner molecules [6] in topological systems will also be provided. [1] Wigner, E. On the Interaction of Electrons in Metals. Phys. Rev. 1934, 46, 1002. [2] Deshpande, V.V.; Bockrath, M. The one-dimensional Wigner crystal in carbon nanotubes. Nat. Phys. 2008, 4, 314. [3] Shapir, I.; Hamo, A.; Pecker, S.; Moca, C.P.; Legeza, O.; Zarand, G.; Ilani, S. Imaging the electronic Wigner crystal in one dimension. Science 2019, 364, 870. [4] Traverso Ziani, N.; Cavaliere, F.; Sassetti, M. Signatures of Wigner correlations in the conductance of a one-dimensional quantum dot coupled to an AFM tip. Phys. Rev. B 2012, 86, 125451. [5] Fiete, G.A. Colloquium: The spin-incoherent Luttinger liquid. Rev. Mod. Phys. 2007, 79, 801. [6] Traverso Ziani, N.; Crépin, F.; Trauzettel, B. Fractional Wigner crystal in the Helical Luttinger Liquid. Phys. Rev. Lett. 2015, 115, 206402.

[34] 2nd of February 2021,[SLIDES]
Fernando Sols,
Protected cat states in a driven superfluid boson gas,
Universidad Complutense de Madrid

open abstract
We investigate the behavior of a one-dimensional Bose-Hubbard gas in both a ring and a hard-wall box, whose kinetic energy is made to oscillate with zero time average, which suppresses first-order particle hopping while allowing even higher-order processes [1]. At a critical value of the driving, the system passes from a Mott insulator to an exotic superfluid phase. The system in the ring has similarities to the Richardson pairing model which can be exploited to understand key features of the interacting boson problem [2]. The superfluid ground state is a macroscopic quantum superposition, or cat state, of two many-body states characterized by the preferential occupation of opposite momentum eigenstates. Interactions give rise to a reduction (or modified depletion) cloud that is common to both macroscopically distinct states. Symmetry arguments permit a precise identification of the two orthonormal many-body branches forming the ground state. In the ring, the system is sensitive to variations of the effective flux but in such a way that the macroscopic superposition is preserved. We discuss other physical aspects that contribute to protect the catlike nature of the ground state. We study the robustness of the effect against variations in the signal shape and switching protocol. [1] G. Pieplow, F. Sols, C. E. Creffield, New J. Phys. 20, 073045 (2018). [2] G. Pieplow, C. E. Creffield, F. Sols, Phys. Rev. Research 1, 033013 (2019).

[35] 2nd of October 2020,[SLIDES]
Duncan O’Dell,
Violent relaxation in quantum fluids with long-range interactions ,
Department of Physics and Astronomy, McMaster University

open abstract
Violent relaxation is a process that occurs in systems with long-range interactions. It has the peculiar feature of dramatically amplifying small perturbations, and rather than driving the system to equilibrium, it instead leads to slowly evolving configurations known as quasistationary states that fall outside the standard paradigm of statistical mechanics. Violent relaxation was originally identified in gravity-driven stellar dynamics; here, I will discuss its extension into the quantum regime by developing a quantum version of the Hamiltonian mean field (HMF) model which exemplifies many of the generic properties of long-range interacting systems. The HMF model can either be viewed as describing particles interacting via a cosine potential, or equivalently as the kinetic XY model with infinite-range interactions, and its quantum fluid dynamics can be obtained from a generalized Gross-Pitaevskii equation. I will also show that singular caustics that form during violent relaxation in the classical model are regulated by interference effects in a universal way described by Thom’s catastrophe theory applied to waves and this leads to emergent length scales and timescales not present in the classical problem. In the deep quantum regime we find that violent relaxation is suppressed altogether by quantum zero-point motion. If time permits I will also discuss soliton solutions of the HMF model. These results are relevant to laboratory studies of self-organization in cold atomic gases with long-range interactions.

[36] 23rd of September 2020,[SLIDES]
Dmitry Petrov,
Université Paris-Saclay, CNRS, LPTMS, 91405, Orsay, France

open abstract
Repulsive Bose-Bose mixtures are known to either mix or phase-separate into pure components. Here we predict a mixed-bubble regime in which bubbles of the mixed phase coexist with a pure phase of one of the components. This is a beyond-mean-field effect which occurs for unequal masses or unequal intraspecies coupling constants and is due to a competition between the mean-field term, quadratic in densities, and a nonquadratic beyond-mean-field correction. We find parameters of the mixed-bubble regime in all dimensions and discuss implications for current experiments. [1] arXiv:2008.05870

[37] 28th of April 2020,[SLIDES]
Claudio Benzoni,
Rayleigh edge waves in two-dimensional chiral crystals,
Technical University of Munich, Germany

open abstract
We investigate, within the framework of linear elasticity theory, edge Rayleigh waves of a two-dimensional elastic solid which breaks time-reversal and parity symmetries due to the Coriolis-Lorentz force. We find that the direction of propagation of the Rayleigh modes is determined not only by the sign of the magnetic field but also by the Poisson ratio of the elastic system. We discover three qualitatively different regions distinguished by the chirality of the low-frequency edge waves and study their universal properties. To illustrate the Rayleigh edge-waves in real time we have carried out finite-difference simulations of the model.

[38] 16th of April 2020,[SLIDES]
G. E. Astrakharchik,
Spontaneous formation of Kagomé lattice in two-dimensional Rydberg atoms,
Departament de Fisica, Universitat Politècnica de Catalunya

open abstract
Two-dimensional Rydberg atoms are modeled at low temperatures by means of the classical Monte Carlo method. The Coulomb repulsion of charged ions competing with the repulsive van der Waals long-range tail is modeled by a number of interaction potentials. We find that under specific conditions the usual triangular crystal becomes unstable with respect to more exotic lattices such as Kagomé, flower, molecular crystal and rectangular chain packings. Ground-state configurations are obtained by means of the annealing procedure and their stability is additionally studied by the normal-mode analysis. While commonly the square lattice is mechanically unstable due to softening of the shear modulus, we were able to find a specific set of parameters for which the square lattice can be made stable.

[39] 3rd of December 2019,[SLIDES]
Raúl Bombín,
BKT transition and stripe phase of a 2D dipolar gas,
Departament de Física, Universitat Politècnica de Catalunya

open abstract
We study a system of bosonic dipoles that are polarized with all their dipole moments forming a certain angle with the plane containing their movement. As a first approach, the superfluid properties of the different phases that appear in the phase diagram of this system were studied at zero temperature (T=0), showing that both for the gas and stripe phase can sustain a superfluid flow. Then, by performing Path Integral Monte Carlo simulations, we extend this study to finite temperature in order to determine the critical temperature $T_c$ at which the transition from superfluid to normal phase occurs. In two dimensional systems this transition is known to be driven by topological vortices that destroy the superfluid signal and follows the Berezinskii–Kosterlitz–Thouless (BKT) scaling. We show that, despite of the anisotropy present when dipoles are tilted, both the gas and the stripe phase follow the BKT scaling allowing us to extract the critical temperature across the phase diagram. We also evaluate the One-Body-Density-Matrix at different temperatures in the stripe phase to show the existence of a quasi-condensate below the critical temperature, which is characteristic of the BKT scenario.

[40] 11th of October 2019,[SLIDES]
Boris A. Malomed,
Nonlinear dissipative photonics with spin-orbit coupling,
Department of Physical Electronics, School of Electrical Engineering, Faculty of Engineering and the Center for Light-Matter Interaction, Tel Aviv University, Tel Aviv, Israel

open abstract
Nonlinear dissipative optics in 2D settings has been a subject of studies for a long time [1]. Currently, a great deal of interest is drawn to 2D patterns in polariton condensates [2]. An important effect in binary polariton condensates, including TE- and TM-polarized photon fields, is spin-orbit coupling (SOC). The present talk aims to summarize recent results predicting robust localized vortex modes (quasi-solitons) supported by the interplay of gain, loss, nonlinearity, and SOC. Dissipative SOC-driven solitons were first obtained in the 2D model including saturable gain (g > 0), loss (a > 0) and SOC constant [3] SOC couples terms with vorticity difference between components . As a result, the model generates two species of stable 2D solitons: vortex-antivortex (VAV) pairs and semi-vortices, the latter ones being a bound state of a zero-vorticity soliton in one component and vortex in the other, as shown in the figure, which displays amplitude and phase patterns in each component: Further, in a physically relevant setting localization can be provided not by self-trapping but by an external trapping potential [4]: This model generates stable VAVs and mixed modes (MMs) states, which combine terms with vorticities 0 and in both components. In nonlinear optical settings per se, SOC can be realized in a dual-core planar waveguide with PT symmetry, represented by the balanced linear gain and loss in the two cores. Stable soliton families of the MM type have been found in this setting [5]. [1] A.G. Vladimirov et al., Numerical investigation of laser localized structures J. Opt. B 1, 101 (1999). [2] C. Schneider, K. Winkler, M.D. Fraser, M. Kamp, Y. Yamamoto, E.A. Ostrovskaya, S. Höfling, Exciton-polariton trapping and potential landscape engineering, Rep. Prog. Phys. 80, 016503 (2017). [3] T. Mayteevarunyoo, B. Malomed, and D. Skryabin, Vortex modes supported by spin-orbit coupling in a laser with saturable absorption, New J. Phys. 20, 113019 (2018). [4] H. Sakaguchi, B. A. Malomed, and D. V. Skryabin, Spin-orbit coupling and nonlinear modes of the polariton condensate in a harmonic trap, New J. Phys. 19, 08503 (2017). [5] H. Sakaguchi and B. A. Malomed, One- and two-dimensional solitons in PT-symmetric systems emulating spin-orbit coupling, New J. Phys. 18, 105005 (2016).

[41] 25th of September 2019,[SLIDES]
Dimitri M. Gangardt,
Full counting statistics and large deviations in thermal 1D Bose gas,
University of Birmingham, UK

open abstract
Above quantum degeneracy temperature one-dimensional bosons behave like classical particles. In particular the distribution of number of particles in an interval, known as Full Counting Statistics (FCS), is Poissonian. This behaviour changes substantially once bosons become degenerate with decreasing temperature. In this talk I will present results for FCS in a parametrically wide range of temperatures and interval lengths which exists in the weakly interacting regime of Lieb-Liniger model. The obtained distribution deviates significantly from Poissonian and, for sufficiently short intervals, the probability of large number fluctuations is strongly enhanced.

[42] 18th of September, 2019,[SLIDES]
Meera Parish,
Quantum impurities in a bosonic medium,
Monash University, Melbourne, Australia

[43] 18th of September 2019,[SLIDES]
G. Bighin,
Far-from-equilibrium dynamics of molecules in 4He nanodroplets: a quasiparticle perspective ,
1IST Austria (Institute of Science and Technology Austria) 2Max Planck Institute of Quantum Optics

open abstract
Angular momentum plays a central role in a plethora of quantum processes, from nuclear collisions to decoherence in quantum dots to ultrafast magnetic switching. We consider a single molecule embedded in a superfluid Helium nanodroplet as a prototype of a fully controllable many-body system in which to reveal angular momentum dynamics: an ultrashort, high-intensity laser pulse can induce molecular axis alignment, creating extreme out-of-equilibrium conditions, while imaging of molecular fragments after Coulomb explosion allows to obtain time-resolved measurements of molecular alignment [1]. The rotational dynamics of a molecule in superfluid Helium cannot be simply understood in terms of interference of rotational molecular states due to the strong interactions with many-body environment: we show that this scenario can be described in terms of the angulon quasiparticle [2,3] – a quantum rotor dressed by a field of many-body excitations – with a very good agreement with experimental data [4] for several molecular species and across a wide range of laser fluences. In the second part of my talk I will introduce a Diagrammatic Monte Carlo approach to angular momentum redistribution in a many-body system [5]. The technique is based on a diagrammatic expansion that merges the usual Feynman diagrams with the angular momentum diagrams known from atomic and nuclear structure theory, thereby incorporating the non-Abelian algebra inherent to quantum rotations. The technique is general and can be applied to a broad variety of systems possessing angular momentum degrees of freedom, thereby establishing a far-reaching connection between Diagrammatic Monte Carlo techniques and molecular simulations. References 1. D. Pentlehner et al., Phys. Rev. Lett. 110, 093002 (2013). 2. R. Schmidt and M. Lemeshko, Phys. Rev. Lett. 114, 203001 (2015). 3. M. Lemeshko, Phys. Rev. Lett. 118, 095301 (2015). 4. I.N. Cherepanov, G. Bighin, L. Christiansen, A.V. Jørgensen, R. Schmidt, H. Stapelfeldt, M. Lemeshko, arXiv:1906.12238. 5. G. Bighin, T. V. Tscherbul and M. Lemeshko, Phys. Rev. Lett. 121, 165301 (2018).

[44] 10th of May 2019,[SLIDES]
Sebastiano Pilati,
Supervised machine learning for ultracold atoms in speckle disorder,
University of Camerino, Italy

open abstract
Due to the availability of big databases, of high performance computers, and of efficient optimization algorithms, the use of machine learning techniques in the industry and in scientific research is growing at an impressive rate. In this work we consider the use of supervised machine techniques to address quantum many-body problems relevant for experiments performed on disordered ultracold atoms. After a broad introduction on supervised machine techniques, I will show how a deep neural network can be trained to make accurate predictions on the low-lying energy levels of noninteracting atomic gases in optical speckle patterns. Interestingly, we find that the neural network is remarkably resilient to the noise added in the training (here, synthetic) data, suggesting that, in the long term, cold-atom quantum simulations could be used to training neural network to solve hard problems that cannot be efficiently addressed using classical computers as, e.g., the many-fermion problem. I will also show recent results on the use of convolutional neural networks, usually employed in image recognition applications, to build a scalable neural network model that allows one to make predictions for system sizes larger than the ones employed during the training stage, providing us with a novel extrapolation technique. I will also highlight the use of transfer learning techniques to speed-up the learning process for large system sizes. S. Pilati, P. Pieri, Scientific Reports 9, 5613 (2019)

[45] 20th of March 2019,[SLIDES]
Andrea Trombettoni,
Off-Diagonal Long-Range Order in One-Dimensional Quantum Systems,
CNR and SISSA, Trieste, Italy

open abstract
A quantum system exhibits off-diagonal long-range order (ODLRO) when the largest eigenvalue of the one-body-density matrix scales as N, where N is the total number of particles. More generally, if the largest eigenvalue scales as N^C to define the scaling exponent C, then C=1 corresponds to ODLRO and C=0 to the single-particle occupation of the density matrix orbitals. When 0

[46] 30th of January 2019,[SLIDES]
Andreas Haller,
Engineering fractional Quantum Hall phases in 1D fermionic lattices,
Institute of Physics, Johannes Gutenberg University, Mainz, Germany Graduate School Materials Science in Mainz, Mainz, Germany

open abstract
Inspired by the recent experimental advances in the study of ultracold atoms trapped in optical lattices, we consider models of fermions hopping in ladder geometries and subject to artificial magnetic fluxes. By applying the concept of resonances in chiral currents, we aim for evidence about fractional Quantum Hall (FQH) phases: In case of nearest-neighbor Hubbard interactions, we identify a gap in the spin sector of the corresponding Luttinger liquid leading to a resonant state at fractional filling factor ν = 1/2. We support our analytic results with matrix product sates (MPS) simulations [1]. [1] A. H., M. Rizzi and M. Burrello, NJP 20, 053007 (2018)

[47] 9th of October 2018,[SLIDES]
Serguey Andreev,
Effective interactions in a quantum Bose-Bose mixture,
ITMO University, St. Petersburg, Russia

open abstract
Application of the methods of Quantum Electrodynamics (QED) to a system of bosons at absolute zero temperature put forward by Spartak Beliaev in 1958 has been one of the most powerful analytical methods in studies of Bose-Einstein condensates. The Beliaev theory provides a prescription of replacement of the actual microscopic interaction by an effective potential which can be used for perturbative expansion of the many-body Hamiltonian. Originally designed for one-component systems, the method has recently been applied to binary Bose mixtures in the context of supersolidity and stabilization of collapsing Bose-Einstein condensates by quantum fluctuations. The present work is aimed at investigation of legitimacy of extrapolation of the Beliaev prescription to two-component systems. We show that quantum scatterings of different components, which until now have been assumed independent, can interfere due to the Andreev-Bashkin entrainment effect. The effect manifests itself in renormalization of the elementary excitations of the system. This result has escaped the earlier considerations based on the Fourier expansion of small-amplitude oscillations of the order parameter. We explain how one can account for the effect by using a properly generalized Bogoliubov approach. In 3D the effect appears in the second order of the perturbation theory, which makes possible using the concept of effective potential in this case. The entrainment arises due to « dressing » of magnons with Bogoliubov phonon modes, by analogy with the physics of Bose polaron. We exploit this fruitful analogy to speculate on possible formation of a magnon crystal in the strongly-interacting regime.

[48] 20th of September 2018,[SLIDES]
Gary Williams,
Superfluid Phase Transition of Liquid 4He in Two and Three Dimensions,
Department of Physics and Astronomy UCLA

open abstract
The 2016 Nobel Prize in Physics highlighted the role of topological excitations in condensed matter. I will review the seminal Kosterlitz-Thouless (KT) theory of the superfluid phase transition in two-dimensional 4He films, and the experimental work at UCLA and Cornell that led to the verification of that theory. At the time the role of the topological quantized vortices in the 2D theory was highly controversial, but is now completely accepted. Further experimental work in my lab putting helium films on porous materials (which are 3D connected) showed a crossover from 2D to 3D critical behavior, where the excitations are vortex loops rather than the vortex pairs of the 2D theory. This understanding then allowed us to formulate a theory of the lambda transition in bulk 3D helium, using the same ideas of KT, but based on the renormalization of vortex loops. Very recently we have studied the superfluid transition of helium films on the surface of carbon nanotubes, and surprisingly have found disagreement with the KT universal prediction, by a factor of 3. This result is still not understood, but we can speculate that this could arise from a 3D corrugation of the helium surface induced by the anisotropic pull from the ordered carbon rings, so that the 2D KT theory may no longer be strictly valid.

[49] 4th of July 2018,[SLIDES]
Mikhail Zvonarev,
Propagation of an impurity through a quantum medium,
Univ Paris-Sud, Laboratoire LPTMS, UMR8626, Orsay, F-91405, France CNRS, Orsay, F-91405, France

open abstract
I will report on recent theoretical and experimental progress in understanding the dynamics of an impurity particle injected into a one-dimensional quantum liquid. I will show that the momentum distribution of the impurity subject to a constant external force exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. As a consequence, the impurity exhibits periodic dynamics that is interpreted as Bloch oscillations, which arise even though the quantum liquid is translationally invariant. I will also discuss a quantum flutter phenomenon, whose essence is that the impurity injected into a liquid with some initial momentum sheds only a part of it to the background gas, and forms a correlated state that no longer decays in time; furthermore, if the initial momentum is large enough, the impurity undergoes long-lived oscillations. The value of the impurity's velocity at infinite time lies between zero and the speed of sound in the gas, and is determined by the injection protocol. This way, the impurity's frictionless motion is a dynamically emergent phenomenon whose description goes beyond accounting for the kinematic constraints of Landau's approach to superfluidity.

[50] 4th of July 2018,[SLIDES]
Nathan L. Harshman,
Anyons from Three-Body Hard-Core Interactions in One Dimension,
Department of Physics, American University, Washington, DC, USA

open abstract
Traditional anyons in two dimensions have generalized exchange statistics governed by the braid group. By analyzing the topology of configuration space, we discover that an alternate generalization of the symmetric group governs particle exchanges when there are hard-core three-body interactions in one-dimension. We call this new exchange symmetry the traid group and demonstrate that it has abelian and non-abelian representations that are neither bosonic nor fermionic, and which also transform differently under particle exchanges than braid group anyons. We show that generalized exchange statistics occur because, like hard-core two-body interactions in two dimensions, hard-core three-body interactions in one dimension create defects with co-dimension two that make configuration space no longer simply-connected. Ultracold atoms in effectively one-dimensional optical traps provide a possible implementation for this alternate manifestation of anyonic physics.

[51] 18th of April 2018,[SLIDES]
Bruno Julia,
Fermionic properties of identical bosons in 2d traps,
Universitat de Barcelona

open abstract
We consider a few number of identical bosons trapped in a 2D isotropic harmonic potential and also the N-boson system when it is feasible [1,2]. The atom-atom interaction is modelled by means of a finite-range Gaussian interaction. The spectral properties of the system are scrutinized, in particular, we derive analytic expressions for the degeneracies and their breaking for the lower-energy states at small but finite interactions. We demonstrate that the degeneracy of the low-energy states is independent of the number of particles in the noninteracting limit and also for sufficiently weak interactions. In the strongly interacting regime, we show how the many-body wave function develops holes whenever two particles are at the same position in space to avoid the interaction, a mechanism reminiscent of the Tonks-Girardeau gas in 1D [3]. The evolution of the system as the interaction is increased is studied by means of the density profiles, pair correlations and fragmentation of the ground state for N = 2, 3, and 4 bosons

[52] 17th of April 2018,[SLIDES]
Marek Tylutki,
Imbalanced Superfluid Fermi Gas on a Lieb Lattice,
Aalto University, Finland

open abstract
We study the Hubbard model with spin imbalance on a Lieb lattice, which features a flat band. The flat band is known to enhance superfluidity due to a diverging density of states. We obtain a phase diagram of this system, and we identify a variety of phases. Apart from the standard BCS and normal states, we observe the Fulde-Ferrell-Larkin-Ovchinnikov type phases with space-dependent pairing function. We also find a thermodynamically stable Sarma phase, where the imbalance coexists with a spatially uniform order parameter. https://arxiv.org/abs/1802.00274

[53] 23rd of February 2018,[SLIDES]
Sebastiano Pilat,
Projective Quantum Monte Carlo Simulations of Ultracold Fermi Gases and adiabatic quantum computers,
University of Camerino, Italy

open abstract
I will review some recent studies on the ground-state properties of strongly-repulsive Fermi gases performed using projective Quantum Monte Carlo techniques. I will focus of the emergence of ferromagnetic phases in clean and in disordered Fermi gases, and on (quasi) antiferromagnetic phases emerging in shallow optical lattices. If time permits, I will highlight recent works in the context of adiabatic quantum computing, discussing how projective QMC simulations can be used to gain insight on the possible superiority of adiabatic quantum computers in solving complex optimization problems, compared to classical optimization methods.

[54] January 31st, 2018,[SLIDES]
Luca Parisi,
Dipartimento di Fisica, Università di Trento and CNR-INO BEC Center, Trento, Italy

open abstract
In recent years it has become possible to investigate interacting many body systems with ultracold atoms. In 3D and 2D reaching strongly correlated regimes is hindered by the instability towards collapse but can be experimentally achieved in quasi-1D system, where the instability is absent [1]. Moreover, in mixtures of two strongly interacting gases you can also have the Andreev-Bashkin effect, an effect in which a superflow in one component induces a supercurrent in the second component. I will first review the general physics of one-dimensional systems before presenting recent Quantum Monte Carlo results, in which we investigate the Andreev-Bashkin effect in one-dimensional mixtures and its consequence on spin excitations [2]. [1] Paredes B, Widera A, Murg V, Mandel O, Fölling S, Cirac I, Shlyapnikov GV, Hänsch TW, Bloch I. "Tonks-Girardeau gas of ultracold atoms in an optical lattice" Nature 429, 277–281 (20 May 2004) [2] Luca Parisi, G.E. Astrakharchik, Stefano Giorgini "Spin dynamics and Andreev-Bashkin effect in mixtures of one-dimensional Bose gases" arXiv:1801.03446

[55] January 30th, 2018,[SLIDES]
Alexandre Dauphin,
Identifying Quantum Phase Transitions with Deep Convolutional Neural Networks,
ICFO– Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology

open abstract
The identification of phases of matter is a challenging task, especially in quantum mechanics, where the complexity of the ground state appears to grow exponentially with the size of the system. We address this problem with state-of-the-art deep learning techniques: adversarial domain adaptation. We derive the phase diagram of the whole parameter space starting from a fixed and known subspace using unsupervised learning. The input data set contains both labeled and unlabeled data instances. The first kind is a system that admits an accurate analytical or numerical solution, and one can recover its phase diagram. The second type is the physical system with an unknown phase diagram. Adversarial domain adaptation uses both types of data to create invariant feature extracting layers in a deep learning architecture. Once these layers are trained, we can attach an unsupervised learner to the network to find phase transitions. We show the success of this technique by applying it on several paradigmatic models: the Ising model with different temperatures, the Bose-Hubbard model, and the SSH model with disorder. The input is the ground state without any manual feature engineering, and the dimension of the parameter space is unrestricted. The method finds unknown transitions successfully and predicts transition points in close agreement with standard methods. This study opens the door to the classification of physical systems where the phases boundaries are complex such as the many-body localization problem or the Bose glass phase.

[56] January 12th, 2018,[SLIDES]
Jordi Mur-Petit,
Dipolar physics in lattices: Quantum magnetism and exotic quantum phases with molecules,
Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom

open abstract
The production and control of ultra-cold molecules has advanced extraordinarily in recent years, with several groups now having reported the production of a broad range of molecules in their absolute ground state [1], and observation of the coherent dynamics between different rotational states of polar molecules [2]. This advances moves us closer to building advanced quantum simulators for exotic strongly-correlated systems. Indeed, polar molecules trapped in optical lattices promise to be a fundamentally new tool for understanding the quantum physics of strongly correlated many-body systems. There is a vast number of theoretical proposals detailing the new and exciting physics to be revealed. For example, such systems will allow us to understand the exotic quantum phases of dipolar quantum gases [3] and to simulate lattices of interacting spins [4]. They will reveal fundamental aspects of quantum magnetism [5], and allow us to explore new regimes of superfluidity [6]. ***In this talk, I will first review recent experimental progress [1,2] and some of the phases that are expected to appear in these systems [3-6]. Then, I will outline our research projects with experimental groups in Durham and Imperial College to investigate the behaviour of generic many-body quantum systems encoded with ultracold molecules [7]. ***[1] See, e.g., S. Ospelkaus et al. [JILA], PRL 104, 030402 (2010); P. K. Molony et al. [Durham], PRL 113, 255301 (2014); M. Guo et al. [Hong Kong], PRL 116, 205303 (2016) [2] B. Yan et al. [JILA], Nature 501, 521 (2013); S. A. Will et al. [MIT], PRL 116, 225306 (2016); S. A. Will et al. [MIT], PRL 116, 225306 (2016); P. D. Gregory et al. [Durham], PRA 94, 041403(R) (2016); T. M. Rvachov et al. [MIT], PRL 119, 143001 (2017). [3] K. Góral et al., PRL 88, 170406 (2002); G. E. Astrakharchik et al., PRL 98, 60405 (2007); H. P. Büchler et al., PRL 98, 060404 (2007); A. Macià et al., PRL 109, 235307 (2012). [4] A. Micheli et al., Nat. Phys. 2, 341 (2006). [5] A.V. Gorshkov et al., PRL 107, 155302 (2011). [6] A. Pikovski et al., PRL 105, 215302 (2010). [7] https://www.qsum.org.uk

[57] 30th of November,[SLIDES]
Raúl Bombín,
Quantum Dipolar Systems,
Universitat Politècnica de Catalunya

open abstract
We analyze the ground state of a two-dimensional quantum system of bosonic dipoles with their dipolar moments all polarized with an static field which forms an angle α with respect to the axis perpendicular to the plane containing the dipoles. The anisotropic character of the dipolar interaction allows the system to reach a stripe phase under certain conditions of density, n, and polarization angle, α. In a previous work, this phase transition by means of the Static Structure Factor which shows that this phase has solid structure when we look in the direction in which the interaction is more repulsive but it is like a gas in the perpendicular direction[1]. Our recent calculations have shown that there are both superfluidity and condensate fraction in the bosonic stripe phase at zero temperature All the calculation have been performed using the variational and diffusion Monte Carlo methods (VMC and DMC) and also Path Integral ground state[2]. Current work is focused on the study of the superfluid to normal fluid transition in bosonic systems at finite temperature, which is of the Kosterlitz-Thouless type, via the Path Integral Monte Carlo method (PIMC). Current work is also being performed in the fermionic system of dipoles, with the aim of drawing regions of the phase diagram where the different phases exist, including the stripe one. References [1] A. Macia, J. Boronat and F. Mazzanti Phys. Rev. A 90, 061601R [2] R. Bombin, J. Boronat and F. Mazzanti arXiv:1708.07673v1 (2017)

[58] Wednesday, 29th of November 2017 at 11:30,[SLIDES]
Luis A. Peña Ardila,
ose Polaron in the strongly interacting Regime,
Aarhus university

open abstract
An impurity immersed in a Bose-Einstein condensate gets dressed by the low-energy excitations of the quantum gas and forms a quasiparticle termed Bose polaron. In the weakly interacting regime, where the impurity-boson coupling strength is small, many theoretical tools including mean-field and perturbative methods describe very well the physics of the Bose Polaron. However, the strong interacting limit where the impurity-boson scattering length is much larger than the inter-particle distance is still in debate. In this talk I address the ground-state properties of the Bose polaron in the strongly interacting limit. I will show that, more sophisticated techniques such as Monte-Carlo methods (QMC) are needed to describe this regime where quantum fluctuations and quantum correlations play an important role. We compare our findings with the most recent experiments in Aarhus and JILA. In the absence of Efimov physics, we also study the universality of the Bose polaron as a function of the gas parameter na3 of the BEC. Being n the density of the Bose and a the boson-boson scattering length. This equation of state allow us to understand the whether the Bose polaron survives when the bosonic bath is switched adiabatically off.

[59] Monday, 30th of October 2017 at 12:00,[SLIDES]
Pietro Massignan,
Bose polarons at finite temperature and strong coupling,
1 Departament de Física, Universitat Politècnica de Catalunya 2 ICFO– Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology

open abstract
The temperature dependence of a mobile impurity in a dilute Bose gas, the Bose polaron, is investigated for wide a range of impurity-bath interactions. Using a diagrammatic resummation scheme designed to include scattering processes important at finite temperature T, we show that the phase transition of the environment to a Bose-Einstein condensate at the critical temperature Tc leads to several non-trivial effects. The attractive polaron present at T=0 fragments into two quasiparticle states for finite temperature whenever |a|≳aB, with a and aB the impurity-boson and boson-boson scattering lengths. While the quasiparticle with higher energy disappears at Tc, the ground state quasiparticle remains well-defined across Tc. Its energy depends non-monotonically on temperature, featuring a minimum at Tc.

[60] Tuesday, 18th of July 2017 at 11:30,[SLIDES]
Giulia De Rosi,
Thermodynamic behavior of a one-dimensional Bose gas at low temperature and superfluidity in neutron-star matter,
INO-CNR BEC Center and Dipartimento di Fisica, Università di Trento, Italy

open abstract
In the first part of the seminar, I will show that the chemical potential of a one-dimensional (1D) interacting Bose gas exhibits a non-monotonic temperature dependence which is peculiar of superfluids. The effect is a direct consequence of the phononic nature of the excitation spectrum at large wavelengths exhibited by 1D Bose gases. For low temperatures T, I demonstrate that the coefficient in the T2 expansion of the chemical potential is defined by the zero-temperature density dependence of the sound velocity and it has been calculated along the crossover between the Bogoliubov weakly-interacting gas and the Tonks-Girardeau gas of impenetrable bosons. The theoretical predictions along the crossover are confirmed by comparison with the exactly solvable Yang-Yang model in which the finite-temperature equation of state is obtained numerically by solving Bethe-ansatz equations. Finally, we have estimated finite-size effects for a 1D ring geometry with periodic boundary conditions at zero-temperature for various thermodynamic functions, pointing out the occurrence of important deviations from the thermodynamic limit. In the second part of the talk, I will show the results of a study of the superfluid gap in pure neutron matter, associated with the formation of Cooper pairs in the 1S0 sector. The interaction responsible of the onset of superfluidity is an effective interaction coming from a nuclear Hamiltonian strongly constrained by phenomenology and obtained from the correlated basis function (CBF) perturbation theory and the formalism of cluster expansions. The calculations have been carried out using an improved version of the CBF effective interaction, in which three-nucleon forces are taken into account using a microscopic model. Our results show that the superfluid transition occurs at values of densities corresponding to the neutron-star inner crust. References: [1] G. De Rosi, G. E. Astrakharchik and S. Stringari "Thermodynamic behavior of a one-dimensional Bose gas at low temperature" Phys. Rev. A 96, 013613 (2017) [2] O. Benhar and G. De Rosi "Superfluid Gap in Neutron Matter from a Microscopic Effective Interaction" arXiv: 1705.06607 (2017)

[61] Friday, 30th of June 2017 at 11:30,[SLIDES]
Mikhail Zvonarev,
Propagation of an impurity through a quantum medium,
Univ Paris-Sud, Laboratoire LPTMS, UMR8626, Orsay, F-91405, France CNRS, Orsay, F-91405, France

open abstract
I will report on recent theoretical and experimental progress in understanding the dynamics of an impurity particle injected into a one-dimensional quantum liquid. I will show that the momentum distribution of the impurity subject to a constant external force exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. As a consequence, the impurity exhibits periodic dynamics that is interpreted as Bloch oscillations, which arise even though the quantum liquid is translationally invariant. I will also discuss a quantum flutter phenomenon, whose essence is that the impurity injected into a liquid with some initial momentum sheds only a part of it to the background gas, and forms a correlated state that no longer decays in time; furthermore, if the initial momentum is large enough, the impurity undergoes long-lived oscillations. The value of the impurity's velocity at infinite time lies between zero and the speed of sound in the gas, and is determined by the injection protocol. This way, the impurity's frictionless motion is a dynamically emergent phenomenon whose description goes beyond accounting for the kinematic constraints of Landau's approach to superfluidity.

[62] Friday, 9th of June 2017 at 12:00,[SLIDES]
Andrii Gudyma,
Monte Carlo Simulations of harmonically trapped atoms with spin-orbit coupling,
Faculty of Science, University of Split, Croatia

open abstract
Recent experimental realizations of the synthetic gauge fields in cold atoms induced interest to description of systems with spin-orbit coupling. In my talk I will discuss development of Monte Carlo algorithms for the systems with spin-orbit coupling and demonstrate their implementation on the example of the few-body trapped system with contact interaction and Rashba and Dresselhaus spin–orbit coupling.

[63] Friday, 9th of June 2017 at 11:00,[SLIDES]
Viktor Cikojevic,
Ground-state properties of dilute Bose-Bose droplets,
Faculty of Science, University of Split, Croatia

open abstract
We report diffusion Monte Carlo results of Bose-Bose mixtures with repulsive intraspecies interaction g11 = g22 and attractive interspecies interaction g12. For a given number of atoms we determine the critical attractive scatering length needed to form a self-bound droplet. The ground-state energies and the density profiles for increasing numbers of particles are determined. Deviations from the mean-field prediction as a function of the ratio of g12/g11 are discussed.

[64] Wednesday, 7th of June 2017 at 11:30,[SLIDES]
Eckhard Krotscheck,
Correlations in the low-density Fermi gas: Fermi-Liquid state, Dimerization, and BCS Pairing,
Department of Physics, University at Buffalo, SUNY

open abstract
We present FHNC-EL ground state calculations for low-density Fermi gases described by two model interactions, of varying strength. We first examine the low-density expansion of the energy and compare with the exact answer by Huang and Yang (H. Huang and C. N. Yang, Phys. Rev. 105, 767 (1957)). It is shown that a locally correlated wave function of the Jastrow-Feenberg type does not recover the quadratic term in the expansion of the energy in powers of a0kF, where a0 is the vacuum s-wave scattering length and kF the Fermi wave number. The problem is cured by adding second-order perturbation corrections in a correlated basis. Going to higher densities and/or more strongly coupled systems, we encounter an instability of the normal state of the system which is characterized by a divergence of the in-medium scattering length. We interpret this divergence as a phonon-exchange driven dimerization of the system, similar to what one has at zero density when the vacuum scattering length a0 diverges. We then study, in the stable regime, the superfluid gap and its dependence on the density and the interaction strength. We identify two different corrections to low-density expansions: One is medium corrections to the pairing interaction, and the other one finite-range corrections. We show that the most important finite-range corrections are a direct manifestation of the many-body nature of the system.

[65] Tuesday, 6th of June 2017 at 15:30,[SLIDES]
Mathias Gartner,
Time-dependent Variational Monte Carlo Simulations,
Institute for Theoretical Physics, Johannes Kepler University, Linz, Austria

open abstract
The novel method of Time-dependent Variational Monte Carlo Simulations (t-VMC), by G. Carleo[1], allows to simulate the time dynamics of quantum many-body systems. In this talk, I will explain the fundamental principles of the t-VMC method, which is based on minimizing the action of the system, using a trial wavefunction depending on time dependent variational parameters. Furthermore, I will show that propagation in imaginary time allows to optimize the trial wavefunction for the ground state of the system, which in turn can be used again as the starting point for simulating real time evolution. Application of t-VMC to simple test systems, as well as the optimization of the ground state wavefunction of a system with modified Lennard Jones interactions, are performed and the results are shown. [1] Carleo, G. (2011). Spectral And Dynamical Properties Of Strongly Correlated Systems (Doctoral dissertation).

[66] Wednesday, 17th of May 2017 at 11:30,[SLIDES]
Pavel Kryuchkov,
Unitary Bose gas with inverse square interaction,
Moscow Engineering Physics Institute

open abstract
Zero-temperature phase diagram of a Bose gas with inverse square interaction is studied by means of diffusion Monte Carlo method. A peculiarity of this system is that the kinetic and potential energies scale similarly with the density, thus the chemical potential can be written as Fermi energy times a constant (Bertsch parameter). Such behavior is similar to unitary limit in two-component Fermi gases. The system properties are independent of the density but are controlled by the strength of interactions. For weak interactions, Bogoliubov theory is derived and used to calculate correlation functions and condensate depletion. For strong interactions a crystal with FCC packing is formed. Low-energy excitations in all regimes are plasmons with square-root dispersion relation.

[67] Wednesday, 22nd of March 2017 at 11:30,[SLIDES]
M.C. Gordillo,
He3 en grafito,
Universidad Pablo de Olavide, Sevilla, Spain

open abstract
En esta charla presentaré los cálculos que hemos realizado para estudiar la adsorción de 3He en grafito. Los sistemas considerados fueron 3He sobre una lámina limpia de grafito, y 3He sobre una capa de 4He que a su vez está en contacto con el grafito. La técnica utilizada fue "diffusion Monte Carlo," lo que implica que nuestros resultados son directamente comparables con experimentos a muy bajas temperaturas. Las capas absorbidas se modelaron como tridimensionales y sometidas al potencial real helio-helio y helio-grafito. Esta descripción realista permitió resolver la discrepancia existente entre los resultados experimentales, que predecían la existencia de un líquido a bajas densidades, y los resultados de previas simulaciones realizados en sistemas completamente planos, que decían que esto era imposible.

[68] Wednesday, 5th of April 2017 at 11:30,[SLIDES]
Fabian Grusdt,
Ultracold polarons: from impurity atoms in a BEC to single holes in an anti-ferromagnetic Mott insulator ,
Harvard University, Cambridge, Massachusetts

open abstract
By doping a fermionic Mott insulator at half filling, it is believed that the anti-ferromagnetic (AFM) state is destroyed and a high-Tc superconducting phase can be reached. To unravel the physics of the hole-doped regime, an important first step is to understand the behavior of a single hole in the AFM. In analogy with the polaron problem, where a mobile impurity interacts with a surrounding bath, one can expect a so-called magnetic polaron to form when a hole is introduced into the AFM. The analogy between these two problems will be the main topic of this talk. After providing an overview of the Bose polaron problem -- an impurity strongly interacting with a Bose-Einstein condensate -- it will be argued that the magnetic polaron is more than just a hole dressed by magnetic excitations. Instead, the strong correlations in this system require a different physical picture, which will be provided in the strong-coupling limit where the hole-hopping is much faster than spin-exchange interactions. Very recently, the Heisenberg AFM has become accessible for quantum-gas microscopes with ultracold fermions in optical lattices. A brief overview of recent experimental progress towards observing single holes and their correlations with the spin environment will also be given.

[69] Wednesday, 1st of March 2017 at 11:30,[SLIDES]
Alexander Fetter,
Vortex Dynamics in Coherently Coupled Bose-Einstein Condensates,
Stanford University

open abstract
In classical hydrodynamics with uniform density, vortices move with the local fluid velocity. This description is rewritten in terms of forces arising from the interactions with other vortices. For example, two positive vortices experience a repulsive interaction and precess in the positive (anti-clockwise) sense. In contrast, coherent rf Rabi coupling between two components of a Bose-Einstein condensate (BEC) acts very differently: for two positive vortices, one in each component, the Rabi coupling induces an attractive interaction. As a result, the positive vortices now precess in the negative (clockwise) sense. In addition, coherent rf Rabi coupling with a single vortex in a two-component trapped BEC induces periodic transfer of vorticity between the two components at the applied Rabi frequency.

[70] Wednesday, 1st of February 2017 at 11:30,[SLIDES]
Robert E. Zillich,
Dynamics of Quantum Many-Body Systems far from Equilibrium,
Institute for Theoretical Physics, Johannes Kepler University, Linz, Austria

open abstract
I give an overview of some of our work on the nonlinear response of bosonic many-body systems: 1. We study interaction quenches of ultra-cold Bose gases via Feshbach resonances, where we find for example that a quench from weakly interacting to strongly interacting can lead to a localization of the perturbation in the pair distribution. Ideas for improvements and further studies using interaction quenches will be discussed. 2. Recent experiments on nonadiabatic alignment of molecules in superfluid 4He droplets have shown that the coupled molecule-He dynamics cannot be understood in terms of past experiments using cw spectroscopy. I will talk about our recent theoretical progress, which is still far from a full theoretical understanding of the nonlinear many-body dynamics of doped 4He droplets. We are developing our own theoretical tools based on the variational principle, but also use quantum Monte Carlo simulations, if feasible. I introduce our tools and methods, in particular the most recent developments for many-body systems far from equilibrium.

[71] Wednesday, 14th of December 2016 at 11:30,[SLIDES]
Giulia De Rosi,
Collective oscillations of trapped atomic gases in low dimensions: a tool for the investigation of collisional processes,
INO-CNR BEC Center and Dipartimento di Fisica, Università di Trento, Italy

open abstract
Since 20 years, both theoretical and experimental investigation of collective oscillations has been carried out in trapped quantum gases. Fermionic and bosonic gases at different interaction, temperature, dimensions and geometrical configurations have been studied. We show a unified description of collective modes for all above atomic gases. All collective frequencies have been calculated by solving a single equation for the flow velocity derived starting from the hydrodynamic equations. Moreover, by using the sum- rule approach, we predict a different excitation signal at high temperature for the dipole compression mode in the hydrodynamic (single frequency) and collisionless (beating of 2 frequencies) regime for a one dimension (1D) harmonically trapped Bose gas. This theoretical prediction opens promising perspectives for the experimental investigation of collisional effects in 1D. References: [1] G. De Rosi and S. Stringari, “Collective oscillations of a trapped quantum gas in low dimensions” , Phys. Rev. A 92 , 053617 (2015). [2] G. De Rosi and S. Stringari, “Hydrodynamic VS collisionless dynamics of a 1D harmonically trapped Bose gas” Accepted for publication in Phys. Rev. A, arXiv:1608.08417 (2016).

[72] Monday, 21st of November 2016 at 11:30,[SLIDES]
Pjotrs Grišins,
Stimulated and spontaneous Hawking effect in superfluid light,
University of Geneva, Switzerland

open abstract
Analog gravity models promise to shed light on the most fundamental phenomena in curved space-times, ranging from particle creation during cosmic expansion to the trans-planckian problem of the Hawking process. In my talk I will introduce the field of analog gravity and review our recent work [1] where we propose an experiment to measure the Hawking effect in a flowing exciton-polariton condensate. Out setup features an acoustic event horizon mimicking a cosmological black hole. We show that with current technology it is possible to detect and measure the temperature of analog Hawking emission, and predict a novel phenomenon of an emergent resonance cavity, a phenomenon that shares some features with a so-called eternal black hole. Finally, we analyze the density correlation pattern and identify the hallmark features of the entanglement of the Hawking radiation. [1] Pjotrs Grišins, Hai Son Nguyen, Jacqueline Bloch, Alberto Amo, and Iacopo Carusotto “Theoretical Study of Stimulated and Spontaneous Hawking Effects from an Acoustic Black Hole in a Hydrodynamically Flowing Fluid of Light” Phys. Rev. B 94, 144518 (2016).

[73] 29th of September 2016,[SLIDES]
Grecia Guijarro,
Gas de Bose en cables multifilamentos,
Universidad Nacional Autónoma de México

open abstract
En este trabajo estudiamos los efectos que ocasionan las estructuras periódicas sobre las propiedades termodinámicas de un gas de Bose sin interacciones. Consideramos estructuras tubulares semi-infinitas y finitas del tipo tubos o cintas multifilamentos. El confinamiento al que está sujeto el gas bosónico se modela usando un número finito de potenciales delta de Dirac en dos direcciones mutuamente perpendiculares, mientras que en la tercera dirección se permite que las partículas se muevan libremente. La estructura que se genera es la de un haz de filamentos de sección transversal rectangular y de paredes permeables excepto las más externas que forman las paredes del cable. El espectro de energías se obtiene de la solución de la ecuación de Schrödinger en 3D con el potencial externo mencionado más la condición de Dirichlet en la frontera del cable. Con la relación de dispersión, es decir, la energía de las partículas como función de su momento, y a través del gran potencial termodinámico se obtienen algunas propiedades termodinámicas usando la estadística de Bose-Einstein. Estudiamos el comportamiento del potencial químico, su primera y segunda derivada, el número de partículas en el estado de mínima energía así como su variación, el calor específico isocórico, la energía interna, entre otras propiedades, todas como función de la temperatura, y para diferentes valores de los parámetros del potencial externo (intensidad de las deltas y separación entre ellas) que definen las características de la estructura.

[74] Friday, 27th of May 2016 at 11:30,[SLIDES]
Emilio Flores Sola,
Finite-size scaling above the upper critical dimension,
1. Coventry University, England, 2. Université de Lorraine, France

open abstract
There should be no remaining doubt regarding the validity of the Renormalization-Group theory, which stands as one of the pillars of modern physics. However, finite-size scaling theory, which is derived from it, remains unclear above the upper critical dimension dc. There, the violation of the hyperscaling relation is an example of how dangerous is the irrelevant variable of the self interaction φ4 term above dc. Such phenomena make the correlation length to play a pivotal role and it can be shown that it scales as a non-trivial power of the linear system size. This relaxation allows us to fix the break down in the finite-size scaling and hyperscaling, through a modified scheme. We investigate this scaling behaviour in low dimensional Ising ferromagnets with long-range interactions both with free and periodic boundaries. Additionally, we discuss the role played by the Fourier modes and we show that the current phenomenological picture is not supported for all thermodynamic observables.

[75] Thursday, 26th of May 2016 at 10:30,[SLIDES]
Yaroslav Lutsyshyn,
Quantum Monte Carlo study of Mg-doped bulk helium-4 at zero temperature ,
University of Rostock

open abstract
Mg-doped helium droplets are believed to have an observable metastable state in which the alkali atoms remain separated by a considerable distance, thus forming so-called “atomic foam”. The exact nature of such a state is not well understood. I will describe our efforts to study the confinement-induced long-distance interaction between a pair of Mg atoms suspended in the superfluid.

[76] Wednesday, 16th of March 2016 at 11:30,[SLIDES]
Carlos Lobo,
Atoms in quadrupole traps: some puzzling results from recent experiments,
Mathematical Sciences, University of Southampton, UK

open abstract
Recent experiments in the group of C. Salomon (ENS-Paris) have shown some interesting behaviour for a classical gas of almost _non-interacting_ atoms trapped in a quadrupole potential, namely that when give a momentum kick to a 3D gas with very weak collision rate, it seems not to react along transverse directions, even though the quadrupole potential is not separable. Indeed the trajectories of individual atoms are highly complex but the gas as a whole seems to preserve a memory of the direction of the kick for very long (perhaps infinite) time. I will discuss the origin of the phenomenon and other issues related to classical irreversibility in the gas kinetics. An additional twist comes from considering a canonical transformation which maps the system onto that of a gas of Weyl fermions. This allows us to make nontrivial statements about the long term dynamics of these exotic particles in harmonic traps.

[77] Wednesday, 2nd of March 2015 at 11:30,[SLIDES]
Petar Stipanović,
Universality of quantum halo states,
Faculty of Science, University of Split, Croatia

open abstract
The results of the recent research in quantum halo systems will be presented. Ground state of weakly bound systems consisting of few particles with a radius extending well into the classically forbidden region is explored, with the goal to test the universality of quantum halo states. The focus of the study are clusters consisting of T↓, D↓, 3He, 4He and alkali atoms, where interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled very precise calculation of both size and binding energy of the trimers and tetramers. Using dimensionless measures of the binding energy and cluster size, studied atomic clusters are compared to other known halos in different fields of physics. Characteristic scaling lengths, which make size-energy ratio to be universal, are selected. In the quantum halo regime, and for large values of scaled binding energies, all dimers and trimers follow almost the same universal line. As the scaled binding energy decreases, tango type trimers separate from Borromean type. Research is extended to tetramers. Furthermore, the structural properties of different trimers are compared with recently published experimental results obtained by Coulomb explosion imaging of diffracted clusters.

[78] Tuesday, 16th of February 2015 at 11:30,[SLIDES]
Eduard Matito,
Harmonium atom as a calibration and development tool in DFT,
Ikerbasque, Donostia International Physics Center (DIPC) and Kimika Fakultatea. Euskal Herriko Unibertsitatea (UPV/EHU), 20018 Donostia, Spain

open abstract
The construction of energy functionals in DFT is a complicated task because there are very few conditions known about the exact functional and they are diffcult to impose in functional expressions. Despite these difficulties in the last years quite many functionals have been put forward, broadening the choice of methods in computational chemistry. In this regard, new theoretical models to explore and calibrate the existing zoo of functionals and construct new expressions are required. Harmonium atom is a model system that replaces the usual electron-nuclear attraction by a harmonic interaction tuned with the confinement parameter strength (ω). The model is realistic as far as the electron correlation is concerned and presents a continuous transit from weak electron-correlation regimes (large ω) to strong-correlation regimes (small ω), thus being a formidable test bed for density functional methods. In this talk, several popular density functionals will be analyzed in the two lowest-lying energy states of three-electron harmonium atom. None of the functionals tested (including a wide variety of LDA, GGAs, meta-GGAs and range-separated functionals) are accurate in the whole range ω values, failing short in the description of the weak correlation limit. Finally, it is shown that new hybridized functional expressions can remedy this pitfall attaining ten μ-hartree accuracy for the whole range of confinement strength values. [1] E. Matito and J. Cioslowski, in preparation. [2] E. Matito, J. Cioslowski and S. Vyboishchikov, Phys. Chem. Chem. Phys. 12, 6712 (2010); J. Cioslowski and E. Matito, J. Chem. Theory Comput. 7 915 (2011) [3] J. Cioslowski, K. Strasburger and E. Matito, J. Chem. Phys. 136 194112 (2012); ibid 141 044128 (2014)

[79] Wednesday, 3rd of February 2015 at 11:30,[SLIDES]
Guillem Ferré,
Phase diagram of a quantum Coulomb wire,
Departament de Fisica, Universitat Politècnica de Catalunya

open abstract
The quantum phase diagram of a one-dimensional Coulomb wire is obtained using the path-integral Monte Carlo method. The exact knowledge of the nodal points of this system permits us to find the energy in an exact way, solving the sign problem which spoils fermionic calculations in higher dimensions. The results obtained allow for the determination of the stability domain, in terms of density and temperature, of the one-dimensional Wigner crystal. At low temperatures, the quantum wire reaches the quantum-degenerate regime, which is also described by the diffusion Monte Carlo method. Increasing the temperature, the system transforms to a classical Boltzmann gas, which we simulate using classical Monte Carlo. At large enough density, we identify a one-dimensional ideal Fermi gas which remains quantum up to higher temperatures than in two-and three-dimensional electron gases. The obtained phase diagram and the energetic and structural properties of this system are relevant to experiments with electrons in quantum wires and to Coulomb ions in one-dimensional confinement. [1] G. Ferré, G. E. Astrakharchik, and J. Boronat Phys. Rev. B 92, 245305 (2015)

[80] 2nd of December 2015,[SLIDES]
Alessio Recati,
INO-CNR BEC Center and Dipartimento di Fisica, Università di Trento, 38123 Povo, Italy and Technische Universität München, James-Franck-Strasse 1, 85748 Garching, Germany

open abstract
Out-of-equilibrium states and quasi-many-body localization in polar lattice gases

[81] 30th of November 2015,[SLIDES]
Marek Tylutki,
Dipartimento di Fisica, Università di Trento and CNR-INO BEC Center, Trento, Italy

open abstract
Mixtures of Ultracold Atomic Gases

[82] 17th of November 2015,[SLIDES]
Natalia Matveeva,
LPMMC, UMR 5493 of CNRS, Université Grenoble Alpes, Grenoble, France

open abstract
Fixed-node diffusion Monte Carlo study of the BCS-BEC crossover in a bilayer system of fermionic dipoles

[83] 4th of November 2015 ,[SLIDES]
Stefano Giorgini,
Dipartimento di Fisica, Università di Trento and CNR-INO BEC Center, Trento, Italy

open abstract
The impurity problem in ultracold atomic gases

[84] 15th of October 2015 ,[SLIDES]
Boris A. Malomed,
Bright solitons from defocusing nonlinearities in optical and matter-wave media,
Department of Physical Electronics, School of Electrical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel

open abstract
The talk aims to give a review of recently obtained results which demonstrate that defocusing cubic media with a spatially inhomogeneous nonlinearity, whose strength increases rapidly enough toward the periphery (faster than r^D in the D-dimensional space, D = 1,2,3, where r is the radial coordinate), give rise to stable solitons (self-trapped localized modes) in all three dimension. These are one-dimensional (1D) fundamental and multihump solitons, 2D dipoles and quadrupoles, including spinning ones, and vortex solitons with an arbitrary topological charge in 2D, and vortex tori (soliton gyroscopes and Hopfions, i.e., intrinsically twisted tori with two independent topological numbers) in 3D. Solitons maintain their coherence in the state of motion, oscillating in the effective nonlinear trapping potential as robust objects. The 3D vortex tori exhibit stable precession and rotation, induced by the application of external torque. In addition to numerically found soliton families, particular solutions can be obtained in an exact analytical form, and accurate analytical approximations are available for the entire families, based on variational and Thomas-Fermi methods. Essentially the same mechanism for the self-trapping of bright solitons under the action of the spatially growing repulsive nonlinearity works in nonlocal media, and in discrete systems too, including the corresponding version of the quantum Bose-Hubbard model. Spontaneous symmetry breaking of the solitons in the 1D and 2D local-nonlinearity modulation profiles with the double-wall structure has been found too. Furthermore, numerical and analytical results demonstrate the existence of stable solitons in a PT-symmetric extension of the 1D model, and of stable dissipative solitons in media combining a uniform linear gain and nonlinear loss whose local strength grows toward the periphery faster than r^D. Such 1D and 2D settings can be implemented both in nonlinear optics and Bose-Einstein condensates (BEC), while the 3D setting may be created in BEC. [1] R. Driben, Y. V. Kartashov, B. A. Malomed, T. Meier, and L. Torner, "Soliton gyroscopes in media with spatially growing repulsive nonlinearity", Phys. Rev. Lett. 112, 020404 (2014) [2] R. Driben, Y. Kartashov, B. A. Malomed, T. Meier, and L. Torner, "Three-dimensional hybrid vortex solitons", New J. Phys. 16, 063035 (2014) [3] Y. V. Kartashov, B. A. Malomed, Y. Shnir, and L. Torner, "Twisted toroidal vortex-solitons in inhomogeneous media with repulsive nonlinearity", Phys. Rev. Lett. 113, 264101 (2014). [4] R. Driben, T. Meier, and B. A. Malomed, "Creation of vortices by torque in multidimensional media with inhomogeneous defocusing nonlinearity", Sci. Rep. 5, 9420 (2015) [5] R. Driben, N. Dror, B. Malomed, and T. Meier, "Multipoles and vortex multiplets in multidimensional media with inhomogeneous defocusing nonlinearity", New J. Phys. 17, 083043 (2015).

[85] 14th of October 2015 ,[SLIDES]
Jean Kormann,
3D Elastic Full Waveform Inversion: Toward Reflection Based Inversion,
Department of Computer Applications in Science and Engineering (CASE) Barcelona Supercomputing Center (BSC)

open abstract
Full Waveform Inversion (FWI) is one of the most challenging procedures to obtain quantitative information of the subsurface. On land seismic surveys, obtaining information from the recorded seismograms requires a full elastic approach. This shortcoming has been reduced as high-performance computing makes 3D elastic FWI more affordable. However, there is still a key issue for overcoming its computational burden and mitigate the requirement of having long-offset data. Here we introduce a Dynamic Offset Control, that is a data selection method for adapting the longer receiver offset to the current inverted frequency. We exemplify our approach through 2D and 3D synthetics, and afterwards, the algorithm was used to invert a real line from a 3D high resolution seismic reflection dataset acquired to image a shallow subsurface. This high resolution dataset images a 500x500 m block located in the Zancara river basin (Spain). The study area is characterized by a relatively heterogeneous geology revealed by the borehole data available and features topographic variations on the order of a few meters. As it is a very shallow target, free-surface effects and topography are critical issues which the code needs to handle appropriately. The obtained 3D velocity models were contrasted with other geophysical data available.

[86] 16th of September 2015,[SLIDES]
Vladimir A. Yurovsky,
"Abandoned symmetry",
Tel Aviv University

open abstract
First applications of the group-theoretical methods in quantum mechanics in works by Wigner, Heitler, and Dirac in 1926-1929 were devoted to the permutation symmetry. A general interest to this topic was lost after the discovery of the Pauli exclusion principle, which allows only permutation-symmetric or antisymmetric wavefunctions for bosons or fermions, respectively, and forbids non-Abelian irreducible representations of the symmetric group, where a wavefunction is transformed to a linear combination of several wavefunctions in the representation. Such representations can appear in physical systems with spinor and spatial degrees of freedom. In the talk I analyze the selection rules and conservation lows, related to the permutation symmetry, as well as the states of spinor gases, where the permutation symmetry properties are pronounced.

[87] 3rd of July,[SLIDES]
Alfredo Vidal Arias,
"Quantum description of one-dimensional hydrogen" (TGF thesis directed by Gregory Astrakharchik and Jordi Boronat),
Universitat Politècnica de Catalunya

[88] 3rd of July,[SLIDES]
Juan Sanchez Baena,
Microscopic Description of Confined Bosonic Dipolar Quantum Gases in 2 Dimensions (TFG thesis directed by Ferran Mazzanti and Jordi Boronat),
Universitat Politècnica de Catalunya

[89] 3rd of July,[SLIDES]
Aleix Gimenez Grau,
"Dynamics of Many-Body Quantum Systems" (TFG thesis directed by Ferran Mazzanti),
Universitat Politècnica de Catalunya

[90] Thursday, 27th of November 2014 at 16:00,[SLIDES]
Andrii Gudyma,
Quantum impurity in one-dimensional Bose gas,
Université Paris-Sud, Laboratoire de Physique Théorique et Modèles Statistiques, UMR CNRS 8626, Orsay, F-91405, France

open abstract
The problem of polaron in a one-dimensional Bose gas at ultralow temperature is addressed. When a single impurity moves in a background of interacting particles, its effective mass changes compared to the bare mass. We study how the effective mass depends on the interaction between the impurity and background. Diffusion Monte Carlo method is used in order to calculate the diffusion coefficient of the impurity in imaginary time. We study system properties under the same conditions as in Florence experiment [1]. The interaction between impurities (K atoms) and the bath (Rb atoms) were controlled by Feshbach and confinement-induced resonances. We find out that zero-temperature predictions agree with the experimental results. This has to be contrasted to the theory proposed in [1], in which the coupling constant had to be artificially corrected by a factor of 3.15. We obtain the polaron mass both for repulsion and attraction with the bath. We explicitly show that the complete three-dimensional simulation and effective one-dimensional descriptions lead to the same result. [1] J. Catani, G. Lamporesi, D. Naik, M. Gring, M. Inguscio, F. Minardi, A. Kantian, and T. Giamarchi “Quantum dynamics of impurities in a one-dimensional Bose gas” Phys. Rev. A 85, 023623 (2012).

[91] Thursday, 25th of September 2014,[SLIDES]
Konstantin Krutitsky,
“Propagation of quantum correlations after a quench in the Mott-insulator regime of the Bose-Hubbard model.” ,
Physics Department, University of Duisburg-Essen, Germany

open abstract
We have developed a new method which allows to study dynamics of quantum correlations in higher dimensional lattices of large size and gives reliable results for long times. The formalism is based on the equations of motion for the reduced density matrices. An infinite set of equations is truncated such that only two-point correlations are taken into account. This approximation is justified by exact diagonalization for small lattices in one and two dimensions. Approximate analytical solutions of the resulting equations are obtained within the framework of a perturbative expansion in powers of the inverse coordination number. Exact solutions are obtained by numerical integration of the equations of motion and show qualitative agreement with the analytical approximations. On the other hand, numerical calculations allow to achieve higher accuracy which is confirmed by comparison with the exact diagonalization. As an application we study the dynamics of interacting bosons in a lattice after quench within the Mott-insulator regime. It is shown that the time evolution of the local particle-number distribution is directly related to the propagation of correlations. In particular, we find the revival of oscillations of the local quantities after the propagation of correlations through the whole system. In two dimensions, the propagation of correlations is anisotropic with the minimal velocity along the lattices axes and maximal along the diagonals. Our method can be easily applied to inhomogeneous lattices which allows to include into consideration the harmonic traps as well as disorder potentials.

[92] Wednesday, 23rd of July 2014 at 11:30,[SLIDES]
Boris A. Malomed,
Stable two-dimensional composite solitons in spin-orbit-coupled self-attractive Bose-Einstein condensates in free space,
Department of Physical Electronics, School of Electrical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel

open abstract
It is commonly known that two-dimensional mean-field models of optical and matter waves with cubic self-attraction cannot produce stable solitons in free space because of the occurrence of collapse in the same setting. By means of numerical analysis and variational approximation, we demonstrate that the two-component model of the Bose-Einstein condensate with the Rashba spin-orbit coupling and cubic attractive interactions gives rise to STABLE solitary-vortex complexes of two types in the FREE SPACE: semivortices (SVs, with a vortex in one component and a fundamental soliton in the other), and mixed modes (MMs, with topological charges 0 and ±1 mixed in both components). The SVs and MMs realize the ground state of the system, provided that the self-attraction in the two components is, respectively, stronger or weaker than the cross attraction between them. The SVs and MMs which are not the ground states are subject to a drift instability. The modes of both types degenerate into unstable Townes solitons when their norms attain the respective critical values, while there is no lower existence threshold for the stable modes. Moving free-space stable solitons are also found in the present non-Galilean-invariant system, up to a critical velocity. Collisions between two moving solitons lead to their merger into a single one. References: Hidetsugu Sakaguchi, Ben Li, and Boris A. Malomed , “Creation of two-dimensional composite solitons in spin-orbit-coupled self-attractive Bose-Einstein condensates in free space”, Phys. Rev. E 89, 032920 (2014).

[93] Friday, 18th of July 2014 at 11:30,[SLIDES]
Marta Abad,
“Persistent currents in two-component condensates”,
INO-CNR BEC Center and Dipartimento di Fisica, Universita di Trento, Italy

open abstract
In this talk we address the stability of persistent currents in two-component Bose-Einstein condensates. We relate the instabilities shown by Bogoliubov excitations to the decay of persistent currents in the miscible regime of the binary mixture, finding a very good agreement with numerical simulations of the Gross-Pitaevskii equations. In particular, we analyze the presence of a partially stable region, where the superflow in the minority component decays while it remains stable in the majority component. The existence of this region could point out in the right direction to explain the experimental results. In addition, we study the behaviour of the system when a velocity difference between the two components drives a dynamical instability, known in the literature as counterflow instability. Finally, we comment on the stability of the currents in the phase separated regime of the binary mixture and also in the presence of a coherent coupling.

[94] Tuesday, 1st of July 2014 at 11:30,[SLIDES]
Leonid Glazman,
Nonlinear Quantum Liquids In One Dimension,
Department of Physics, Yale University, USA

open abstract
The conventional description of one-dimensional quantum fluids is based on the Luttinger liquid theory. In that theory, the true energy-momentum relation of particles making up the fluid is replaced by a linear one. This simplification is crucial for the theory, and abandoning it has proven to be difficult. The talk presents a breakthrough which allows one to circumvent the difficulty. The new theory describes dynamic responses of a fluid consisting of particles with a generic spectrum. It also provides a pathway for developing kinetic theory of a quantum fluid. The developed new description is applicable to a diverse group of systems, including, for example, electrons in quantum wires, one-dimensional spin liquids, and cold atomic gases in one-dimensional traps.

[95] Wednesday, 12th of March 2014 at 11:30,[SLIDES]
Andrii Gudyma,
LPTMS, Paris-Sud University

open abstract
Breathing modes of one-dimensional trapped BEC
One-dimensional ultracold atomic gas of bosons is considered. Frequencies of low-lying excitations are calculated for the parameters of the model relevant for the recent experiment from Innsbruck group [Science 325, 1224 (2009)]. Perturbative methods are combined with the exact Bethe Ansatz solution for the model. Analysis of results are made for both the weak coupling and the Tonks-Girerdeau regimes. Analytical calculations are complement with the results from the quantum Monte Carlo simulations. On the example of the two-particle system compared different methods of calculation of the breathing mode. Theoretical results confronted with data from experiment.

[96] Wednesday, 5th of March 2014 at 11:30,[SLIDES]
Riccardo Rota,
Dipartimento di Fisica e INO-CNR BEC center Universita` degli Studi di Trento

open abstract
“Quantum Monte Carlo approach to dynamic properties of ultracold Bose systems”
Using quantum Monte Carlo methods, we perform zero-temperature calculations of the dynamic structure factor of a system of Bose particles interacting via a hard-sphere potential. The hard-sphere model is able to capture, in the dilute regime, the essential properties of ultracold atoms with positive scattering length, and, at higher densities, it can been used to characterize strongly interacting systems where the hard-core repulsion at short distances is the leading part of the interatomic potential, like superfluid helium. With increasing density, we observe the appearance of an incoherent, multiphonon contribution in the spectral function at frequencies higher than the single quasi-particle peak and a crossover of the dispersion of elementary excitations from a Bogoliubov-like spectrum to a phonon-maxon-roton curve. The behavior of the dynamic structure factor at the same density in the stable solid and metastable gas phase above the freezing point is also discussed.

[97] Tuesday, 18th of February 2014 at 16:00,[SLIDES]
Guido Pupillo,
Université de Strasbourg and CNRS, Strasbourg, France

open abstract
“Dynamics of blockaded Rydberg gases in low dimensions”
We discuss the quantum phases and dynamics of a gas of two-dimensional Bosons with finite-range soft-core interactions. For low densities, the system is shown to form a solid in which superfluidity is provided by delocalized zero-point defects. This provides the first example of continuous-space supersolidity consistent with the Andreev-Lifshitz-Chester scenario. We further discuss the connection between quantum mechanical supersolid behaviour and a novel mechanism for a glass formation.

[98] Tuesday, 18th of February 2014 at 16:00,[SLIDES]
Guido Pupillo,
Universite de Strasbourg and CNRS, Strasbourg, France

open abstract
“Dynamics of blockaded Rydberg gases in low dimensions”
We discuss the quantum phases and dynamics of a gas of two-dimensional Bosons with finite-range soft-core interactions. For low densities, the system is shown to form a solid in which superfluidity is provided by delocalized zero-point defects. This provides the first example of continuous-space supersolidity consistent with the Andreev-Lifshitz-Chester scenario. We further discuss the connection between quantum mechanical supersolid behaviour and a novel mechanism for a glass formation.

[99] Wednesday, 5th of February 2014 at 11:30,[SLIDES]
Robert E. Zillich,
Institute for Theoretical Physics, Johannes Kepler University, Altenbergerstrasse 69, 4040 Linz, Austria

open abstract
“Dipolar Multilayers”
We approximate a dipolar Bose gas in a one-dimensional optical lattice as bosons residing on different 2D planes, thus effectively mapping the system on a multi-component 2D Bose gas. The long range of the dipole interaction can lead to appreciable inter-layer correlations up to the limit where dipoles from different planes form bound states. We study the dependence of these correlations on density, distance between layers, and polarization direction, and their effect on the excitation spectrum.

[100] Monday, 11th of November 2013 at 12:00,[SLIDES]
Leticia Tarruell,
ICFO-The Institute of Photonic Sciences, Barcelona (Spain)

open abstract
"Exploring synthetic quantum materials with ultracold fermions in a tunable-geometry optical lattice"
Ultracold atomic gases in optical lattices provide the opportunity of engineering synthetic quantum materials in a clean and highly controlled environment. On the one hand, these systems can be viewed as experimental quantum simulators for exploring challenging problems of condensed matter physics. On the other hand, they also allow for the realization of completely novel materials of interesting properties, without counterpart in solid-state systems.

In my talk, I will present recent experiments where an ultracold Fermi gas trapped in an optical lattice of tunable geometry is used to explore both aspects.

By loading non-interacting atoms into a honeycomb structure, we realize artificial graphene and observe the presence of two Dirac points in the band structure [1]. The flexibility of our experimental approach allows us to adjust the properties of these Dirac points at will, moving them inside the Brillouin zone and changing the effective mass of the associated Dirac fermions. Furthermore, we observe how the two Dirac points annihilate each other when coming too close together, a situation which is presently out of reach in solid-state samples.

Preparing instead a repulsively interacting gas of atoms in two different internal states, we implement the Fermi-Hubbard model and aim at simulating quantum magnetism in this system. In particular, we explore experimentally how certain crystal geometries favor the emergence of short-range magnetic order, and directly probe the nearest-neighbor spin correlations of the system [2]. In a dimerized lattice, the correlations manifest as an excess number of singlets as compared to triplets, whereas in an anisotropic simple cubic lattice we observe the appearance of antiferromagnetic correlations along one spatial axis.

[1] L. Tarruell, D. Greif, T. Uehlinger, G. Jotzu and T. Esslinger, "Creating, moving and merging Dirac points with a Fermi gas in a tunable honeycomb lattice", Nature 483, 302â€“305 (2012).
[2] D. Greif, T. Uehlinger, G. Jotzu, L. Tarruell and T. Esslinger, "Short-range quantum magnetism of ultracold fermions in an optical lattice", Science 340, 1307-1310 (2013)

[101] Wednesday, 18th of September 2013 at 11:30,[SLIDES]
Mikhail Zvonarev,
Univ Paris-Sud, Laboratoire LPTMS, UMR8626, Orsay, F-91405, France; CNRS, Orsay, F-91405, France

open abstract
"Dynamics of quantum particles in one-dimensional quantum liquids"
What is the dynamics of a particle injected into a gas of quantum particles of other type? I will address this question in one spatial dimension in two different situations. In one case, an external particle is injected with a finite momentum, while as in the other the particle is subjected to a constant driving force. In the former case I will demonstrate that, contrary to conventional picture of relaxation dynamics, the particle never comes to a complete stop. Instead, for initially supersonic velocity long-lived oscillations develop and the velocity saturates to a value almost independent of initial conditions. In the latter case long-lived oscillations and velocity saturation take are also present, though their physical origin is different from the one in the former case. The methods used, namely Bethe Ansatz and numerics, will be discussed in detail along with the state-of-the-art experiments.

Reference:
Nature Physics 8, 881 (2012)
arXiv:1303.3583

[102] Monday, 16th of September 2013 at 11:30,[SLIDES]
Boris A. Malomed,
Tel Aviv University, Tel Aviv, Israel

open abstract
"Suppression of the quantum-mechanical collapse by repulsive interactions in bosonic gases"
The quantum-mechanical collapse (alias "fall onto the center" of particles attracted by potential -V/r2) is a well-known issue in the quantum theory. We demonstrate that, in a rarefied gas of quantum particles attracted by the above-mentioned potential, the repulsive nonlinearity induced by collisions between the particles prevents the collapse, recreating the missing ground state. Further, a phase transition in the ground state is found at a critical value of V. The setting may be realized in the 3D gas of dipolar bosons attracted by a central charge, including the case of the binary gas. The addition of the harmonic trapping potential gives rise to a tristability, in the case when the Schroedinger equation still does not lead to the collapse. In the 2D setting, the cubic nonlinearity is not strong enough to prevent the collapse; however, the quintic term does it. The analysis is also extended to the 3D anisotropic setting, with the dipoles polarized by an external uniform field.

Results were partly published in:
H. Sakaguchi and B. A. Malomed, Suppression of the quantum-mechanical collapse by repulsive interactions in a quantum gas, Phys. Rev. A 83, 013907 (2011);
H. Sakaguchi and B. A. Malomed, Suppression of the quantum collapse in an anisotropic gas of dipolar bosons, Phys. Rev. A 84, 033616 (2011).

[103] Wednesday, 24th of April 2013 at 11:30,[SLIDES]
Andrea Trombettoni,
CNR-IOM DEMOCRITOS & SISSA - Trieste, Italy

open abstract
"Quantum simulations with ultracold atoms in optical lattices"
In this talk I will present two examples of quantum simulations with ultracold atoms in optical lattices. In the first part I discuss the properties of ultracold bosons in optical lattices at half-filling: using a similarity Hamiltonian renormalization procedure, we determine an effective spin-1/2 representation of the Bose-Hubbard model. This mapping allows to give analytical estimates of the correlation functions of the Bose-Hubbard model, which are in good agreement with DMRG results. The discussed analysis shows that, also at finite interaction, the 1D Bose-Hubbard model with suitably chosen parameters may be seen as a quantum simulator of the XXZ chain. In the second part I will discuss anisotropic Ginzburg-Landau and Lawrence-Doniach models describing a layered superfluid ultracold Fermi gas in optical lattices. We derive the coefficients of the anisotropic Ginzburg-Landau and the mass tensor as a function of anisotropy, filling and interaction, showing that near the unitary limit the effective anisotropy of the masses is significantly reduced. The anisotropy parameter is shown to vary in realistic setups in a wide range of values. We also derive the Lawrence-Doniach model - often used to describe the 2D-3D dimensional crossover in layered superconductors - for a layered ultracold Fermi gas, obtaining a relation between the interlayer Josephson couplings and the Ginzburg-Landau masses.

[104] Tuesday, 26th of February 2013 at 11:30,[SLIDES]
Omjyoti Dutta,
Institut de Ciències Fotòniques (ICFO)

open abstract
"Beyond standard Hubbard models"
A natural description of ultracold gases trapped in optical lattices is given by the so-called single-band Hubbard model. We will discuss the additional effects of interaction to induce intra- as well as inter-band scattering to neighbouring sites, which shows up as density dependent correlated tunneling processes. Such processes can spontaneously give rise to various exotic phases. As examples, we will present
1) spontaneous creation of non-Fermi liquid smectic metal phases in dipolar Fermions, and
2) dynamical generation of exotic lattices in Fermi-Fermi mixtures.

[105] Thursday, 24th of January 2013 at 11:30,[SLIDES]
Stephan Humeniuk,
Institut de Ciències Fotòniques (ICFO)

open abstract
"Introduction to Stochastic Series Expansion Quantum Monte Carlo"
The seminar is intended to be a technical introduction into the Stochastic Series Expansion Monte Carlo method [1] which is an alternative to Path Integral Monte Carlo and a method of choice for lattice spin systems. I will review the SSE representation of the partition function and the different update mechanisms, i.e. diagonal and off-diagonal update as well as the worm algorithm that allows to calculate off-diagonal correlation functions. Finally, I will mention a scheme that allows to treat long-range and frustrated interactions in quantum Ising systems efficiently [2].
[1] Phys. Rev. B 59, R14157 (1999)
[2] Phys. Rev. E 68, 056701 (2003)

[106] Wednesday, 5th of December 2012 at 11:30,[SLIDES]
O. N. Osychenko,
Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, Campus Nord B4-B5, E-08034, Barcelona, Spain

open abstract
"Diffusion Monte Carlo study of phase transitions at zero temperature."
This seminar is devoted to application of diffusion Monte Carlo method to quantum phase transitions. The first problem that we study is the phase diagram of Yukawa systems. It is known that in the case of a large mass imbalance, the light fermions introduce an effective Yukawa potential between the heavy fermions. It was predicted that this effective interaction might lead to crystallization in 2D systems, although no estimations were done in 3D case. For the first time a fully quantum mechanical calculation is done in order to find the exact zero-temperature phase diagram of Yukawa particles. The melting and solidification curves show that there is a possibility for a reentrant gas-solid-gas transition, caused by a competition between long-range Coulomb part of the Yukawa potential (melts at large densities) and exponential screening (melts at small densities). In order to localize more accurately the phase transition point in the high density regime we apply the Ewald summation technique.

As well, we present a quantum Monte Carlo study of a bulk system of bosonic Rydberg atoms, that is alkali atoms with a single electron residing on a very high orbital, interacting through van der Waals 1/|r|6 interaction potential. Our aim is to understand how much of the behavior of a real system comes from this leading term, and to which extent the 1/|r|6 interaction of the excited atoms in the cloud can describe crucial properties of experimentally relevant systems like mixtures of excited and ground state atoms.

It the last part we present the results of quantum Monte Carlo calculations of atomic para-hydrogen p-H2 zero temperature below its solidification curve in a range of densities. In this study our principal motivation was to understand better the properties of the metastable liquid/glass phase at low temperatures.

[107] Thursday, 15th of November 2012 at 12:00,[SLIDES]
Miguel Angel García-March,
Dept. d'Estructura i Constituents de la Materia Univ. de Barcelona

open abstract
"Strongly correlated ultracold bosons as impurities immersed in a Bose-Einstein condensate."
Impurities, such as fermions or ions, immersed in a cloud of weakly interacting bosons forming a condensate have attracted recent interest because they conform versatile, highly controllable, and experimentally feasible systems for studying quantum correlations. We present an exact many body description of a mixture of ultracold bosons in which a few strongly interacting atoms play the role of impurities immersed in a second species of a much bigger cloud of weakly interacting atoms. The latter species acts as a tunable environment, whose effect over the impurities can be controlled through the interspecies interactions. This framework permits us to describe the behaviour of the quantum correlations between both species through the process of phase separation or along with the dynamics of the impurities within the environment. We also assess the possibility of mimicking the tools of quantum optics to analyse these quantum correlations. Finally, we derive a semiclassical description of the system in terms of two non-linear coupled differential equations where the nonlinearity depends on the interaction strength. Experimental feasibility and technological applications are henceforth discussed.

[108] Wednesday, 17th of October 2012 at 11:30,[SLIDES]
Natalia Matveeva,
Dipartimento di Fisica, Università di Trento, Italy

open abstract
"Liquid and crystal phase of dipolar fermions in two dimensions."
The liquid and crystal phase of a single-component Fermi gas with dipolar interactions are investigated using quantum Monte Carlo methods in two spatial dimensions and at zero temperature. The dipoles are oriented by an external field perpendicular to the plane of motion, resulting in a purely repulsive 1/r^3 interaction. In the liquid phase we calculate the equation of state as a function of the interaction strength and other relevant properties characterizing the Fermi-liquid behavior: effective mass, discontinuity at the Fermi surface and pair correlation function. In the high density regime we calculate the equation of state of the Wigner crystal phase and the critical density of the liquid to solid first order phase transition. Close to the freezing density we also search for the existence of a stripe phase, but such a phase is never found to be energetically favorable.

[109] Wednesday, 10th of October 2012 at 11:30,[SLIDES]
Krzysztof Jachymski,
Faculty of Physics, University of Warsaw, Poland

open abstract
"Localization of ultracold bosons in a quasiperiodic optical lattice."
Ultracold gases provide a unique opportunity to study Anderson localization in disordered trapping potential. Both the properties of the trap and the strength of the interactions can be controlled during the experiment.

I will discuss the properties of ultracold bosons in a one-dimensional quasiperiodic optical lattice, where the localization occurs for sufficiently strong disorder. The transition can be detected with a nondestructive method using off-resonant light scattering.

[110] Friday, 7th of September 2012 at 10:30,[SLIDES]
Gianluca Bertaina,
Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

open abstract
"Quantum Monte Carlo study of a resonant Bose-Fermi mixture."
We study resonant Bose-Fermi mixtures at zero temperature, with different relative concentrations of the bosons. We use for the first time a Quantum Monte Carlo method with Fixed-Node approximation, to explore the system from the weak to the strong coupling limit. A repulsive interaction among bosons is introduced to provide stability to the bosonic component. Beyond the unitarity limit, the resonant attractive interaction supports a bound fermionic dimer. At the many-body level, increasing the boson-fermion coupling the system undergoes a quantum phase transition from a state with condensed polaronic bosons immersed in a Fermi sea, to a normal Fermi-Fermi mixture of the composite fermions and the bare fermions in excess. We obtain the equation of state and we characterize the momentum distributions both in the weakly and in the strongly interacting limits, finding interesting signatures of the different many-body ground states. We compare Quantum Monte Carlo results to T-matrix calculations and we draw a preliminary phase diagram as a function of the Bose-Fermi scattering length and the concentration of the bosons.

[111] Thursday, 26th of July 2012 at 10:30,[SLIDES]
D. M. Gangardt,
School of Physics and Astronomy, University of Birmingham, UK

open abstract
"Bloch oscillations without lattice: dynamics of mobile impurities in one-dimensional quantum liquids."
A quantum impurity, i.e. a particle different from the others, moving in quantum liquid in one dimension is an interesting many-body problem. The dynamics of such mobile impurity is very rich due to its strong interactions with collective degrees of freedom of the liquid. One of the manifestations of this is the periodicity of the dispersion law of the impurity, similar to that of a particle in a lattice. Under action of an external force the motion of the particle exhibits Bloch oscillations. We discuss the role of dissipative processes and show that as a result the Bloch oscillations become superimposed with a drift. Our main finding is that the drift velocity and the period of the oscillations are extremely sensitive to the temperature as well as the underlying microscopic interactions.

[112] Friday, 11th of May 2012 at 11:30,[SLIDES]
Yaroslav Lutsyshyn,
Institute of Physics University of Rostock 18051 Rostock, Germany

open abstract
"Fast quantum Monte Carlo on GPU."
I will describe recent efforts to parallelize quantum Monte Carlo for use with graphical processing units (GPUs). Modern graphical cards often feature GPUs with several hundreds of single computing cores. Arrays of such cores are designed to execute in parallel using single instruction-multiple data parallel model. Such vector parallelization suits well the applications in scientific computing. GPUs produced by Nvidia, a market leader in such GPU products, may be conveniently programmed with an extensions of the standard programming languages called CUDA (Compute Unified Device Architecture) framework.

I will report my recent work in parallelizing variational Monte Carlo for use with the Nvidia GPU cards. Acceleration of up to fifty times was achieved when comparing GPU execution with a single-core CPU run. Thanks to such an improvement in speed, variational calculations of a liquid helium system with over ten thousand particles can be comfortably performed on a regular desktop workstation. The talk will focus on details of the NVidia GPU architecture and its relation to the CUDA programming model. I will also cover some practical considerations that are important for starting a new CUDA project.

[113] Wednesday, 7th of March 2012 at 15:00,[SLIDES]
Leandra Vranješ Markić,
Faculty of Science, University of Split, HR-21000 Split, Croatia

open abstract
"Quantum Monte Carlo simulations of 4He clusters adsorbed on the surface of graphene."
Graphene, a single layer of graphite, which has demonstrated many surprising physical properties, is also of interest as a novel adsorber.

The talk will give an overview of our recent study of 4HeN clusters adsorbed on one and both sides of a graphene sheet. Different models for the interaction potentials will be discussed. We have modelled interactions of 4He clusters with graphene using an averaged helium-carbon potential that depends only on the distance to the graphene sheet, and a potential constructed as a sum of individual helium-carbon interactions. That way, we assess the effect of corrugation on the binding properties of helium clusters, both using isotropic and anisotropic interaction potentials. In addition, we asses the influence of the substrate mediated McLachlan interaction.

All the calculations have been performed using quantum Monte Carlo methods. At zero temperature the ground-state properties of 4HeN for 2â‰¤ Nâ‰¤100 have been determined using variational and diffusion Monte Carlo calculations. We find that clusters adsorbed on both sides of graphene are correlated. In addition, we observe the changes in the size of the clusters. For selected clusters, calculations have been performed also at finite temperature by path integral Monte Carlo simulations.

[114] Tuesday 28th of February 2012 at 12:00,[SLIDES]
Diana Hufnagl,
Institut fur Theoretische Physik Johannes Kepler Universitat Linz - Austria

open abstract
"Staibility and Excitations of Dipolar Quantum Gases"
Due to the recent experimental advances in generating Feshbach-associated molecular quantum gases, the field of ultracold dipolar gases has become a topic of great interest. In this talk we present our current work in this field. We study polarized dipolar Bose systems in a pancake shaped trap, where we also introduce a tunnel barrier to go from the limit of a quasi-2D layer, to the limit of two quasi-2D layers. We discuss instabilities due to dimerization in one layer and the associated signature of a roton in the spectrum. For the case of two layers we investigate for example the dependence of the stability on the distance of these layers. Recently we also investigated quantum gases other than dipolar gases, namely Rydberg-dressed atoms in two dimensions, where we study instabilities caused by the formation of a droplet lattice. We will also briefly mention work on the BEC-BCS crossover and the effects of molecular rotation.

[115] Thursday, 19th of January 2012 at 11:00,[SLIDES]
Sebastiano Pilati,
The Abdus Salam International Center for Theoretical Physics Trieste - Italy

open abstract
"Quantum many-body physics with ultracold gases in optical lattices."
In experiments performed with ultracold gases one can manipulate the main parameters that characterize the sample. This allows to induce various intriguing quantum phenomena due to strong inter-particle correlations. In this talk, we will review some recent quantum Monte Carlo studies of Bose and Fermi gases with resonant inter-particle interactions. For bosonic systems in optical lattices, we investigate the superfluid to insulator transition by using a novel continuous-space quantum Monte Carlo technique, and compare our results with more conventional theories based on simplified discrete lattice models. For fermions, we apply Kohn-Sham Density Functional Theory to investigate quantum magnetism of repulsive Fermi gases in optical lattices. We discuss how these calculations could provide a new link between experiments with ultracold atomic gases and the simulation of strongly correlated materials.

[116] Wednesday, 26th of October 2011 at 12:00,[SLIDES]
Konstantin Krutitsky,
Physics Department, University of Duisburg-Essen, Germany

open abstract
"Excitation dynamics in a lattice Bose gas within the time-dependent Gutzwiller mean-field approach."
Studies of excitations of ultracold atoms in optical lattices play an important role in the understanding of their physical properties and dynamical behavior. For instance, the celebrated quantum phase transition from the superfluid into the Mott-insulator is accompanied by the opening of the gap in the excitation spectrum. We have studied the ground state, stationary excitation modes, as well as the dynamics of the lattice Bose gas using the time-dependent Gutzwiller mean-field approach valid in the superfluid as well as in the Mott-insulator phases.

The modes of collective excitations are calculated within the framework of the linear-response theory considering small perturbations of the many-body ground state. The lowest excitation modes of the Mott-insulator display energy gaps which become smaller when we approach the phase boundary. The lowest-energy excitation of the superfluid is a Goldstone mode that appears due to the spontaneous breaking of the phase symmetry. It has a phonon-like dispersion relation characterized by the sound velocity which is described by the hydrodynamic relation. In this mode, the condensate and the normal fractions oscillate in phase. The second mode of the superfluid has a gap and the oscillations of the condensate and the normal components are out-of-phase. We have performed simulations of the sound-wave propagation in the linear-response regime and beyond.

Dark solitons of ultracold bosons in the vicinity of the Mott-insulator -- superfluid phase transition are studied as well. Making use of the Gutzwiller ansatz we have found antisymmetric eigenstates corresponding to standing solitons, as well as propagating solitons created by phase imprinting. Near the phase boundary, superfluidity has either a particle or a hole character depending on the system parameters, which greatly affects the characteristics of both types of solitons.

[117] Friday, 16th of September 2011 at 15:00,[SLIDES]
Miguel A. Cazalilla,
Centro de Física de Materiales CSIC-UPV/EHU. Paseo Manuel de Lardizabal 5, E-20018 San Sebastian, Spain; Donostia International Physics Center (DIPC)

open abstract
"Dynamical Theory of Superfluidity in One Dimension."
A theory accounting for the dynamical aspects of the superfluid response of one dimensional (1D) quantum fluids is reported. In long 1D systems, the onset of superfluidity is related to the dynamical suppression of quantum phase slips at low temperatures. The effect of this suppression as a function of frequency and temperature is discussed within the framework of the relevant correlation function that is accessible experimentally, namely the momentum response function. Application of these results to the understanding of the superfluid properties of helium confined in nanometer-size pores, dislocations in solid 4He, and ultra-cold atomic gases is also briefly discussed.

[118] Wednesday, 23rd of March 2011 at 11:00,[SLIDES]
Henry R. Glyde,
Department of Physics and Astronomy University of Delaware Newark, Delaware 19711

open abstract
"Bose-Einstein Condensation and Superfluidity in Nanoscale Liquid and Solid Helium."
Helium was first liquefied in the famous Kamerlingh Onnes laboratories in Leiden, Holland in 1908. More than a century of fascinating discovery followed revealing many remarkable phenomena such as Bose-Einstein condensation (BEC), superfluidity and well defined phonon-roton modes. Superflow of helium confined in porous media has always been an important part of this history. In 2004 superflow behavior was observed in solid helium. In this talk we review neutron scattering measurements of BEC, phonon-roton modes and superflow in bulk liquid helium. We then focus on helium confined in porous media and make the case for the existence of a new insulating phase that contains localized regions of BEC in porous media. This "localized BEC" phase lies between the superfluid and normal liquid phase and is induced by disorder. Recent measurements of BEC in bulk liquid helium under pressure up to the liquid/solid interface will also be presented as well as earlier attempts to observe BEC in solid helium.

[119] Wednesday, 13th of April 2011 at 11:30,[SLIDES]
O. N. Osychenko,
Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya

open abstract
"Monte Carlo study of quantum phase diagram of Rydberg atoms with repulsive 1/r6 interaction."
Recently the methods of laser cooling of atoms have given rise to a new wave of experiments on ultracold (~100 ÂµK) Rydberg atoms [1,2], in particular on alkali bosonic atoms as Rb87. These experiments revealed a suppression of excitations, called "van der Waals blockade", that was proposed as a perspective means to implement quantum gates between atom qubits. We study the quantum phase diagram of bosons interacting via repulsive van der Waals 1/r6 potential. The critical density for zero temperature gas-crystal phase transition is obtained from diffusion Monte Carlo calculations. If effective mass entering in the kinetic energy were taken to be of the order of the mass of a Rb atom, then the typical experimental conditions would correspond to being deeply in the phase of a classical crystal. Effects of the temperature are studied in the classical crystal using path integral Monte Carlo and classical Monte Carlo methods.

[1] Axel Grabowski, Rolf Heidemann, Robert LÃ¶w, JÃ¼rgen Stuhler, Tilman Pfau "High resolution Rydberg spectroscopy of ultracold Rubidium atoms'",
Fortschr. Phys., 54, 765, (2005).
[2] Vera Bendkowsky, BÃ¶rn Butscher, Johannes Nipper, James P. Shaffer, Robert LÃ¶w, Tilman Pfau "Observation of ultralong-range Rydberg molecules",
Nature, 458, 1005 (2009).

[120] Wednsday, 6th of April 2011 at 11:30 UPC Campus Nord, B4-212 (aula seminari),[SLIDES]
Carles Calero,
Institut de Ciència de Materials de Barcelona, ICMAB-CSIC

open abstract
"Ionic transport through a biological nanochannel."
Understanding transport of charged solutes across transmembrane channels is a problem of paramount importance in biophysics, since it is crucial to regulate many cell functions [1]. An important property of biological nanochannels is their preference to the passage of either cations or anions through them, the so-called selectivity. New experiments on the selectivity of a biological nanochannel (the OmpF bacterial porin, a model channel) observe a reversal of selectivity in the presence of multivalent cations [2]. In my talk, I will discuss the results of new high-performance all-atom molecular dynamics simulations of the ionic transport across this biological nanochannel in the presence of electrolyte which provide an explanation for the experimental observations [3]. I will also discuss a method based on the calculation of first passage time quantities that was used to obtain macroscopic parameters which characterize the diffusive dynamics of the ions in the interior of the channel from the MD simulations' trajectories [4].

[1] B. Hille, Ion Channels of Excitable Membranes (Sinaure, Sunderland, 2001)
[2] A. Alcaraz et al., Biophys. J. 96, 56 (2009)
[3] M. Aguilella-Arzo, J. Faraudo, C. Calero, Soft Matter 6, 6079 (2010)
[4] C. Calero, J. Faraudo, M. Aguilella-Arzo, Phys. Rev. E 83, 021908 (2011)

[121] Thursday, 31st of March 2011 at 11:00,[SLIDES]
Lluís Masanes,
ICFO - Institut de Ciències Fotòniques

open abstract
"A system equilibrates when diagonalizing its hamiltonian is sufficiently hard."
In classical systems there is a relation between integrability and thermalization, but this is not well understood in the quantum case. Closed quantum systems never equilibrate to a stationary state. However, in some circumstances, the system equilibrates locally: the reduced density matrix of a subsystem evolves to a stationary state. The lack of integrability of a quantum system can be quantified by the computational complexity of diagonalizing its hamiltonian. It can be shown that, if this complexity is at least quadratic then local equilibration holds. If this complexity is sublinear then local equilibration does not hold.

[122] Friday, 4th of March 2011 at 10:30,[SLIDES]
Yaroslav Lutsyshyn,
Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya

open abstract
"Quantum Monte Carlo calculations of point defect formation in solid helium."
Interest in the influence of strong quantum effects on the properties of crystalline solids has rekindled since the recent discovery of supersolidity and a host of accompanying effects in solid helium-4. While the exact nature of these effects remains unknown, they are oft-believed to owe to defects populating the experimental samples. In this talk I will describe our efforts to characterize quantum solids with first principles quantum many-body methods, and will report the results for properties of vacancy formation in solid helium.

Y. Lutsyshyn et al., Phys. Rev. B 82, 180506(R) (2010)

[123] Monday, 14th of February 2011 at 11:00,[SLIDES]
Prof. Andrey Mishchenko,
Cross-Correlated Materials Research Group, RIKEN (The Institute of Physical and Chemical Research), Japan

open abstract
"Sharp Transition for Single Polarons in the One-Dimensional Su-Schrieffer-Heeger Model"
We study [1] a single polaron in the Su-Schrieffer-Heeger (SSH) model using four different techniques (three numerical and one analytical). Polarons show a smooth crossover from weak to strong coupling, as a function of the electron-phonon coupling strength, in all models where this coupling depends only on phonon momentum q. In the SSH model the coupling also depends on the electron momentum k; we find it has a sharp transition, at a critical coupling strength, between states with zero and nonzero momentum of the ground state. All other properties of the polaron are also singular. This result is representative of all polarons with coupling depending on k and q, and will have important experimental consequences (e.g., in angle-resolved photoemission spectroscopy and conductivity experiments).

[1]. D. J. J. Marchand et al, Phys. Rev. Lett. v. 105, 266605 (2010).

[124] September 10th, 2010,[SLIDES]
Lubos Mitas,
Electronic structure quantum Monte Carlo: pfaffians and many-body nodes of ground and excited states,
North Carolina State University

[125] Monday, 5 July 2010,[SLIDES]
Victor Gurarie,
SU(N) magnetism with cold atoms and chiral spin liquids,
Department of Physics University of Colorado Boulder CO 80305

open abstract
Certain cold atoms, namely the alkaline earth-like atoms whose electronic degrees of freedom are decoupled from their nuclear spin, can be thought of as quantum particles with an SU(N)-symmetric spin. These have recently been cooled to quantum degeneracy in the laboratories around the world. A new world of SU(N) physics has thus become accessible to experiment, including that described by the SU(N) Hubbard model in various dimensions as well as many others. We show that the Mott insulator of such cold atoms is a SU(N) symmetric antiferromagnet of the type not commonly studied in the literature. We further show that in 2 dimensions, this antiferromagnet is a chiral spin liquid, a long sought-after topological state of magnets, with fractional and non-Abelian excitations.

[126] May 19, 2010,[SLIDES]
Doerte Blume,
[Lecture 6] Dipolar gases: Long-range and angle-dependent interactions,
Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814

open abstract
Since the first experimental realization of gaseous atomic Bose-Einstein condensates in 1995, the field has progressed tremendously. Nowadays, degenerate Bose and Fermi gases can be prepared with controllable interaction strengths by tuning a magnetic field in the vicinity of a Feshbach resonance. This tunability has opened the door for a variety of studies at the interface of atomic, nuclear, chemical, condensed matter and astrophysics. This course will explore fewbody aspects of Bose and Fermi gases with tunable interaction strengths and draw parallels to other few-body systems such as weakly-bound nuclei. A number of theoretical frameworks such as scattering theory and a hyperspherical approach will be introduced and then applied to bosonic and fermionic few-body systems. The course is aimed at graduate students and postdoctoral researchers with a solid foundation in quantum mechanics, and designed to benefit not only those individuals who are currently already engaged in the broad field of cold atom physics but also those individuals who want to get a first exposure to the field of cold atom physics and related fewbody aspects. The course will consist of several interrelated lectures as listed below. Additional lectures will be scheduled once the course has started. Each lecture will start with an introduction that places the topic into a broader context. Next, the theoretical formalism will be introduced with an emphasize on the development of simple physical pictures. Lastly, the physical implications will be illustrated by discussing a variety of examples from the literature and the instructor's own research.

[127] May 18, 2010,[SLIDES]
Doerte Blume,
[Lecture 5] Bosonic few-body systems: What is different?,
Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814

open abstract
Since the first experimental realization of gaseous atomic Bose-Einstein condensates in 1995, the field has progressed tremendously. Nowadays, degenerate Bose and Fermi gases can be prepared with controllable interaction strengths by tuning a magnetic field in the vicinity of a Feshbach resonance. This tunability has opened the door for a variety of studies at the interface of atomic, nuclear, chemical, condensed matter and astrophysics. This course will explore fewbody aspects of Bose and Fermi gases with tunable interaction strengths and draw parallels to other few-body systems such as weakly-bound nuclei. A number of theoretical frameworks such as scattering theory and a hyperspherical approach will be introduced and then applied to bosonic and fermionic few-body systems. The course is aimed at graduate students and postdoctoral researchers with a solid foundation in quantum mechanics, and designed to benefit not only those individuals who are currently already engaged in the broad field of cold atom physics but also those individuals who want to get a first exposure to the field of cold atom physics and related fewbody aspects. The course will consist of several interrelated lectures as listed below. Additional lectures will be scheduled once the course has started. Each lecture will start with an introduction that places the topic into a broader context. Next, the theoretical formalism will be introduced with an emphasize on the development of simple physical pictures. Lastly, the physical implications will be illustrated by discussing a variety of examples from the literature and the instructor's own research.

[128] May 14th, 2010,[SLIDES]
Robert E. Zillich,
Dipolar quantum gases,
Institut für Theoretische Physik, Johannes Kepler Universität, Linz, Austria

[129] May 14, 2010,[SLIDES]
Doerte Blume,
[Lecture 4] Unequal-mass Fermi gases: Universal or non-universal?,
Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814

open abstract
Since the first experimental realization of gaseous atomic Bose-Einstein condensates in 1995, the field has progressed tremendously. Nowadays, degenerate Bose and Fermi gases can be prepared with controllable interaction strengths by tuning a magnetic field in the vicinity of a Feshbach resonance. This tunability has opened the door for a variety of studies at the interface of atomic, nuclear, chemical, condensed matter and astrophysics. This course will explore fewbody aspects of Bose and Fermi gases with tunable interaction strengths and draw parallels to other few-body systems such as weakly-bound nuclei. A number of theoretical frameworks such as scattering theory and a hyperspherical approach will be introduced and then applied to bosonic and fermionic few-body systems. The course is aimed at graduate students and postdoctoral researchers with a solid foundation in quantum mechanics, and designed to benefit not only those individuals who are currently already engaged in the broad field of cold atom physics but also those individuals who want to get a first exposure to the field of cold atom physics and related fewbody aspects. The course will consist of several interrelated lectures as listed below. Additional lectures will be scheduled once the course has started. Each lecture will start with an introduction that places the topic into a broader context. Next, the theoretical formalism will be introduced with an emphasize on the development of simple physical pictures. Lastly, the physical implications will be illustrated by discussing a variety of examples from the literature and the instructor's own research.

[130] May 13, 2010,[SLIDES]
Doerte Blume,
[Lecture 3] Universal behavior of small two-component Fermi gases with equal masses.,
Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814

open abstract
Since the first experimental realization of gaseous atomic Bose-Einstein condensates in 1995, the field has progressed tremendously. Nowadays, degenerate Bose and Fermi gases can be prepared with controllable interaction strengths by tuning a magnetic field in the vicinity of a Feshbach resonance. This tunability has opened the door for a variety of studies at the interface of atomic, nuclear, chemical, condensed matter and astrophysics. This course will explore fewbody aspects of Bose and Fermi gases with tunable interaction strengths and draw parallels to other few-body systems such as weakly-bound nuclei. A number of theoretical frameworks such as scattering theory and a hyperspherical approach will be introduced and then applied to bosonic and fermionic few-body systems. The course is aimed at graduate students and postdoctoral researchers with a solid foundation in quantum mechanics, and designed to benefit not only those individuals who are currently already engaged in the broad field of cold atom physics but also those individuals who want to get a first exposure to the field of cold atom physics and related fewbody aspects. The course will consist of several interrelated lectures as listed below. Additional lectures will be scheduled once the course has started. Each lecture will start with an introduction that places the topic into a broader context. Next, the theoretical formalism will be introduced with an emphasize on the development of simple physical pictures. Lastly, the physical implications will be illustrated by discussing a variety of examples from the literature and the instructor's own research.

[131] May 12, 2010,[SLIDES]
Doerte Blume,
[Lecture 2] Theoretical frameworks for treating cold few-particle systems: Two-body scattering and hyperspherical perspective.,
Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814

open abstract
Since the first experimental realization of gaseous atomic Bose-Einstein condensates in 1995, the field has progressed tremendously. Nowadays, degenerate Bose and Fermi gases can be prepared with controllable interaction strengths by tuning a magnetic field in the vicinity of a Feshbach resonance. This tunability has opened the door for a variety of studies at the interface of atomic, nuclear, chemical, condensed matter and astrophysics. This course will explore fewbody aspects of Bose and Fermi gases with tunable interaction strengths and draw parallels to other few-body systems such as weakly-bound nuclei. A number of theoretical frameworks such as scattering theory and a hyperspherical approach will be introduced and then applied to bosonic and fermionic few-body systems. The course is aimed at graduate students and postdoctoral researchers with a solid foundation in quantum mechanics, and designed to benefit not only those individuals who are currently already engaged in the broad field of cold atom physics but also those individuals who want to get a first exposure to the field of cold atom physics and related fewbody aspects. The course will consist of several interrelated lectures as listed below. Additional lectures will be scheduled once the course has started. Each lecture will start with an introduction that places the topic into a broader context. Next, the theoretical formalism will be introduced with an emphasize on the development of simple physical pictures. Lastly, the physical implications will be illustrated by discussing a variety of examples from the literature and the instructor's own research.

[132] May 11, 2010,[SLIDES]
Doerte Blume,
[Lecture 1]: Theoretical frameworks for treating cold few-particle systems: Two-body scattering and hyperspherical perspective.,
Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814

open abstract
Since the first experimental realization of gaseous atomic Bose-Einstein condensates in 1995, the field has progressed tremendously. Nowadays, degenerate Bose and Fermi gases can be prepared with controllable interaction strengths by tuning a magnetic field in the vicinity of a Feshbach resonance. This tunability has opened the door for a variety of studies at the interface of atomic, nuclear, chemical, condensed matter and astrophysics. This course will explore fewbody aspects of Bose and Fermi gases with tunable interaction strengths and draw parallels to other few-body systems such as weakly-bound nuclei. A number of theoretical frameworks such as scattering theory and a hyperspherical approach will be introduced and then applied to bosonic and fermionic few-body systems. The course is aimed at graduate students and postdoctoral researchers with a solid foundation in quantum mechanics, and designed to benefit not only those individuals who are currently already engaged in the broad field of cold atom physics but also those individuals who want to get a first exposure to the field of cold atom physics and related fewbody aspects. The course will consist of several interrelated lectures as listed below. Additional lectures will be scheduled once the course has started. Each lecture will start with an introduction that places the topic into a broader context. Next, the theoretical formalism will be introduced with an emphasize on the development of simple physical pictures. Lastly, the physical implications will be illustrated by discussing a variety of examples from the literature and the instructor's own research.

[133] 22 April 2010 at 11:00,[SLIDES]
Lev P. Pitaevskii,
Landau as a teacher,
University of Trento, Italy; Kapitza Institute for Physical Problems, Moscow, Russia

open abstract
Landau was a prominent physicist who made fundamental contributions to many areas of theoretical physics. He was awarded the 1962 Nobel Prize in Physics for his development of a mathematical theory of superfluidity. Landau was the principal founder of a great tradition of theoretical physics in the Soviet Union, which is sometimes referred to as the "Landau school". Pitaevskii has worked in Landau’s department during 1955-1962 starting as Ph.D. student and continuing as a researcher. They interacted virtually every day until Landau’s scientific career was brought to an abrupt end by a tragic car accident on January 7, 1962. Pitaevskii coauthored several volumes of the famous Landau course on Theoretical Physics and is currently editor of the course. Pitaevskii will tell us about his experience working together with L. D. Landau, about the method which Landau used to select his students, about the atmosphere and the rules in his department, about his seminars and his style in discussions.

[134] 2 December 2009,[SLIDES]
Claudio Cazorla,
THEORETICAL STUDY OF RARE GASES (RG) AND RG-X (X=He,H2) MIXTURES UNDER PRESSURE,
Department of Earth Sciences, UCL, London, UK Materials Simulation Laboratory, London, UK

open abstract
Rare Gases (RG=He, Ne, Ar, Kr, Xe) are ubiquitous in Earth, solar system planets, stars and interstellar medium so understanding them is necessary to understand the composition and evolution of the universe. From a technological point of view RGs are also important because they present remarkable mechanical and reactivity properties that have been exploited in hydrostatic pressure-transmitter medium applications in high-pressure experiments and synthesis processes. At normal P-T conditions, RGs are considered among the most simple systems because they posses closed-shell electronic structure which prevent them from reacting with other atomic species and conducting electricity. Nevertheless, in both low P-T and high P-T regimes RGs exhibit very intriguing behaviour which defies and challenges both classical and modern physics conceptions [1]. In this talk, I will review recent theoretical work done on He and RG-He mixtures under pressure [2,3]. Key ideas of the computational approaches used on these investigations, namely density functional theory (DFT) and the diffusion Monte Carlo method (DMC), will be also highlighted. In particular, I will talk about the phase diagram and physical properties of Ne(He)2, a compound which is observed to stabilize in the MgZn2 Laves structure at low P and for which we recently have predicted a series of solid-solid phase transitions induced by effect of pressure. We also have conducted investigations on the Ar(H2)2 system [4] where it has been suggested possible chemical-induced metallization of hydrogen at compressions already within the reach of present diamond-anvill-cell (DAC) facilities (P~175GPa). Interestingly, based on preliminary DFT and ab initio molecular dynamics results, we predict a new solid-solid phase boundary on the phase diagram of this compound and unravel a novel type of melting behaviour at extreme P-T conditions. [1] C. Cazorla et al., New J. of Phys. 11, 013047 (2009) [2] C. Cazorla and J. Boronat, J. of Phys.: Condens. Matt. 20, 015223 (2008) [3] C. Cazorla, D. Errandonea and E. Sola, Phys. Rev. B 80, 064105 (2009)

[135] Tuesday, 22nd of September 2009 at 11:00,[SLIDES]
Antun Balaž,
Институт за физику, Универзитет у Београду (Institute of Physics, Belgrade - Serbia)

open abstract
"High order actions in Path Integral Monte Carlo"
Recently, we have developed an efficient recursive approach for analytically calculating the short-time expansion of the propagator to extremely high orders for a general many-body quantum system [1]. Here we apply this technique for numerical study of thermodynamical properties of a rotating ideal Bose gas of 87Rb atoms in an anharmonic trap [2]. First, the energy spectrum of the system is obtained by the exact diagonalization of the discretized short-time propagator. Then the condensation temperature, ground-state occupancy, density profiles and the time-of-flight absorption pictures are calculated for varying rotation frequencies, including the critical and over-critical regime. The obtained results improve previous semiclassical calculations and agree well with Path Integral Monte Carlo simulations.

[1] A. Balaž, A. Bogojević, I. Vidanović, A. Pelster, PRE 79, 036701 (2009)
[2] V. Bretin, S. Stock, Y. Seurin, J. Dalibard, PRL 92, 050403 (2004)

[136] Tuesday, 15th of September 2009 at 16:00,[SLIDES]
Marvin D. Girardeau,
School of Optical Sciences, University of Arizona, Tucson, AZ

open abstract
"Fermionization, bosonization, and correlations in ultracold Bose and Fermi gases in tight waveguides "
Ultracold gases in tight de Broglie waveguides provide highly controllable realizations of one-dimensional quantum gases. A historical overview of Tonks-Girardeau (TG) gas of impenetrable bosons: from analytic prediction a long time ago to recent experimental realizations is presented. Fermi-Bose mapping is constructed for the ground-state wave function permitting to find an exact many-body solution for TG gas. Another exactly solvable system of fermions with strong attractions is explained: fermionic Tonks-Girardeau (FTG) gas. Superconductive long-range order is discussed. Generalized FB mapping is introduced: Spin-aligned fermions <-> bosons. Finally, Bose gases A and B with FTG AB interactions are also studied.

[137] 1st of June 2009,[SLIDES]
Mikko Saarela,
Department of Physics, University of Oulu, Finland

open abstract
Lecture series in Universitat Politecnica de Catalunya, 2009
Course on Many-Body theory "Charged quantum fluids from charged Bose gas to electron-hole liquid"

[138] 22 April 2009,[SLIDES]
Konstantin Krutitsky,
University of Duisburg-Essen, Physics Department

open abstract
"Quasi-2D bosons in correlated random potentials"
Laser speckles provide a unique possibility to create fully controlled two-dimentional disorder potentials for neutral atoms. This type of disorder has exponential probability distribution and finite spatial correlation length. In this talk, I shall present our resent results on the superfluidity and Bose-Einstein condensation of interacting bosons in the presence of the laser speckles at zero temperature obtained using Monte Carlo methods in continuum as well as Gross-Pitaevskii equation and Bogoliubov theory.

[139] 22nd of July 2008,[SLIDES]
Prof. Yurii E.Lozovik,
Institute of Spectroscopy, Russian Academy of Sciences and Moscow Institute of Physics and Technology, Russia

open abstract
Course "BOSE-EINSTEIN COHERENCE IN SEMICONDUCTORS AND GRAPHENE" : Lecture 3 "Physical properties, possible coherent phases and applications of graphene"
- Moore law.
- Allotrope forms of carbon
- Discovery of graphene
- Problem of existence of 2D membrane and 2Dcrystal
- Orbitals and chemical bonds
- Band structure and envelope function of graphene
- Berry phase and absence of backscattering
- Controlling by gate
- Problems of quantum dot and transistor based on graphene
- Optical properties of graphene
- Pecularity of correlations in graphene
- Coherent phases and collective properties
- Peculiarities of coherent phase in graphene
- Landau quantization and anomalous QHE in graphene
- Magnetoexciton condensation
- Possible applications. NEMS

[140] 21st of July 2008,[SLIDES]
Prof. Yurii E.Lozovik,
Institute of Spectroscopy, Russian Academy of Sciences and Moscow Institute of Physics and Technology, Russia

open abstract
Course "BOSE-EINSTEIN COHERENCE IN SEMICONDUCTORS AND GRAPHENE" : Lecture 2 "Bose condensation and superfluidity of cavity polaritons"
- Exciton -polaritons in optical microcavity â€“ entanglement of cavity photons and 2D excitones
- Bose-condensation and polariton laser
- Gross-Pitaevskii-type system of equations for two-component entangled system
- Landau method to calculate superfluid density Ï s - inapplicability to polaritons
- Derivation of effective action of polariton system as system of two interacting Bose-field from linear response theory
- Kosterlitz â€“Thouless transition of polariton system to superfluid state
- Polariton traps "Smoking gun proofs" of polariton BEC

[141] 20th of July 2008,[SLIDES]
Prof. Yurii E.Lozovik,
BOSE-EINSTEIN COHERENCE IN SEMICONDUCTORS AND GRAPHENE : Lecture 1,
Institute of Spectroscopy, Russian Academy of Sciences and Moscow Institute of Physics and Technology, Russia

open abstract
Course "BOSE-EINSTEIN COHERENCE IN SEMICONDUCTORS AND GRAPHENE" : Lecture 1 "Possible phases, Bose condensation and superfluidity of excitons in nanostructures: theory, present experimental status and problems"
- Matter versus exciton matter - similarities and dissimilarities.
- Short history of Bose condensate
- Physical realizations. Coupled quantum wells.
- Phases and regimes for exciton system in coupled quantum wells.
- BCS regime
- Rare exciton system
- BEC-BCS crossover
- Strongly correlated phenomena and phases in CQW
- Exciton crystal and supersolid
- Exciton traps.
- Definition of superfluidity in finite system
- Local density approximation.
- Coherent phenomena
- Present experimental status of exciton BEC

[142] 19th of June 2008 at 11:00,[SLIDES]
Gora Shlyapnikov,
Laboratoire de Physique Théorique et Modéles Statistiques, Univ. Paris-Sud, Orsay, France

open abstract
"Ultracold Quantum gases" Dmitry Petrov and Gora Shlyapnikov: Lecture 6 "Dipolar quantum gases. Fermionic dipoles"
- BCS limit. Superfluid transition in a single-component dipolar Fermi gas
- Collective and single-particle exzcitations
- 2D dipolar Fermi gas. Superfluid-non-superfluid quantum transition
- 1D dipolar Fermi gas. Quantum Kosterlitz-Thouless transition

[143] 18th of June 2008 at 11:00,[SLIDES]
Gora Shlyapnikov,
Laboratoire de Physique Théorique et Modéles Statistiques, Univ. Paris-Sud, Orsay, France

open abstract
"Ultracold Quantum gases" Dmitry Petrov and Gora Shlyapnikov: Lecture 5 "Dipolar quantum gases. Bosonic dipoles"
- Dipolar systems to be studied. Dilute regime. Scattering problem
- Dipolar Bose-Einstein condensate. Dynamical instability
- Trapped dipolar BEC. Gross-Pitaevskii equation. Stability diagram.
- 2D and 1D dipolar BEC. Roton-maxon spectrum and instability
- Can one obtain a supersolid state in free space?

[144] 17th of June 2008 at 11:00,[SLIDES]
Gora Shlyapnikov,
Laboratoire de Physique Théorique et Modéles Statistiques, Univ. Paris-Sud, Orsay, France

open abstract
"Ultracold Quantum gases" Dmitry Petrov and Gora Shlyapnikov: Lecture 4 "Strongly interacting Fermi gases"
- Strongly interacting regime. Unitarity limit.
- Universal thermodynamics
- Experiments with two-species Fermi gases. Monte Carlo studies.
- Strongly interacting regime in Fermi mixtures. What changes on the BEC and on the BCS sides of the resonance?

[145] 13th of June 2008 at 11:00,[SLIDES]
Dmitry Petrov,
Laboratoire de Physique Théorique et Modéles Statistiques, Univ. Paris-Sud, Orsay, France

open abstract
"Ultracold Quantum gases" Dmitry Petrov and Gora Shlyapnikov: Lecture 3 "Weakly bound dimers"
- 4-body problem. Born-Oppenheimer and exact treatment
- Hybrid Born-Oppenheimer technique. Formation of trimers

[146] 12th of June 2008 at 11:00,[SLIDES]
Dmitry Petrov,
Laboratoire de Physique Théorique et Modéles Statistiques, Univ. Paris-Sud, Orsay, France

open abstract
"Ultracold Quantum gases" Dmitry Petrov and Gora Shlyapnikov: Lecture 2 "Three-body problem"
- Born-Oppenheimer approximation
- Efimov effect
- Three-body problem near a narrow resonance
- Few-body problem in a lattice, exotic trimer states
- Exact (not Born-Oppenheimer) approach to three-body problem

[147] 10th of June 2008 at 11:00,[SLIDES]
Dmitry Petrov,
Laboratoire de Physique Théorique et Modéles Statistiques, Univ. Paris-Sud, Orsay, France

open abstract
- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials">">" placeholder="- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials">" placeholder="- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials" placeholder=""Ultracold Quantum gases" Dmitry Petrov and Gora Shlyapnikov: Lecture 1 "Scattering theory"
- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials">">">" placeholder="" placeholder="- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials" placeholder=""Ultracold Quantum gases" Dmitry Petrov and Gora Shlyapnikov: Lecture 1 "Scattering theory"
- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials">">" placeholder="- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials">" placeholder="- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials" placeholder=""Ultracold Quantum gases" Dmitry Petrov and Gora Shlyapnikov: Lecture 1 "Scattering theory"
- Interatomic forces. Van der Waals potential. Length and energy scales
- Ultracold scattering. Scattering amplitude, length and volume
- Resonant scattering. Width of the resonance
- Short-range pseudopotentials">">">">

[148] [SLIDES]