ITP > Tilman Enss > Many-body theory seminar

Tilman Enss | Many-body theory seminar

The research seminar of the Enss group takes place on Wednesdays at 14.00h in Philosophenweg 19 (seminar room).

Winter term 2024/25

Wednesday 20 November 2024, 14.15h, Phil19 SR
Journal club: Absorbing state dynamics of stochastic gradient descent
Dr. Giacomo Gori (Universität Heidelberg)

Abstract: journal club on Zhang and Martiniani, arXiv:2411.11834.

Summer term 2024

Wednesday 12 June 2024, 14.15h, Phil12 kHS
Journal club: Inducing Resonant Interactions in Ultracold Atoms with a Modulated Magnetic Field
Tilman Enss (Universität Heidelberg)

Abstract: journal club on D. Hudson Smith, PRL 115, 193002 (2015).
Wednesday 29 May 2024, 14.15h, Phil19 SR
Journal club: Critical Probability Distributions of the Order Parameter from the Functional Renormalization Group
Charalampos Pozoukidis (Universität Heidelberg)

Abstract: journal club on I. Balog et al., PRL 129, 210602 (2022).
Wednesday 22 May 2024, 14.15h, Phil19 SR
Journal club: Unscrambling the physics of out-of-time-order correlators
Tilman Enss (Universität Heidelberg)

Abstract: journal club on B. Swingle, Nature Phys. 14, 988 (2018).
Wednesday 8 May 2024, 14.15h, Phil19 SR
Dynamics of the open O(N) model
Dr. Johannes Lang (Universität zu Köln)

Abstract:
The evolution of open systems at late times differs fundamentally from that of closed systems. While the latter thermalize to a temperature determined by the energy density of the initial state in the former, a competition between this mechanism and equilibration with the environment emerges. In this talk, I will derive the dynamic theory in the symmetry-broken phase of an O(N)-symmetric field theory based on an expansion of the two-particle irreducible (2PI) effective action to next-to-leading order in 1/N and highlight the analogies and differences to the corresponding theory for closed systems. This approach culminates in the open-system Boltzmann equation valid in the absence of quasi-particles. Specifically for the O(N) model interactions are screened efficiently at small momenta and, therefore, the late-time evolution is effectively collisionless.
Wednesday 24 April 2024, 14.15h, Phil19 SR
Critical emergent geometry: numerical evidences, local formulation, and theoretical localization strategies
Dr. Giacomo Gori (Universität Heidelberg)

Winter term 2023/24

Wednesday 27 March 2024, 14.15h, Phil19 SR
Journal club: Ensemble inequivalence in long-range quantum systems
Tilman Enss (Universität Heidelberg)

Abstract: journal club on N. Defenu et al., arXiv:2403.06673.
Wednesday 06 March 2024, 14.15h, Phil19 SR
Journal club: Universal quantum dynamics of Bose polarons
Dr. Moritz Drescher (Universität Heidelberg)

Abstract: journal club on J. Etrych et al., arXiv:2402.14816.
Tuesday 20 February 2024, 14.00h, Phil19 SR
Universality of one-dimensional Lévy flights
Hagen Koch (Universität Heidelberg)
Wednesday 07 February 2024, 14.15h, Phil19 SR
Journal club: Observation of hydrodynamization and local prethermalization in 1D Bose gases
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Y. Le et al., Nature 618, 494 (2023).
Wednesday 31 January 2024, 14.15h, Phil19 SR
Journal club: Accelerating analysis of Boltzmann equations using Gaussian mixture models
Tilman Enss (Universität Heidelberg)

Abstract: journal club on P. Dolgirev et al., arXiv:2304.09911.
Wednesday 24 January 2024, 14.15h, Phil19 SR
Journal club: Time-reparametrization invariance: from Glasses to toy Black Holes
Dr. Giacomo Gori (Universität Heidelberg)

Abstract: journal club on J. Kurchan, arXiv:2401.03186.
Wednesday 20 December 2023, 14.15h, Phil19 SR
Complex-valued in-medium potential between heavy impurities in ultracold atoms
Dr. Keisuke Fujii (Universität Heidelberg)

Abstract:
We formulate the induced potential in a finite temperature cold atomic medium between two heavy impurities, or polarons, which is shown to be complex-valued in general. The imaginary part of the complex-valued potential describes a decoherence effect, and thus, the resulting Schrödinger equation for the two polarons acquires a non-Hermitian term. We apply the developed formulation to two representative cases of polarons interacting with medium particles through the s-wave contact interaction: (i) the normal phase of single-component (i.e., spin-polarized) fermions using the fermionic field theory, and (ii) a superfluid phase using the superfluid effective field theory, which is valid either for a Bose-Einstein condensate (BEC) of a single-component Bose gas or for the BEC-BCS crossover in two-component fermions at a low-energy regime. Computing the leading-order term, the imaginary part of the potential in both cases is found to show a universal r^-2 behavior at long distance. We propose three experimental ways to observe the effects of the universal imaginary potential in cold atoms.
[1] Y. Akamatsu et al., arXiv:2312.08241.
Wednesday 13 December 2023, 14.15h, Phil19 SR
Journal club: Universal four-component Fermi gas in one dimension
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Y. Nishida and D.T. Son, Phys. Rev. A 82, 043606 (2010).
Wednesday 6 December 2023, 14.15h, Phil19 SR
Introduction to Multi-vectors
Dr. Moritz Drescher (Universität Heidelberg)
Wednesday 29 November 2023, 14.15h, Phil19 SR
Non-abelian invariants in periodically-driven quantum rotors
Volker Karle (IST Austria)

Abstract:
This presentation explores the role of topological invariants in the non-equilibrium dynamics of periodically-driven quantum rotors. Under generic driving, quantum rotors exhibit dynamical localization, a prominent example of quantum chaos[1]. Utilizing Floquet theory, we analyze the non-linear response of these systems, transitioning from static eigenstates to non-equilibrium Floquet states. In a recent publication[2], we have generalized the model to include three-dimensional rotations and diverse laser pulses, inspired by experiments[3] on closed-shell diatomic molecules driven by periodic, far-off-resonant laser pulses. This approach uncovers a complex phase space with both localized and delocalized Floquet states. We demonstrate that the localized states are topological in nature, originating from Dirac cones protected by reflection and time-reversal symmetry. These states can be modified through laser strength adjustments, making them observable in current experiments through molecular alignment and observation of rotational level populations. Notably, in scenarios involving higher-order quantum resonances leading to multiple Floquet bands, the topological charges become non-Abelian. This results in the remarkable finding that the exchange of Dirac cones across different bands is non-commutative, enabling non-Abelian braiding. This phenomenon is linked to the recently identified non-Abelian topological Euler invariant[4], paving the way for the study of controllable multi-band topological physics in gas-phase experiments with small molecules, as well as for classifying dynamical molecular states by their topological invariants.
[1] Casati, G., & Chirikov, B. (Eds.). Quantum Chaos: Between Order and Disorder. Cambridge University Press (1995).
[2] Karle, V., Ghazaryan, A., & Lemeshko, M. Topological Charges of Periodically Kicked Molecules. Physical Review Letters, 130, 103202 (2023).
[3] Bitter, M., & Milner, V. Control of quantum localization and classical diffusion in laser-kicked molecular rotors. Physical Review A, 95, 013401 (2017).
[4] Bouhon, A., Bzdušek, T., & Slager, R. J. Geometric approach to fragile topology beyond symmetry indicators. Physical Review B, 102, 115135 (2020).
Wednesday 8 November 2023, 14.15h, Phil19 SR
Journal club: Universal scaling of the dynamic BKT transition in quenched 2D Bose gases
Dr. Keisuke Fujii (Universität Heidelberg)

Abstract: journal club on Sunami et al., Science 382, 443 (2023).
Wednesday 18 October 2023, 14.15h, Phil19 SR
Journal club: Quantum Bose-Fermi droplets
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Rakshit et al., SciPost Phys. 6, 079 (2019).

Summer term 2023

Thursday 6 July 2023, 14.15h, Phil19 SR jointly with complex systems seminar
Beyond the Cooper pairs paradigm: fermion quadrupling condensates in multicomponent superconductors
Dr. Ilaria Maccari (Stockholm University, Sweden)
Wednesday 21 June 2023, 14.15h, Phil19 SR
Journal club: The Nishimori line and Bayesian statistics
Dr. Giacomo Gori (Universität Heidelberg)

Abstract: journal club on Yukito Iba, J. Phys. A: Math. Gen. 32, 3875 (1999).
Wednesday 7 June 2023, 14.15h, Phil19 SR
Journal club: Quantum physics in connected worlds
Tilman Enss (Universität Heidelberg)

Abstract: journal club on J. Tindall et al., Nat. Comm. 13, 7445 (2022).
Wednesday 24 May 2023, 14.15h, Phil19 SR
Journal club: Dissipationless flow in a Bose-Fermi mixture
Dr. Keisuke Fujii (Universität Heidelberg)

Abstract: journal club on Z.Z. Yan et al., arXiv:2304.07663.
Wednesday 26 April 2023, 14.15h, Phil19 SR
Strongly coupled Bose-Fermi mixtures in 2D
Dr. Félix Rose (Universität Heidelberg)

Abstract:
I present a theoretical study of the phase diagram of strongly coupled two-dimensional Bose-Fermi mixtures that can be realized in two-dimensional semiconductors and ultracold atoms as a function of the particle densities, focusing on the limit of small boson density. To that effect, we used a functional renormalization group approach. At finite boson density, the fermions and bosons can form a fermionic composite molecule with a well-defined Fermi surface. Beyond a certain critical density, this molecular phases gives way to a mixed phase where polarons are hybridized with molecular degrees of freedom. We predict frequency- and momentum-resolved spectral functions.
[1] J. von Milczewski, F. Rose, and R. Schmidt, Functional-renormalization-group approach to strongly coupled Bose-Fermi mixtures in two dimensions, Phys. Rev. A 105, 013317 (2022).
Wednesday 19 April 2023, 14.15h, Phil19 SR
Journal club: Non-reciprocal phase transitions
Dr. Mrinal Sarkar (Universität Heidelberg)

Abstract: journal club on Fruchart et al., Nature 592, 363 (2021).

Winter term 2022/23

Wednesday 29 March 2023, 14.15h, Phil19 SR
Journal club: Nonequilibrium strong-coupling theory for a driven-dissipative ultracold Fermi gas in the BCS-BEC crossover region
Sarah Görlitz (Universität Heidelberg)

Abstract: journal club on Kawamura et al., Phys. Rev. A 101, 013602 (2020).
Wednesday 22 March 2023, 14.15h, Phil19 SR
Introduction to Random Matrix Theory
Eric Jacob (Universität Heidelberg)
Thursday 16 March 2023, 14.15h, Phil19 SR
Diagrammatic Monte-Carlo approach to many-flavour unitary fermions
Dr. Gunnar Möller (University of Kent, UK)

Abstract:
The difficulty of understanding strongly correlated matter is often related to the absence of a natural small parameter providing a starting point for a perturbative treatment. Large-N approaches provide a prominent theoretical tool, which seeks to address this issue for fermionic theories by introducing a generalised theory with many fermion flavours, that then becomes amenable to perturbative treatments.
We demonstrate how to implement this concept in conjunction with a diagrammatic Monte-Carlo approach, using a many fermion-flavour generalisation of the unitary Fermi gas [1] for illustration. This system provides an ideal test case thanks to existing diagrammatic Monte-Carlo techniques developed in [2,3], which yield a numerically exact solution thanks to explicit resummation of the high-order asymptotics [4].
We present results on the extended large-N generalisation of the theory [5,6], and demonstrate how to apply diagMC in this setting. Using the resummation technique developed by Rossi et al [4], we show that the convergence radius in the Borel plane is enlarged as a function of fermion flavours, thus facilitating the convergence of the series in the vicinity of the transition into the superfluid phase.
We argue that the combination of large-N field theory techniques with high-order numerical resummations opens up a new avenue for investigations of strongly interacting systems more generally. In the introduction to this talk, we will additionally review recent developments in diagrammatic Monte-Carlo techniques.
[1] W. Zwerger, Springer (2012).
[2] K. van Houcke et al., Nat. Phys. 8, 366 (2012).
[3] K. van Houcke et al., Phys. Rev. B 99, 035140 (2019).
[4] R. Rossi et al., Phys. Rev. Lett. 121, 130405 (2018).
[5] M.Y. Veillette et al., Phys. Rev. A 75, 043614 (2007).
[6] P. Nikolic and S. Sachdev, Phys. Rev. A 75, 033608 (2007).
Wednesday 15 March 2023, 14.15h, Phil19 SR
Introduction to the Toric Code
Nikita Titov (Universität Heidelberg)
Wednesday 8 February 2023, 14.15h, Phil19 SR
Journal club: Floquet prethermalization and regimes of heating in a periodically driven, interacting quantum system
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Weidinger et al., Sci. Rep. 7, 45382 (2017).
Wednesday 1 February 2023, 16.15h, online
Strongly interacting impurities in a dilute Bose condensate
Nikolay Yegovtsev (University of Colorado Boulder, USA)

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. This construction can be generalized to include the first order correction in inverse powers of the impurity-boson scattering length around the unitary point. Finally, I will discuss our recent results on comparison between the Gross-Pitaevskii equation and the Diffusion Monte Carlo as well as a calculation of the effective mass of the polaron at unitarity.
[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.
[2] Nikolay Yegovtsev, Pietro Massignan, and Victor Gurarie. Strongly interacting impurities in a dilute Bose condensate. Phys. Rev. A 106, 033305.
[3] Nikolay Yegovtsev, Grigori Astrakharchik, Pietro Massignan, and Victor Gurarie. Validity of the Gross-Pitaevskii equation for Bose polarons [in preparation].
Wednesday 25 January 2023, 14.15h, Phil19 SR
Journal club: Thermography of the superfluid transition in a strongly interacting Fermi gas
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Yan et al., arXiv:2212.13752.
Wednesday 11 January 2023, 14.15h, Phil19 SR
Planckian transport
Dr. Félix Rose (Universität Heidelberg)

Abstract: introduction to transport in quantum critical systems.
Wednesday 7 December 2022, 14.15h, Phil19 SR
Journal club: Statistical theory of transport by strongly interacting lattice fermions
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Mukerjee et al., PRB 73, 035113 (2006).
Wednesday 23 November 2022, 14.15h, online
Critical geometry
Dr. Giacomo Gori (Universität Heidelberg)

Abstract: introduction to the geometric properties of critical systems.
Wednesday 9 November 2022, 14.15h, Phil19 SR
p-Wave Fermi gases in Confined Geometries
Dr. Jeff Maki (University of Trento, Italy)

Abstract:
The most convenient way to create one-dimensional (1D) spin polarized Fermi gases is by harmonic confinement in the two transverse directions. These 1D spin polarized Fermi gases have been proposed to be an ideal platform for studying universal physics, where the effective range does not play an important role. This is in contrast to their three-dimensional (3D) counterparts. Not only is harmonic confinement useful for studying 1D physics but it allows one to study the whole 1D-3D crossover. In this talk we examine the 1D-3D crossover for spin-polarized Fermi gases with p-wave interactions. We show that there are two types of low-dimensional interactions reminiscent of 1D odd-wave and even-wave scattering. Both interactions are present and necessary for describing the 1D-3D crossover and leave important experimental signatures on the RF transfer rate that have been experimentally observed.
Thursday 27 October 2022, 14.15h, Phil19 SR jointly with complex systems seminar
Competing orders in graphene-based heterostructures
Dr. Laura Classen (MPI-FKF Stuttgart, Germany)

Abstract:
Motivated by graphene-based heterostructures like intercalated graphene and twisted bilayer graphene, I consider competing orders around Van Hove fillings in 2D hexagonal systems that arise due to amplified interactions from the increased density of states. This includes mechanisms for superconductivity from repulsive interactions, and in particular the possibility to obtain a topological d+id superconducting state. I discuss how the conventional Van Hove scenario qualitatively changes when accounting for the following effects in the different hexagonal heterostructures: a flattened dispersion along one direction at high-order Van Hove points, additional orbital or valley degrees of freedom, or incommensurate positions and non-local cluster interactions.
Wednesday 19 October 2022, 14.15h, Phil19 SR
Synchronization in the Kuramoto model: Effect of a nearest-neighbor interaction and stochastic resetting
Dr. Mrinal Sarkar (Universität Heidelberg)

Abstract:
Synchronization is one of the fascinating emergent phenomena in complex dynamical systems comprising a large population of interacting degrees of freedom. This phenomenon abounds in nature. We study such an emergent behavior within the ambit of the celebrated Kuramoto model. The model comprises a large collection of phase oscillators of distributed frequencies that are globally coupled through the sine of their phase differences. A nearest-neighbor interaction in the model affects the synchronization dynamics dramatically, which I will discuss in this talk in three different contexts. First, we explore the effect of competing interactions by including a nearest-neighbor interaction in the Kuramoto model on a one-dimensional periodic lattice that results in a rich phase diagram with the existence of a nonequilibrium tricritical point [1]. Second, we consider a system of identical Kuramoto oscillators on a two-dimensional periodic square lattice. Relaxation dynamics in such a system is studied when the oscillators interact only with their nearest neighbors. For a random initial condition, the system ends up either in a completely phase-synchronized state or a phase-locked state characterized by the presence of topological defects in the phase-field of the oscillators [2]. Finally, we investigate the effect of stochasticity on emergent behavior in such a system. In the presence of dichotomous noise as a stochastic force, the dynamics exhibits for any non-zero finite noise correlation time a nonequilibrium Berezinskii-Kosterlitz-Thouless (BKT)-like transition as one tunes the noise strength, similar to the equilibrium one in the presence of Gaussian white noise [3]. In the last part of my talk, I will discuss how stochastic resetting dramatically modifies the phase diagram of the Kuramoto model, allowing, in particular, for the emergence of a synchronized phase even in parameter regimes for which the bare model does not support such a phase [4].
[1] M. Sarkar and S. Gupta, Phys. Rev. E 102, 032202 (2020).
[2] M. Sarkar and N. Gupte, Phys. Rev. E 103, 032204 (2021).
[3] M. Sarkar, Chaos 31, 083102 (2021).
[4] M. Sarkar and S. Gupta, Chaos 32, 073109 (2022).

Summer term 2022

Wednesday 28 September 2022, 14.15h, Phil19 SR
Dynamical Systems
Dr. Mrinal Sarkar (Universität Heidelberg)

Abstract: Introduction to the Kuramoto model.
Thursday 15 September 2022, 14.15h, Phil19 SR
Journal club: Observing emergent hydrodynamics in a long-range quantum magnet
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Joshi et al., Science 376, 720 (2022).
Wednesday 20 July 2022, 11.00h, Phil19 SR
Journal club: Universality in Anderson localization on random graphs with varying connectivity
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Sierant et al., arXiv:2205.14614.
Wednesday 15 June 2022, 14.15h, Phil19 SR
Using a space-time mapping for probing heating suppression in periodically driven many-body quantum systems
Dr. Etienne Wamba (Wissenschaftskolleg Berlin and University of Buea, Cameroon)

Abstract:
While space-time mappings were long used to map a solvable problem into its simpler form, we introduced a non-traditional use of exact space-time mappings which consists in exactly mapping time evolutions of different quantum many-body systems both with[1] and without[2] dissipation and applicable in quantum gas experiments. In this talk, I will discuss a mean-field model of many-body systems with rapid periodic driving. The driving is a certain periodic but anharmonic modulation of the gas's two-body interaction, at a particular frequency, which makes it possible to map the Floquet experiment onto an evolution with slowly varying parameters. Such a mapping between a Floquet evolution and a slow process allows us to investigate non-equilibrium many-body dynamics and examine how rapidly driven systems may avoid heating up, at least when mean-field theory is still valid. We learn that rapid periodic driving may not yield to secular heating because the time evolution of the system has a kind of hidden adiabaticity, inasmuch as it can be mapped exactly onto that of an almost static system[3].
[1] E. Wamba, A. Pelster, and J. R. Anglin, Phys. Rev. A 94, 043628 (2016).
[2] E. Wamba and A. Pelster, Phys. Rev. A 102, 043320 (2020).
[3] E. Wamba, A. Pelster, and J. R. Anglin, arxiv:2108.07171.
Wednesday 8 June 2022, 14.30h, online
Dissipative processes at the acoustic horizon
M.L. Chiofalo (U Pisa, Italy) with D. Grasso (Pisa), M. Mannarelli (Gran Sasso), and S. Trabucco (Gran Sasso)

on their recent work Chiofalo et al., arXiv:2202.13790.
Wednesday 1 June 2022, 14.15h, Phil12 SR 306
Universal van der Waals force between heavy polarons in superfluids
Dr. Keisuke Fujii (Universität Heidelberg)

Abstract: We investigate the long-range behavior of the induced Casimir interaction between two spinless heavy impurities, or polarons, in superfluid cold atomic gases. With the help of effective field theory of a Galilean invariant superfluid, we show that the induced impurity-impurity potential at long distance universally shows a relativistic van der Waals-like attraction (1/r⁷) resulting from the exchange of two superfluid phonons. We also clarify finite temperature effects from the same two-phonon exchange process. The temperature T introduces the additional length scale c_s/T with the speed of sound c_s. Leading corrections at finite temperature scale as T⁶/r for distances r<c_s/T smaller than the thermal length. For larger distances the potential shows a nonrelativistic van der Waals behavior (T/r⁶) instead of the relativistic one. Our EFT formulation applies not only to weakly coupled Bose or Fermi superfluids but also to that composed of strongly-coupled unitary fermions with a weakly coupled impurity. The sound velocity controls the magnitude of the van der Waals potential, which we evaluate for the fermionic superfluid in the BCS-BEC crossover.
[1] K. Fujii, M. Hongo, and T. Enss, arXiv:2206.01048.
Wednesday 11 May 2022, 14.15h, online
Kuramoto model with additional nearest-neighbor interactions: Existence of a nonequilibrium tricritical point
Mrinal Sarkar (IIT Madras, India)

Abstract:
Synchronization is one of the fascinating emergent phenomena in complex dynamical systems comprising a large population of interacting degrees of freedom. The Kuramoto model serves as a paradigm to study the phenomenon of synchronization. The model comprises a large collection of phase oscillators of distributed frequencies that are globally coupled through the sine of their phase differences. In this talk, I will discuss the dramatic effect brought about by the inclusion of a nearest-neighbor interaction in the model. In particular, we explore the effect of competing interactions by including a nearest-neighbor interaction in the Kuramoto model on a one-dimensional periodic lattice. As is known, for unimodal and symmetric frequency distributions, while the mean-field Kuramoto model exhibits a continuous phase transition between low-coupling incoherent phases and high-coupling synchronized phases, the model with solely nearest-neighbor interactions does not exhibit any synchronized phase on a one-dimensional periodic lattice. However, in a setting where both are present, while the mean-field interaction favors global synchrony, the nearest-neighbor interaction may have cooperative or competitive effects depending on its sign and magnitude. For unimodal and symmetric frequency distributions, we demonstrate that the inclusion of a nearest-neighbor interaction results in a rich phase diagram in that the model in the stationary state exhibits, in contrast to the usual Kuramoto model, both continuous and first-order transitions between synchronized and incoherent phases, with the transition lines meeting at a tricritical point. Our results are based on numerical integration of the dynamics as well as an approximate theory involving appropriate averaging of fluctuations in the stationary state.
[1] M Sarkar and S Gupta, Phys. Rev. E 102, 032202 (2020).
Wednesday 27 April 2022, 14.15h, Phil19 SR
Journal club: Dissipative processes at the acoustic horizon
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Chiofalo et al., arXiv:2202.13790.
Wednesday 6 April 2022, 14.15h, Phil19
Journal club: Mediated interactions between ions in quantum degenerate gases
Dr. Keisuke Fujii (Universität Heidelberg)

Abstract: journal club on Ding et al., arXiv:2203.02768.

Winter term 2021/22

Wednesday 30 March 2022, 14.15h, Phil19 SR
Diagrammatic Monte Carlo for electronic correlation in molecules: high-order many-body perturbation theory with low scaling
Dr. Giacomo Bighin (Universität Heidelberg)

Abstract:
We present a low-scaling diagrammatic Monte Carlo approach to molecular correlation energies. Using combinatorial graph theory to encode many-body Hugenholtz diagrams, we sample the Møller-Plesset (MPn) perturbation series, obtaining accurate correlation energies up to n = 5, with quadratic scaling in the number of basis functions. Our technique reduces the computational complexity of the molecular many-fermion correlation problem, opening up the possibility of low-scaling, accurate stochastic computations for a wide class of many-body systems described by Hugenholtz diagrams. (arXiv:2203.12666)
Thursday 17 March 2022, 14.15h, Phil19 SR, jointly with Complex Systems Seminar
Metastability and discrete spectrum of long-range systems
Dr. Nicolò Defenu (ETH Zurich, Switzerland)

Abstract:
The linear dynamics of closed quantum systems produce well-known difficulties in the definition of quantum chaos. This leads to several issues in the theoretical justification of the equilibration and thermalization dynamics observed in closed experimental systems. In the case of large harmonic baths these issues are partially resolved due to the continuous nature of the spectrum, which produces divergent Poincaré recurrence times. As is shown in the following, such a scenario does not apply to long-range interacting models, whose spectrum remains discrete up to the thermodynamic limit, in contradiction with the textbook description of infinitely extended systems [PNAS July 27, 2021 118 (30) e2101785118].
Wednesday 9 March 2022, 14.15h, online
Complex scaling flows in quench dynamics of interacting particles
Dr. Giacomo Gori (Universität Heidelberg)

Abstract:
Many-body systems driven out of equilibrium can exhibit scaling flows of the quantum state. For a sudden quench to resonant interactions between particles we construct a new class of analytical scaling solutions for the time evolved wave function with a complex scale parameter. These solutions determine the exact dynamical scaling of observables such as the pair correlation function, the contact and the fidelity. We give explicit examples of the nonequilibrium dynamics for two trapped fermions or bosons quenched to unitarity, for ideal Bose polarons, and for resonantly interacting three-body systems. These solutions reveal universal scaling properties of interacting many-body systems that arise from the buildup of correlations at short times after the quench.
Wednesday 23 February 2022, 14.15h, Phil19 SR and online
Journal club: Stochastic-field approach to the quench dynamics of the one-dimensional Bose polaron
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Jager et al., PRR 3, 033212 (2022).
Wednesday 9 February 2022, 14.15h, Phil19 SR and online
Journal club: Second sound attenuation near quantum criticality
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Li et al., Science 375, 528 (2022).
Wednesday 19 January 2022, 14.15h, online
Quantum Brownian motion and the Bose polaron problem
Daniel Arrufat Vicente (Universität Heidelberg)
Wednesday 1 December 2021, 14.15h, Phil19 SR
Journal club: Non-relativistic conformal invariance in mesoscopic two-dimensional Fermi gases
Dr. Jeff Maki (University of Hong Kong)

Abstract: journal club on Berkassy et al., arXiv:2111.09893.
Wednesday 17 November 2021, 14.15h, Phil19 SR and online
Disorder in order: localization in a randomless cold atom system
Dr. Félix Rose (MPQ Garching, Germany)

Abstract:
We present a mapping between the Edwards model of disorder, describing a single particle subjected to the potential of randomly-positioned scatterers, and the Bose polaron problem of a light impurity interacting with a Bose-Einstein condensate (BEC) of heavy atoms. The time evolution of the impurity emulates the disorder-averaged dynamics of the Edwards model. The mapping offers a novel experimental setup to investigate the physics of Anderson localization. It is valid in any space dimensions and can be extended to include interactions between bosons, several particles, and arbitrary scattering and confining potentials. We focus on the case of an impurity interacting with a one-dimensional BEC through a repulsive contact interaction. The polaron model is studied by means of a variational approach, which can be benchmarked against the corresponding, exactly-solvable disorder model. While the simple Chevy Ansatz misses the localization physics entirely, a more involved coherent state Ansatz combined with the Lee-Low-Pines transform captures the qualitative physics of disorder.
Tuesday 16 November 2021, 11.15h, Phil19 SR and online
Criticality and Phase Diagram of Quantum Long-Range Systems
Prof. Andrea Trombettoni (University of Trieste and SISSA, Trieste)

Abstract:
Several recent experiments in atomic, molecular and optical systems motivated an huge interest in the study of quantum long-range spin systems. The goal of the talk is to present a general description of their critical behavior and phases, devising a treatment valid in d dimensions with an exponent d+sigma for the power-law decay of the couplings in the presence of an O(N) symmetry. By introducing a convenient ansatz for the effective action, one can determine the phase diagram for the N-component quantum rotor model with long-range interactions, with N=1 corresponding to the Ising model. The phase diagram in the sigma-d plane shows a nontrivial dependence on sigma. As a consequence of the fact that the model is quantum, the correlation functions are genuinely strongly anisotropic in the spatial and time coordinates for sigma smaller than a critical value and in this region the isotropy is not restored even at the criticality. Results for the correlation length exponent nu and the dynamical critical exponent z and a comparison with numerical findings for them are presented.
Thursday 11 November 2021, 11.15h, Phil19 SR and online
The Bulk Viscosity in 1D Spin Polarized Fermi Gases
Dr. Jeff Maki (University of Hong Kong)

Abstract:
Often the low-energy physics of 1D Fermi gases are described by the Luttinger Liquid (LL) model. In this model the excitations of the system are bosonic soundwaves which are undamped. As a result the LL model is integrable and lacks dissipation. In order to observe dissipation, one needs to explicitly break the symmetries of the LL model with further interactions. In this talk I present an analysis of the 1D spin polarized Fermi gas with odd-wave interactions. This system clearly provides two-body interactions beyond the LL model. We show how these interactions induce dissipation through a finite bulk viscosity, and contrast our approach to other well known methods like kinetic theory.
Wednesday 3 November 2021, 14.15h, Phil19 SR
Journal club: Entropy Production and the Role of Correlations in Quantum Brownian Motion
Tilman Enss (Universität Heidelberg)

Abstract: journal club on A. Colla et al., arXiv:2108.02599.
Thursday 21 October 2021, 14.15h, Phil19 SR
An impurity in a heteronuclear two-component Bose mixture
Dr. Giacomo Bighin (Universität Heidelberg)

Abstract:
We study the fate of an impurity in an ultracold heteronuclear Bose mixture, focusing on the experimentally relevant case of a 41K-87Rb mixture, with the impurity in a 41K hyperfine state. Our work provides a comprehensive description of an impurity in a BEC mixture with contact interactions across its phase diagram. We present results for the miscible and immiscible regimes, as well as for the impurity in a self-bound quantum droplet. Here, varying the interactions, we find novel, exotic states where the impurity localizes either at the center or at the surface of the droplet.
[1] G. Bighin et al., arXiv:2109.07451.
Monday 11 October 2021, 11.30h, Phil19 Seminar room
Quantum and thermal stability of quasiperiodic patterns of ultracold Bose gases
Prof. Tommaso Macrì (Universidade Federal do Rio Grande do Norte, Brazil)

Abstract:
The search for spontaneous pattern formation in equilibrium phases with genuine quantum properties is a leading direction of current research. We investigate the effect of quantum fluctuations - zero point motion and exchange interactions - on the phases of an ensemble of bosonic particles with local and nonlocal interactions via path-integral Monte Carlo simulations to determine their ground state properties. In the high-density limit we observe patterns with 12-fold rotational symmetry compatible with periodic approximants of quasicrystalline phases. In the second part, I present results for a system of 2D trapped bosons in a quasiperiodic potential at finite temperature. Alongside the superfluid, normal fluid and insulating phases, we demonstrate the existence of a Bose glass phase, which is found to be robust to thermal fluctuations for a set of parameters within reach of current experiments with quasi-2D optical confinement.

Summer term 2021

Wednesday 29 September 2021, 14.15h, online
Probing the Correlated Spin-Dynamics of Ultracold Atoms: Magnetic and Polaronic Properties
Dr. Georgios Koutentakis (Universität Hamburg)

Abstract:
In this talk I will go through two topics relevant for ultracold experiments with spinor atoms. First I will outline the main facets of ferromagnetism emerging in one-dimensional systems. Here, by exploring the dynamics of few-body systems [1, 2], we were able to expose and characterize the underlying ferromagnetic (Hund) and antiferromagnetic (Anderson) spin-exchange mechanisms. The competition of these spin-interactions under certain conditions results in the system exhibiting distinctly ferromagnetic behavior. Our results provide a generalization to the well-enstablished Stoner model of ferromagnetism and might be relevant for studies in higher dimensional setups. Subsequently, I will analyze the dynamics of the Bose polaron in one-dimension exhibiting the phenomenon of the temporal orthogonality catastrophe [3] and provide spectroscopic means for its identification [3, 4]. This phenomenon is manifested when strong repulsive bath-impurity interactions are switched on in a rapid manner, resulting to a rapid evolution of the system state towards a configuration orthogonal to the initial one. The occurrence of the temporal orthogonality catastrophe signifies the dynamical decay of the Bose polaron and the tendency of the system to an eigenstate thermalized state.
[1] G. M. Koutentakis, S. I. Mistakidis, and P. Schmelcher, New J. Phys. 21, 053005 (2019).
[2] G. M. Koutentakis, S. I. Mistakidis, and P. Schmelcher, New J. Phys. 22, 063058 (2020).
[3] S. I. Mistakidis, G. C. Katsimiga, G. M. Koutentakis, Th. Busch, and P. Schmelcher, Phys. Rev. Lett. 122, 183001 (2019).
[4] S. I. Mistakidis, G. C. Katsimiga, G. M. Koutentakis, Th. Busch, and P. Schmelcher, Phys. Rev. Research 2, 033380 (2020).
Wednesday 22 September 2021, 14.15h
Journal club: Idealized hydrodynamics
Tilman Enss (Universität Heidelberg)

Abstract: journal club on Zhe-Yu Shi et al., arXiv:2011.01415.
Tuesday 27 July 2021, 14.00h, online
Dynamics of two-dimensional Bose-Einstein condensates
Nikita Titov (Universität Heidelberg)
Wednesday 21 July 2021, 14.00h, online
A Variational Approach for Quantum Annealing within the LHZ Architecture
Maximilian Kramer (Universität Heidelberg)
Monday 12 to Friday 16 July 2021, hybrid
Workshop on Low Dimensional Quantum Many Body Systems
at Internationales Wissenschaftsforum Heidelberg (IWH), Germany
Wednesday 7 July 2021, 14.15h, online
Brownian motion in field theoretic formulation
Dr. Keisuke Fujii (Universität Heidelberg)
Wednesday 30 June 2021, 14.15h, online
Density functional theory for dilute quantum gases
Dr. Giacomo Bighin (Universität Heidelberg)
Wednesday 9 June 2021, 14.15h, online
High-order diagrammatic expansion around BCS theory: Polarized superfluid phase of the attractive Hubbard model
Dr. Félix Werner (ENS Paris, France)

Abstract:
In contrast to conventional QMC methods, expansions of intensive quantities in series of connected Feynman diagrams can be formulated directly in the thermodynamic limit. Over the last decade, diagrammatic Monte Carlo algorithms made it possible to reach large expansion orders and to obtain state-of-the-art results in the normal phase of various key models of interacting fermions.
We report first results inside a superfluid/superconducting phase, namely the s-wave superfluid phase of attractive Hubbard model in 3D [1]. Spontaneous symmetry breaking is realized by expanding around the BCS Hamiltonian. All diagrams up to order ~12 are summed thanks to the connected determinant algorithm [2] with anomalous propagators. Working on the BCS side of the strongly correlated regime, we observe convergence of the expansion, and benchmark the results against determinant diagrammatic Monte Carlo [3]. In presence of a polarizing Zeeman field (where unbiased benchmarks are unavailable due to the sign problem), we observe a first-order superfluid-to-normal phase transition, and a significant polarization of the superconducting phase at higher temperature. We also discuss the large-order behavior of the expansion and its relation to Goldstone and instanton singularities.
[1] G. Spada, R. Rossi, F. Simkovic, R. Garioud, M. Ferrero, K. Van Houcke, F. Werner, arXiv:2103.12038.
[2] R. Rossi, PRL 119, 045701 (2017).
[3] E. Burovski, N. Prokof'ev, B. Svistunov, M. Troyer, PRL 96, 160402 (2006).
Wednesday 26 May 2021, 13.00h, online
Role of effective range in p-wave Fermi gases
Prof. Shizhong Zhang (University of Hong Kong)

Abstract:
In this talk, I focus on the role of effective range in a single component Fermi gas, interacting primarily via p-wave scattering channel. We show how the p-wave effective range affects several physical quantities, including the bulk/shear viscosity and the three-body recombination rate.
Wednesday 28 April 2021, 14.15h, online
Transport properties of resonant Fermi gases
Dr. Keisuke Fujii (Universität Heidelberg)

Abstract:
We investigate the transport properties of two-component Fermi gases near the two-body resonance in two and three dimensions. We particularly focus on their transport coefficients, such as the bulk viscosity, the shear viscosity, and the thermal conductivity, for an arbitrary scattering length. In this talk, we will discuss two topics: (i) proposal of a novel method to measure the bulk viscosity using a spacetime-dependent scattering length [1], and (ii) consistency between the quantum virial expansion and the kinetic theory in the calculation of the transport coefficients [2,3].
[1] KF & Y. Nishida, Phys. Rev. A 98, 063634 (2018); [arXiv:1807.07983].
[2] KF & Y. Nishida, Phys. Rev. A 102, 023310 (2020); [arXiv:2004.12154].
[3] KF & Y. Nishida, [arXiv:2103.10123] (2021).
Wednesday 21 April 2021, 13.45h, online
Magnetization profiles at the upper critical dimension as solutions of the integer Yamabe problem
Dr. Giacomo Gori (Universität Heidelberg)

Abstract: introduction to A. Galvani et al., arXiv:2103.12449.
Wednesday 14 April 2021, 14.15h, online
Self-stabilized Bose polarons
Tilman Enss (Universität Heidelberg)

Abstract: introduction to R. Schmidt et al., arXiv:2102.13616.
Wednesday 31 March 2021, 14.15h, online
Journal club: Bose-Einstein Condensation on the Surface of a Sphere
Dr. Giacomo Bighin (Universität Heidelberg)

Abstract: Journal club on Tononi et al., PRL 123, 160403 (2019) and PRL 125, 010402 (2020).
Wednesday 24 March 2021, 14.15h, online
Journal club: Dynamical quantum Cherenkov transition of fast impurities in quantum liquids
Tilman Enss (Universität Heidelberg)

Abstract: Journal club on K. Seetharam et al., arXiv:2101.00030.
Wednesday 17 March 2021, 14.15h, online
Journal club: Maximum Energy Growth Rate in Dilute Quantum Gases
Tilman Enss (Universität Heidelberg)

Abstract: Journal club on Ran Qi et al., arXiv:2102.07051.
Tuesday 28 July 2020, 10.00h, online
Collective excitations of trapped Bose-Einstein condensed gases: short vs long range interactions
Dr. Sukla Pal (University of Otago, New Zealand)
Thursday 23 July 2020, 11.00h, online
Conformal Symmetry and the Dynamics of Unitary Fermi Gases
Dr. Jeff Maki (University of Hong Kong)
Thursday 16 July 2020, 14.15h, online — jointly with Complex Systems seminar
Rotational coherence spectroscopy and far-from-equilibrium dynamics of molecules in 4He nanodroplets
Dr. Giacomo Bighin (Institute of Science and Technology Austria)

Abstract:
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 [1,3,4].
Using the concept of quasiparticle and the recently-introduced angulon quasiparticle [2,3], we provide a better understanding of the rotational dynamics of the molecule, even far from equilibrium or when they are strongly affected by the interaction with the many-body environment. The dynamical theory we develop shows a very good agreement with experimental data [3,4] for several molecular species and across a wide range of laser fluences, contributing to advancing the understanding of angular momentum dynamics in a many-body environment.
[1] D. Pentlehner et al., Phys. Rev. Lett. 110, 093002 (2013).
[2] R. Schmidt and M. Lemeshko, Phys. Rev. Lett. 114, 203001 (2015).
[3] I.N. Cherepanov, GB, L. Christiansen, A.V. Jørgensen, R. Schmidt, H. Stapelfeldt, M. Lemeshko, arXiv:1906.12238.
[4] A.S. Chatterley, ..., GB et al., Phys. Rev. Lett. 125, 013001 (2020).
Thursday 9 July 2020, 14.15h, online
Journal club: Evidence for Unbounded Growth of the Number Entropy in Many-Body Localized Phases
Dr. Tilman Enss (Universität Heidelberg)

Abstract: Journal club on M. Kiefer-Emmanouilidis et al., PRL 124, 243601 (2020).
Thursday 2 July 2020, 14.15h, online
Complex networks with tuneable dimensions as a universality playground
Dr. Giacomo Gori (Universität Heidelberg)

Abstract:
Universality is one of the key concepts in understanding critical phenomena. However, for interacting inhomogeneous systems described by complex networks a clear understanding of the relevant parameters for universality is still missing. Here we discuss the role of a fundamental network parameter, the spectral dimension, neglected in previous investigations. For this purpose, we construct a complex network model where the probability of a bond between two nodes is proportional to a power law of the nodes' distances. By explicit computation we prove that the spectral dimension for this model can be tuned continuously from 1 to infinity, and we discuss related network connectivity measures. We propose our model as a tool to probe universal behaviour on inhomogeneous structures and comment on the possibility that the universal behaviour of correlated models on such networks mimics the one of continuous field theories in fractional euclidean dimensions. We suggest that similar structures could be engineered in atomic, molecular and optical devices in order to tune universal properties to a desired value.
[1] A.P. Millán, G. Gori, F. Battiston, T. Enss and N. Defenu, arXiv:2006.10421.
Thursday 18 June 2020, 14.15h, online
Strong correlations in the normal phase of an attractive Fermi gas
Dr. Michele Pini (U Camerino, Italy)

References:
M. Pini et al., Phys. Rev. B 99, 094502 (2019) and arXiv:1912.04802.

Winter term 2019

Thursday 20 February 2020, 11.00h, SR Phil19
Dynamics of the quantum long-range interacting Ising chain
Giulia Piccitto (SISSA Trieste, Italy)

Abstract:
In this talk I will briefly introduce the problem of nonequilibrium dynamics of quantum many-body systems. In particular, I will focus on the dynamics of the long-range interacting Ising chain, both in the presence and absence of dissipation. Focusing on the closed Ising spin chains with power-law decaying interactions and accounting for short-range correlations by a cluster mean field theory I will show that the latter are responsible for the emergence of a chaotic dynamical region. Analyzing the fine details of the phase diagram, I will show that the resulting chaotic dynamics bears close analogies with that of a tossed coin. Finally I will show some preliminary results on the dissipative dynamics of fully connected Ising chains.
Wednesday 12 February 2020, 10.30h, INF 226 (CQD lounge) — jointly with Weidemüller group
Journal club: Polaron bubble stabilised by medium-induced three-body interactions
Dr. Tilman Enss (Universität Heidelberg)

Abstract: Journal club on P. Naidon, arXiv:2001.08352 (2020).
Thursday 6 February 2020, 14.15h, SR Phil19 — jointly with Complex Systems seminar
One-dimensional disordered bosons
Prof. Nicolas Dupuis (Univ. Paris, France)

Abstract:
After a general introduction to the field theory of disordered systems, which emphasizes the need of considering different copies (replicas) of the system within a functional approach, we discuss the phase diagram of a one-dimensional disordered Bose fluid. Using bosonization and a nonperturbative functional renormalization-group approach, we find that the localized phase (dubbed Bose-glass) is described by a fully attractive strong-disorder fixed point characterized by a singular disorder correlator whose functional dependence assumes a cuspy form. We show that this reveals the glassy properties (pinning, "shocks" and "avalanches") of the Bose-glass phase due to the existence of metastable states, as well as the crucial role of quantum tunneling between different metastable configurations leading to the existence of rare superfluid regions.
Monday 3 February 2020, 14.15h, SR Phil19
The low-temperature phase in the two-dimensional long-range XY model
Fabiana Cescatti (Univ. Parma, Italy)
Wednesday 18 December 2019, 14.15h, SR Phil19
Transport in the unitary Fermi gas
Dr. Tilman Enss (Universität Heidelberg)
Journal club: Dynamics of relaxation and dressing of a quenched Bose polaron
Moritz Drescher (Universität Heidelberg)

Abstract: Journal club on D. Boyanovsky et al., PRA 100, 043617 (2019).
Monday 02 December 2019, 15.30h, INF 226 (CQD lounge) — jointly with Weidemüller group
Journal club: Trapped Bose-Einstein Condensates with Attractive s-wave Interaction
Eleonora Lippi (Universität Heidelberg)

Abstract: Journal club on Tao Shi et al., arXiv:1909.02432.
Wednesday 27 November 2019, 14.15h, SR Phil19
Bulk viscosity and contact correlations in attractive Fermi gases
Dr. Tilman Enss (Universität Heidelberg)

Abstract:
The bulk viscosity determines dissipation during hydrodynamic expansion. It vanishes in scale invariant fluids, while a nonzero value quantifies the deviation from scale invariance. For the dilute Fermi gas the bulk viscosity is given exactly by the correlation function of the contact density of local pairs. As a consequence, scale invariance is broken purely by pair fluctuations. These fluctuations give rise also to logarithmic terms in the bulk viscosity of the high-temperature nondegenerate gas. For the quantum degenerate regime I report numerical Luttinger-Ward results for the contact correlator and the dynamical bulk viscosity throughout the BEC-BCS crossover. The ratio of bulk to shear viscosity ζ/η is found to exceed the kinetic theory prediction in the quantum degenerate regime. Near the superfluid phase transition the bulk viscosity is enhanced by critical fluctuations and has observable effects on dissipative heating, expansion dynamics and sound attenuation.
[1] Phys. Rev. Lett. 123, 205301 (2019).
Wednesday 20 November 2019, 14.15h, SR Phil19
Detecting hidden and composite orders in layered models via machine learning
Dr. Nicolò Defenu (Universität Heidelberg)

Abstract:
We use machine learning to study layered spin models where composite order parameters may emerge as a consequence of the interlayerer coupling. We focus on the layered Ising and Ashkin-Teller models, determining their phase diagram via the application of a machine learning algorithm to the Monte Carlo data. Remarkably our technique is able to correctly characterize all the system phases also in the case of hidden order parameters, i.e., order parameters whose expression in terms of the microscopic configurations would require additional preprocessing of the data fed to the algorithm. Within the approach we introduce, owing to the construction of convolutional neural networks, naturally suitable for layered image-like data with arbitrary number of layers, no preprocessing of the Monte Carlo data is needed, also with regard to its spatial structure. The physical meaning of our results is discussed and compared with analytical data, where available. Yet, the method can be used without any a priori knowledge of the phases one seeks to find.
Wednesday 06 November 2019, 14.15h, SR Phil19
Journal club: Quantum unbinding near a zero temperature liquid-gas transition
Dr. Tilman Enss (Universität Heidelberg)

Abstract: Journal club on W. Zwerger, J. Stat. Mech. (2019) 103104.
Wednesday 30 October 2019, 14.15h, SR Phil19
Superfluidity and quasicrystals with nonlocal interactions
Prof. Tommaso Macrì (Universidade Federal do Rio Grande do Norte, Natal, Brazil)

Abstract:
In recent years, propelled by the progress in the field of quantum simulations with ultracold atoms, there has been an increasing interest of the condensed matter community in what is generally called quasicrystal lattices, long-range ordered but non-periodic structures. Besides retaining intrinsic relevant questions that range from the stability of tiled structures at zero temperature to their relation to fractal lattices, quasicrystals have also shown to support quantum phases of matter such as superconductors and Bose-Einstein condensates. Nonetheless, in spite of important works that address the emergence of quasicrystalline order in classical systems, a deeper understanding of the role of quantum fluctuations in these structures still lacks. Here we present our proposal to realize quasi-crystalline states in ultracold setups with non-local interactions.
Wednesday 23 October 2019, 14.15h, SR Phil19
Journal club: Bose polaron in spherical trap potentials: Spatial structure and quantum depletion
Moritz Drescher (Universität Heidelberg)

Abstract: Journal club on J. Takahashi et al., PRA 100, 023624 (2019).
Wednesday 16 October 2019, 14.15h, SR Phil19
Complex dynamical neural networks and emergent synchronization phenomena
Dr. Ana Paula Millán Vidal (University of Granada, Spain)

Summer term 2019

Thursday 4 July 2019, 14.15h, SR Phil19 (jointly with Theory of Complex Systems)
Geometry of Bounded Critical Phenomena
Dr. Giacomo Gori (Univ Padova, Italy)

Abstract:
What would you do if you were a system at criticality confined in a bounded domain? Of course you would forget about details of the interaction, lattice spacing flowing to an RG fixed point. Besides attaining this bulk universal behavior you would also try (boundary condition permitting) to forget about the confinement becoming "as uniform as possible". Implementing this requirement in absolute geometric language, the one used by general relativity, we obtain novel predictions for the structure of one- and two-point correlators. These predictions are tested successfully against numerical experiments yielding a precise estimate of a critical exponent of the Ising model in three dimensions.
Thursday 27 June 2019, 14.00h, SR Phil19
Quench dynamics in strongly interacting trapped Fermi gases
Till Johann (Universität Heidelberg)
Wednesday 5 June 2019, 14.00h, SR Phil19
Role of the repulsive Fermi polaron in spin transport
Hans Böhringer (Universität Heidelberg)
Wednesday 29 May 2019, 14.00h, SR Phil19
Bose polaron as an instance of quantum Brownian motion
Moritz Drescher (Universität Heidelberg)

Abstract: Journal club on A. Lampo et al., arXiv:1704.07623.
Thursday 23 May 2019, 9.00h, SR Phil16
Far-from-equilibrium dynamics of molecules in 4He nanodroplets: a quasiparticle perspective
Dr. Giacomo Bighin (IST Austria)

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. Here 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.
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 - a quantum rotor dressed by a field of many-body excitations - with a very good agreement with experimental data for several molecular species and across a wide range of laser fluences. The dynamical theory we develop contributes to advancing the understanding of angular momentum dynamics in a many-body environment, with applications ranging from ultracold molecules to condensed matter.
Friday 17 May 2019, 12.00h, Phil12;SR106 (jointly with STRUCTURES Jour fixe)
Applied string theory: Understanding strange metals in the lab with virtual black holes
Prof. Koenraad Schalm (Institute Lorentz for Theoretical Physics, Leiden University)

Abstract:
The Anti-de-Sitter/Conformal Field Theory correspondence, also known as AdS/CFT, has given us an unprecedented new holographic window in strongly coupled physics. In particular, the existence of charged black holes in AdS predicts the existence of novel quantum critical fixed points distinct from the conventional theory of critical phenomena. I will review how the distinct features of these novel quantum critical points show a remarkable resemblance with the profoundly mysterious behavior of exotic strange metal states of quantum matter, e.g. in high Tc superconductors. Recent experiments of the past two years strongly indicate that this resemblance is more than superficial. This has put us at the cusp of a new era in theoretical physics: we will present the case that current experiments can and will test a holographic gravity model as the theory of the strange metal state.
Thursday 16 May 2019, 14.15h, SR Phil19 (jointly with Theory of Complex Systems)
Quantum Chaos, hydrodynamics and black hole scrambling
Prof. Koenraad Schalm (Institute Lorentz for Theoretical Physics, Leiden University)

Abstract:
For perturbative scalar field theories, the late-time-limit of the out-of-time-ordered correlation function that measures (quantum) chaos is shown to be equal to a Boltzmann-type kinetic equation that measures the total gross (instead of net) particle exchange between phase space cells, weighted by a function of energy. This derivation gives a concrete form to numerous attempts to derive chaotic many-body dynamics from ad hoc kinetic equations. As in conventional Boltzmann transport, which follows from the dynamics of the net particle number density exchange, the kernel of this kinetic integral equation is also set by the 2-to-2 scattering rate. This provides a mathematically precise statement of the known fact that in dilute weakly coupled gases late-time transport and early-time scrambling (or ergodicity) are controlled by the same physics.
Surprisingly infinitely strongly coupled, large-Nc theories with a holographic dual also possess this relation between early- and late-time physics. The gravitational shock wave computation used to extract the scrambling rate in strongly coupled quantum theories with a holographic dual is directly related to probing the system's hydrodynamic sound modes. At a special point along the sound dispersion relation curve, the residue of the retarded longitudinal stress-energy tensor two-point function vanishes. This pole-skipping point encodes the Lyapunov exponent of quantum chaos.
Wednesday 8 May 2019, 14.00h, SR Phil19
Quantum Many-Body Conformal Dynamics
Dr. Tilman Enss (Universität Heidelberg)

Abstract: Journal club on J. Maki and F. Zhou, arXiv:1904.11549.
Tuesday 16 April 2019, 14.15h, SR Phil19
Vortex physics in 2D disordered superconductors
Dr. Ilaria Maccari (University of Rome "La Sapienza")

Abstract:
The study of quasi-two-dimensional (2D) superconductors can hardly avoid to take into account the presence of vortices. Many real systems, of both conventional and unconventional superconductors, undergo the superconductor-insulator transition via the Berezinskii-Kosterlitz-Thouless (BKT) transition, at which the phase coherence of the condensate is destroyed by means of free-vortex proliferation. When a finite flux of magnetic field is applied to films, the role of vortices becomes even more crucial for the understanding of the superconducting (SC) transition. Indeed, in this case the transition is no longer driven by vortex-antivortex unbinding, but rather by the melting of the 2D Abrikosov lattice of vortices. Such melting, as predicted by the BKT theory, and afterwards refined by Halperin, Nelson and Young (BKTHNY), can occur in two steps via an intermediate state called hexatic phase. In real systems, the observation of the two-step BKTHNY melting could be hindered by the presence of additional ingredients, such as random pinning, and also competing phases. In [1], we have shown that the 2-dimensional vortex lattice in a-MoGe thin film follows the BKTHNY sequence of melting as the magnetic field is increased. Identifying the signatures of various transitions on the bulk transport properties of the superconductor, we construct a vortex phase diagram for a 2D superconductor. From a theoretical standpoint, the classical XY model with a transverse magnetic field is a very promising candidate to capture the physics experimentally observed and to explore stronger disorder regimes. It constitutes hence a perfect playground for the investigation of the long-standing issue of the emergent glassy-state of the vortex lattice in the presence of disorder.
[1] I. Roy, S. Dutta, A. N. Roy Choudhury, S. Basistha, I. Maccari, S. Mandal, J. Jesudasan, V.Bagwe, C. Castellani, L. Benfatto, P. Raychaudhuri, Phys. Rev. Lett. 122, 047001 (2019).
Wednesday 3 April 2019, 14.00h, SR Phil19
Expansion dynamics in strongly interacting Fermi gases
Dr. Tilman Enss (Heidelberg University)

Winter term 2018/2019

Tuesday 12 February 2019, 10.00h, KIP SR 2.404 jointly with Oberthaler group
Junctions of weakly-coupled 1D strongly-interacting bosonic systems
Dr. Andrea Trombettoni (CNR-IOM and SISSA, Trieste)

Abstract:
After very briefly reviewing the use of ultracold atoms for the implementation of quantum devices, I discuss an example of junctions made by 1D strongly interacting systems weakly coupled between them. I will focus on properties of 1D Bose gases and then of junctions of Tonks-Girardeau gases. When three Tonks-Girardeau gases are coupled, one can exactly map their Hamiltonian by means of a suitable Jordan-Wigner transformation into the Hamiltonian of the multichannel Kondo model. I will also show recent results on the experimental realization of Y-geometries with holographic traps, and comment about recent progress in atomtronics.
Wednesday 6 February 2019, 14.00h, SR Phil19
Second sound and superfluidity in ultracold quantum gases
Dr. Vijay Singh (Hamburg University)

Abstract:
Ultracold atom systems are well-controlled and tunable quantum systems, and thereby enable us to explore quantum many-body effects, such as superfluidity, or second sound. In this talk, I will examine second sound and superfluidity in ultracold quantum gases using analytical and simulation techniques. I will report on the second sound measurements in the BEC-BCS crossover and provide a theoretical description of the second sound velocity on the BEC side of the system [1]. Here, I will demonstrate that the second sound velocity vanishes at the superfluid-thermal boundary, which is a defining feature of second sound. In the second part of this talk, I will investigate superfluidity of ultracold quantum gases via laser stirring. I will present the stirring experiments in the BEC- BCS crossover and provide a quantitative analysis of the breakdown of superfluidity [2]. I will then investigate superfluidity of 2D Bose gases across the Kosterlitz-Thouless transition and provide a quantitative understanding of the experiments performed in the Dalibard group [3]. I will also present the noise correlations of 2D Bose gases in short time of flight and use them to determine the superfluid phase of the recent experiments at Hamburg [4].
[1] D. Hoffmann, V. P. Singh, T. Paintner, W. Limmer, L. Mathey, and J. H. Denschlag, Second sound in the BEC-BCS crossover, forthcoming.
[2] W. Weimer, K. Morgener, V. P. Singh, J. Siegl, K. Hueck, N. Luick, L. Mathey, and H. Moritz, Phys. Rev. Lett. 114, 095301 (2015); V. P. Singh et al., Phys. Rev. A 93, 023634 (2016).
[3] V. P. Singh, C. Weitenberg, J. Dalibard, and L. Mathey, Phys. Rev. A 95, 043631 (2017).
[4] V. P. Singh and L. Mathey, Phys. Rev. A 89, 053612 (2014).
Wednesday 23 January 2019, 12.00h, SR Phil19
Quasiparticle origin of dynamical quantum phase transitions
Dr. Jad Halimeh (MPIPKS Dresden and TU München)

Abstract:
We consider one- and two-dimensional Ising models with varying interaction ranges. Using matrix product state techniques, we study the dynamics of these systems and show a direct connection between the type of lowest-energy quasiparticles in the spectrum of the quench Hamiltonian and the type of nonanalyticities occuring in the Loschmidt return rate, a dynamical analog of the free energy. Our results also show a clear connection between the type of nonanalyticities and the phase of the long-time steady state in addition to how the order parameter decays at intermediate times. In particular, we discuss anomalous nonanalyticities that occur with no underlying local signature in the order parameter dynamics, unlike the traditional regular nonanalyticities that always correspond to zero crossings of the order parameter. Moreover, we demonstrate how dynamical quantum phase transitions can be used to extract the equilibrium physics of the model from short-time dynamics.
Wednesday 23 January 2019, 11.00h, SR Phil19
Quasi-localized excitations induced by confinement in translationally-invariant quantum chains
Alessio Lerose (SISSA Trieste, Italy)

Abstract:
We show that quantum confinement can induce spatial quasi-localization of excitations and slow dynamics even in the absence of quenched disorder. By means of numerical computations based on matrix product states and exact diagonalization, we study the nonequilibrium evolution in quantum Ising chains with longitudinal fields, in long-range quantum Ising chains, and in U(1) lattice gauge theories in one dimension. We demonstrate the emergence of regimes characterized by quasi-many-body localization and long-lived excitations at high energy. We capture these anomalous nonequilibrium dynamics via effective analytical descriptions or via exact mappings to models exhibiting weak ergodicity breaking. These phenomena can be tested in quantum simulators with trapped ions and Rydberg atoms.
References: arXiv:1806.09674, arXiv:1811.05513, and work in preparation (Feb 2019).
Wednesday 16 January 2019, 14.00h, SR Phil19
Spectral functions in QCD: Calculation and Application
Nicolas Wink (Heidelberg University)
Wednesday 9 January 2019, 14.00h, SR Phil19
Relaxation dynamics of disordered Heisenberg spins realized by Rydberg atoms
Dr. Martin Gärttner (Heidelberg University)
Wednesday 12 December 2018, 15.40h, INF 226 room 01.210 — jointly with Weidemüller group
Boiling a Unitary Fermi Liquid
Manuel Gerken (Heidelberg University)

Abstract: Journal club on S. Yan et al., arXiv:1811.00481.
Wednesday 28 November 2018, 14.00h, SR Phil19
Deviations from off-diagonal long-range order and mesoscopic condensation in one-dimensional quantum systems
Andrea Colcelli (SISSA Trieste, Italy)

Abstract:
A quantum system exhibits off-diagonal long-range order (ODLRO) when the largest eigenvalue λ₀ of the one-body-density matrix scales as λ₀ ~ N, where N is the total number of particles. Putting λ₀ ~ 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<C<1, C can be used to quantify deviations from ODLRO. In this talk I will present the study of the exponent C in a variety of one-dimensional bosonic and anyonic systems.
Wednesday 31 October 2018, 15.30h, INF 226 room 01.210 — jointly with Weidemüller group
Real space dynamics of attractive and repulsive polarons in Bose-Einstein condensates
Moritz Drescher (Heidelberg University)

Abstract:
We investigate the formation of a Bose polaron when a single impurity in a Bose-Einstein condensate is quenched from a non-interacting to an attractively interacting state in the vicinity of a Feshbach resonance. We use a beyond-Fröhlich Hamiltonian that is able to cover both sides of the resonance and the Lee-Low-Pines variational ansatz to compute the time-evolution of Boson density profiles in position space. We find that on the repulsive side of the Feshbach resonance, the system keeps oscillating with a characteristic frequency for which we derive an implicit equation and discuss to what extent this can be interpreted as a competition between a molecular and a repulsive polaron state. If the impurity is introduced at finite velocity, it is periodically slowed down or even arrested before speeding up again.
Wednesday 24 October 2018, 14.00h, SR Phil19
Motion of an impurity particle in a boson superfluid
Moritz Drescher (Heidelberg University)

Abstract: Journal club on Girardeau, Phys. Fluids 4, 279 (1961).
Wednesday 17 October 2018, 14.00h, SR Phil19
Transport in strange metals
Dr. Tilman Enss (Heidelberg University)

Summer term 2018

Tuesday 02 October 2018, 14.15h, SR Phil19
Universality in the epsilon expansion
Dr. Alessandro Codello (CP3-Origins Odense and INFN Bologna)

Abstract:
After reviewing the Functional reformulation of the standard Perturbative RG (FPRG), I'll describe the classification of universality classes in arbitrary dimension within the epsilon-expansion and the relative determination of CFT data. In the single component case, universality classes are represented by renormalizable scalar QFTs with self-interacting potentials of highest monomial phi^m below their upper critical dimensions dc = 2m/(m-2). For even integers, m >= 4 these theories coincide with the Landau-Ginzburg description of multi-critical phenomena and interpolate with the unitary minimal models in d = 2, while for odd m the theories are non-unitary and start at m = 3 with the Lee-Yang universality class. An important outcome of this analysis is the realization of the existence of a new non-trivial family of d = 3 universality classes with upper critical dimension dc = 10/3. Subsequently, I will show how the FPRG formalism allows a straightforward generalization to the multicomponent case, with almost no need for additional computations. The classification of multicomponent universality classes is far from complete and I will discuss the present state of knowledge with few examples, including Potts and O(N) models.
Tuesday 18 September 2018, 14.00h, SR Phil19
RG induced inflation
Dr. Nicolò Defenu (Heidelberg University)
Tuesday 07 August 2018, 10.00h, SR Phil19
Journal club: Dynamical formation of Bose polarons
Dr. Tilman Enss (Heidelberg University)

Abstract: Journal club on K.K. Nielsen et al., arXiv:1806.09933.
Thursday 19 July 2018, 15.00h, INF 226 room 01.106 (Glas Box)— jointly with Weidemüller group
Composite, rotating impurities interacting with a many-body environment: analytical and numerical approaches
Dr. Giacomo Bighin (IST Vienna, Austria)

Abstract:
The angulon quasiparticle formalizes the concept of a composite, rotating impurity in a quantum many-body environment and has proven useful in the description of several experimental settings, from ultracold molecules in a BEC to molecules in He nanodroplets. I introduce a diagrammatic formalism, merging Feynman diagrams with the angular momentum diagrams known from atomic and nuclear structure theory, describing angular momentum redistribution in a many-body system. Then, motivated by recent experiments on laser-induced alignment of molecules in He nanodroplets, I introduce a finite-temperature variational approach to angulon dynamics, showing that the far-from-equilibrium dynamical response of molecular impurities can be rationalized in terms of angulons.
Tuesday 17 July 2018, 14.15h, SR Phil19
Many-body physics with quantum impurities in cold atoms and beyond
Dr. Richard Schmidt (Max-Planck Institute for Quantum Optics, Garching)

Abstract:
When an impurity is immersed into an environment, it changes its properties due to its interactions with the surrounding medium. The impurity is dressed by many-body excitations and forms a quasiparticle, the polaron. Depending on the character of the environment and the form of interactions, different types of polarons are created. In this talk, I will review recent experimental and theoretical progress on studying the many-body physics of polarons in ultracold atomic systems [1], and discuss related polaronic phenomena encountered in two-dimensional semiconductors [2] and the study of rotating molecules in superfluid Helium [3]. In the second part of the talk I will then focus on impurities interacting with bosonic quantum gases. Specifically, I will discuss progress on the theoretical description of Rydberg excitations coupled to Bose-Einstein condensates. In such systems the interaction between the Rydberg atom and the Bose gas is mediated by the Rydberg electron. This gives rise to a new polaronic dressing mechanisms, where instead of collective excitations, molecules of gigantic size dress the Rydberg impurity. We develop a functional determinant approach [4] to describe the dynamics of such Rydberg systems which incorporates atomic and many-body theory. Using this approach we predict the appearance of a superpolaronic state which has recently been observed in experiments [5,6].
[1] R. Schmidt, M. Knap, D. A. Ivanov, J.-S. You, M. Cetina, and E. Demler, Rep. Prog. Phys. 81, 024401 (2018). [2] M. Sidler et al., Nature Physics 13, 255 (2017). [3] R. Schmidt, and M. Lemeshko, Phys. Rev. Lett. 114, 203001 (2015). [4] R. Schmidt, H. Sadeghpour, and E. Demler, Phys. Rev. Lett. 116, 105302 (2016). [5] F. Camargo et al., Phys. Rev. Lett. 120, 083401 (2018). [6] R. Schmidt et al., Phys. Rev. A 97, 022707 (2018).
Wednesday 11 July 2018, 14.00h, SR Phil19
Journal club: Strong-coupling Bose polarons out of equilibrium: Dynamical renormalization-group approach
Moritz Drescher (Heidelberg University)

Abstract: Journal club on Grusdt et al., Phys. Rev. A 97, 033612 (2018).
Wednesday 4 July 2018, 14.00h, SR Phil19
Many-body properties from a few-body perspective
Dr. Tilman Enss (Heidelberg University)
Wednesday 27 June 2018, 14.00h, SR Phil19
Journal club: Collisional Properties of a Polarized Fermi Gas with Resonant Interactions
Hans Böhringer (Heidelberg University)

Abstract: Journal club on G.M. Bruun et al., Phys. Rev. Lett 100, 240406 (2008).
Wednesday 6 June 2018, 14.00h, SR Phil19
Dynamical Vertex Approximation for the attractive Hubbard model
Dr. Lorenzo Del Re (SISSA, Trieste, and Technical University Vienna)

Abstract: In this seminar, I will present an extension of the formalism of the Dynamical Vertex Approximation (DΓA) [1], a diagrammatic approach including many-body correlations beyond the Dynamical Mean-Field Theory [2], to treat the case of attractive on-site interactions. I will first introduce the method from a theoretical point of view and briefly review some of the already published results for the repulsive case [3-5]. Then, I will derive the equations for the attractive case proving the validity of the derivation by showing, both analytically and numerically, that the results obtained in the particle-hole symmetric case fully preserve the exact mapping between the attractive and the repulsive models [6]. Furthermore, I will show an application of the extended algorithm to the attractive Hubbard model in three dimensions, for different fillings and interaction values. Specifically, I will focus on the parameter region in the proximity of the second-order transition to the superconducting and charge-density wave phase, respectively, and show (i) their phase-diagrams, (ii) their critical behavior, as well as (iii) the effects of the strong non-local correlations on the single-particle properties.
[1] A. Toschi et al., Phys. Rev. B 75, 045118 (2007); [2] A. Georges et al., Rev. Mod. Phys. 68, 13 (1996); [3] G. Rohringer et al., Phys. Rev. Lett. 107, 256402 (2011); [4] T. Schäfer et al., Phys. Rev. B 91, 125109 (2015); [5] G. Rohringer et al., arXiv:1705.00024 (2017); [6] L. Del Re, M. Capone, A. Toschi, in preparation (2018).
Wednesday 30 May 2018, 14.00h, SR Phil19
Journal club: Casimir interaction among heavy fermions in the BCS-BEC crossover (part 2)

Abstract: Journal club on Y. Nishida, Phys. Rev. A 79, 013629 (2009).
Wednesday 23 May 2018, 14.00h, SR Phil19
Dynamical critical scaling of long-range interacting quantum magnets
Dr. Nicolò Defenu (Heidelberg University)

Abstract: Slow variations (quenches) of the magnetic field across the paramagnetic-ferromagnetic phase transition of spin systems produce heat. In short-ranged systems the heat exhibits a universal power-law scaling as a function of the quench rate, known as Kibble-Zurek (KZ) scaling. Attempts to extend this hypothesis to long-range interacting systems have lead to seemingly contradicting results. In this work we analyse slow quenches of the magnetic field in the Lipkin-Meshkov-Glick model, which describes fully-connected quantum spins. We determine the quantum contribution to the residual heat as a function of the quench rate by means of a Bogoliubov expansion about the mean-field value and calculate the exact solution. For a quench which ends at the quantum critical point we identify two regimes: the adiabatic limit for finite-size chains, where the scaling is dominated by the Landau-Zener tunneling, and the Kibble-Zurek scaling. For a quench symmetric about the critical point, instead, there is no Kibble-Zurek scaling.
Wednesday 16 May 2018, 14.00h, SR Phil19
Bosonic mixtures in two dimensions
Volker Karle (Heidelberg University)

Abstract: In this talk a two-component bosonic gas in two dimensions at low temperatures with zero-range repulsive interaction is considered. In our work we focus on the coexistence phase with superfluid behavior in both components, where a new phenomenon appears: The non-dissipative drag, also called Andreev-Bashkin effect, as a result of the interbosonic interactions, which leads to a modification of the usual BKT transition. Quantum fluctuations of the elementary excitations lead to further corrections of the phase boundary. We study the renormalization of the densities at finite temperatures using standard RG methods.
Wednesday 09 May 2018, 13.00h, SR Phil19
Journal club: Casimir interaction among heavy fermions in the BCS-BEC crossover
Dr. Tilman Enss (Heidelberg University)
Abstract: Journal club on Y. Nishida, Phys. Rev. A 79, 013629 (2009).
Wednesday 02 May 2018, 13.00h, Phil12 gHS
SFB workshop
Wednesday 18 April 2018, 16.00h, CQD lounge — jointly with Weidemüller group
The Efimov effect in Li-Cs mixtures
Dr. Bing Zhu (Heidelberg University)
Wednesday 11 April 2018, 13.30h, SR Phil19
Scale invariance and the Quantum anomaly in the 2D Fermi gas
Dr. Nicolò Defenu (Heidelberg University)

Winter term 2017/18

Wednesday 17 January 2018, 14.00h, SR Phil19
Multiply quantised vortices in fermionic superfluids: angular momentum, unpaired fermions, and spectral asymmetry
Dr. Sergej Moroz (Technical University Munich)

Abstract: Quantized vortices are a hallmark of superfluids and superconductors. In this seminar I will talk about the orbital angular momentum L_z of an s-wave paired superfluid in the presence of an axisymmetric multiply quantised vortex. For vortices with winding number |k| > 1, I will argue that in the weak-pairing BCS regime, L_z is significantly reduced from its value L_z=ℏ N k/2 in the BEC regime, where N is the total number of fermions. This deviation results from the presence of unpaired fermions in the BCS ground state, which arise as a consequence of spectral flow along the vortex sub-gap states.
Wednesday 13 December 2017, 14.00h, SR Phil19
Soliton friction and pairing in superfluids
Dr. Johannes Hofmann (Cambridge University, UK)

Abstract: I shall discuss two examples of interaction effects in quantum gases.
First, I shall discuss the interaction of a collective quantum object - a soliton in a one-dimensional Bose gas - with its thermal environment. Intuitively, one could think of this object as a large pollen in a fluid, expecting Brownian motion to affect the soliton dynamics. Yet, because of the underlying integrability of the problem, it was long thought that such an interaction does not exist. It turns out, however, that there remains a more subtle interplay between soliton and thermal gas which gives rise to a damping force similar to the radiation force exerted on an accelerated charge in electrodynamics, called the Abraham-Lorentz force.
The second part of the talk will discuss interaction effects in mesoscopic Fermi gases relevant to ongoing experiments in Heidelberg as well as experiments on SrTiO₃ nanostructures. While Fermi gases with a variable interaction typically realize a BEC-BCS crossover, finite particle number or confinement can give rise to additional fluctuation effects. I will introduce some aspects of mesoscopic superfluids and discuss how fluctuation effects show up in experiments.
Wednesday 25 October 2017, 14.00h, SR Phil19
Hard-core bosons in flat band systems
Moritz Drescher (Heidelberg University)
Wednesday 18 October 2017, 11.15h, SR Phil19
Collective oscillations of a trapped atomic gas in low dimensions and thermodynamics of one-dimensional Bose gas
Dr. Giulia De Rosi (University of Trento, Italy)

Abstract: Ultracold atoms are exceptional tools to explore the physics of quantum matter. In fact, the high degree of tunability of ultracold Bose and Fermi gases makes them ideal systems for quantum simulation and for investigating macroscopic manifestations of quantum effects, such as superfluidity.
In ultracold gas research, a central role is played by collective oscillations. They can be used to study different dynamical regimes, such as superfluid, collisional, or collisionless limits or to test the equation of state of the system. In this talk, I will present a unified description of collective oscillations in low dimensions covering both Bose and Fermi statistics, different trap geometries and zero as well as finite temperature, based on the formalism of hydrodynamics and sum rules.
I will discuss the different behaviour exhibited by the second excited breathing mode in the collisional regime at low temperature and in the collisionless limit at high temperature in a one-dimensional (1D) trapped Bose gas with repulsive contact interaction. I will show how this mode exhibits a single-valued excitation spectrum in the collisional regime and two different frequencies in the collisionless limit. Our predictions could be important for future research related to the thermalization and damping phenomena in this low-dimensional system. I will show that 1D uniform Bose gases exhibit a non-monotonic temperature dependence of the chemical potential characterized by an increasing-with-temperature behaviour at low temperature. This is due to the thermal excitation of phonons and reveals an interesting analogy with the behaviour of superfluids. Finally, I will discuss our research on a gas with a finite number N of atoms in a ring geometry at zero temperature. I will discuss explicitly the deviations of the thermodynamic behaviour in the ring from the one in the large N limit.