Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 67, Number 7
Monday–Friday, May 30–June 3 2022; Orlando, Florida
Session Q08: Quantum Gases in Low Dimensions IIRecordings Available

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Chair: Cheng Chin, Chicago Room: Salon 7/8 
Thursday, June 2, 2022 8:00AM  8:12AM 
Q08.00001: BerezinskiiKosterlitzThouless transitions of an easyplane ferromagnetic spin1 Bose gas Andrew Underwood, Lewis Williamson, Andrew Groszek, Xiaoquan Yu, Peter B Blakie In twodimensional Bose gases thermal fluctuations preclude the formation of longrange order. Consequently, the superfluid transition is of the BerezinskiiKosterlitzThouless (BKT) type, characterized by the emergence of quasilongrange order driven by the binding of vortexantivortex pairs. Spinor Bose gases provide a unique platform to study BKT transitions, on account of their ability to simultaneously exhibit phase and spin ordering. Here we utilize a stochastic GrossPitaevskii model to investigate the BKT transitions in an easyplane ferromagnetic spin1 Bose gas. We find that in general two BKT transitions occur with decreasing temperature: first mass superfluidity is established, followed at a lower temperature by spin superfluidity. We demonstrate that these transitions are associated with the binding of mass and spin vortexantivortex pairs respectively. Furthermore, we develop a phase diagram dependent on quadratic Zeeman energy, which demonstrates that the mass and spin superfluid transitions become indistinguishable in the isotropic ferromagnetic phase. 
Thursday, June 2, 2022 8:12AM  8:24AM 
Q08.00002: Curvatureinduced BCSBEC crossover of atomic Fermi superfluid in a spherical bubble trap ChihChun Chien, Yan He, Hao Guo The spherical bubble traps in microgravity allows cold atoms to be confined on a spherical surface. By considering a twocomponent Fermi gas with attractive interactions, we derive and analyze the BCSLeggett theory of atomic Fermi superfluid on a thin spherical shell. Despite the flat dispersion within each angular momentum number and jumps between adjacent levels of an ideal Fermi gas on a spherical shell, the properly normalized gap and chemical potential of Fermi superfluid exhibit universal behavior regardless of the geometries. The conventional BCSBEC crossover induced by increasing the interactions occurs on a sphere. However, a curvatureinduced BCSBEC crossover emerges when the particle number and interaction strength are fixed but the sphere is shrinking. Different from a 3D Fermi superfluid, the pairing energy scale of a 2D Fermi superfluid is determined by the twobody binding energy. When placed on a 2D compact geometry like the spherical shell, an increase of the curvature leads to an increase of the Fermi energy and causes a reduction of the ratio between the pairing and kinetic energies. Therefore, the system is pushed towards the BCS limit as the sphere shrinks. The curvatureinduced BCSBEC crossover thus demonstrates the effects of compact geometries in low dimensions on manybody systems. 
Thursday, June 2, 2022 8:24AM  8:36AM Withdrawn 
Q08.00003: Equation of state of a twodimensional spherical Bose gas Andrea Tononi We analyze the scattering problem of identical bosonic particles confined on a spherical surface. At low scattering energies, and for a radius much larger than the healing length, we express the contact interaction strength in terms of the swave scattering length. Adopting this relation, we are then able to regularize the zeropoint energy of the spherical Bose gas and to obtain its equation of state, which includes the corrections due to the finite radius of the sphere, and that coincides with the flatcase result in the infiniteradius limit. Our results are relevant for modeling the ongoing microgravity experiments with twodimensional bubbletrapped BoseEinstein condensates. 
Thursday, June 2, 2022 8:36AM  8:48AM 
Q08.00004: ParticleImbalanced Bose Mixtures in the Droplet Regime Ilias Englezos, Simeon I Mistakidis, Peter Schmelcher We unravel the transition of the bound state behavior of attractively interacting twocomponent bosonic mixtures from particle balanced settings towards the impurity limit. A multitude of phases is found to occur depending on the interplay between the intercomponent attraction and the particleimbalance. For weak attractions (droplet regime) and larger imbalances the flattop density profile of the majority component becomes progressively localized and it is restricted within the spatial region where the minority one is located. In contrast, turning to stronger attractions, the highly localized solitonlike state of the majority species features a spatial delocalization and eventually exhibits a flattened background for sufficiently large particle imbalance. A depletion of the density profile accompanied by strong twobody correlations is evinced for increasing repulsion of the minority component. The aforementioned structural deformations are explained by constructing a suitable effective potential approach within the modified GrossPitaevskii model, while the few to manybody crossover is probed through an abinitio variational approach. 
Thursday, June 2, 2022 8:48AM  9:00AM 
Q08.00005: Spin dynamics in lowdimensional quantum gases Arko Roy Cooled to nanoKelvin temperatures, ultracold dilute atomic gases manifest as macroscopic quantum systems. Observable in experiments, atomic quantum gases give direct access to the physics of lowdimensional quantum phenomena. Be it the simulators of condensed matter systems or components of novel quantum technologies, it is of imense value to know the transport properties of ultracold atomic quantum gases to a fine detail. In this talk, we shall discuss that thermal fluctuations in a twodimensional spinor system, in particular binary BoseBose mixtures, are much more important than in three dimensions. Not only they inhibit true BoseEinstein condensation, as already well known, but they also smoothen all sharp features in the spin properties at finite temperature. 
Thursday, June 2, 2022 9:00AM  9:12AM 
Q08.00006: Exploring quantum dynamics in homogeneous twodimensional quantum gases HIKARU TAMURA, ChengAn Chen, ChenLung Hung Realization of a homogeneous quantum gas loaded in an arbitrarily painted box potential could unveil intricate quantum manybody dynamics inaccessible using samples localized in conventional harmonic traps. In equilibrium, trap uniformity permits realization of manybody phases that may only exist in a narrow parameter space. Moreover, inducing nonequilibrium quantum dynamics via interaction quenches in a painted box trap could control or inhibit global mass transport and disentangle its effect from the spreading of quantum correlations across a sample. In this talk, we discuss realizations of novel quantum dynamics in tunable cesium atomic quantum gases confined in a twodimensional (2D) box trap. In the first example, we perform interaction quenches to attractive values and observe for the first time 2D matterwave Townes solitons that form from a modulational instability exhibiting universal scaling behaviors in its early time quantum dynamics. Using a homogeneous gas loaded in a 2D optical lattice, in the second example we discuss how by employing control of local chemical potential and interaction parameters to investigate the elusive quantum critical dynamics across a superfluidMott insulator quantum critical point. 
Thursday, June 2, 2022 9:12AM  9:24AM 
Q08.00007: Stronglyinteracting bosons at 2D1D dimensional crossover Hepeng Yao, Lorenzo Pizzino, Thierry Giamarchi Quantum gases at dimensional crossover exhibit fruitful physics which reflects special properties of noninteger dimensions. While various fascinating researches have been carried out in the tightbinding limit [13], the smooth dimensional crossover for stronglyinteracting bosons in continuous lattice, which is beyond meanfield theory and strongly adapted to current generation of experiments, is rarely studied. In this talk, I will present our study about stronglyinteracting bosons under continuous potential at 2D1D dimensional crossover [4]. Using exact quantum Monte Carlo calculations, we investigate this dimensional crossover by computing longitudinal and transverse superfluid fractions as well as the superfluid correlation as a function of temperature, interactions and potential. We find the special behavior of longitudinal superfluidity at the crossover which signatures the interplay of dimensionalities. Moreover, we show the correlation function evolves from a BerezinskiiKosterlitzThouless (BKT) to TomonagaLuttinger liquid (TLL) type, with the coexistence of 2D and 1D behaviors appearing at the dimensional crossover. In the end, I will discuss how the consequences of the dimensional crossover can be investigated in cold atomic gases experiments. 
Thursday, June 2, 2022 9:24AM  9:36AM 
Q08.00008: Probing sineGordon dynamics in coupled spin chains Elisabeth Wybo, Michael Knap, Alvise Bastianello The recent progress in the control and manipulation of synthetic quantum matter has sparked a large interest in the realization of effective theories that form a lowenergy description of a wide class of systems. A very prominent example of such an effective theory is the quantum sineGordon field theory, which naturally describes many onedimensional quantum systems at low temperatures. 
Thursday, June 2, 2022 9:36AM  9:48AM 
Q08.00009: Compressibility and the Equation of State of an Optical Quantum Gas in a Box Erik Busley, Leon Espert Miranda, Andreas Redmann, Christian Kurtscheid, Kirankumar Karkihalli Umesh, Frank Vewinger, Martin Weitz, Julian Schmitt The compressibility of a medium, quantifying its response to mechanical perturbations, is a fundamental quantity determined by the equation of state. For gases of material particles, studies of the mechanical response are well established, in fields from classical thermodynamics to cold atomic quantum gases. In the quantum degenerate ideal Bose gas in two dimensions, however, a peculiar prediction so far has remained unseen: the compressibility of the gas is expected to become infinitely large. Here we demonstrate a measurement of the equation of state as well as the compressibility of a homogeneously trapped twodimensional quantum gas of light inside a nanostructured dyefilled optical microcavity. Upon reaching quantum degeneracy we observe signatures of BoseEinstein condensation in the finitesize system, causing a sharp increase of the density response to an external force, hiniting at the infinite compressibility of the uniform twodimensional Bose gas. 
Thursday, June 2, 2022 9:48AM  10:00AM 
Q08.00010: Excitonic TonksGirardeau and chargedensitywave phases in monolayer semiconductors Rafal Oldziejewski, Alessio Chiocchetta, Johannes Knörzer, Richard Schmidt Excitons in twodimensional semiconductors provide a novel platform for fundamental studies of manybody interactions. In particular, dipolar interactions between spatially indirect excitons may give rise to strongly correlated phases of matter that so far have been out of reach of experiments in ultracold gases. Here, we show that excitonic fewbody systems in atomically thin transitionmetal dichalcogenides confined to a onedimensional geometry undergo a crossover from a TonksGirardeau to a chargedensitywave regime. To this end, we take into account realistic system parameters and predict the effective excitonexciton interaction potential. We find that the pair correlation function of excitons contains key signatures of the manybody crossover already at small exciton numbers and show that photoluminescence spectra provide readily accessible experimental fingerprints of these strongly correlated quantum manybody states. We then study the system within the Luttinger Liquid theory that agrees remarkably well with fewbody exact calculations in predicting spatial correlation functions and blueshifting of the photoluminescence spectra for an increasing number of excitons. Finally, we predict the excitation spectrum of the system for all densities in the manybody limit and study the temporalspatial paircorrelation functions of emitted photons. Our findings showcase TMDs as an alternative platform for manybody dipolar physics in lowdimensions that may outperform most alternative experimental settings in different tasks and that may complement studies using ultracold molecules that have yet to reach the required densities and control to realize their full potential. 
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