Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B02: Strongly Interacting Bose and Fermi Gases 
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Sponsoring Units: DAMOP Chair: Eduardo IbarraGarciaPadilla, Rice University Room: 105 
Monday, March 2, 2020 11:15AM  11:27AM 
B02.00001: Fermionic superfluidity in confined onedimensional spinimbalanced systems: A configurationspace HartreeFockBogoliubov approach Kelly Patton, Daniel E Sheehy We study pairing and density correlations in imbalanced onedimensional Fermi systems with shortrange interactions that are spatially confined by either a harmonic or a hardwall (or "box") trapping potential. It has been hoped that such systems, which can be realized using ultracold atomic gases, would exhibit the longsoughtafter FuldeFerrellLarkinOvchinnikov (FFLO) superfluid phase. Our approach applies a general HartreeFockBogoliubov (HFB) transformation to handle spatiallyinhomogeneous pairing and density correlations on an equal footing to yield predictions for the ground state of confined 1D Fermi gases in harmonic and hardwall traps. We find that while both cases yield a spatially modulated FFLO pairing amplitude in the imbalanced regime, in the case of a harmonic trap the corresponding signature in the local density is rather weak. In contrast, in the hardwall case, we find a strong reflection of the FFLO pairing in the local in situ densities. In particular, we find that the excess spins are strongly localized near nodes in the pairing amplitude, thus creating an unmistakable signature of the FFLO state in a hardwall box trap. 
Monday, March 2, 2020 11:27AM  11:39AM 
B02.00002: Ferromagnetism in the SU(n) Hubbard model with nearly flat band Kensuke Tamura, Hosho Katsura Recently, the SU(n) (n>2) Hubbard model describing multicomponent fermions with SU(n) symmetry has been a focus of interest, as it is expected to exhibit a rich phase diagram. However, very little is known rigorously about the model with n>2. Here we study the model on a onedimensional Tasaki lattice and derive rigorous results for the ground states. We first study the model with a flat band at the bottom of the singleparticle spectrum. We prove that the ground states are SU(n) ferromagnetic when the number of particles is half the number of lattice sites, generalizing the previous result in Ref. [1]. To discuss SU(n) ferromagnetism in a nonsingular setting, we perturb the flatband model and make the bottom band dispersive. Then we find that SU(n) ferromagnetism in the ground states of the perturbed model at the same filling can be proved if each local Hamiltonian (independent of the system size) is positive semidefinite (p.s.d.). Furthermore, we prove that the local Hamiltonian is p.s.d. for sufficiently large interaction and band gap [2]. 
Monday, March 2, 2020 11:39AM  11:51AM 
B02.00003: Onedimensional Spinpolarized Fermi Gas near resonances Yuta Sekino, Yusuke Nishida We study a quantum field theory for resonantly interacting spinless fermions in one dimension. This fermions are known to correspond to onedimensional bosons with deltafunction interaction [1]. We perform the renormalization group analysis and clarify that threebody coupling, which is zero in the ultraviolet theory, emergently appears in the infrared limit [2]. This running coupling allows us to rederive the energy relaiton, i.e., the expresson of energy in terms of a momentum distribution and contact parameters characterizing local corretions of the system. The obtained energy relation is consistent with that previously derived in the first quantization formalism [3]. 
Monday, March 2, 2020 11:51AM  12:03PM 
B02.00004: Dynamical Fermionization and Scaling Behaviour for a Strongly Repulsive Spinor Gas after Quench Shah Saad Alam, Tim Skaras, Li Yang, Han Pu Dynamical fermionization has been theoretically demonstrated for trapped 1D bosonic and anyonic gases in the TonksGirardeau limit. It refers to the phenomenon where, after the initial harmonic confinement is turned off, the momentum distribution of the system asymptotically approaches that of a trapped Fermi gas. Evidence of dynamical fermionization was experimentally shown for 1D hard core bosonic gases recently. We extend this study to a harmonically confined 1D spinor gas in the hard core and strongly repulsive regimes, and analytically prove the existence of dynamical fermionization. We further discuss numerical investigation of two particle and few particle calculations for specific spinor systems. Finally, we present the Tan contact for a strongly interacting spinor system, as well as its scaling during expansion. 
Monday, March 2, 2020 12:03PM  12:15PM 
B02.00005: Dynamics of Macroscopic Quantum Tunneling from Superfluid to Mott Insulating Regimes Diego Alcala, Marie McLain, Lincoln Carr In 1928 quantum tunneling was discovered to explain alpha decay, and in 2002 the MottSuperfluid quantum phase transition was experimentally observed. How would the transition between a Superfluid and Mott insulator alter the tunneling dynamics of a manybody system? Specifically, we study bosons in a quasi onedimensional metastable trap, modeled by the BoseHubbard Hamiltonian, using matrix product state methods which grant access to manybody observables, and compare to meanfield, which fails for strong interactions. We quantify how the barrier and interaction energies can amplify or reduce number fluctuations by an order of magnitude. Bond entropy is found to maximize when nearly half of the atoms have escaped in a Superfluid, while Mottdominated interactions result in a maximum when only one quarter of the atoms have escaped. Mottdominated dynamics also produce strong, longrange, and offdiagonal correlations. We find significantly different time scales in observables, i.e., when bond entropy and fluctuations maximize. Periodic fluctuations in time derivatives are found for several observables, scaling with the size of the metastable trap, Finally, preliminary results suggest that interaction energies can alter the escape velocity of atoms. 
Monday, March 2, 2020 12:15PM  12:27PM 
B02.00006: On the possible existence of an effective momentummomentum coupling in a correlated electronic system Amir O. Caldeira, Thais Victa Trevisan, Gustavo Monteiro In this talk, we present a study on how to partly reincorporate the effects of localized binding electrons on the dynamics of their itinerant counterparts in Hubbardlike Hamiltonians. This is done by relaxing the constraint that the former should be entirely frozen in the chemical bonds between the underlying lattice sites through the employment of a BohrOppenheimer ansatz for the wavefunction of the whole electronic system. Accordingly, the latter includes itinerant as well as binding electron coordinates. It is then argued that going beyond the adiabatic approximation, which will be properly justified in due time, we are able to show that the net effect of virtual transitions of binding electrons between their ground and excited states is to furnish the itinerant electrons with an effective interelectronic momentummomentum interaction. Once expressed in a localized orbital basis, this term generates new twobody processes which cannot be found even among those neglected in obtaining the Hubbard Hamiltonian. Although we have applied these ideas to the specific case of rings, we are sure they can be easily generalized to higher dimensional systems sharing the required properties of which we have made use herein. 
Monday, March 2, 2020 12:27PM  12:39PM 
B02.00007: Superfluid vs. MottInsulator Phase in Imperfect MultiRods Lattices Omar Abel RodríguezLópez, M. A. Solís We calculate the ground state (GS) energy and the static structure factor at zero temperature of an interacting Bose gas confined by a onedimensional, periodic, multirods lattice created by an external KronigPenney potential. We employ the Diffusion Monte Carlo (DMC) method to estimate the physical properties exactly up to a statistical error. In the limit of zero external potential, we recover the results for the wellknown LiebLiniger model. Using the Luttinger Liquid formalism for the lowenergy properties of 1D systems, we find a phase transition from the superfluid state to the Mott insulator state as the lattice height increases. Also, we report that the introduction of a barrier defect in the lattice favors the superfluid phase with respect to the Mott insulator phase. 
Monday, March 2, 2020 12:39PM  12:51PM 
B02.00008: Optimized pairing from repulsive interactions in FermiHubbard ladders and its static and dynamic signatures Thomas Koehler, Adrian Kantian Experiments on FermiHubbard models, implemented via latticeconfined ultracold gases, are moving towards temperatures where their charge gap and possibly their spin gap can be resolved. It is thus important to obtain accurate quantitative theory for those systems in order to optimize the chance of observing any possible unconventional pairing from repulsive interactions at the given temperatures of the ongoing experiments. 
Monday, March 2, 2020 12:51PM  1:03PM 
B02.00009: Matrix Product States in the Continuum and Cold Atomic Gases Joseph Peacock, Aleksandar Ljepoja, C. J. Bolech Cold atomic gases are an ideal laboratory to explore the physics of interacting degenerate quantum gases due to the high degree of tunability possible in the experiments. In particular, special trapping arrangements allow, among other things, to control the effective dimensionality of the systems. In this talk we present an update on the continuum formulation of Matrix Product States (cMPS) to describe one dimensional dilute quantum gases. The goal is to develop cMPS as an accurate predictive tool to plan and analyse past and future experiments. To that end, we shall present results for the cases of trapped singlespecies and multispecies bosonic and fermionic atoms, as well as their mixtures. When available, we make quantitative comparisons of cMPS results with the exact results for solvable cases. 
Monday, March 2, 2020 1:03PM  1:15PM 
B02.00010: Spin Imbalance Effect in a Mixture of Spinor Fermions and HardCore Bosons Ramón Guerrero Suárez, Juan Mendoza Arenas, Roberto Franco Pe?aloza, Jereson Silva Valencia Mixtures of bosons and fermions have been the subject of research since the first studies of ^{3}He^{4}He system. With the current cold atoms setups, these types of mixtures can be realized and the inter and intraspecies interactions can be readily tuned. By means of a BoseFermi Hubbard Hamiltonian, mixtures of bosons and fermions can be studied theoretically. Different superfluids, Mott Insulators, phase separation, spin and charge density waves, together with Wigner crystals have been reported for these systems. We study numerically the effect that spin imbalance can have in these mixtures, specifically for a system of spinor fermions and in the hardcore bosonic limit. Using DMRG we explore the phase diagram of the ground state, finding new insulator states that appear only when there is imbalance in the system. This happens for both attractive and repulsive intraspecies interactions and for both attractive and repulsive fermionic interactions, implying that the new insulator states are due to the coupling between bosons and fermions. 
Monday, March 2, 2020 1:15PM  1:27PM 
B02.00011: Ab initio auxiliaryfield quantum Monte Carlo study of finitetemperature properties of the twodimensional Fermi ga YuanYao He, Hao Shi, Shiwei Zhang Finitetemperature properties of the twodimensional unpolarized Fermi gas with a zerorange attractive interaction are studied by a numerically exact auxiliaryfield quantum Monte Carlo method [1]. This system has generated strong experimental and theoretical interest as a clean and wellcontrolled testground for a rich set of physics combining strong interaction and superfluidity in two dimensions. To reliably reach the continuum limit, we adopt a new finitetemperature algorithm [1], which has computational scaling as linear in lattice size instead of cubic as in the standard algorithm. Numerically exact results for the equation of state, contact parameter, momentum distributions as well as pairing properties are obtained across the BCSBEC crossover, spanning the entire temperature range and connecting with exact zerotemperature results [2]. We also investigate the BerezinskiiKosterlitzThouless transition and possible pseudogap physics in this system. 
Monday, March 2, 2020 1:27PM  1:39PM 
B02.00012: Universal intrinsic highrank spin Hall effects Junpeng Hou, Chuanwei Zhang Spin Hall effect (SHE) is one of the key concepts in modern condensedmatter physics since its first discovery more than two decades ago. In this work, we introduce the concept dubbed highrank spin Hall effect, in which the usual (charge) Hall effects and SHEs are incorporated as rank0 and rank1 SHEs. The first nontrivial example is then rank2 SHE and we showcase a minimal intrinsic model in a spin1 Fermionic system (a threecomponent Fermion or triplydegenerate point), which exhibits an universal rank2 spin Hall conductivity e/4π. As a generalization to larger spin, we further provide another model in a spin3/2 system. A simple experimental setup is proposed based on recently experimentally realized 2dimensional spinorbit coupling in cold atoms and the effects of Zeeman fields are investigated. Our work reveals interesting spin transport phenomena in largespin systems and may lead to novel applications in spintronic and quantummechanical devices. 
Monday, March 2, 2020 1:39PM  1:51PM 
B02.00013: Beyond meanfield corrections to the quasiparticle spectrum of superfluid Fermi gases Senne Van Loon, Jacques Tempere, Hadrien Kurkjian The notion of quasiparticles is an essential tool for the study of interacting manybody systems. In superfluid Fermi gases, two types of elementary excitations can be identified: the fermionic branch of broken pairs, and the bosonic collective mode describing the collective motion of the pairs. These can be observed in systems of ultracold fermionic atoms, where, due to Fesbach resonances, a whole range of superfluids can be studied. Measurements of the quasiparticle spectrum are already available [1], though the theoretical study of corrections to the fermionic branch remains limited [2]. Here, we investigate this quasiparticle branch in the BCSBEC crossover and calculate the quasiparticle lifetime and energy shift due to its coupling with the collective mode. Close to the minimum of the branch the quasiparticles are undamped, allowing us to find the energy correction in a selfconsistent way, that we express in experimentally relevant quantities. 
Monday, March 2, 2020 1:51PM  2:03PM 
B02.00014: Prediction of Exotic Electron Transport Properties using the Shape Effect of the Fermi Surface Elena Derunova, Yan Sun, Mazhar Ali The intrinsic anomalous Hall and spin Hall effects (AHE/SHE) provided evidence of exotic electronic transport phenomena relating to a topological connection between bands around the Fermi energy. Currently, the calculation of Berry curvature and the Kubo formalism based on Green functions is used to quantify bands’ topological connection and predict the AHE, SHE and other effects. Here we show that the topological connection of the bands also leads to particular shapes of the corresponding Fermi surfaces (FS) and that geometric analysis of these shapes can be used to predict transport properties. For the AHE/SHE we developed a qualitative indicator, Η_{F}, based on Gaussian curvature of the FS and found that Η_{F} is linearly correlated with the experimentally measured AHE (Rsquare 0.97) in a variety of famous AHE compounds as well correlated with the Kubo calculated SHE. We show that consideration of the geodesic flow on the FS gives rise to FS based equations in a semiclassical style, which contain information about the electron transport properties aside from longitudinal conduction. Going beyond just AHE/SHE, a full understanding of the Shape Effect of the FS (SEFS) opens the door to simple prediction of exotic electron transport property in materials in a novel and facile way. 

B02.00015: Superfluidity of interacting fermions in optical lattices: Interplay of population imbalance, dimensionality, and latticecontinuum mixing Qijin Chen, Jibiao Wang, Lin Sun In this talk, I will discuss the superfluid behavior of ultracold atomic Fermi gases in 1D and 2D optical lattices (OL), subject to a short range pairing interaction, and show the highly unusual behavior as a consequence of the interplay between population imbalance, dimensionality and more importantly latticecontinuum mixing. These systems are different from pure 3D continuum or 3D lattices, in that each lattice "site" now contains many fermions. Using a pairing fluctuation theory, we demonstrate that this feature leads to unexpected enhancement of pair hopping in the presence of population imbalace and thus possible enhancement of Tc on the BEC side of the unitary limit. For 1DOL, the superfluid phase exists only for a very limited range of parameters, and the truncated momentum space in the lattice dimension in combination with the enhanced pair hopping may give rise to enhance Tc in the BEC regime, with an constant BEC asymptote. For 2DOL, the further truncated momentum space helps to strongly suppress pairing fluctuation contributions to the pseudogap, so that Tc gradually approaches its high meanfield value in the deep BEC regime. 
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