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 Ibarra-Garcia-Padilla, Rice University Room: 105 |
Monday, March 2, 2020 11:15AM - 11:27AM |
B02.00001: Fermionic superfluidity in confined one-dimensional spin-imbalanced systems: A configuration-space Hartree-Fock-Bogoliubov approach Kelly Patton, Daniel E Sheehy We study pairing and density correlations in imbalanced one-dimensional Fermi systems with short-range interactions that are spatially confined by either a harmonic or a hard-wall (or "box") trapping potential. It has been hoped that such systems, which can be realized using ultracold atomic gases, would exhibit the long-sought-after Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid phase. Our approach applies a general Hartree-Fock-Bogoliubov (HFB) transformation to handle spatially-inhomogeneous pairing and density correlations on an equal footing to yield predictions for the ground state of confined 1D Fermi gases in harmonic and hard-wall 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 hard-wall 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 hard-wall 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 multi-component 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 one-dimensional Tasaki lattice and derive rigorous results for the ground states. We first study the model with a flat band at the bottom of the single-particle 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 non-singular setting, we perturb the flat-band 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 semi-definite (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: One-dimensional Spin-polarized 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 one-dimensional bosons with delta-function interaction [1]. We perform the renormalization group analysis and clarify that three-body 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 Tonks-Girardeau 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 Mott-Superfluid quantum phase transition was experimentally observed. How would the transition between a Superfluid and Mott insulator alter the tunneling dynamics of a many-body system? Specifically, we study bosons in a quasi one-dimensional meta-stable trap, modeled by the Bose-Hubbard Hamiltonian, using matrix product state methods which grant access to many-body observables, and compare to mean-field, 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 Mott-dominated interactions result in a maximum when only one quarter of the atoms have escaped. Mott-dominated dynamics also produce strong, long-range, and off-diagonal 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 meta-stable 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 momentum-momentum 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 Hubbard-like 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 Bohr-Oppenheimer 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 inter-electronic momentum-momentum interaction. Once expressed in a localized orbital basis, this term generates new two-body 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. Mott-Insulator Phase in Imperfect Multi-Rods Lattices Omar Abel Rodríguez-López, 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 one-dimensional, periodic, multi-rods lattice created by an external Kronig-Penney 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 well-known Lieb-Liniger model. Using the Luttinger Liquid formalism for the low-energy 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 Fermi-Hubbard ladders and its static and dynamic signatures Thomas Koehler, Adrian Kantian Experiments on Fermi-Hubbard models, implemented via lattice-confined ultra-cold 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 single-species and multi-species 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 Hard-Core 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 3He-4He 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 Bose-Fermi 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 hard-core 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 auxiliary-field quantum Monte Carlo study of finite-temperature properties of the two-dimensional Fermi ga Yuan-Yao He, Hao Shi, Shiwei Zhang Finite-temperature properties of the two-dimensional unpolarized Fermi gas with a zero-range attractive interaction are studied by a numerically exact auxiliary-field quantum Monte Carlo method [1]. This system has generated strong experimental and theoretical interest as a clean and well-controlled 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 finite-temperature 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 BCS-BEC crossover, spanning the entire temperature range and connecting with exact zero-temperature results [2]. We also investigate the Berezinskii-Kosterlitz-Thouless transition and possible pseudogap physics in this system. |
Monday, March 2, 2020 1:27PM - 1:39PM |
B02.00012: Universal intrinsic high-rank spin Hall effects Junpeng Hou, Chuanwei Zhang Spin Hall effect (SHE) is one of the key concepts in modern condensed-matter physics since its first discovery more than two decades ago. In this work, we introduce the concept dubbed high-rank spin Hall effect, in which the usual (charge) Hall effects and SHEs are incorporated as rank-0 and rank-1 SHEs. The first non-trivial example is then rank-2 SHE and we showcase a minimal intrinsic model in a spin-1 Fermionic system (a three-component Fermion or triply-degenerate point), which exhibits an universal rank-2 spin Hall conductivity e/4π. As a generalization to larger spin, we further provide another model in a spin-3/2 system. A simple experimental setup is proposed based on recently experimentally realized 2-dimensional spin-orbit coupling in cold atoms and the effects of Zeeman fields are investigated. Our work reveals interesting spin transport phenomena in large-spin systems and may lead to novel applications in spintronic and quantum-mechanical devices. |
Monday, March 2, 2020 1:39PM - 1:51PM |
B02.00013: Beyond mean-field 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 many-body 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 BCS-BEC 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 self-consistent 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, ΗF, based on Gaussian curvature of the FS and found that ΗF is linearly correlated with the experimentally measured AHE (R-square 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. |
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B02.00015: Superfluidity of interacting fermions in optical lattices: Interplay of population imbalance, dimensionality, and lattice-continuum 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 lattice-continuum 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 mean-field value in the deep BEC regime. |
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