Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A36: Interacting Fermi Gases I |
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Sponsoring Units: DAMOP Chair: Congjun Wu, University of California, San Diego Room: 211 |
(Author Not Attending)
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A36.00001: Dynamical instabilities and transient short-range order in the fermionic Hubbard model Johannes Bauer, Mehrtash Babadi, Eugene Demler We study the dynamics of magnetic correlations in the half-filled fermionic Hubbard model following a fast ramp of the repulsive interaction. We use Schwinger-Keldysh self-consistent second-order perturbation theory to investigate the evolution of single-particle Green's functions and solve the non-equilibrium Bethe-Salpeter equation to study the dynamics of magnetic correlations. This approach gives us new insights into the interplay between single-particle relaxation dynamics and the growth of antiferromagnetic correlations. Depending on the ramping time and the final value of the interaction, we find different dynamical behavior which we illustrate using a dynamical phase diagram. Of particular interest is the emergence of a transient short-range ordered regime characterized by the strong initial growth of antiferromagnetic correlations followed by a decay of correlations upon thermalization. The discussed phenomena can be probed in experiments with ultracold atoms in optical lattices. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A36.00002: Polarization induced phase separation and re-entrant transition of two component lattice fermions in one dimension Theja DeSilva By investigating the compressibility of one dimensional lattice fermions at various filling factors, we study phase separation and re-entrant transition within the framework of Bethe ansatz method. We model the system by repulsive Hubbard model and calculate compressibility as functions of polarization for arbitrary values of chemical potential, temperature, and interaction strength. For filling factors $ 0 < n < 1$, the compressibility is a non-monotonic function of polarization at all thermodynamic parameters. The compressibility reveals phase transition into phase separated state for both low and intermediate temperatures, as well as intermediate interactions as one increases the polarization. For certain filling factors, we find re-entrant transition into the mixed phase at a higher polarization. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A36.00003: Size and shape of Mott regions for fermionic atoms in a two-dimensional optical lattice Tiago Mendes Santos, Thereza Paiva, Raimundo R. dos Santos We investigate the harmonic-trap control of size and shape of Mott regions in the Fermi Hubbard model on a square optical lattice. The use of Lanczos diagonalization on clusters with twisted boundary conditions, followed by an average over 50-80 samples, drastically reduce finite-size effects in some ground state properties; calculations in the grand canonical ensemble together with a local-density approximation (LDA) allow us to simulate the radial density distribution. We have found that as the trap closes, the atomic cloud goes from a metallic state, to a Mott core, and to a Mott ring; the coverage of Mott atoms reaches a maximum at the core-ring transition. A ``phase diagram'' in terms of an effective density and the on-site repulsion is proposed, as a guide to maximize the Mott coverage. We also predict that the usual experimentally accessible quantities, the global compressibility and the average double occupancy (rather, its density derivative) display detectable signatures of the core-ring transition. Some spin correlation functions are also calculated, and predict the existence N\'eel ordering within Mott cores and rings. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A36.00004: Ground-state properties of spin-imbalanced Fermions in three-dimensional optical lattices Peter Rosenberg, Simone Chiesa, Shiwei Zhang The past two decades have seen remarkable progress in cold atom physics. Novel experimental techniques have made it possible to simulate many condensed matter models. One system that has received considerable focus is ultra-cold atoms in an optical lattice with unequal populations of two hyperfine states. This system is an ideal candidate for the experimental realization of the elusive Fulde-Ferrell-Larkin-Ovchinnikov phase. We investigate the phase diagram of this system using Hartree-Fock-Bogoliubov theory. Detailed numerical calculations are performed to determine the ground-state properties systematically for different values of density, spin polarization and interaction strength. We first consider the high density and low polarization regime, in which the effect of the optical lattice is most evident. We then proceed to the low density and high polarization regime where the effects of the underlying lattice are less significant and the system begins to resemble a continuum Fermi gas. We explore the effects of density, polarization and interaction on the character of the phases in each regime and highlight the qualitative differences between the two regimes. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A36.00005: Spin-balanced Fulde-Ferrell superfluids in driven fermionic optical lattices Zhen Zheng, Chunlei Qu, Xubo Zou, Chuanwei Zhang The Fulde-Ferrell-Lakin-Ovchinnikov (FFLO) states, Cooper pairs with finite center-of-mass momenta, were predicted to exit in a variety of systems, but so far unambiguous experimental evidence is still lacking. Current schemes for generating FFLO pairing are based on either large Zeeman field or a combination of spin-orbit coupling and small Zeeman field to induce Fermi surface mismatch. The existence of Zeeman field will necessarily induce spin imbalances of the system and suppress the order parameter. In cold atomic gases, both schemes currently face certain practical experimental issues. In this talk, we propose to realize Fulde-Ferrell superfluids in a spin-balanced fermionic optical lattice where s- and p-orbital bands of the lattice are coupled by an additional weak moving optical lattice. We show that such coupling naturally leads to a spin-independent asymmetric Fermi surface, which, together with the s-wave scattering interaction between two spins, yields an FF type of superfluid pairing. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A36.00006: Two-leg fermionic Hubbard model with a state-dependent hopping Shun Uchino, Thierry Giamarchi We study a two-leg fermionic Hubbard ladder model with a state-dependent hopping. We find that, contrarily to the case without a state-dependent hopping, for which the system has a superfluid nature regardless of the sign of the interaction at incommensurate filling, in the presence of such a hopping a spin-triplet superfluid, spin- density wave and charge-density wave phases emerge. We examine our results in the light of periodically-driven optical lattices in cold atoms. and give protocols allowing to realize the spin-triplet superfluid elusive in the cold atoms [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A36.00007: Attractive Hofstadter-Hubbard model with imbalanced chemical and vector potentials Menderes Iskin We study the interplay between the Hofstadter butterfly, strong interactions and Zeeman field within the mean-field Bogoliubov-de Gennes theory in real space, and explore the ground states of the attractive single-band Hofstadter-Hubbard Hamiltonian on a square lattice, including the exotic possibility of imbalanced vector potentials. We find that the cooperation between the vector potential and superfluid order breaks the spatial symmetry of the system, and flourish stripe-ordered Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like superfluid and supersolid phases that can be distinguished and characterised according to their coexisting pair-density (PDW), charge-density (CDW) and spin-density (SDW) wave orders. We also discuss confined systems and comment on the likelihood of observing such stripe-ordered phases by loading neutral atomic Fermi gases on laser-induced optical lattices under laser-generated artificial gauge fields. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A36.00008: Induced p-wave Superfluidity in Imbalanced Fermi Gases in a Synthetic Gauge Field Heron Caldas, Mucio Continentino We study pairing formation and the appearance of induced spin-triplet p-wave superfluidity in dilute three-dimensional imbalanced Fermi gases in the presence of a uniform non-Abelian gauge field. This gauge field generates a synthetic Rashba-type spin-orbit interaction which has remarkable consequences in the induced p-wave pairing gaps. Without the synthetic gauge field, the p-wave pairing occurs in one of the components due to the induced (second-order) interaction via an exchange of density fluctuations in the other component. We show that this p-wave superfluid gap induced by density fluctuations is greatly enhanced due to the Rashba-type spin-orbit coupling. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A36.00009: The Realization of 3D Weyl Semimetal Phase in Optical lattice and its Detection Wen-Yu He, Shi-Zhong Zhang, Kam Tuen Law We describe a method to realize 3D Weyl semimetal phase in multilayer-coupled honeycomb optical lattices, with energy offset between two sublattices turned on. Two Raman beams are utilized to assist coupling between neighbor sites and generate synthetic magnetic flux both vertically and horizontally. Both one and two pairs of Weyl points can be achieved by tuning the unconventional interlayer coupling strength and the detuning in the Raman processes. We demonstrate that the detection for the emergence of Weyl points can be done through measuring the Landau Zener transitions of atoms from the lower occupied bands to the upper unoccupied bands. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A36.00010: Nambu-Goldstone modes of an ultracold $^6Li-$ $^{40}K$ mixture in an optical lattice Zlatko Koinov, Shanna Pahl, Rafael Mendoza A low-energy theory of the Nambu-Goldstone excitation spectrum and the corresponding speed of sound of an interacting Fermi mixture of Lithium-6 and Potassium-40 atoms in a two-dimensional optical lattice at finite temperatures with the Fulde-Ferrell order parameter is presented. We assume that the interacting fermions are in a sufficiently deep periodic lattice potential described by the Hubbard Hamiltonian. The discussion is restricted to the BCS side of the Feshbach resonance where the Fermi atoms exhibit superfluidity. The quartic on-site Hubbard interaction is decoupled via a Hubbard-Stratonovich transformation. The numerical solution of the Bethe-Salpeter equation in the generalized random phase approximation shows that the two-species Fermi gas has a superfluid phase revealed by two rotonlike minima in the asymmetric collective-mode energy. At some values of polarization, interacting strength and temperature, the dispersion relation of the Nambu-Goldstone excitation $\omega(Q)$ initially bends upward as the quasimomentum $Q$ increases before bending over. Due to this anomalous dispersion one long-wavelength phonon can decay into another one by absorbing a second phonon Landau damping), or one phonon can decay into two others (the Beliaev damping). [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A36.00011: Unusual robust phase coherence in a coupled boson-fermion system Maciej Maska, Nandini Trivedi We consider a coupled boson-fermion model in two dimensions, that describes itinerant fermions hybridizing with localized bosons composed of pairs of tightly bound opposite-spin fermions. We trace out the fermionic degrees of freedom and perform a Monte Carlo simulation for the effective classical Hamiltonian of boson phases. We find that the fermions not only generate an effective long-range temperature-dependent boson-boson coupling that generates a finite phase stiffness, but remarkably the phase stiffness is considerably more robust than that described by the XY model. Our analysis further shows that the inter-vortex interaction in the effective model is a power law rather than logarithmic as in the XY model. As one of the possible explanations for this persistent phase stiffness we consider the long range Berry phases carried by the itinerant fermions. Our results are relevant for resonance superfluids in the BCS-BEC crossover regime and also certain aspects of the high temperature superconductivity. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A36.00012: Shear viscosity to entropy density ratios and implications for (im)perfect fluidity in Fermionic and Bosonic superfluids Rufus Boyack, Hao Guo, K. Levin Recent experiments on both unitary Fermi gases and high temperature superconductors (arxiv:1410.4835 [cond-mat.quant-gas], arxiv:1409.5820 [cond-mat.str-el].) have led to renewed interest in near perfect fluidity in condensed matter systems. This is quantified by studying the ratio of shear viscosity to entropy density. In this talk we present calculations of this ratio in homogeneous bosonic and fermionic superfluids, with the latter ranging from BCS to BEC. While the shear viscosity exhibits a power law (for bosons) or exponential suppression (for fermions), a similar dependence is found for the respective entropy densities. As a result, strict BCS and (true) bosonic superfluids have an analogous viscosity to entropy density ratio, behaving linearly with temperature times the (T-dependent) dissipation rate; this is characteristic of imperfect fluidity in weakly coupled fluids. This is contrasted with the behavior of fermions at unitarity which we argue is a consequence of additional terms in the entropy density thereby leading to more perfect fluidity. (arXiv:1407.7572v1 [cond-mat.quant-gas]) [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A36.00013: Odd frequency Bosonic and Fermionic condensate Alexander Balatsky We introduce the concept of the odd-frequency Bose Einstein Condensate (BEC), characterized by the odd frequency/time two-boson expectation value. To illustrate the concept of odd frequency BEC we present simple classification of pair boson condensates that explicitly permits this state. We point qualitative differences of odd-frequency BEC with conventional BEC and introduce the order parameter and wave function for the odd-frequency BEC [1]. This step extends the classification of the odd frequency states typically discussed in the context of odd frequency fermion pairing [2]. \\[4pt] [1] Odd-frequency Two Particle Bose-Einstein Condensate A.V. Balatsky, arXiv:1409.4875 \newline [2] Odd-frequency superconducting pairing in multiband superconductors, Annica M. Black-Schaffer and Alexander V. Balatsky, Phys. Rev. B 88, 104514, (2013). [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A36.00014: Vortex Dynamics in Atomic BECs: Some Recent Developments Panayotis Kevrekidis In the present work, we will briefly discuss a series of recent experiments by a number of groups enabling the examination of a small number of vortices (including ones of different charges) in atomic BECs. We will use a particle method to try to understand the dynamics of these vortices, in the appropriate limits. We will also briefly demonstrate a so-called generating function method that allows to connect the problem of identifying the vortex centers to the theory of classical orthogonal (such as Hermite) polynomials. This formalism will be shown to be quite useful in identifying vortex polygons and other complex vortex patterns. TIme permitting, generalizations to 3D settings and vortex rings will be briefly touched upon. [Preview Abstract] |
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