Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M35: Spin-orbit Coupling and the BEC-BCS Crossover |
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Sponsoring Units: DAMOP Chair: Cheng Chin, University of Chicago Room: 702 |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M35.00001: Interaction-Tuned Dynamical Transitions in a Rashba Spin-Orbit-Coupled Fermi Gas Juraj Radic, Stefan Natu, Victor Galitski We consider the time evolution of the magnetization in a Rashba spin-orbit-coupled Fermi gas, starting from a fully-polarized initial state. We model the dynamics using a Boltzmann equation, which we solve in the Hartree-Fock approximation. The resulting non-linear system of equations gives rise to three distinct dynamical regimes with qualitatively different asymptotic behaviors of the magnetization at long times. The distinct regimes and the transitions between them are controlled by the interaction strength: for weakly interacting fermions, the magnetization decays to zero. For intermediate interactions, it displays undamped oscillations about zero and for strong interactions, a partially magnetized state is dynamically stabilized. The dynamics we find is a spin analog of interaction induced self-trapping in double-well Bose Einstein condensates. The predicted phenomena can be realized in trapped Fermi gases with synthetic spin-orbit interactions. [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M35.00002: Cavity-Assisted Dynamical Spin-Orbit Coupling in Cold Atoms Lin Dong, Lu Zhou, Biao Wu, B. Ramachandhran, Han Pu In this work, we consider the effect of spin-orbit coupling in ultracold atoms induced by a quantized light field inside an optical cavity, where the back-action from the atom to the cavity light field plays an essential role. The Raman coupling gives rise to effective spin-orbit interaction which couples atom's center-of-mass motion to its pseudospin degrees of freedom. Meanwhile, the cavity photon is dynamically affected by the atom. This system possesses remarkable features. For example, loop structure may emerge in dispersion curves, effective nonlinearity from the atom-photon feedback induces dynamical instability, etc. To understand the intriguing physics, we analytically computed the critical condition for forming loops and performed stability and dynamical analysis. Furthermore, we propose to demonstrate dynamical instability experimentally in terms of counting sudden change of photon number inside the cavity. From a practical point of view, all the ingredients proposed in this work has been demonstrated in various labs. Hence our proposal can be readily tested in experiment. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M35.00003: Vortices and vortex states of Rashba spin-orbit coupled condensates Predrag Nikolic The Rashba spin-orbit coupling in two spatial dimensions is captured by a static SU(2) gauge field with a non-zero magnetic Yang-Mills flux. This SU(2) analogue of magnetic field enables two-dimensional topological insulators (TI) reminiscent of integer quantum Hall states. An outstanding question is whether non-Abelian fractional TIs could exist as well. We explore this from the point of view that quantum melting of a vortex lattice can produce fractional incompressible liquids when the number of flux quanta per particle is not small. Physical systems in which an SU(2) vortex lattice melting could perhaps be arranged include two or three-component bosonic cold atoms in optical lattices, as well as solid-state heterostructures with a conventional or Kondo TI quantum well. This talk will discuss the types of vortices and vortex lattices that could exist in these systems as ``parent'' states to fractional quantum liquids. Analytical arguments based on conservation laws reveal several possibilities for vortex states, some of which do not break the time-reversal symmetry. We will present mean-field numerical results that paint certain vortex states as excellent metastable or ground states of a microscopic lattice model. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M35.00004: The Pairing of Rashba Spin-orbit Coupled Fermi Gas in Optical Lattice Xiaosen Yang, Ho-Kin Tang, Jinhua Sun, Hai-Qing Lin We make an urgent advance using determinant quantum Monte Carlo (DQMC) simulations on Rashba spin-orbit coupled Fermi gases in square optical lattice, which is free of the sign-problem. We show that the Berezinskii-Kosterlitz-Thoules phase transition temperature is firstly enhanced and then suppressed by Rashba spin-orbit coupling at strong attraction region. At weak attraction region, Rashba spin-orbit coupling always suppresses the transition temperature. We also show that the spin susceptibility becomes anisotropic and retain finite at zero temperature. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M35.00005: Wavefunction Vortex Attachment via Matrix Products: Application to Atomic Fermi Gases in Flat Spin-Orbit Bands Vito Scarola Ultracold atomic gases in the presence of strong spin-orbit coupling present challenging many-body problems. For very strong spin orbit coupling, interaction effects dominate. The resulting many-body problem is non-perturbative but progress can be made with validated wavefunctions that properly account for the location of wavefunction vortices. I will discuss a new method to construct and validate Jastrow-correlated wavefunctions in arbitrary bases. The method implements vortex insertion in terms of matrix products. The approach was tested on a model of a dilute gas of Rashba spin-orbit coupled fermions in the presence of slow rotation. Validated wavefunctions show that vortices in slowly rotating atomic fermi gases with strong spin-orbit coupling cluster near the system center and should therefore be directly visible in time-of-flight imaging. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M35.00006: Novel magnetic phases for two-component Bose-Hubbard model with synthetic spin-orbit coupling in one dimension Xiaoqun Wang, Jize Zhao, Ping Zhang, Shijie Hu, Jun Chang We present a new phase diagram for the two-component Bose-Hubbard model with a synthetic spin-orbit coupling in one dimension by employing the density-matrix renormalization group method. A ferromagnetic long-range order emerges in both the Mott insulator and superfluid phases. It results from the spontaneous breaking of the $Z_2$ symmetry, when the spin-orbit coupling term becomes comparable to the hopping kinetic energy and the inter-component interaction is smaller than the intra-component one. These novel effects are expected to be detectable with the present realization of synthetic spin-orbit coupling in experiments. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M35.00007: Dilute spin-orbit Fermi gases Daniel Maldonado-Mundo, Lianyi He, Patrik \"Ohberg, Manuel Valiente We study repulsive Fermi gases with Rashba spin-orbit coupling in two and three dimensions when they are dilute enough that a single branch of the spectrum is occupied in the non-interacting ground state. We develop an effective renormalizable theory for fermions in the lower branch and obtain the energy of the system in three dimensions to second order in the renormalized coupling constant. We then exploit the non-Galilean-relativistic nature of spin-orbit coupled gases. We find that at finite momentum, the two-dimensional Fermi sea is deformed in a non-trivial way. Using mean-field theory to include interactions, we show that the ground-state of the system acquires a finite momentum, and is consequently deformed, when the interaction is stronger than a critical value. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M35.00008: Spin-Orbit Coupled Dengenerate Fermi Gases with Topological Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase Chun Fai Chan, Ming Gong The spin-orbit coupled degenerate Fermi gas provides an ideal platform for the search of topological matters and associated topological excitations. Recently, it has been shown that the topological Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, in which the Cooper pairs carry finite center-of-mass momentum, can be realized in the present of in-plane and out-of-plane Zeeman field. In this work, we study the topological phase transition and topological edge modes in the degenerate Fermi gas system. We first show that the in-plane Zeeman field creates an s-wave pairing channel, thus the effective pairing is a $s+p$ wave pairing in the topological FFLO phase regime. Then we study the phase diagram in the parameter space and discuss how these phases can be reached in experiments. At last we study topological excitations in a slab geometry, and discuss their possible experimental measurement issues. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M35.00009: Unconventional superfluidity in spin-orbit coupled ultracold atomic Fermi gases Xia-Ji Liu, Lei Jiang, Han Pu, Yan Chen, Hui Hu Ultracold atoms has been proven to be an ideal table-top system to reveal novel states of quantum matter. The latest development of engineering synthetic spin-orbit coupling in ultracold atoms has created a new frontier that is endowed with a strong interdisciplinary character. This is a system that has a close connection to new functional materials such as topological insulators and has the potential to perform topological quantum computation based on Majorana fermions. Here we report our recent theoretical work on spin-orbit coupled atomic Fermi gases. We predict a new anisotropic state of matter which consists of exotic quasi-particles with anisotropic effective mass. In the superfluid phase, these exotic quasi-particles exhibit salient features in the momentum distribution, single-particle spectral function and spin structure factor. By applying an external Zeeman field, novel states of matter known as topological superfluids or inhomogeneous Fulde-Ferrell superfluids can form. We propose that strong nonmagnetic or magnetic impurity scattering, created by a narrow dimple laser beam, can induce a universal mid-gap bound state in topological superfluids. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M35.00010: BCS-BEC Crossover and topological phase transition in Fermi Gases driven by Spin-Orbit Coupling and Zeeman field Yi-Xiang Yu, Jinwu Ye, Wuming Liu In this work, we investigate 3D and 2D Fermi gases with uniaxial, Rashba-type and isotropic spin-orbit coupling (SOC). By calculating the chemical potentials and cooper-pair sizes, we find that the increasing Rashba and isotropic SOC can drive a crossover from BCS side to BEC side, while uniaxial SOC will not affect the properties of the many-body system. According to recent experiments, we also consider both a zeeman field and SOC simultaneously appearing in a 3D and 2D Fermi gas. We find the zeeman field can drive the system from the normal state to the topological superfluid states in Rashba SOC case. In 3D Rashba case, there are two topological superfluid phases which have different number of Weyl Fermions. At the same time, our results also show that the zeeman field can drive a converse crossover from BEC side to BCS side. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M35.00011: Magnetic excitations and spin-gap phenomenon in the BCS-BEC crossover regime of an ultracold Fermi gas Hiroyuki Tajima, Takashi Kashimura, Ryo Hanai, Ryota Watanabe, Yoji Ohashi We investigate the uniform spin susceptibility $\chi$ and strong-coupling corrections in the BCS-BEC crossover regime of an ultracold Fermi gas. Within the framework of an extended $T$-matrix theory,\footnote{T.Kashimura, R.Watanabe, and Y.Ohashi, Phys. Rev. A \textbf{86}, 043622 (2012).} we show that $\chi$ exhibits non-monotonic temperature dependence in the normal state, and is suppressed near the superfluid phase transition temperature $T_{\rm c}$. This spin-gap phenomenon is found to be deeply related to the pseudogap phenomenon appearing in the single-particle density of states. To characterize this magnetic phenomenon, we introduce the spin-gap temperature $T_{\rm s}$ as the temperature at which $\chi$ takes a maximum value. Determining $T_{\rm s}$ in the entire BCS-BEC crossover region, we identify the spin-gap regime in the phase diagram of a Fermi gas with respect to the temperature and the strength of a pairing interaction. Since the spin-gap is crucial key phenomenon in high-$T_{\rm c}$ cuprates, our results would be useful for the study of this many-body phenomenon using ultracold Fermi gases, as well as in observing the pseudogap phenomenon through the spin-gap phenomenon. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M35.00012: Exact calculations of strongly paired fermions in two dimensions Simone Chiesa, Shiwei Zhang The problem of strongly interacting superfluid Fermi gas has attracted considerable interest, especially in three-dimensions at unitarity. Although the corresponding problem in two-dimensions does not have a unitarity limit per se, it is expected to offer a rich interplay between the interaction strength and density. A quantitative understanding is important, particularly in light of its possible experimental realization with cold atoms. To this end, we have carried out auxiliary-field quantum Monte Carlo simulations on large system sizes. The ground-state energy is obtained for an unpolarized gas with a zero-range attractive interaction. The calculations are exact, and are performed using a BCS trial wave function that greatly reduces the statistical fluctuation. We present the calculated equation of state as a function of $k_F a$, and make comparisons with BCS and other results. Other ground-state observables will also be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M35.00013: Theory of BCS-BEC crossover in altracold atomic Fermi gases in the presence of impurities Qijin Chens We present a theory of BCS-BEC crossover in ultracold atomic Fermi gases in the presence of nonmagnetic impurities, for variable impurity strength from the Born to the unitary limit. The particle-particle scattering T-matrix and the impurity scattering T-matrix will both be considered self-consistently at the same time, in either a 3D continuum or an optical lattice. Result of $T_c$, the chemical potential $\mu$ and the excitation gap $\Delta$ as well as the order parameter $\Delta_{SC}$, will be presented as a function of impurity strength and impurity density, and will also be compared with the case of $d$-wave pairing such as in high $T_c$ superconductors. References: Q.J. Chen and J.R. Schrieffer, Phys. Rev. B 66, 014512 (2002). [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M35.00014: Superfluid phase transition and effects of mass imbalance in the BCS-BEC crossover regime of an ultracold Fermi gas: A self-consistent T-matrix theory Ryo Hanai, Yoji Ohashi We investigate a two-component Fermi gas with mass imbalance ($m_\uparrow\ne m_\downarrow$, where $m_\sigma$ is an atomic mass in the $\sigma$-component) in the BCS-BEC crossover region. Including pairing fluctuations within a self-consistent $T$-matrix theory, we examine how the superfluid instability is affected by the presence of mass imbalance. We determine the superfluid region in the phase diagram of a Fermi gas in terms of the temperature, the strength of a pairing interaction, and the ratio of mass imbalance. The superfluid phase transition is shown to always occur even when $m_\uparrow\ne m_\downarrow$.\footnote{R.Hanai and Y.Ohashi, J. Low Temp. Phys., DOI 10.1007/s10909-013-0909-3.} This behavior of $T_{\rm c}$ is quite different from the previous result in an extended $T$-matrix theory,\footnote{R.Hanai, \textit{et. al.}, Phys. Rev. A (2013) in press.} where $T_{\rm c}$ vanishes at a certain value of $m_\uparrow/m_\downarrow>0$ in the BCS regime. Since Fermi condensates with mass imbalance have been discussed in various systems, such as a cold Fermi gas, an exciton(polariton) condensate, as well as color superconductivity, our results would be useful for further understandings of these novel Fermi superfluids. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M35.00015: Quasiparticle Berry curvature and Chern numbers in spin-orbit-coupled bosonic Mott insulators Clement Wong, Rembert Duine We study the ground-state topology and quasiparticle properties in bosonic Mott insulators with two- dimensional spin-orbit couplings in cold atomic optical lattices. We show that the many-body Chern and spin-Chern number can be expressed as an integral of the quasihole Berry curvatures over the Brillouin zone. Using a strong-coupling perturbation theory, for an experimentally feasible spin-orbit coupling, we compute the Berry curvature and the spin Chern number and find that these quantities can be generated purely by interactions. We also compute the quasiparticle dispersions, spectral weights, and the quasimomentum space distribution of particle and spin density, which can be accessed in cold-atom experiments and used to deduce the Berry curvature and Chern numbers. Physical Review A \textbf{88}, 053631 (2013) [Preview Abstract] |
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