Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M36: Interacting Fermi Gases II |
Hide Abstracts |
Sponsoring Units: DAMOP Chair: Joseph Thywissen, University of Toronto Room: 211 |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M36.00001: Superfluidity in Strongly Interacting Spin-Polarized Fermi Gases Ben A. Olsen, Melissa Revelle, Jacob A. Fry, Randall G. Hulet, Daniel E. Sheehy We report measurements of phase separation in a harmonically trapped, spin polarized two-component Fermi gas. The interactions in the gas are varied using a magnetically-tuned Feshbach resonance between the weakly-interacting BCS and strongly-interacting BEC regimes. Using spin-selective imaging, we measure the density profiles for the two lowest hyperfine levels of $^6$Li, with the superfluid phase being indicated by an unpolarized central core. We determine phase boundaries for the superfluid transition as a function of interactions and polarization, finding results that are consistent with earlier experimental results in the crossover regime\footnote{M. Zwierlein et al., Science {\bf 311}, 492 (2006); N. Navon et al., Science {\bf 328}, 729 (2010)}. We also explore the BEC side of resonance, where we compare our measurements to Quantum Monte Carlo simulations\footnote{G. Bertaina and S. Giorgini, PRA {\bf 79}, 013616 (2009)}, and also the deep BCS regime, where few theoretical predictions are available. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M36.00002: Equation of State of One-Dimensional Fermions in Harmonic Traps Casey Berger, Eric Anderson, Joaquin Drut We test a novel numerical method for computing the ground state energy of fermions in a harmonic trapping potential. The new technique combines hybrid Monte Carlo and a Gauss-Hermite discretization instead of a uniform lattice. Use of the harmonic oscillator basis and Gauss-Hermite points avoids the problem of edge effects and spurious copies that arise from periodic boundary conditions. This study sets the stage for calculations in higher dimensions, relying on non-uniform Fast Fourier Transform algorithms for acceleration. Based on this method we determine the ground-state energy of unpolarized few-body systems constrained to one-dimensional motion. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M36.00003: Strongly interacting fermions in 1D Li Yang, Liming Guan, Han Pu Under second order degenerate perturbation theory, we show that the physics of N fermions with arbitrary spin in one dimension in Tonks-Girardeau (TG) and super-Tonks-Girardeau(sTG) regions can be described by super-exchange interaction. An effective spin chain Hamiltonian (none-translational-symmetric Sutherland model) can be obtained from this procedure. For spin-1/2 particles, this model is the none-translational-symmetric Heisenberg model, where a transition between Heisenberg anti-ferromagnetic (AFM) and ferromagnetic (FM) states is expected to occur when the interaction strength is tuned from TG to sTG limit. We show that the FM and AFM states can be distinguished in two different methods: the first is based on their distinct response to a spin-dependent magnetic gradient, and the second is based on their distinct momentum distribution. We examine the validity of the spin-chain model by comparison with results obtained from unbiased techniques such as exact diagonalization and TEBD. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M36.00004: 1D-3D Crossover in a Spin-Imbalanced Fermi Gas Melissa Revelle, Ben A. Olsen, Jacob Fry, Randall G. Hulet We experimentally study the phases of an ultracold two-spin component gas of atomic fermions ($^{6}$Li) confined to 1D tubes formed by a 2D optical lattice. The atoms are prepared in the lowest two hyperfine sublevels where their interactions are tuned by a Feshbach resonance. We previously observed phase separation into a partially-polarized superfluid core and either fully-paired or fully-polarized wings (depending on the spin polarization)\footnote{Y.A. Liao et al., Nature 467, 567 (2010).}. In 3D, the phase separation is inverted, such that the cloud center is fully paired\footnote{G. B. Partridge et al., Science 311, 503 (2006); Y. Shin et al., Phys. Rev. Lett. 97, 030401 (2006).}. We investigate the transition from a 1D to 3D gas by varying the lattice depth and interaction strength which changes the ratio of the tunneling rate between the tubes to the pair binding energy. We are exploring a region of parameter space that is believed to be the most promising region for the exotic FFLO superfluid phase\footnote{M. Parish et al., PRL 99, 250403 (2007).}. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M36.00005: Stability of Fulde-Ferrell-Larkin-Ovchinnikov states in ultracold atomic Fermi gases Jibiao Wang, Qijin Chen The elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states have attracted enormous attention in the condensed matter and AMO communities. They have not been observed experimentally in three-dimensional (3D) Fermi gases, largely due to their predicted small region in the phase space and very low temperature required. In this talk, we will discuss the stability of the FFLO states in 3D homogeneous Fermi gases, both in equal-mass and mass imbalanced systems, within a pairing fluctuation theory. We find that the effective mass of noncondensed pairs in the directions perpendicular to the FFLO wavevector is negative, leading to instability of the FFLO states previously predicted in the literatures. Treatment beyond the $T$-matrix level and further symmetry breaking factors such as optical lattices and spin-orbit coupling may be necessary in order to find stable FFLO states. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M36.00006: Strong-coupling ansatz for the one-dimensional Fermi gas in a harmonic potential Meera Parish, Jesper Levinsen, Pietro Massignan, Georg Bruun The 1D Fermi gas with repulsive short-range interactions provides an important model of strong correlations and is often amenable to exact methods. However, in the presence of confinement, no exact solution is known for an arbitrary number of strongly interacting fermions. Here, we propose a novel ansatz for generating the lowest-energy wavefunctions of the repulsive 1D Fermi gas in a harmonic potential near the Tonks-Girardeau limit of infinite interactions. We specialize to the case of a single impurity interacting with $N$ majority particles, where we may derive analytic forms of the approximate wavefunctions. Comparing with exact numerics, we show that the overlap between the wavefunctions from our ansatz and the exact ones in the ground-state manifold exceeds 0.9997 for $N\leq 8$. Moreover, the overlap for the ground-state wavefunction extrapolates to 0.9999 as $N\to\infty$. Thus our ansatz is essentially indistinguishable from numerically exact results in both the few- and many-body limits. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M36.00007: Two-dimensional harmonically trapped fermionic atomic gases Daniel E. Sheehy, Qin-Qin Lu Recent experiments [1,2] have probed superfluidity of two-dimensional fermionic atomic gases confined to a harmonic trapping potential. While theoretical studies of such systems often incorporate the presence of the trap using the local density approximation, here we present a different approach based on a Bardeen-Cooper-Schrieffer-type variational wavefunction in which the single-particle states undergoing pairing are the exact eigenfunctions of the single-particle problem (i.e., harmonic oscillator wavefunctions). Our approach, following earlier work in the 1D case[3], allows the calculation of several experimentally-relevant observables, such as the local densities and noise correlations. [1] A.T. Sommer, L.W. Cheuk, M.J.H. Ku, W.S. Bakr, and M.W. Zwierlein, Phys. Rev. Lett. {\bf 108}, 045302 (2012). [2] Y. Zhang, W. Ong, I. Arakelyan, and J.E. Thomas, Phys. Rev. Lett. {\bf 108}, 235302 (2012). [3] S. Kudla, D.M. Gautreau, and D.E. Sheehy, http://arxiv.org/abs/1404.4081 [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M36.00008: Pseudogap phenomenon near the phase transition from $p_x$- to $p_x+ip_y$-wave Fermi superfluid Daisuke Inotani, Yoji Ohashi We discuss an ultracold superfluid Fermi gas with a $p$-wave Feshbach resonance (FR). In this system, it has been predicted that the split of $p_x$-, $p_y$-, and $p_z$-wave channels in the $p$-wave FR by a dipole-dipole interaction leads to multi-superfluid phases. While the $p_x$-wave state appears below $T_{\rm c}$, the $p_x+ip_y$-wave state is expected to become more stable below a certain temperature ($\equiv T_{\rm c}^{p_x+ip_y} |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M36.00009: Quantum Monte Carlo Simulation of one dimensional SU(N) Fermion system Shenglong Xu, Congjun Wu One dimensional interacting fermionic systems with $SU(N)$ symmetry have been realized in ultracold atom systems. The interplay between the dimensionality and symmetry provides a platform to search for unconventional phenomena. Inspired by recent experiments, we use quantum Monte Carlo to explore the metallic phase, nature of the metal-insulator transition as well as possible exotic magnetic orders of such systems. The effects of trapping potential are also discussed. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M36.00010: Stoner ferromagnetism of a strongly interacting Fermi gas in the quasirepulsive regime Lianyi He, Xia-Ji Liu, Xu-Guang Huang, Joseph Carlson, Hui Hu Recent advances in rapidly quenched ultracold atomic Fermi gases near a Feshbach resonance arise a number of interesting problems, in the context of observing the long-sought Stoner ferromagnetic phase transition. The possibility of experimentally obtaining a ``quasirepulsive'' regime in the upper branch of the energy spectrum due to the rapid quench is currently debated and theoretically, the Stoner transition has mainly been investigated at zero temperature or high polarization, due to the limited theoretical approaches in the strongly repulsive regime. Here, we develop a nonperturbative large-N expansion theory for a quasirepulsive Fermi gas near resonance and present a finite temperature phase diagram for its Stoner instability. Our results agree well with the known quantum Monte-Carlo simulations at zero temperature and recover the virial expansion prediction at high temperature for arbitrary interaction strengths. At resonance, we find that the unitary Fermi gas undergoes the Stoner transition at about 1.5TF, where TF is the Fermi degeneracy temperature. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M36.00011: Three-Component Fermi Gases in Two Dimensions Thomas Kirk, Meera Parish We study a three-component Fermi gas in two spatial dimensions using a two-channel model. We investigate both few- and many-body properties of the gas, and we construct the ground-state phase diagram as a function of the effective range and coupling strength. We discuss how such a system may be experimentally realisable in the context of cold-atomic gases. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M36.00012: Spin susceptibility and effects of fluctuating Cooper pairs in the BCS-BEC crossover regime of a superfluid Fermi gas Hiroyuki Tajima, Ryo Hanai, Yoji Ohashi We theoretically discuss the spin susceptibility $\chi$ and effects of strong-coupling corrections in the BCS-BEC crossover regime of an ultracold Fermi gas. Using an extended $T$-matrix approximation, we calculate $\chi$ over the entire BCS-BEC crossover region, showing that this magnetic quantity is very sensitive to pairing fluctuations in both the normal and the superfluid phase. In the normal state, it is suppressed by preformed singlet Cooper pairs near $T_{\rm c}$, being similar to the spin-gap phenomenon in high-$T_{\rm c}$ cuprates. Below $T_{\rm c}$, on the other hand, pairing fluctuations enhance $\chi$, in the sense that the suppression of this quantity by the superfluid order is weakened due to partial dissociation of Cooper pairs. From these, we determine the region where pairing fluctuations strongly affect spin excitations in the phase diagram of a Fermi gas with respect to the temperature and the strength of a pairing interaction. We also compare our results with the recent experiments on a $^6$Li Fermi gas. Our results indicate that the spin susceptibility is a useful observable in understanding strong-coupling properties of an ultracold Fermi gas in the BCS-BEC crossover region. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M36.00013: Exact Ground-state Properties of Atomic Fermi Gases in Two Dimensions Hao Shi, Simone Chiesa, Shiwei Zhang Experimental realization of the interacting Fermi gas in three dimensions and the precise comparisons it has allowed with theory and computation have lead to rapid advances in the study of strongly paired fermions. Two-dimensional systems are being realized and offer many new opportunities. We perform exact calculations on the two-dimensional strongly interacting, unpolarized Fermi gas with a zero-range attractive interaction. A new auxiliary-field approach is used which is exact and accelerates the sampling of imaginary-time paths by a force bias technique. The method allows calculations with long imaginary-time and large lattice sizes. We present the calculated equation of state, the contact parameter, the condensate fraction, and the pairing correlation function. The structure of the pairing wave function is determined as a function of $k_F a$, we will also discuss the effect of spin-orbit coupling. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M36.00014: Absence of spin order in a two-dimensional orbital optical lattice Zhenyu Zhou, Vincent Liu, Erhai Zhao Mott insulators with both spin and orbital degeneracy are pertinent to a family of transition metal oxides. The intertwined spin and orbital fluctuations can lead to exotic phases such as quantum spin-orbital liquids. Here we consider two-component spin 1/2 fermionic atoms with strong repulsive interaction on the $p$-band of the square optical lattice. We derive the spin-orbital exchange for quarter filling of the $p$-band in the Mott limit, and show it frustrates the development of long range spin order. Exact diagonalization indicates a spin-disordered ground state with ferro-orbital order. The system dynamically decouples into individual Heisenberg spin chains, each realizing a Luttinger liquid accessible at higher temperatures compared to atoms confined to the $s$-band. Our model serves as an example of how orbital order enhances quantum fluctuations to prevent spin order and leads to dimension reduction in a quantum gas system. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M36.00015: Intrinsic p-wave pair amplitude in a trapped ultracold s-wave superfluid Fermi gas Yuki Endo, Daisuke Inotani, Ryo Hanai, Yoji Ohashi We theoretically discuss the possibility that an unconventional p-wave Cooper-pair amplitude is induced in the ordinary s-wave superfluid Fermi gas. Using a Hubbard model, we numerically show that such a phenomenon really occurs, when both the spatial and (pseudo) spin inversion symmetries of the system are broken. In an ultracold Fermi gas, this situation is shown to be realized, when two spin states (that describe two atomic hyperfine states contributing to s-wave Cooper pairs) feel different trap potentials. Thus, in this situation, when the s-wave pairing interaction is suddenly changed to a p-wave one by using the Feshbach resonance technique, at least just after this tuning, the p-wave superfluid Fermi gas is expected to be realized (which is characterized by the superfluid order parameter given by the product of the p-wave pair amplitude and the p-wave pairing interaction). In this talk, we also clarify the optimal condition to obtain the p-wave pair amplitude in the BCS-BEC crossover region, using the BCS-Leggett strong-coupling theory. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2020 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
1 Research Road, Ridge, NY 11961-2701
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700