Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B35: Fermi Gases |
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Sponsoring Units: DAMOP Chair: Lawrence Cheuk, Massachusetts Institute of Technology Room: 702 |
Monday, March 3, 2014 11:15AM - 11:27AM |
B35.00001: Observation of antiferromagnetic correlations in the Fermi-Hubbard model R.A. Hart, P.M. Duarte, T.L. Yang, X. Liu, R.G. Hulet, T.C.L. Paiva, D. Huse, R.T. Scalettar, N. Trivedi The physics of high temperature superconductors is not well understood, although it is known that the undoped parent compounds of many of them are antiferromagnetic (AF) insulators. The Fermi-Hubbard model at half filling (one atom per lattice site) is known to exhibit a phase transition to an antiferromagnetic insulator at a low temperature. We realize the Fermi-Hubbard model by loading ultracold $^{6}$Li atoms into a three-dimensional red-detuned optical lattice. We have compensated the confining potential of the lattice with blue-detuned laser beams in order to evaporatively cool the atoms. We have cooled sufficiently to observe AF correlations using spin-sensitive Bragg scattering of near-resonant light. Comparison with Quantum Monte Carlo (QMC) calculations indicates that the temperature is between 2-3 $T_{N}$, where short-range correlations begin to develop. Bragg scattering combined with QMC provides sensitive thermometry in a previously unexplored regime. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B35.00002: Phase diagram of a one-dimensional spin-full Bose-Fermi mixture at large boson densities Alberto Nocera, Roman M. Lutchyn, Adrian E. Feiguin We determine the ground state phase diagram of a one dimensional Bose-Fermi Hubbard model with spin-full fermions using the Density Matrix Renormalization Group (DMRG) method. We focus on the regime with one fermion per site, and deep into the superfluid phase. We study the effects of the boson-fermion interaction on the fermionic pairing, as a function of the interaction strength, hopping, and bosonic density. We identify the regime in which fermionic superfluidity dominates, and a phase with coexisting CDW and bosonic superfluidity. At high boson densities we find a fermionic Wigner crystal coexisting with bosonic superfluidity. We analyze the structure of the Cooper pairs and the bosonic cloud that acts as the glue. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B35.00003: Anisotropic Weyl Fermions from Quasiparticle Excitation Spectrum of a 3D Fulde-Ferrell Superfluid Yong Xu, Ruilin Chu, Chuanwei Zhang Weyl fermions, first proposed for describing massless chiral Dirac fermions in particle physics, have not been observed yet in experiments. Recently, much effort has been devoted to explore Weyl fermions around band touching points of single particle energy dispersions in certain solid state materials (named Weyl semimetals), similar as graphene for Dirac fermions. Here we show that such Weyl semimetals also exist in the quasiparticle excitation spectrum of a three-dimensional (3D) spin-orbit coupled Fulde-Ferrell (FF) superfluid. By varying Zeeman fields, the properties of Weyl fermions, such as their creation and annihilation, number and position, as well as anisotropic linear dispersions around band touching points, can be tuned. We study the manifestation of anisotropic Weyl fermions in sound speeds of FF fermionic superfluids, which are detectable in experiments. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B35.00004: $d_{xy}$-density wave in fermion-fermion cold atom mixtures Chen-Yen Lai, WenMin Huang, David Campbell, Shan-Wen Tsai Using a functional renormalization group (FRG) calculation, we predict number density wave instabilities in a doubly-degenerate Fermi-Fermi mixture on a square optical lattice. We take both inter-species and intra-species interactions into account and treat them on an equal footing. When the two species of fermions are both at half-filling, we find two out-of-phase conventional ($s$-wave) number density waves, which arise from from a sufficiently strong on-site inter-species repulsion. Moving only one species away from half-filling, we discover that an unconventional $d_{xy}$-density emerges. When both species are away from half-filling, a superconductivity instability becomes dominant. Apart from the detailed FRG calculation, we develop an intuitive minimal model to capture the physical mechanism, which emerges from the density imbalance between the two species of fermions in the vicinity of half-filling. Our study sheds light on the search for unconventional density waves in strong correlated systems. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B35.00005: Quantum Monte Carlo study of antiferromagnetic correlations in the Positive $U$ Fermi-Hubbard model Thereza Paiva, Russell Hart, Pedro Duarte, Ernie Yang, Xinxing Liu, Randy Hulet, David Huse, Richard Scalettar, Nandini Trivedi We use determinantal quantum Monte Carlo (QMC) simulations$^{1,2}$ to investigate the fermion Hubbard model as a function of filling, temperature and population imbalance. We find that (i) the structure factor is not very sensitive to population imbalance for the ranges of temperatures currently available in the experiments; (ii) at half filling for a large range of $U$ the antiferromagnetic structure factor collapses onto a universal curve. This scaling behavior along with QMC data for other scattering angles allows us to directly compare with experimental Bragg scattering data and put constraints on the experimental Neel ordering temperature. $^1$ ``Fermions in 2D Optical Lattices: Constraints on entropy for observing antiferromagnetism and superfluidity", T. Paiva, R.T. Scalettar, M. Randeria, and N. Trivedi, Phys. Rev. Lett. 104, 066406 (2010). $^2$ ``Fermions in 3D Optical Lattices: Cooling Protocol to Obtain Antiferromagnetism", T. Paiva, Yen Lee Loh, N. Trivedi, M. Randeria and R.T. Scalettar, Phys. Rev. Lett. 107, 086401 (2011) [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B35.00006: Extended $s$-wave superfluid of repulsively interacting three-component fermionic atoms in optical lattices Sei-ichiro Suga, Kensuke Inaba We investigate pairing symmetry of the superfluid state in repulsively interacting three-component (colors) fermionic atoms in optical lattices. This superfluid state appears, when two of the color-dependent three repulsions are much stronger than the other close to half filling [1]. We evaluate the effective pairing interaction by collecting random-phase-approximation-type diagrams and ladder diagrams, and solve the Eliashberg equation within weak-coupling theory in square optical lattices. We find that pairing symmetry is an extended $s$-wave, although in the phase diagram the superfluid state is adjacent to the color-density wave or paired Mott insulator at half filling. The $k$-dependence of the superfluid order parameter is caused by quantum fluctuations of the staggered color-density wave. When the difference in the three repulsions is decreased, paring symmetry changes from an extended $s$-wave to a $d$-wave. We expect $^{6}$Li, $^{171}$Yb, $^{173}$Yb atoms and their mixtures in optical lattices to be possible candidates for observing this superfluid state.\\[0pt] [1] K. Inaba and S. Suga, \textit{Phys. Rev. Lett.} \textbf{108} (2012) 255301. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B35.00007: Suppressing the loss of ultracold molecules via the continuous quantum Zeno effect Bihui Zhu, Bryce Gadway, Michael Foss-Feig, Johannes Schachenmayer, Michael Wall, Kaden Hazzard, Bo Yan, Steven Moses, Jacob Covey, Deborah Jin, Jun Ye, Murray Holland, Ana Rey We develop theoretical methods to explain the recently observed suppression of chemical reactions between two rotational states of fermionic KRb molecules confined in 1D tubes with a superimposed optical lattice along them [Yan {\it et al}., Nature 501, 521 (2013)]. The loss suppression is a consequence of both lattice confinement and the continuous quantum Zeno effect, which in this case takes place in the regime where the two-body loss is larger than other energy scales in the lattice. To quantitatively analyze the experiment, we derive a renormalized single-band model which accounts for 3D multi-band effects, and formulate from it a rate equation and mean-field theory validated by comparing with numerically exact t-DMRG. We demonstrate that the renormalized model captures the measured dependence of the loss rate on all lattice parameters, allowing us to determine the filling fraction. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B35.00008: Pair correlations in the two-dimensional Fermi gas Vudtiwat Ngampruetikorn, Jesper Levinsen, Meera M. Parish We consider the two-dimensional Fermi gas at finite temperature with attractive short-range interactions. Using the virial expansion, which provides a controlled approach at high temperatures, we determine the spectral function and contact for the normal state. Our calculated spectra are in qualitative agreement with recent photoemission measurements [M. Feld et al., Nature 480, 75 (2011)], thus suggesting that the observed pairing gap is a feature of the high-temperature gas rather than being evidence of a pseudogap regime just above the superfluid transition temperature. We further argue that the strong pair correlations result from the fact that the crossover to bosonic dimers occurs at weaker interactions than previously assumed. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B35.00009: FFLO state of two-dimensional imbalanced Fermi gases Daniel Sheehy Trapped fermionic atomic gases exhibit superfluid states, akin to superconductivity in a metal, due to the pairing of two species of atomic fermion. In recent years, there has been much experimental and theoretical interest in studying the behavior of fermionic superfluids under an imposed population imbalance that disrupts such pair formation and superfluidity and can lead to new phases, including phase separation, imbalanced normal (Fermi liquid), and the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state predicted to exhibit a spatially-modulated superfluid state to accommodate the population imbalance. I will present recent theoretical results on fermionic atomic gases confined to a quasi two-dimensional (2D) geometry, showing that the FFLO phase in may more stable in 2D than in the bulk (3D) case (similar to the case of quasi-1D imbalanced gases), providing another possible setting for observing the FFLO state. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B35.00010: Disordered Mott Insulator in heavy-light Fermi mixture in optical lattices Anzi Hu, Maciej Maska, Jim Freericks, Charles Clark Ultracold mixtures of different atomic species have great promise of realizing novel many-body phenomena beyond Hubbard model. In a mixture of femionic atoms with large mass differences, a disordered Mott insulator can be formed as the result of the repulsive interaction between two species. The disorder Mott insulator leads to an incompressible total density of the mixture while the relative density is still compressible. Based on strong-coupling expansion and Monte Carlo calculations, we show that this phase can exist for a broad parameter region for ultracold mixtures confined by a harmonic trap and a three-dimensional optical lattice. The realization of such phase can lead to new ways of quantum control in ultracold mixtures. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B35.00011: Evaporative cooling in a compensated optical lattice P.M. Duarte, R. Hart, T.L. Yang, X. Liu, R.G. Hulet We present experimental results of evaporative cooling in a three-dimensional, red-detuned optical lattice. The lattice is compensated by the addition of three blue-detuned gaussian beams which overlap each of the lattice laser beams, but are not retro-reflected~\footnote{C.~J.~M. Mathy, et al., ``Enlarging and cooling the N\'eel state in an optical lattice,'' Phys. Rev. A {\bfseries 86,} 023606 (2012).}. The intensity of the compensating beams can be used to control the difference between the chemical potential in the lattice and the threshold for evaporation. We start with a two spin component degenerate Fermi gas of $^{6}$Li atoms at a temperature $<0.05 T_{F}$ in a dimple potential, which is obtained by rotating the polarization of the lattice retro beams to prevent the formation of standing waves. The temperature of the cloud is measured by releasing it from the dimple and fitting the momentum distribution to a Thomas-Fermi profile. We perform round-trip measurements into, and out of the lattice to study the adiabaticity of the loading as well as the effect of the compensating beams. Using the compensated lattice potential, we have reached temperatures low enough to produce antiferromagnetic spin correlations, which we detect via Bragg scattering of light. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B35.00012: Energy, decay rate, and effective masses for a moving polaron in a Fermi sea: Explicit results in the weakly attractive limit Trefzger Christian, Castin Yvan We study the properties of an impurity of mass $M$ moving through a spatially homogeneous three-dimensional fully polarized Fermi gas of particles of mass $m$. In the weakly attractive limit, where the effective coupling constant $g\to0^-$ and perturbation theory can be used, both for a broad and a narrow Feshbach resonance, we obtain an explicit analytical expression for the complex energy $\Delta E(\mathbf{K})$ of the moving impurity up to order two included in $g$. This also gives access to its longitudinal and transverse effective masses $m_\parallel^*(\mathbf{K})$, $m_\perp^*(\mathbf{K})$, as functions of the impurity wave vector $\mathbf{K}$. Depending on the modulus of $\mathbf{K}$ and on the impurity-to-fermion mass ratio $M/m$ we identify four regions separated by singularities in derivatives with respect to $\mathbf{K}$ of the second-order term of $\Delta E(\mathbf{K})$, and we discuss the physical origin of these regions. Remarkably, the second-order term of $m_\parallel^*(\mathbf{K})$ presents points of non-differentiability, replaced by a logarithmic divergence for $M=m$, when $\mathbf{K}$ is on the Fermi surface of the fermions. We also discuss the third-order contribution and relevance for cold atom experiments. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B35.00013: Persistence of Bose condensate after Fermi surface destruction in a Bose-Fermi Mixture Eric Duchon, Shizhong Zhang, Soon-Yong Chang, Mohit Randeria, Nandini Trivedi We propose a single variational wave function to investigate the ground state properties of a Bose-Fermi mixture with equal boson and fermion population. We use variational and diffusion quantum Monte Carlo techniques to study this mixture as a function of increasing attraction between bosons and fermions. Sandwiched between the expected states in weak and the strong coupling limits, we find evidence for a novel state at intermediate coupling for which we make two predictions: (I) a complete destruction of the atomic Fermi surface and emergence of a molecular Fermi sea that coexists with a remnant of the Bose-Einstein condensate, and (II) evidence for fermion pairing correlations mediated by bosons. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B35.00014: Unconventional superfluid in a two-leg fermonic ladder Shun Uchino, Akiyuki Tokuno, Thierry Giamarchi We show that a novel unconventional superfluid triggered by a spin-orbit coupling is realized in a repulsively interacting fermonic ladder system. A competition between spin singlet and triplet pairings occurs due to the breaking of inversion symmetry. We show that both superfluid orders decay algebraically with the same exponent except for the special coupling constants for which a dominant superfluid is determined solely by the spin-orbit coupling. We also propose an experiment to observe such phases with cold atoms. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B35.00015: Quantum simulation of correlated-hopping models with fermions in optical lattices M. Di Liberto, C.E. Creffield, G.I. Japaridze, C. Morais Smith By using a modulated magnetic field in a Feshbach resonance for ultracold fermionic atoms in optical lattices, we show that it is possible to engineer a class of models usually referred to as correlated-hopping models. These models differ from the Hubbard model in exhibiting additional density-dependent interaction terms that affect the hopping processes. In addition to the spin-SU(2) symmetry, they also possess a charge-SU(2) symmetry, which opens the possibility of investigating the $\eta$-pairing mechanism for superconductivity introduced by Yang for the Hubbard model. We discuss the known solution of the model in 1D (where $\eta$ states have been found in the degenerate manifold of the ground state) and show that, away from the integrable point, quantum Monte Carlo simulations at half filling predict the emergence of a phase with coexisting incommensurate spin and charge order. [Preview Abstract] |
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