Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y31: Ferromagnetism and Spin-imbalances in Quantum Gases |
Hide Abstracts |
Sponsoring Units: DAMOP Chair: Chuanwei Zhang, University of Washington Room: E141 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y31.00001: Ferromagnetism in spin-1/2 Bose and Fermi gases confined by elongated potential Shintaro Takayoshi, Masahiro Sato, Shunsuke Furukawa Internal degrees of freedom in many-body systems generally provoke a variety of phenomena, as electron spins in solids generate various magnetic structures. As the simplest system with internal degrees of freedom we study one-dimensional two-component (pseudospin-1/2) Bose/Fermi gas, which could be realized by using ultra-cold atoms. If a strong repulsion is introduced between two components, the spontaneous population imbalance (i.e., ferromagnetism) is expected to appear. However, it is known that the existent weak- and strong-coupling theories cannot capture the nature of the imbalanced phase and the ordering. We have thoroughly studied them [1] by combining numerical approaches (exact diagonalization and infinite time-evolving block decimation) with some analytic methods in an efficient manner. It is shown that (i) the universality class of the ferromagnetic transition drastically changes from a first order to an Ising type when an inter-component hopping is introduced, and (ii) the imbalanced phase has a gapless ``charge'' mode and a gapped ``spin'' one. [1] S.Takayoshi, M.Sato, and S.Furukawa, arXiv:0911.3157. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y31.00002: A repulsive atomic gas in a harmonic trap on the border of itinerant ferromagnetism Gareth Conduit, Ben Simons Alongside superfluidity, itinerant (Stoner) ferromagnetism remains one of the most well-characterized phases of correlated Fermi systems. A recent experiment has reported the first evidence for novel phase behavior on the repulsive side of the Feshbach resonance in a two-component ultracold Fermi gas. By adapting recent theoretical studies to the atomic trap geometry, we show that an adiabatic ferromagnetic transition would take place at a weaker interaction strength than is observed in experiment. This discrepancy motivates a simple non-equilibrium theory that takes account of the dynamics of magnetic defects and three-body losses. The formalism developed displays good quantitative agreement with experiment. Accepted for publication in Phys. Rev. Lett. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y31.00003: Nearly Ferromagnetic Phases in Ultracold Fermi Gases: Simulations of $^3$ Helium Kathryn Levin, Chih-Chun Chien, Hao Guo We outline a proposal for a future experimental program based on previous studies which purport to observe itinerant ferromagnetism in an ultracold atomic Fermi gas. Here we consider smaller positive scattering lengths (corresponding to moderately strong repulsive interactions), near to, but before the instability is surpassed. This simulation of a Hubbard gas Hamiltonian is thought to capture the physics of the classic Fermi liquid $^3$Helium. As such, it will shed light on the long standing puzzle about whether this Fermi liquid is nearly ferromagnetic or nearly localized. Natural extensions to the polarized case are of considerable interest as simulations of polarized $^3$Helium. We discuss how to deduce the Landau parameters, how to include trap effects and the expected nature of the collective modes. Because the ground state of $^3$Helium is a $p$-wave superfluid, such studies should also be viewed as relevant to understanding how non $s$-wave superfluidity might be associated with Hubbard models in general. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y31.00004: Fermi-liquid properties and instabilities of strongly imbalanced Fermi gases Kelly R. Patton, Daniel E. Sheehy Recent experiments [Schirotzek et al., PRL ${\bf 102}$, 230402 (2009)] involving highly imbalanced ultracold atomic gases have revealed so-called spin or Fermi polarons. These quasiparticles are composed of spinful atoms correlated with a ``cloud'' of atoms of opposite spin. These correlations lead to a renormalization of the free or bare atom's properties. Theoretically, these quasiparticles have been well described by a variational wave function consisting of a single impurity atom interacting with the remaining Fermi sea. Using diagrammatic many-body theory we extend these results and investigate the dependence of the polaron's Fermi liquid properties on finite temperature, as well as increased polaron density. Furthermore, we investigate instabilities of this normal Fermi liquid state, such as transitions to a superfluid or phase-separated state, as the temperature is lowered and/or the density of polarons is increased. [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y31.00005: Magnetization dynamics in cold-atom ferromagnets Inti Sodemann, Dmytro Pesin, Allan MacDonald Recent experiments appear to demonstrate the realization of the ferromagnetic state in ultracold two-component Fermi gases with repulsive interactions [Gyu-Boong Jo et al., Science \textbf {325}, 1521 (2009)], providing a new arena for the study of ferromagnetism distinct from that of the solid state. We address the question of the magnetization dynamics at large length scales in the framework of Landau-Liftshitz equations modifed to account for some features of cold-atom ferromagnetism. In particular, the role of an external magnetic field is played by the hyperfine splitting along the trap, and randomness in the latter induces an effective magnetic damping in the plane perpendicular to this field. We study the preparation of non-uniform magnetic textures using intentionally applied non-uniform energy splittings, and their decay processes into the uniform magnetization state. In particular, we compare the decay process of magnetic spiral textures with the decay of supercurrent due to phase slips. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y31.00006: Ferromagnetism in Two-component Fermi gases: Variational and Green's Function Monte Carlo Studies Nandini Trivedi, Soon-Yong Chang, Mohit Randeria We study the possibility of a ferromagnetic instability in both repulsive and attractive two-component Fermi gases using lowest-order constrained variational (LOCV), variational Monte Carlo (VMC), and fixed-node Green's function Monte Carlo (GFMC) methods. For repulsive interactions, where the range $r_0$ is of order the scattering length $a >0 $, we find clear evidence for a ferromagnetic Stoner instability at $k_F a \sim {\cal O}(1)$. The occurrence of ferromagnetism is robust though the precise value of $k_F a$ at the instability is not universal and depends upon the shape of the potential. To model the recent experiments [1], where the underlying interactions are attractive with $|a| \gg r_0$, one must be on the repulsive excited branch on the $a>0$ side of the Feshbach resonance. We write the many-body wavefunction as a suitable Jastrow factor times a fermionic determinant, with a nodal structure that ensures the system is on the excited branch. We will report on the possible ferromagnetic instability in this Fermi-liquid state and its implications for experiments. \bigskip [1] G-B. Jo {\it et al.}, Science {\bf 325}, 1521 (2009). [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y31.00007: Signature of the FFLO phase in the collective modes of a trapped ultracold Fermi gas Jonathan Edge, Nigel Cooper We study theoretically the collective modes of a two component Fermi gas with attractive interactions in a quasi-one-dimensional harmonic trap. We focus on an imbalanced gas in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. Using a mean field theory, we study the response of the ground state to time-dependent potentials. For potentials with short wavelengths, we find dramatic signatures in the large-scale response of the gas which are characteristic of the FFLO phase. This signature is present both for zero temperature and for small but finite temperature. This response provides an effective way to detect the FFLO state in experiments. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y31.00008: Universal Spin Transport in a Strongly Interacting Fermi Gas Ariel Sommer, Andre Schirotzek, Mark Ku, Martin Zwierlein We study the collision and subsequent diffusion of the two spin components of a strongly interacting Fermi gas. The spin components are initially fully separated via a Stern-Gerlach gradient pulse. By quickly turning on an external magnetic field in the vicinity of a Feshbach resonance we control the interaction between the two spin states in the subsequent collision. From our measurements, we find the spin diffusion coefficient as a function of the interaction strength between the two components. It attains a minimum at the Feshbach resonance on the order of h/m, where h is Planck's constant and m is the atomic mass. At positive scattering length, atoms in the two spin states form molecules as they mix. We observe the formation of molecules and the evolution of the atomic and molecular populations using spatially resolved RF spectroscopy. Our experiment may shed light on the question of whether stable, ferromagnetic spin domains can exist in repulsively interacting Fermi mixtures. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y31.00009: Paired Fraction of Ultracold Atoms in the BEC-BCS Crossover Yean-an Liao, Wenhui Li, T. Paprotta, A.S.C. Rittner, R.G. Hulet We report quantitative measurements of the paired fraction of a two-spin mixture of $^{6}$Li atoms as a function of interaction strength and temperature. The interaction strength is tuned from the molecular BEC regime to the BCS regime using a Feshbach resonance. The fraction of atoms that are paired is measured by photo-exciting the pairs to a spatially small vibrational level of an electronically excited state of the Li$_{2}$ diatomic molecule\footnote{G. B. Partridge {\it et al.}, {\it Phys. Rev. Lett.} {\bf 95}, 020404 (2005).}. We find that a fraction of the gas is rapidly excited, while the remaining atoms undergo a relatively slow photoassociative process. The photoexcitation rate is a function of time, therefore, shows a kink, which we interpret as the depletion of pairs. The location of the kink is independent of the photo-excitation laser intensity, as long as it is fast compared to the pair reformation time. At sufficiently low temperature, we find the paired fraction varies from near unity at the BEC limit, to below our limits of detectability on the BCS side. At unitarity, we observe preformed pairs above \textit{T}$_{c}$, a phenomenon shared with high-temperature superconductors. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y31.00010: Metastable states of Fermionic condensates in highly elongated traps at unitarity Leslie Baksmaty, Hong Lu, Han Pu, Carlos Bolech We present results for a 3-dimensional fully self-consistent meanfield Bogoliubov-deGennes study of finite trapped samples of degenerate spin polarized Fermionic gases. Our findings indicate that for a large enough sample in a cigar-shaped trap and in the unitary regime, there are typically three types of solutions which are almost degenerate and possess the the ff. properties: (i) The solution most similar to the semi-classical approximation (LDA) is consistently the lowest in energy. (ii) However, the other two metastable solutions connected by a second-order transition are much more consistent with experimental observations and (iii) one of these metastable solutions supports an FFLO state. These results are highly relevant to recent experiments where significant and unexpected distortions were observed in density profiles. We submit that these metastable solutions are relevant false vacua because given the slow relaxation times, the actual states observed in an experiment could be a strong function of the experimental procedure. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y31.00011: Effect of trap shape on imbalanced fermi superfluids Masaki Tezuka, Yoichi Yanase, Masahito Ueda By solving the Bogoliubov-de Gennes equations with coupling-constant renormalization appropriate for an elongated trapped system with a chemical potential difference, we show that the maximum population imbalance ratio for condensation to occur, $P_{\mathrm{CC}}$, does not increase with the trap aspect ratio $\lambda$. This is also confirmed by our simulation based on the real-space self-consistent $T$-matrix approximation (RSTA). Moreover, while the deformation of the cloud shape from that expected from the trap shape increases, it stays minor for extreme values of $\lambda$. This finding indicates that, despite the apparent discrepancy between the MIT and Rice experiments over the value of $P_{\mathrm{CC}}$ and the breakdown of local density approximation, the equilibrium state of the system for the aspect ratio in the Rice experiment would be closer to that of MIT. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y31.00012: The BCS-BEC crossover and the disappearance of FFLO-correlations in a spin-imbalanced, 1D Fermi gas Fabian Heidrich-Meisner, Adrian Feiguin, Ulrich Schollwoeck, Wilhelm Zwerger We present a numerical study of the one-dimensional BCS-BEC crossover of a spin-imbalanced Fermi gas. The crossover is described by the Bose-Fermi resonance model in a real space representation. Our main interest is in the behavior of the pair correlations, which, in the BCS limit, are of the Fulde-Ferrell-Larkin-Ovchinnikov type, while in the BEC limit, a superfluid of diatomic molecules forms that exhibits quasi-condensation at zero momentum. We use the density matrix renormalization group method to compute the phase diagram as a function of the detuning of the molecular level and the polarization. As a main result, we show that FFLO-like correlations disappear well below full polarization close to the resonance. The critical polarization depends on both the detuning and the filling. Heidrich-Meisner, Feiguin, Schollwoeck, Zwerger, arXiv:0908.3074 [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y31.00013: Non-Fermi liquid fixed point for an imbalanced gas of fermions in $1+\epsilon$ dimensions Andrew James, Austen Lamacraft We consider a gas of two species of fermions with population imbalance. Using the renormalisation group in $d=1+\epsilon$ dimensions, we show that for spinless fermions and $\epsilon > 0$ a fixed point appears at finite attractive coupling where the quasiparticle residue $Z$ vanishes, and identify this fixed point with the transition to Larkin--Ovchinnikov--Fulde--Ferrell order (inhomogeneous superconductivity). When the two species of fermions also carry spin degrees of freedom we find a repulsive fixed point, indicating a transition to spin density wave order. [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y31.00014: Effect of three-body loss on itinerant ferromagnetism in an atomic Fermi gas Gareth Conduit, Ehud Altman A recent experiment has provided the first evidence for itinerant ferromagnetism in an ultracold atomic gas of fermions with repulsive interactions. However, the gas in this regime is also subject to significant three-body loss. We adopt an extended Hertz-Millis theory to account for the effect of loss on the transition and on the ferromagnetic state. We find that the losses damp quantum fluctuations and thereby significantly increase the critical interaction strength needed to induce ferromagnetism. This effect may resolve a discrepancy between the experiment and previous theoretical predictions of the critical interaction strength. We further illuminate the impact of loss by studying the collective spin excitations in the ferromagnet. Even in the fully polarized state, where loss is completely suppressed, spin waves acquire a decay rate proportional to the three-body loss coefficient. [Preview Abstract] |
Friday, March 19, 2010 10:48AM - 11:00AM |
Y31.00015: DMRG study of spin-imbalanced fermionic cold atoms trapped on optical ladders --- evolution of spin-aligned domains Masahiko Okumura, Susumu Yamada, Masahiko Machida, Hideo Aoki DMRG is used to study spin-imbalanced fermionic cold atoms that reside on a two-leg optical ladder and trapped to a finite region to explore the spin structure. We find that the ground state comprises phase-separated regions that include fully spin-polarized, partially polarized, and non-polarized domains for strong on-site repulsions. We determine the conditions for realizing large itinerant ferromagnetic (fully polarized metallic) regions. [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. |
© 2024 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
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700