Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P45: Unitary Fermi Gases and the BCS-BEC Crossover |
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Sponsoring Units: DAMOP Chair: Ken O'Hara, The Pennsylvania State University Room: A310 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P45.00001: Universal physics in the dilute Fermi gases Soon Yong Chang, Mohit Randeria, Nandini Trivedi Using Quantum Monte Carlo techniques, we investigate universal properties of the repulsive upper branch as well as the superfluid ground state across the Feshbach resonance. We test the Tan relations by computing (a) the equation of state, (b) the short distance behavior of the two-particle density matrix, and (c) the large-$k$ tail of the momentum distribution $n(k)$. We have used twisted boundary conditions to improve the approach to the thermodynamic limit, which allows us to probe $n(k)$ at a dense set of $k$ values. We find consistent estimates of the $1/k_F a$ dependence of the contact $C$ at $T=0$ from all three methods. We show that, just like the superfluid ground state across the BCS-BEC crossover, the repulsive upper branch also obeys these relations, albeit with different values of $C$. This reflects the universal behavior of dilute Fermi gases with a short range potential, in contrast to, for instance, the hard sphere gas in the $k_F a \sim 1$ regime. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P45.00002: Thermal distribution and the contact of Fermi gases at large scattering lengths Joaquin Drut, Timo Lahde, Timour Ten A few years ago, Tan and others derived a set of exact relations valid for strongly interacting non-relativistic Fermi gases in the regime of short interaction range and large scattering length. Recent developments have shown that a central quantity in these identities, the so-called ``contact'' C, actually plays a crucial role in the characterization of these systems, as it determines multiple thermodynamic properties as well as linear-response sum rules. However, computing the ``contact'' presents a challenge as it requires non-perturbative methods such as Quantum Monte Carlo. In this contribution, we present our first results for C as a function of temperature in the limit of infinite scattering length. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P45.00003: Investigating the Effect of Density Inhomogenity on Photoemission Spectroscopy Tara Drake, John Gaebler, Rabin Paudel, Jayson Stewart, Deborah Jin Ultracold atomic gases realize clean and controllable model systems for investigating many-body quantum physics. However, trapped gases are intrinsically spatially inhomogeneous in their density, and in many cases, one would like to compare measurements of these systems with theoretical understanding for a homogeneous gas. In particular, density inhomogeneity can complicate the interpretation of data taken in momentum space, as the original spatial information is lost during time of flight expansion. The effect of density inhomogeneity due to a harmonic trapping potential is studied in a degenerate gas of 40K atoms. Using a method to select only the atoms in the center of the trap, we study how a more homogenous sample changes what can be seen in time of flight experiments, including photoemission spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P45.00004: Spectral functions in ultracold Fermi gases William Schneider, Mohit Randeria We study the fermion spectral function in the superfluid state across the BEC-BCS crossover and in the normal Fermi liquid phase in highly imbalanced Fermi gases. We focus on features that can be measured in momentum-resolved radio frequency spectroscopy experiments. We go beyond mean field theory and include the effects of Gaussian order parameter fluctuations in a manner that gives excellent agreement with asymptotically exact results for the $T=0$ equation of state in the BEC and BCS limits, as well as quantum Monte Carlo (QMC) results near unitarity. We show that sharp Bogoliubov quasiparticles, with a substantial coherent spectral weight, exist near unitarity. We argue that this is true generally even beyond the Gaussian approximation. In addition, quasiparticle scattering and interaction with collective modes produces incoherent spectral weight. We show that the dispersion is strongly renormalized at unitarity with its minimum shifted up from its mean field value $\sqrt{2 m \mu}$ and compare our results with existing QMC data. We discuss how the spectral function changes qualitatively compared with its mean field form as $1/(k_Fa)$ increases and the chemical potential changes sign. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P45.00005: Short-range correlations and entropy in ultracold atomic Fermi gases Zhenhua Yu, Georg Bruun, Gordon Baym We relate short-range correlations in ultracold atomic Fermi gases to the entropy of the system over the entire temperature, T, vs. coupling strength, .1/kF a, plane. In the low temperature limit the entropy is dominated by phonon excitations and the correlations increase as T4. In the BEC limit, we calculate a boson model within the Bogoliubov approximation to show explicitly how phonons enhance the fermion correlations. In the high temperature limit, we show from the virial expansion that the correlations decrease as 1/T. By interpolating between the two limits, we predict that the correlations reach a maximum at a finite temperature. We infer the general structure of the isentropes of the Fermi gas in the T,.1/kF a plane, and the temperature dependence of the correlations in the unitary, BEC, and BCS limits. Our results compare well with measurements of the correlations via photoassociation experiments at higher temperatures. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P45.00006: Momentum Resolved Radio Frequency Spectroscopy in Trapped Polarized Gases Kathryn Levin, Chih-Chun Chien, Hao Guo With recent advances in momentum resolved radio frequency (RF) spectroscopy, both experiment and theory, one can consider doing analogous experiments on polarized Fermi gases. In this talk we present predictions for the behavior of the fermionic spectral functions in the majority and minority bands. By truncating the integrated trap contributions at varying radii, the spectral functions will reflect the increase in the local polarization from nearly zero at the center to large values at the edges. We present predictions for these spectral functions and discuss their implications for future experiments.\\[4pt] [1] Stewart, J T et al. Nature 454, 744 (2008)\\[0pt] [2] Chen, Q. and Levin, K. Phys. Rev. Lett. 102, 190402 (2009)\\[0pt] [3] Chen, Q. et al. Rep. Prog. Phys. 72, 122501 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P45.00007: Observation of shock waves in a unitary Fermi gas James Joseph, Manas Kulkarni, Alexander Abanov, John Thomas We study the nonlinear hydrodynamics of a strongly interacting (unitary) Fermi gas comprising a 50-50 mixture of the lowest two hyperfine states of $^6$Li near a broad Feshbach resonance at 834 G. The gas is is cooled via forced evaporation in a cigar-shaped CO$_2$ laser trap with a repulsive optical sheet potential at the center creating two separate clouds. When the repulsive potential is turned off and the two clouds collide we observe exotic nonlinear hydrodynamics distinguished by the formation of a very sharp and stable density peak at the center of the trap and subsequent evolution into a box-like shape with sharp edges. We attribute these characteristics to shock-wave formation in the unitary gas. By solving the hydrodynamic equations numerically we can reproduce the time dependence of the observed density profiles. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P45.00008: Theory of shock waves in a unitary Fermi gas Alexander Abanov, Manas Kulkarni, James Joseph, John Thomas We present here evidence of shock wave formation during the collision of two clouds of a unitary Fermi gas. A unitary Fermi gas is an ideal test ground for extreme quantum hydrodynamics. By its very nature the system exhibits universality, i.e., the properties of the gas, constrained by an underlying scale invariance, depend entirely on the density and temperature. The nonlinear hydrodynamics exhibited in this experiment is understood by using a dimensionally reduced quasi- 1D form of the quantum hydrodynamic equations of motion. We found a near perfect agreement with the experiment. The evidence of shock wave formation is a hallmark of nonlinear physics in a universal quantum hydrodynamic system. The hydrodynamic approach works well deep in the nonlinear regime even at low density and for a system far from equilibrium. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P45.00009: Finite Size Effects in the BCS-BEC Crossover from Functional Renormalization Michael Scherer We apply the functional renormalization group approach to the BCS-BEC crossover for an ultracold gas of fermionic atoms. Formulated in terms of a scale-dependent effective action, the functional RG interpolates continuously between the atomic or molecular microphysics and the macroscopic physics on large length scales. A systematic derivative expansion provides for both a description of the many-body physics and its expected universal features as well as an accurate account of the few-body physics and the associated BEC and BCS limits. Furthermore we put the system into a finite volume and employ periodic boundary conditions to study the effect of a finite size on the few-body scattering physics as well as the critical temperature for the phase transition to superfluidity. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P45.00010: Effects of particle-hole channel on the behavior of BCS-BEC crossover Qijin Chen BCS-BEC crossover is effected by increasing pairing strength between fermions from weak to strong. Such pairing is associated primarily with the particle-particle channel. Effects of the particle-hole channel is often dropped. On the other hand, Gor'kov et al argued that the particle-hole channel can cause a substantial reduction in both Tc and the pairing gap. However, this result has largely been neglected until recent years when BCS-BEC crossover has been realized experimentally in ultracold Fermi gases. In this talk, we study the effects of the particle-hole channel on BCS-BEC crossover in a $G_0G$ scheme. While in the BCS limit, such effects may be approximated by a shift in the pairing strength, the situation becomes more complex as the interaction becomes stronger where the gap is no longer very small. References: Q.J. Chen, I. Kosztin, B. Janko, and K. Levin, Phys. Rev. Lett. 81, 4708 (1998); Q.J. Chen, J. Stajic, S.N. Tan, and K. Levin, Physics Reports 412, 1-88 (2005). [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P45.00011: Microscopic Approach to Viscosities in Superfluid Fermi Gases: From BCS to BEC Peter Scherpelz, Hao Guo, Dan Wulin, Chih-Chun Chien, Kathryn Levin We compute the shear viscosity, $\eta$, in a BCS-BEC crossover scheme which is demonstrably consistent, via sum rules, with conservation laws. The onset of a normal state pairing gap and the contribution from bosonic (non-condensed pair) degrees of freedom lead to a considerable reduction in the magnitude of these viscosities at general temperatures $T$. When quantitatively compared with shear viscosity experiments (we independently infer an estimated lifetime from radio frequency data) the agreement is reasonable, as is a comparison of $\eta/s$, where $s$ is the trap entropy density. Our fermionic picture is to be contrasted with that of others in the literature which presume that Goldstone bosons are crucial. As in conventional BCS superconductors, we show these Goldstone bosons do not couple to transverse probes such as the shear viscosity. As a result our calculated viscosity at low $T$ becomes arbitrarily small, rather than exhibiting the upturn predicted by others. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P45.00012: Triply degenerate quantum mixture of $^{41}$K, $^{40}$K and $^{6}$Li Peyman Ahmadi, Cheng-Hsun Wu, Ibon Santiago, Jee Woo Park, Martin Zwierlein We report the observation of a triply quantum degenerate mixture of $^{41}$K, $^{40}$K and $^{6}$Li atoms. It is demonstrated that bosonic $^{41}$K atom is an efficient coolant for sympathetic cooling of fermionic $^{40}$K and $^{6}$Li atoms. The $^{40}$K and $^{6}$Li mixture provides access to a strongly correlated Fermi-Fermi mixture allowing us to study superfluidity and Cooper pairing with imbalanced masses. We also present our investigation of $^{41}$K and $^{40}$K, a Bose-Fermi mixture where a 12 G p-wave resonance and a 40 G s-wave resonance are observed. Negligible differential gravitational sag between $^{41}$K and $^{40}$K makes these resonances excellent candidates for studying unexplored properties of Bose-Fermi mixtures such as Boson mediated Cooper pairing. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P45.00013: The superfluid-insulator transition in disordered Fermi gases near unitarity Sarang Gopalakrishnan Superfluids, whether composed of weakly interacting fermions (i.e., in the BCS limit) or bosons (i.e., in the BEC limit), undergo quantum phase transitions into an insulating phase in the presence of strong disorder. In the BCS limit, such a transition occurs when the disorder is strong enough to overcome the fermions' kinetic energy; in the BEC limit, it occurs when the disorder is strong enough to overcome the bosons' interaction energy. We address the fate of the disorder-driven superfluid-insulator transition in the intermediate ``unitary'' regime, discuss the conditions under which the superfluid-insulator phase boundary is non-monotonic in this regime, and investigate the properties of the insulating phase. Our analysis is quantitatively valid at high densities or for narrow Feshbach resonances, but its qualitative implications are expected to hold beyond these regimes; it can also be adapted to show that the superfluid-insulator transition occurs at infinitesimally weak disorder for a unitary Fermi gas in four dimensions. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P45.00014: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P45.00015: Population imbalance and pairing in the BCS-BEC crossover of three-component ultracold fermions Tomoki Ozawa, Gordon Baym We investigate the phase diagram and the BCS-BEC crossover of a homogeneous three-component ultracold Fermi gas with a U(3) invariant attractive interaction. We show that the system at sufficiently low temperatures exhibits population imbalance, as well as fermionic pairing. We describe the crossover in this system, connecting the weakly interacting BCS regime of the partially population-imbalanced fermion pairing state and the BEC limit with three weakly interacting species of molecules, including pairing fluctuations within a t-matrix calculation of the particle self-energies. [Preview Abstract] |
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