Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T40: Strongly Interacting Quantum Gases 
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Sponsoring Units: DAMOP Chair: Qi Zhou Room: 349 
Thursday, March 21, 2013 8:00AM  8:12AM 
T40.00001: Symmetry methods for harmonically trapped, interacting particles Nathan Harshman We present a new method for exploiting the symmetries of interacting fewbody systems trapped in harmonic potentials to achieve efficient numerical calculations of energy eigenstates. Precision experiments with ultracold atoms trapped in deep optical wells, as well as connections to recombination loss rates in trapped BECs, have driven experimental interest in this topic. Our method has two key elements. First, transformations from the particle observables into the centerofmass/Jacobi observables can be implemented using the $\mathrm{U}(Nd)$ symmetries of $N$ harmonic oscillators in $d$ dimensions. Second, particle exchange symmetries are realized geometrically as orthogonal transformations in Jacobi relative hypercoordinates. Despite this apparent mathematical complexity, the results are easy to implement and interpret, and the method provides simple classifications of particle clustering in configurations and eigenstates. As a side benefit, the entanglement spectroscopy of fewbody systems with tunable interactions can be explored. [Preview Abstract] 
Thursday, March 21, 2013 8:12AM  8:24AM 
T40.00002: Unitary thermodynamics calculated from thermodynamic geometry George Ruppeiner Degenerate atomic Fermi gases of atoms near a Feshbach resonance show universal thermodynamic properties, which are here calculated with the geometry of thermodynamics, and the thermodynamic curvature $R$. Unitary thermodynamics is expressed as the solution to a pair of ordinary differential equations, a ''superfluid'' one valid for small entropy per particle $z\equiv S/N k_B$, and a ''normal'' one valid for large $z$. These two solutions are joined at a secondorder phase transition at $z=z_c$. Define the internal energy per particle in units of the Fermi energy as $Y=Y(z)$. For small $z$, $Y(z)=y_0+y_1 z^{\alpha }+y_2 z^{2 \alpha}+\cdots,$ where $\alpha$ is a constant exponent, $y_0$ and $y_1$ are scaling factors, and the series coefficients $y_i$ ($i\ge 2$) are determined uniquely in terms of $(\alpha, y_0, y_1)$. For large $z$ the solution follows uniquely if, in addition, we specify $z_c$, with $Y(z)$ diverging as $z^{5/3}$. The four undetermined parameters $(\alpha,y_0,y_1,z_c)$ were determined by fitting the theory to experimental data taken by a Duke University group on $^6$Li in an optical trap with a Gaussian potential. The best fit of this theory to the data has $\chi^2\sim1$. [Preview Abstract] 
Thursday, March 21, 2013 8:24AM  8:36AM 
T40.00003: Density and particlehole fluctuation effects on the position of Feshbach resonances in atomic Fermi gases Qijin Chen Feshbach resonances have been the key to achieve tunable effective pairing interaction strength in atomic Fermi gases. Most important experiments, as well as their theoretical explanations, rely on precise determination of the locations of these resonances. For the extensively studied $^6$Li and $^{40}$K Fermi gases, the positions of the widely used $s$wave Feshbach resonances have been regarded as being measured with high precision. In this talk, we show that due to inevitable particlehole fluctuations, there is a significant density effect on the resonance locations. For a $^6$Li gas with a realistic $T_F = 1$ $\mu$K, the shift in location in terms of magnetic field can be as high as 8G at low temperature $T$, and this effect does not necessarily go away at high $T$. This will cause important consequences as to whether and how the scattering length taken from the literature need to be recalibrated for the concrete parameters specific to a given experiment. References: Q.J. Chen, arXiv:1109.2307. [Preview Abstract] 
Thursday, March 21, 2013 8:36AM  8:48AM 
T40.00004: Scale Invariance in 2D BCSBEC Crossover Rajdeep Sensarma, Edward Taylor, Mohit Randeria In 2D BCSBEC crossover, the frequency of the breathing mode in a harmonic trap , as well as the lower edge of the radio frequency spectroscopy response, show remarkable scaleinvariance throughout the crossover regime, i.e. they are independent of the coupling constant. Using functional integral methods, we study the behaviour of these quantities in the 2D BCSBEC crossover and comment on the possible reasons for this scale independence. [Preview Abstract] 
Thursday, March 21, 2013 8:48AM  9:00AM 
T40.00005: Apparent LowEnergy Scale Invariance in TwoDimensional Fermi Gases Edward Taylor, Mohit Randeria Recent experiments on a 2D Fermi gas find an undamped breathing mode oscillating at twice the trap frequency over a wide range of parameters [1]. To understand this seemingly scaleinvariant behavior in a system with an energy scale, the dimer binding energy, we derive two exact results valid across the entire BCSBEC crossover at all temperatures [2]. We relate both the shift of the mode frequency from its scaleinvariant value as well as a sum rule characterizing the lowenergy spectral weight in the bulk viscosity to a single parameter. This parameter characterizes the deviation from scale invariance at low energies and remarkably, vanishes exactly at zero temperature within meanfield BCS theory. Only thermal and quantum fluctuations contribute a nonzero value for this parameter and hence, break the lowenergy, effective scale invariance. We discuss reasons why, in 2D with an interaction that depends logarithmically on the density, these fluctuations contribute very weakly. \\[4pt] [1] E. Vogt, M. Feld, B. Frohlich, D. Pertot, M. Koschorreck, and M. Kohl, Phys. Rev. Lett. 108, 070404 (2012). \\[0pt] [2] E. Taylor and M. Randeria, Phys. Rev. Lett. 109, 135301 (2012). [Preview Abstract] 
Thursday, March 21, 2013 9:00AM  9:12AM 
T40.00006: Probing the Contact Locally in a Trapped Unitary Fermi Gas Rabin Paudel, Yoav Sagi, Tara Drake, Deborah Jin The inherent density inhomogeneity of a trapped gas can complicate interpretation of experiments and can wash out sharp features. This is especially important for a Fermi gas, where interaction effects as well as the local Fermi energy, or Fermi momentum, depend on the density. We report on experiments that use optical pumping with shaped light beams to spatially select the center part of a trapped gas for probing. This technique is compatible with momentum resolved measurements. For a weakly interacting Fermi gas of $^{40}$K atoms, we present measurements of the momentum distribution that reveal for the first time a sharp Fermi surface. We then apply this technique to a strongly interacting Fermi gas at the Feshbach resonance, where we measured the temperature dependence of the Tan's contact locally in the trapped gas. [Preview Abstract] 
Thursday, March 21, 2013 9:12AM  9:24AM 
T40.00007: Magnetic properties and pseudogap phenomenon in an ultracold Fermi gas with population imbalance Takashi Kashimura, Ryota Watanabe, Yoji Ohashi We discuss the magnetic properties of an ultracold Fermi gas with population imbalance. In the unpolarized case, the photoemisson spectroscopy have observed a gaplike (pseudogap) structure in the normal state above the superfluid phase transition temperature, such an anomalous structure has not been detected in the highlypolarized regime. In this talk, we discuss how the poseudogap phenomenon is affected by the polarization of the system. Within the framework of an extendend $T$matrix theory, we calculate the polarization dependence of DOS to show that the pseudogap gradually disappeas with increasing the polarization rate. In a highlypolarized regime, the system is simply described as a gas of longlived quasiparticles. We also show that the calculated polarization as a function of an effective ``magnetic'' field $h=(\mu_\uparrow  \mu_\downarrow)/2$ agrees well with the experimental data [where $\mu_\sigma$ is the chemical potential of atoms with pseudospin $\sigma(=\uparrow, \downarrow)$]. [Preview Abstract] 
Thursday, March 21, 2013 9:24AM  9:36AM 
T40.00008: Spin Diffusion in a Cold Fermi Gas Close to Unitarity Hua Li, Kevin Bedell, Jason Jackiewicz We study the transport properties of a normal two component Fermi gas with strong attractive interactions close to the unitary limit. In particular, we compute its spin diffusion coefficient in the extreme low temperature limit. To calculate the spin diffusion coefficient we need the scattering amplitudes. The scattering amplitudes are calculated from the Landau parameters. These parameters are obtained from the local version of the induced interaction model for computing Landau parameters. The leading order finite temperature corrections to the spin diffusion coefficient are also calculated. At temperatures close to the BCS transition temperature, pairing fluctuations are considered in calculating the scattering amplitudes. A minimum is found on the calculated temperature dependent spin diffusion coefficient curve. The position and magnitude of this minimum is sensitive to the Landau parameter F0a. Upon choosing a proper value of F0a, we are able to present a good match between the theoretical result and the experimental measurement which has a minimum with a value of order h/m being observed at some finite temperature below the Fermi temperature. [Preview Abstract] 
Thursday, March 21, 2013 9:36AM  9:48AM 
T40.00009: FuldeFerrellLarkinOvchinnikov states in FermiFermi mixtures Jibiao Wang, Qijin Chen FuldeFerrellLarkinOvchinnikov (FFLO) states have been of great interest in the study of population imbalanced atomic Fermi gases. It has been known that the phase space of FFLO states for an equalmass Fermi gas in three dimension (3D) is rather small and thus has not been observed experimentally. In this talk, we will explore possible effects of mass imbalance as in a FermiFermi mixture on the FFLO phases for a 3D homogeneous case. In particular, we will use a pairing fluctuation theory in which incoherent pairing fluctuations constitute a key ingredient of the theory and thus lead naturally to the appearance of a pseudogap when the pairing interaction becomes strong. We will present various phase diagrams related to the FFLO states at both zero and finite temperatures, throughout the BCSBEC crossover, and show that a large mass ratio may indeed enhance FFLO type of pairing and make it easier to detect such states experimentally. References: Y. He, C.C. Chien, Q.J. Chen, and K. Levin, Phys. Rev. A 75, 021602(R) (2007); Q.J. Chen, Y. He, C.C. Chien, and K. Levin, Phys. Rev. B 75, 014521 (2007). [Preview Abstract] 
Thursday, March 21, 2013 9:48AM  10:00AM 
T40.00010: Observation of Feshbach resonances between ultracold Na and Rb atoms Fudong Wang, Dezhi Xiong, Xiaoke Li, Dajun Wang Absolute groundstate $^{23}$Na$^{87}$Rb molecule has a large electric dipole moment of 3.3 Debye and its two body exchange chemical reaction is energetically forbidden at ultracold temperatures. It is thus a nice candidate for studying quantum gases with dipolar interactions. We have built an experiment setup to investigate ultracold collisions between Na and Rb atoms as a first step toward the production of ground state molecular samples. Ultracold mixtures are first obtained by evaporative cooling of Rb and sympathetic cooling of Na. They are then transferred to a crossed dipole trap and prepared in different spin combinations for Feshbach resonance study. Several resonances below 1000 G are observed with both atoms prepared in either $\leftF = 1, m_F = 1\right\rangle$ or $\leftF = 1, m_F = 1\right\rangle$ hyperfine states. Most of them are within 30 G of predicted values$^{\S}$ based on potentials obtained by high quality molecular spectroscopy studies. This work is supported by RGC Hong Kong. \\\noindent ${\S}$ E. Tiemann, private communications [Preview Abstract] 
Thursday, March 21, 2013 10:00AM  10:12AM 
T40.00011: Quantum Phase Transitions in a BoseFermi Mixture Eric Duchon, Shizhong Zhang, SoonYong Chang, Mohit Randeria, Nandini Trivedi Motivated by the recent experimental realization of stable BoseFermi mixtures with broad Feshbach resonances, we investigate possible quantum phases and phase transitions in this system using variational Monte Carlo. Within a singlechannel model appropriate near broad Feshbach resonances, we show that as the bosonfermion coupling increases, the BoseEinstein condensate disappears and the atomic Fermi surface is destroyed while the Fermi surface of the composite molecules emerges. We calculate the momentum distribution of atomic and molecular fermions and demonstrate that the atomic fermion's quasiparticle weight $Z$ vanishes at a critical coupling. [Preview Abstract] 
Thursday, March 21, 2013 10:12AM  10:24AM 
T40.00012: From the Cooper problem to canted supersolids in BoseFermi mixtures Lode Pollet, Peter Anders, Philipp Werner, Matthias Troyer, Manfred Sigrist We calculate the phase diagram of the BoseFermi Hubbard model on the $3d$ cubic lattice at fermionic half filling and bosonic unit filling by means of singlesite dynamical meanfield theory (DMFT). For fast bosons, this is equivalent to the Cooper problem in which the bosons can induce $s$wave pairing between the fermions. We also find miscible superfluid and canted supersolid phases depending on the interspecies coupling strength. In contrast, slow bosons favor fermionic charge density wave structures for attractive fermionic interactions. These competing instabilities lead to a rich phase diagram within reach of cold gas experiments. [Preview Abstract] 
Thursday, March 21, 2013 10:24AM  10:36AM 
T40.00013: Closed Channel Amplitude in a Many Body Feshbach System Nicolas Lopez, Eddy Timmermans, ShanWen Tsai Near a narrow Feshbach resonance (with magnetic field width 10 mG or smaller) the ultracold atom interactions acquire an effective range that can be comparable to the average interparticle distance. Although requiring a more accurate magnetic field control than their broad counterparts, the narrow Feshbach resonances can free cold atom physics from its straightjacket of the contact interaction paradigm. The finiterange effects can give rise to roton features in the phonon dispersion of dilute BoseEinstein condensates (BEC's) and BEC's can support a ground state with modulated density patterns that breaks translational symmetry. We show that the finite range interaction is the consequence of the timedelay in atomatom collisions. The narrow regime is also the parameter region in which the interacting atoms can spend a significant fraction of their time in the spinrearranged (also called ``closed'') channel. To study the interaction physics we describe two atoms in a harmonic trap, interacting near a narrow resonance. We find the fraction of time that the atoms spend in the closed channel at fixed magnetic field and we extend this result to estimate the fraction of time that a distinguishable atom moving through a BEC spends in the closed channel state, quasibound to BECatoms. [Preview Abstract] 

T40.00014: ABSTRACT WITHDRAWN 
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