Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A16: Quantum Fermi Gases and Feshbach Resonances |
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Sponsoring Units: DAMOP Chair: Martin Zwierlein, Massachusetts Institute of Technology Room: 317 |
Monday, March 16, 2009 8:00AM - 8:12AM |
A16.00001: Theory of rf Spectroscopy in ultracold Fermi Gases William Schneider, Vijay Shenoy, Mohit Randeria We calculate the rf spectroscopy line shape for three states of the ultracold Fermi gas: (a) the equal spin population superfluid state in the BCS-BEC crossover, (b) the normal Fermi liquid state in a highly imbalanced gas, and (c) the normal Fermi liquid state for a repulsive, balanced gas. We address the question of how rf spectroscopy can make a sharp distinction between a normal Fermi liquid and a paired superfluid at $T = 0$. We also describe the role of final state interactions and of finite temperature effects. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A16.00002: BCS-BEC crossover in an optical lattice Parag Ghosh, Roberto Diener, Mohit Randeria We model fermions with an attractive interaction in an optical lattice with a single-band Hubbard model away from half-filling with on-site attraction $U$ and nearest neighbor hopping $t$. We use a large $N$ theory with Sp($2N$) symmetry to study the fluctuations beyond mean field theory. At $T=0$, we calculate across the crossover various observables, including chemical potential, gap, ground state energy, speed of sound and compressibility. The superfluid density $\rho_s$ is found to have non-trivial $U/t$ dependence in this lattice system. We show that the transition temperature $T_c$ scales with the energy gap in the weak coupling limit but crosses over to a $t^2/U$ scaling in the BEC limit, where phase fluctuations controlled by $\rho_s$ determine $T_c$. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A16.00003: Pseudogap phenomena in the BCS-BEC crossover regime of atomic Fermi gases Shunji Tsuchiya, Ryota Watanabe, Yoji Ohashi We study pseudogap behavior of atomic Fermi gases in the BCS-BEC crossover. Including paring fluctuations, we calculate the fermionic density of states above the superfluid transition temperature Tc, based on the strong coupling theory developed by Nozieres and Schmitt-Rink. We show that the gap structure appears in the density of states above Tc in the crossover region, and it evolves as the attractive interaction strength increases. We also clarify the temperature dependence of the pseudogap, which disappears as the temperature rises, and determine the pseudogap region in the phase diagram. We discuss the origin of the pseudogap by examining the behaviors of quantities such as spectral function and self-energy. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A16.00004: Universal properties of ultracold Fermi gases Shizhong Zhang, Anthony Leggett We present some general considerations on the properties of a two-component ultra-cold Fermi gas along the BEC-BCS crossover. It is shown that the interaction energy and the free energy can be written in terms of a single dimensionless function $h({\xi,\tau})$, where $\xi=-(k_Fa_s)^{-1}$ and $\tau=T/T_F$. The function $h(\xi,\tau)$ incorporates all the many-body physics and naturally occurs in other physical quantities as well. In particular, we show that the average rf-spectroscopy shift $\overline{\d\o}(\xi,\tau)$ and the molecular fraction $f_c(\xi,\tau)$ in the closed channel can be expressed in terms of $h(\xi,\tau)$ and thus have identical temperature dependence. The conclusions should have testable consequences in future experiments. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A16.00005: Probing the Spectral Function Using Momentum Resolved Radio Frequency Spectroscopy in Trapped Fermi Gases Qijin Chen, Kathy Levin A measurement of the centrally important spectral function has not been possible in ultracold Fermi gases until recent momentum resolved radio frequency (RF) spectroscopy experiments in $^{40}$K. These new experiments can be exploited to test many-body theories underlying general quantum simulations performed on Fermi gases. Here we establish the underlying physics of these RF measurements. We show that, by providing a clear dispersion signature of pairing, they remove ambiguity plaguing the interpretation of previous RF studies. Our calculated spectral intensities are in semi-quantitative agreement with the data. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A16.00006: Temperature and final state effects in radio frequency spectroscopy experiments on atomic Fermi gases Yan He, Chih-chun Chien, Qijin Chen, Kathy Levin We present a simple and systematic characterization of the radio frequency (RF) spectra of homogeneous, paired atomic Fermi gases at general temperatures, $T$, in the presence of final state interactions. The spectra, consisting of possible bound states and positive as well as negative detuning ($\nu$) continua, satisfy exactly the zeroth- and first-moment sum rules at all $T$. We show how to best extract the pairing gap and how to detect the $\nu <0$ continuum arising from thermally excited quasiparticles, not yet seen experimentally. We explain semi-quantitatively recent RF experiments on ``bound-bound'' transitions, predicting effects of varying temperature. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A16.00007: The Higgs resonance in fermionic condensates Roman Barankov The Higgs mode appears in the spectrum of fermionic condensates described by the BCS model as a result of the energy dispersion of interaction. Specifically, the mode enters the spectral gap of quasi-particle excitations when the pairing of fermions is enhanced at the Fermi energy. Conversely, it becomes a resonance in the quasi-particle continuum with a finite lifetime, when the pairing is suppressed on the energy scale small compared to the equilibrium gap. The exponential decay of the mode converts into algebraic decay for a smooth suppression. We confirm our analytical results by numerical analysis of the pairing dynamics. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A16.00008: Dynamic Cluster Monte Carlo Study of the Single-Particle Spectra of Strongly-Interacting Fermion Gases Shiquan Su, Daniel E. Sheehy, Juana Moreno, Mark Jarrell We study Feshbach-resonantly interacting fermions near unitarity within the context of the attractive Hubbard model. Our principal focus is the single-particle spectral function for such strongly-interacting fermions, recently probed in radio-frequency spectroscopy and photoemission experiments in cold-atom systems. To obtain quantitatively-accurate results on unitary gases, we apply the Dynamical Cluster Approach (DCA) and the Maximum Entropy method to study this system both in the pair-formation temperature region and in the low-temperature condensed state. Different Quantum Monte Carlo approaches emphasizing different observables are used as the quantum solver in the DCA approach, and the data from different approaches are compared to each other. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A16.00009: Feshbach-Einstein condensates Valy Rousseau, Peter Denteneer We investigate the phase diagram of a two-species Bose-Hubbard model describing atoms and molecules on a lattice, interacting via a Feshbach resonance. We identify a region where the system exhibits an exotic super-Mott phase and regions with phases characterized by atomic and/or molecular condensates. Our approach is based on a recently developed exact quantum Monte Carlo algorithm, the Stochastic Green Function (SGF) algorithm with tunable directionality. We confirm some of the results predicted by mean-field studies, but we also find disagreement with these studies. In particular, we find a phase with an atomic but no molecular condensate, which is missing in all mean-field phase diagrams. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A16.00010: Strongly-correlated fermionic matter in the dilute limit Bogdan Mihaila, Andres Cardenas We study ``the ground-state properties of the many-body system composed of spin-1/2 fermions interacting via a zero-range, infinite scattering length contact interaction.'' The above is referred to sometimes as the \emph{George Bertsch problem}, and is of particular interest in astrophysics in connection with the equation of state for neutron matter and has been revisited recently with the advent of experimental studies of the BCS to BEC crossover in ultracold fermionic atom gases. We will show that new insights into the solution to this problem are obtained in the context of a coupled-cluster (exp S) expansion approach to calculating the equation of state for dilute fermionic systems and that present state-of-the-art Monte Carlo calculations have not yet provided the definitive answer. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A16.00011: Superfluid equation of state of dilute composite bosons or how to include $3$ and $4$-body problems in the {\it many}\,body problem Xavier Leyronas, Roland Combescot We show how the $3$ and $4$-body problems emerge in the BEC limit of the BEC-BCS crossover, where we treat explicitely dimers as made of two fermions. We give the argument leading, at zero temperature, to the calculation of the equation of state. We find that, when expanding the chemical potential in powers of the density $n$ up to the Lee-Huang-Yang order, proportional to $n^{3/2}$, the result is identical to the one of elementary bosons in terms of the dimer-dimer scattering length $a_M$, the composite nature of the dimers appearing only in the next order term proportional to $n^2$. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A16.00012: A study of momentum entanglement and negativity in Bardeen-Cooper-Schrieffer states at finite temperature Chun KIt Chung, Chi Kwong Law We study the momentum entanglement between the spin-up and spin- down particles of the homogeneous Bardeen-Cooper-Schrieffer (BCS) state at finite temperature. To achieve this, we construct from the BCS state the partial transposition $\rho_2^{\mathrm{T} _A}$ of the two particle density matrix in momentum space. The structure of $\rho_2^{\mathrm{T}_A}$ and its corresponding negativity ${\cal N}_2$ are examined. We show that $\rho_2^ {\mathrm{T}_A}$ consists of infinitely many decoupled $2 \times 2 $ submatrices, and momentum entanglement coexists with the pairing order parameter $\Delta$. It is found that pairs with momenta slightly above a surface related to the Fermi energy contribute this entanglement most significantly. We propose an entanglement witness operator as a measurable quantity to detect momentum entanglement in BCS states. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A16.00013: Matterwave Probe for Detecting Fermi Superfluidity Satyan Bhongale, Han Pu We propose a matter wave probe for detecting BCS type superfluidity within a trapped two-component Fermi gas. While, previous theoretical/experimental attempts have addressed Fermi superfluidity via a global measurement, for example by demonstrating a vortex lattice, the current proposal allows for a local measurement of the pairing gap. For this, we study the phase diagram of a mixture of Bose-Einstein condensate and an interacting two-component Fermi gas. We identify regions of the parameter space where the Bose-Fermi mixture is unstable resulting in phase separation. We show that, under proper conditions, by employing a tunable scattering resonance, the phase separation phenomenon can be exploited as a robust probe of ``local'' fermion superfluidity. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A16.00014: Detecting Onset of BCS-Superfluidity Using a BEC Probe B. Ramachandhran, S.G. Bhongale, H. Pu Recent experiments\footnote{Zwierlein et al. Nature 435, 1047, (2005), and references therein} have used Feshbach resonance to tune the interactions in a two-component ultracold Fermi gas to obtain (Bardeen-Cooper-Schrieffer) BCS-type pairing and hence superfluidity. For this degenerate gas, we propose using Bose Einstein Condensate (BEC) as a matter wave probe of the BCS superfluid state. Towards this end, we explore the phase diagram of a 3-dimensional mixture of BEC and a two-component superfluid fermi gas at finite temperature. In particular, we identify the regime in which the homogenous mixture becomes unstable against phase separation. We show that, under proper conditions, this spatial phase separation phenomenon occuring in the presence of the BEC can be used to probe the ``local'' value of the superfluid Gap parameter and possibly help detect the onset temperature of the BCS superfluidity. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A16.00015: ABSTRACT WITHDRAWN |
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