Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session K2: The BCS-BEC Crossover in Trapped Fermi Gases |
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Chair: Pierre Meystre, University of Arizona Room: Burnham Yates Conference Center Ballroom II |
Friday, May 20, 2005 10:30AM - 11:06AM |
K2.00001: Molecular Probe of the BCS/BEC Crossover in $^6$Li Invited Speaker: We have used the broad Feshbach resonance in $^6$Li to create a strongly-interacting Fermi gas and to study the crossover between a Bose-Einstein condensate (BEC) of diatomic molecules and a superfluid of paired fermions. The atoms are prepared in a two-spin component mixture that can be brought into a collisional resonance with a bound molecular state by tuning a magnetic field. A pure BEC of weakly-bound ``dressed'' molecules is created on the low-field side of the resonance. These dressed molecules are a hybridization of pairs of free atoms in the triplet channel and the bound state in the singlet channel. The details of this hybridization have significant implications for the nature of a Fermi superfluid near a Feshbach resonance as it determines the nature of the pairs; that is, whether they are small and molecular in character, or large extended objects akin to Cooper pairs. We use optical molecular spectroscopy to probe the molecular content of the many-body state at magnetic fields near the Feshbach resonance. The optical probe projects the closed, singlet-channel molecular character of the paired state onto a vibrational level of an excited molecule. We find that the molecular contribution in the crossover region can be $\sim$10\%, which is much larger than expected. This large bare molecule fraction is seemingly unexplained by two-body physics. We expect that the large molecular content will have a significant effect on the properties of a resonant Fermi superfluid, including its critical temperature and excitation spectrum. We have also observed that the gas responds to the optical probe when the probe frequency is tuned to the bound-bound bare molecule transition, rather than to the transition from the dressed molecules. The latter transition frequency is magnetic field dependent, while the former is not. When driving the bound-bound transition, the condensate undergoes coherent oscillations as a function of the probe duration that persist for many ms. Although not fully understood, we believe that these oscillations are a collective response of the paired gas. [Preview Abstract] |
Friday, May 20, 2005 11:06AM - 11:42AM |
K2.00002: Feshbach Molecule Formation in Finite-Temperature Quantum Gases Invited Speaker: An exciting development in the field of ultracold atomic gases is the ability to create diatomic molecules by adjusting a Feshbach resonance in the interatomic potential. An extraordinary application of this capability has been to dynamically traverse the BEC-BCS crossover in Fermi gases. While a great deal of attention has focused on equilibrium properties in the {\it{superfluid}} regime, a complete theoretical understanding of the dynamics of molecule formation in a {\it{normal}} gas is still lacking. In a recent article [Williams {\it{et al.}}, J. Phys. B: At. Mol. Opt. Phys. {bf{37}}, L351 (2004)], we presented coupled Boltzmann-like kinetic equations for the atoms and molecules. In this talk, we use our approach to understand the saturation behavior of the molecular conversion efficiency that is observed in experiments (eg. Hodby {\it{et al.}}, cond- mat/0411487). [Preview Abstract] |
Friday, May 20, 2005 11:42AM - 12:18PM |
K2.00003: Pairing of fermionic atoms in a strongly interacting quantum gas Invited Speaker: The formation of composite bosons by pairing fermions leads to intriguing phenomena in physics, with superconductivity and $^3 $He superfluidity being prominent examples. In ultracold gas of fermionic atoms, formation and condensation of diatomic molecules are recently realized near magnetically tunable Feshbach resonances. This achievement opens exciting possibilities to explore the crossover from molecular Bose- Einstein condensate (BEC) to fermionic superfluid in the Bardeen- Cooper-Schrieffer (BCS) state. We report on the observation of the pairing gap in an ultracold gas of $^6$Li atoms by a precision radio frequency spectroscopy. Starting with a molecular Bose-Einstein condensate, we control the two-body interactions and monitor the binding energy of the atom pairs as the system enters the BEC-BCS crossover regime. The dependence of the pairing gap on the temperature and the Fermi energy in the crossover regime provides strong evidences that a molecular Bose-Einstein condensate with positive scattering lengthes can be smoothly and isentropically converted into a fermionic superfluid with negative scattering lengthes. We propose a mean-field approach to describe the atom pairs in the strong interaction regime. By introducing an effective potential which characterizes the overlap of the pair wave functions, the mean field equation allows us to calculate the chemical potential and the equation of states. The results excellently agree with the recent quantum Monte Carlo calculations. We show that the smooth crossover from the bosonic mean field interactions between molecules to the Fermi pressure among atoms is associated with the evolution of the atomic correlation function. [Preview Abstract] |
Friday, May 20, 2005 12:18PM - 12:54PM |
K2.00004: Density Profiles And Thermodynamics In Strongly Interacting Fermi Gases Invited Speaker: We study trapped Fermi gases near Feshbach resonances at general temperatures $T$ to determine their density profiles and thermodynamics. The theoretical formalism (based on the concept of BCS-BEC crossover physics applied to the conventional mean field ground state) and the experimental comparisons are highlighted in this talk. That the profiles fit a Thomas-Fermi (TF) functional form is a key observation which derives from including the, generally ignored, non-condensed pair excitations (along with the usual gapped fermionic excitations, as well as condensed pairs). We demonstrate how a TF fit to experimental profiles can be used to extract a physical temperature scale, thus setting the groundwork for analyzing experimental thermodynamical data. We discuss the low temperature power laws in the entropy and energy $E(T)$ associated with both fermionic and non-condensed pair excitations of the condensate and demonstrate very good agreement with recent measurements of both the density profiles and $E(T)$ at all $T$. \\ This talk is based on the following two papers: Jelena Stajic, Qijin Chen and K. Levin, cond-mat/0408104 and Phys. Rev. Lett (to be published) and J. Kinast, A. Turlapov, J.E. Thomas, Qijin Chen, Jelena Stajic and K. Levin, published online in Science Express on January 27, 2005 [DOI:10.1126/science.1109220], to be published in Science. [Preview Abstract] |
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