Session J19: Invited Session: Strongly Interacting Cold Fermi Gases

From ultracold Fermi Gases to Neutron Stars

Ultracold dilute atomic gases can be considered as model systems to address some pending problem in Many-Body physics that occur in condensed matter systems, nuclear physics, and astrophysics. We have developed a general method to probe with high precision the thermodynamics of locally homogeneous ultracold Bose and Fermi gases [1,2,3]. This method allows stringent tests of recent many-body theories. For attractive spin 1/2 fermions with tunable interaction ($^{6}$Li), we will show that the gas thermodynamic properties can continuously change from those of weakly interacting Cooper pairs described by Bardeen-Cooper-Schrieffer theory to those of strongly bound molecules undergoing Bose-Einstein condensation. First, we focus on the finite-temperature Equation of State (EoS) of the unpolarized unitary gas. Surprisingly, the low-temperature properties of the strongly interacting normal phase are well described by Fermi liquid theory [3] and we localize the superfluid phase transition. A detailed comparison with theories including recent Monte-Carlo calculations will be presented. Moving away from the unitary gas, the Lee-Huang-Yang and Lee-Yang beyond-mean-field corrections for low density bosonic and fermionic superfluids are quantitatively measured for the first time. Despite orders of magnitude difference in density and temperature, our equation of state can be used to describe low density neutron matter such as the outer shell of neutron stars. \\[4pt] [1] S. Nascimb\`{e}ne, N. Navon, K. Jiang, F. Chevy, and C. Salomon, \textit{Nature} \textbf{463}, 1057 (2010) \\[0pt] [2] N. Navon, S. Nascimb\`{e}ne, F. Chevy, and C. Salomon, \textit{Science} \textbf{328}, 729 (2010) \\[0pt] [3] S. Nascimb\`{e}ne, N. Navon, S. Pilati, F. Chevy, S. Giorgini, A. Georges, and C. Salomon, Phys. Rev. Lett. \textbf{106}, 215303 (2011)   

Spin-Imbalance in One and Three-Dimensional Fermi Gases

The FFLO modulated superconductor state was independently proposed by Fulde and Ferrell, and Larkin and Ovchinnikov, as a way of accommodating the excess spin in a spin-polarized superconductor. The pairs in the FFLO state have non-zero center-of-mass momentum, which can produce a periodic structure with a spatially-modulated order parameter. Although there is some evidence for FFLO pairing in certain heavy fermion compounds that are able to accommodate both magnetic and superconducting order, conclusive experimental proof remains elusive. Motivated by the search for exotic paired states, we have performed experiments with spin-imbalanced ultracold atomic Fermi gases in both 3D and 1D. We use two hyperfine levels of $^6$Li to emulate the spin-up and down states. The $s$-wave pairing interactions are controlled via a magnetically-tuned collisional (Feshbach) resonance. In 3D, we find that the gas phase separates into an evenly paired BCS-like core, surrounded by the excess spin-up atoms [1]. For the 1D experiment, a two-dimensional optical lattice was used to create an array of 1D tubes that are each filled with $\sim$200 atoms. The weak axial confinement again produces a phase separation, but in contrast to 3D we find a partially-polarized central core surrounded by either fully-paired or fully-polarized wings, depending on the degree of overall spin-polarization [2]. Theory predicts that the partially-polarized phase is an FFLO superfluid. We are currently trying to obtain direct evidence for FFLO pairing, which may be revealed in the pair momentum distribution found by releasing the atoms axially and allowing them to expand in time-of-flight. We are also exploring the 1D-3D dimensional crossover that occurs when the coupling between tubes is reduced. \\[4pt] [1] G.B. Partridge \emph{et al}, Science \textbf {311}, 503 (2006). \\[0pt] [2] Y.A. Liao \emph{et al}, Nature \textbf {467}, 567 (2010).   

Onset of a Pseudogap Regime in Ultracold Fermi Gases

We show, using an ab initio approach based on Quantum Monte Carlo technique, that the pseudogap regime emerges in ultracold Fermi gases close to the unitary point. We locate the onset of this regime at a value of the interaction strength slightly to the BCS side of the unitary point. We determine the evolution of the gap as a function of temperature and interaction strength in the Fermi gas around the unitary limit and show that our results exhibit a remarkable agreement with the recent wave-vector resolved radio frequency spectroscopy data. Our results indicate that a finite temperature structure of the Fermi gas around unitarity is complicated and involves the presence of preformed Cooper pairs, which however do not contribute to the long range order.   

Strongly Repulsive Quantum Gases

Advances in cold atom experiments have shown that the properties of repulsive Fermi and Bose gases are far more intricate than generally expected, as these systems can produce molecules even in the weakly interacting regime. Recent experiments, however, reveal some general yet puzzling properties of these gases in the strongly repulsive regime. In this talk, we show that these properties are direct consequences of statistics, and are fundamental properties of quantum gases [1]. We shall also discuss the related issue of itinerant ferromagnetism, and discuss the physical settings in which ferromagnetism can be found. \\[4pt] [1] V. B. Shenoy and Tin-Lun Ho, arXiv: 1106.0960, to appear in Physical Review Letters