Session H1: Fermi Gases and Magnetism

Spin Susceptibility of a Fermi Gas with Strong Repulsive Interactions

The possibility of itinerant ferromagnetism in degenerate Fermi gases with strong repulsive interactions has recently been the subject of vigorous theoretical and experimental work. In this study we characterize a nonequilibrium system shortly after the sudden switch-on of strong repulsion. Using the recently-developed technique of speckle imaging, we measure its spin susceptibility and the corresponding pair correlations. As expected, the system develops substantial anticorrelations between the two spin species, but our observations are inconsistent with significant formation of ferromagnetic domains within the samples studied.   

Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture

We report on the expansion of an ultracold Fermi-Fermi mixture of $^6$Li and $^{40}$K under conditions of strong interactions controlled via an interspecies Feshbach resonance. We study the expansion of the mixture after release from the trap and, in a narrow magnetic field range, we observe two phenomena related to hydrodynamic behavior. The common inversion of the aspect ratio is found to be accompanied by a collective effect where both species stick together and expand jointly despite of their widely different masses. Our work constitutes a major experimental step for a controlled investigation of the many-body physics of this novel strongly interacting quantum system.   

Universal features of strongly polarized mass imbalanced fermi gases

I will discuss the universal phase diagram of strongly polarized two-component fermi gases as a function of mass imbalance r between the two species. Concretely, I will describe the ground state of a single ``spin- down'' impurity atom interacting attractively with a ``spin-up'' atomic Fermi gas. By constructing variational wave functions for polarons, molecules and trimers, we performed a detailed study of the transitions between each of these dressed bound states as a function of mass ratio and interaction strength. We found that the presence of a Fermi sea enhances the stability of the p-wave trimer, which can be viewed as a Fulde-Ferrell-Larkin- Ovchinnikov (FFLO) molecule that has bound an additional majority atom. For sufficiently large r, the transitions lie outside the region of phase separation in imbalanced Fermi gases and should thus be observable in experiment, unlike the well- studied equal-mass case.   

Evaporative Depolarization and Deformation of 3D Imbalanced Fermi Gases

We previously observed phase separation of a spin-imbalanced 3D Fermi gas, with a spatially deformed paired core surrounded by a shell of polarized atoms.\footnote{G. B. Partridge \textit{et al.}, \textit{Science} \textbf{311}, 503 (2006); G. B. Partridge \textit{et al.}, \textit{PRL} \textbf{97}, 190407 (2006).} By studying the dynamics of evaporation, we find that trap depth anisotropy and fast evaporation produces the deformation. The deformed state is remarkably metastable, with a superfluid-normal transition which is extended to a much higher polarization than observed in the MIT and ENS experiments.\footnote{M. Zwierlein \textit{et al.}, \textit{Science} \textbf{311}, 492 (2006); S. Nascimb$\grave{e}$ne \textit{et al.}, \textit{PRL} \textbf{103}, 170402 (2009).} Our findings confirm the evaporative depolarization mechanism suggested previously,\footnote{M. M. Parish \textit{et al.}, \textit{PRA} \textbf{80}, 063605 (2009)} in which preferential evaporation from the center of an elongated trap reduces the chemical potential difference in the central core relative to the surrounding polarized phases.   

Polarized fermionic superfluidity in the crossover from 3D to 1D

We present results for a self-consistent bogoliubov-deGennes treatment of polarized fermionic superfluid at strong interaction in an anisotropic trap with cylindrical symmetry. Our studies track the nature of superfluidity as the radial confinement is increased and system goes from 3 to 1 dimension. We observe significant FFLO regions in both the 3d and 1d region which was previously unexpected.   

Anderson Localization of Ultracold Fermionic K

We experimentally observe stationary, exponential localization of spin-polarized ultracold fermionic atoms in the presence of a 3-D disordered potential generated from optical speckle. A localized component and a mobile component of the gas emerge after release from a harmonic trap into the speckle potential. The density of the localized component decays exponentially in space with a decay constant depending on the initial temperature of the gas, while the mobile component expands ballistically. Exact numerical simulation of classical trajectories in the 3D potential has excluded diffusion or a percolation threshold as possible explanations. The exponential localization in combination with the presence of a mobility edge, indicated by the temperature dependence of the fraction of the cloud localized, make our observations qualitatively consistent with Anderson localization in 3D.   

Classical Magnetic Impurity in Ultracold Fermi Superfluids

In cold atom experiments, we have tools to create spin dependent optical potential, which gives us the possibility to achieve classical magnetic impurity in cold atom systems. Here we study the physics of magnetic impurity. A localized magnetic impurity can induce a mid-gap bound state, the Yu-Shiba state, in superfluid Fermi gas. We propose a modified RF spectroscopy to measure the local density of states, as a cold-atom analog of STM, which may be used to detect Yu-Shiba state. In addition, magnetic impurity can locally induce population imbalance in the system, potentially providing a method to realize FFLO-like state in a controlled way. We demonstrate such a possibility by solving the self-consist Bogoliubov-de Gennes equations.   

Fermionization of two Distinguishable Fermions

Recently, our group has demonstrated the ability to prepare few-fermion systems of in well-defined quantum states with near unity fidelity. These systems consist of 1-10 ultracold atoms confined in an optical microtrap [1]. Combined with the ability to control the interparticle interactions with Feshbach resonances this gives us a level of experimental control unavailable in any other mesoscopic quantum system. Here we report on our studies of two distinguishable fermions with resonant interactions confined in a 1-D harmonic oscillator potential. We find that for diverging coupling strength $g_{1D} \rightarrow \pm \infty$ the system undergoes a process of fermionization, similar to a 1-D Bose gas in the Tonks-Girardeau regime. \\[4pt] [1] F. Serwane et. al., arXiv:1101.2124v1