Session W6: Strong Interacting Fermi Gases with Spin Asymmetry

Experiments in spin-polarized Fermi gases-- pairing without superfluidity?

Fermionic superfluidity requires pairing of fermions. The nature of fermionic pairing in the strongly interacting regime both in the superfluid and possibly in the normal phase is of interest to condensed matter, nuclear and high energy physics. The experimental realization of high temperature superfluidity in ultracold Fermi gases opens a new approach to explore strongly interacting fermions both in the superfluid and normal phases. One question of relevance for example to superfluidity of quarks in cold baryonic matter as well as superconductivity has been the stability of the superfluid against an imbalance between the two strongly interacting fermionic components. An imbalance can be caused by different masses of the fermions or an externally applied magnetic field to a superconductor. In our experiments a density imbalance between two fermionic spin components is introduced. We will present the phase diagram of a spin-polarized Fermi gas of 6Li atoms at unitarity, mapping out the superfluid phase versus temperature and density imbalance. The nature of the phase transition changes from first-order to second-order at a tricritical point. At zero temperature, there is a quantum phase transition from a fully-paired superfluid to a partially-polarized normal gas at a critical spin polarization, known the Chandrasekhar-Clogston limit of superfluidity. These observations together with the implementation of an in situ ideal gas thermometer provide quantitative tests of theoretical calculations on the stability of resonant superfluidity. Pairing correlations in the superfluid and normal phases were explored in radio-frequency spectroscopy experiments. We studied how pairing correlations evolve across the superfluid to normal phase transition both as a function of temperature and spin imbalance. Even at spin imbalances above the Chandrasekhar-Clogston limit a gap in the single-particle excitation spectrum is observed. This indicates that the system is in a correlated state and the minority component is paired. The influence of final state interactions on the rf spectra will be discussed. Using a new superfluid 6Li spin mixture we demonstrate that pair dissociation spectra in the BEC-BCS crossover resemble asymmetric molecular dissociation spectra. Work done in collaboration with Y. Shin, A. Schirotzek and W. Ketterle, Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, MIT, Cambridge, MA 02139.   

Theory of RF Spectroscopy in Strongly Interacting Fermi Gases

Radio frequency (RF) spectroscopy is an extremely powerful probe of the many-body state of a gas of cold atoms. For example spectra of cold Fermi gases have been used as evidence of superfluidity, and of pairing fluctuations in the normal state. Despite the large amount of information they contain, spectra of trapped gasses are not completely trivial to analyze. I will discuss the theory of RF spectroscopy, showing that the link between pairing and the observed spectra is very indirect, and that many of the ``pairing'' features occur even in a gas with no pairing whatsoever. I will also describe the important role played by final state interactions.   

Phase separation in a spin polarized Fermi gas at the BEC-BCS crossover

A strongly interacting ultra-cold gas of fermionic $^6$Li with unequal numbers of two spin components exhibits two distinct low temperature paired states.\footnote{G. B. Partridge, Wenhui Li, Y. A. Liao, R. G. Hulet, M. Haque and H. T. C. Stoof, \emph{Phys. Rev. Lett.} \textbf{97}, 190407 (2006) } Phase separation, where a uniformly paired core is maintained in the center of the trap by the expulsion of excess unpaired atoms, is observed at the lowest temperatures up to large number imbalance. Sharp boundaries, consistent with a first-order phase transition, are observed between the core and the unpaired atoms. Moreover, the superfluid core deforms markedly, becoming less elongated due to surface tension at the superfluid/normal boundary. At higher temperature, the core remains unpolarized up to a critical polarization, but does not deform. This temperature dependence is consistent with a tri-critical point in the phase diagram. Additionally, we are exploring the possibility that the large critical imbalance for loss of phase separation is a result of relatively small particle number ($10^5$) and high aspect ratio (30) elongated confinement. \vspace{1ex} \newline To date, no evidence for the long sought Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state has been observed in ultracold atomic gases. It is predicted that in 3D the FFLO phase occupies a very small region of the phase diagram,\footnote{D. E. Sheehy and L. Radzihovsky, \emph{Ann. Phys.} \textbf{322}, 1790 (2007)} whereas in 1D, the stability of the FFLO state is believed to be enhanced. Though the current optical potential is elongated, it is still in the 3D regime, and so a 2D optical lattice potential has been constructed to provide an array of 1D tubes. We will present results of our studies of the polarized Fermi gas in this 1D geometry.   

Few-body physics of trapped unequal mass fermions

The behavior of a two-component dilute Fermi gas exhibits an interesting dependence on the mass ratio between the two species. Our study tackles this system with 3-20 particles, using two independent techniques. First, an essentially exact diagonalization for 3-6 particles determines both the ground state and also the pattern of excited state energies, and our analysis permits an extraction of the dimer-dimer scattering length and effective range. Secondly, the nature of the system ground state is studied as a function of the mass ratio and the number of particles, up to N=20, using fixed-node diffusion Monte Carlo (DMC) techniques. By using two different solution techniques in their overlapping range of applicability from N=3-6, we are able to assess the accuracy of the nodal surface employed in the fixed-node DMC calculation. Physical properties such as the excitation gap will be analyzed over this range of particle number, and the intriguing unitarity limit is also considered.   

Exploring an ultracold Fermi-Fermi mixture: interspecies Feshbach resonances of $^6$Li-$^{40}$K

We report on the observation of interspecies Feshbach resonances in an ultracold mixture of two fermionic species, $^6$Li and $^{40}$K. Interpretation of the data unambiguously assigns molecular bound states to the various resonances and fully characterizes the ground-state scattering properties in any combination of spin states. Using this knowledge we hope to be able to produce $^6$Li-$^{40}$K molecules, cool them to quantum degeneracy, and study their BEC-BCS crossover. \newline \newline In collaboration with: F. Schreck, Institut fuer Quantenoptik und Quanteninformation, Oesterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria; E. Wille, Institut fuer Quantenoptik und Quanteninformation, Oesterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria and Institut fuer Experimentalphysik und Forschungszentrum fuer Quantenphysik, Universitaet Innsbruck, 6020 Innsbruck, Austria; F.M. Spiegelhalder, G. Kerner, D. Naik, A. Trenkwalder, G. Hendl, Institut fuer Quantenoptik und Quanteninformation, Oesterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria; R. Grimm, Institut fuer Quantenoptik und Quanteninformation, Oesterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria and Institut fuer Experimentalphysik und Forschungszentrum fuer Quantenphysik, Universitaet Innsbruck, 6020 Innsbruck, Austria; T.G. Tiecke, J.T.M. Walraven,Van der Waals-Zeeman Institute of the University of Amsterdam, 1018 XE, The Netherlands; S.J.J.M.F. Kokkelmans, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; E. Tiesinga, P.S. Julienne, Joint Quantum Institute, National Institute of Standards and Technology and University of Maryland, Gaithersburg, Maryland 20899-8423, USA