Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X6: Pairing in Imbalanced Fermi Mixtures |
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Sponsoring Units: DCOMP DAMOP Chair: Richard Scalettar, University of California, Davis Room: Ballroom C2 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X6.00001: Correlated phases in the Fermi Hubbard model with spin and mass imbalance Invited Speaker: One dimensional attractive fermions with unequal spin populations provide a direct realization of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid state, in which Cooper pairs condense at finite momentum. The presence of an additional mass asymmetry between the two components gives rise to multi-particle bound states, including trimers made of one light and two heavy fermions. I first discuss the stability of these bound states through the exact solution of the three-body problem. Based on Density Matrix Renormalization Group simulations and bosonization theory, I then show that at finite and commensurate densities the ground state of the system is a Luttinger liquid of trimers. In this new phase superconducting FFLO correlations are exponentially suppressed. Finally I explain how the mass asymmetry changes the topology of the grand-canonical phase diagram of the Fermi Hubbard model. [GO, E. Burovski and T. Jolicoeur, PRL 104, 065301 (2010); E. Burovski, GO and T. Jolicoeur, PRL 103, 215301 (2009)] [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X6.00002: The phase diagram of imbalanced Fermi gases Invited Speaker: Recent experimental and theoretical research has focused on the phases of strongly interacting Fermi gases under an imposed population imbalance between the fermion species. The large difference in chemical potential between the majority and minority species disrupts conventional singlet s-wave pairing, yielding a rich phase diagram including regions of phase separation, Fulde-Ferrell-Larkin-Ovchinnikov superfluidity and magnetic superfluidity. I will discsuss these predicted phases, as well as the behavior at large imbalance where the minority species can induce an effective attraction among the majority fermions and a resulting instability towards p-wave superfluidity. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X6.00003: Pair formation in Fermi systems with population imbalance in one- and two-dimensional optical lattices Invited Speaker: I will discuss pairing in fermionic systems in one- and two-dimensional optical lattices with population imbalance. This will be done in the context of the attractive fermionic Hubbard model using the Stochastic Green Function algorithm in d=1 while for d=2 we use Determinant Quantum Monte Carlo. This is the first exact QMC study examining the effects of finite temperature which is very important in experiments on ultra-cold atoms. Our results show that, in the ground state, the dominant pairing mechanism is at nonzero center of mass momentum, i.e. FFLO. I will then discuss the effect of finite temperature in the uniform and confined systems and present finite temperature phase diagrams. The numerical results will be compared with experiments. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X6.00004: Spin imbalanced Fermi gases in 1D and the crossover to 3D Invited Speaker: The search for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, a polarized superfluid with a spatially varying order parameter, has generated large interest in both condensed matter and cold atoms communities. To date, there has been only indirect experimental evidence of FFLO in the heavy fermion superconductor CeCoIn5. A strongly interacting 3D polarized Fermi gas exhibits three phases at low temperature: an unpolarized superfluid, a partially polarized and a fully polarized normal phase, which phase separate in an optical trap. There is no experimental evidence for an FFLO phase in the 3D system and theory predicts that it occupies only a small region in the phase diagram. In a 1D polarized Fermi gas, the FFLO phase is predicted to occupy a large region of the phase diagram. We have implemented a 2D optical lattice in order to explore the phase diagram of an imbalanced spin mixture of $^6$Li. In in-situ density distributions, we observe in the center of the cloud a partially polarized region surrounded by an either fully polarized or an unpolarized superfluid shell depending on the spin imbalance. The density profiles are quantitatively well described by a finite temperature Bethe ansatz and can be used to extract the 1d phase diagram of the imbalanced 1d Fermi gas. Moreover, the quantitative agreement of experiment and theory paves the way for directly observing the elusive FFLO phase in the system. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X6.00005: Universal Spin Transport in Strongly Interacting Fermi Gases Invited Speaker: Ultracold gases of fermionic atoms have emerged as a unique platform to study strongly interacting fermion systems. Here we study spin transport in a two-state mixture of fermionic atoms near a Feshbach resonance. Starting with two separate spin domains in an atom trap, we observe the subsequent evolution of the spin mixture towards the eventual ground state, a superfluid of fermion pairs. Initially, the gas clouds of unlike spins almost perfectly bounce off each other, despite densities a million times thinner than air. Only over several seconds, about 100 000 collision times, the spins slowly diffuse into each other and, below a critical temperature, form a superfluid of fermion pairs. We determine the transport properties in this gas as a function of interaction strength and temperature. In particular, we find the spin diffusion coefficient in the strongly interacting, degenerate regime to take on the universal value for a ``perfect fluid'', $D \simeq \hbar/m = \frac{(100 \mu \rm m)^2}{{\rm s}}$, where $m$ is the mass of the $^6$Li atoms. At high temperatures, we find the universal law $D = \alpha \frac{\hbar}{m} \left(T/T_F\right)^{3/2}$ with a constant $\alpha$. The ratio of spin conductivity and spin diffusion coefficient yields the spin susceptibility in these gases, showing the Curie law at high temperatures and a departure from the compressibility at low temperatures, that we interpret as a signature for entering the Fermi liquid regime. Our transport experiments near and far equilibrium have implications on other strongly interacting Fermi systems, suggesting a fundamental lower limit to the spin diffusion coefficient, in the absence of localization, on the order of $\hbar/m$ - a conjecture already made by Onsager. Our spin susceptibility measurements appear to exclude a ferromagnetic ground state on the repulsive side of the Feshbach resonance. [Preview Abstract] |
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