Density and spin response function of a normal Fermi gas at unitarity

Magnetic Phase Transition in a Mixture of Two Interacting Superfluid Bose Gases at Finite Temperature

The miscibility condition for a binary mixture of two interacting Bose-Einstein condensates is shown to be deeply affected by interaction driven thermal fluctuations. These give rise to a first order phase transition to a demixed phase with full spatial separation of the two condensates, even if the mixture is miscible at zero temperature. Explicit predictions for the isothermal compressibility, the spin susceptibility, and the phase transition temperature T_{M} are obtained in the framework of Popov theory, which properly includes beyond mean-field quantum and thermal fluctuations in both the spin and density channels. For a mixture of two sodium condensates occupying the hyperfine states |F=1⟩,|m_{F}=1⟩ and |F=1,m_{F}=-1⟩, respectively, T_{M} is predicted to occur at about 0.7 times the usual BEC critical temperature.

Review of pseudogaps in strongly interacting Fermi gases

A central challenge in modern condensed matter physics is developing the tools for understanding nontrivial yet unordered states of matter. One important idea to emerge in this context is that of a 'pseudogap': the fact that under appropriate circumstances the normal state displays a suppression of the single particle spectral density near the Fermi level, reminiscent of the gaps seen in ordered states of matter. While these concepts arose in a solid state context, they are now being explored in cold gases. This article reviews the current experimental and theoretical understanding of the normal state of strongly interacting Fermi gases, with particular focus on the phenomonology which is traditionally associated with the pseudogap.

Quantum mechanical limitations to spin diffusion in the unitary Fermi gas

We compute spin transport in the unitary Fermi gas using the strong-coupling Luttinger-Ward theory. In the quantum degenerate regime the spin diffusivity attains a minimum value of D(s) Symbol: see text] 1.3 ħ/m approaching the quantum limit of diffusion for a particle of mass m. Conversely, the spin drag rate reaches a maximum value of Γ(sd) [Symbol: see text] 1.2k(B)T(F)/ħ in terms of the Fermi temperature T(F). The frequency-dependent spin conductivity σ(s)(ω) exhibits a broad Drude peak, with spectral weight transferred to a universal high-frequency tail σ(s) (ω → ∞) = ħ(1/2)C/3π(mω)(3/2) proportional to the Tan contact density C. For the spin susceptibility χ(s)(T) we find no downturn in the normal phase.

Dynamic spin response of a strongly interacting Fermi gas

We present an experimental investigation of the dynamic spin response of a strongly interacting Fermi gas using Bragg spectroscopy. By varying the detuning of the Bragg lasers, we show that it is possible to measure the response in the spin and density channels separately. At low Bragg energies, the spin response is suppressed due to pairing, whereas the density response is enhanced. These experiments yield the first independent measurements of the spin-parallel and spin-antiparallel dynamic and static structure factors, which provide insight into the different features of density and spin response functions. At high momentum the spin-antiparallel dynamic structure factor displays a universal high frequency tail, proportional to ω(-5/2), where ℏω is the probe energy.

Establishing the presence of coherence in atomic fermi superfluids: spin-flip and spin-preserving Bragg scattering at finite temperatures

We show how in ultracold Fermi gases the difference between the finite temperature T structure factors, called S_(ω,q), associated with spin and density, reflects coherent order at all ω, q, k(F)a, and T. This observation can be exploited in two photon Bragg scattering experiments on gases which are subject to variable attractive interactions. Our calculations incorporate spin and particle number conservation laws which lead to compatibility at general T with two f-sum rules. Because of its generality a measurement of S_(ω,q) can be a qualitative, direct, in situ approach for establishing superfluid order.