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arXiv:1201.2972 · Physical Review A

Density, spin, and pairing instabilities in polarized ultracold Fermi gases

Inti Sodemann, D. A. Pesin, Allan H. MacDonald

Published Mar 21, 2012• 15 citations
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Abstract

We study the influence of population imbalance on the pairing, spin, and density instabilities of a two-component ideal Fermi gas after a sudden quench of interactions near a Feshbach resonance. Over a large region of parameters the pairing instability is dominated by finite-momentum pairing, suggesting the possibility of observing Fulde-Ferrell-Larkin-Ovchinnikov--like states in the unstable initial dynamics. Long-wavelength density instabilities are found on the Bardeen-Cooper-Schrieffer side of the resonance, and are interpreted as a precursor of the phase separation expected at equilibrium. On the Bose-Einstein-condensate side of the resonance, the pairing instability is present for scattering lengths larger than a critical value that is only weakly dependent on population imbalance and always smaller than the scattering length at which the Stoner-like spin instability occurs.

DOI
10.1103/physreva.85.033628

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We describe the dynamics of two component non-interacting ultracold Fermions which are initially in thermal equilibrium and undergo a rapid quench to either the repulsive or attractive side of a Feshbach resonance. The short time dynamics is dominated by the exponentially growing collective modes. We study the Stoner instability and formation of ferromagnetic textures on the repulsive side, and the pairing instability towards BCS or FFLO-like states (determined by the population imbalance) on the attractive side. In each case, we evaluate the growth rate of unstable modes and predict the typical lengthscale of textures to be formed.
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We consider a trapped Fermi gas with population imbalance at finite temperatures and map out the detailed phase diagram across a wide Feshbach resonance. We take the Larkin-Ovchinnikov-Fulde-Ferrel (LOFF) state into consideration and minimize the thermodynamical potential to ensure stability. Under the local density approximation, we conclude that a stable LOFF state is present only on the BCS side of the Feshbach resonance, but not on the BEC side or at unitarity. Furthermore, even on the BCS side, a LOFF state is restricted at low temperatures and in a small region of the trap, which makes a direct observation of a LOFF state a challenging task.
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