Lianyi He, Xu-Guang Huang
We present a theoretical study of the bulk superfluid properties and the collective modes in two-component Fermi gases with s-wave attractive interaction and synthetic Rashba spin-orbit coupling. At zero Zeeman field, the system undergoes a crossover from the ordinary fermionic superfluid to the Bose-Einstein condensation of a novel type of bound molecules (referred to as rashbons) when the spin-orbit coupling strength is tuned from weak to strong. We show that the bulk superfluid properties and the properties of the collective modes manifest this crossover. At large spin-orbit coupling, the superfluid density and the velocity of the Goldstone sound mode become universal, that is, independent of the strength of the s-wave attraction. We also determine the two-body interactions among the rashbons. At nonzero Zeeman field, the system undergoes quantum phase transitions to some exotic superfluid phases which are topologically nontrivial. For the two-dimensional system, we show that the quantum phase transition is related to the gapless fermionic spectrum which causes infrared singularities and the phase transition is of third order. The superfluid density and the sound velocity behave oppositely in the normal and the topological superfluid phases: They are suppressed by the Zeeman field in the normal superfluid phase, but get enhanced in the topological superfluid phase. The sound velocity also shows non-analytical behavior across the phase transition. Around the phase transition, the massive Higgs mode is softened. For the three-dimensional system, the singularities caused by the gapless fermionic spectrum are weakened and the phase transition is of higher than third order. However, the superfluid density and the sound velocity have similar behaviors to the two-dimensional system.
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http://arxiv.org/abs/1207.2810
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