Superfluidity and collective modes in Rashba spin-orbit coupled Fermi gases

TL;DR

This paper theoretically investigates superfluidity and collective modes in Rashba spin-orbit coupled Fermi gases, revealing a crossover from BCS/BEC to rashbon condensates, and explores topological phase transitions induced by Zeeman fields.

Contribution

It introduces a comprehensive effective action formalism for collective modes in spin-orbit coupled Fermi gases and analyzes the superfluid properties across different coupling regimes and phases.

Findings

Superfluid density and sound velocity show crossover behavior with spin-orbit coupling strength.

Zeeman field induces topological phase transitions with nonanalytic thermodynamic responses.

Superfluid properties are suppressed or enhanced depending on the phase and dimensionality.

Abstract

We present a theoretical study of the superfluidity and the corresponding collective modes in two-component atomic Fermi gases with s-wave attraction and synthetic Rashba spin-orbit coupling. The general effective action for the collective modes is derived from the functional path integral formalism. By tuning the spin-orbit coupling from weak to strong, the system undergoes a crossover from an ordinary BCS/BEC superfluid to a Bose-Einstein condensate of rashbons. We show that the properties of the superfluid density and the Anderson-Bogoliubov mode manifest this crossover. At large spin-orbit coupling, the superfluid density and the sound velocity become independent of the strength of the s-wave attraction. The two-body interaction among the rashbons is also determined. When a Zeeman field is turned on, the system undergoes quantum phase transitions to some exotic superfluid phases which are topologically nontrivial. For the two-dimensional system, the nonanalyticities of the thermodynamic functions and the sound velocity across the phase transition are related to the bulk gapless fermionic excitation which causes infrared singularities. The superfluid density and the sound velocity behave nonmonotonically: they are suppressed by the Zeeman field in the normal superfluid phase, but get enhanced in the topological superfluid phase. The three-dimensional system is also studied.