Anomalous isothermal compressibility in spin-orbit coupled degenerate Fermi gases

TL;DR

This paper investigates the unusual behavior of isothermal compressibility in spin-orbit coupled degenerate Fermi gases, revealing anomalous peaks in the gapless Weyl phase and linking thermodynamic properties to topological phases.

Contribution

It formulates the isothermal compressibility in spin-orbit coupled Fermi gases, identifying anomalous peaks in the Weyl phase and connecting thermodynamics to topological superfluid phases.

Findings

Anomalous peaks in compressibility occur in the gapless Weyl phase.

Compressibility behavior varies between BCS and BEC regimes.

Predictions include measurable responses in sound velocity and density fluctuations.

Abstract

The spin-orbit coupling (SOC) in degenerate Fermi gases can fundamentally change the fate of $s$-wave superfluids with strong Zeeman field and give rise to topological superfluids and associated Majorana zero modes. It also dramatically changes the thermodynamic properties of the superfluids. Here we report the anomalous isothermal compressibility $\kappa_T$ in this superfluids with both SOC and Zeeman field. We formulate this quantity from the Gibbs-Duhem equation and show that the contribution of $\kappa_T$ comes from the explicit contribution of chemical potential and implicit contribution of order parameter. In the Bardeen-Cooper-Schrieffer (BCS) limit, this compressibility is determined by the density of state near the Fermi surface; while in the Bose Einstein condensate (BEC) regime it is determined by the scattering length. Between these two limits, we find that the anomalous peaks can only be found in the gapless Weyl phase regime. This anomalous behavior can be regarded as a remanent effect of phase separation. The similar physics can also be found in the lattice model away from half filling. These predictions can be measured from the anomalous response of sound velocity and fluctuation of carrier density.