Density and spin modes in imbalanced normal Fermi gases from collisionless to hydrodynamic regime

TL;DR

This paper investigates how mass and population imbalance influence density and spin collective modes in a normal Fermi gas, revealing crossover behaviors from collisionless to hydrodynamic regimes and highlighting the effects on dynamic structure factors.

Contribution

It provides a detailed analysis of collective mode crossover behaviors in imbalanced Fermi gases using the Boltzmann equation and structure factors, highlighting new effects of imbalance.

Findings

Mass imbalance shifts the crossover to higher temperatures.

A peak in the spin dynamic structure factor appears only in the collisionless regime.

Population imbalance allows spin response to persist across regimes.

Abstract

We study mass and population imbalance effect on density (in-phase) and spin (out-of-phase) collective modes in a two-component normal Fermi gas. By calculating eigenmodes of the linearized Boltzmann equation as well as the density/spin dynamic structure factor, we show that mass and population imbalance effects offer a variety of collective mode crossover behaviors from collisionless to hydrodynamic regimes. The mass imbalance effect shifts the crossover regime to the higher-temperature, and a significant peak of the spin dynamic structure factor emerges only in the collisionless regime. This is in contrast to the case of mass and population balanced normal Fermi gases, where the spin dynamic response is always absent. Although the population imbalance effect does not shift the crossover regime, the spin dynamic structure factor survives both in the collisionless and hydrodynamic regimes.