The effect of population imbalance on the Berezinskii-Kosterlitz-Thouless phase transition in a superfluid Fermi gas

TL;DR

This paper investigates how population imbalance affects the Berezinskii-Kosterlitz-Thouless superfluid transition in a two-dimensional Fermi gas, revealing heightened sensitivity and complex phase behavior compared to three-dimensional systems.

Contribution

It provides a detailed analysis of the imbalance effects on 2D BKT transition, including phase diagrams and the identification of phase separation and tricritical points.

Findings

Superfluidity is more sensitive to imbalance in 2D than in 3D.

Imbalance induces phase separation and tricritical points.

Strong-coupling regime shows pronounced imbalance effects.

Abstract

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism describes the breakdown of superfluidity in a two-dimensional Bose gas or a two-dimensional gas of paired fermions. In the latter case, a population imbalance between the two pairing partners in the Fermi mixture is known to influence pairing characteristics. Here, we investigate the effects of imbalance on the two-dimensional BKT superfluid transition, and show that superfluidity is even more sensitive to imbalance than for three dimensional systems. Finite-temperature phase diagrams are derived using the functional integral formalism in combination with a hydrodynamic action functional for the phase fluctuations. This allow to identify a phase separation region and tricritical points due to imbalance. In contrast to superfluidity in the three-dimensional case, the effect of imbalance is also pronounced in the strong-coupling regime.