$\mathcal{PT}$-Symmetry Enhanced Berezinskii-Kosterlitz-Thouless Superfluidity

TL;DR

This paper explores how $ ext{PT}$-symmetry in non-Hermitian 2D Fermi gases enhances superfluidity and raises the BKT transition temperature, revealing novel effects of complex spectra on topological phase transitions.

Contribution

It demonstrates that $ ext{PT}$-symmetry can enhance superfluid density and increase BKT transition temperature in non-Hermitian 2D Fermi gases, a novel finding in low-dimensional topological physics.

Findings

$ ext{PT}$-symmetry guarantees real superfluid density despite complex spectra.

Imaginary Zeeman field increases BKT transition temperature.

Transition temperature is higher in weak interaction regimes.

Abstract

Berezinskii-Kosterlitz-Thouless (BKT) transition, the topological phase transition to a quasi-long range order in a two-dimensional (2D) system, is a hallmark of low-dimensional topological physics. The recent emergence of non-Hermitian physics, particularly parity-time ($\mathcal{PT}$) symmetry, raises a natural question about the fate of low-dimensional orders (e.g., BKT transition) in the presence of complex energy spectrum. Here we investigate the BKT phase transition in a 2D degenerate Fermi gas with an imaginary Zeeman field obeying $\mathcal{PT}$-symmetry. Despite complex energy spectrum, $\mathcal{PT}$-symmetry guarantees that the superfluid density and many other quantities are real. Surprisingly, the imaginary Zeeman field enhances the superfluid density, yielding higher BKT transition temperature than that in Hermitian systems. In the weak interaction region, the transition temperature can be much larger than that in the strong interaction limit. Our work showcases a surprising interplay between low-dimensional topological defects and non-Hermitian effects, paving the way for studying non-Hermitian low-dimensional phase transitions.