Density induced BCS-Bose evolution in gated two-dimensional superconductors: The Berezinskii-Kosterlitz-Thouless transition as a function of carrier density

TL;DR

This paper explores how superconductivity in gated two-dimensional materials transitions from BCS to Bose pairing as carrier density varies, emphasizing the BKT transition mechanism and aligning theory with experimental data.

Contribution

It introduces a detailed analysis of the BCS-Bose crossover in 2D superconductors, highlighting the role of the BKT transition and providing quantitative agreement with experiments.

Findings

Superconducting transition is governed by BKT vortex-antivortex pairing.

The BCS to Bose evolution is a smooth crossover, not a phase transition.

Theoretical results match experimental data on LixZrNCl at various densities.

Abstract

We discuss the evolution from BCS to Bose superconductivity versus carrier density in gated two-dimensional s-wave superconductors. We show that the transition from the normal to the superconducting state is controlled by the Berezinskii-Kosterlitz-Thouless vortex-antivortex pairing mechanism, and that the evolution from high carrier density (BCS pairing) to low carrier density (Bose pairing) is just a crossover in s-wave systems. We compare our results to recent experiments on the superconductor LixZrNCl, a lithium intercalated layered nitride, and obtain very good quantitative agreement at low and intermediate densities.