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

Abstract

We discuss the evolution from BCS to Bose superconductivity versus carrier density in gated two-dimensional s-wave superconductors. We investigate the carrier density dependence of the critical temperature, superfluid density, order parameter, chemical potential and pair size. 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.

Andreev reflection spectroscopy in strongly paired superconductors

“…While most solid-state superconductors reside firmly in the BCS regime, certain strongly correlated materials such as cuprates have been rationalized in terms of the BCS-BEC paradigm [22]. Further, a new generation of experiments probing low-carrierdensity materials including iron-based compounds [23][24][25][26], Li x ZrNCl [27,28] and moiré graphene systems [29][30][31][32][33][34] has revealed signatures consistent with proximity to a BEC state-opening a new experimental frontier for unconventional superconductivity. Developing probes that can unambiguously identify BEC superconductors and distinguish possible competing phases therefore poses a pressing problem.…”