Gate-controlled BCS-BEC crossover in a two-dimensional superconductor

TL;DR

This study demonstrates the BCS-BEC crossover in a 2D superconductor, ZrNCl, using ionic gating to tune carrier density, revealing a pseudogap phase and scaling behavior consistent with theoretical predictions.

Contribution

It provides experimental evidence of the BCS-BEC crossover in a 2D superconductor via gate tuning, establishing ZrNCl as an ideal platform for studying this phenomenon.

Findings

Observation of pseudogap phase at low doping

Scaling of T_BKT/T_F consistent with BCS-BEC crossover theory

Gate doping enables tuning across the BCS-BEC regime

Abstract

The Bardeen-Cooper-Schrieffer (BCS) condensation and the Bose-Einstein condensation (BEC) are the two extreme limits of the ground state of the paired fermion systems. We report crossover behavior from the BCS condensation to the BEC realized in the two-dimensional (2D) superconductor, electron doped layered material ZrNCl. The phase diagram, established by simultaneous experiments of resistivity and tunneling spectra under the ionic gating, demonstrates the pseudogap phase at the low doping regime. In the low carrier density limit, $T_{\rm BKT}$ (Berezinskii-Kosterlitz-Thouless transition temperature for 2D superconductors) scales as $T_{\rm BKT}/T_{\rm F} = 0.12$, where $T_{\rm F}$ is the Fermi temperature, which is consistent with the theoretical upper bound expected in the BCS-BEC crossover regime. The present results indicate that the gate-doped semiconductor provides an ideal platform for the 2D BCS-BEC crossover without any added complexity, such as magnetic orders and density waves.