Gate-controlled low carrier density superconductors: Toward the two-dimensional BCS-BEC crossover

TL;DR

This study demonstrates that lightly doped two-dimensional superconductors, specifically lithium-intercalated layered nitrides, show increased critical temperatures and pseudogap states, providing new insights into the BCS-BEC crossover.

Contribution

It reports the first observation of enhanced superconductivity and pseudogap formation in low-doped 2D nitrides, advancing understanding of the BCS-BEC crossover in such systems.

Findings

Critical temperature Tc increases with decreasing doping.

Observation of pseudogap state below 35.5 K.

Superconducting coupling strength reaches 5.9.

Abstract

Superconductivity mostly appears in high carrier density systems and sometimes exhibits common phase diagrams in which the critical temperature Tc continuously develops with carrier density. Superconductivity enhanced in lightly doped regime has seldom been reported, although it is an ideal direction towards the crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensate (BEC) limits, where the behavior of Cooper pairs changes dramatically. Here we report transport properties and superconducting gaps in single-crystalline lithium-intercalated layered nitrides (LixHfNCl and LixZrNCl) down to the low-doping regime, enabled by a combination of ionic gating device and tunneling spectroscopy. Upon the reduction of doping, both systems display the increase of Tc, up to 24.9 K especially in LixHfNCl, with the concomitant enhancement of two-dimensionality leading to a pseudogap state below 35.5 K as well as the increase of superconducting coupling strengths 2D/kBTc reaching 5.9. Such behavior in the carrier density region as low as 10^20 cm-3 indicates that lightly doped two-dimensional superconductors exhibit the unprecedented nature that is distinct from that in conventional superconductors, and offer a new route to access BCS-BEC crossover.