Onset Temperatures for Superconducting Fluctuations in Te-annealed FeTe$_{1-x}$Se$_x$ Single Crystals: Evidence for the BCS-BEC Crossover

TL;DR

This study investigates the onset temperatures of superconducting fluctuations in Te-annealed FeTe$_{1-x}$Se$_x$ single crystals, revealing high fluctuation temperatures that suggest the system is in the BCS-BEC crossover regime.

Contribution

It provides new experimental evidence of high superconducting fluctuation temperatures in FeTe$_{1-x}$Se$_x$, supporting its placement in the BCS-BEC crossover regime.

Findings

High $T_{scf}$ values up to 40 K for $x$=0.4

Superconductivity in FeTe$_{1-x}$Se$_x$ is within the BCS-BEC crossover regime

Superconducting fluctuations are significantly above $T_c$

Abstract

Recently, the superconductors' community has witnessed an unsettled debate regarding whether iron-based superconductors, in particular FeSe and FeSe$_{1-x}$S$_x$, are in the Bardeen-Cooper-Shrieffer (BCS) - Bose-Einstein condensation (BEC) crossover regime. Nonetheless, one particular system, FeTe$_{1-x}$Se$_x$, has been less investigated in this regard owing to the screening of its intrinsic superconducting properties by the inevitable iron excess. Herein, the onset temperatures for superconducting fluctuations ($T_{scf}$) are investigated by measuring the magnetoresistance (MR) of Te-annealed, high-quality FeTe$_{1-x}$Se$_x$ ($x$ = 0.1, 0.2, 0.3, and 0.4) single crystals. The results reveal very high $T_{scf}$ values for these crystals. Particularly for $x$ = 0.4, $T_{scf}$ reaches approximately 40 K, which is 2.7 times larger than $T_c$. This indicates that the superconductivity of the FeTe$_{1-x}$Se$_x$ system is well within the BCS-BEC crossover regime.