Upper critical fields of the 11-system iron-chalcogenide superconductor FeSe$_{0.25}$Te$_{0.75}$

TL;DR

This study measures the upper critical magnetic fields of FeSe$_{0.25}$Te$_{0.75}$, revealing strong-coupling superconductivity with Pauli limiting behavior and large superconducting gap, using high-field resistivity experiments.

Contribution

First detailed high-field measurements of the upper critical field in FeSe$_{0.25}$Te$_{0.75}$, showing deviations from WHH model and evidence of strong-coupling superconductivity.

Findings

Upper critical fields are smaller than WHH model predictions.

Evidence of Pauli limiting behavior in the compound.

Indication of a large superconducting gap and strong-coupling superconductivity.

Abstract

We have performed electrical resistivity measurements of a polycrystalline sample of FeSe$_{0.25}$Te$_{0.75}$, which exhibits superconductivity at $T_{\rm c} \sim 14$ K, in magnetic fields up to 55 T to determine the upper critical field $\mu_{0}H_{\rm c2}$. In this compound, very large slopes of $\mu_{0}H_{\rm c2}$ at the onset, the mid-point, the zero-resistivity temperatures on superconductivity are determined to be -13.7, -10.1, and -6.9 T/K, respectively. The observed $\mu_{0}H_{\rm c2}(T)$s of this compound are considerably smaller than those expected from the Werthamer-Helfand-Hohenberg model, manifesting the Pauli limiting behavior. These results suggest that this compound has a large Maki parameter, but it is smaller than that calculated for a weak-coupling superconductor, indicating a large superconducting gap of this compound as a strong-coupling superconductor.