Weak superconducting fluctuations and small anisotropy of the upper critical fields in an Fe1.05Te0.85Se0.15 single crystal

TL;DR

This study investigates the upper critical fields of an Fe$_{1.05}$Te$_{0.85}$Se$_{0.15}$ superconductor, revealing weak fluctuations, small anisotropy, and strong Pauli paramagnetic effects, with nearly isotropic critical fields at low temperatures.

Contribution

It provides new insights into the anisotropy and fluctuation effects in Fe(Te,Se) superconductors, especially regarding the small anisotropy of the upper critical field.

Findings

Weak superconducting fluctuations observed.

Nearly isotropic upper critical field of about 47 T.

Strong Pauli paramagnetic effect evidenced.

Abstract

We have investigated the temperature dependence of the resistive upper critical fields ($\mu_{0}H_{\rm c2}(T)$) for an Fe$_{1.05}$Te$_{0.85}$Se$_{0.15}$ single crystal, which exhibit superconductivity at $T_{\rm c} \sim 14 K, in magnetic fields of up to 55 T. Two-dimensional feature and superconducting fluctuations of the samples are found to be weak, because the resistive broadening effect on applied magnetic fields of up to 14 T is small. The Pauli paramagnetic effect is obviously evidenced by the strong suppression of the $\mu_{0}H_{\rm c2}^{ab}(T)$ ($H \parallel ab$) curve and nearly isotropic $\mu_{0}H_{\rm c2}(0) \approx 47 T is seen for both $H \parallel ab$ and $H \parallel c$. This fact is almost identical to the results of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystals reported previously. We have discussed that the small anisotropy of the upper critical field at low temperatures in Fe$_{1+y}$(Te,Se) systems against the variation of the Te/Se ratio.