Evolution of the Pauli spin-paramagnetic effect on the upper critical fields of K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ single crystals

TL;DR

This study investigates how sulfur substitution affects the upper critical magnetic fields in K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ single crystals, revealing the evolution of the Pauli spin-paramagnetic effect and multiband influences with increasing S content.

Contribution

It provides a detailed analysis of the evolution of the Pauli spin-paramagnetic effect on the upper critical fields in these crystals using WHH theory, highlighting the interplay of multiband effects and anisotropy.

Findings

$ ext{H}_{c2}$ decreases with more sulfur content.

Spin-paramagnetic effect dominates for $H\parallel ab$ and diminishes with S.

Multiband effects influence $H_{c2}$ for $H\parallel c$ and weaken with S.

Abstract

We have studied the temperature dependence of the upper critical fields, $\mu_{0}H_{c2}$, of K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ single crystals up to 60 T. The $\mu _{0}H_{c2}$ for $H\parallel ab$ and $H\parallel c$ decrease with increasing sulfur content. The detailed analysis using Werthamer-Helfand-Hohenberg (WHH) theory including the Pauli spin-paramagnetic effect shows that $\mu _{0}H_{c2}$ for $H\parallel ab$ is dominated by the spin-paramagnetic effect, which diminishes with higher S content, whereas $\mu _{0}H_{c2}$ for $H\parallel c$ shows a linear temperature dependence with an upturn at high fields. The latter observation can be ascribed to multiband effects that become weaker for higher S content. This results in an enhanced anisotropy of $\mu _{0}H_{c2}$ for high S content due to the different trends of the spin-paramagnetic and multiband effect for $H\parallel ab$ and $H\parallel c$, respectively.