Evolution of the Pauli spin-paramagnetic effect on the upper critical fields of single-crystallineKxFe2−ySe2−zSz

Abstract

We have studied the temperature dependence of the upper critical fields ${\ensuremath{\mu}}_{0}{H}_{c2}$ of ${K}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2\ensuremath{-}z}{S}_{z}$ single crystals up to 60 T. The ${\ensuremath{\mu}}_{0}{H}_{c2}$ for $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$ decrease with increasing sulfur content. The detailed analysis using Werthamer-Helfand-Hohenberg theory including the Pauli spin-paramagnetic effect shows that ${\ensuremath{\mu}}_{0}{H}_{c2}$ for $H\ensuremath{\parallel}ab$ is dominated by the spin-paramagnetic effect, which diminishes with higher S content, whereas ${\ensuremath{\mu}}_{0}{H}_{c2}$ for $H\ensuremath{\parallel}c$ 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 ${\ensuremath{\mu}}_{0}{H}_{c2}$ for high S content due to the different trends of the spin-paramagnetic and multiband effect for $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$, respectively.