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

We have studied the temperature dependence of the upper critical fields, $μ_{0}H_{c2}$, of K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ single crystals up to 60 T. The $μ_{0}H_{c2}$ 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 $μ_{0}H_{c2}$ for $H\parallel ab$ is dominated by the spin-paramagnetic effect, which diminishes with higher S content, whereas $μ_{0}H_{c2}$ 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 $μ_{0}H_{c2}$ 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.

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