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, $\mu_{0}H_{c2}$, 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.