Evolution of the Pauli spin-paramagnetic effect on the upper critical fields of single-crystalline KxFe2-y,Se2-zSz.

We have studied the temperature dependence of the upper critical fields μ0Hc2 of KxFe2-ySe2-zSz single crystals up to 60 T. The μ0Hc2 for H ∥ ab and H ∥ c decrease with increasing sulfur content. The detailed analysis using Werthamer-Helfand-Hohenberg theory including the Pauli spin-paramagnetic effect shows that μ0Hc2 for H ∥ ab is dominated by the spin-paramagnetic effect, which diminishes with higher S content, whereas μ0Hc2 for H ∥ 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 μ0Hc2 for high S content due to the different trends of the spin-paramagnetic and multiband effect for H ∥ ab and H ∥ c, respectively. [ABSTRACT FROM AUTHOR]