Evolution of the Pauli spin-paramagnetic effect on the upper critical fields of KₓFe₂₋ySe₂₋zSz single crystals

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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 (WHH) 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.