Single Crystal Growth, Transport Phenomena, and Upper Critical Field of Alkali Metal-Based K\(_x\)Fe\(_{2-y}\)(Se,S)\(_2\) Iron Chalcogenides

We have grown large (8–10 mm in dimension) single crystals of K\(_{0.8}\)Fe\(_{1.7}\)(Se\(_{0.73}\)S\(_{0.27}\))\(_2\) composition with partial isovalent (Se,S) substitution related to the 122-Se family of iron-chalcogenide superconductors. High quality and homogeneity of the superconducting properties below \(T_c \approx 26\) K were confirmed using X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, transport, and magnetic probes. Using detailed magnetotransport R(T) probes in magnetic fields up to 16 T in two field orientations (in-plane \(H \parallel ab\) and out-of-plane \(H \parallel c\)), we determined temperature dependences of the upper critical field \(\mu _0H_{c2}^{ab}(T)\) and \(\mu _0H_{c2}^c(T)\), magnetic anisotropy \(\gamma \approx 3.5\) in the vicinity of \(T_c\), and estimate zero-temperature values \(\mu _0H_{c2}^{ab}(0) \approx 99\) T, \(\mu _0H_{c2}^c(0) \approx 28\) T.