Upper critical fields in a FeSe0.5Te0.5 superconducting single crystal

A single crystal with a nominal composition FeSe0.5Te0.5 was obtained by the Bridgman method. A quartz ampulla with the sample inside was vacuum-sealed and maintained at 1050 °C for 37 h to homogenize the sample. Subsequently, the quartz ampulla with the sample was moved with a speed of 2.2 mm/h to a furnace which was at 450 °C. X-ray diffraction confirmed the tetragonal structure of the grown single crystal with the cleavage plane corresponding to the ab plane. Resistance measurements were carried out with magnetic fields from 0 to 9 T, applied parallel to the c axis and ab plane, respectively. A zero-field critical temperature Tc = 14 K was determined. The upper critical field vs. temperature phase diagram was built for temperatures where the resistance drops to 90%, 50%, and 10% of the normal state resistance. The linear extrapolation to T = 0 K gave upper critical fields of 57.2, 51.8, and 46.0 T for Hǁc axis and 109.6, 95.5, and 80.9 T for Hǁab. Applying the Werthamer–Helfand–Hohenberg (WHH) theory, ...