Upper Critical Field of a Two-Band SrFe2 − xNixAs2 Superconductor.

The upper critical field of an iron-based SrFe_{2 − x}Ni_xAs₂ superconductor single crystal is measured for the first time in longitudinal and transverse magnetic fields up to 16 T in the basal ab plane and to it along the c axis. The H_c2(0) values are 18 and 25 T for H ∥ c and H ∥ ab, respectively. The anisotropy of the upper critical field γ(T) = H_c2^ab / H_c2^c decreases monotonically to 1.4 with decreasing temperature. The temperature dependence of H_c2 is not described fully by the Werthamer–Helfand–Hohenberg model taking into account the influence of orbital and spin paramagnetic effects. However, an effective two-band model reproduces well the measured temperature dependence. This indicates the two-band nature of this superconductor, where one of the bands is almost isotropic. According to the two-band model approximation, the contribution from the anisotropic band dominates near T_c, whereas the main contribution at low temperatures comes from the isotropic band with a lower diffusion coefficient. [ABSTRACT FROM AUTHOR]