The impact of Maki parameter and spin orbit scattering constant in the WHH model on upper critical magnetic fields in Ni-and co-doped pnictide bulk superconductors

Abstract

Werthamer, Helfand, and Hohenberg (WHH) model is applied to the measured upper critical magnetic fields of Ni-and Co-doped pnictide bulk superconductors. This approach incorporates both orbital and paramagnetic effects in the upper critical field’s temperature dependence. The relative importance of both the orbital and paramagnetic effects in the suppression of the superconductivity is described by the Maki parameter. In most superconductors, the Maki parameter is usually much less than unity suggesting that the influence of the paramagnetic effect is small. However, in materials with heavy electron effective mass, in which the Fermi energy is small, or in layered materials in a magnetic field parallel to the layers, the Maki parameter can be larger than unity. This is the case for our bulk pnictides. By fitting the measured temperature dependence of the upper critical field, we derive the Maki parameter and the spin orbit scattering constant and examine the correlation between the Maki parameter, the spin orbit scattering constant, and the reduced upper critical field for our curve fits vs. temperature. The obtained Maki parameters indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant. At the same time, our spin orbit scattering constants indicate that spin-orbit scattering needs to be also included in description of the Hc2(T) data in these materials.