Competing effects of mass anisotropy and spin-Zeeman coupling on the upper critical field of a mixed d- and s-wave superconductor

Based on the linearized Eilenberger equations, the upper critical field (Formula presented) of mixed d- and s-wave superconductors has been microscopically studied with an emphasis on the competing effects of mass anisotropy and spin-Zeeman coupling. We find the mass anisotropy always enhances (Formula presented) while the Zeeman interaction suppresses (Formula presented) As required by the thermodynamics, we find (Formula presented) is saturated at zero temperature. We compare the theoretical calculations with recent experimental data of (Formula presented).