Upper critical field of a mixed d- and s-wave superconductor

Based on the linearized gap equation, the upper critical field (H_c2) of a mixed d- and s-wave superconductor is microscopically obtained from the superconducting transition temperature (T_c) down to zero temperature. It is shown that near T_c, H_c2 increases almost linearly with decreasing temperature. Moreover, the mixed d- and s-wave superconductor has a larger value of H_c2 than that of a pure d-wave superconductor in the whole temperature region. Interestingly, our calculation unambiguously shows that H_c2 finally becomes saturated. This implies that an admixture of the s-wave component could not be the origin of its upward curvature which has been reported in some experiments and some calculations based on the Ginzburg-Landau equations.