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. [S0163-1829(98)50126-9].