UPPER CRITICAL-FIELD OF A MIXED D-WAVE 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 fr om the superconducting transition temperature (T-c) down to zero tempe rature. It is shown that near T-c, H-c2 increases almost linearly with decreasing temperature. Moreover, the mixed d- and s-wave superconduc tor has a larger value of H-c2 than that of a pure d-wave superconduct or in the whole temperature region. Interestingly, our calculation una mbiguously shows that H-c2 finally becomes saturated. This implies tha t 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)50 126-9].