THE UPPER CRITICAL-FIELD OF AN ANHARMONIC SUPERCONDUCTOR

We investigate the orbital upper critical field of a dirty superconductor with strong anharmonic lattice vibrations. We show that anharmonicity gives rise to an anomalously large upper critical field with an upward curvature in the temperature dependence of H(c2)(T). It is found that the magnitude of the corresponding electron-phonon coupling constant lambda is quite reasonable. Possible implications for the layered oxide superconductors as short coherence length and large gap are briefly discussed.