Anomalous magnetic field dependence of the superconducting condensation energy in YBa2Cu3O7 single crystals

The anisotropic magnetostriction and thermal expansion of untwinned YBa2Cu3O7 single crystals have been studied using capacitance dilatometry for H parallel c along all crystallographic axes. The thermodynamical analysis is possible due to the high crystal quality. The magnetostriction coefficient λi=(1/Li)dLi/dH (i=a,b,c) is reversible above 55 K, providing important information about the pressure dependencies of the thermodynamical critical field Hc(T). The magnetic field dependence of the zero-temperature superconducting condensation energy is obtained by using the fact that the length (volume) difference between normal and superconducting states, Ln-Ls, provides a direct measure of the uniaxial pressure (hydrostatic pressure) dependence of the superconducting condensation energy. In conventional BCS superconductors, the superconducting condensation energy, as well as the magnitude of Ln-Ls, decreases monotonically with increasing field due to the increasing density of normal-state vortex cores. We find practically no field dependence of Ln-Ls as T approaches zero, which implies that the superconducting pairing energy is nearly field-independent in magnetic field up to 10 T. (orig.)