THERMODYNAMIC EVIDENCE FOR A DENSITY-OF-STATES PEAK NEAR THE FERMI LEVEL IN YBA2CU3O7-Y

Thermodynamic properties such as the specific-heat discontinuity at the transition temperature (T(c)), DELTAC/T(c), and the zero-temperature critical field, H(c)(0), of oxygen-deficient YBa2Cu3O7-y are analyzed to show that, within the framework of the BCS formalism, the density of states (DOS) at the Fermi level is peaked at y = 0. The energy dependence of the DOS is considered for several models and found to be consistent with the Fermi level lying close to a two-dimensional van Hove singularity.