ELECTRONIC SPECIFIC-HEAT OF SUPERCONDUCTORS WITH VAN-HOVE SINGULARITIES - EFFECTS OF A MAGNETIC-FIELD AND THERMAL FLUCTUATIONS

The mean-field electronic specific heat of a superconductor in the pre sence of a magnetic field is calculated. We consider an energy spectru m presenting saddle points in the band structure. They correspond to V an Hove singularities in the density of states. We use a three-dimensi onal band structure by introducing a coupling energy between the CuO2 planes. Both s-wave and d-wave energy-gap-parameter cases are investig ated. It is noticed that the specific-heat jump should increase or dec rease with the field, respectively. It is found that the d-wave model seems to be qualitatively and quantitatively more realistic. The influ ence of a magnetic field on the jump of the electronic specific heat o f various high-T-c superconductors such as YBa2Cu3O7-x, HgBa2Ca2Cu3O8, and Tl2Ba2CuO6+delta near T-c is compared to mean-field results. Fluc tuations are extracted and analyzed. It is pointed out that the mean-f ield background should be extracted before analyzing the fluctuations. Critical fluctuation regimes, Gaussian, lowest Landau level, and XY r egimes as well as the effective dimensionality of the systems are obta ined. The field regimes are given on the T-c line.