Specific Heat of Type-II Superconductors in Magnetic Fields Near T-c

The GLAG theory predicts that the value of the specific heat C-M in the mixed state just below the transition to the normal state, is a function of the Ginzburg-Landau (GL) parameter to, even for vanishingly small magnetic fields (T approximate to To). CM is thus different from the specific heat C-s in the pure superconducting state (H = 0) for all K except kappa congruent to 1.9. Experimental results of the authors generally confirm this paradoxical prediction which is not in contradiction with the thermodynamics of phase transitions. To experimentally clarify this behavior close to T-c, Nb80Mo20 (kappa congruent to 4) and Pb98In2 (kappa congruent to 0.8) specimens are studied in fields much smaller than those usually applied (down to 17 Oe). A new method for measuring specific heat in decreasing as well as increasing temperature is introduced. Very good agreement with the theory is found for fields higher than similar to 120 Oe while a deviation is observed close to T-c for lower fields where values of the Abrikosov lattice parameter would become larger than 1 mu. The sense and the magnitude of the deviation are such that C-M and C-S would be equal for fields of 20 Oe and below. Possible explanations are discussed. It is suggested that a fundamental modification of the vortex state structure occurs in small fields close to T-c.