SPECIFIC-HEAT OF YBA2CU3O7-DELTA CERAMICS WITH SINGLE AND DOUBLE SUPERCONDUCTING TRANSITIONS IN MAGNETIC-FIELDS UP TO 14-T

The specific heat of porous YBa2Cu3Oapproximate to 7.00 ceramics with two well separated steps at the superconducting transition was measure d in magnetic fields up to 14 T. The same measurements were performed for an YBa2Cu3Oapproximate to 6.93 ceramics from a similar batch annea led reversibly to obtain one single sharp transition. The results show that both components of the double transition, as well as the single transition, are transformed in the same way under the application of a field, both in position and width. It is concluded that both componen ts of the split transition are of superconducting origin, and are rela ted with the existence of two kinds of homogeneous domains, the typica l sizes of which are large compared to the superconducting coherence l ength. Depending on the criterion chosen for the definition of Tc(H), the angle-averaged critical-field slopes mu(0) partial derivative H-c2 /partial derivative T\(Tc) are found to range from -5.4 to -10.1 T/K f or all transitions. The slopes for both components of the split transi tion are equal within +/- 10%.