SPECIFIC-HEAT OF HG-BASED HIGH-TEMPERATURE SUPERCONDUCTORS NEAR T-C

The specific heat of single crystals of HgBa2Ca2Cu3O8+delta and HgBa2C uO4+delta have been measured in fields up to 7 T-c by a sensitive ac t echnique capable of measuring microgram samples. For both Hg-based sup erconductors there is evidence of strong thermal fluctuations in the s pecific heat with the mean-field step being replaced by a quasi-logari thmic divergence of C at T-c in zero field. The almost symmetric shape of the anomaly is in sharp contrast to that observed in YBa2Cu3O6.95, and seems to be intrinsic to the quasi-two-dimensiona1 high-T-c super conductors. In increasing field there is a strong reduction in the hei ght of the anomaly with the peak position being only weakly shifted to lower temperature. The applicability of fluctuation theories to the d ata will be discussed.