Specific heat of Hg-based high temperature superconductors near T-c

The specific heat of single crystals of HgBa2Ca2Cu3O8+delta and HgBa2CuO4+delta have been measured in fields up to 7 T-c by a sensitive ac technique capable of measuring microgram samples. For both Hg-based superconductors there is evidence of strong thermal fluctuations in the specific heat with the mean-field step being replaced by a quasi-logarithmic 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 superconductors. In increasing field there is a strong reduction in the height of the anomaly with the peak position being only weakly shifted to lower temperature. The applicability of fluctuation theories to the data will be discussed.