POSSIBLE EXISTENCE OF A VANHOVE SINGULARITY NEAR THE FERMI-SURFACE OF HGBA2CACU2O6+DELTA

The superconducting transition temperatures (T(c)'s) of HgBa2CaCU206+delta with different dopings and different ambient T(c)'s were examined magnetically under hydrostatic pressures. We observed that T(c) increases with pressure linearly at a rate of 0.22+/-0.02 K/kbar up to 18 kbar for as-synthesized samples with an ambient T(c) approximately 111 K, but at a similar initial rate only below - 4 kbar, and at a reduced rate of 0.11+/-0.02 K/kbar above, for the oxygenated samples with an ambient T(c) approximately 1 1 7 K. The observation suggests the possible existence of a Van Hove singularity in the electronic energy spectrum of HgBa2CaCU206+delta and a possible pressure-induced Fermi-surface-topology change in the oxygenated compound.