Possible existence of a Van Hove singularity near the Fermi surface of HgBa2CaCu2O6+

The superconducting transition temperatures (Tcs) of HgBa2CaCu2O6+ with different dopings and different ambient Tcs were examined magnetically under hydrostatic pressures. We observed that Tc increases with pressure linearly at a rate of 0.220.02 K/kbar up to 18 kbar for as-synthesized samples with an ambient Tc111 K, but at a similar initial rate only below 4 kbar, and at a reduced rate of 0.110.02 K/kbar above, for the oxygenated samples with an ambient Tc117 K. The observation suggests the possible existence of a Van Hove singularity in the electronic energy spectrum of HgBa2CaCu2O6+ and a possible pressure-induced Fermi-surface-topology change in the oxygenated compound.