Pressure dependence of T_c in HgBa₂cuo_4+δ up to 18 GPa with 0.17<δ<0.3

The superconducting transition temperature T_c of several HgBa₂CuO_4+δ samples with 0.17<δ<0.3 has been measured as the function of the quasi-hydrostatic pressure P up to 18 GPa. The observed T_c^max under pressure is nearly δ-independent around the optimum doping level, demonstrating directly that the pressure-induced charge-transfer only plays a minor role in the unusual pressure effects of the Hg-based cuprates. The δ-range, over which HgBa₂CuO_4+δ is superconducting, significantly shrinks under pressure, and the T_c-δ correlation seems to evolve from parabolic at ambient pressure to truncated-triangle like above 10 GPa.