Hydrostatic pressure on HgBa₂CaCu₂O _6+δ and HgBa₂Ca₂Cu₃O _8+δ

The superconducting transition temperature T_c and its pressure dependence dT_c/dP of HgBa₂CaCu₂O _6+δ (Hg-1212) and HgBa₂Ca₂Cu ₃O_8+δ (Hg-1223) were measured up to 17 kbar. T _c increases with pressure approximately linearly for both compounds before oxidation. However, the nonlinearity in the T_c-P correlation shows up after oxidation in both compounds. For Hg-1212, the average dT _c/dP decreases as oxygen doping increases, while it increases in Hg-1223. These observations are in conflict with the modified pressure-induced charge-transfer model, but might be attributed to the possible existence of fine electronic structure. These observations suggest that a T_c much higher than 140 K might be achievable in Hg-1223 by means of higher physical or chemical pressure.