Pressure effect on the Tc of HgBa2CuO4+ with 0.07 0.39

The superconducting transition temperature (Tc) of HgBa2CuO4+ (Hg 1:2:0:1) has been determined under pressures up to 1.7 GPa over a wide range. We found that the linear pressure effect on Tc (dTc/dP) is constant at +2.0±0.1 K/GPa for <0.3, but displays a drastic change around 0.3, coinciding with the onset of a quadratic pressure effect on Tc and the boomerang path previously reported. Comparison of the present observations for <0.3 with the inverse parabolic carrier density (n) dependence of Tc suggests that the inverse parabolic Tc(n) relation is less universal than originally envisioned, and that the observed large dTc/dP of optimally doped Hg 1:2:0:1 and other members of the homologous series is a result of the change of electron bands induced by pressure.