δ-Influence on the pressure-effect on T_c of HgBa₂CuO_4+δ and the inverse parabolic T_c-relation

We have observed a δ-independent linear pressure effect on T_c of HgBa₂CuO_4+δ under hydrostatic pressures up to 1.7 GPa for δ≤0.30 but a nonlinear effect for δ>0.30. These results, together with previous doping data, show a possible non-rigidity of the electron bands of the compounds, based on which previous phenomenological models of the pressure effect on T_c are built, and thus suggest only a restricted universality for the inverse parabolic relation between T_c and charge carriers per CuO₂-layer. In other words, factors in addition to charge carriers can affect high temperature superconductivity.