PRESSURE EFFECT ON THE T-C OF HGBA2CUO4+DELTA WITH 0.07-LESS-THAN-OR-EQUAL-TO-DELTA-LESS-THAN-OR-EQUAL-TO-0.39

The superconducting transition temperature (T-c) of HgBa2CuO4+delta (Hg 1:2:0:1) has been determined under pressures up to 1.7 GPa over a wide delta range. We found that the linear pressure effect on T-c (dT(c)/dP) is constant at +2.0+/-0.1 K/GPa for delta<0.3, but displays a drastic change around delta similar to 0.3, coinciding with the onset of a quadratic pressure effect on T-c and the ''boomerang path'' previously reported. Comparison of the present observations for delta<0.3 with the inverse parabolic carrier density (n) dependence of T-c suggests that the inverse parabolic T-c(n) relation is less universal than originally envisioned, and that the observed large dT(c)/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.