Relaxation effects in the transition temperature of superconducting HgBa2CuO4+delta

In previous studies on a number of underdoped and overdoped high-temperatur e superconductors, including YBa2Cu3O7-y and Tl2Ba2CuO6+delta, the transiti on temperature T-c has been found to change with time in a manner which dep ends on the sample's detailed temperature and pressure history. This relaxa tion behavior in T-c is believed to originate from rearrangements within th e oxygen sublattice. In the present high-pressure studies on HgBa2CuO4+delt a to 0.8 GPa we find clear evidence for weak relaxation effects in strongly underdoped and overdoped samples (Tc similar or equal to 40-50 K) with an activation energy E-A(1 bar)similar or equal to 0.8-0.9 eV. For overdoped H gBa2CuO4+delta E-A increases under pressure more rapidly than previously ob served for YBa2Cu3O6.41, yielding an activation volume of + 11+/-5 cm(3); t he dependence of T-c on pressure is markedly nonlinear, an anomalous result for high-T-c superconductors in the present pressure range, giving evidenc e for a change in the electronic and/or structural properties near 0.4 GPa.