In previous studies on a number of under- and overdoped high temperature superconductors, including YBa_{2}Cu_{3}O_{7-y} and Tl_{2}Ba_{2}CuO_{6+\delta}, the transition temperature T_c has been found to change with time in a manner which depends on the sample's detailed temperature and pressure history. This relaxation behavior in T_c is believed to originate from rearrangements within the oxygen sublattice. In the present high-pressure studies on HgBa_{2}CuO_{4+\delta} to 0.8 GPa we find clear evidence for weak relaxation effects in strongly under- and overdoped samples (Tc≃40−50K) with an activation energy EA(1bar)≃0.8−0.9eV. For overdoped HgBa_{2}CuO_{4+\delta} E_{A} increases under pressure more rapidly than previously observed for YBa_{2}Cu_{3}O_{6.41}, yielding an activation volume of +11 \pm 5 cm^{3}; the dependence of T_c on pressure is markedly nonlinear, an anomalous result for high-T_c superconductors in the present pressure range, giving evidence for a change in the electronic and/or structural properties near 0.4 GPa.
@article{arxiv.cond-mat/9905332,
title = {Relaxation Effects in the Transition Temperature of Superconducting HgBa2CuO4+delta},
author = {S. Sadewasser and J. S. Schilling and J. L. Wagner and O. Chmaissem and J. D. Jorgensen and D. G. Hinks and B. Dabrowski},
journal= {arXiv preprint arXiv:cond-mat/9905332},
year = {2009}
}