Influence of oxygen doping on the sign of the slope of T-c vs pressure curves in Tl2Ba2CuO6+x

We report on resistive measurements of the critical temperature of Tl2Ba2Cu O6+x single crystals with x approximate to 0 ("optimally doped", T-c = 89 K ) and x > 0 ("overdoped", T-c = 40 K) under the applied quasihydrostatic pr essure up to 1.1 GPa. The pressure has been found to shift T, upwards for t he optimally doped sample and downwards for the overdoped one. We discuss t he reasons leading to the change of the sign of dT(c)/dP upon overdoping, a nd the ramifications of our data for the existing models put forward to exp lain the influence of oxygen doping and pressure on T,. (C) 2001 Published by Elsevier Science B.V.