OXYGEN ORDERING EFFECTS AND THE SUPERCONDUCTING TRANSITION-TEMPERATURE T-C OF YBA2CU3OX UNDER PRESSURE

The superconducting transition temperature T-c of YBa2Cu3Ox depends strongly on the oxygen content x and the degree of oxygen ordering in the CuO-chains. This oxygen ordering can be influenced drastically by storing an oxygen deficient sample at temperatures T-s above 240K. At lower temperatures the degree of oxygen ordering is preserved, even when pressure is applied. Therefore applying pressure at low temperatures close to T-c results in a T-c increase that is not affected by oxygen ordering. This intrinsic pressure effect (dT(c)/dp)(i) depends strongly on the oxygen content x and shows a distinct maximum of 7.4K/GPa at x=6.72. For samples close to the maximum oxygen content x=7.0 the intrinsic pressure effect is almost zero. For some single crystals negative (dT(c)/dp)(i) values have been observed at this oxygen content. If a sample is stored at temperatures T-s above 240K pressure induced oxygen rearrangement is possible. This causes a T-c relaxation under pressure. The T-c increase due to a completed oxygen reordering under pressure (dT(c)/dp)(0) has a maximum at x=6.72, too. At x close to 7.0 this additive pressure effect vanishes according to the lack of oxygen ordering possibilities.