Hydrostatic and uniaxial pressure effect on Tc of YBa2Cu3Ox

The Variation of the transition temperature Tc of YBa2Cu3Ox with hydrostatic He-gas pressure depends on the oxygen content x. The pressure effect dTc/dp increases from small negative values at x=7 to dTc/dp=7.4 K/GPa at x=6.7. For oxygen contents below x=6.7 dTc/dp drops to 3 K/GPa and remains nearly constant1. The charge transfer model2 cannot explain the drop at x=6.7.