PRESSURE-DEPENDENCE OF TC AND CHARGE-TRANSFER IN YBA2CU3OX (6.35 LESS-THAN-OR-EQUAL-TO X LESS-THAN-OR-EQUAL-TO 7) SINGLE-CRYSTALS

The superconducting critical temperature T(c) of YBa2Cu3Ox single crystals (6.3 5 less than or similar to x less than or similar to 7) has been measured resistively as a function of pressure P (0 less-than-or-equal-to P less than or similar to 20 kbar). The initial rate dT(c)/dP exhibits three distinct regimes, a narrow plateau near x almost-equal-to 7 with dT(c)/dP almost-equal-to 0.04 K/kbar, a plateau in the range 6.4 less than or similar to x less than or similar to 6.8, where dT(c)/dP-0.43 K/kbar, and a maximum value approximately 0.8 K/kbar at x almost-equal-to 6.35. An analysis of the T(c)(x,P) data using a phenomenological model yields a pronounced peak near x=6.8 in d-dELTA-n(h)(x)/dP, where DELTA-n(h)(x) is the change in the hole density in the CUO2 Planes relative to the value corresponding to the fully oxygenated sample.