HYDROSTATIC-PRESSURE DEPENDENCE OF THE SUPERCONDUCTING TRANSITION-TEMPERATURE TO 7-GPA IN BI2CA1SR2CU2O8+Y AS A FUNCTION OF OXYGEN-CONTENT

The dependence of the superconducting transition temperature T(c) on hydrostatic pressure to 7 GPa has been measured on Bi2Ca1Sr2Cu2O8+y single crystals for three different oxygen concentrations using a diamond-anvil cell and highly densified helium as pressure medium. In all three cases, a markedly nonlinear behavior in T(c)(P) is observed. Whereas the increasingly oxygen-rich samples with T(c)(0) = 88 K and 80 K both show an initial increase in T(c) at the rate dT(c)/dP = + 1.2 K/GPa followed by maximum in T(c)(P) at 1.5 GPa and 4 GPa, respectively, the oxygen-poor sample with T(c)(0) = 54 K exhibits little change in T(c) to 1.5 GPa, followed by a downward step and slow decrease at higher pressures. A markedly different pressure dependence T(c)(P) is observed if no pressure medium is used so that the anvils press directly onto the sample. It is not possible to account for the present results using a simple charge-transfer model.