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

The dependence of the superconducting transition temperature T(c) on h ydrostatic pressure to 7 GPa has been measured on Bi2Ca1Sr2Cu2O8+y sin gle crystals for three different oxygen concentrations using a diamond -anvil cell and highly densified helium as pressure medium. In all thr ee cases, a markedly nonlinear behavior in T(c)(P) is observed. Wherea s the increasingly oxygen-rich samples with T(c)(0) = 88 K and 80 K bo th 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, th e 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 observe d 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 usi ng a simple charge-transfer model.