PRESSURE-DEPENDENCE OF TC AND ANISOTROPIC FEATURES IN THE FAMILY Y2BA4CU6+0O14+0, Y2BA4CU6+1O14+1, Y2BA4CU6+2O14+2

The family Y2Ba4Cu6 + nO14 + n (n = 0,1,2) shows a wide range of the transition temperature Tc. The pressure dependences of Tc (dTc/dp) differ by one order of magnitude. We have shown that: 1. a) YBa2Cu3O7 (123) has a differential dTc/dp from 0.03 K/kbar to 0.43 K/kbar, depending on the oxygen content. 2. b) YBa2Cu3.5O7.5 (247) has a dTc/dp of 0.5 K/kbar with a leveling off of Tc(p) at high pressure. 3. c) YBa2Cu4O8 (124) has up to 18 kbar a dTc/dp of 0.57 K/kbar without a diminished pressure effect. Further, we report on the change of Tc when Y is partially replaced by Sc or In. We have also investigated the 90 K compound (Y,Ba,Ca)3Cu4O8 looking for a still high pressure dependence. We discuss the connection of dTc/dp with the strong shift of the apical oxygen towards the CuO plane at applied pressure, as detected by neutron diffraction. The n = 1 and n = 2 members have the great advantage, not to be twinned. So it was possible for the first time to resolve the anisotropy of the a- and b direction. We have measured the polarized reflectance R(ω) with respect to the a- and b direction of single crystals from 0.001 to 10 eV. The resistivity in a- and b direction (ρa and ρb) at ambient and high pressure reveals an anisotropy of a factor three and a deviation from the linear temperature behavior of ρ(T). © 1990.