Pressure dependence of T c and anisotropic features in the family Y 2Ba 4Cu 6 + nO 14 + n ( n = 0,1,2)

Abstract

The family Y 2Ba 4Cu 6 + nO 14 + n (n = 0,1,2) shows a wide range of the transition temperature T c. The pressure dependences of T c (dT c/dp) differ by one order of magnitude. We have shown that: 1. a) YBa 2Cu 3O 7 (123) has a differential dT c/dp from 0.03 K/kbar to 0.43 K/kbar, depending on the oxygen content. 2. b) YBa 2Cu 3.5O 7.5 (247) has a dT c/dp of 0.5 K/kbar with a leveling off of T c(p) at high pressure. 3. c) YBa 2Cu 4O 8 (124) has up to 18 kbar a dT c/dp of 0.57 K/kbar without a diminished pressure effect. Further, we report on the change of T c when Y is partially replaced by Sc or In. We have also investigated the 90 K compound (Y,Ba,Ca) 3Cu 4O 8 looking for a still high pressure dependence. We discuss the connection of dT c/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 ( ρ a and ρ b ) at ambient and high pressure reveals an anisotropy of a factor three and a deviation from the linear temperature behavior of ρ(T).