INPLANE ANISOTROPY OF THE PENETRATION DEPTH DUE TO SUPERCONDUCTIVITY ON THE CU-O CHAINS IN YBA2CU3O7-DELTA, Y2BA4CU7O15-DELTA, AND YBA2CU4O8

Muon spin relaxation studies on a variety of polycrystalline YBa2Cu3O7-δ, Y2Ba4Cu7O15-δ, and YBa2Cu4O8 samples show that the δ-dependent depolarization rate (and hence nsm*) is greatly enhanced when the CuO chains are free of disorder (δ→0). Ca substitution allows the doping on the planes and chains to be independently controlled and the enhanced depolarization rate can be unambiguously attributed to condensation of mobile carriers on the chains as Cooper pairs, thus increasing the total condensate density ns. The in-plane London penetration depth is highly anisotropic: λa=155 nm but λb falls as low as 80 nm when δ→0. © 1995 The American Physical Society.