In-Plane Anisotropy of the Penetration Depth Due to Superconductivity on the Cu-O Chains in YBa2Cu3O7−δ, Y2Ba4Cu7O15−δ, and YBa2Cu4O8

Abstract

Muon spin relaxation studies on a variety of polycrystalline $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$, ${\mathrm{Y}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_7{\mathrm{O}}_{15-\delta }$, and Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ samples show that the $\delta$-dependent depolarization rate (and hence $\frac{n_s}{m^{*}}$) is greatly enhanced when the CuO chains are free of disorder ($\delta \to 0$). 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 $n_s$. The in-plane London penetration depth is highly anisotropic: ${\lambda }_a=155\mathrm{}$ nm but ${\lambda }_b$ falls as low as 80 nm when $\delta \to 0$.