a-b plane anisotropy in YBCO

Özet

The zero temperature in plane penetration depth in an untwinned single crystal of optimally doped $YBa_2Cu_3O_{6.93}$ is highly anisotropic. This fact has been interpreted as evidence that a large amount of the condensate resides in the chains. On the other hand, the temperature dependence of $\lambda_a(T)/\lambda_a(0)$ and $\lambda_b,(T)/\lambda_b(0)$ (where the b-direction is along the chains) are not very different. This constrains theories and is, in particular, difficult to understand within a proximity model with d-wave pairing only in the $CuO_2$ plane and none on the CuO chains but instead supports a more three dimensional models with interplane interactions.