cAxis Superfluid Response of Copper Oxide Superconductors

A novel interplay between d-wave superconducting order parameter symmetry and the underlying Cu $3d$ orbital based electronic structure of copper oxides leads to a striking anisotropy in the superfluid response of these systems. In clean tetragonal materials the c axis penetration depth increases as T⁵ at low temperature, in contrast to linear T behavior in the ab plane. Disorder is a relevant perturbation which causes all components of the superfluid response to depend quadratically on temperature at low temperature. However, the crossover temperature scale from the intrinsic d-wave behavior to the disorder dominated behavior for the in-plane response may be different from that for the out-plane response.