Effects of the CuO chains on the anisotropic penertration depth of   YBa2Cu4O8
  The temperatuer dependence of the magnetic penetration depth of grain aligned YBa2Cu4O8 has been measured along the ab plane and c-axis. Both $\lambda_ab$ and $\lambda_c$ vary as \sqrt{T} up to 0.4Tc implying a square root density of states at low energy. The results are discussed in terms of a proximity model of alternating stacked superconducting and normal layers 
Christos Panagopoulos, Jeffery L Tallon, and Tao Xiang