Surface Impedance Studies of YBCO
A. Hosseini, S. Kamal, D.A. Bonn, Ruixing Liang, W.N. Hardy
Published in PRL: Phys. Rev. Lett., 81, 1298 (1998).

New measurements of the surface impedance in YBa₂Cu₃O_7-x show that the c-axis penetration depth and conductivity below T_c exhibit behaviour different from that observed in the planes. The c-axis penatration depth never has the linear temperature dependence seen in the ab-plane. Instead of the conductivity peak seen in the planes, the c-axis microwave conductivity falls to low values in the superconducting state, then rises slightly below 20 K. These results show that c-axis transport remains incoherent below T_c, even though this is one of the least anisotropic cuprate superconductors.