New measurements of surface impedance in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ show that the $\hat{c}$-axis penetration depth and conductivity below $T_c$ exhibit behavior different from that observed in the planes. The $\hat{c}$-axis penetration depth never has the linear temperature dependence seen in the ab plane. Instead of the conductivity peak seen in the planes, the $\hat{c}$-axis microwave conductivity falls to low values in the superconducting state, then rises slightly below 20 K. These results show that $\hat{c}$-axis transport remains incoherent below $T_c$, even though this is one of the least anisotropic cuprate superconductors.

• Received 23 March 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.1298