We have improved a microwave self-resonant technique to measure surface resistance ${\mathit{R}}_{\mathit{s}}$ directly and surface inductance ${\mathit{L}}_{\mathit{s}}$ indirectly. For films prepared by the laser ablation technique we observed that ${\mathit{R}}_{\mathit{s}}$ at 21 GHz decreased by about three orders of magnitude as the temperature decreased from 90 to 80 K reaching a low value of 4.9×${10}^{\mathrm{-}4}$ Ω. We measured the London penetration depth λ and coherence length ξ, and found that both λ and ξ are anisotropic. Their values depended on the direction of the microwave electric field relative to the c axis. We deduced the value of ${\lambda }_{\mathrm{\parallel }}$(0) to be about 1800 Å, ${\lambda }_{\mathrm{\parallel }}$(86.5) about 8000 Å, and ${\lambda }_{\mathrm{\perp }}$(86.5) about 26 000 Å, where ${\lambda }_{\mathrm{\parallel }}$(0) is the penetration depth (as T→0 K) for the electromagnetic electric field parallel and ${\lambda }_{\mathrm{\perp }}$ perpendicular to the film plane. In addition ${\xi }_{\mathrm{\parallel }}$ was determined to be 129 Å and ${\xi }_{\mathrm{\perp }}$ equal to 40 Å at 86.5 K. The anisotropic factor γ is about 3.

• Received 12 November 1993

DOI:https://doi.org/10.1103/PhysRevB.49.9924