The microwave surface impedance of ultra-pure superconducting metals

Quasiparticle relaxation and non-local effects on the surface impedance Z(s) of pure metals are analysed for arbitrary relaxation times tau and mean free paths I. The analysis predicts a non-monotonic variation of the differential loss tangent (partial derivativeR(s)/partial derivative tau)/(partial derivativeX(s)/partial derivative tau) in the normal state, which distinguishes it from the classical limits. In the superconducting state, Z(s) can be described by a two-fluid model incorporating a Drude-type conductivity and an effective relaxation time tau (e). The t-values derived from theory and data measured on Sr2RuO3-crystals using a hollow dielectric resonator technique at 10 GHz are consistent, and in accordance with DC resistivity data.