Electrodynamic response of Sr2 RuO4

The surface impedance Zs = Rs +i Xs of a number of Sr2 RuO4 crystals has been measured in the frequency range f∼2-15 GHz using a dielectric resonator technique. Zs conforms to the classical skin effect only above 20 K, because the quasiparticle relaxation rate 1 τ approaches 2πf at lower temperatures. Below Tc, Rs decreases while Xs initially increases and peaks at a frequency-dependent temperature. The microwave properties are quite unlike those of conventional and high- Tc superconductors and imply a finite quasiparticle fraction at T=0 with a temperature-independent relaxation time throughout the superconducting state.