Electrodynamic response of Sr2Ru04

The surface impedance Z(s)=R-s+iX(s) of a number of Sr2RuO4 crystals has been measured in the frequency range f similar to 2-15 GHz using a dielectric resonator technique. Z(s) conforms to the classical skin effect only above 20 K, because the quasiparticle relaxation rate 1/tau approaches 2 pi f at lower temperatures. Below T-c, R-s decreases while X-s initially increases and peaks at a frequency-dependent temperature. The microwave properties are quite unlike those of conventional and high-T-c superconductors and imply a finite quasiparticle fraction at T=0 with a temperature-independent relaxation time throughout the superconducting state.