Microwave surface impedance measurements of Sr2 RuO4: The effect of impurities

The temperature, purity, and frequency dependence of the real and imaginary components of the complex surface impedance of three single crystals of the proposed p -wave superconductor Sr2 RuO4, with Tc values of 1.40, 1.24, and 0.74 K, have been measured over a range of microwave frequencies from 4-16 GHz down to 100 mK. The unusual superconducting microwave properties can be described by a multiband, two-fluid model consistent with electron relaxation times remaining largely unchanged on entering the superconducting state and energy gaps on all bands having the same temperature dependence. In addition, the model is consistent with nonmagnetic impurity scattering strongly suppressing the superconducting fractions leading to large residual losses at low temperatures and an increase in penetration depth. Model independent derivations of the complex microwave conductivity are shown to be largely consistent with such a scenario.