Microwave spectroscopy of novel superconductors

A hollow dielectric resonator technique has been developed for accurate measurements of the complex microwave surface impedance of novel superconductors from 2 to 20 GHz down to 100 mK. Illustrative measurements of the antiferromagnetic ordering and spin-dynamics of Gd ions in GdBa 2Cu 3O 7 and of microwave penetration and losses in MgB 2 wires and high quality single crystals of Sr 2RuO 4 are presented. Measurements on Sr 2RuO 4 imply an unchanged electron relaxation time τ on entering the superconducting state, a power law temperature dependence of the normal state fraction implying nodes in the superconducting order parameter, and residual low temperature losses on increasing frequency.