The Low Temperature Microwave Properties of GdBa₂Cu₃O_7-δ and Sr₂RuO₄

Measurements of the microwave surface resistance R_s and reactance X_s of GdBa₂CU₃O_7-δ and Sr₂RuO₄ single crystals are reported in the temperature range 0.4 to 25 K at frequencies from 10 to 20 GHz, using a hollow dielectric resonator technique. The GdBa₂Cu₃O_7-δ measurements are interpreted in terms of antiferromagnetic alignment of the Gd spins, resulting in a Nèel transition at 2.25 K, with a strongly temperature spin relaxation time both above and below the transition. Above T_c = 1.45 K, the normal state surface impedance of the suspected p-wave superconductor Sr₂RuO₄ agrees rather well with that expected from dc measurements; however, below T_c, we observe an unusual increase in the surface reactance, which we attribute to a long electron relaxation time. Using a two-fluid model, we obtain a normal fluid component that decreases almost linearly with temperature and extrapolates to a finite low-temperature value.