Microwave surface resistance measurements on the heavy fermion superconductor UPt3

The relative change of the microwave surface resistance has been measured as a function of temperature in the superconducting and normal states of UPt₃ using a cavity perturbation technique. With our frequency range of ∼9–38 GHz, the microwave photon energy (ℏω) is ∼20–95% of the estimated zero-temperature (BCS) energy gap (2Δ₀) of UPt₃. The surface resistance shows a change in slope as the signature of T_c and has a significantly weaker temperature dependence in the superconducting state than would be predicted by the Mattis–Bardeen theory. At higher frequencies, the slope tends to change by a smaller amount. No indication of collective modes at any of the frequencies was observed down to the lowest temperatures measured (∼0.1 K).