Microwave surface impedance measurements on the heavy fermion superconductor UPt₃

We have used cavity perturbation techniques to study the microwave surface impedance of the heavy fermion intermetalic compound UPt₃ in the superconducting and normal states. The surface impedance as a function of temperature was measured in the frequency range from ∼ 9 to 38 GHz. These microwave photon energies are about 20% to 95% of the estimated zero-temperature energy gap of UPt₃. A change of slope was observed in the resistance vs. temperature curves at T_c. No evidence for a collective mode was observed for all the frequencies down to the lowest temperature reached ( ∼ 0.1 K).