Microwave impedance measurements on the heavy fermion superconductors UPt3 and UBe13

The complex microwave impedance in the frequency range of ∼8–36 GHz was measured in the superconducting state of two well characterized heavy fermion single crystals, UBe13 and UPt3. This microwave photon energy range is of the same order of magnitude as the zero-temperature (BCS) energy gap, 15% to 60% for the case of UBe13, and is higher than previous measurements.