MICROWAVE COMPLEX IMPEDANCE MEASUREMENTS ON THE HEAVY-FERMION SUPERCONDUCTOR UBE13

The complex microwave impedance in the frequency range of similar to 8 -36 GHz was measured in the superconducting state of a well-characteri zed heavy fermion single crystal of UBe13. This microwave photon energ y ranges roughly from 15% to 60% of the zero-temperature (BCS) energy gap and is higher than previous measurements. Large changes are observ ed in the complex impedance when the sample is cooled below T-c for al l frequencies measured. The surface impedance cannot be described by t he Mattis-Bardeen theory in the dirty limit for a conventional superco nductor.