Ultrasound Studies of Superconducting UB13 and (Th,U) Be13-Observation of a New Peak in The Sound Velocity

We report on low frequency ultrasound studies of UBe13 and (Th,U) BE13. At low frequencies (10-100 MHZ) and at low acoustic powers we have observed a new resonance peak well below Tc. This new feature moves lower in temperature at higher frequencies and disappears for frequencies in exceeds of 100 MHZ. The observation of this feature also depends in a sensitive way on the acoustic power used and disappears at high powers. Our results suggest observation of a new collective mode or domain wall resonances as recently discussed by Joynt, Rice and Ueda for anisotropic superconductivity.