Ultrasonic studies of the heavy fermion compounds

A review of ultrasonic measurements on the three heavy fermion compounds UPt₃, URu₂Si₂ and CeCu₆ is given. These compounds show one or more of the three types of co-operative behavior; Kondo-like effect, antiferromagnetism and superconductivity; all these features are observed as ultrasonic anomalies. Both UPt₃ and CeCu₆ show a strong coherence effect which is akin to a Kondo-like transition in the electronic system. Velocity anomalies are found at the antiferromagnetic transition for URu₂Si₂. UPt₃ also has an antiferromagnetic transition which is believed to influence the subsequent superconducting ordering. For UPt₃, the superconductivity is of an unconventional nature; multiple superconducting phases have been observed and a complete phase diagram has been obtained from high resolution velocity measurements.