Listening to Phase Transitions - Ultrasound Techniques in the Study of Heavy Fermion Systems.*

One of the early triumphs of the BCS theory of superconductivity was that it successfully predicted the behavior of ultrasound attenuation in tin and indium. Since then ultrasound techniques have been used to study conventional and unconventional superconductivity, as well as a range of other phases.

In this talk I would like to present results from ultrasound attenuation and speed of sound measurements on U₆Fe under hydrostatic pressures of up to ~7 kBar. U₆Fe is on the light side of what are considered heavy fermion systems with an effective electronic mass of ~20m_e, and has, at ambient pressure, a superconducting (SC) as well as a charge density wave (CDW) transition at about 4K and 9K respectively. 

By tuning the pressure we are able to gradually suppress the CDW, revealing an interesting interplay between it and SC. Upon the SC transition a peak is seen in the ultrasound attenuation, similar to what has been observed in much heavier fermionic systems, but unexpected from the point of view of BCS theory. We wish to investigate what these results imply for the SC gap structure.