Interaction of Ultrasound with Elementary Excitations of HCP HELIUM-4.
I have made high resolution measurements of the velocity and attenuation of longitudinal sound at 10, 30 and 50 MHz in extremely high quality ^4 He crystals at low densities. The 10 MHz data shows the characteristic effects of as broad vacancy wave band. The vacancy wave-phonon relaxation time appears to be temperature independent, suggesting the vacancy waves are correlated. I found that the vacancy formation energy approaches zero at the minimum in the melting curve, and at the melting temperature as well. The 30 and 50 MHz data have this same behavior, but in addition show the effects of some resonant coupling. The form of the coupling suggests another traveling wave, possibly second sound on the vacancy gas. This may be a sign that the system is supersolid. The higher frequencies also showed signs of Akheiser attenuation. Below 150 mK, an attenuation peak occurs as well. The velocity in this region decreases with decreasing temperature. These may be a consequence of a relaxation effect.
- Pub Date:
- Physics: Condensed Matter