Thermal Pulses and Second Sound in Hexagonal Close-Packed Helium-4.

摘要

The propagation of heat as a temperature wave has been observed in hcp 4He crystals grown at pressures of 25, 50, 80,and 130atmospheres. Large high quality crystals were grown by pressurization at a constant temperature. High resolution data was taken with very small input pulses by using a boxcar integrator. Pulse shapes were studied from the onset of second sound down to 80 mK where phonons cross the sample chamber ballistically. The phenomenon of 'differentiation'in the second region has been observed for the first time in the solid,and the failure to observe it in earlier work has been explained. Pulse broadening measurements give a direct measure of the dependence of the normal process relaxation time on temperature and firmly establish a 1/(T cubed) power law. (Author)