PHONON SPECTRUM OF HCP HE4

The phonon dispersion relation of the quantum crystal hcp He4 has been studied for wave vectors along the [101̄0] and [0001] symmetry directions. Of the 10 branches of the spectrum along these directions, we have observed phonon groups from all but two, the TO[101̄0] and LO [0001]. The LO and LA [101̄0] branches are only partially complete. The experimental results have been compared with a calculation for the spectrum by Gillis, Koehler, and Werthamer; the agreement is found to be reasonable. The excitations are found to be well defined, except for the LO [101̄0] branch, where we observe a wave-vector-dependent relaxation. The single crystals of helium used for the experiment were held at 1.03 ± 0.09°K and had a molar volume of 21.1 ± 0.1 cm3. © 1968 The American Physical Society.