Lattice dynamics of fccHe4

The lattice dynamics of solid fcc ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, at a molar volume of 11.7 ${\mathrm{cm}}^3$, has been investigated using inelastic-neutron-scattering techniques. Well-defined phonon peaks, whose widths could be accounted for by the instrumental resolution, were observed in the vicinity of the zone center; however, these phonon peaks, when taken on opposite sides of a reciprocal-lattice point, exhibited intensity differences which could not be accounted for by conventional harmonic theory. These observations indicate the presence of interference terms between one-phonon and multiphonon scattering. From approximately halfway to the zone boundary the phonon peaks of the $L\left[100\right]\mathrm{}$, $T_2\mathrm{}\left[111\right]\mathrm{}$, and $L\left[111\right]\mathrm{}$ branches exhibited appreciable broadening which was particularly pronounced in the vicinity of the point $X$ of the Brillouin zone. Close to the zone boundary these phonons were found to be asymmetric with a rather long tail on the high-energy side. These observations are in qualitative agreement with the theoretical calculations of the scattering function. The measured dispersion curves, along the [100], [110], and [111] symmetry directions, were found to be in good agreement with Horner's theory. The latter theory is found to be in more satisfactory agreement with the experimental observations than the cutoff theories used in the heavier rare-gas solids.