Anharmonic effects in neutron scattering studies of lattice excitations in BCC4He

We report high resolution measurements of lattice excitations of bcc solid He-4 using inelastic neutron scattering. The resulting dispersion relations of the phonons agree well with Path Integral Monte Carlo Simulations which we carried out in parallel with the experimental work. In addition, we studied the recently discovered two "optic-like" excitation branches. One of these branches is weakly dispersive, while the second one seems dispersionless. The dispersive optical branch interacts with the phonons. We suggest that this interaction may be responsible for some of the anharmonic effects reported in the past, such as phonon interference. We argue that the agreement of experimental results with the "Self Consistent Phonon Theory" may be improved if the coupling between the phonons and one of the "optic-like" excitation branches is included in the calculations.