Neutron scattering study of the excitation spectrum of solid helium at ultra-low temperatures

There has been a resurgence of interest in the properties of solid helium due to the recent discovery of non-classical rotational inertia (NCRI) in solid ⁴He by Chan and coworkers below 200 mK which they have interpreted as a transition to a ‘supersolid’ phase. We have carried out a series of elastic and inelastic neutron scattering measurements on single crystals of hcp ⁴He at temperatures down to 60 mK. While we have found no direct evidence of any change in the excitation spectrum at low temperatures, we have found that the excitation spectrum of solid ⁴He shows several interesting features, including extra branches in addition to the phonon branches. We interpret these extra branches as single particle excitations due to propagating vacancy waves, which map on to the famous ‘roton minimum’ long known in the excitation spectrum of superfluid liquid ⁴He. The results show that in fact solid ⁴He shares several features in common with the superfluid.