New excitations in bcc $^{4}$He - an inelastic neutron scattering study

We report results of inelastic neutron scattering experiments on bcc solid $^{4}$He (Pelleg et al. \textit{Phys. Rev. B.} \textbf{73}, 180301(R)(2006)). In the experiments, we studied the excitation spectrum of the solid, including the phonon branches and the recently discovered ``optic-like'' branch ( T.Markovich et al., \textit{Phys. Rev. Lett.} \textbf{88}, 195301(2002)). We were able to determine that the new ``optic-like'' branch has an intrinsic dispersion, hence it is a propagating mode. This excitation also couples to the usual phonons. In addition, in the new experiments we discovered another ``optic-like'' branch. The second ``optic-like'' branch is dispersionless, with an energy around 1 meV ($\sim$~11K). This excitation does not seem to couple to phonons. Hence, the properties of the two ``optic-like'' branches seem different. Since one expects only 3 acoustic phonon branches in a monoatomic cubic crystal, these two new branches must represent some different type of excitations. Some potential interpretations, based on Path Integral Monte Carlo simulations, will be presented.