Observation of a new excitation in bcc solid 4He by inelastic neutron   scattering
  We report neutron scattering measurements of the phonons in bcc solid 4He. In general, only 3 accoustic phonon branches should exist in a monoatomic cubic crystal. In addition to these phonon branches, we found a new ''optic-like'' mode along the [110] direction. One possible interpretation of this new mode is in terms of localized excitations unique to a quantum solid. 
T. Markovich, E. Polturak, J. Bossy, and E. Farhi