  Recent theoretical and experimental work on solid He-4 has focused on
vacancies, or other lattice defects and whether these can form a Bose-Einstein
condensate within the solid. The earliest suggestion that this could happen was
based on the conjecture that the ground state of the solid at T = 0 K would
include vacancies. Although no direct evidence for zero point vacancies has
been presented, a variety of experimental observations have been interpreted as
being due to thermally activated vacancies. Here we directly probe the
excitation spectrum of hcp solid He-4 using inelastic neutron scattering. We
identify a branch of delocalized excitations, with both longitudinal and
transverse dispersion that coexist with acoustic phonons. While the energy gap
is larger and the characteristic wavevector is shifted to coincide with the
position of the (100) Bragg peak, the longitudinal effective mass of this mode
is within 15% of that for rotons in superfluid He-4. The dispersion relation
intersects the longitudinal acoustic phonon half way to the zone boundary, and
this distinguishes the mode from a conventional optic phonon. Our results
provide direct evidence for coherent delocalized non-phonon excitations in
solid helium, which we identify as delocalized vacancy modes.
