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Abstract: Recent theoretical and experimental work on solid He-4 has focused onvacancies, or other lattice defects and whether these can form a Bose-Einsteincondensate within the solid. The earliest suggestion that this could happen wasbased on the conjecture that the ground state of the solid at T = 0 K wouldinclude vacancies. Although no direct evidence for zero point vacancies hasbeen presented, a variety of experimental observations have been interpreted asbeing due to thermally activated vacancies. Here we directly probe theexcitation spectrum of hcp solid He-4 using inelastic neutron scattering. Weidentify a branch of delocalized excitations, with both longitudinal andtransverse dispersion that coexist with acoustic phonons. While the energy gapis larger and the characteristic wavevector is shifted to coincide with theposition of the 100 Bragg peak, the longitudinal effective mass of this modeis within 15% of that for rotons in superfluid He-4. The dispersion relationintersects the longitudinal acoustic phonon half way to the zone boundary, andthis distinguishes the mode from a conventional optic phonon. Our resultsprovide direct evidence for coherent delocalized non-phonon excitations insolid helium, which we identify as delocalized vacancy modes.



Autor: E. Blackburn, S. K. Sinha, C. Broholm, J. R. D. Copley, R. W. Erwin, J. M. Goodkind

Fuente: https://arxiv.org/







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