Excitations in Few-Atomic-Layer Adsorbed Helium Films: The Two-Dimensional Roton

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Abstract
Inelastic neutron scattering from helium adsorbed on Graphon indicates the presence of two types of excitations: one bulklike, which appears to be associated with the normal density part of the film; and a second, at lower energy, which is thought to originate in the high-density liquid layer immediately adjacent to the solid layers of the film. The latter is believed to be responsible for the reduced roton gap inferred from bulk studies of few-atomic-layer superfluid films.