Localized Collective Excitations in Superfluid Helium in Vycor

Inelastic neutron scattering has been used to probe the collective excitations of superfluid helium in porous Vycor glass for 0.5 less than or equal to T(K) less than or equal to 1.9. The confined roton displays a temperature dependence similar to that of the bulk roton but with larger widths. Additional scattering has been observed near the roton minimum but with a substantially lower energy than the bulk roton. This scattering is consistent with a "two-dimensional roton," predicted to occur at the solid-liquid interface and inferred from measurements of the heat capacity and superfluid fraction.