Excitations beyond the roton of liquid 4He in aerogel

The dynamic structure factor, S(Q,omega), for wavevectors, 2.0 less than or equal to Q less than or equal to 3.6 Angstrom(-1) of liquid He-4 in 95% porous aerogel has been measured by inelastic neutron scattering methods. The aerogel was grown with deuterated materials and the multiple scattering involving the aerogel was negligible. S(Q, omega) in the superfluid phase consists of a single peak plus broad intensity at higher energy omega, as in bulk superfluid He-4. The single peak is identified with the phonon-roton excitation at higher Q. The weight in the peak, Z(Q), and the excitation energy dispersion curve, omega(Q), has the same basic wavevector dependence as in the bulk. The energy omega(Q) is 2-3% below the bulk value at the end point and the peak is unobservable beyond Q = 3 Angstrom(-1) within the present statistical precision. No peak is observed at T = 2.3 K in normal He-4 suggesting, as in bulk He-4, that the characteristic excitation at higher Q is associated with the superfluid phase.