Excitations beyond the roton of liquid He-4 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% po rous aerogel has been measured by inelastic neutron scattering methods. The aerogel was grown with deuterated materials and the multiple scattering in volving the aerogel was negligible. S(Q, omega) in the superfluid phase con sists of a single peak plus broad intensity at higher energy omega, as in b ulk superfluid He-4. The single peak is identified with the phonon-roton ex citation at higher Q. The weight in the peak, Z(Q), and the excitation ener gy 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 po int and the peak is unobservable beyond Q = 3 Angstrom(-1) within the prese nt statistical precision. No peak is observed at T = 2.3 K in normal He-4 s uggesting, as in bulk He-4, that the characteristic excitation at higher Q is associated with the superfluid phase.