Elementary excitations of liquid He in aerogel

The phonon-roton excitations of liquid He immersed in aerogel of 95% porosity have been measured using inelastic neutron scattering. Excitations having wave vectors at the phonon (Q50.2 Å ), maxon, roton, and beyond the roton (Q52.4 Å ) regions of the dispersion curve were investigated at temperatures between T50.5 K and T52.25 K. Aerogel grown with deuterated materials not exposed to air was used and measurements in bulk liquid He were made under the same conditions for direct comparison. S(Q ,v) in He in aerogel is very similar to that in bulk He. There is no evidence for any low-energy excitations below the phonon-roton curve or of an energy gap in the dispersion curve vQ at small Q values. At T50.5 K and at the maxon, roton, and at Q52.4 Å, the He excitation energies vQ in aerogel lie a few meV below the bulk values and the width GQ of S(Q ,v) has a small finite value (GQ;5 meV!. The temperature dependence of the energies vQ and widths GQ are similar, but not identical, to those in the bulk. @S0163-1829~98!01218-1#