Elementary excitations of liquid 4He in aerogel

The phonan-roton excitations of liquid He-4 immersed in aerogel of 95% porosity have been measured using inelastic neutron scattering. Excitations having wave vectors at the phonon (Q=0.2 Angstrom(-1)), maxon, roton, and beyond the roton (Q = 2.4 Angstrom(-1)) regions of the dispersion curve were investigated at temperatures between T = 0.5 K and T = 2.25 K. Aerogel grown with deuterated materials not exposed to air was used and measurements in bulk liquid He-4 were made under the same conditions for direct comparison. S(Q,omega) in He-4 in aerogel is very similar to that in bulk He-4. There is no evidence for any low-energy excitations below the phonon-roton curve or of an energy gap in the dispersion curve omega(Q) at small Q values. At T = 0.5 K and at the maxon, roton, and at Q = 2.4 Angstrom(-1), the He-4 excitation energies omega(Q) in aerogel lie a few mu eV below the bulk values and the width Gamma(Q) of S(Q,omega) has a small finite value (Gamma(Q) similar to 5 mu eV). The temperature dependence of the energies omega(Q) and widths omega(Q) are similar, but not identical, to those in the bulk. [S0163-1829(98)01218-1].