Excitations Beyond the Roton of Liquid 4He in Aerogel

Author: Azuah R.T.   Glyde H.R.   Beamish J.R.   Adams M.A.  

Publisher: Springer Publishing Company

ISSN: 0022-2291

Source: Journal of Low Temperature Physics, Vol.117, Iss.1-2, 1999-10, pp. : 113-125

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Abstract

The dynamic structure factor, S(Q, ω), for wavevectors, 2.0≤Q≤3.6 Å-1 of liquid 4He 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, ω) in the superfluid phase consists of a single peak plus broad intensity at higher energy ω, as in bulk superfluid 4He. The single peak is identified with the phonon-roton excitation at higher Q. The weight in the peak, ZQ, and the excitation energy dispersion curve, ωQ, has the same basic wavevector dependence as in the bulk. The energy ωQ is 2–3% below the bulk value at the end point and the peak is unobservable beyond Q=3 Å-1 within the present statistical precision. No peak is observed at T=2.3 K in normal 4He suggesting, as in bulk 4He, that the characteristic excitation at higher Q is associated with the superfluid phase.