No abstract

No abstract

Abstract. First measurements are reported on pure surface excitations in thin superfluid 4 He films on silica aerogel. The ripplon dispersion curve is found to be the same for helium on graphite and silica aerogel substrates. However, the layered roton line width in helium films on aerogel shows a pronounced broadening with respect to that found in helium films on graphite. 61.12.Ex Surface excitations were first measured on graphite [1,2] and modelled using a many-body theory [3]. The excitations present in very thin films are composed of ripplons and layered rotons. The ripplon is an excitation of the liquid-gas interface of the film, while the layered roton is an excitation bound to the layered structure of the liquid film itself. In the case of a very thin film only one layered structure and one layered roton exist adjacent to the solid-liquid interface. The liquid film is formed on top of a solid helium layer: on graphite, for instance, firstly two solid layers of helium are formed upon condensing helium in the sample. These solid layers also exist on aerogel substrate, but they are strongly modified due to the heterogeneous surface of this substrate. On a given substrate one can find a coverage of helium at which only the two above-mentioned excitations, ripplons and layered rotons, exist, i.e. no bulk excitations, e.g. phonons, maxons or rotons are observed. The ripplon dispersion relation is determined by the liquid-gas-interface properties, and therefore it shows essentially the same behaviour on graphite and on silica aerogel. On the other hand, the properties of the layered roton are expected to be affected by the nature of the solid-liquid interface; they are most conveniently determined at this particular coverage. The layered roton gap energy is determined by the substrate-helium interaction [3]; the curvature reflects its two-dimensional character through the liquid structure factor and the effective mass [3,4]; and its line width is anomalously broad. The last feature may have importance in relation to the bulk helium behaviour in aerogel, seen * Corresponding author. (Fax: +33-476/207120, E-mail: lauter@ill.fr) when the aerogel is filled with helium, because the temperature dependence of the superfluid component shows near the lambda transition a different exponent with respect to helium in graphite [5]. PACS:

An investigation of the dispersion of excitations in a quantum liquid, superfluid He4, is carried out. An attempt is made to systematize the published experimental data that indicate a substantially different nature of excitations with wave vectors corresponding to different parts of the dispersion curve of liquid He4. Neutron spectroscopy data are analyzed in relation to a certain physical hypothesis concerning the formation of such a spectrum, and it is found that the majority of the known experimental facts can be explained in framework of that hypothesis. Particular attention is paid to a comparison of the experimental data obtained on the DIN-2PI time-of-flight spectrometer (at the IBR-2 Reactor, Dubna) with the results obtained at foreign research centers.

No abstract