Surface excitations in liquid helium nanofilms
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RYSTALLINE MATERIALS AND LIQUIDS
Surface Excitations in Liquid Helium Nanofilms I. V. Kalinina, M. Kozab, H. Lauterb, V. V. Lauter-Pasyukc, d, and A. V. Puchkova a
Leœpunskiœ Institute of Physics and Power Engineering, Obninsk, Kaluga oblast, 249033 Russia b Institut Laue–Langevin, Grenoble, France c Technical University, Munich, Germany d Joint Institute for Nuclear Research, ul. Zholio-Kyuri 6, Dubna, Moscow oblast, 141980 Russia e-mail: [email protected] Received November 27, 2006
Abstract—The structural dynamic factor S(Q, ω) of liquid 4He has been measured by inelastic neutron scattering in films of different thickness at temperatures from 0.6 to 0.05 K. The measurements were performed on an IN6 spectrometer (Institut Laue–Langevin, France). Analysis of the obtained experimental data on S(Q, ω) made it possible to establish the main parameters of surface excitations arising in a helium film and their dependences on the film temperature and thickness. The measurements showed that surface excitations arise in a helium film at a thickness of about two to three atomic layers, and the film thickness plays a decisive role in the formation of surface excitations. PACS numbers: 61.12.Ex, 67.40.Db, 67.40.-w DOI: 10.1134/S1063774507030224
INTRODUCTION It is known that macroscopic properties of liquid helium (specific heat and the density of the superfluid component) placed in porous media differ from the properties of massive liquid helium. There are theoretical suggestions that the change in the macroscopic properties of helium in porous media is due to specific excitations propagating in a thin helium layer adjacent to the surface of the porous material. The inelastic neutron scattering studies of helium in porous media confirmed the existence of such surface excitations [1]. As is expected from the theoretical estimates [2], the change of the macroscopic properties of helium is related specifically with the formation of new surface excitations since limitation of the characteristic sizes of massive helium to average sizes of pores almost does not affect the parameters of excitations of massive helium. Currently, some types of surface excitations propagating in helium films are known. These are replons, surface rotons, and surface rotons under conditions of confined geometry (we will refer to them as confined rotons). Replons are excitations propagating along the liquid/gas interface. They are long-wavelength excitations manifesting themselves in S(Q, ω) at momentum transfers up to ~1.5 Å–1. Surface rotons are formed at the solid/liquid interface, propagate in the liquid helium film, and are observed in the range of wave vectors characteristic of rotons in massive helium but at the energies lower than roton energies by ~0.1 meV. Confined rotons are observed almost in the same range of wave vectors as the surface rotons but with a larger
uncertainty in energy, in the form of a wide continuum. These excitations propagate in a helium film located in a closed volume. Generally, this volume is a pore incompletely covered by
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