Excitations of the Surface of Liquid 4He
The surface of liquid 4He at zero temperature and the 4He vapor-liquid interface at temperatures T > 0 are currently the objects of substantial experimental and theoretical interest. Part of this activity concentrates on studying ground state properties, such as the thickness of the 4He surface and interface, for which some experimental data over a range of temperatures are now available.1, 2 Other experimental and theoretical work explores the elementary excitations supported by these inhomogeneous 4He systems. The spectrum of excitations of the surface of liquid 4He in the short wavelength regime has been measured in recent experiments on 4He films adsorbed on a substrate.3, 4 It is well established by earlier measurements5–9 that at long wavelengths the dispersion relation of 4He surface modes may be described correctly by the laws of hydrodynamics.
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