Landau Model for Films and Interfaces of Superfluid Helium-4 at T = 0k

We use a Landau theory appropriate to an inhomogeneous superfluid at temperature T = 0 to describe structural and dynamical effects at a gas/superfluid interface and for a superfluid film on an inert attractive substrate. The parameters of the theory are determined phenomenologically by fitting measured bulk properties of the homogeneous superfluid. The theory then predicts in a consistent way both static properties (density profile, interface/surface tension) and excitations (ripplons, phonons, and their associated wavefunctions). The simplicity of the theory makes the connection between the symmetries of the system and the form of the excitation spectra particularly transparent. In Chapter II, we take the Landau free-energy functional to be local. This restriction precludes description of roton effects. Numerical results are, as a consequence, not quantitative; however, calculations are easy enough so that generic features of the spectra and wavefunctions can be illustrated conveniently. In Chapter III, we allow nonlocality in the free-energy functional, making it possible to incorporate roton effects. Two solid-like near-substrate layers then appear in the film profiles, followed by liquid. The surface tension obtained is more realistic than that of the local model. A surface excitation spectrum with a roton-like minimum has been obtained for both a gas/liquid interface and a film on a graphite substrate. The third-sound velocities c(,3) vs. (DELTA)(mu) show oscillations characteristic of the layer structure, which are consistent with measured data if a proper substrate potential is chosen. A possible form of the effective van der Waals potential of a graphite substrate, which can explain most experimental observations, is suggested.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Ji, Guangda

• Physics, Fluid and Plasma

Language dc:language

eng

Identifier

(UMI)AAI8701515

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/77394