visiting fellowship Professor Adamenko 2007

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Strongly interacting phonon systems are usually isotropic and only recently have anisotropic systems been seriously investigated. We continue to discover and measure new and interesting effects which are due to the anisotropy, these include: 1. the creation of a beam of high energy phonons which have a narrower angular width than the low energy phonons which create them, 2. discovering that a phonon pulse forms a sheet with a uniform energy density which get wider as the heater dimensions are reduced, 3.showing that collisions between two phonon sheets, which leads to the creation of a hot line in the liquid helium, depends strongly on the angle between the sheets, and 4. measuring the rich behacviour of the angular dependence of phonon pulses as a function of pressure. These effects all stem from the fact that the injected excitations only occupy a small solid angle in momentum space. This coupled with strongly anisotropic interactions, makes the system qualitatively different to the usual isotropic system. The low energy phonons strongly interact and come into a dynamic equilibrium and we find theoretically that the drift velocity of the pulse is as an important and fundamental parameter as the temperature of the phonons. This application is for funds to continue to develop the theory with Professor Adamenko and his group, to explain some of our experimental results. We shall analyse roton second sound in anisotropic roton systems with a view to measuring the roton density, calculate the rate of scattering between high energy phonons and start formulating a theory for the scattering between high- energy phonon and rotons, start developing a theory for low-energy phonon propagation at all angles to the heater normal and at pressures 0-24 bar, and for the creation of rotons by a heater and by condensation.