Evidence for two-dimensional phonons in a thin metal film

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Abstract
In this paper we describe an experiment that uses a moderate dc electric field to heat electrons in a thin metal film that can have its acoustic coupling to the surroundings changed in situ. The acoustic coupling is found to have a substantial effect on the electron-heating characteristics. This illustrates the importance of considering phonon escape from the film in this type of experiment. Using a rate equation to model the data obtained for different conditions of phonon escape from the film, we have found that in certain circumstances there is strong evidence to suggest that low-frequency phonons in the film are two dimensional.