DIRECT GENERATION OF ULTRASOUND BY ELECTROMAGNETIC RADIATION IN METALS IN A MAGNETIC FIELD
Abstract
A study of the amplitude of an electromagnetically generated acoustic wave in a metal in the presence of a magnetic field is presented. The incident electromagnetic wave propagates normally to a plane metal surface and the dc magnetic field is applied in this same direction. The effects on acoustic generation, under these conditions, of the manner in which the conduction electrons are scattered at the metal surface are investigated. The calculation of the electric field inside the metal, under anomalous skin effect conditions, involves the solution of an integral equation by the Fredholm determinantal method. It is shown that the experimentally observed nonmonotonic behavior of the acoustic flux as a function of the applied magnetic field can be explained within the framework of the free-electron model, if diffuse scattering is assumed. The theory is then extended to include effects due to the periodic potential of the crystal lattice. It is concluded that besides the Lorentz force, the collision-drag force and the Bragg reaction force, an additional "deformation" reaction force acts on the positive ions. In the effective-mass approximation, the latter can to a considerable extent cancel the effects of the Bragg reaction force. It is shown that these reaction forces do not significantly alter the free-electron predictions concerning sound propagation. Furthermore, they leave the position and strength of the helicon-phonon interaction unchanged.
Degree
Ph.D.
Subject Area
Condensation
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