Electron Relaxation Time Anisotropy in Copper

Abstract

A method for determining electron relaxation time anisotropies in metals is suggested and presented along with preliminary data taken at 4.2°K using a single crystal grown from 99.999% pure copper. The analysis is based on the behavior of the attenuation of ultrasonic waves by conduction electrons in the high magnetic field limit. The data are discussed in terms of the Pippard model of the Fermi surface of copper. The technique also allows a rather direct test of the free electron theory of ultrasonic attenuation in that shear wave wave measurements are used in determining the total attenuation caused by the conduction electrons. It is suggested that a study of high-field shear wave attenuation will allow the total electronic attenuation to be found in any metal, whereas previously it has been possible to determine this quantity only for superconductors. On the rough experimental model used it is found that the relaxation time of electrons in neck orbits is several times larger than that of the other orbits studied. The relaxation times are of the order of ${10}^{-10\mathrm{}}$ sec, and are impurity limited at 4.2°K. Mean free paths are found to be about 20 times smaller than estimated from the number of magnetoacoustic oscillations.