Determination of the Fermi Velocity by the High-Field Tilt Effect

The attenuation of a sound wave in a semimetal in the presence of a magnetic field is examined when the magnetic field is tilted from the direction perpendicular to the direction of propagation of the sound wave. It is found that in the limit of high magnetic fields (${\omega }_c\tau \ge 10ql$) the behavior of the atenuation with the angle of tilt gives a point-by-point determination of the velocity on the Fermi surface. A detailed treatment of a two-spherical-band model is given and the case of a general Fermi surface is examined qualitatively.