  In this work we present the results of theoretical analysis of magnetic
quantum oscillations of the velocity and attenuation of high frequency
ultrasound waves traveling in quasi-two-dimensional conductors. We chose a
geometry where the wave vector of the longitudinal sound wave and the external
magnetic field are directed along the axis of symmetry of the Fermi surface.
Assuming a moderately weak Fermi surface corrugation we showed that the
oscillating correction to the sound velocity may include a special term besides
an ordinary contribution originating from quantum oscillations of the charge
carriers density of states at the Fermi surface. This additional term is
generated by a "phase stability" resonance occurring when the charge carriers
velocity in the direction of the wave propagation equals the sound velocity.
The two oscillating contributions to the sound velocity are shown to differ in
phase and shape, and they may have the same order of magnitude. The appearance
of the extra term may bring significant changes in magnetic quantum
oscillations of the velocity of sound in Q2D conductors, especially at low
temperatures.

Materials Science