Quantum oscillations in the high frequency magnetoacoustic response of a quasi-two-dimensional metal

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 (Q2D) conductors. We chose a geometry where both the wavevector 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 carrier density of states at the Fermi surface. This additional term is generated by a 'phase stability' resonance occurring when the charge carrier 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 about significant changes in magnetic quantum oscillations of the velocity of sound in Q2D conductors, especially at low temperatures.