Influence of the piezoelectric plate surface metallization on phase velocity of the backward acoustic waves

The fundamental interaction between electrical and mechanical subsystems in condensed matter is universally assumed to feature positive coupling. This means that electrically shorting a piezoelectric surface invariably slows down acoustic waves. However, it has been previously theoretically predicted that electrical shorting of the plate surface leads to an increase in the phase velocity of backward acoustic waves, which are characterized by oppositely directed phase and group velocities. This work provides the first experimental confirmation of this phenomenon. Through theoretical analysis and direct measurement, the effect of a metal screen approaching the YX LiNbO₃ plate surface on the phase velocity of both forward and backward acoustic waves was investigated. The results demonstrate that, contrary to forward waves, the phase velocity of the backward acoustic wave increases as a metal plate approaches the piezoelectric surface at fixed frequency. This phenomenon establishes that an increase in phase velocity upon electrical shorting of the piezoelectric plate surface is indeed a hallmark of the piezoactive backward acoustic waves. It is important for development of acoustoelectronic devices based on such waves.