Acoustoelectric Origin of the Acoustic Orbital Hall Effect

The orbital Hall effect, which generates orbital currents, has emerged as a key mechanism for the angular momentum transport in solids. Its acoustic analogue, the acoustic orbital Hall effect, has recently been observed in which a surface acoustic wave (SAW) drives an orbital current transverse to its propagation direction. However, their microscopic mechanism has remained elusive. Here, we reveal that the acoustic orbital Hall effect in Ti is driven by an acoustoelectric mechanism. Using Ti/Ni bilayers on a piezoelectric LiNbO3 substrate, we observe a clear phase shift in the magnetic field angle dependence of the acoustic orbital Hall voltage as the SAW propagation direction is varied relative to the crystallographic axes. This behavior is consistent with orbital current generation by the in-plane electric field associated with the acoustoelectric evanescent wave. Moreover, from simultaneously measured acoustic orbital Hall and acoustic orbital pumping signals, we determined the efficiency of converting lattice dynamics into orbital transport.