Nanoscale symmetry protection of the reciprocal acoustoelectric effect.

Neumann’s principle states that all physical properties of a material are bound by its symmetry. While bulk crystals conform to well-defined point and space groups, phenomena at a substrate’s surface could have less apparent symmetry origins. Here we experimentally explore both reciprocal and non-reciprocal types of acoustoelectric effects driven by surface acoustic waves (SAW). The non-reciprocal acoustoelectric voltage is connected to the natural single-phase unidirectional transducer from device engineering. On the other hand, reciprocal acoustoelectric effect exists in certain SAW configurations that are of different symmetry origins. Half of the configurations have a valid reciprocity-preserving symmetry element of either a mirror plane or a two-fold rotational axis that is perpendicular to the substrate surface. The other half of the configurations do not possess a reciprocity-preserving symmetry element of the half-space but relate to the first scenario through an interchange of SAW propagation and the surface normal directions. Here, it is the symmetric construction in the equation of motion, which holds at nanoscale distances, to be responsible for the correspondence between two scenarios and extends the cases of reciprocal SAW states. This correspondence traces its physical origin to the SAW composition of compression and shear waves along two orthogonal directions respectively.