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 follow 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 (AE) effects driven by surface acoustic waves (SAW). The non-reciprocal AE voltage is connected to the natural single-phase unidirectional transducer from device engineering. On the other hand, reciprocal AE 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 an even-order rotational axis that is perpendicular to the substrate surface. The other half of the configurations do not possess reciprocity-preserving symmetry operations of the half-space but have the SAW propagation and the surface normal directions interchanged from the first scenario. Here, the reciprocity of SAW states is protected by the symmetric structure of the nanoscale strain tensor. The correspondence between two types of configurations has its origin imbedded in the SAW composition of both compression and shear waves along two orthogonal directions respectively.