Missed surface waves in non-piezoelectric solids

TL;DR

This paper reveals the existence of shear surface waves in non-piezoelectric solids due to flexoelectric coupling, which were previously considered impossible, and discusses their properties and potential experimental detection.

Contribution

It demonstrates that shear surface waves can propagate in non-piezoelectrics via flexoelectric effects, challenging classical elasticity theory.

Findings

Flexo-SW can exist in all crystalline dielectrics due to flexoelectric coupling.

Penetration depth of Flexo-SW depends on flexocoupling strength and can reach tens of microns.

Surface waves can be distinguished from bulk phonons in thin films of 20-50 nm thickness.

Abstract

The physical processes taking place at the surface and near the surface of solids is so rich and versatile that sometimes they seem to be the inexhaustible subject of fundamental research. In particular, since the discovery by Lord Rayleigh surface waves in solids focus increased attention of scientists, because their experimental and theoretical studies can serve as the source of unique information about the surface impact on the dynamics and structure of the atomic lattice, structural instabilities and phase transitions induced by the surface, and explore the properties of phonons in spatially-confined systems The existence of purely shear surface wave is impossible in non-piezoelectrics within the framework of the classical theory of elasticity, because the Rayleigh surface waves have different polarization and are the mixture of shear and dilatational waves. We showed that the "forbidden" shear surface wave (shortly Flexo-SW) can propagate near the surface of all crystalline dielectrics due to the omnipresent flexoelectric coupling. The appearance and penetration depth of the Flexo-SW is ruled by the flexocoupling strength. In particular the penetration depth of transverse acoustic mode diverges in the absence of the flexoelectric coupling and so these waves become indistinguishable from the bulk waves. For paraelectrics such as strontium titanate with typical flexoeletric coefficients (~2V) the penetration depth of the Flexo-SW can reach more than tens microns at THz frequencies. The circumstances can explain the absence of experimental observations of the missed surface waves in thick layers and bulk materials. However we predict that the peaks of neutron intensity corresponding to the surface Flexo-SW and bulk phonon modes can be separated in non-piezoelectric thin films of thickness ~(20 - 50)nm.