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The physical processes taking place at the surface and near the surface ofsolids is so rich and versatile that sometimes they seem to be theinexhaustible subject of fundamental research. In particular, since thediscovery by Lord Rayleigh surface waves in solids focus increased attention ofscientists, because their experimental and theoretical studies can serve as thesource of unique information about the surface impact on the dynamics andstructure of the atomic lattice, structural instabilities and phase transitionsinduced by the surface, and explore the properties of phonons inspatially-confined systems The existence of purely shear surface wave isimpossible in non-piezoelectrics within the framework of the classical theoryof elasticity, because the Rayleigh surface waves have different polarizationand are the mixture of shear and dilatational waves. We showed that the"forbidden" shear surface wave (shortly Flexo-SW) can propagate near thesurface of all crystalline dielectrics due to the omnipresent flexoelectriccoupling. The appearance and penetration depth of the Flexo-SW is ruled by theflexocoupling strength. In particular the penetration depth of transverseacoustic mode diverges in the absence of the flexoelectric coupling and sothese waves become indistinguishable from the bulk waves. For paraelectricssuch as strontium titanate with typical flexoeletric coefficients (~2V) thepenetration depth of the Flexo-SW can reach more than tens microns at THzfrequencies. The circumstances can explain the absence of experimentalobservations of the missed surface waves in thick layers and bulk materials.However we predict that the peaks of neutron intensity corresponding to thesurface Flexo-SW and bulk phonon modes can be separated in non-piezoelectricthin films of thickness ~(20 - 50)nm.

Missed surface waves in non-piezoelectric solids