Explicit secular equations for piezoacoustic surface waves: Rayleigh modes

The existence of a two-partial Rayleigh wave coupled to an electrical field in 2 mm piezoelectric crystals is known but has rarely been investigated analytically. It turns out that the Z cut X propagation problem can be fully solved, up to the derivation of the secular equation as a polynomial in the squared wave speed. For the metallized (unmetallized) boundary condition, the polynomial is of degree 10 (48). The relevant root is readily identified and the full description of the mechanical and electrical fields follows. The results are illustrated in the case of the superstrong piezoelectric crystal, potassium niobate, for which the effective piezoelectric coupling coefficient is calculated to be about 0.1.