Possibility of anomalous absorption of transverse ultrasonic waves near the Curie points of triglycine-sulfate-type ferroelectrics

We predict that anomalous absorption of transverse ultrasonic waves σ_yz~e^i(kz-ωt), σ_xy~e^i(kx-ωt) is possible near T_c in triglycine-sulfate-type ferroelectrics in spite of the fact that there is no piezoelectric or electrostrictive coupling between σ_yz (or σ_xy) and P_y. A new coupling term νσ_yz∂P_y∂z allowed by symmetry is introduced to account for this absorption. The Landau-Khalatnikov theory is applied to obtain in both phases κ~ω⁴τ²(1+ω²τ²), where τ~|T-T_c|^-1.