Theory of acoustical activity

In this article the lattice dynamical expression for the gyrotropic parameter of the acoustical activity along an acoustic axis is deduced using the long-wavelength method of Born and Huang. Adopting the central force approximation we present a theoretical demonstration for the empirical rule that acoustical activity is related to a helix arrangement of atoms. The experimental facts obtained by inelastic neutron scattering are discussed, and the deviation from a ${q}^{2}$ law for the frequency splitting of the two circularly polarized branches is attributed to the anharmonic interactions using an approach of the many-body problem.

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