Softening electromagnetic and sound waves at phase transitions in crystals with linear magnetoelectric effects

Abstract

Whereas the order parameter of a phase transition is transformed either as the components of the electric or magnetic polarisation or as a linear combination of the deformation tensor components, the velocities of the electromagnetic waves and of sound, which propagate in a definite symmetry direction, become zero when approaching a critical point. A rather large class of such phase transitions of the second kind, the order parameter of which is transformed simultaneously as the electric and magnetic polarisations or as both polarisation and the components of the deformation tensor, is studied. In the latter case the bound electromagnetic and sound waves stand for softening hydrodynamic modes. It is shown that at such 'combined' phase transitions qualitatively new effects arise in the behaviour of the softening hydrodynamic waves. The strongest anomalies are observed at magnetoelectric phase transitions.