A general theorem by Goldstone implies low frequency excitations corresponding to the long wavelength fluctuations of a symmetry breaking older parameter. In multiferroic materials simultaneous magnetic and ferroelectric order have been observed and have attracted much attention recently because of the fundamental interest of systems with coupled order parameters and because of their potential for cross electric and magnetic functionality. These materials exhibit novel coupled magnon-phonon excitations called electromagnons. These are magnons that can be excited by the electric field, wherein electric dipole spectral weight is transferred from the phonons down to the magnon frequency. Electromagnons therefore provide a mechanism for enhancing the static dielectric constant. Are Electromagnons the Goldstone Bosons of multiferroics? As it happens the story is more complex and that the mechanism for electromagnons is not always the same as the mechanism for multiferrocity. This raises the possibility that electromagnons are more general than multiferroicity. The phenomenology of electromagnons will be reviewed.
Host: Sang-Wook Cheong