Dennis Drew
CNAM, Physics
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.