96 research outputs found
Griffiths phase manifestation in disordered dielectrics
We predict the existence of Griffith phase in the dielectrics with
concentrational crossover between dipole glass (electric analog of spin glass)
and ferroelectricity. The peculiar representatives of above substances are
, , or relaxor ferroelectrics like
. Since this phase exists above
ferroelectric phase transition temperature (but below that temperature for
ordered substance), we call it "para-glass phase". We assert that the
difference between paraelectric and para-glass phase of above substances is the
existence of clusters (inherent to "ordinary" Griffiths phase in Ising magnets)
of correlated dipoles. We show that randomness play a decisive role in
Griffiths (para-glass) phase formation; this phase does not exist in a mean
field approximation.Comment: 5 pages, 4 embedded postscript figure
Role of dimensionality in spontaneous magnon decay: easy-plane ferromagnet
We calculate magnon lifetime in an easy-plane ferromagnet on a tetragonal
lattice in transverse magnetic field. At zero temperature magnons are unstable
with respect to spontaneous decay into two other magnons. Varying ratio of
intrachain to interchain exchanges in this model we consider the effect of
dimensionality on spontaneous magnon decay. The strongest magnon damping is
found in the quasi-one-dimensional case for momenta near the Brillouin zone
boundary. The sign of a weak interchain coupling has a little effect on the
magnon decay rate. The obtained theoretical results suggest possibility of
experimental observation of spontaneous magnon decay in a quasi-one-dimensional
ferromagnet CsNiF. We also find an interesting enhancement of the magnon
decay rate for a three-dimensional ferromagnet. The effect is present only for
the nearest-neighbor model and is related to effective dimensionality reduction
in the two-magnon continuum.Comment: 6 pages, 6 figure
Spontaneous magnon decays in planar ferromagnet
We predict that spin-waves in an easy-plane ferromagnet have a finite
lifetime at zero temperature due to spontaneous decays. In zero field the
damping is determined by three-magnon decay processes, whereas decays in the
two-particle channel dominate in a transverse magnetic field. Explicit
calculations of the magnon damping are performed in the framework of the
spin-wave theory for the square-lattice ferromagnet with an anisotropy
parameter . In zero magnetic field the decays occur for
with . We also discuss possibility
of experimental observation of the predicted effect in a number of
ferromagnetic insulators.Comment: 6 pages, 6 figures, to appear in Europhysics Letter
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