521 research outputs found
Defects as a reason of continuity of normal-incommensurate phase transitions
Almost all normal-incommensurate phase transitions observed experimentally
are continuous. We show that there is not any theoretical reason for this
general behaviour in perfect crystals. A normal-incommensurate phase transition
that is not too far from the mean-field tricritical point should be
discontinuous and it is highly improbable that so far reported
normal-incommensurate phase transitions lie very far from this point. To
understand this behaviour we study influence of defects on a hypothetical
first-order normal-incommensurate phase transition in a pure material. We have
found that this influence is strikingly different from that on other kinds of
first-order phase transitions. The change of the discontinuity of the order
parameter at the transition is negative and formally diverges within our
approximate theory. At the same time the diminishing of the phase transition
temperature remains finite. We interpret these results as an indication that at
least some of the observed seemingly second-order normal-incommensurate
transitions would be first-order transitions in defectless crystals.Comment: 17 pages, 1 figur
Effects of anisotropic elasticity in the problem of domain formation and stability of monodomain state in ferroelectric films
We study cubic ferroelectrics films that become uniaxial with a polar axis
perpendicular to the film because of a misfit strain due to a substrate. The
main present result is the analytical account for the elastic anisotropy as
well as the anisotropy of the electrostriction. They define, in particular, an
orientation of the domain boundaries and stabilizing or destabilizing effect of
inhomogeneous elastic strains on the single domain state. We apply the general
results to perovskite systems like BaTiO3/SrRuO3/SrTiO3 films and find that at
least not far from the ferroelectric phase transition the equilibrium domain
structure consists of the stripes along the cubic axes or at 45 degrees to
them. We have also showed that in this system the inhomogeneous strains
increase stability with regards to the small fluctuations of the metastable
single domain state, which may exist not very close to the ferroelectric
transition. The latter analytical result is in qualitative agreement with the
numerical result by Pertsev and Kohlstedt [Phys. Rev. Lett. 98, 257603 (2007)],
but we show that the effect is much smaller than those authors claim. We have
found also that under certain conditions on the material constants, which are
not satisfied in the perovskites but are not forbidden either, a checkerboard
domain structure can be realized instead of the stripe-like one and that the
polarization-strain coupling decreases stability of a single domain state
instead of increasing it. The single domain state is metastable at certain
large thicknesses and becomes suitable for memory applications at even larger
thicknesses when the lifetime of the metastable state becomes sufficiently
large.Comment: 20 pages, 6 figure
Formation and rapid evolution of domain structure at phase transitions in slightly inhomogeneous ferroelectrics
We present the first analytical study of stability loss and evolution of
domain structure in inhomogeneous ferroelectric samples for exactly solvable
model. The model assumes a short-circuited capacitor with two regions with
slightly different critical temperatures Tc1 > Tc2, where Tc1-Tc2 << Tc1, Tc2.
Even a tiny inhomogeneity like 10-5 K may result in splitting the system into
domains below the phase transition temperature. At T < Tc2 the domain width
is proportional to (Tc1 - T)/(Tc1 - Tc2) and quickly increases with lowering
temperature. The minute inhomogeneities in Tc may result from structural
(growth) inhomogeneities which are always present in real samples and a similar
role can be played by inevitable temperature gradients.Comment: 5 pages, 2 figures, discussion expanded and references added to
experiments on graded ferroelectrics and ferroelectric superlattice
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