93 research outputs found
Multidomain switching in the ferroelectric nanodots
Controlling the polarization switching in the ferroelectric nanocrystals,
nanowires and nanodots has an inherent specificity related to the emergence of
depolarization field that is associated with the spontaneous polarization. This
field splits the finite-size ferroelectric sample into polarization domains.
Here, based on 3D numerical simulations, we study the formation of 180 polarization domains in a nanoplatelet, made of uniaxial ferroelectric
material, and show that in addition to the polarized monodomain state, the
multidomain structures, notably of stripe and cylindrical shapes, can arise and
compete during the switching process. The multibit switching protocol between
these configurations may be realized by temperature and field variations
Pairing state in multicomponent superconductors
We use the microscopic weak coupling theory to predict the pairing state in
superconductors of cubic, hexagonal, or tetragonal symmetry, where the order
parameter is multicomponent, i.e., transforms according to either a
2-dimensional or a 3-dimensional representation of the crystal point group. We
show that the superconducting phase usually breaks the time-reversal symmetry
for singlet multicomponent superconductors. The superconducting order parameter
for triplet superconductors in most cases turns out to be non-magnetic.Comment: 7 page
Effects of Magnetic Order on the Upper Critical Field of UPt
I present a Ginzburg-Landau theory for hexagonal oscillations of the upper
critical field of UPt near . The model is based on a
representation for the superconducting order parameter,
, coupled to an in-plane AFM order parameter,
. Hexagonal anisotropy of arises from the weak in-plane
anisotropy energy of the AFM state and the coupling of the superconducting
order parameter to the staggered field. The model explains the important
features of the observed hexagonal anisotropy [N. Keller, {\it et al.}, Phys.
Rev. Lett. {\bf 73}, 2364 (1994).] including: (i) the small magnitude, (ii)
persistence of the oscillations for , and (iii) the change in
sign of the oscillations for and (the temperature at the
tetracritical point). I also show that there is a low-field crossover
(observable only very near ) below which the oscillations should vanish.Comment: 9 pages in a RevTex (3.0) file plus 2 postscript figures (uuencoded).
Submitted to Physical Review B (December 20, 1994)
Origin of Ferroelastic Domains in Free-Standing Single Crystal Ferroelectric Films
The origin of the unusual 90^o ferroelectric / ferroelastic domains,
consistently observed in recent studies on meso and nanoscale free-standing
single crystals of BaTiO3 [Schilling et al., Physical Review B, 74, 024115
(2006); Schilling et al., Nano Letters, 7, 3787 (2007)], has been considered. A
model has been developed which postulates that the domains form as a response
to elastic stress induced by a surface layer which does not undergo the
paraelectric-ferroelectric, cubic-tetragonal phase transition. This model was
found to accurately account for the changes in domain periodicity as a function
of size that had been observed experimentally. The physical origin of the
surface layer might readily be associated with patterning damage, seen in
experiment; however, when all evidence of physical damage is removed from the
BaTiO3 surfaces by thermal annealing, the domain configuration remains
practically unchanged. This suggests a more intrinsic origin, such as the
increased importance of surface tension at small dimensions. The effect of
surface tension is also shown to be proportional to the difference in hardness
between the surface and the interior of the ferroelectric. The present model
for surface tension induced twinning should also be relevant for finely grained
or core-shell structured ceramics
Antiferromagnetism and Superconductivity in UPt_3
The short ranged antiferromagnetism recently seen in UPt_3 is proved
incompatible with two dimensional (2D) order parameter models that take the
antiferromagnetism as a symmetry breaking field. To adjust to the local moment
direction, the order parameter twists over very long length scales as per the
Imry-Ma argument. A variational solution to the Ginzburg-Landau equations is
used to study the nature of the short ranged order. Although there are still
two transitions, the lower one is of first order -- in contradiction to
experiments. It is shown that the latent heat predicted by the 2D models at the
lower transition is too large not to have been seen. A simple periodic model is
numerically studied to show that the lower transition can not be a crossover
either.Comment: To appear in Journal of Physics: Condensed Matter. 9 pages, 2 figure
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