593 research outputs found
Theoretical prediction of multiferroicity in double perovskite YNiMnO
We put forward double perovskites of the RNiMnO family (with a
rare-earth atom) as a new class of multiferroics on the basis of {\it ab
initio} density functional calculations. We show that changing from La to Y
drives the ground-state from ferromagnetic to antiferromagnetic with
spin patterns. This E-type ordering
breaks inversion symmetry and generates a ferroelectric polarization of few
. By analyzing a model Hamiltonian we understand the microscopic
origin of this transition and show that an external electric field can be used
to tune the transition, thus allowing electrical control of the magnetization.Comment: 4 pages, 3 figure
Temperature independent diffuse scattering and elastic lattice deformations in relaxor PbMg1/3Nb2/3O3
The results of diffuse neutron scattering experiment on PbMg1/3Nb2/3O3 single
crystal above the Burns temperature are reported. It is shown that the high
temperature elastic diffuse component is highly anisotropic in low-symmetry
Brillouin zones and this anisotropy can be described using Huang scattering
formalism assuming that the scattering originates from mesoscopic lattice
deformations due to elastic defects. The qualitative agreement between this
model and the experimental data is achieved with simple isotropic defects. It
is demonstrated that weak satellite maxima near the Bragg reflections can be
interpreted as the finite resolution effect.Comment: 7 pages, 7 figure
Dual structures in PZN-xPT ferroelectric relaxors
We performed x-ray diffraction studies on a series of
(1-)Pb(ZnNb)O-PbTiO (PZN-PT) single crystals
with different incident photon energies, and therefore different penetration
depths. Our results show that outer-layers of to 50 m thick are
present in all samples. The structure of those outer-layers is different from
that of the inside of the crystals, by having much greater (rhombohedral)
distortions. With increasing , rhombohedral-type lattice distortions
develop, both in the outer-layer and the inside.Comment: submitted to Appl. Phys. Let
High frequency polarization switching of a thin ferroelectric film
We consider both experimentally and analytically the transient oscillatory
process that arises when a rapid change in voltage is applied to a
ferroelectric thin film deposited on an substrate.
High frequency () polarization oscillations are observed
in the ferroelectric sample. These can be understood using a simple
field-polarization model. In particular we obtain analytic expressions for the
oscillation frequency and the decay time of the polarization fluctuation in
terms of the material parameters. These estimations agree well with the
experimental results
Universal neural field computation
Turing machines and G\"odel numbers are important pillars of the theory of
computation. Thus, any computational architecture needs to show how it could
relate to Turing machines and how stable implementations of Turing computation
are possible. In this chapter, we implement universal Turing computation in a
neural field environment. To this end, we employ the canonical symbologram
representation of a Turing machine obtained from a G\"odel encoding of its
symbolic repertoire and generalized shifts. The resulting nonlinear dynamical
automaton (NDA) is a piecewise affine-linear map acting on the unit square that
is partitioned into rectangular domains. Instead of looking at point dynamics
in phase space, we then consider functional dynamics of probability
distributions functions (p.d.f.s) over phase space. This is generally described
by a Frobenius-Perron integral transformation that can be regarded as a neural
field equation over the unit square as feature space of a dynamic field theory
(DFT). Solving the Frobenius-Perron equation yields that uniform p.d.f.s with
rectangular support are mapped onto uniform p.d.f.s with rectangular support,
again. We call the resulting representation \emph{dynamic field automaton}.Comment: 21 pages; 6 figures. arXiv admin note: text overlap with
arXiv:1204.546
Diffuse Neutron Scattering Study of a Disordered Complex Perovskite Pb(Zn1/3Nb2/3)O3 Crystal
Diffuse scattering around the (110) reciprocal lattice point has been
investigated by elastic neutron scattering in the paraelectric and the relaxor
phases of the disordered complex perovskite crystal-Pb(Zn1/3Nb2/3)O3(PZN). The
appearance of a diffuse intensity peak indicates the formation of polar
nanoregions at temperature T*, approximately 40K above Tc=413K. The analysis of
this diffuse scattering indicates that these regions are in the shape of
ellipsoids, more extended in the direction than in the direction.
The quantitative analysis provides an estimate of the correlation length, \xi,
or size of the regions and shows that \xi ~1.2\xi , consistent with
the primary or dominant displacement of Pb leading to the low temperature
rhombohedral phase. Both the appearance of the polar regions at T*and the
structural transition at Tc are marked by kinks in the \xi curve but not
in the \xi one, also indicating that the primary changes take place in a
direction at both temperatures.Comment: REVTeX file. 4 pages, 3 figures embedded, New version after referee
cond-mat/010605
Diffusion Processes on Small-World Networks with Distance-Dependent Random-Links
We considered diffusion-driven processes on small-world networks with
distance-dependent random links. The study of diffusion on such networks is
motivated by transport on randomly folded polymer chains, synchronization
problems in task-completion networks, and gradient driven transport on
networks. Changing the parameters of the distance-dependence, we found a rich
phase diagram, with different transient and recurrent phases in the context of
random walks on networks. We performed the calculations in two limiting cases:
in the annealed case, where the rearrangement of the random links is fast, and
in the quenched case, where the link rearrangement is slow compared to the
motion of the random walker or the surface. It has been well-established that
in a large class of interacting systems, adding an arbitrarily small density
of, possibly long-range, quenched random links to a regular lattice interaction
topology, will give rise to mean-field (or annealed) like behavior. In some
cases, however, mean-field scaling breaks down, such as in diffusion or in the
Edwards-Wilkinson process in "low-dimensional" small-world networks. This
break-down can be understood by treating the random links perturbatively, where
the mean-field (or annealed) prediction appears as the lowest-order term of a
naive perturbation expansion. The asymptotic analytic results are also
confirmed numerically by employing exact numerical diagonalization of the
network Laplacian. Further, we construct a finite-size scaling framework for
the relevant observables, capturing the cross-over behaviors in finite
networks. This work provides a detailed account of the
self-consistent-perturbative and renormalization approaches briefly introduced
in two earlier short reports.Comment: 36 pages, 27 figures. Minor revisions in response to the referee's
comments. Furthermore, some typos were fixed and new references were adde
Ground State of Relaxor Ferroelectric
High energy x-ray diffraction measurements on Pb(ZnNb)O
(PZN) single crystals show that the system does not have a rhombohedral
symmetry at room temperature as previously believed. The new phase (X) in the
bulk of the crystal gives Bragg peaks similar to that of a nearly cubic lattice
with a slight tetragonal distortion. The Bragg profile remains sharp with no
evidence of size broadening due to the polar micro crystals (MC). However, in
our preliminary studies of the skin, we have found the expected rhombohedral
(R) phase as a surface state. On the other hand, studies on an electric-field
poled PZN single crystal clearly indicate a rhombohedral phase at room
temperature.Comment: 11 pages with 3 figure
On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3
Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2) TiO3-0.06BaTiO(3) (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature-dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric-toantiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we propose that the commonly observed two dielectric anomalies are attributed to thermal evolution of ferroelectric polar nanoregions of R3c and P4bm symmetry, which coexist nearly throughout the entire temperature range and reversibly transform into each other with temperature.open1128
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