124 research outputs found
Correlation Between Structure And C-Afm Contrast Of 180-Degree Domain Walls In Rhombohedral Bati03
Using Landau-Ginzburg-Devonshire theory we describe 180-degree domain wall
structure, intrinsic energy and carrier accumulation in rhombohedral phase of
BaTiO3 as a function of the wall orientation and flexoelectric coupling
strength. Two types of domain wall structures (phases of the wall) exist
depending on the wall orientation. The low-energy 'achiral' phase occurs in the
vicinity of the {110} wall orientation and has odd polarization profile
invariant with respect to inversion about the wall center. The second 'chiral'
phase occurs around {211} wall orientations and corresponds to mixed parity
domain walls that may be of left-handed or right-handed chirality. The
transformation between the phases is abrupt, accompanied with 20-30% change of
the domain wall thickness and can happen at fixed wall orientation with
temperature change. We suggest that the phase transition may be detected
through domain wall thickness change or by c-AFM. The structure of the domain
wall is correlated to its conductivity through polarization component normal to
the domain wall, which causes free carriers accumulation. Depending on the
temperature and flexoelectric coupling strength relative conductivity of the
wall becomes at least one order of magnitude higher than in the single-domain
region, creating c-AFM contrast enhancement pronounced and detectable.Comment: 31 pages, 10 figures, Supplementary material
Domain wall conduction in multiaxial ferroelectrics
The conductance of domain wall structures consisting of either stripes or
cylindrical domains in multi-axial ferroelectric-semiconductors is analyzed.
The effects of the domain size, wall tilt and curvature, on charge
accumulation, are analyzed using the Landau-Ginsburg Devonshire (LGD) theory
for polarization combined with Poisson equation for charge distributions. Both
the classical ferroelectric parameters including expansion coefficients in
2-4-6 Landau potential and gradient terms, as well as flexoelectric coupling,
inhomogeneous elastic strains and electrostriction are included in the present
analysis. Spatial distributions of the ionized donors, free electrons and holes
were found self-consistently using the effective mass approximation for the
respective densities of states. The proximity and size effect of the electron
and donor accumulation/depletion by thin stripe domains and cylindrical
nanodomains are revealed. In contrast to thick domain stripes and thicker
cylindrical domains, in which the carrier accumulation (and so the static
conductivity) sharply increases at the domain walls only, small nanodomains of
radius less then 5-10 correlation length appeared conducting across entire
cross-section. Implications of such conductive nanosized channels may be
promising for nanoelectronics.Comment: 39 pages, 11 figures, 3 tables, 4 appendice
Numerical study of the minimum of the thermodynamic potential of a ferroelectric barium titanate
The macroscopic polarization is the most essential concept in any phenomenological description of dielectric media. The presence of a spontaneous macroscopic polarization is the defining property of a ferroelectric material that is central to whole physics of ferroelectrics. Based solely on symmetry considerations, Landau theory can provide a reliable description of a system equilibrium behavior near a phase transition. This phenomenological approach allows giving relations between measurable quantities using a minimal set of input parameters that can be determined by comparison with physical experiment or numerical first-principles calculation. Now, the problem of true presentation of Landau-Devonshire potential for different materials is outstanding. The properties of Landau-Devonshire thermodynamic potential (TP) for ferroelectric materials in this article is investigated on the example of barium titanate. The previously approved topological method of research of nonlinear properties of ferroelectrics is applied. The cellular complexes approach use for interpretation of TP extremes. The new representation of the 8-order polynomial TP compared with previously used 6-order representation. For the 8 order potential the structure of metastable states for barium titanate is presented in the vicinity of first-order phase transition point at T = 5°C. New method proposed for the qualitative description of the metastable states structure that TP generates. Based on the method the temperature regions are pointed where the 6 and 8 order potentials predicting for the metastable states are qualitatively different
COMPUTER SIMULATION OF LOCAL MOBILITY IN DENDRIMERS WITH ASYMMETRIC BRANCHING BY BROWNIAN DYNAMICS METHOD
The Brownian dynamics method has been used to study the effect of the branching asymmetry on the local orientational mobility of segments and bonds in dendrimers in good solvent. “Coarse-grained” models of flexible dendrimers with different branching symmetry but with the same average segment length were considered. The frequency dependences of the rate of the spin-lattice relaxation nuclear magnetic resonance (NMR) [1/T1H(H)] for segments or bonds located at different distances from terminal monomers were calculated. After the exclusion of the contribution of the overall dendrimer rotation the position of the maxima of the frequency dependences [1/T1H(ωH)] for different segments with the same length doesn’t depend on their location inside a dendrimer both for phantom models and for models with excluded volume interactions. This effect doesn’t depend also on the branching symmetry, but the position of the maximum [1/T1H(ωH))] is determined by the segment length. For bonds inside segments the positions of the maximum [1/T1H(ωH)] coincide for all models considered. Therefore, the obtained earlier conclusion about the weak influence of the excluded volume interactions on the local dynamics in the flexible symmetric dendrimers can be generalized for dendrimers with an asymmetric branching
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