103 research outputs found
Static conductivity of charged domain wall in uniaxial ferroelectric-semiconductors
Using Landau-Ginzburg-Devonshire theory we calculated numerically the static
conductivity of both inclined and counter domain walls in the uniaxial
ferroelectrics-semiconductors of n-type. We used the effective mass
approximation for the electron and holes density of states, which is valid at
arbitrary distance from the domain wall. Due to the electrons accumulation, the
static conductivity drastically increases at the inclined head-to-head wall by
1 order of magnitude for small incline angles theta pi/40 by up 3 orders of
magnitude for the counter domain wall (theta=pi/2). Two separate regions of the
space charge accumulation exist across an inclined tail-to-tail wall: the thin
region in the immediate vicinity of the wall with accumulated mobile holes and
the much wider region with ionized donors. The conductivity across the
tail-to-tail wall is at least an order of magnitude smaller than the one of the
head-to-head wall due to the low mobility of holes, which are improper carries.
The results are in qualitative agreement with recent experimental data for
LiNbO3 doped with MgO.Comment: 20 pages, 6 figures, 1 appendi
Time-dependent conduction current in lithium niobate crystals with charged domain walls
We present the experimental study of the increase and decrease of the abnormal conduction current appeared during polarization reversal at elevated temperatures (120-250 °C) in stoichiometric and MgO doped lithium niobate single crystals. It is shown that the conduction current is caused by existence of the through charged domain walls. The time dependence of the conduction current has been measured in low electric field immediately after partial switching. The maximal value of the conduction current in crystal with through charged domain walls is of 4-5 orders of magnitude higher than in initial single domain state. The activation energy is 1.1 eV. © 2013 AIP Publishing LLC
"Head-to-head" and "tail-to-tail" 180-degree domain walls in an isolated ferroelectric
"Head-to-head" and "tail-to-tail" 180-degree domain-walls in a finite
isolated ferroelectric sample are theoretically studied using Landau theory.
The full set of equations, suitable for numerical calculations is developed.
The explicit expressions for the polarization profile across the walls are
derived for several limiting cases and wall-widths are estimated. It is shown
analytically that different regimes of screening and different dependences for
width of charged domain walls on the temperature and parameters of the system
are possible, depending on spontaneous polarization and concentration of
carriers in the material. It is shown that the half-width of charged domain
walls in typical perovskites is about the nonlinear Thomas-Fermi
screening-length and about one order of magnitude larger than the half-width of
neutral domain-walls. The formation energies of "head-to-head" walls under
different regimes of screening are obtained, neglecting the poling ability of
the surface. It is shown that either "head-to-head" or "tail-to-tail"
configuration can be energetically favorable in comparison with the monodomain
state of the ferroelectric if the poling ability of the surface is large
enough. If this is not the case, the existence of charged domain walls in bulk
ferroelectrics is merely a result of the domain-growth kinetics. Size-effect
corresponding to the competition between state with charged domain wall, single
domain state, multidomain state, and the state with the zero polarization is
considered. The results obtained for the case of an isolated ferroelectric
sample were compared with the results for an electroded sample. It was shown
that charged domain wall in electroded sample can be either metastable or
stable, depends on the work function difference between electrodes and
ferroelectric and the poling ability of the electrode/ferroelectric interface.Comment: 47 pages, 10 figure
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
Conductivity of twin walls - surface junctions in ferroelastics: interplay of deformation potential, octahedral rotations, improper ferroelectricity and flexoelectric coupling
Electronic and structural phenomena at the twin domain wall-surface junctions
in the ferroelastic materials are analyzed. Carriers accumulation caused by the
strain-induced band structure changes originated via the deformation potential
mechanism, structural order parameter gradient, rotostriction and flexoelectric
coupling is explored. Approximate analytical results show that inhomogeneous
elastic strains, which exist in the vicinity of the twin walls - surface
junctions due to the rotostriction coupling, decrease the local band gap via
the deformation potential and flexoelectric coupling mechanisms. This is the
direct mechanism of the twin walls static conductivity in ferroelastics and, by
extension, in multiferroics and ferroelectrics. On the other hand,
flexoelectric and rotostriction coupling leads to the appearance of the
improper polarization and electric fields proportional to the structural order
parameter gradient in the vicinity of the twin walls - surface junctions. The
"flexo-roto" fields leading to the carrier accumulation are considered as
indirect mechanism of the twin walls conductivity. Comparison of the direct and
indirect mechanisms illustrates complex range of phenomena directly responsible
for domain walls static conductivity in materials with multiple order
parameters.Comment: 35 pages, 11 figures, 3 table, 3 appendices Improved set of
rotostriction coefficients are used in calculation
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
DNA methylation profiling of ovarian carcinomas and their in vitro models identifies HOXA9, HOXB5, SCGB3A1, and CRABP1 as novel targets
<p>Abstract</p> <p>Background</p> <p>The epigenetics of ovarian carcinogenesis remains poorly described. We have in the present study investigated the promoter methylation status of 13 genes in primary ovarian carcinomas (n = 52) and their <it>in vitro </it>models (n = 4; ES-2, OV-90, OVCAR-3, and SKOV-3) by methylation-specific polymerase chain reaction (MSP). Direct bisulphite sequencing analysis was used to confirm the methylation status of individual genes. The MSP results were compared with clinico- pathological features.</p> <p>Results</p> <p>Eight out of the 13 genes were hypermethylated among the ovarian carcinomas, and altogether 40 of 52 tumours were methylated in one or more genes. Promoter hypermethylation of <it>HOXA9</it>, <it>RASSF1A</it>, <it>APC</it>, <it>CDH13</it>, <it>HOXB5</it>, <it>SCGB3A1 (HIN-1)</it>, <it>CRABP1</it>, and <it>MLH1 </it>was found in 51% (26/51), 49% (23/47), 24% (12/51), 20% (10/51), 12% (6/52), 10% (5/52), 4% (2/48), and 2% (1/51) of the carcinomas, respectively, whereas <it>ADAMTS1</it>, <it>MGMT</it>, <it>NR3C1</it>, <it>p14</it><sup><it>ARF</it></sup>, and <it>p16</it><sup><it>INK</it>4<it>a </it></sup>were unmethylated in all samples. The methylation frequencies of <it>HOXA9 </it>and <it>SCGB3A1 </it>were higher among relatively early-stage carcinomas (FIGO I-II) than among carcinomas of later stages (FIGO III-IV; <it>P </it>= 0.002, <it>P </it>= 0.020, respectively). The majority of the early-stage carcinomas were of the endometrioid histotype. Additionally, <it>HOXA9 </it>hypermethylation was more common in tumours from patients older than 60 years of age (15/21) than among those of younger age (11/30; <it>P </it>= 0.023). Finally, there was a significant difference in <it>HOXA9 </it>methylation frequency among the histological types (<it>P </it>= 0.007).</p> <p>Conclusion</p> <p>DNA hypermethylation of tumour suppressor genes seems to play an important role in ovarian carcinogenesis and <it>HOXA9</it>, <it>HOXB5</it>, <it>SCGB3A1</it>, and <it>CRABP1 </it>are identified as novel hypermethylated target genes in this tumour type.</p
A possible role for HLA-G in development of uteroplacental acute atherosis in preeclampsia
HLA-G, a non-classical HLA molecule expressed by extravillous trophoblasts, plays a role in the maternal immune tolerance towards fetal cells. HLA-G expression is regulated by genetic polymorphisms in the 3' untranslated region (3'UTR). Low levels of HLA-G in the maternal circulation and placental tissue are linked to preeclampsia. Our objective was to investigate whether variants of the 3'UTR of the HLA-G gene in mother and fetus are associated with acute atherosis, a pregnancy specific arterial lesion of the decidua basalis that is prevalent in preeclampsia. Paired maternal and fetal DNA samples from 83 normotensive and 83 preeclamptic pregnancies were analyzed. We sequenced the part of the HLA-G 3'UTR containing a 14-bp insertion/deletion region and seven single nucleotide polymorphisms (SNPs). Associations with acute atherosis were tested by logistic regression. The frequency of heterozygosity for the 14-bp polymorphism (Ins/Del) and the +3142 SNP (C/G) variant in the fetus are associated with acute atherosis in preeclampsia (66.7 % vs. 39.6 %, p = 0.039, and 69.0 % vs. 43.4 %, p = 0.024). Furthermore, the fetal UTR-3 haplotype, which encompasses the 14-bp deletion and the +3142G variant, is associated with acute atherosis in preeclampsia (15 % vs. 3.8 %, p = 0.016). In conclusion, HLA-G polymorphisms in the fetus are associated with acute atherosis. We hypothesize that these polymorphisms lead to altered HLA-G expression in the decidua basalis, affecting local feto-maternal immune tolerance and development of acute atherosis
Modeling of dielectric hysteresis loops in ferroelectric semiconductors with charged defects
We have proposed the phenomenological description of dielectric hysteresis
loops in ferroelectric semiconductors with charged defects and prevailing
extrinsic conductivity. Exactly we have modified Landau-Ginsburg approach and
shown that the macroscopic state of the aforementioned inhomogeneous system can
be described by three coupled equations for three order parameters. Both the
experimentally observed coercive field values well below the thermodynamic one
and the various hysteresis loop deformations (constricted and double loops)
have been obtained in the framework of our model. The obtained results
quantitatively explain the ferroelectric switching in such ferroelectric
materials as thick PZT films.Comment: 21 pages, 10 figures, sent to Journal of Physics: Condensed Matte
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