90 research outputs found
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
Thermodynamics of nanodomain formation and breakdown in Scanning Probe Microscopy: Landau-Ginzburg-Devonshire approach
Thermodynamics of tip-induced nanodomain formation in scanning probe
microscopy of ferroelectric films and crystals is studied using the
Landau-Ginzburg-Devonshire phenomenological approach. The local redistribution
of polarization induced by the biased probe apex is analyzed including the
effects of polarization gradients, field dependence of dielectric properties,
intrinsic domain wall width, and film thickness. The polarization distribution
inside subcritical nucleus of the domain preceding the nucleation event is very
smooth and localized below the probe, and the electrostatic field distribution
is dominated by the tip. In contrast, polarization distribution inside the
stable domain is rectangular-like, and the associated electrostatic fields
clearly illustrate the presence of tip-induced and depolarization field
components. The calculated coercive biases of domain formation are in a good
agreement with available experimental results for typical ferroelectric
materials. The microscopic origin of the observed domain tip elongation in the
region where the probe electric field is much smaller than the intrinsic
coercive field is the positive depolarization field in front of the moving
counter domain wall. For infinitely thin domain walls local domain breakdown
through the sample depth appears. The results obtained here are complementary
to the Landauer-Molotskii energetic approach.Comment: 35 pages, 8 figures, suplementary attached, to be submitted to Phys.
Rev.
Precise Measurement of the Pi+ -> Pi0 e+ nu Branching Ratio
Using a large acceptance calorimeter and a stopped pion beam we have made a
precise measurement of the rare Pi+ -> Pi0 e+ Nu,(pi_beta) decay branching
ratio. We have evaluated the branching ratio by normalizing the number of
observed pi_beta decays to the number of observed Pi+ -> e+ Nu, (pi_{e2})
decays. We find the value of Gamma(Pi+ -> Pi0 e+ Nu)/Gamma(total) = [1.036 +/-
0.004(stat.) +/- 0.004(syst.) +/- 0.003(pi_{e2})] x 10^{-8}$, where the first
uncertainty is statistical, the second systematic, and the third is the pi_{e2}
branching ratio uncertainty. Our result agrees well with the Standard Model
prediction.Comment: 4 pages, 5 figures, 1 table, revtex4; changed content; updated
analysi
Spin physics with antiprotons
New possibilities arising from the availability at GSI of antiproton beams,
possibly polarised, are discussed. The investigation of the nucleon structure
can be boosted by accessing in Drell-Yan processes experimental asymmetries
related to cross-sections in which the parton distribution functions (PDF) only
appear, without any contribution from fragmentation functions; such processes
are not affected by the chiral suppression of the transversity function
. Spin asymmetries in hyperon production and Single Spin Asymmetries
are discussed as well, together with further items like electric and magnetic
nucleonic form factors and open charm production. Counting rates estimations
are provided for each physical case. The sketch of a possible experimental
apparatus is proposed.Comment: Presented for the proceedings of ASI "Spin and Symmetry", Prague,
July 5-10, 2004, to be published in Czech. J. Phys. 55 (2005
Precise Measurement of pi+ -> e+ nu Branching Ratio
The PEN Collaboration is conducting a new measurement of the pi+ -> e+ nu
branching ratio at the Paul Scherrer Institute, with the goal uncertainty of
delta B/B(pie2)=5E-4 or lower. At present, the combined accuracy of all
published pie2 decay measurements lags behind the theoretical calculation by a
factor of 40. In this contribution we report on the PEN detector configuration
and its performance during two development runs done in 2007 and 2008.Comment: pdflatex, 11 pages, 5 figures, to be published in "Progress in
High-Energy Physics and Nuclear Safety", NATO Science for Peace Series: B -
Physics and Biophysic
Precise Measurement of the Pion Axial Form Factor in the Pi+ -> e+ nu gamma Decay
We have studied radiative pion decays Pi+ -> e+ nu gamma in three broad
kinematic regions using the PIBETA detector and a stopped pion beam. Based on
Dalitz distributions of 42,209 events we have evaluated absolute Pi -> e nu
gamma branching ratios in the three regions. Minimum chi^2 fits to the integral
and differential (E(e+),E(gamma)) distributions result in the axial-to-vector
weak form factor ratio of gamma = F_A/F_V = 0.443(15),or F_A = 0.0115(4) with
F_V = 0.0259. However, deviations from Standard Model predictions in the
high-E(gamma)/low-E(e+) kinematic region indicate the need for further
theoretical and experimental work.Comment: 5 pages, 4 figures, 1 table, revtex
- …