50,512 research outputs found
Finite size and intrinsic field effect on the polar-active properties of the ferroelectric-semiconductor heterostructures
Using Landau-Ginzburg-Devonshire approach we calculated the equilibrium
distributions of electric field, polarization and space charge in the
ferroelectric-semiconductor heterostructures containing proper or incipient
ferroelectric thin films. The role of the polarization gradient and intrinsic
surface energy, interface dipoles and free charges on polarization dynamics are
specifically explored. The intrinsic field effects, which originated at the
ferroelectric-semiconductor interface, lead to the surface band bending and
result into the formation of depletion space-charge layer near the
semiconductor surface. During the local polarization reversal (caused by the
inhomogeneous electric field induced by the nanosized tip of the Scanning Probe
Microscope (SPM) probe) the thickness and charge of the interface layer
drastically changes, it particular the sign of the screening carriers is
determined by the polarization direction. Obtained analytical solutions could
be extended to analyze polarization-mediated electronic transport.Comment: 35 pages, 12 figures, 1 table, 2 appendices, to be submitted to Phys.
Rev.
A simple electrostatic model applicable to biomolecular recognition
An exact, analytic solution for a simple electrostatic model applicable to
biomolecular recognition is presented. In the model, a layer of high dielectric
constant material (representative of the solvent, water) whose thickness may
vary separates two regions of low dielectric constant material (representative
of proteins, DNA, RNA, or similar materials), in each of which is embedded a
point charge. For identical charges, the presence of the screening layer always
lowers the energy compared to the case of point charges in an infinite medium
of low dielectric constant. Somewhat surprisingly, the presence of a
sufficiently thick screening layer also lowers the energy compared to the case
of point charges in an infinite medium of high dielectric constant. For charges
of opposite sign, the screening layer always lowers the energy compared to the
case of point charges in an infinite medium of either high or low dielectric
constant. The behavior of the energy leads to a substantially increased
repulsive force between charges of the same sign. The repulsive force between
charges of opposite signs is weaker than in an infinite medium of low
dielectric constant material but stronger than in an infinite medium of high
dielectric constant material. The presence of this behavior, which we name
asymmetric screening, in the simple system presented here confirms the
generality of the behavior that was established in a more complicated system of
an arbitrary number of charged dielectric spheres in an infinite solvent.Comment: 15 pages, 6 figure
Extended Classical Over-Barrier Model for Collisions of Highly Charged Ions with Conducting and Insulating Surfaces
We have extended the classical over-barrier model to simulate the
neutralization dynamics of highly charged ions interacting under grazing
incidence with conducting and insulating surfaces. Our calculations are based
on simple model rates for resonant and Auger transitions. We include effects
caused by the dielectric response of the target and, for insulators, localized
surface charges. Characteristic deviations regarding the charge transfer
processes from conducting and insulating targets to the ion are discussed. We
find good agreement with previously published experimental data for the image
energy gain of a variety of highly charged ions impinging on Au, Al, LiF and KI
crystals.Comment: 32 pages http://pikp28.uni-muenster.de/~ducree
Towards time-dependent, non-equilibrium charge-transfer force fields: Contact electrification and history-dependent dissociation limits
Force fields uniquely assign interatomic forces for a given set of atomic
coordinates. The underlying assumption is that electrons are in their
quantum-mechanical ground state or in thermal equilibrium. However, there is an
abundance of cases where this is unjustified because the system is only locally
in equilibrium. In particular, the fractional charges of atoms, clusters, or
solids tend to not only depend on atomic positions but also on how the system
reached its state. For example, the charge of an isolated solid -- and thus the
forces between atoms in that solid -- usually depends on the counterbody with
which it has last formed contact. Similarly, the charge of an atom, resulting
from the dissociation of a molecule, can differ for different solvents in which
the dissociation took place. In this paper we demonstrate that such
charge-transfer history effects can be accounted for by assigning discrete
oxidation states to atoms. With our method, an atom can donate an integer
charge to another, nearby atom to change its oxidation state as in a redox
reaction. In addition to integer charges, atoms can exchange "partial charges"
which are determined with the split charge equilibration method.Comment: 11 pages, 7 figure
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Electroluminescence excitation mechanisms in an epoxy resin under divergent and uniform field
Electroluminescence excitation mechanisms have been investigated in epoxy resin under divergent and uniform field situations. Metallic wires embedded in the resin were used to produce field divergence whereas film samples were metallised to obtain a uniform field. Electroluminescence under divergent field was stimulated by an impulse voltage. Light was emitted on the positive and negative fronts of the square pulses when the field exceeded 20 kV/mm at the wire surface, with equal intensity and without polarity dependence. There was evidence of space charge accumulation around the wires in multiple-pulse experiments. Charge injection and extraction occurring at both fronts of the pulse provide the condition for EL excitation. Further excitation of the EL during the plateau of the voltage pulse is prevented by the opposite field of the trapped charge. Field computation with and without space charge supports the proposed interpretation. A DC voltage was used for the uniform field experiments. A continuous level of electroluminescence is found at 175 kV/mm. Charging/discharging current measurements and space charge profile analyses using the pulsed electro-acoustic (PEA) technique were performed at different fields up to the EL level. Dipolar orientation generates a long lasting transient current that prevents the conduction level being reached within the experimental protocol (one hour poling time). The continuous EL emission is nevertheless associated with a regime where the conduction becomes dominant over the orientational polarization. Polarization and space charge contribute to the PEA charge profiles. Homo-charge injection at anode and cathode is seen at 20 kV/mm and a penetration of positive space charge in the bulk is detected above 100 kV/mm, suggesting an excitation of the continuous EL by bipolar charge recombination
Surface potential at a ferroelectric grain due to asymmetric screening of depolarization fields
Nonlinear screening of electric depolarization fields, generated by a stripe
domain structure in a ferroelectric grain of a polycrystalline material, is
studied within a semiconductor model of ferroelectrics. It is shown that the
maximum strength of local depolarization fields is rather determined by the
electronic band gap than by the spontaneous polarization magnitude.
Furthermore, field screening due to electronic band bending and due to presence
of intrinsic defects leads to asymmetric space charge regions near the grain
boundary, which produce an effective dipole layer at the surface of the grain.
This results in the formation of a potential difference between the grain
surface and its interior of the order of 1 V, which can be of either sign
depending on defect transition levels and concentrations. Exemplary acceptor
doping of BaTiO3 is shown to allow tuning of the said surface potential in the
region between 0.1 and 1.3 V.Comment: 14 pages, 11 figures, submitted to J. Appl. Phy
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