886 research outputs found
Electrical current distribution across a metal-insulator-metal structure during bistable switching
Combining scanning electron microscopy (SEM) and electron-beam-induced
current (EBIC) imaging with transport measurements, it is shown that the
current flowing across a two-terminal oxide-based capacitor-like structure is
preferentially confined in areas localized at defects. As the thin-film device
switches between two different resistance states, the distribution and
intensity of the current paths, appearing as bright spots, change. This implies
that switching and memory effects are mainly determined by the conducting
properties along such paths. A model based on the storage and release of charge
carriers within the insulator seems adequate to explain the observed memory
effect.Comment: 8 pages, 7 figures, submitted to J. Appl. Phy
Characterization of the Active Site of Acetylcholinesterases by Application of Sterically Modified Acetylcholine Homologues
Our aim was to find steric limitations for the model of the
active site of acetylcholinesterase and cho1inesterase. For thi<s purpose
we used homologues of acetylcholine with hydrocarbon chains
of increasing size at the cationic head. Catalysis of hydrolysis
ceased for both enzymes, when the methyl groups of the cationic
head of acetylcholine were substituted by three n-propyl groups.
With data already documented in the literature and our additional
experiments, under the same conditions, we were able to present
models of the active sites for both enzymes
Characterization of the Active Site of Acetylcholinesterases by Application of Sterically Modified Acetylcholine Homologues
Our aim was to find steric limitations for the model of the
active site of acetylcholinesterase and cho1inesterase. For thi<s purpose
we used homologues of acetylcholine with hydrocarbon chains
of increasing size at the cationic head. Catalysis of hydrolysis
ceased for both enzymes, when the methyl groups of the cationic
head of acetylcholine were substituted by three n-propyl groups.
With data already documented in the literature and our additional
experiments, under the same conditions, we were able to present
models of the active sites for both enzymes
DISTRIBUTION AND KINETICS OF 14C-VECURONIUM IN RATS AND MICE
The distribution and kinetics of 14C-vecuronium were studied in rats and mice. 14C-Vecuronium accumulated rapidly in the liver. Both unchanged and metabolized vecuronium were excreted with the bile into the intestines and stomach. Reabsorption in the gut was probably responsible for an enterohepatic increase in radioactivity in the liver after one hour. Excretion through the kidneys increased continuously from low values after the initial peak. Binding in compartments with acid mucopolysaccharides such as cartilage, connective tissue etc., was less important. Bloodbrain barrier and placenta were permeable only to a small degre
Simultaneous dynamic electrical and structural measurements of functional materials
A new materials characterization system developed at the XMaS beamline, located at the European Synchrotron Radiation Facility in France, is presented. We show that this new capability allows to measure the atomic structural evolution (crystallography) of piezoelectric materials whilst simultaneously measuring the overall strain characteristics and electrical response to dynamically (ac) applied external stimuli
Size Dependence of Metal-Insulator Transition in Stoichiometric Fe3O4 Nanocrystals
Magnetite (Fe3O4) is one of the most actively studied materials with a famous
metal-insulator transition (MIT), so-called the Verwey transition at around 123
K. Despite the recent progress in synthesis and characterization of Fe3O4
nanocrystals (NCs), it is still an open question how the Verwey transition
changes on a nanometer scale. We herein report the systematic studies on size
dependence of the Verwey transition of stoichiometric Fe3O4 NCs. We have
successfully synthesized stoichiometric and uniform-sized Fe3O4 NCs with sizes
ranging from 5 to 100 nm. These stoichiometric Fe3O4 NCs show the Verwey
transition when they are characterized by conductance, magnetization, cryo-XRD,
and heat capacity measurements. The Verwey transition is weakly size-dependent
and becomes suppressed in NCs smaller than 20 nm before disappearing completely
for less than 6 nm, which is a clear, yet highly interesting indication of a
size effect of this well-known phenomena. Our current work will shed new light
on this ages-old problem of Verwey transition.Comment: 18 pages, 4 figures, Nano Letters (accepted
Thermodynamic theory of epitaxial ferroelectric thin films with dense domain structures
A Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory is
presented, which enables the thermodynamic description of dense laminar
polydomain states in epitaxial ferroelectric thin films. The theory explicitly
takes into account the mechanical substrate effect on the polarizations and
lattice strains in dissimilar elastic domains (twins). Numerical calculations
are performed for PbTiO3 and BaTiO3 films grown on (001)-oriented cubic
substrates. The "misfit strain-temperature" phase diagrams are developed for
these films, showing stability ranges of various possible polydomain and
single-domain states. Three types of polarization instabilities are revealed
for polydomain epitaxial ferroelectric films, which may lead to the formation
of new polydomain states forbidden in bulk crystals. The total dielectric and
piezoelectric small-signal responses of polydomain films are calculated,
resulting from both the volume and domain-wall contributions. For BaTiO3 films,
strong dielectric anomalies are predicted at room temperature near special
values of the misfit strain.Comment: 19 pages, 8 figure
Tunneling electroresistance effect in ferroelectric tunnel junctions at the nanoscale
Stable and switchable polarization of ferroelectric materials opens a
possibility to electrically control their functional behavior. A particularly
promising approach is to employ ferroelectric tunnel junctions where the
polarization reversal in a ferroelectric barrier changes the tunneling current
across the junction. Here, we demonstrate the reproducible tunneling
electroresistance effect using a combination of Piezoresponse Force Microscopy
(PFM) and Conducting Atomic Force Microscopy (C-AFM) techniques on
nanometer-thick epitaxial BaTiO3 single crystal thin films on SrRuO3 bottom
electrodes. Correlation between ferroelectric and electronic transport
properties is established by the direct nanoscale visualization and control of
polarization and tunneling current in BaTiO3 films. The obtained results show a
change in resistance by about two orders of magnitude upon polarization
reversal on a lateral scale of 20 nm at room temperature. These results are
promising for employing ferroelectric tunnel junctions in non-volatile memory
and logic devices, not involving charge as a state variable.Comment: 18 pages, 4 figure
Hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxides
Using a linear combination of atomic orbitals approach, we report a
systematic comparison of various Density Functional Theory (DFT) and hybrid
exchange-correlation functionals for the prediction of the electronic and
structural properties of prototypical ferroelectric oxides. It is found that
none of the available functionals is able to provide, at the same time,
accurate electronic and structural properties of the cubic and tetragonal
phases of BaTiO and PbTiO. Some, although not all, usual DFT
functionals predict the structure with acceptable accuracy, but always
underestimate the electronic band gaps. Conversely, common hybrid functionals
yield an improved description of the band gaps, but overestimate the volume and
atomic distortions associated to ferroelectricity, giving rise to an
unacceptably large ratio for the tetragonal phases of both compounds.
This super-tetragonality is found to be induced mainly by the exchange energy
corresponding to the Generalized Gradient Approximation (GGA) and, to a lesser
extent, by the exact exchange term of the hybrid functional. We thus propose an
alternative functional that mixes exact exchange with the recently proposed GGA
of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] which, for solids, improves
over the treatment of exchange of the most usual GGA's. The new functional
renders an accurate description of both the structural and electronic
properties of typical ferroelectric oxides.Comment: 13 pages, 4 figures, 7 table
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