838 research outputs found
Structural, electronic and magnetic properties of SrRuO under epitaxial strain
Using density functional theory within the local spin density approximation,
structural, electronic and magnetic properties of SRO are investigated. We
examine the magnitude of the orthorhombic distortion in the ground state and
also the effects of applying epitaxial constraints, whereby the influence of
large (in the range of ) in-plane strain resulting from coherent
epitaxy, for both [001] and [110] oriented films, have been isolated and
investigated. The overall pattern of the structural relaxations reveal coherent
distortions of the oxygen octahedra network, which determine stability of the
magnetic moment on the Ru ion. The structural and magnetic parameters exhibit
substantial changes allowing us to discuss the role of symmetry and
possibilities of magneto-structural tuning of \SRO-based thin film structures.Comment: 11 page
Low-temperature phase transformations of PZT in the morphotropic phase-boundary region
We present anelastic and dielectric spectroscopy measurements of
PbZr(1-x)Ti(x)O(3) with 0.455 < x < 0.53, which provide new information on the
low temperature phase transitions. The tetragonal-to-monoclinic transformation
is first-order for x < 0.48 and causes a softening of the polycrystal Young's
modulus whose amplitude may exceed the one at the cubic-to-tetragonal
transformation; this is explainable in terms of linear coupling between shear
strain components and tilting angle of polarization in the monoclinic phase.
The transition involving rotations of the octahedra below 200 K is visible both
in the dielectric and anelastic losses, and it extends within the tetragonal
phase, as predicted by recent first-principle calculations.Comment: 4 pages, 4 figure
Polarity control of carrier injection at ferroelectric/metal interfaces for electrically switchable diode and photovoltaic effects
We investigated a switchable ferroelectric diode effect and its physical
mechanism in Pt/BiFeO3/SrRuO3 thin-film capacitors. Our results of electrical
measurements support that, near the Pt/BiFeO3 interface of as-grown samples, a
defective layer (possibly, an oxygen-vacancy-rich layer) becomes formed and
disturbs carrier injection. We therefore used an electrical training process to
obtain ferroelectric control of the diode polarity where, by changing the
polarization direction using an external bias, we could switch the transport
characteristics between forward and reverse diodes. Our system is characterized
with a rectangular polarization hysteresis loop, with which we confirmed that
the diode polarity switching occurred at the ferroelectric coercive voltage.
Moreover, we observed a simultaneous switching of the diode polarity and the
associated photovoltaic response dependent on the ferroelectric domain
configurations. Our detailed study suggests that the polarization charge can
affect the Schottky barrier at the ferroelectric/metal interfaces, resulting in
a modulation of the interfacial carrier injection. The amount of
polarization-modulated carrier injection can affect the transition voltage
value at which a space-charge-limited bulk current-voltage (J-V) behavior is
changed from Ohmic (i.e., J ~ V) to nonlinear (i.e., J ~ V^n with n \geq 2).
This combination of bulk conduction and polarization-modulated carrier
injection explains the detailed physical mechanism underlying the switchable
diode effect in ferroelectric capacitors.Comment: Accepted for publication in Phys. Rev.
Concentration phase diagram of Ba(x)Sr(1-x)TiO3 solid solutions
Method of derivation of phenomenological thermodynamic potential of solid
solutions is proposed in which the interaction of the order parameters of
constituents is introduced through the account of elastic strain due to misfit
of the lattice parameters of the end-members. The validity of the method is
demonstrated for Ba(x)Sr(1-x)TiO3 system being a typical example of
ferroelectric solid solution. Its phase diagram is determined using
experimental data for the coefficients in the phenomenological potentials of
SrTiO3 and BaTiO3. In the phase diagram of the Ba(x)Sr(1-x)TiO3 system for
small Ba concentration, there are a tricritical point and two multiphase points
one of which is associated with up to 6 possible phases.Comment: 8 pages, 3 figure
Metal-Ferroelectric-Metal heterostructures with Schottky contacts I. Influence of the ferroelectric properties
A model for Metal-Ferroelectric-Metal structures with Schottky contacts is
proposed. The model adapts the general theories of metal-semiconductor
rectifying contacts for the particular case of metal-ferroelectric contact by
introducing: the ferroelectric polarization as a sheet of surface charge
located at a finite distance from the electrode interface; a deep trapping
level of high concentration; the static and dynamic values of the dielectric
constant. Consequences of the proposed model on relevant quantities of the
Schottky contact such as built-in voltage, charge density and depletion width,
as well as on the interpretation of the current-voltage and capacitance-voltage
characteristics are discussed in detail.Comment: 14 pages with 4 figures, manuscript under revision at Journal of
Applied Physics for more than 1 year (submitted May 2004, first revision
September 2004, second revision May 2005
Octahedral Tilt Instability of ReO_3-type Crystals
The octahedron tilt transitions of ABX_3 perovskite-structure materials lead
to an anti-polar (or antiferroelectric) arrangement of dipoles, with the low
temperature structure having six sublattices polarized along various
crystallographic directions. It is shown that an important mechanism driving
the transition is long range dipole-dipole forces acting on both displacive and
induced parts of the anion dipole. This acts in concert with short range
repulsion, allowing a gain of electrostatic (Madelung) energy, both
dipole-dipole and charge-charge, because the unit cell shrinks when the hard
ionic spheres of the rigid octahedron tilt out of linear alignment.Comment: 4 page with 3 figures included; new version updates references and
clarifies the argument
Influence of the electric field on the latent heat of the ferroelectric phase transition in KDP
The specific heat, heat flux (DTA trace) and dielectric constant of KDP
ferroelectric crystal have been measured simultaneously for various electric
fields with a conduction calorimeter. The specific heat presents a strong
anomaly but these simultaneous measurements allow us to evaluate the latent
heat accurately. Latent heat decreases with field and the value of critical
electric field --that where latent heat disappears-- is estimated to be
(0.44\pm0.03) kV/cm. Incidentally, we have measured simultaneously the
dielectric permittivity which suggests that latent heat is developed as domains
are growing.Comment: 7 pages, 6 figures, ReVTeX, twocolumn format, to appear in J. Phys.
Cond. Matte
Electronic structure, magnetism and superconductivity of MgCNi
The electronic structure of the newly discovered superconducting perovskite
MgCNi is calculated using the LMTO and KKR methods. The states near the
Fermi energy are found to be dominated by Ni-d. The Stoner factor is low while
the electron-phonon coupling constant is estimated to be about 0.7, which
suggests that the material is a conventional type of superconductor where T
is not affected by magnetic interactions. However, the proximity of the Fermi
energy to a large peak in the density of states in conjunction with the
reported non-stoichiometry of the compound, has consequences for the stability
of the results.Comment: 3 pages, 4 figure
Robotic geometric and volumetric inspection of high value and large scale aircraft wings
Increased demands in performance and production rates require a radical new approach to the design and manufacturing of aircraft wings. Performance of modern robotic manipulators has enabled research and development of fast automated non-destructive testing (NDT) systems for complex geometries. This paper presents recent outcomes of work aimed at removing the bottleneck due to data acquisition rates, to fully exploit the scanning speed of modern 6-DoF manipulators. The geometric assessment of the parts is carried out with a robotised dynamic laser scanner encoded through an absolute laser tracker. This method allows scanning speeds up to 330mm/s at 1mm pitch. State of the art ultrasonic instrumentation has been integrated into a large robot cell to enable fast data acquisition, high scan resolutions and accurate positional encoding. A fibre optic connection between the ultrasonic instrument and the server computer enables data transfer rates up to 1.6 GB/s. The robotic inspection system presented herein is currently being tested for industrial exploitation. The adopted system integration strategies allow traditional ultrasonic phased array scanning as well as full matrix capture (FMC) and other novel scanning approaches (e.g. multi-Tx phased array). Scan results, relative to a 1.2m x 3m carbon fibre sample, are presented. The system shows a reference scanning rate of 25.3m2/hour with an 8Tx/8Rx PA approach and an ultrasonically reachable scanning rate over 100m2/hour with the novel techniques
Dynamics of cubic-tetragonal phase transition in KNbO perovskite
The low-energy part of the vibration spectrum in KNbO was studied by cold
neutron inelastic scattering in the cubic phase. In addition to acoustic
phonons, we observe strong diffuse scattering, which consists of two
components. The first one is quasi-static and has a temperature-independent
intensity. The second component appears as quasi-elastic scattering in the
neutron spectrum indicating a dynamic origin. From analysis of the inelastic
data we conclude that the quasi-elastic component and the acoustic phonon are
mutually coupled. The susceptibility associated with the quasi-elastic
component grows as the temperature approaches T
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