770 research outputs found
Impact of elasticity on the piezoresponse of adjacent ferroelectric domains investigated by scanning force microscopy
As a consequence of elasticity, mechanical deformations of crystals occur on
a length scale comparable to their thickness. This is exemplified by applying a
homogeneous electric field to a multi-domain ferroelectric crystal: as one
domain is expanding the adjacent ones are contracting, leading to clamping at
the domain boundaries. The piezomechanically driven surface corrugation of
micron-sized domain patterns in thick crystals using large-area top electrodes
is thus drastically suppressed, barely accessible by means of piezoresponse
force microscopy
Materials Contrast in Piezoresponse Force Microscopy
Piezoresponse Force Microscopy contrast in transversally isotropic material
corresponding to the case of c+ - c- domains in tetragonal ferroelectrics is
analyzed using Green's function theory by Felten et al. [J. Appl. Phys. 96, 563
(2004)]. A simplified expression for PFM signal as a linear combination of
relevant piezoelectric constant are obtained. This analysis is extended to
piezoelectric material of arbitrary symmetry with weak elastic and dielectric
anisotropies. This result provides a framework for interpretation of PFM
signals for systems with unknown or poorly known local elastic and dielectric
properties, including nanocrystalline materials, ferroelectric polymers, and
biopolymers.Comment: 20 pages, 3 figures, 1 table, accepted to Appl. Phys. Lett. (without
Appendices), algebraic errors were correcte
Quantum Flexoelectricity in Low Dimensional Systems
Symmetry breaking at surfaces and interfaces and the capability to support
large strain gradients in nanoscale systems enable new forms of
electromechanical coupling. Here we introduce the concept of quantum
flexoelectricity, a phenomenon that is manifested when the mechanical
deformation of non-polar quantum systems results in the emergence of net dipole
moments and hence linear electromechanical coupling proportional to local
curvature. The concept is illustrated in carbon systems, including
polyacetylene and nano graphitic ribbons. Using density functional theory
calculations for systems made of up to 400 atoms, we determine the
flexoelectric coefficients to be of the order of ~ 0.1 e, in agreement with the
prediction of linear theory. The implications of quantum flexoelectricity on
electromechanical device applications, and physics of carbon based materials
are discussed.Comment: 15 pages, 3 figure
Magnetodielectric coupling in Mn3O4
We have investigated the dielectric anomalies associated with spin ordering
transitions in the tetragonal spinel MnO, using thermodynamic,
magnetic, and dielectric measurements. We find that two of the three magnetic
ordering transitions in MnO lead to decreases in the temperature
dependent dielectric constant at zero applied field. Applying a magnetic field
to the polycrystalline sample leaves these two dielectric anomalies practically
unchanged, but leads to an increase in the dielectric constant at the
intermediate spin-ordering transition. We discuss possible origins for this
magnetodielectric behavior in terms of spin-phonon coupling. Band structure
calculations suggest that in its ferrimagnetic state, MnO corresponds
to a semiconductor with no orbital degeneracy due to strong Jahn-Teller
distortion.Comment: 6 pages, 7 figure
Towards a microscopic theory of toroidal moments in bulk periodic crystals
We present a theoretical analysis of magnetic toroidal moments in periodic
systems, in the limit in which the toroidal moments are caused by a time and
space reversal symmetry breaking arrangement of localized magnetic dipole
moments. We summarize the basic definitions for finite systems and address the
question of how to generalize these definitions to the bulk periodic case. We
define the toroidization as the toroidal moment per unit cell volume, and we
show that periodic boundary conditions lead to a multivaluedness of the
toroidization, which suggests that only differences in toroidization are
meaningful observable quantities. Our analysis bears strong analogy to the
modern theory of electric polarization in bulk periodic systems, but we also
point out some important differences between the two cases. We then discuss the
instructive example of a one-dimensional chain of magnetic moments, and we show
how to properly calculate changes of the toroidization for this system.
Finally, we evaluate and discuss the toroidization (in the local dipole limit)
of four important example materials: BaNiF_4, LiCoPO_4, GaFeO_3, and BiFeO_3.Comment: replaced with final (published) version, which includes some changes
in the text to improve the clarity of presentatio
Simulation of sub-millimetre atmospheric spectra for characterizing potential ground-based remote sensing observations
The sub-millimetre is an understudied region of the Earth's atmospheric electromagnetic spectrum. Prior technological gaps and relatively high opacity due to the prevalence of rotational water vapour lines at these wavelengths have slowed progress from a ground-based remote sensing perspective; however, emerging superconducting detector technologies in the fields of astronomy offer the potential to address key atmospheric science challenges with new instrumental methods. A site study, with a focus on the polar regions, is performed to assess theoretical feasibility by simulating the downwelling clear-sky sub-millimetre spectrum from 30 mm (10 GHz) to 150 μm (2000 GHz) at six locations under annual mean, summer, winter, daytime, nighttime and low humidity conditions. Vertical profiles of temperature, pressure and 28 atmospheric gases are constructed by combining radiosonde, meteorological reanalysis, and atmospheric chemistry model data. The sensitivity of the simulated spectra to the choice of water vapour continuum model and spectroscopic line database is explored. For the atmospheric trace species hypobromous acid (HOBr), hydrogen bromide (HBr), perhydroxyl radical (HO2) and nitrous oxide (N2O) the emission lines producing the largest change in brightness temperature are identified. Signal strengths, centre frequencies, bandwidths, estimated minimum integration times and maximum receiver noise temperatures are determined for all cases. HOBr, HBr and HO2 produce brightness temperature peaks in the mK to K range, whereas the N2O peaks are in the K range. The optimal sub-millimetre remote sensing lines for the four species are shown to vary significantly between location and scenario, strengthening the case for future hyperspectral instruments that measure over a broad wavelength range. The techniques presented here provide a framework that can be applied to additional species of interest and taken forward to simulate retrievals and guide the design of future sub-millimetre instruments
Contrast Mechanisms for the Detection of Ferroelectric Domains with Scanning Force Microscopy
We present a full analysis of the contrast mechanisms for the detection of
ferroelectric domains on all faces of bulk single crystals using scanning force
microscopy exemplified on hexagonally poled lithium niobate. The domain
contrast can be attributed to three different mechanisms: i) the thickness
change of the sample due to an out-of-plane piezoelectric response (standard
piezoresponse force microscopy), ii) the lateral displacement of the sample
surface due to an in-plane piezoresponse, and iii) the electrostatic tip-sample
interaction at the domain boundaries caused by surface charges on the
crystallographic y- and z-faces. A careful analysis of the movement of the
cantilever with respect to its orientation relative to the crystallographic
axes of the sample allows a clear attribution of the observed domain contrast
to the driving forces respectively.Comment: 8 pages, 8 figure
Scaled free energies, power-law potentials, strain pseudospins and quasi-universality for first-order structural transitions
We consider ferroelastic first-order phase transitions with
order-parameter strains entering Landau free energies as invariant polynomials,
that have structural-variant Landau minima. The total free energy
includes (seemingly innocuous) harmonic terms, in the {\it
non}-order-parameter strains. Four 3D transitions are considered,
tetragonal/orthorhombic, cubic/tetragonal, cubic/trigonal and
cubic/orthorhombic unit-cell distortions, with respectively, and 2; and and 6. Five 2D transitions are also considered, as
simpler examples. Following Barsch and Krumhansl, we scale the free energy to
absorb most material-dependent elastic coefficients into an overall prefactor,
by scaling in an overall elastic energy density; a dimensionless temperature
variable; and the spontaneous-strain magnitude at transition .
To leading order in the scaled Landau minima become
material-independent, in a kind of 'quasi-universality'. The scaled minima in
-dimensional order-parameter space, fall at the centre and at the
corners, of a transition-specific polyhedron inscribed in a sphere, whose
radius is unity at transition. The `polyhedra' for the four 3D transitions are
respectively, a line, a triangle, a tetrahedron, and a hexagon. We minimize the
terms harmonic in the non-order-parameter strains, by substituting
solutions of the 'no dislocation' St Venant compatibility constraints, and
explicitly obtain powerlaw anisotropic, order-parameter interactions, for all
transitions. In a reduced discrete-variable description, the competing minima
of the Landau free energies induce unit-magnitude pseudospin vectors, with values, pointing to the polyhedra corners and the (zero-value) center.Comment: submitted to PR
High-pressure neutron study of the morphotropic PZT: phase transitions in a two-phase system
In piezoelectric ceramics the changes in the phase stabilities versus stress
and temperature in the vicinity of the phase boundary play a central role. The
present study was dedicated to the classical piezoelectric,
lead-zirconate-titanate (PZT) ceramic with composition
Pb(ZrTi)O at the Zr-rich side of the morphotropic phase
boundary at which both intrinsic and extrinsic contributions to
piezoelectricity are significant. The pressure-induced changes in this
two-phase (rhombohedral +monoclinic at room temperature and
above 1 GPa pressures) system were studied by high-pressure neutron
powder diffraction technique. The experiments show that applying pressure
favors the phase, whereas the phase transforms continuously to the
, which is favored at elevated temperatures due to the competing entropy
term. The phase transformation is discontinuous. The
transformation contributes to the extrinsic piezoelectricity. An important
contribution to the intrinsic piezoelectricity was revealed: a large
displacement of the cations (Zr and Ti) with respect to the oxygen anions
is induced by pressure. Above 600 K a phase transition to a cubic phase took
place. Balance between the competing terms dictates the curvature of the phase
boundary. After high-pressure experiments the amount of rhombohedral phase was
larger than initially, suggesting that on the Zr-rich side of the phase
boundary the monoclinic phase is metastable.Comment: 6 figure
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