Pieter J. Swart

Phenomenological theory of a single domain wall in uniaxial trigonal ferroelectrics: Lithium niobate and lithium tantalate

A phenomenological treatment of domain walls based on the Ginzburg-Landau-Devonshire theory is developed for uniaxial trigonal ferroelectrics, lithium niobate and lithium tantalate. The contributions to the domain-wall energy from polarization and strain as a function of orientation are considered. Analytical expressions are developed that are analyzed numerically to determine the minimum polarization, strain, and energy configurations of domain walls. It is found that hexagonal

Hysteresis and avalanches in two-dimensional foam rheology simulations.

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Calculating the performance of 1–3 piezoelectric composites for hydrophone applications: An effective medium approach

walls are preferred over

Experimental study of internal fields and movement of single ferroelectric domain walls

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Graph-based linear scaling electronic structure theory

walls in both materials. This agrees well with experimental observation of domain geometries in stoichiometric composition crystals.

Role of elastic compatibility in martensitic texture evolution

Foams have unique rheological properties that range from solidlike to fluidlike. We study two-dimensional noncoarsening foams of different disorder under shear in a Monte Carlo simulation, using a driven large-Q Potts model. Simulations of periodic shear on an ordered foam show several different response regimes. At small strain amplitudes, bubbles deform and recover their shapes elastically, and the macroscopic response is that of a linear elastic cellular material. For increasing strain amplitude, the energy-strain curve starts to exhibit hysteresis before any topological rearrangements occur, indicating a macroscopic viscoelastic response. When the applied strain amplitude exceeds a critical value, the yield strain, topological rearrangements occur, the foam starts to flow, and we observe macroscopic irreversibility. We find that the dynamics of topological rearrangements depend sensitively on the structural disorder. Structural disorder decreases the yield strain; sufficiently high disorder changes the macroscopic response of a foam from a viscoelastic solid to a viscoelastic fluid. This wide-ranging dynamical response and the associated history effects of foams result from avalanchelike rearrangement events. The spatiotemporal statistics of rearrangement events do not display long-range correlations for ordered foams or at low shear rates, consistent with experimental observations. As the shear rate or structural disorder increases, the topological events become more correlated and their power spectra change from that of white noise toward 1/f noise. Intriguingly, the power spectra of the total stored energy also exhibit this 1/f trend.

The local atomic structure and phonons in Ba0.5Sr0.5TiO3

A new method is presented for evaluating the performance of 1–3 polymer/piezoelectric ceramic composites for hydrophone applications. The Poisson’s ratio effect, i.e., the enhancement of the hydrostatic performance which can be achieved by mixing piezoelectric ceramics with polymers, is studied in detail. Using an “effective medium” approach, algebraic expressions are derived for the composite hydrostatic charge coefficient dh, the hydrostatic figure of merit dhgh, and the hydrostatic electromechanical coupling coefficient kh in terms of the properties of the constituent materials, the ceramic volume fraction, and a microstructural parameter p. The high contrast in stiffness and dielectric constants existing between the two phases can be exploited to derive simple, geometry-independent approximations which explain quantitatively the Poisson’s ratio effect. It is demonstrated that the stiffness and the Poisson’s ratio of the polymer matrix play a crucial role in enhancing hydrophone performance. Using a di...

Ferroelectric domain kinetics in congruent LiTaO3

Abstract We have made a direct and submicron scale measurement of the strain, internal electric field of single 180° domain walls in ferroelectric LiTaO 3 using a collection mode Near-Field Scanning Optical Microscope. We unambiguously identify single domain walls by simultaneously recording nanometer scale topographic and optical images. The birefringence at the domain wall is used in the optical imaging. From the internal field structure we estimate the interface energy. We have also studied the pinning and movement of a single 180° domain wall under an uniform applied electric field. The domain wall was observed to curve between the pinning defects with a radius of curvature uniquely given by the material parameters and the applied electric field. The change in birefringence with applied field is related to the second order electro-optic and piezo-optic coefficients and is used to infer the orientation of the internal field.

Exploring Network Structure, Dynamics, and Function using NetworkX

We show how graph theory can be combined with quantum theory to calculate the electronic structure of large complex systems. The graph formalism is general and applicable to a broad range of electronic structure methods and materials, including challenging systems such as biomolecules. The methodology combines well-controlled accuracy, low computational cost, and natural low-communication parallelism. This combination addresses substantial shortcomings of linear scaling electronic structure theory, in particular with respect to quantum-based molecular dynamics simulations.

Direct Observation of Pinning and Bowing of a Single Ferroelectric Domain Wall

Abstract We employ the time-dependent Ginzburg-Landau (TDGL) method to analyze the time evolution of strain fields in a model for materials with martensitic phase transformations. The free energy functional is expressed in terms of the components of the strain tensor, and its functional derivatives with respect to these components give their rate of change. However, the components of the strain tensor are not independent fields; rather, they are related by the Saint-Venant compatibility condition. This condition imposes constraints on the variations of the strain tensor components needed to obtain the equations of motion. Incorporating these constraints in the TDGL procedure introduces extra terms that effectively act as long-range, anisotropic elastic interactions. The latter govern the types of elastic textures that may emerge during a martensitic transformation. The results from the numerical solution of these evolution equations exhibit fine and coarse tweed, twinning, and tip-splitting.