Flexoelectric properties of ferroelectrics and the nanoindentation size-effect

Recent works have established the critical role of flexoelectricity in a variety of size-dependent physical phenomena related to ferroelectrics including giant piezoelectricity at the nanoscale, dead-layer effect in nanocapacitors, dielectric properties of nanostructures among others. Flexoelectricity couples strain gradients to polarization in both ordinary and piezoelectric dielectrics. Relatively few experimental works exist that have determined flexoelectric properties and they all generally involve some sort of bending tests on micro-specimens. In this work, we present a straightforward method based on nanoindentation that allows the evaluation of flexoelectric properties in a facile manner. The key contribution is the development of an analytical model that, in conjunction with indentation load–displacement data, allows an estimate of the flexoelectric constants. In particular, we confirm the experimental results of other groups on BaTiO₃ which differ by three orders of magnitude from atomistic predictions. Our analytical model predicts (duly confirmed by our experiments) a strong indentation size-effect due to flexoelectricity.     

Dissipative homoclinic loops of two‐dimensional maps and strange attractors with one direction of instability

We prove that when subjected to periodic forcing of the form certain two‐dimensional vector fields with dissipative homoclinic loops generate strange attractors with Sinai‐Ruelle‐Bowen measures for a set of forcing parameters (μ, ρ, ω) of positive Lebesgue measure. The proof extends ideas of Afraimovich and Shilnikov and applies the recent theory of rank 1 maps developed by Wang and Young. We prove a general theorem and then apply this theorem to an explicit model: a forced Duffing equation of the form © 2011 Wiley Periodicals, Inc.     

Qualitative properties of positive singular solutions to nonlinear elliptic systems with critical exponent

We studied the asymptotic behavior of local solutions for strongly coupled critical elliptic systems near an isolated singularity. For the dimension less than or equal to five we prove that any singular solution is asymptotic to a rotationally symmetric Fowler type solution. This result generalizes the celebrated work due to Caffarelli, Gidas and Spruck [1] who studied asymptotic proprieties to the classic Yamabe equation. In addition, we generalize similar results by Marques [12] for inhomogeneous context, that is, when the metric is not necessarily conformally flat.     

Microscopic structure and structuring of perovskite surfaces and interfaces: SrTiO3, RBa2Cu3O7-δ

3 surfaces and bicrystal interfaces and the growth of YBa₂Cu₃O_7-δ thin films on such substrates using scanning tunneling microscopy and X-ray diffraction. Proper annealing of SrTiO₃ in oxygen and/or ultrahigh vacuum produces uniformly terminated, atomically flat and well-ordered surfaces. For vicinal SrTiO₃(001) surfaces the particular annealing sequence and miscut angle sensitively determines the resulting step structure and thus the microscopic surface morphology. Steps of SrTiO₃(001) surfaces can be adjusted to a height of one, two, or multiple times the unit-cell height (a_STO=0.3905 nm). The growth of YBa₂Cu₃O_7-δ films on these substrates by pulsed laser deposition was traced from the initial nucleation to a thickness of about 300 nm. The morphology, texture, and defect structure of the films is determined by the specific structure and morphology of the pristine substrate. Anisotropic, planar defects, originating from substrate step edges, strongly modify the electronic transport properties of the film leading to critical currents up to ≈9×10⁷ A/cm² at 4 K as well as pronounced transport anisotropies. Surfaces and interface energy terms are discussed, which also determine the observed structure of bicrystal boundaries. Received: 16 April 1998/Accepted: 21 August 1998     

High temperature limit of the order parameter correlation functions in the quantum Ising model

In this paper we use the exact results for the anisotropic two-dimensional Ising model obtained by Bugrii and Lisovyy [A.I. Bugrii, O.O. Lisovyy, Theor. Math. Phys. 140 (2004) 987] to derive the expressions for dynamical correlation functions for the quantum Ising model in one dimension at high temperatures.     

Helium cosmic ray flux measurements at Mars

The helium energy spectrum in Martian orbit has been observed by the MARIE charged particle spectrometer aboard the 2001 Mars Odyssey spacecraft. The orbital data were taken from March 13, 2002 to October 28, 2003, at which time a very intense Solar Particle Event caused a loss of communication between the instrument and the spacecraft. The silicon detector stack in MARIE is optimized for the detection of protons and helium in the energy range below , which typically includes almost all of the flux during SPEs. This also makes MARIE an efficient detector for GCR helium in the energy range of 50–. We will present the first fully normalized flux results from MARIE, using helium ions in this energy range.     

3D periodic structures grown on silicon by radiation of a pulsed Nd:YAG laser and their field emission properties

Periodic three-dimensional structures were successfully grown on single crystal Si wafers either bare or Au-covered under their exposure to a pulsed radiation of a Nd:YAG laser in vacuum. The structures protrude above the initial wafer surface for 10 μm while their spatial period is about 70 μm. The coupling of the laser radiation to Si surface is related to the thermal non-linear absorption of the near band gap radiation. The structures exhibit an efficient field emission with an average emission current of 5 mA/cm² and is sensitive to the post-treatment of samples. The drawbacks of the emission current densities are discussed.     

Random cluster models for icosahedral phase alloys

We present results on computer generated random cluster models for icosahedral phase alloys. By the application of physically motivated constraints on the local atomic cluster configurations, the model achieves long range translational order comparable to, or greater than, that found in simple icosahedral alloys such asi-AlMnSi andi-AlLiCu. The parallel and perpendicular space structures are explored in some detail including a comparison with experimental powder diffraction patterns and an examination of the phason fluctuations. The latter are shown to decrease markedly with increased constraint on the local environment, but the remaining phason strain would seem finally not to vanish with increasing model size. Our model is compared with other cluster models for quasicrystalline materials and is shown to possess a density and connectivity very close to those of Elser's (best) model and those predicted by Henley for a canonical tiling. The relation of this model to recently discovered icosahedral phase alloys with resolution limited diffraction peak widths, which are essentially free of phason strain, is also discussed.