Results 1 - 10 of 12502
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[en] We present ab initio calculations of the longitudinal flexoelectricity for BaTiO3 and SrTiO3 using a direct approach. The calculated value for SrTiO3 agrees with recently reported measurements. For BaTiO3, however, the theoretical values are smaller than the measured ones; possible reasons for the discrepancy are discussed. (fast track communication)
[en] Opto-thermionic refrigeration is an environment-friendly method. To improve the opto-thermionic refrigeration efficiency, a system with a perovskite oxide multilayer of p-BaTiO3/BaTiO3/SrTiO3/BaTiO3/n-BaTiO3 is proposed based on the model of opto-thermionic refrigeration. Because the Auger coefficient of SrTiO3 is two orders smaller than that of GaAs, the calculated refrigeration power of the p-BaTiO3/BaTiO3/SrTiO3/BaTiO3/n-BaTiO3 system is much higher than that of the p-AlGaAs/AlGaAs/GaAs/AlGaAs/n-AlGaAs system. A refrigeration power of as high as 39 W cm-2 is obtained numerically in the p-BaTiO3/BaTiO3/SrTiO3/BaTiO3/n-BaTiO3 system, which suggests a potential application of the oxide structures in opto-thermionic refrigeration.
[en] The optical properties of amorphous and polycrystalline BaxSr1-xTiO3 (BST) thin films fabricated on Si(100) substrates by a sol-gel spin-coating technique were investigated by spectroscopic ellipsometry (SE). The spectrum of the extinction coefficient k was obtained by using the refractive index and structure parameters determined by SE in the photon energy range of 2.1-5.2 eV. A four-phase fitting model was employed to describe the optical properties of the BST thin films; the spectra of their optical constants and the band gap energy Eg were determined by means of optimization. In addition, the refractive index dispersion data related to the short-range-order structure of the films agreed well with the single-oscillation energy model. The dependence of the refractive index, k and Eg on annealing temperature were analysed. The variation of optical band gap energy with composition was investigated by changing the content of barium in the films. (author)
[en] The authors have studied strain-relaxation in BaTiO3/SrTiO3 superlattices(SLs) on SeTiO3(001) using in-plane x-ray diffraction. When the total thickness of the SLs exceeds critical thickness (tc), the mechanism of strain-relaxation depends on the thickness of the sublayers in the SLs.Under the different relaxation mechanism, the strains contribute differently to optical nonlinearity. The present results have been supported by the calculations on the principle of energy minimization
[en] An artificial superlattice of BaTiO3/LaNiO3 (BTO/LNO) was grown epitaxially on an Nb-doped SrTiO3 (001) single-crystalline substrate with a dual-gun rf magnetron sputtering system. The structural characteristics of the superlattice films were studied mainly by X-ray reflectivity. Formation of a superlattice structure was confirmed from the appearance of Bragg peaks separated by Kiessig fringes in X-ray reflectivity curves and a crystal truncation rod (CTR) spectrum. The surface and interfacial roughness of the superlattice were derived from the fitted curves for specular reflectivity. A conformal relationship between consecutive BTO and LNO layers was observed from off-specular scattering. According to the fitted results and atomic-force microscopy images, the evolution of surface structure follows the Stranski-Krastanov (S-K) growth mode, in which the surface roughness increased with the thickening of sublayers
[en] A procedure is presented for accurately determining the thickness, optical functions, and surface-roughness characteristics of thin-film insulators from two-channel spectroscopic polarization-modulation ellipsometry data. For films with minimal surface roughness, the optical functions can be determined over the entire measured spectrum; for rougher films, the analysis of the spectroscopic ellipsometry data yields meaningful values of the optical functions only in the transparent region. In general, the films must be transparent in a given range of wavelengths sampled by the ellipsometer so that at least two interference oscillations can be observed. The use of the procedure is illustrated with the determination of the optical functions of SrTiO3 and BaTiO3 thin films grown on MgO, and of SiOx films grown on Si. For SrTiO3 and BaTiO3, the thin-film results are compared with the measured optical functions of the respective bulk materials
[en] The structural relaxation and formation energies of intrinsic vacancies in cubic-BaTiO3 were studied by using a first-principles plane-wave-based pseudopotential calculation. The calculated defect formation energies and the atomic relaxations were compared with previously reported data for SrTiO3 [Tanaka T, Matsunaga K, Ikuhara Y, Yamamoto T. Phys Rev B 2003;68:205213]. It was found that the formation energies of the partial Schottky species, VTi2-+VO2+ and VBa2-+VO2+, are the lowest in the oxidized condition. In contrast, as the oxygen chemical potential decreased, namely the reduced condition, the O vacancy becomes dominant in cubic-BaTiO3. Concerning the atomic relaxation, it was found that BaTiO3 and SrTiO3 show a difference in magnitude. It was also found that the overall vacancy formation energy in BaTiO3 is higher than that in SrTiO3
[en] This dissertation deals with Periodically Segmented Waveguides (PSW), which are applied on KTiOP04 (KTP) crystals, by chemical ion-exchange process. In these waveguides, the crystal polarity and refractive index are periodically modulated to obtain Quasi Phase Matching (QPM) between the fundamental and second-harmonic waves. PSW is a relatively new optical device which exhibits unique optical properties in comparison with a continuous waveguide. The possibility of utilizing the KTP-PSW as a compact, cw, blue-violet, source by doubling infra-red light, is the main motivation for studying the optical properties of KTP segmented waveguides. Nevertheless, much attention in this work is also given to the study of linear optical properties of KTP-PSW, most of which, to my best knowledge, has not been studied yet. Controlling and understanding the linear optical properties of KTP-PSW, are required, for applying the PSW as an optical device by its own, and for control and characterization of the non-linear optical properties of the waveguide. In this work the dependence of the linear optical properties of KTP-PSW on geometrical parameters (period size, duty cycle and waveguide width) were studied. The experimental measured parameters include the PSW near field and the Bragg reflections, which appear due lo the grating structure of the waveguide. The possibility of controlling the wavelength and intensity, of the segmented waveguide Bragg reflections of regular period and super-period, is shown theoretically and experimentally. An unexpected dependence was found, by the experimental measurement, between the index profile and the ion-exchanged segment area,. The segmented waveguide dispersion curve, neff(λ) in the infra-red region was found, A main part of the research work is dedicated to the study of nonlinear characteristics of PSW. The different factors, which effect the Second Harmonic Generation (SHG), are measured experimentally and analyzed. The experimental results are compared to the theoretical model for the various PSW parameters used. By measuring the QPM wavelength the PSW dispersion curve, in the visible region, is defined, as well. An optimal KTP-PSW structure, for doubling infra-red light to blue, is suggested. Based on the doubling efficiency experimental results, the PSW nonlinear coefficient (which is different from the KTP coefficient) is found. Experimental results of higher order, SHG lateral modes, are shown, as well. Non uniform segmented waveguide is another subject studied in this dissertation. Three new structures which enable to obtain unique optical properties which can not be achieved by regular uniform PSW, are suggested and tested. The first structure is designed to get a PSW which does not have Bragg reflections. In the second structure, the Bragg reflections of the waveguide are not sensitive to the photolithography process. In the third structure the QPM spectrum is broadened considerably. Good agreement, between the experimental and the calculated results, are found. An accurate measurement of the PSW effective index can be obtained, by measuring the Bragg reflection wavelength. This quality can be used to evaluate the substrate crystal reflective index and the continuous waveguide effective index, by a new method which is proposed in this work. The KTP dispersion curve, nc(λ) , which was found experimentally (by this new method) is compared to experimental values obtained from KTP bulk (non waveguide) QPM. It was found that the KTP dispersion curve obtained in this work is more accurate then the values given in the literature. The continuous waveguide dispersion curve was also tested by the SHG wavelength obtained from continuous waveguide frequency doublers. Good agreement, between the experimental and the calculated result, was also found in this case
[en] Layered compounds have been synthesized and structurally characterized for the n=5 and 6 members of the perovskite-related family La4Srn-4TinO3n+2 by combining X-ray diffraction and transmission electron microscopy. Their structure can be regarded as comprising [(La,Sr)5Ti5O17] and [(La,Sr)6Ti6O20] perovskite blocks joined by crystallographic shears along the a-axis, with consecutive blocks shifted by 1/2 p. The n=5 member is similar to the previously reported n=5 member of other AnBnO3n+2-related series. The n=6 member, which has only been briefly reported in other systems previously, is also a well-behaved member of this AnBnO3n+2 series
[en] The effect of mechanical stress on the direct piezoelectric properties of pre-poled (1 − x)(Na0.5Bi0.5)TiO3–xBaTiO3 (NBT–xBT) in the range 4% ≤ x ≤ 13% was studied in situ using a mechanical load frame. Prior to mechanical loading, compositions near the morphotropic phase boundary (MPB, x = 6–7% BT) exhibited enhanced ferroelectric and piezoelectric properties compared to compositions further from the MPB. Specifically, the lowest ferroelectric coercive field and highest piezoelectric coefficient within this composition range occur at x = 7% BT. During mechanical compression, the MPB compositions exhibited the lowest depoling stress. The results demonstrate that, while favorable piezoelectric and ferroelectric properties can be obtained at compositions near the MPB, these compositions are also the most susceptible to the effects of mechanical depoling. Ferroelastic domain wall motion is suggested as the primary factor that may be responsible for these behaviors.