The crystalline orientation and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films mediated by the intermediate layer of LaNiO3

c-Axis-oriented and (1 1 7)-oriented Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films are successfully controlled by the intermediate layer of LaNiO₃ (LNO) with chemical solution deposition (CSD), respectively. X-ray diffraction (XRD) demonstrates that the structure and orientation of LNO thin films have a strong effect on the orientation of BLT thin films. Scanning electron microscopy suggests that BLT thin films on LNO electrode exhibit crack-free, uniform size grains and dense microstructure. A crystalline orientation dependent remanent polarization is observed in BLT thin films, and it is found that the remanent polarization (2P_r) of (1 1 7)-oriented films is larger than that of c-axis-oriented films. Our research directly demonstrates that the vector of the main spontaneous polarization in these layered perovskite materials (BLT) is along a-axis.     

Lanthanum-doped Bi4Ti3O12 ceramics prepared by high-pressure technique

We present an effective way in this paper to increase the density of lanthanum doped bismuth titanate ceramics, Bi4-xLaxTi3O12 (BLT), thereby significantly improving the performance of the BLT ceramics. Dense BLT ceramicses, Bi4-xLaxTi3O12 (x = 0.25, 0.5, 0.75, 1.0), are prepared by using nanocrystalline powders fabricated by a-gel method and high-pressure technique. The microstructures of the BLT ceramicses prepared separately by conventional-pressure and high-pressure techniques are investigated by using x-ray diffraction and transmission electron microscope. The influence of La-doping on the densification of bismuth titanate ceramics is investigated. The experimental results indicate that the phase compositions of all samples with various lanthanum dopings sintered at 900°C possess layer-structure of Bi4Ti3O12 . The green compacts are pressed under 2.5 GPa, 3.0 GPa, 3.5 GPa and 4.0 GPa, separately. It is found that the density of BLT ceramics is significantly increased due to the decreasing of porosity in the green compacts by high-pressure process.     

Dependence of excess bismuth content in precursor sols on ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films fabricated by chemical solution deposition

Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were fabricated on Pt/Ti/SiO₂/Si(1 0 0) substrates by chemical solution deposition (CSD), and the dependence of ferroelectric and dielectric properties of the as-deposited BLT thin films on excess Bi content in precursor sols was studied. It is found that the prepared BLT thin film shows the best polarization-electric field, capacitance-voltage and dielectric constant (?_r)-frequency characteristics, when the value of excess Bi content in precursor sols is 10%. In detail, its remnant polarization (2P_r) value is 40 μC/cm², the capacitance tunability is 21% measured at room temperature under conditions of an applied voltage of 8 V and measurement frequency of 10 kHz, and the ?_r is 696 at 100 kHz frequency.     

Sparse Bayesian reconstruction method for multispectral bioluminescence tomography

We present a sparse Bayesian reconstruction method based on multiple types of a priori information for multispectral bioluminescence tomography （BLT）. In the Bayesian approach, five kinds of a priori information are incorporated, reducing the ill-posedness of BLT. Specifically, source sparsity characteristic is considered to promote reconstruction results. Considering the computational burden in the multispectral case, a series of strategies is adopted to improve computational efficiency, such as optimal permissible source region strategy and node model of the finite element method. The performance of the proposed algorithm is validated by a heterogeneous three-dimensional （3D） micron scale computed tomography atlas and a mouse-shaped phantom. Reconstructed results demonstrate the feasibility and effectiveness of the proposed algorithm.     

Recent advances in bioluminescence tomography: methodology and system as well as application

Optical molecular imaging has been rapidly developed to noninvasively visualize in vivo physiological and pathological processes involved in normal and suffering organisms at the cellular and molecular levels, in which advanced optical imaging technology and modern molecular biology are being combined to provide a state‐of‐the‐art tool for preclinical biomedical research. Among optical molecular imaging modalities, bioluminescence tomography (BLT) has experienced considerable growth and attracted much attention in recent years for its excellent performance, unique advantages, and high cost‐effectiveness. This article focuses on the genesis and development of BLT, especially for its computational methodology, imaging system, and biomedical application. An overview of the advantages and challenges of the conventional planar bioluminescence imaging technique is first described in comparison with currently available molecular imaging modalities. The imaging algorithms for inverse source reconstruction are classified and summarized according to different a priori knowledge, followed by a simple depiction of the uniqueness theorems of BLT solution. Diverse imaging systems for obtaining three‐dimensional quantitative information of internal bioluminescent sources are then reviewed. The latest application examples of BLT in tumor study and drug discovery are introduced and compared with other mature imaging technologies. Finally, the paper is concluded and an attractive prospect for BLT is predicted.     

Morphology and microstructure of all-epitaxial ferroelectric tri-layered (Bi,La)4Ti3O12/Pb(Zr0.4Ti0.6)O3/ (Bi,La)4Ti3O12 thin films on SrTiO3(011)

The morphology and microstructure of all-epitaxial (Bi,La)₄Ti₃O₁₂/Pb(Zr_0.4Ti_0.6)O₃/(Bi,La)₄Ti₃O₁₂ (BLT/PZT/BLT) tri-layered ferroelectric films, grown on (011)-oriented SrTiO₃ (STO) substrates by pulsed laser deposition, are investigated by transmission electron microscopy (TEM). X-ray diffraction and electron diffraction patterns demonstrate that the epitaxial relationship between BLT, PZT and STO can be described as ; . Cross-sectional TEM images show that the growth rate of BLT is nearly two times that for PZT at the same growth conditions, and 90° ferroelectric domain boundaries lying on {110} planes are observed in the PZT layer. The 90° ferroelectric domains in the PZT layer extend up to 600 nm in length. Long domains penetrate into the neighboring columnar grain through the columnar grain boundary, whereas others are nucleating at the columnar grain boundaries. The roughness of the PZT/BLT interfaces appears to depend on the viewing direction, i.e., it is different for different azimuthal directions. Planar TEM investigations show that the grains in the top BLT layer have a rod-like morphology, preferentially growing along the [110]_BLT direction. The grain width is rather constant at about 90 nm, whereas the length of the grains varies from 150 to 625 nm. These morphological details point to the important role the crystal anisotropy of BLT plays for the growth and structure of the tri-layered films. PACS 81.15-z; 68.37.Lp; 77.84.-s     

High resolution optical coherence tomography by ?1-optimization

An algebraic optical coherence tomography (OCT) method is presented in this paper for high resolution tomography imaging. The method achieves high resolution by formulating the frequency domain OCT (FD-OCT) into a ?₁-optimization based algebraic reconstruction problem. The performance of the proposed algebraic method is evaluated by simulation and experiment, and compared to the FD-OCT method. It is shown that the proposed method can deliver high resolution beyond the coherence length and imaging depth bigger than the ones of the FD-OCT method. The computational load is greatly reduced compared to the traditional algebraic reconstruction methods.     

Bi3.25La0.75Ti3O12 (BLT) nanotube capacitors for semiconductor memories

We report results of fabrication and examination of Bi_3.25La_0.75Ti₃O₁₂ (BLT) ferroelectric nanotubes. BLT nanotubes are suggested for developing 3D ferroelectric nanotube capacitors which could be used in high-density memory applications. BLT nanotubes were prepared by template-wetting process using polymeric sources where anodic aluminum oxide had been used as a template. After annealing, tubular BLT structures were crystallized inside the pores of the template. By selective etching of the template, released BLT nanotubes have been obtained. Crystallization and nucleation of the nanotubes were analyzed by XRD and FE-SEM techniques.     

Non-convex algorithm for sparse and low-rank recovery: Application to dynamic MRI reconstruction

In this work we exploit two assumed properties of dynamic MRI in order to reconstruct the images from under-sampled K-space samples. The first property assumes the signal is sparse in the x-f space and the second property assumes the signal is rank-deficient in the x-t space. These assumptions lead to an optimization problem that requires minimizing a combined l_p-norm and Schatten-p norm. We propose a novel FOCUSS based approach to solve the optimization problem. Our proposed method is compared with state-of-the-art techniques in dynamic MRI reconstruction. Experimental evaluation carried out on three real datasets shows that for all these datasets, our method yields better reconstruction both in quantitative and qualitative evaluation.     

The effect of microwave annealing on the electrical characteristics of lanthanum doped bismuth titanate films obtained by the polymeric precursor method

Lanthanum doped bismuth titanate thin films (Bi_3.25La_0.75Ti₃O₁₂ - BLT) were produced by the polymeric precursor method and crystallized in a domestic microwave oven and in conventional furnace. Using platinum coated silicon substrates configuration, ferroelectric properties of the films were determined with remanent polarization P_r and a coercive field E_c of 3.9 μC/cm² and 70 kV/cm for the film annealed in the microwave oven and 20 μC/cm² and 52 kV/cm for the film annealed in conventional furnace, respectively. The films annealed in conventional furnace exhibited excellent retention-free characteristics at low infant periods indicating that BLT thin films can be a promise material for use in non-volatile memories. On the other hand, the pinning of domains wall causes a strong decay at low infant periods for the films annealed in the microwave furnace which makes undesireable the application for future FeRAMS memories.     

Smoothly Clipped Absolute Deviation (SCAD) regularization for compressed sensing MRI Using an augmented Lagrangian scheme

PurposeCompressed sensing (CS) provides a promising framework for MR image reconstruction from highly undersampled data, thus reducing data acquisition time. In this context, sparsity-promoting regularization techniques exploit the prior knowledge that MR images are sparse or compressible in a given transform domain. In this work, a new regularization technique was introduced by iterative linearization of the non-convex smoothly clipped absolute deviation (SCAD) norm with the aim of reducing the sampling rate even lower than it is required by the conventional l₁ norm while approaching an l₀ norm.Materials and MethodsThe CS-MR image reconstruction was formulated as an equality-constrained optimization problem using a variable splitting technique and solved using an augmented Lagrangian (AL) method developed to accelerate the optimization of constrained problems. The performance of the resulting SCAD-based algorithm was evaluated for discrete gradients and wavelet sparsifying transforms and compared with its l₁-based counterpart using phantom and clinical studies. The k-spaces of the datasets were retrospectively undersampled using different sampling trajectories. In the AL framework, the CS-MRI problem was decomposed into two simpler sub-problems, wherein the linearization of the SCAD norm resulted in an adaptively weighted soft thresholding rule with a sparsity enhancing effect.ResultsIt was demonstrated that the proposed regularization technique adaptively assigns lower weights on the thresholding of gradient fields and wavelet coefficients, and as such, is more efficient in reducing aliasing artifacts arising from k-space undersampling, when compared to its l₁-based counterpart.ConclusionThe SCAD regularization improves the performance of l₁-based regularization technique, especially at reduced sampling rates, and thus might be a good candidate for some applications in CS-MRI.     

High-throughput screening of ferroelectric materials for non-volatile random access memory using multilayer perceptrons

During the last several years, the development of combinatorial technology has enabled synthesis of a huge amount of chemical compounds in a short time. The large number of variables makes the direct human interpretation of data derived from combinatorial experimentation for high-throughput screening (HTS) very difficult. Artificial neural networks using multilayer perceptrons (MLP) have been successfully applied to the regression problems with various material data. In this work, MLP model was applied to HTS of ferroelectric materials including Bi_4−xLa_xTi₃O₁₂ (BLT) and Bi_4−xCe_xTi₃O₁₂ (BCT). The model using MLP was made to predict the ferroelectric properties of whole feasible experimental conditions. Once a neural network model with high accuracy and good generalization performance was established, we could predict the expected optimal reaction conditions with the best characteristics. The highest gradient value obtained using MLP model is higher than the maximum value found from experiments, thereby accelerating the discovery of the optimal compositions and post-annealing time of BCT and BLT.     

Variational methods in radiative transfer problems

An efficient variational-iterative method is applied to the problem of diffuse reflection by a plane-parallel inhomogeneous atmosphere with isotropic scattering. The emergent intensity I(τ = 0; μ, μ₀) with μ = μ₀ corresponds to the maximum of an associated functional. It is, however, shown that I(τ = 0; μ, μ₀) computed by the variational method alone has relatively large errors when μ ≠ μ₀. Such deficiencies are removed by a combined variational-iterative method. The interdependence of the iterative and variational methods is also investigated. They are shown to play a complementary role to each other. The proper choice of trial functions is emphasized in light of computational efficiency and flexibility. Two distinct classes of trial functions: the polynomials, and the step functions are investigated as possible choices of trial functions. The latter choice is shown to be far more efficient in computation. Numerical results for both approximate emergent intensities and source functions are presented and found to be in good agreement with the exact solutions. Simple analytic two-step function approximations of the source function and intensities are also presented for the case of a two-layer inhomogeneous model.     

Ferroelectric properties of Bi3.25−x/3La0.75Ti3−xNbxO12 films prepared by pulsed laser deposition

Bi₄Ti₃O₁₂ (BiT), Bi_3.25La_0.75Ti₃O₁₂ (BLT), Bi_4−x/3Ti_3−xNb_xO₁₂ (BTN) and Bi_3.25−x/3La_0.75Ti_3−xNb_xO₁₂ (BLTN) thin films have been prepared by pulsed laser deposition. BTN and BLTN films exhibit a maximum in the remanent polarization P_r at a Nb content x=0.018. At this Nb content, the BLTN film has a P_r value (25 μC/cm²) that is much higher than that of BiT and a coercive field similar to that of BiT. The polarization of this BLTN film is fatigue-free up to 10⁹ switching cycles. The high fatigue resistance is mainly due to the substitution of Bi³⁺ ions by La³⁺ ions at the A site and the enhanced P_r arises largely from the replacement of Ti⁴⁺ ions by Nb⁵⁺ ions at the B site. The mechanisms behind the effects of the substitution at the two sites are discussed.     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

An algorithm for sparse MRI reconstruction by Schatten p-norm minimization

In recent years, there has been a concerted effort to reduce the MR scan time. Signal processing research aims at reducing the scan time by acquiring less K-space data. The image is reconstructed from the subsampled K-space data by employing compressed sensing (CS)-based reconstruction techniques. In this article, we propose an alternative approach to CS-based reconstruction. The proposed approach exploits the rank deficiency of the MR images to reconstruct the image. This requires minimizing the rank of the image matrix subject to data constraints, which is unfortunately a nondeterministic polynomial time (NP) hard problem. Therefore we propose to replace the NP hard rank minimization problem by its nonconvex surrogate — Schatten p-norm minimization. The same approach can be used for denoising MR images as well.Since there is no algorithm to solve the Schatten p-norm minimization problem, we derive an efficient first-order algorithm. Experiments on MR brain scans show that the reconstruction and denoising accuracy from our method is at par with that of CS-based methods. Our proposed method is considerably faster than CS-based methods.     

Magnetic surface reconstruction in thin film: Possibilities of production and exploitation

Various theoretical techniques are considered for the production of magnetic surface reconstruction (MSR) in ferromagnetic thin films. The problem is discussed within the framework of the surface field model assuming the existence of a unidirectional anisotropy on the surface described by an effective field K_s It is shown that knowledge of the conditions for the occurrence of MSR together with complementary information accessible from critical spin wave resonance permits the complete determination of the properties of the field K_s.     

New fast and memory-sparing method for rigorous electromagnetic analysis of 2D periodic dielectric structures

A method is presented that performs the exact electromagnetic analysis of 2D periodic dielectric structures of arbitrary profile or index distribution and possibly large period. The generalized source method is used to formulate the problem of light diffraction in the form of a volume integral equation reduced to a linear equation system, which is solvable by known fast algorithms. The calculation time and required memory are linearly proportional to the total number N_o of considered diffraction orders instead of N_o³ typical for conventional methods. Numerical examples are provided to demonstrate the potential of the method for the analysis of complex periodic structures.     

Integral transform method in radiative transfer

A method based on integral Fourier transform technique is proposed for obtaining the radiation intensity in a nonconservative, isotropically scattering, finite slab with a general internal isotropic source and with general transparent boundary conditions. The theory of the method is first presented and it is shown to provide an exact analytical solution for the problem. Numerical results for various physical situations are then reported and compared with previous other results wherever possible. The case of the universal functions Θ(τ) and Θ_g(τ) is also considered.     

Composite pulses for RF phase encoded MRI: A simulation study

BackgroundIn B₁ encoded MRI, a realistic non-linear phase RF encoding coil will generate an inhomogeneous B₁ field that leads to spatially dependent flip angles. The non-linearity of the B₁ phase gradient can be compensated for in the reconstruction, but B₁ inhomogeneity remains a problem. The effect of B₁ inhomogeneity on tip angles for conventional, B₀ encoded MRI, may be minimized using composite pulses. The objective of this study was to explore the feasibility of using composite pulses with non-linear RF phase encoding coils and to identify the most appropriate composite pulse scheme.MethodsRF encoded signals were simulated via the Bloch equation for various symmetric, asymmetric and antisymmetric composite pulses. The simulated signals were reconstructed using a constrained least squares method.ResultsRoot mean square reconstruction errors varied from 6% (for an asymmetric composite pulse) to 9.7% (for an antisymmetric composite pulse).ConclusionAn asymmetric composite pulse scheme created images with fewer artifacts than other composite pulse schemes in inhomogeneous B₀ and B₁ fields making it the best choice for decreasing the effects of spatially varying flip angles. This is contrary to the conclusion that antisymmetric composite pulses are the best ones to use for spin echo sequences in conventional, B₀ encoded, MRI.