Intensity-dependent beam-fanning in high-gain photorefractive barium titanate crystal

Abstract

A phenomenological multi-wave mixing model has been used along with a more general solution of Kukhtarev's model to explain intensity-dependent beam-fanning in photorefractive (PR) BaTiO₃ crystal. The variation of trap density owing to the photoexcitation of charge carriers has been considered to explain the intensity-dependent effect. Theoretical results predicted by the model show a confirmation with experimental results obtained from PR crystal, and also show some of the controlling mechanisms that enable use of beam-fanning for some potential applications.     

Enhancement of Phase Modulation Detection Sensitivity in Ring Photorefractive Configurations

Abstract

An adaptive ring photorefractive configuration, which can increase remarkably the sensitivity of the homodyne detection of high frequency light phase modulation, is proposed and analysed. For typical BaTiO₃ photorefractive crystals, the enhancement factor can reach 100. However, the practical results can be limited by the light losses in the ring cavity and, in particular, inside the photorefractive sample.     

Scanning Kerr Microscopy of the Spin Hall Effect in <Emphasis Type="Italic">n</Emphasis>-Doped GaAs with Various Doping Concentration

We investigated the doping concentration (N _D) dependence of the extrinsic spin Hall effect (SHE) in n-doped GaAs with N _D raging from 3×10¹⁶ cm⁻³ to 5×10¹⁷ cm⁻³. By using scanning Kerr microscopy (SKM) measurements, we observed the Kerr rotation signal due to the spin accumulation near the channel edges in all the samples with different N _D. Moreover, the position and in-plane magnetic field dependence of the Kerr rotation signal are found to vary with N _D. We analyzed the N _D dependence of the spin Hall conductivity by taking account of the N _D-dependent spin lifetime based on the typical drift-diffusion model.     

Ac conductivity of BaTiO3 containing (90V2O5-10P2O5) oxide glasses dispersed with nanocrystalline particles

The frequency dependent AC conductivity (_ac) of some new multicomponent (1 – x) (90V₂O₅-10P₂O₅) + xBaTiO₃ (x = 0.1 to 0.9) type glasses containing nanocrystalline BaTiO₃ particles (observed from the transmission electron microscopic study) has been reported in the temperature range 80–400 K. The behavior of _ac is broadly similar to what has been observed previously in many other amorphous semiconductors and polymers (namely nearly linear frequency dependence and weak temperature dependence). Analysis of the experimental conductivity data shows that Long's overlapping large polaron tunneling (OLPT) mechanism is the most probable mechanism of conduction for the present BaTiO₃ doped glasses. The Quantum mechanical tunneling (QMT) model and the classical hopping of electrons over a barrier can, however, explain the AC conductivity data only below _D/2–160 K (_D is the Debye temperature). Reasonable values of the relaxation time and barrier heights related to the models have been obtained from the fits of the conductivity data.     

Charge-field hysteresis of BaTiO3: PVDF composites

Hysteresis phenomena in BaTiO₃: PVDF composites with different weight fractions of BaTiO₃ and PVDF were studied in the temperature range 20–180°C. A sinusoidal electric field of up to 100 kV ^–1 at 50 Hz was used. The saturation polarization, P _sat, remanant polarization, P _r, and coercive field, E_c, were observed to increase with increasing ceramic content of the composite samples. An increase of polarization with temperature was observed for all composites up to 80°C. This has been explained on the basis of an increase in the conductivity of the PVDF phase.     

Investigation of Ultrasound Attenuation in Impurity-Helium Solids Containing Liquid Helium

The attenuation of ultrasound in Impurity-Helium (Im-He) solids created upon introduction of impurity atoms or molecules (D ₂ , N ₂ , Kr) into a volume of superfluid helium has been investigated. The observed features of attenuation show that a porous substance consisting of a loosely interconnected continuous network is created in superfluid helium. This network is formed by impurity particles encapsulated in solidified helium. Analysis of attenuation allows us to conclude that Im-He solid samples have a wide distribution of pores from 8 nm to 800 nm. It was established that the character of attenuation in D ₂ -He samples is considerably different from that in heavier Im-He solids, for which two maxima of attenuation were sometimes observed. A sharp peak was observed at T_c0 very close to the bulk helium lambda transition temperature and a second broad peak occurred at T_cc0 . This behavior is similar to that predicted theoretically for liquid helium in restricted fractal porous media.     

Multiferroic Properties of (Bi0.9Gd0.1FeO)1−x (BaTiO3) x Ceramics

Conventional solid-state reaction method has been employed for the synthesis of polycrystalline (Bi_0.9Gd_0.1FeO)_1?x (BaTiO₃) _x for x=0.1, 0.2 and 0.3, ceramics samples. The effect of BaTiO₃ content on the multiferroic properties of Gd-doped BiFeO₃ ceramics has been presented. Pure perovskite phase with high density has been obtained by optimizing the synthesis approach, calcination and sintering strategies. Structural analysis carried out using X-ray diffraction confirms the formation of desired morphotropic phase. The dielectric properties have been investigated at different concentration of BaTiO₃ as function of temperature, revealing that by increasing the BaTiO₃ content dielectric constant increases while dielectric losses decrease. Magnetic study shows that initially saturation magnetization increases with increase in BaTiO₃ content up to x=0.1; however, afterwards it decreases for higher concentration of BaTiO₃. According to ferroelectric measurements, PE loops (with low coercive field) are observed at room temperature. The remnant polarization (P _r ) has been found to be 0.169, 0.619 and 0.760 μC/cm², respectively, for samples with x=0.1, 0.2 and 0.3. Magnetoelectric coupling in as-synthesized samples has been indirectly deduced by an anomaly observed at magnetic transition temperature.     

Impurities in single crystal indium phosphide

The impurity concentrations in present low carrier concentration indium phosphide single crystals (N _D-N _A=2×10¹⁵ to 5×10¹⁵cm^–3) grown by the Czochralski technique have been measured by spark-source mass spectrometry and radio-gamma activation analysis and compared with both the polycrystalline source material and the excess indium produced during compounding and growth. The predominant impurities are shown to be carbon, oxygen and silicon but the segregation of lesser impurities into the excess indium has allowed some nineteen other elements which are likely to be present in indium phosphide to be identified. No consistent correlation is evident between the measured concentration of specific impurities and the ionized donor (N _D) and acceptor (N _A) impurity levels as determined from the free-electron concentration (N _D-N _A) and Hall mobility at 77 K using the Brooks-Herring theory.     

Non-adiabatic polaron hopping conduction in semiconducting V2O5-Bi2O3 oxide glasses doped with BaTiO3

BaTiO₃-doped (5–40 wt %) 90V₂O₅-10Bi₂O₃ (VB) glasses have been prepared by a quick quenching technique. The d.c. electrical conductivities, _d.c., of these glasses have been reported in the temperature range 80–450 K. The electrical conductivity of these glasses, which arises due to the presence of V⁴⁺ and V⁵⁺ ions, has been analysed in the light of the small-polaron hopping conduction mechanism. The adiabatic hopping conduction valid for the undoped VB glasses (with 80–95 mol % V₂O₅), in the high-temperature region, is changed to a non-adiabatic hopping mechanism in the BaTiO₃-doped VB glasses. At lower temperatures, however, a variable range hopping (VRH) mechanism dominates the conduction mechanism in both the glass systems. Such a change-over from adiabatic to non-adiabatic conduction mechanism is a new feature in transition metal oxide glasses. Various parameters, such as density of states at the Fermi level N(E_F), electron wave-function decay constant, , polaron radius, r _p, and its effective mass, m _p ^* , etc., have been obtained for all the glass samples from a critical analysis of the electrical conductivity data satisfying the theory of polaron hopping conduction.     

Surface,structural and electrical properties of BaTiO3 films grown on p-Si substrates by low pressure metal organic chemical vapour deposition

Metal organic chemical vapour deposition of BaTiO₃ using Ba(tmhd)₂, Ti(OC₃H₇)₄ and N₂O, where tmhd equals 2,2,6,6-tetramethyl-3,5-heptanedionate, via pyrolysis at relatively low temperatures (370C) was performed in order to produce BaTiO₃ insulator gates. Scanning electron microscopy showed that the surfaces of the BaTiO₃ films had very smooth morphologies. Atomic force microscopy showed that the BaTiO₃ thin film was polycrystalline. X-ray diffraction results indicated that BaTiO₃ crystalline films grew on Si(100) with [110] orientation. High resolution transmission electron microscopy measurements showed that the BaTiO₃ films were polycrystalline, and an interfacial layer in the BaTiO₃/Si interface was formed. The stoichiometry and atomic structure of the BaTiO₃ films were investigated by Auger electron spectroscopy and transmission measurements, respectively. Room temperature capacitance-voltage measurements clearly revealed metal-insulator-semiconductor behaviour for samples with BaTiO₃ insulator gates, and interface state densities at the BaTiO₃/p-Si interface were approximately high, 10¹¹ eV^–1 cm^–2, at the middle of the Si energy gap.     

A MODEL STUDY OF NARROW NANOTUBES RELATED TO C36, C32, C20, AND C16: SEMIEMPIRICAL AND NONEMPIRICAL TREATMENTS

ABSTRACT

Very recently, narrow nanotubes have been observed with diameters of 5 or even 4?Å. In this report we perform calculations of narrow model nanotubes capped by fragments of D _2d and D _6h C₃₆ fullerene cages or by fragments of C₃₂ and C₁₆ quasi-fullerene cages with two four-membered rings, or finally by a fragment of dodecahedral C₂₀. The computations can reproduce the observed diameters of the narrow nanotubes. The results also indicate that fragments of C₃₂, used as caps instead of C₃₆, can lead to quite competitive energetics. Thus, a novel possibility that some of the narrow nanotubes can contain four-membered rings at their tips is suggested.     

Mode Conversion by Photorefractive Damage in Planar Optical Waveguides: Application to Fully Confined Modes

Abstract

A theoretical analysis of mode conversion induced by photorefractive damage in optical waveguides is presented. The model applies to any geometry and uses the well known coupled-wave approach which has been extended to anisotropic media. The light-induced coupling coefficients have been expressed in terms of the photorefractive space charge field. The formulae have been, then, applied to calculate the initial conversion rate of a fully confined mode in a planar LiNbO₃ waveguide perpendicular to the c axis. The photorefractive field developed by illumination has been obtained under a short-time (initial) approximation, taking into account all transport processes. Some numerical estimates on mode conversion rates are also given and compared to available experimental data.     

Effect of phosphor doping on the microstructure and dielectric properties of barium titanate ceramics

The effect of phosphor doping on the sintering behaviour, microstructure and dielectric properties of BaTiO₃ has been investigated. Diisopropyl phosphinate is added to high-purity BaTiO₃ powder prepared by the wet chemical method with a final amount of 0.14 wt.% P₂O₅ with respect to the BaTiO₃. Phosphor-doped BaTiO₃ ceramics with a high density and uniform grain size have been produced by using wet processing and pressureless sintering without any binder. A scanning electron microscope, thermometric analysis, X-ray diffraction and an impedance analyser have been used to determine the microstructure as well as the dielectric properties. The phosphor cations can form a liquid phase belonging to the ternary system BaO-TiO₂-P₂O₅, leading to the formation of BaTiO₃ ceramics with high density at low temperature. Phosphor-doped BaTiO₃ ceramics with a high density of 96% D_th are obtained by sintering at 1200°C with a soaking time of 2 h. The dielectric constants of samples sintered at 1150 and 1200°C are as high as 6100 and 5500, respectively; the Curie temperature of samples decreases with decreasing sintering temperature. Doping with a small amount of phosphor can improve the sintering and dielectric properties of BaTiO₃ ceramics.     

Synthesis and characterization of nanostructured ferroelectric compounds

Nanosized BaTiO₃, PbTiO₃, and LiNb₂O₃ have been synthesized by ligand–precursor route using tartaric acid, TEA, and EDTA. The average particle sizes are 15, 18, and 21 nm for BaTiO₃, PbTiO₃, and LiNb₂O₃, respectively. The transition temperatures are 55, 127, and 223 °C for BaTiO₃, PbTiO₃, and LiNb₂O₃, respectively, and corresponding values of the dielectric constant are 5394, 6150, and 1400, respectively. The transition temperature and dielectric constant are lowered from the values of polycrystalline samples due to the high degree of defects in nanostructured materials.     

An electron spin resonance study of Mn2+ doped calcium hydrazine carboxylate monohydrate

Single crystals of calcium hydrazine carboxylate, monohydrate have been studied by ESR of Mn²⁺ doped in the calcium sites. X-band ESR indicated a large crystal field splitting necessitating experiments at Q band. The analysis shows two magnetically inequivalent (but chemically equivalent) sites withg _xx = 2.0042±0.0038,g _yy=2.0076 ±0.0029,g _zz=2.0314±0.001,A _zz=0.0099±0.0002 cm⁻¹,A _xx=0.0092±0.0002 cm⁻¹,A _yy=0.0082±0.0002 cm⁻¹,D=3/2D _zz=0.0558±0.0006 cm⁻¹, andE=1/2 (D _yy−D _yy)=0.0127±0.0002 cm⁻¹. One of the principal components of the crystal field, (D _zz), is found to be along the Ca ↔ Ca direction in the structure and a second one, (D _xx), along the perpendicular to the plane of the triangle formed by three neighbouring calciums. TheA tensor is found to have an orientation different from that of theg andD tensors reflecting the low symmetry of the Ca²⁺ sites.     

Multi-Gaussian Ultrasonic Beam Modeling for Multiple Curved Interfaces—an ABCD Matrix Approach

ABSTRACT

A multi-Gaussian beam model uses a superposition of Gaussian beams to simulate the waves radiated from an ultrasonic transducer. We show that propagation and reflection/transmission laws for Gaussian beams in fluids and elastic solids can be written in the form of A , B , C , D matrices that are analogous to the A, B, C, D scalars used in Gaussian optics. This representation leads to simple expressions for a Gaussian beam even after that beam has been transmitted or reflected at multiple curved interfaces and produces a highly modular multi-Gaussian beam model that is also computationally very efficient. Some examples of the use of this model for both planar and curved interfaces are given.     

Mechanochemical synthesis of BaTiO<Subscript>3</Subscript> from barium titanyl oxalate

The effect of mechanochemical processing in air and water on the physicochemical transformations of barium titanyl oxalate has been studied using X-ray diffraction, thermal analysis, FTIR spectroscopy, temperature-programmed argon desorption, and particle size measurements. The results demonstrate that mechanochemical processing of barium titanyl oxalate in air leads to the formation of structurally imperfect barium titanate. During subsequent air calcination at 550°C, this material transforms into well-crystallized cubic BaTiO₃, whereas thermal decomposition of barium titanyl oxalate only yields cubic BaTiO₃ starting at 800°C. Mechanochemical processing in water leads to partial amorphization of barium titanyl oxalate, and conversion of the product to BaTiO₃ requires heat treatment at 700°C. All of the BaTiO₃ samples obtained via mechanochemical processing have a larger specific surface in comparison with samples prepared by conventional calcination of barium titanyl oxalate or other known processes.     

Improved current limiters based on mixed-phase BaTiO3 ceramic semiconductors

Very stable and highly reproducible current-limiting characteristics have been observed for polycrystalline ceramics prepared from sintering mixtures of coarse-grained, donor-doped BaTiO₃ (tetragonal) as the major phase and ultrafine, undoped cubic perovskite such as BaSnO₃, BaZrO₃, SrTiO₃ or BaTiO₃ (cubic) as the minor phase. The initial linear current-voltage (I-V) relationship becomes current-limiting with increase in applied potential and the consequent onset of thermal equilibrium. The strong current maximum of theI-V curve of donor-doped BaTiO₃ can be eliminated when the ceramics are constituted of mixed phases. The voltage drop at the insulating grain boundaries minimizes the temperature gradient between the interior and the surface, and subdues the thermal runaway. The magnitude of the limiting current, and hence the power-handling capacity, can be varied with the controlled addition of grain boundary layer modifiers and by optimizing the processing parameters. The dielectric constant versus temperature or voltage variation in power dissipation with ambient temperature and resistivity-temperature relations point to the necessity of the mixed phase character for the current-limiting property.     

Volume variation of Gruneisen parameters offcc transition metals

The volume variation of the Gruneisen parameters of tenfcc transition metals, up to 40% compression, has been studied on the basis of a model approach proposed by Antonovet al. The results are reasonably good for six metals except for Rh, Ag, Au and Ni when compared with available experimental and other theoretical values. The model requires an appropriate modification for Rh, Ag, Au and Ni.     

CdSe/CdS core/shell quantum dots as sensitizer of a photorefractive polymer composite

Abstract

CdSe/CdS core/shell, tri-n-octylphosphine oxide passivated, quantum dots are used to sensitize a photorefractive polymer composite. The composite also consists of poly(N-vinylcarbazole) as the nominally charge transporting matrix and an electro-optic chromophore. The efficacy of sensitization and consequent photorefractive performance is investigated using transmission spectroscopy and ellipsometry, two-beam coupling and degenerate four-wave mixing experiments. The photorefractive nature of the photo-induced grating is confirmed by the observation of asymmetric two-beam coupling. Four-wave mixing reveals record diffraction efficiencies for a nano-particle-sensitized photorefractive polymer at the field levels applied (1.3% at 70 V.μm^?1). A recently developed analytical technique is used to extract space-charge field rise time values from degenerate four-wave mixing transients. In turn, analysis of the dependence of the rise time on applied field is used to determine the zero-field charge dissociation efficiency to be 3.6 × 10^?5 ± 0.5 × 10^?5. It is further shown that the magnitude of this parameter accounts for most of the difference in photorefractive response rate between the present material and a similar C₆₀ sensitized composite.