The crystallographic symmetries of single BaPb1−xBixO3 crystals grown from BaCO3-PbO2-Bi2O3 solutions

Single crystals of Perovskite-type oxide BaPb_1−xBi_xO₃ are grown from BaCO₃-PbO₂-Bi₂O₃ solutions which are weighed in two kinds of mixing ratios: X/2 mol % BaCO₃ − (100−X) mol % PbO₂ − X/2 mol % Bi₂O₃ and (10+X/2) mol % BaCO₃ − (90−X) mol % PbO₂ − X/2 mol % Bi₂O₃ These room temperature crystal structures are examined by using an X-ray powder diffraction method. The crystals grown from X/2−(100−X)−X/2 mol % solutions are orthorhombic at room temperature, while the structures are tetragonal with crystals grown from relatively Ba rich and Pb poor ( (10+X/2)−(90−X)−X/2 mol % ) solutions. This result indicates that the difference in the mixing ratio of the initial materials brings about a drastic structural change. The orthorhombic and the tetragonal crystals of x0.25 exhibit superconducting transition at 10K and 12K, respectively. The transition temperature in the latter is 2K higher than the former. In the light of this result, some difference between orthorhombic and tetragonal crystals is considered to influence superconductivity in this system.     

Effect of magnetic ordering on optical and magneto-optical properties of Fe/Cr superlattices

Optical reflectance/transmittance and magneto-optical Kerr effect of Fe/Cr superlattices (SL) are studied by using the first principles full-potential linearized augmented plane-wave method. Equilibrium atomic structures are optimized through total energy and atomic force approaches. Pronounced peaks in the ellipticity and optical spectra are obtained around 3000 cm⁻¹ for the ferromagnetic SL due to the inter-band electron excitations. The change in magnetic ordering from ferromagnetic to antiferromagnetic cases alters optical functions, namely, the reflectance, R and transmittance T. The calculated ΔR/R and ΔT/T have a pronounced maximum around 2000 cm⁻¹ and decay almost linearly thereafter toward high photon energies, a result that agrees qualitatively well with experiment.     

A theoretical analysis of refractive-index grating waveguide structures for integrated optics

Based upon equivalent transmission line method, a perturbation analysis on the guiding properties of refractive-index gratings is presented. By numerical calculation, the leakage parameter which influences the leakage energy into the diffracted orders due to the grating scatter, is achieved as functions of the grating period, thickness of the waveguide and index modulation Δn. Our results reveal that, although the index modulation Δn is much less than that of the etched or deposited grating, refractive-index gratings can still reach a high leakage parameter because of the prominent increase of the grating height.     

Multilayer relaxation of Rh(311)

The multilayer relaxation of the Rh(311) surface was investigated by means of LEED structure determination both for vertical and surface parallel (registry) relaxations. Excellent agreement between experimental and calculated spectra could be achieved mirrored by a minimum Pendry R-factor R = 0.174. The first three layer spacings are oscillatorily relaxed by Δd₁₂/d₀ = −14.5 ± 1.8%, Δd₂₃/d₀ = +4.9 ±2.0% and Δd₃₄/d₀ = −1.0 ±2.0%. There seems to be a coherent registry shift of the fir Δs = 0.03 ± 0.07 Å which, however, is within the error limits of the structure determination. Moreover, an energy dependent inner potential is detected. The results are discussed in comparison to equivalent surfaces of other materials as well as for the less open surfaces of rhodium.     

Light-enhanced reactivation of passivated boron in hydrogen treated silicon

The thermal reactivation of the boron acceptor in hydrogenated silicon is strongly enhanced if the anneals are performed under illumination. The changes of the inactive boron concentration R versus annealing time t satisfy the equation dR/dt = -rR²/(N_A − R)², where N_A is the total boron concentration. Under strong illumination the annealing parameter r is proportional to the light intensity.     

Intensity dependence of two-center nonvolatile holographic recording in LiNbO3:Cu:Ce crystals

Nonvolatile photorefractive gratings have been recorded in LiNbO₃:Cu:Ce crystals by using a He–Ne laser (633 nm) for recording and an argon ion laser (458 nm) for sensitizing. The sensitizing light increases the recording sensitivity by a−bexp(−I_s/c) and saturation behavior will appear with high enough intensity of sensitizing light. The recording light increases the slope of η^1/2 as a function of time during the initial stages of hologram formation by sublinear I^x_r (x<1) and thus the recording light decreases the recording sensitivity. The dependence of saturation diffraction efficiency on the intensities of the recording and sensitizing light shows that there is a maximum dynamic range of the recording process.     

磁共振碳谱研究:应用分子子图指数估计与预测卤代烃化学位移

系统研究了核磁共振碳谱(¹³C NMR)与化学位移规律及其定量构谱关系(QSSR).本文提出了一组含多元素组成的分子子图指数矢量(VMSG),并发现它与卤代烃化学位移(CSRX)有很好线性相关性.采用多元线性回归(MVLR)进行准确估计与预测,结果良好.     

Study of in fiber Mach–Zehnder interferometer based on two spaced 3-dB long period gratings surrounded by a refractive index higher than that of silica

The paper deals with the response of a Mach–Zehnder interferometer composed by two 3-dB long period gratings (LPG) with different external refractive indices higher than that of silica (leaky configuration). A rough analysis of the cladding propagation arising in leaky configuration has allowed us to establish a model that is sufficient to describe the interferometer couplings evolution at the resonance. In particular, we have shown that the interferometer sensitivity depends strongly on its phase-shift Δ and is optimized for Δ equal to π [2π] or 3π/4 [2π]. An experimental study was undertaken and allowed us to validate our model. Finally, a comparison with a single LPG demonstrated an increased sensitivity to the external index, suggesting the conception of a new fiber sensor operating with external refractive indices higher than that of silica.     

ESR of double-perovskite Sr2FeMoO6

We present an ESR study of Sr₂FeMoO₆ in the paramagnetic region. A single line at g≈2 was associated with Fe³⁺ ions. The intensity follows Curie–Weiss law in the whole T range. For T >500 K a secondary line is attributed to ferromagnetic (FM) impurities. The line width is described by ΔH_pp(T)=ΔH_pp(∞)(1−Θ/T) with a high value for ΔH_pp(∞). The absence of narrowing effects is a signature of double-exchange (DE) interactions and indicates that DE drives the FM ordering at a relatively high T_c.     

Crystal structure determination of Pb2Sr2YCu3O8+δ (δ=0, 1.67) by powder neutron diffraction

Crystal structures of Pb₂Sr₂YCu₃O_8+δ(δ=0.0 and 1.67) were determined by powder neutron diffraction. For δ=0, it had been indicated by powder X-ray diffraction that the structure was a monoclinic one. The R factors of the present analysis for the monoclinic structure were about 3/4 of those for the previous orthorhombic ones. The structure was almost the same as the previous X-ray result except z(O(2)). For the annealed phase, the tetragonal and the orthorhombic structures had been presented by X-ray diffraction and by neutron diffraction, respectively. The R factors of the present analysis for the orthorhombic structure were about 5/6 of those for the tetragonal structure. The distance between the Pb plane and the oxygen-deficient Cu plane was found to become larger in proportion to δ, though there occurs a phase separation in the low-δ region.     

Spectral simulation for infrared surface electromagnetic wave spectroscopy

Infrared surface electromagnetic wave spectra, which are obtained by the Otto and Kretschmann configurations, are calculated by the Fresnel formula of stratified layers. The calculated reflectivity is plotted as a function of incidence angle, air gap thickness, metal layer thickness, and wavenumbers. From these calculations, optimum experimental variables for both configurations are predicted in the infrared region. At the optimum angle of incidence, the z-component of the electric field intensity was found to be maximum at the surface of silver. The calculated SEW spectra of a PVAc film on silver show a large value of ΔR and a strong frequency dependence. The Otto configuration has advantages in the mid-infrared region.     

Incoherent bremsstrahlung in nuclei

We have studied the processes A(e, e′γ)X in nuclei, or incoherent bremsstrahlung, and determined expressions for the cross section in terms of the same nuclear response functions R_L, R_T, which appear in inclusive electron scattering (e, e′) in nuclei. Calculations of the cross sections are carried out using a Fermi gas model, complemented by the local-density approximation, to evaluate the response functions. We have carried out a study which shows that the reaction can be used to determine reliably the response functions from experimental data. On the other hand we have compared the incoherent bremsstrahlung with the coherent one in order to see the limits to the tagging technique, which produces monochromatic photons based on the assumption of the dominance of the coherent process. We observe that at energies E_γ < 1 GeV the dominance of the coherent process extends to relatively large scattering angles, making the present technique completely safe. However, as the energy of the electron increases, the region of dominance of the coherent process is reduced to smaller scattering angles. These results should be of use when extending the tagging technique to planned or future electron facilities.     

Generalized Einstein manifolds

A Finslerian manifold is called a generalized Einstein manifold (GEM) if the Ricci directional curvature R(u,u) is independent of the direction. Let F⁰(M, g_t) be a deformation of a compact n-dimensional Finslerian manifold preserving the volume of the unitary fibre bundle W(M). We prove that the critical points g₀ F⁰(g_t) of the integral I(g_t) on W(M) of the Finslerian scalar curvature (and certain functions of the scalar curvature) define a GEM. We give an estimate of the eigenvalues of Laplacian Δ defined on W(M) operating on the functions coming from the base when (M, g) is of minima fibration with a constant scalar curvature H admitting a conformal infinitesimal deformation (CID). We obtain λ ≥ H/(n − 1) (Δf = λf). If M is simply connected and λ = H/(n − 1), then (M, g) is Riemannian and is isometric to an n-sphere. We first calculate, in the general case, the formula of the second variationals of the integral I (g_t) for G = g₀, then for a CID we show that for certain Finslerian manifolds, I″(g₀) > 0. Applications to the gravitation and electromagnetism in general relativity are given. We prove that the spaces characterizing Einstein-Maxwell equations are GEMs.     

Thermal transport properties in a He---He mixture near and below the superfluid transition

Measurements of the thermal conductivity and of X/T are reported for a ³He---⁴He mixture with a ³He mole fraction X = 0.622. At T_λ, X/T passes through a sharp peak. A comparison with the theory of Khalatnikov is presented. The relaxation times τ(T) to reach steady state conditions show qualitatively the same behaviors as X/T.     

A continuum model of a solid with a free surface: I. Normal mode analysis of the model

A complete analysis is made of the normal modes of an isotropic continuum model of a solid which occupies the region (0, 0, 0) < (x, y, z) < (X, Y, Z), the limit (X, Y, Z) → (∞,∞,∞) being taken. The surface z = 0 is a free surface and that z = Z is fixed; cyclic boundary conditions are used in the x and y directions. The modal displacements (vectors), modal frequencies (as a function of the wave-vectors), and wave-vector-densities are obtained for all types of normal modes; the work is a modification and extension of that of earlier workers.     

Differential operators on a Riemann surface with projective structure

Let X be a Riemann surface equipped with a projective structure and a theta characteristic on X, or in other words, is a holomorphic line bundle equipped with a holomorphic isomorphism with the holomorphic cotangent bundle Ω_X. The complement of the zero section in the total space of the line bundle has a natural holomorphic symplectic structure, and using , this symplectic structure has a canonical quantization. Using this quantization, holomorphic differential operators on X are constructed. The main result is the construction of a canonical isomorphism

, n≥0, provided i[−2(k−1),0].     

Impurity effects on adhesive energies

Some simple arguments are made about the effect of impurities on adhesive interactions at solid junctions. A universal adhesive force relation is derived for brittle adhesion. The adhesive binding energy, ΔE, is an important parameter in brittle adhesion forces. ΔE has also been shown by others to be important when there is plastic flow. We found that impurity effects on ΔE are determined by the segregation energies to the junction and to the free surfaces. At low temperatures, if it is energetically more favorable for impurities to segregate to the surfaces than to the junction, then the impurities will decrease ΔE. The converse is also true. For example, for self junctions which are in registry, ΔE is decreased if surface segregation is exothermic and increased if it is endothermic. These segregation energy relationships are consistent with the results of a number of experiments on the effects of impurities on adhesion forces and grain boundary embrittlement.     

Interference between resonance and potential scattering of high energy electrons from crystal surfaces

A solution of the tight-binding model of resonance diffraction of high-energy electrons from a crystal surface is found which describes the behaviour of the reflectivity in the vicinity of the intersection of a resonace parabola and a horizontal Kikuchi line. A simple analytical formula is obtained which makes it possible to evaluate the wave function for the case where both resonance and potential contributions to the reflectivity are of the same order of magnitude. The intensity of the specular reflection is shown to be directly related to the effective displacement Δ of the beam in the direction parallel to the surface, and a general formula suitable for evaluation of Δ from calculated RHEED rocking curves is derived.     

Optimizing the bandwidth and noise performance of distributed multi-pump Raman amplifiers

Based on hybrid genetic algorithm (HGA), the signal bandwidth of the distributed multi-pump Raman amplifiers is optimized, and the corresponding noise figure is obtained. The results show that: (1) the optimal signal bandwidth Δλ decreases with the increase of the span length L, e.g., Δλ is 79.6 nm for L=50 km and 41.5 nm for L=100 km under our simulated conditions; (2) the relationship between Δλ and L is approximately linear; (3) the equivalent noise figure can be negative and increases with the extension of L; (4) there are one or several global maximum signal bandwidth on the determinate conditions; (5) to realize the fixed Δλ, several candidates can be obtained by means of HGA, as has important applications on the design of distributed multi-pump Raman amplifiers in practice.     

Density of the superfluid fraction near the λ point in He---He mixtures by oscillating disk method

The critical exponents ζ(X) and amplitudes κ(X) of superfluid fraction _s/_λ in ³He---⁴He mixtures were determined by oscillating disk method approaching 10⁻⁴ K to lambda temperatures determined from velocity minima of first sound.