Study of electronic structures and absorption bands of BaMgF4 crystal with F colour centre

The electronic structures of BaMgF 4 crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc-Slater theory,using a numerically discrete variational (DV-Xα) method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band. The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV,which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands.     

boldmath Study of electronic structures and absorption bands of BaMgF4 crystal with F colour centre

The electronic structures of BaMgF₄ crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band. The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV, which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands.     

Influence of Strain-Reducing Layer on Strain Distribution of Self-Organized InAs/GaAs Quantum Dot and Redshift of Photoluminescence Wavelength

A systematic investigation about the strain distributions around the InAs/GaAs quantum dots using the finite element method is presented. A special attention is paid to influence of an Ino.2 Gao.sAs strain reducing layer. The numerical results show that the horizontal- and vertical-strain components and the biaz~ial strain are reinforced in the InAs quantum dot due to the strain-reducing layer. However, the hydrostatic strain in the quantum dot is reduced. In the framework of eight-band k · p theory, we study the band edge modifications due to the presence of a strain reducing layer. The results demonstrate that the strain reducing layer yields the decreasing band gap, i.e., the redshift phenomenon is observed in experiments. Our calculated results show that degree of the redshift will increase with the increasing thickness of the strain-reducing layer. The calculated results can explain the experimental results in the literature, and further confirm that the long wavelength emission used for optical fibre communication is realizable by adjusting the dependent parameters. However, based on the calculated electronic and heavy-hole wave function distributions, we find that the intensity of photoluminescence will exhibits some variations with the increasing thickness of the strain-reducing layer.     

Structural, electronic, and optical properties of ZnOl_xSex alloys using first-principles calculations

The structural, electronic, and optical properties of binary ZnO, ZnSe compounds, and their ternary ZnOl_xSex alloys are computed using the accurate full potential linearized augmented plane wave plus local orbital （FP-LAPW ＋ lo） method in the rocksalt （B 1） and zincblende （B3） crystallographic phases. The electronic band structures, fundamental energy band gaps, and densities of states for ZnO1_xSex are evaluated in the range 0 〈 x 〈 1 using Wu-Cohen （WC） generalized gradient approximation （GGA） for the exchange-correlation potential. Our calculated results of lattice parameters and bulk modulus reveal a nonlinear variation for pseudo-binary and their ternary alloys in both phases and show a considerable deviation from Vegard＇s law. It is observed that the predicted lattice parameter and bulk modulus are in good agreement with the available experimental and theoretical data. We establish that the composition dependence of band gap is semi-metallic in B1 phase, while a direct band gap is observed in B3 phase. The calculated density of states is described by taking into account the contribution of Zn 3d, O 2p, and Se 4s, and the optical properties are studied in terms of dielectric functions, refractive index, reflectivity, and energy loss function for the B3 phase and are compared with the available experimental data.     

Structural,electronic, and optical properties of ZnO_(1-x)Se_x alloys using first-principles calculations

The structural, electronic, and optical properties of binary ZnO, ZnSe compounds, and their ternary ZnO 1-x Se x alloys are computed using the accurate full potential linearized augmented plane wave plus local orbital (FP-LAPW + lo) method in the rocksalt (B1) and zincblende (B3) crystallographic phases. The electronic band structures, fundamental energy band gaps, and densities of states for ZnO 1-x Se x are evaluated in the range 0 ≤ x ≤ 1 using Wu-Cohen (WC) generalized gradient approximation (GGA) for the exchange-correlation potential. Our calculated results of lattice parameters and bulk modulus reveal a nonlinear variation for pseudo-binary and their ternary alloys in both phases and show a considerable deviation from Vegard's law. It is observed that the predicted lattice parameter and bulk modulus are in good agreement with the available experimental and theoretical data. We establish that the composition dependence of band gap is semi-metallic in B1 phase, while a direct band gap is observed in B3 phase. The calculated density of states is described by taking into account the contribution of Zn 3d, O2p, and Se 4s, and the optical properties are studied in terms of dielectric functions, refractive index, reflectivity, and energy loss function for the B3 phase and are compared with the available experimental data.     

Electronic and optical properties of CdS/CdZnS nanocrystals

Cd1-x ZnxS nanocrystals are prepared by a co-precipitation method with different atomic fractions of Zn.The texture,structural transformation and optical properties with increasing x value in Cd1-x ZnxS are studied with scanning electron microscopy,electron diffraction patterning,and absorption spectra respectively.Quantum confinement in a strained CdS/Cd1-xZnxS related nanodot with various Zn content values is investigated theoretically.Binding energies on exciton bound CdS/CdxZn1-xS quantum dot are computed,with consideration of the internal electric field induced by the spontaneous and piezoelectric polarizations,and thereby the interband emission energy is calculated as a function of the dot radius.The optical band gap from the UV absorption spectrum is compared with the interband emission energy computed theoretically.Our results show that the average diameter of composite nanoparticles ranges from 3 nm to 6 nm.The X-ray diffraction pattern shows that all the peaks shift towards the higher diffracting angles with an increase in Zn content.The lattice constant gradually decreases as the Zn content increases.The strong absorption edge shifts towards the lower wavelength region and hence the band gap of the films increases as the Zn content increases.The values of the absorption edge are found to shift towards the shorter wave length region and hence the direct band gap energy varies from 2.5 eV for the CdS film and 3.5 eV for the ZnS film.Our numerical results are in good agreement with the experimental results.     

Electronic and optical properties of pure and Ce3+-doped CaS single crystals: a first-principles prediction

This paper investigates the electronic and optical properties for pure and Ce 3+-doped CaS crystals by using the first-principles total energy calculations.The results show that CaS:Ce has a direct band gap of 2.16 eV,and the top of the valence band is determined by S 3p states and the bottom of the conduction band is determined by Ce 4f states,respectively.Our results validate that the yellow emission from CaS:Ce is produced by doped cerium and the green emission quenches at 12.5% cerium concentration.The Ce-S bond shows more covalent character than the Ca-S bond.     

Electronic Structures of PbMoOa Crystals with F-Type Colour Centres

Electronic structures of PbMoO4 crystals containing F-type colour centres with the lattice structure optimized are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory, using a numerically discrete variational （DV-Xa） method. The calculated results show that F and F＋ centres have donor energy levels in the forbidden band. The optical transition energies are 2.166eV and 2.197eV, respectively, corresponding to the 580nm absorption bands in PbMoO4 crystal. The 580nm absorption band in PbMoO4 is originated from the F-type colour centres.     

Influences of anionic and cationic dopants on the morphology and optical properties of PbS nanostructures

Selenium and zinc are used as anionic and cationic dopant elements to dope PbS nanostructures. The undoped and doped PbS nanostructures are grown using a thermal evaporation method. Scanning electron microscopy （SEM） results show similar morphologies for the undoped and doped PbS nanostructures. X-ray diffraction （XRD） patterns of three sets of the nanostructures indicate that these nanostructures each have a PbS structure with a cubic phase. Evidence of dopant incorporation is demonstrated by X-ray photoelectron spectroscopy （XPS）. Raman spectra of the synthesized samples con- firm the XRD results and indicate five Raman active modes, which relate to the PbS cubic phase for all the nanostructures. Room temperature photoluminescence （PL） and UV-Vis spectrometers are used to study optical properties of the undoped and doped PbS nanostructures. Optical characterization shows that emission and absorption peaks are in the infrared （IR） region of the electromagnetic spectrum for all PbS nanostructures. In addition, the optical studies of the doped PbS nanos- tructures reveal that the band gap of the Se-doped PbS is smaller, and the band gap of the Zn-doped PbS is bigger than the band gap of the undoped PbS nanostructures.     

Seasonal variability of cloud optical depth over northwestern China derived from CERES/MODIS satellite measurements

The seasonal variability of cloud optical depth over northwestern China derived from Clouds and the Earth's Radiant Energy System(CERES)Single Scanner Footprint(SSF)Aqua Moderate Resolution Imaging Spectroradiometer(MODIS)Edition 1B data from July 2002 to June 2004 is presented.The regions of interest are those with Asia monsoon influence,the Tianshan and Qilian Mountains,and the Taklimakan Desert.The results show that the instantaneous measurements presented here are much higher than the previous results derived from International Satellite Cloud Climatology Project(ISCCP) D2 monthly mean data.Generally the measurements of cloud optical depth are the highest in summer and the lowest in winter,however,Taklimakan Desert has the lowest measurements in autumn.The regional variation is quite significant over northwestern China.     

H0323+022天体的光变特性和黑洞质量研究

从大量文献资料中，收集了BL Lac天体H0323+022光学V波段的大量观测数据，获得了H0323+ 022从1982年至2003年的长期光变曲线.给出了从2000年至2003年，使用云南天文台1米望远 镜和上海天文台的156米望远镜，对H0323+022进行光学波段近4年的监测数据，观测发现 了H0323+022在光学R波段有7min的短时标光变.色指数B—V与V波段的星等值有较强的相关性 ，对H0323+022 CCD测光的结果与Bai（1998）观测的结果ΔR=063 ΔI=081是 一致的.由观测所发现的短时标光变，推算出了BL Lac天体H0323+022的中心黑洞质量和辐射 区域. 关键词： BL Lac天体 光变曲线 短时标光变 辐射区域 中心黑洞质量     

Observation of active galactic nuclei on the GT-48 CrAO γ-ray telescope in 2004

The results of observations of active galactic nuclei on the γ-ray telescope GT-48 (Crimean Astrophysical Observatory) in 2004 are reported. Fluxes of very-high-energy γ rays (E ≥ 10¹² eV) from the 3C 66A, Mkn 421, Mkn 501, and 1H 1426 objects have been recorded at a level exceeding 4σ. The upper values of γ-ray fluxes for the active nuclei of the 1ES 1959 and BL Lac galactics are presented. Analysis of the data on the 3C66A object confirmed the existence of γ-ray fluxes from this source. This object was previously discovered in Crimea.     

Coordinated observation of the massive system 4U1700+24=HD154791

Summary We report results from multifrequency observations of the X-ray source 4U1700+24/HD154791. This source was observed simultaneously with the EXOSAT and IUE satellites on March 25, 1985, and it is continuously monitored in the optical band at the Bologna Astronomical Observatory since July 1984. Our observations show source features which are different from those measured during previous observations. The X-ray spectrum is significantly softer than in previous measurements, and much more absorbed; high excitation lines in UV spectra (C IV and N V) have disappeared; the optical spectrum, despite a fairly good photometric stability of the M giant HD154791 in V and B bands also shows differences with respect to previous observations. We discuss our results in the framework of both a binary model and a coronal model. We point out that our observations strengthen the hypothesis that the X-ray emission is due to mass accretion onto a compact object orbiting around the M giant star HD154791. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Very high energy γ rays from close active galactic nuclei Mkn 421, Mkn 501, and NGC 1275

The results of observations of two types of metagalactic sources, BLLacs Mkn 421 (z = 0.031) and Mkn 501 (z = 0.034) and the Seyfert galaxy NGC 1275 (z = 0.017) are reported. These observations have been carried out with the SHALON mirror telescope (Lebedev Institute of Physics, Russian Academy of Sciences). The γ-ray fluxes (with energies E > 0.8 TeV) from Mkn 421 and Mkn 501, averaged over the entire observation period, were estimated to be (0.63 ± 0.14) × 10^?12 and (0.86 ± 0.13) × 10^?12 cm^?2 s^?1, respectively. The flux from the source NGC 1275 was (0.78 ± 0.13) × 10^?12 cm^?2 s^?1. The energy spectra of γ rays and images of the sources are reported.     

Mkn 501 X射线和TeV射线准周期震荡的可能机制

我们提出了对流占主导的吸积流（ADAF）盘的辐射线是Mkn 501中从X射线到Tevγ射线的源光子；而且此盘的不稳定性可以解释Mkn 501中在X射线和Tevγ射线光变曲线23天的准周期光变。在这个模型中,光学到X射线辐射进入喷流，后由于喷流内相对论电子的作用而转化成高能光子。在这个过程中，发生在ADAF盘中的不稳定性导致源光子的准周期变化，从而导致了X射线和γ射线的变化。     

Hole-interface optical phonon relaxation rates with valence band-mixing effects

We theoretically investigate the hole-interface optical phonon scattering rates for a InGaAs-AlGaAs quantum well structure, taking into account the valence-band mixing. The dispersion relation and the electrostatic potentials for interface optical phonon modes are obtained based on the macroscopic dielectric continuum model. For the hole dispersion relation, the Luttinger-Kohn Hamiltonian is used. The hole-interface optical phonon interaction is evaluated by the Fermi's golden rule taking into account the Bloch overlap factor.Our results show that the hole-interface phonon scattering rates within the parabolic band approximation are different from those including valence band mixing effects. Especially, in the low energy region, the hole-interface phonon scattering rates within the parabolic band approximation are overestimated very significantly.     

Tachyonic cascade spectra of supernova remnants and TeV blazars

The superluminal spectral densities of relativistic electrons in uniform motion are derived, semiclassically and in second quantization. The effect of electron spin on the tachyonic radiation field, a Proca field with negative mass-square, is studied. There is a longitudinally polarized spectral component due to the negative mass-square of the tachyonic quanta. The radiation densities are averaged with electron distributions, and high- and low-temperature expansions are obtained. Spectral fits to the γ-ray spectra of the Crab Nebula, the supernova remnant RX J1713.7–3946, and the BL Lacertae objects H1426+428, 1ES 1959+650, Mkn 501, and Mkn 421 are performed. In contrast to TeV photons, the extragalactic tachyon flux is not attenuated by interaction with the background light; there is no absorption of tachyonic γ-rays, as tachyons do not interact with infrared photons. The curvature of the TeV spectra in double-logarithmic plots is caused by the Boltzmann factor of the electron densities generating the tachyon flux. The extended spectral plateau in the GeV band, visible in the spectral maps of the two Galactic supernova remnants as well as in the flare spectra of the BL Lacertae objects, is reproduced by the tachyonic radiation densities. Estimates of the electron populations in the supernova remnants and active galactic nuclei are inferred from the spectral fits, such as power-law indices, electron temperatures, and source counts. Upper bounds on the Lorentz factors in the source populations are derived and compared to the breaks in the high-energy cosmic-ray spectrum. PACS  95.30.Gv; 11.10.Lm; 98.70.Sa; 03.50.Kk     

基于聚合经验模态分解方法的OJ 287 射电流量变化周期分析

BL Lac 天体OJ 287 是一个重要的Blazar天体, 光变具有准周期特点. 本文从密歇根大学射电天文台数据库收集了OJ 287射电波段4.8, 8.0和 14.5 GHz超过30年时间的观测数据. 由于天文观测资料的复杂性, 一些传统寻找周期的方法不是太理想. 近几年迅速发展起来的聚合经验模态分解(EEMD) 特别适合于具有非线性和非平稳动态变化特性的Blazar天体光变规律的研究. 运用EEMD方法对OJ 287 的3个射电波段流量分别进行多时间尺度分解, 各获得6个代表不同时间尺度局部特征信息的本征模态函数分量和一个趋势项. 研究结果表明: 4.8 GHz射电流量变化的主要周期是18.9, 11.9, 5.7 和2.4年; 8.0 GHz 的主要周期是12.2, 5.2和2.4 年; 14.5 GHz的主要周期是21.8, 12.0, 4.3和 2.4 年. 尽管3个波段流量变化主要周期大小有一定的差别, 但是存在着近似相同的变化周期12.0 和2.4年, 说明这3个波段可能来源于相同的辐射机制, 也证实了OJ 287 射电波段和光学波段具有相同的长光变周期12.0 年. 这些结论表明: EEMD 作为一种全新的非线性、非平稳信号处理方法, 可以更精确地提取Blazar天体中不同光变周期的分量. 关键词： OJ 287 光变周期 聚合经验模态分解     

A defect model for the optical and EPR activity of the T-center in yttrium-stabilized zirconia

Abstract

Direct quantitative correlations have been recently observed between the T-centre EPR signal and an optical absorption band at 370 nm, in X-irradiated YSZ, suggesting that the colour centre and the paramagnetic centre might be the same entity. In order to confirm this hypothesis, theoretical modeling was undertaken, where by appropriate refinement the known electronic structure of the T-centre is shown to be compatible with an optical transition, having an oscillator strength of the correct magnitude.