铜铁镁三掺铌酸锂晶体的第一性原理研究

利用基于密度泛函理论的第一性原理,研究了Cu:Fe:Mg:LiNbO3晶体及对比组的电子结构和光学特性.研究显示,单掺铜或铁铌酸锂晶体的杂质能级分别由Cu 3d轨道或Fe 3d轨道贡献,禁带宽度分别为3.45和3.42 eV;铜、铁共掺铌酸锂晶体杂质能级由Cu和Fe的3d轨道共同贡献,禁带宽度为3.24 eV,吸收峰分别在3.01,2.53和1.36 eV处;Cu:Fe:Mg:LiNbO3晶体中Mg^2+浓度低于阈值或高于阈值(阈值约为6.0 mol%)的禁带宽度分别为2.89 eV或3.30 eV,吸收峰分别位于2.45 eV,1.89 eV或2.89 eV,2.59 eV,2.24 eV.Mg^2+浓度高于阈值,会使吸收边较低于阈值情况红移;并使得部分Fe^3+占Nb位,引起晶体场改变,从而改变吸收峰位置和强度.双光存储应用中可选取2.9 eV作为擦除光,2.5 eV作为读取和写入光,选取Mg^2+浓度达到阈值的三掺晶体在增加动态范围和灵敏度等参量以及优化再现图像的质量等方面更具优势.     

Er:YCOB和Er:Yb:YCOB晶体的光谱分析

分别测量了Er YCOB晶体和ErYb YCOB晶体的室温吸收光谱.讨论了Yb3+离子对Er3+离子的敏化作用.     

The radiothermoluminescence of (ZnS:CdS:Ag:Ni:Co) phosphors irradiated with beta-rays

The radiothermoluminescence characteristics have been studied for a number of (ZnS:CdS::Ag:Ni:Co) phosphors, with various cobalt impurity contents, after exposure to⁹⁰Sr beta-rays at room temperature. Thermoluminescent spectra show a peak wavelength of 7000 Å, which remains unchanged with increase in cobalt concentration. Thermoluminescence curves exhibit a single well-defined peak appearing at temperatures far above room temperature. This peak shifts to lower temperatures with increasing cobalt content. The relation between thermoluminescence response and dose is linear over the dose range of 1 to 500 rads. Thermoluminescence measurements made after storing the irradiated phosphor for 48 hours in dark have revealed an information loss of 4% over this storage period. The radiothermoluminescence characteristics, storage capacity and storage stability of these sulphide phosphors compete reasonably with those of lithium fluoride over the dose range of 1 rad to 500 rads.     

Electron-paramagnetic-resonance of ZnTe:Ga and ZnTe:Sn

Photosensitive EPR measurements ofS-state centers of gallium and tin in ZnTe single crystals are reported. In addition to large hyperfine interaction with the impurity nuclei the spectra show partially resolved superhyperfine structures arising from three shells of tellurium ligands. Both HF and SHF splittings decrease with increasing temperature. The samples exhibitp-type photoconductivity with the same spectral sensitivity observed with the generation and depletion of the EPR-spectrum.     

Laser phenomena in DCEL of ZnS:Cu:Nd:Cl thin films

The possibility of the occurrence of laser phenomena in the DCEL of thin films of ZnS:Cu:Nd:Cl, designed for display purposes, has been investigated. Evidence of stimulated emission at ～ 1080 nm has been found in both DCEL and cathodoluminescence emission of the films and has been attributed to a tendency to population inversion involving the ${}^4\text{F}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ energy level and a sub-level of the ${}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ energy level. In addition, the expected directional nature of the stimulated emission has been shown to be present at the higher applied voltages.     

a-C:N:H纳米尖端荧光产生的机理

用CH4,H2和NH3为反应气体,利用等离子体增强热丝化学气相沉积在沉积有碳膜的Si衬底上制备了a-C:N:H纳米尖端,并用扫描电子显微镜和微区Raman光谱仪对碳膜和纳米尖端进行了表征。结果表明:Raman谱中含有与碳和氮相关的峰,且纳米尖端的Raman谱比碳膜的Raman谱有很强的荧光背景。Raman谱中的峰说明沉积的碳膜和纳米尖端是a-C:N:H薄膜和a-C:N:H尖端。a-C:N:H纳米尖端的Raman谱中强荧光背景的产生表明其在激发光源照射的过程中发射了强荧光,对a-C:N:H纳米尖端产生强荧光的机理进行了探讨。     

Essay: Cosmic Censorship: The Role of Quantum Gravity

The cosmic censorship hypothesis introduced by Penrose thirty years ago is still one of the most important open questions in classical general relativity. In this essay we put forward the idea that cosmic censorhip is intrinsically a quantum gravity phenomena. To that end, we construct a gedanken experiment in which cosmic censorship is violated within the purely classical framework of general relativity. We prove, however, that quantum effects restore the validity of the conjecture. This suggests that classical general relativity is inconsistent and that cosmic censorship might be enforced only by a quantum theory of gravity.     

Mechanism of photostimulated luminescence: BaFCl:Eu2+

We propose a new parallel model based on the analysis of photostimulated luminescence (PSL) process and existing theoretical models. While solving this model, the general expression for PSL is gained. BaFCl:Eu²⁺, one of the PSL materials, is prepared and a series of spectra are measured. We compared the theoretical and experimental results in terms of the decay process, the difference between two kinds of color centers and the relationship between irradiation dose and luminescent intensity. Supported by the National Nature Science Foundation of China.     

双掺Nd:Ce:YAG晶体激光输出特性

 为提高固体激光器的能量利用率,增大输出能量,将双掺Nd:Ce:YAG晶体的输出特性与普通Nd:YAG晶体进行了对比研究。分析了Nd³⁺和Ce³⁺的吸收光谱对激光晶体初始阈值反转粒子数的影响,结果显示:双掺Nd:Ce:YAG晶体可以提高晶体对泵浦光能量的利用率及激光器的输出能量,且可降低阈值泵浦能量。并分别检测了Nd:Ce:YAG激光晶体与Nd:YAG晶体的输出激光能量和阈值泵浦能量,实验结果表明：在输入电压为750 V时,Nd:Ce:YAG晶体与Nd:YAG晶体的输出能量分别为651.5 mJ 和390.4 mJ,能量利用率分别为2.31%和1.38%,激光振荡需要的泵浦能量阈值分别为10.56 mJ和15.21 mJ,且普通Nd:YAG晶体的斜效率为0.36%,而双掺Nd:Ce:YAG晶体的为0.49% 。     

Laser characteristics of Cr:Nd:GdCOB self-frequency-doubling crystal

The phase matching angle of GdCOB crystal is calculated. By using Xenon flash lamp as pump source, we have realized the free run from 1.06 μm to 0.53 μm in self-frequency-doubling Cr:Nd:GdCOB (bi-doped) crystal and Nd:GdCOB (uni-doped) crystal. The experimental results show that the threshold energies for bi-doped crystal and uni-doped crystal are 0.92 J and 1.00 J, respectively; when the pump energy is 10 J, the output energies of green laser for two kinds of crystals are 2.46 mJ and 1.96 mJ, respectively. The output energy of Cr:Nd:GdCOB crystal has an increase of 25% in comparison to that of Nd:Gd:COB crystal. In addition, we discuss ways to improve efficiency.     

Thermally stimulated luminescence of CaS : Bi : Pd phosphors

CaS phosphors co-activated with Bi⁺³ and Pd⁺² impurities in varying concentrations have been prepared and their thermoluminescence has been systematically studied. The general features of the curves are discussed and the activation energies have been estimated in three different ways. The escape frequency factor is determined using the theoretical model of Randall and Wilkins. Conclusions are drawn regarding the type of kinetics involved in the thermoluminescence process.     

Role of disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN

We present calculations of magnetic exchange interactions and critical temperature T(c) in Ga1-xMnxAs, Ga1-xCrxAs, and Ga1-xCrxN. The local spin-density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg Hamiltonion, but no significant further approximations are made. We show the following: (i) configurational disorder results in large dispersions in the pairwise exchange interactions; (ii) the disorder strongly reduces T(c); (iii) clustering in the magnetic atoms, whose tendency is predicted from total-energy considerations, further reduces T(c), while ordering the dopants on a lattice increases it. With all the factors taken into account, T(c) is reasonably predicted by the local spin-density approximation in Mn:GaAs without the need to invoke compensation by donor impurities.     

The 1: 2: 3: 4 theorem and the ground state of free polarons

As a consequence of the electrostatic nature of the electron longitudinal optical phonon interaction, the electron-phonon interaction matrix is independent of the mass of the electron. On the other hand the dimensionless coupling constant α does depend on this mass. This fact enables us to establish a generalization of the 1: 2: 3: 4 theorem of Pekar for the whole range of the coupling constant and to derive a ground state theorem for free polarons which relates the long wavelenght behaviour of the current-current correlation function with the ground state energy.     

Hamiltonian systems with detuned 1:1:2 resonance: Manifestation of bidromy

We consider a generalization of the 1:1:2 resonant swing-spring [see H. Dullin, A. Giacobbe, R.H. Cushman, Physica D 190 (2004) 15] which is suggested both by the symmetries of this system and by its physical and in particular molecular realizations [see R.H. Cushman, H.R. Dullin, A. Giacobbe, D.D. Holm, M. Joyeux, P. Lynch, D.A. Sadovskií, B.I. Zhilinskií, Phys. Rev. Lett. 93 (2004) 024302-1-024302-4]. Our generic integrable system is detuned off the exact Fermi resonance 1:2. The three-dimensional (3D) image of its energy-momentum map EM consists either of two or three qualitatively different non-intersecting 3D regions: a regular region at low vibrational excitation, a region with monodromy similar to that studied for the exact resonance, and in some cases—an intermediate region in which the 3D set of regular values of EM is partially self-overlapping while remaining connected. In the presence of this latter region, the system has an interesting property which we called bidromy. We analyze monodromy and bidromy for a concrete integrable classical Hamiltonian system of three coupled oscillators and for its quantum analog. We also show that the bifurcation involved in the transition from the regular region to the region with monodromy can be regarded as a special resonant equivariant analog of the Hamiltonian Hopf bifurcation.     

Comparison of spectroscopic properties of Yb:YAP and Yb:YAG crystals

The Yb:YAG and Yb:YAP crystals have been grown by Czochralski method. The absorption spectra and the fluorescence spectra of Yb:YAG and Yb:YAP crystals have been investigated. It is shown that the Yb:YAG crystal has better laser properties and smaller threshold power than Yb:YAP crystal. In addition, the absorption cross-section of the Yb:YAP crystal is 2.16 times of that of the Yb:YAG crystal,so laser diode pumped Yb:YAG lasing can be easily realized. Because YAP single crystal is anisotropic, it is provided with polarization characteristics.     

Review: Gas of Wormholes: A Possible Ground State of Quantum Gravity

In order to gain insight into the possible Ground State of Quantized Einstein's Gravity, we have derived a variational calculation of the energy of the quantum gravitational field in an open space, as measured by an asymptotic observer living in an asymptotically flat space-time. We find that for Quantum Gravity (QG) it is energetically favourable to perform its quantum fluctuations not upon flat space-time but around a "gas" of wormholes of mass m _p, the Planck mass (m _p 10¹⁹ GeV) and average distance l _p, the Planck length a _p(a _p 10^–33 cm). As a result, assuming such configuration to be a good approximation to the true Ground State of Quantum Gravity, space-time, the arena of physical reality, turns out to be well described by Wheeler's quantum foam and adequately modeled by a space-time lattice with lattice constant l _p, the Planck lattice.     

Large-scale simulations of a-Si:H: the origin of midgap states revisited

Large-scale classical and quantum simulations are used to generate a-Si:H structures. The bond-resolved density of the occupied electron states discloses the nature of microscopic defects responsible for levels in the gap. Highly strained bonds give rise to band tails and midgap states. The latter originate mainly from stretched bonds, in addition to dangling bonds, and can act as hole traps. This study provides strong evidence for photoinduced degradation (Staebler-Wronski effect) driven by strain, thus supporting recent work on a-Si, and sheds light on the role of hydrogen.     

Microwave-Assisted Combustion Synthesis of ZnO:Eu Nanoparticles: Effect of Fuel Types

Nanoparticles of Europium oxide doped with Zinc oxide were synthesized via microwave-assisted combustion method. Citric acid as a simultaneous fuel and chelating agent and glycine as a fuel and mixture of these fuels were sleeted. X-Ray diffraction patterns (XRD) indicated the formation of ZnO structure with a few amount of Eu₂O₃ phase. Fourier transformation infra red (FTIR) spectra reveal the increase of ZnO₄ bonds with glycine content of fuels mixture. Scanning electron microscope (SEM) images showed the conversion of nanosphere to spongy-like structure with respect to change of fuel mixtures from citric to glycine. From transmission electron microscopy (TEM) nanoparticles of a mean size 30 nm are observed Green fluorescence emission of different samples was due to activation of self activated center of ZnO structure through transition of electron from Eu³⁺ to V_zn sites.