光子晶体缺陷结构的特性

基于平面波展开法,设计了二维正方晶格和三角晶格光子晶体缺陷结构,数值模拟了TE模、TM模式的光子晶体禁带特性,计算了二维三角晶格光子晶体缺陷结构的模式分布。结果表明,光子晶体缺陷结构能够形成很好的禁带特性和模式结构,研究结论为光子晶体波导器件的开发提供参考。     

Molecular-dynamics calculation of the thermal conductivity coefficient of the germanium single crystal

The thermal conductivity coefficient of the germanium crystal lattice has been calculated by molecular dynamics simulation. Calculations have been performed for both the perfect crystal lattice and the crystal lattice with defects such as monovacancies. For the perfect germanium single crystal, the dependence of the thermal conductivity coefficient on the lattice temperature has been obtained in the temperature range of 150–1000 K. The thermal conductivity coefficient of the germanium lattice has been calculated as a function of the monovacancy concentration.     

A kinetic model for the premelting of a crystalline structure

An analytical kinetic approach to examine the premelting phenomenon is suggested by using a first passage time analysis. Premelting is considered to occur when the time of formation of a Frenkel type defect in the surface monolayer becomes sufficiently small. The mean time of defect formation on the surface lattice, i.e., the mean time necessary for a selected (surface-located) molecule to leave its lattice site and form a Frenkel defect, is calculated by using a first passage time analysis. The model is illustrated by numerical calculations for a crystalline structure composed of molecules interacting via the Lennard-Jones (LJ) potential. The lattice vectors in the plane parallel to the free surface of the crystal were assumed to be equal (to the lattice parameter) and the angle between them was varied. The model predictions of the Tammann temperature (of premelting) are very sensitive to the parameters of the LJ potential. In all the cases considered, the temperature dependence of the mean first passage time has two clearly distinct regimes: at low temperatures the dependence is sharp and at high temperatures it is weak.     

Two-dimensional photonic crystals with a lattice defect: Spectrum of defect modes, localization of light, and formation of evanescent waves

Properties of an electromagnetic field localized in the defect modes of two-dimensional photonic crystals are studied. The defect-mode spectrum of these structures is calculated, electromagnetic field localization and channeling effects are analyzed, and the properties of the field inside and beyond a photonic crystal with a lattice defect are also studied. The calculations show that the electromagnetic field is localized in the defect mode of a photonic crystal in a region smaller than the wavelength. The dependence of the defect-mode spectrum on the parameters of the photonic crystal is investigated and possibilities for controlling the spectrum of defect modes are indicated. It is shown that the optical field leaving a photonic crystal possesses the properties of a evanescent wave, which means that spatial resolution substantially greater than the wavelength of the radiation can be achieved in the near field and opens up possibilities for using photonic crystals with a lattice defect in near-field optical microscopy. The possibility of externally controlling an optical field localized in the defect modes of a photonic crystals is demonstrated.     

用正电子湮没技术研究形变铁的恢复

本文介绍应用正电子湮没寿命和多普勒加宽方法研究60％形变铁等时退火各阶段中缺陷的恢复行为,实验结果说明正电子湮没平均寿命τ及多普勒加宽的S参数都是退火温度的函数,完整晶体中湮没寿命τ_f=111±lPS,缺陷中湮没寿命τ_d=162±lps,本文还根据捕获模型计算了正电子的捕获速率及τ₁,结果证明与理论预期的完全一致,经计算得出捕获正电于的相对缺陷浓度的范围为10^-7-10^-4数量级。 关键词：     

氘化对KH2PO4晶体微观缺陷影响的正电子湮没研究

采用传统降温法从不同程度氘化(x=0, 0.51, 0.85)的生长溶液中生长氘化KH₂PO₄(KDP) 晶体, 利用正电子湮没技术(正电子寿命谱和多普勒展宽谱)、结合X射线衍射谱(XRD) 结构分析, 对KDP晶体氘化生长的微观缺陷进行了研究, 讨论了氘化程度对晶体内部微观结构特性、缺陷类型和浓度的影响. XRD结果显示晶胞参数a, b值随氘含量的增加而增加, c值无明显变化; 正电子寿命谱结果发现随着氘化浓度的提高, KDP晶体内部中性填隙缺陷以及氧缺陷不断增加, 引起晶体晶格畸变; 氢空位、K空位、杂质替位缺陷不断发生缔合反应形成复合缺陷, 缺陷浓度不断减少; 团簇、微空洞等大尺寸缺陷也在不断发生聚合反应, 缺陷浓度表现为不断减少. 多普勒实验结果表明随着氘化程度的提升, 晶体内部各类缺陷表现为同步变化. 实验结果表明, KDP晶体在低浓度氘化生长(50%以内)下缺陷反应较弱, 而在高浓度氘化(50%以上)下的缺陷反应显著增强.     

Neutron scattering from a substitutional mass defect

The dynamic structure factor is calculated for a low concentration of light mass scatterers substituted in a cubic crystal matrix. A new numerical method for the exact calculation is demonstrated. We derive a local density of states for the low momentum transfer limit, and derive the shifts and widths of the oscillator peaks in the high momentum transfer limit. We discuss the limitations of an approximation which decouples the defect from the lattice.     

Calculating the dipolon binding energy in alkaline earth metal oxides and the ground state energy of F c + color centers (dipolon+electron) in MgO

The binding energy of the simplest neutral crystal lattice defect, a dipolon consisting of neighboring unlike vacancies, is calculated for the alkaline earth metal oxides having an NaCl structure (MgO, CaO, SrO, BaO) using a method developed by the author. The calculated values are found to correlate with a number of physical properties of these crystals. The ground state energy of an MgO crystal with an F _c ⁺ color center (dipolon1electron) is calculated using two independent quantum-mechanical methods. The results of these calculations are essentially identical. Fiz. Tverd. Tela (St. Petersburg) 41, 1976–1978 (November 1999)     

Analytical optimization of the lattice parameter using the binding energy calculated in the quasi-classical approximation

The dependence of the static binding energy of a crystal on the crystal structure as calculated in the initial quasi-classical approximation is shown to allow the equilibrium lattice parameter to be found analytically. The application of this method to boron nitride modifications leads to lattice parameters that coincide with experimental values to within several percent. This method gives lattice parameters of 2.66, 3.49, and 2.44 Å for the BN hexagonal layer and the cubic c-BN and “ideal” wurtzite-like w-BN crystals, respectively. The corresponding binding energies are estimated to be 23.2, 14.1, and 13.6 eV/mol, respectively.     

Crystal equilibrium and the electronic contribution to the elastic constants of potassium and rubidium

Electronic contribution to the elastic constants of K and Rb are calculated by applying the crystal equilibrium condition to an eight parameter lattice dynamical model. For comparison, these quantities have also been calculated according to Fuch's criterion. The two results are found to be in fairly good agreement. Also, the calculated phonon frequencies agree well with the experimental data.     

纳米镍粉体的晶格膨胀

采用阳极弧放电等离子体技术成功制备了各种不同晶粒尺寸的纳米镍粉体材料，并利用X射线衍射(XRD)、透射电子显微镜(TEM)和相应选区电子衍射(SAED)等测试手段对所制备的样品的晶体结构、晶格参数、形貌、粒度进行性能表征. 实验结果表明：阳极弧等离子体法制备的镍纳米粉的晶体结构与相应的块物质相同，为fcc结构的晶态. 不同晶粒尺寸的纳米镍粉体的晶格常数均大于完整单晶镍的晶格常数，晶格畸变表现为晶格膨胀. 晶格常数和晶胞体积随着晶粒尺寸的减小而增大，晶格畸变量与晶粒尺寸的倒数成正比. 镍纳米粉体的晶格膨胀主要是由于受到表面能和表面张力的作用而引起的，可以利用纳米晶体的热力学理论作定性解释. 关键词： 镍纳米粉体 晶格常数 晶格膨胀 晶胞体积     

Local strain effects for isolated pairs of ortho-H2 molecules in para hydrogen

The crystal fields resulting from the local strains associated with a defect consisting of an isolated pair of ortho-H₂ molecules in solid para hydrogen are calculated. The Kanzaki method for the determination of the static properties of lattice defects is used. It is found that the crystal field parameters depend on the pair state |F,±M > and differ considerably for in-plane and out-of-plane pairs. Theories of Luryi, et al. and Harris, et al. for the energy levels of pairs of ortho-H₂ molecules in nearly pure para hydrogen are compared with our results.     

Defect structure of β-LiAl

Lattice parameter and density measurements have been made on a series of Li-Al alloys extending over the LiAl β-phase. The lattice parameter of the slowly cooled β-phase alloys varied almost linearly with composition within the phase field. Theoretical density calculations for several single-defect models from lattice parameter data did not yield a good fit with the experimental density data. Rather, a model based on the coexistence of two types of defects, namely, vacancies in the lithium sublattice and lithium antistructure atoms in the aluminum sublattice, characterized the defect structure. Calculated concentrations of these defects varied with departure from stoichiometry and showed good agreement with the concentration of lithium vacancies estimated from NMR measurements. Compositional dependence of the vacancy concentration and the lithium diffusion coefficient produced a good correlation and strongly suggested that the lithium atoms diffuse via a vacancy mechanism. It is hypothesized that the defect structure of this compound may be related to the electronic structure of this so-called NaTl phase.     

3-d Model for strain ordering in steel: I Static effects

We present a spring-defect model in 3-dimensions to describe the connection between elastic distortion and interstitial carbon ordering associated with phase transition from a body centred cubic (BCC) to body centered tetragonal (BCT) structure in BCC metals such as α-iron. The presence or the absence of the carbon is modelled in terms of a pseudo spinŝ=+1or -l.An Ising interaction between carbon atoms is recovered after eliminating the lattice degrees of freedom, which is longranged. The coupling between the spin and lattice degrees of freedom allows for a systematic study of ferroelasticity and the variation of the lattice parameter with carbon concentration. The mean field results for the paraelastic to ferroelastic transition, lattice parameter and static compliance are presented. The significant feature of this calculation is not only a derivation of the defect-defect interaction, but also an explicit calculation of the strain dipole tensor associated with each defect, from a microscopic model.     

The amount and location of amorphous component in polyethylene single crystals

Polyethylene single crystals from dilute solutions usually have a density ρ which is less than that of the ideal crystal lattice (ρ_c = 1.000 g/cm³). This density defect can be formally assigned to an “amorphous” component (1-α) = (ρ_c-ρ)/(ρ_c-ρa) and may be caused by vacancies in the crystal lattice and/or by the surface disorder connected with chain folds. The two-phase concept of single crystal structure assumes a very nearly ideal lattice core and two amorphous layers on the fold-containing surfaces. It is supported by density and heat content data of annealed samples, by small-angle X-ray scattering, by wide-line NMR investigations, and by the results of fuming nitric acid treatment.

The maximum thickness of the amorphous surface layer as a function of temperature may be estimated from the free energy requirement of such a layer. With regular chain folds one has a high concentration of gauche conformations yielding a high surface energy. The need for such a concentration disappears in the amorphous layer with loose loops. This gain in energy may be spent for surface melting. Still more important is the gain in entropy caused by the random distribution of loop lengths.     

Thermal properties of high-k Hf_1-xSi_xO₂

Classical atomistic simulations based on the lattice dynamics theory and the Born core-shell model are performed to systematically study the crystal structure and thermal properties of high-k Hf1-xSixO2 . The coefficients of thermal expansion, specific heat, Grneisen parameters, phonon densities of states and Debye temperatures are calculated at different temperatures and for different Si-doping concentrations. With the increase of the Si-doping concentration, the lattice constant decreases. At the same time, both the coefficient of thermal expansion and the specific heat at a constant volume of Hf1-xSixO2 also decreases. The Grneisen parameter is about 0.95 at temperatures less than 100 K. Compared with Si-doped HfO2 , pure HfO2 has a higher Debye temperature when the temperature is less than 25 K, while it has lower Debye temperature when the temperature is higher than 50 K. Some simulation results fit well with the experimental data. We expect that our results will be helpful for understanding the local lattice structure and thermal properties of Hf1-xSixO2 .     

First-Principles Study on the Electronic Structures for Y^3＋：PbWO4 Crystals

The possible defect models of Y^3＋：PbWO4 crystals are discussed by defect chemistry and the most possible substituting positions of the impurity Y^3＋ ions are studied by using the general utility lattice program （GULP）. The calculated results indicate that in the lightly doped Y^3＋ ：PWO crystal, the main compensating mechanism is [2Ypb^＋ ＋ VPb^2-], and in the heavily doped Y^3＋ ：PWO crystal, it will bring interstitial oxygen ions to compensate the positive electricity caused by YPb^＋, forming defect clusters of [2Ypb^＋ ＋Oi^2-] in the crystal. The electronic structures of Y3＋ ：PWO with different defect models are calculated using the DV-Xα method. It can be concluded from the electronic structures that, for lightly doped cases, the energy gap of the crystal would be broadened and the 420nm absorption band will be restricted; for heavily doped cases, because of the existence of interstitial oxygen ions, it can bring a new absorption band and reduce the radiation hardness of the crystal.     

Effect of nonlocal elasticity on internal friction peaks observed during martensite transformation

The internal friction associated with martensite is calculated using elastic interaction energy between dislocations and solute atoms in nonlocal elasticity during low temperature aging process. The relaxation strength depends on the lattice parameter of the crystal as well as the temperature and the heating rate. The peak heights increase with increasing lattice parameter. The proposed model can demonstrate more realistically the shape of the change of internal friction versus temperature when nonlocal elasticity is included.     

Magnon mechanism of defect reactions in solids

A phenomenological theory is developed that describes the effect of a magnetic field on the defect reactions in a solid. The theory is based on a concept regarding the lattice magnetism according to which a defect induces a magnetoactive (magnon) branch in the spectrum of elementary excitations of a crystal lacking a magnetic structure in the absence of defects. The probability of a defect complex disintegrating in a magnetic field is calculated in terms of the magnon mechanism of the reaction.     

A study of ionic solids by means of new density-functional theory techniques

Summary The lattice parameter, the bulk modulus and the dissociation energy of NaCl and MgO are calculated by a method which allows the direct determination of the total energy and of the charge density of a crystal. An extensive comparison with the results of standard band structure calculations is reported. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.