低温相偏硼酸钡(β-BaB_2O_4)晶体倍频系数的计算

本文从“离子基团理论”出发,并考虑A位离子晶格电场对(B_3O_6)~(3-)环的影响,采用CNDO/S近似计算了(B_3O_6)~(3-)基团的分子轨道,进而求得低温相偏硼酸钡(β-BaB_2O_4)晶体的倍频系数。计算结果与实验值符合,由此进一步证实了这一论点:β-BaB_2O_4晶体的倍频系数主要来自(B_3O_6)~(3-)基团中共轭π轨道的贡献。计算结果还表明,CNDO/S型近似是一种计算晶体中共价键基团分子轨道的有效方法。     

二维正方晶格多点缺陷声子晶体实验研究

基于超声浸水透射技术, 实验研究了有限尺寸二维正方晶格钢/水声子晶体多点缺陷模态性质. 利用COMSOL Multiphysics软件建立该声子晶体有限元计算方法, 求解了9×9超胞多点缺陷声子晶体能带结构, 把缺陷局域模态频率与数值仿真和实验结果进行对比, 结果表明: 实验数据和理论值能够很好符合. 进一步分析发现, 点缺陷数量影响声波局域效应、本征模态和传播特性, 为设计有限尺寸声波器件提供理论依据. 关键词： 声子晶体 多点缺陷 实验研究 有限元     

下地幔压力条件下(Mg0.97, Fe0.03)O方镁铁矿的光学性质

本文采用第一性原理方法, 计算了(Mg_0.97, Fe_0.03) O方镁铁矿的理想晶体和含镁-氧离子双空位点缺陷晶体在下地幔压力条件下的光学性质. 吸收光谱数据表明, 方镁铁矿理想晶体的结果与晶体场等理论预测的结果相似:压力诱导铁自旋态的转变将导致方镁铁矿吸收谱有巨大蓝移, 并在近红外光区出现了透明现象. 然而, 在方镁铁矿中含有点缺陷时, 其结果与晶体场等理论预测结果存在本质差异:铁自旋态的转变将导致在近红外光区的吸收性显著增强. 含点缺陷晶体折射率的计算结果表明, 压力、波数及铁自旋态的转变对(Mg_0.97, Fe_0.03) O方镁铁矿的折射率有较明显地影响. 本文结果不仅对探索下地幔方镁铁矿在高压下的光学性质有重要的参考价值, 而且还指明高压吸收光谱的测量可能是准确获得铁自旋态信息一个好的手段.     

低温相偏硼酸钡(β-BaB2O4)晶体倍频系数的计算

本文从“离子基团理论”出发,并考虑A位离子晶格电场对(B₃O₆)^3-环的影响,采用CNDO/S近似计算了(B₃O₆)^3-基团的分子轨道,进而求得低温相偏硼酸钡(β-BaB₂O₄)晶体的倍频系数。计算结果与实验值符合,由此进一步证实了这一论点:β-BaB₂O₄晶体的倍频系数主要来自(B关键词：     

漏斗型完全光子带隙光波导单向传输

光波单向传输器件在全光计算和信息处理方面具有重要应用.本文提出一种具有完全光子带隙的硅基空气孔光子晶体漏斗型光波导结构,在光通信波段可实现单向传输特性.漏斗型光波导可打破光波对称传输,引入点缺陷通过模式转换与失配进一步抑制反向透射,并研究了不同的点缺陷类型与位置对反向透射的影响.采用时域有限差分法进行数值计算,优化选取了最佳的点缺陷模式.结果显示,单柱型点缺陷在向左移动5a(a为光子晶体晶格常数,a=470 nm)时,横电(TE)偏振光在工作波长1550 nm处正向透射率为0.716,透射对比度为0.929,工作带宽为111 nm.另外,本文提出的光波单向传输器件结构简单、工艺要求低,有望为集成光路中单向传输器件设计提供新的解决方案.     

二维异质结光子晶体中含近邻点缺陷的弯曲波导的可调谐滤波特性

应用时域有限差分(FDTD)法,对光在二维异质结光子晶体中含两个近邻点缺陷直角弯曲波导的传输特性进行了数值模拟研究。计算结果表明,具有特定本征共振频率的光子晶体微腔(点缺陷),可将在其邻近波导中传播的相同频率成分的光波"引入"到微腔中。"引入"的频率依赖于异质结光子晶体弯曲波导和微腔的参量,从而能够实现调变"引入"频率。通过改变点缺陷周围介质材料(液晶)的折射率或相关介质柱半径,可分别实现约31 nm或12 nm的波长调谐范围。计算结果为光通信中的调谐、上/下载滤波器,提供了一条新的设计思路和依据。     

二维磁振子晶体中点缺陷模的耦合性质研究

在超原胞近似下, 利用平面波展开法数值计算了多点缺陷的二维磁振子晶体带结构及其部分缺 陷模的磁化强度场分布. 研究结果表明, 点缺陷模间的能量发生了耦合, 使自旋波沿着缺陷方向传播. 利用该性质可将含多点缺陷体的二维磁 振子晶体作为自旋波导波器件的制作材料.     

冲击压缩下蓝宝石光学性质的晶向效应

本文基于第一性原理计算方法,研究了a向、c向和r向蓝宝石理想晶体和含氧离子空位点缺陷晶体在0-180 GPa冲击压力范围内的光学性质.波长在1550 nm处理想晶体的折射率数据表明,在蓝宝石Corundum、Rh2O3以及CalrO_3相区,其折射率分别表现出强、弱以及强的晶向效应.波长在0-250 nm范围内理想晶体的能量损失谱结果指明,在Corundum和Rh_2O_3结构相区,其晶向效应不明显;在CalrO_3结构相,主峰附近的波段范围内,蓝宝石的能量损失谱有一定的晶向效应:c和r向的主峰强度基本相同,但a向主峰强度明显高于c和r向主峰强度.缺陷晶体数据表明:氧离子空位点缺陷对蓝宝石折射率和能量损失谱晶向效应的影响较微弱.     

一种紧凑的、可调的、基于缺陷共振的光开关

采用平面波展开法(PWM)和时域有限差分(FDTD)法,研究光在含点缺陷的光子晶体波导中的传输特性.计算结果表明,通过引入两共振腔(点缺陷),处于共振频率附近的光波将被完全反射回光子晶体波导.由于共振频率随点缺陷的折射率的变化而改变,这种现象可用来设计可调光开关. 关键词： 光子晶体波导 缺陷共振 光开关 透射率     

固/固型二维正方晶格声子晶体缺陷态研究*

固/固型声子晶体在抑制噪音和隔离振动等工程领域有着潜在的应用。利用有限元方法对存在缺陷的二维正方晶格金/环氧树脂声子晶体进行了研究,数值计算结果表明弹性波在点缺陷处局域,带隙中出现多条缺陷模,点缺陷数目的增加对声子晶体局域特性产生显著影响;弹性波沿着线缺陷传播形成波导,改变线缺陷结构可以改变弹性波传播方向和实现信号的分离。对声子晶体缺陷特性的研究可为声学滤波器和波导等器件的设计提供参考。     

Ab initio calculations of electronic and optical properties in O-doped LiF crystal

We present a first-principle study of electronic and optical properties in pure LiF and O-doped LiF crystals. The pure LiF crystal exhibits a wide band gap while the O-doped LiF crystal shows the less band gap due to the contribution of O 2p. Some optical constants, such as dielectric functions, reflectivity and the refractive index, have been performed. The calculated reflectivity and refractive index from the pure LiF crystal agree with the experimental and recently calculated results in the low-energy range. Meanwhile, the optical properties have also been predicted from the O-doped LiF crystal. The absorption band in 200 nm has been observed, which is relatively close to the experimental result.      

Selective adsorption of 3He and 4He on clean surfaces of NaF and LiF

Atomic beam scattering was used to study the gas-surface interaction between helium and ionic crystals. In particular, the binding energies of the selectively adsorbed states were measures for two isotopes of helium on the (001) surfaces of NaF and LiF cleaved in vacuum. Consistent results were obtained by avoiding incident conditions where band structure effects invalidate the free particle approximation. The energy levels can be fit to a 9-3 model potential.     

高压下LiF和NaF的结构稳定性及其电子和光学性质的第一性原理研究

基于密度泛函理论，采用全势线性缀加平面波加局域轨道方法，计算模拟了LiF高压下的相变行为，预测其在450GPa附近发生由NaCl结构(B1)到CsCl结构(B2)的结构相变.同时还计算了高压下LiF不同相的电学特性，LiF的复介电函数以及介电常数随压强变化关系.通过比较能带结构的变化行为，得出LiF在53GPa附近还存在等结构相变，即由直接带隙结构变为间接带隙结构.将LiF的计算结果与另外一个同构化合物NaF进行了比较讨论. 关键词： LiF 压致相变 从头计算     

Exciton states in CaF2, SrF2, and BaF2

We have applied the pseudopotential theory to the calculation of exciton states and binding energies for the ionic crystals CaF₂, SrF₂, and BaF₂, using a transfer model. Because of the satisfying agreement between our results and the experimental data in each case, we assume this pseudopotential formalism will describe transfer exciton states and energies in other ionic crystals.     

Interface electronic structure of alkali halide contacts

The electronic structure of contacts of different ionic crystals was calculated for LiCl, NaCl and KCL using the LCAO method within the next-nearest-neighbour approximation. No localized states (interface states) were found which leads to the conclusion that the experimentally observed influence of such contacts on photoelectron spectra is mainly due to the changed position of the bottom of conduction band in the interface region. Calculated values are compared with measured shifts of photoelectron spectra and a reasonable qualitative agreement is obtained. We suggest that the contact of two ionic crystals can be treated as a classical Schottky barrier.     

Electronic structure of BiSeI clusters

The valence band (VB) energy levels of BiSeI crystals have been calculated and compared to the X-ray photoelectron spectra of SbSI crystals. A molecular cluster consisting of 20 molecules of BiSeI has been used to calculate the total VB density of states by the density functional theory (DFT) method. The spectrum of total VB density of states of the (BiSeI)₂₀ cluster weighted by atomic photoemission cross-sections has been compared with the experimental X-ray photoelectron VB spectrum of SbSI type crystals. The cluster model calculation has showed that the core level energy splitting depends on the difference of ionic charges of the same atoms at the edges of the (BiSeI)₂₀ cluster.     

Intraband radioluminescence of LiF crystals

Spectra of high-energy electrons are calculated in the electron-hole ionization-passive region of lithium fluoride crystals for the conditions of intense irradiation by short pulses of accelerated electrons. The yield of intraband radioluminescence of these crystals is estimated in comparison with the yield of this kind of luminescence of more extensively studied NaCl crystals. The calculations demonstrate that the yield of radioluminescence determined by electron transitions in the conduction band of LiF crystals is two orders of magnitude weaker than the yield of analogous luminescence of NaCl crystals. This is explained, first, by special features of the energy band structure and, second, by the form of the energy dependence of the density of states in the conduction band of LiF crystals. The yield of hole-type intraband radioluminescence is estimated for various assumed relations between the widths of the valence and the forbidden bands.     

Luminescence depolarization mechanisms for color centers with the luminescence peak at 670 nm in gamma-irradiated LiF:Mg crystals

Processes of tunneling and thermal depolarization of luminescence of color centers with two absorption bands at 385 and 560 nm and the emission band peaked at 670 nm were studied in gamma-irradiated LiF:Mg crystals. As was previously concluded, relaxed excited states of the center have trigonal symmetry and are characterized by four equivalent trigonal Jahn-Teller minima corresponding to the C ₃ crystallographic axes. The luminescence depolarization observed at 110–120 K results both from the thermally stimulated transition and the tunnel transition of the system between the Jahn-Teller minima. Temperature dependence of the polarization degree was calculated and compared with experimental data.     

EPR of Mn4+ in LiF:U,Mn

A new manganese center in LiF single crystals doped with U₃O₈ and MnO₂ has been observed at room and liquid nitrogen temperatures. The trigonal EPR spectrum consists of one very anisotropic fine structure transition with six well-resolved hyperfine components. An analysis of the spectra recorded at X and Q bands shows that the zero field splitting is larger than the electronic Zeeman interaction.By comparing a theoretical calculation of the g values and the hyperfine structure with the experimental X and Q band data it was concluded that the ion is Mn⁴⁺. The presence of Mn⁴⁺ in these crystals confirms an assumption of Runciman regarding the possibility of introducing high valency ions in alkali fluorides containing uranium with an excess of oxygen.     

X-ray scattering factors for ionic crystals with complete electron shells

Scattering factors for ionic crystals with complete electron shells were considered by employing radially deformed atomic wave functions for “crystal ions”. The deformation was taken into account by the scaling method introduced by Mansikka in his earlier work. The correction due to the overlap between the deformed ions in crystals were evaluated by means of the formulae presented in our previous papers. Scattering factors for LiF and NaF crystals were computed and compared with experimental data. The effect of deformation is comparable in magnitude with the overlap ones.