Split interstitials in computer models of single-crystal and amorphous copper

The effect of interstitial atoms in a dumbbell configuration on elastic moduli of single crystal copper has been investigated using molecular dynamics simulation. It has been shown that shear components of the dipole tensor and λ-tensor increase when the concentration of split interstitials exceeds 0.6–0.7%. This is associated with their interaction responsible for a significant change in the distribution of orientations of split interstitials and, hence, for a change in the type of the induced local symmetry breakings of the face-centered cubic structure. It has been found that, in the model of amorphous copper, there are “defects” (elastic dipoles) with properties similar to those of split interstitials in the single crystal. The deviatoric component of their λ-tensor is more than an order of magnitude greater than the dilatation component and is responsible for the decreased value of the shear modulus and thermal effects in noncrystalline metallic materials.     

Calculation of torsional and rotational raman spectra of hydrogen peroxide

We present calculated intensity distributions in torsional, rotational, and torsional-rotational Raman lines in spectra of hydrogen peroxide. Ab initio calculations of polarizability tensor components as functions of internal rotation angle were carried out in the HF/6-311G approximation. It is shown that the structure and transformational properties of the polarizability tensor components of hydrogen peroxide in extended molecular symmetry group G₄(EM) permit formation of purely rotational and torsional and rotational-torsional Raman spectra. Common expressions to calculate Raman line intensities governed by torsional and rotational motions of the non-rigid symmetric top molecule are obtained. The torsional components of the line intensities have been calculated by estimating the appropriate matrix elements. The contribution of rotational components has been calculated using the 3j-symbols technique.     

Bond dynamics in arbitrary lattices

We treat the quantum mechanics/dynamics of lattices in arbitrary dimensions, with arbitrary topology - whether crystalline or amorphous - whether elastic or inelastic. The method is a generalization of a transformation which has been successful for one- or two-dimensional structures, from site to bond coordinates. Our formulation clarifies the role of defects in melting phenomena.     

The inertial mass tensor of a polaron in an isotropic medium

Owing to a funadmentally erroneous approach to calculations of the effective polaron mass (calculations that use a model without spatial dispersion of the lattice polarizability), the polaron inertial mass has never before been distinguished from the mass as a measure of kinetic energy. In this paper we derive an expression for the tensor of the inertial mass of a large polaron. The tensor is found to be fully determined by two components: the longitudinal component, corresponding to the case where the force acting on the polaron is parallel to the polaron velocity, and the transverse component, corresponding to the case where the acceleration is perpendicular to the polaron velocity. The components of the polaron inertial mass tensor depend quasirelativistically on the polaron velocity due to the quasirelativistic compression of the polarization field in the direction of motion, which constitutes the effect of spatial dispersion of the lattice polarizability. We derive a formula that approximates the dependence of the components of the polaron mass tensor on all the parameters: the frequency and dispersion of the phonons, the polaron velocity, and the effective dielectric constant. Zh. éksp. Teor. Fiz. 115, 180–186 (January 1999)     

The rovibrational Raman spectrum of water vapour v 1 and v 3

The OH stretching region in the Raman spectrum of water vapour has been observed and reproduced by computer simulation using rovibrational wave functions derived from the analysis of high-resolution absorption spectra. Relative intensity measurements allow the determination of the depolarization ratio and the ratio of the two non-vanishing second-order irreducible tensor components for v ₁. These quantities have been combined with the literature value for the cross section of this band to calculate the principal cartesian components of the v ₁ derived polarizability tensor. The cross section for the v ₃ vibration was also determined, and these results are considered in terms of the bond polarizability theory.     

Calculation of electronic ion polarizability and parameters of interionic repulsion in LiNbO3 crystals

A method for calculating the anisotropy of the electronic polarizability of O^2? ions in oxygen octahedral ferroelectrics has been proposed. It is shown that, in the case of mixed ion-covalent bonds in the crystal, the calculation of the polarizability within the approximation of isolated ions gives incorrect results. The principal components of the polarizability tensor for the O^2? ions and the electronic polarizability of the Nb⁵⁺ ions are calculated for the LiNbO₃ ferroelectric. Analysis of the stability of the LiNbO₃ structure confirms the results obtained.     

The ΔE effect in iron-boron and iron-chromium-boron amorphous invar aloys

Temperature- and field-dependence of elastic properties of amorphous Fe-B and Fe-Cr-B Invar alloys have been investigated and Elinvar characteristics have been found together with a large ΔE effect. These results have been revealed that the elastic properties of these amorphous alloys are very different those of the crystalline Fe-Ni Invar alloy.     

On the determination of the volume fraction of the crystalline phase in amorphous-crystalline alloys

The possibility of determining volume fractions of crystalline and amorphous phases of partially crystalline alloys from X-ray diffraction data has been discussed. The crystallization of an amorphous microwire of the Fe_73.9B_13.2Si_10.9C₂ composition has been investigated. The crystallization leads to the formation of α-Fe and Fe(Si). An analysis has been made of the X-ray diffraction patterns recorded for a series of samples with different contents of the crystalline and amorphous phases. The angular range has been determined and the calibration graph has been constructed, which can be used to determine the volume fractions of the amorphous and crystalline components in amorphous-crystalline samples.     

Local field anisotropy in Langmuir-Blodgett films of cadmium arachidate

Lorentz tensor and local field tensor components for uniaxial Langmuir-Blodgett films of cadmium arachidate from 3 to 100 nm thick on silver, polymethyl methacrylate, and glass substrates have been experimentally determined using the data on the film refractive index dispersion in the visible range. The constraints from below on the mean value and anisotropy of the molecular polarizability caused by the intermolecular interactions in the film have been established.     

Atomistic origin of urbach tails in amorphous silicon

Exponential band edges have been observed in a variety of materials, both crystalline and amorphous. In this Letter, we infer the structural origins of these tails in amorphous and defective crystalline Si by direct calculation with current ab initio methods. We find that exponential tails appear in relaxed models of diamond silicon with suitable extended defects that emerge from relaxing point defects. In amorphous silicon (a-Si), we find that structural filaments of short bonds and long bonds exist in the network, and that the tail states near the extreme edges of both band tails are also filamentary, with much localization on the structural filaments. We connect the existence of both filament systems to structural relaxation in the presence of defects and of topological disorder.     

Orientational order and evolution of the properties of biomolecules in anisotropic media

Experimental values of the Lorentz tensor components and the local field of a light wave have been obtained for anisotropic films of polypeptide and DNA. These data indicate an increase in the mean value and anisotropy of the polarizability of biomolecules with respect to an isotropic solution. These changes in the polarizability as an indicator of the response of biomolecules to external actions, as well as their chemical and biological activity, specify the direction of the evolution of the properties of biomolecules in orientationally ordered media.     

Vacuum correlators of the electromagnetic field and the Casimir-Polder forces in the field of a cosmic string

Vacuum correlators of the electric and magnetic fields are calculated in the geometry of a cosmic string. Formulas for the vacuum expectation values for the squares of field components are derived. The forces acting on an atom due to the vacuum fluctuations (Casimir-Polder forces) are investigated. For atoms with isotropic tensor of polarizability these forces are attractive with respect to the string. In the anisotropic case, depending on the eigenvalues of the polarizability tensor, the Casimir-Polder forces can be either attractive or repulsive.     

The determination of stresses in thin films; modelling elastic grain interaction

X-ray diffraction is frequently employed for the analysis of mechanical stresses in polycrystalline specimens. To this end, suitable so-called diffraction elastic constants are needed for determining the components of the mechanical stress tensor from measured lattice strains. These diffraction elastic constants depend on the single-crystal elastic constants of the material considered and the so-called grain interaction, describing the distribution of stresses and strains over the crystallographically differently oriented crystallites composing the specimen. Well-known grain interaction models, as due to Voigt, to Reuss, to Neerfeld and Hill and to Eshelby and Kröner, may be applied to bulk specimens, but they are generally not suitable for thin films. In this paper, an average 'effective' grain interaction model is proposed that consists of a linear combination of basic extreme models including new models specially suited to thin films. Experimental verification has been achieved by X-ray diffraction strain measurements performed on a sputter-deposited copper film. This is the first time that anisotropic grain interaction has been analysed quantitatively.     

Molecular Dynamics as a tool to interpret macroscopic amorphization-induced swelling in silicon carbide

We present here an investigation of the irradiation-induced swelling of SiC using Classical Molecular Dynamics simulations. Heavy ion irradiation has been assumed to affect the material in two steps: (a) creation of local atomic disorder, modeled by the introduction of extended amorphous areas with various sizes and shapes in a crystalline SiC sample at constant volume (b) induced swelling, determined through relaxation using Molecular Dynamics at constant pressure. This swelling has been computed as a function of the amorphous fraction introduced. Two different definitions of the amorphous fraction were introduced to enable meaningful comparisons of our calculations with experiments and elastic modeling. One definition based on the displacements relative to the ideal lattice positions was used to compare the Molecular Dynamics results with data from experiments combining ion implantations and channeled Rutherford Backscattering analyses. A second definition based on atomic coordination was used to compare the Molecular Dynamics results to those yielded by a simplified elastic model. The simulation results using the lattice-based definition of the amorphous fraction compare very well with the experimental results. This proves that the modeling in two steps chosen for the creation of the amorphous regions is reasonable. Moreover, the results show very clearly that SiC swelling does not scale linearly with the amorphous fraction introduced. Two swelling regimes are observed relatively to the size of the amorphous area. Comparison of the elastic model with the Molecular Dynamics results using the coordination-based definition of the amorphous fraction has also enabled us to shed light on the swelling mechanisms and has shown that amorphization-induced swelling exhibits an elastic behavior. Furthermore, scalings for the swelling as a function of the two amorphous fractions considered, which can be used by larger scale models, have been determined. Finally, our study shows that classical Molecular Dynamics calculations enable one to connect the results of the available experiments with the elastic calculations and to get further insight into the swelling mechanisms.     

基于键极化加和模型及其实验修正方法的超拉曼微观极化率张量元微分的简化方案-C2v对称性

相干反斯托克斯拉曼光谱(CARS) 和相干反斯托克斯超拉曼光谱(CAHRS)等高阶非线性光谱技术已经应用在动力学过程、基因表达谱筛选、高分辨率光谱分析等诸多领域。但因其涉及到的高阶微观极化率张量元数量众多,对其光谱信号进行定量分析非常困难。文献通过理论分析将CARS和CAHRS的微观极化率张量元分解为拉曼微观极化率张量元微分α′_i′j′和超拉曼微观极化率张量元微分β′_i′j′k′的乘积,将高阶光谱定量分析的困难简化为对低阶光谱的分析。该研究处理了具有C_2v对称性分子基团,提出一种利用键极化加和模型及其实验修正的方法简化超拉曼微观极化率张量元微分β′_i′j′k′比值的方案。首先利用键极化加和模型方法,在对C_2v分子基团局域模式假设的基础上,进行单键的局域模式假设,通过对单键伸缩振动进行简正振动模式的分析,并与单键的偶极矩表达式进行比较,得到单键的超拉曼微观极化率张量元微分β′_i′j′k′。在单键β′_i′j′k′结果的基础上利用键极化加和模型,对C_2v分子基团进行对称性分析,将基团中两个单键伸缩振动的β′_i′j′k′耦合,得到C_2v分子基团对称振动和反对称振动两种简正振动模式的超拉曼微观极化率张量元微分β′_i′j′k′的表达式。在此基础上对C_2v分子基团对称振动和反对称振动的β′_i′j′k′表达式进行理论推导,结合对应振动模式下不同偏振组合的超拉曼退偏率ρ_HR-SS和ρ_HR-AS及HV偏振组合时实验测量的超拉曼光谱强度比,得到超拉曼实验修正的C_2v分子基团β′_i′j′k′之间的比值。再结合文献中拉曼实验修正的拉曼微观极化率张量元微分α′_i′j′之间的比值,即可得到C_2v分子基团CARS和CAHRS的微观极化率张量元之间的比例关系,为定量分析高阶非线性光谱信息提供理论基础。     

典型高分子材料的固体核磁共振研究

本论文通过固体核磁共振（NMR）谱及动力学参量的测量,并结合X-射线衍射技术和DSC测量等研究了两种典型高分子材料的相结构、链的运动以及相与相之间的关系.  乙烯-醋酸乙烯共聚物( EVA) 是最主要的乙烯共聚物之一. 研究发现,EVA的相组成非常复杂,共有5个不同的组分. 除了PE中所观察到的常规单斜晶相和刚性的正交晶相外,我们发现还存在第三个晶相分量－运动性较强的晶相（SOCP,可能是转动相）. 它不仅拥有自己的熔点,而且它的化学位移和分子运动性不同于刚性正交晶相（LOCP）. 另一方面,非晶相也由两种不同的分量组成：运动受限的各相异性的非晶界面相和高度可动的橡胶型的非晶相. 我们进一步详细研究了EVA中的晶区链动力学和非晶区的低温冻结行为. 实验发现,在正交晶相中,高分子链以180° flip-flop方式运动,同时伴随沿链方向的平移型跳跃运动,并引起正交晶相和非晶相之间的长程链扩散,通过NOE的测量证实了这种相间链扩散的存在,并进一步通过实验证实这种相间链扩散是一种受限扩散而不是自由扩散. 同时非晶相的两个组分具有不同的低温冻结行为：当温度低于-弛豫转变温度时,橡胶型的非晶相中的长程分子运动被冻结,但仍存在分子的局域运动;而界面非晶在低温时冻结成一种有序取向结构,并用质子自旋扩散实验证实该有序结构与正交晶相相邻近.  少量纳米级片层状粘土分散在聚合物中就可赋予材料许多优异的性能,我们用固体NMR技术对EVA/REC复合材料的结构和其中粘土的分散性质进行研究,发现上述复合材料中所形成的晶体类型不仅依赖于各组分的性质还依赖于所形成的复合材料的类型.  偏氟乙烯/三氟乙烯共聚物（P（VDF-TrFE））是最主要的铁电高聚物之一. 我们利用变温固体¹⁹F MAS NMR 谱及弛豫数据的测量详细研究了电子辐照对P(VDF-TrFE)共聚物的分子结构、构型、运动性以及相变等的影响. 发现,电子辐照不仅改变了分子链段的构型和运动性,同时也改变了局部分子化学结构. 电子辐照促使铁电相向顺电相(或者非晶相)转变,与此同时诱发了富含VDF和含-TrFE链段从全反式的构型到混合的反式-旁式构型的转变. 电子辐照加剧顺电区域中的分子运动而在高温熔融态中(>100 ℃）,分子的运动反而受限.     

Polarized Raman Scattering in Silicon Layers on Sapphire

The intensities of polarized components of Raman light scattering are measured for different crystallographic directions in epitaxial layers of silicon on sapphire. The experiment shows that the scattered radiation has a strong depolarized component, which is directly related to the occurrence of defects in the epitaxial layer. A model of an ensemble of crystallites is proposed to describe a defect crystal silicon layer on sapphire. It is shown that the characteristic value of disordering of crystalline domains in the epitaxial layer can be determined from the ratio between the intensities of the polarized and depolarized components. From measurements of the Raman scattering intensity as a function of the crystallographic direction, the anisotropy of the Raman scattering tensor is determined. Numerical values for two samples are obtained.     

Electric field gradient for interstitial positive muons in fcc metals: An AB Initio calculation of size effect

The electric field gradients (EFGs) resulting from interstitial point defects in fcc metals have been investigated. The defect induced charge density, used to evaluate the valence effect EFG, is calculated self-consistently in the density functional formalism. An ab initio calculation of the size effect EFG is carried out for a positive muon at an octahedral site in the fcc lattice in the elastic continuum model. The components of the strain field tensor are evaluated assuming the lattice of dressed point ions interacting through the screened Coulomb potential. No adjustable parameter has been used. The theoretical results are in good agreement with the experimental values within experimental uncertainties. It is emphasized that both the strain and conduction electron contributions are equally important in the estimation of the electric field gradient.     

Optical properties of GaAs films deposited via pulsed ion ablation

Optical reflectance and absorbance of gallium arsenide films formed on polycrystalline corundum, quartz glass, and copper foil are investigated in the energy interval of 1.1–6.2 eV. The films have been deposited from ablation plasma induced by a high-power ion beam. The exponential and interband absorbance spectra of the material of films are determined by defects in the GaAs crystalline lattice and the intricate composition of the material with predominance of nanocrystalline inclusions in the amorphous phase. Films deposited on polycor at the plasma flame center with the use of a low-resistance target have optimal properties for application in devices of optoelectronics and solar power engineering. Thermal vacuum treatment at 300–850 K modifies the optical properties of films owing to annealing of defects and changing of the structural-phase composition of a material.