Electronic and magnetic structures of chain structured iron selenide compounds

Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2 （2212＊） and BaFe2Se3 （123＊） are studied by the first-principles calculations. We find that while all these compounds are composed of one-dimensional （1D） Fe chain （or ladder） structures, their electronic structures are not close to be quasi-lD. The magnetic exchange couplings between two nearest-neighbor （NN） chains in 2212＊ and between two NN two-leg-ladders in 123＊ are both antiferromagnetic （AFM）, which is consistent with the presence of significant third NN AFM coupling, a common feature shared in other iron-chalcogenides, FeTe （11＊） and KyFe2-xSe2 （122＊）. In magnetic ground states, each Fe chain of 2212＊ is ferromagnetic and each two-leg ladder of 123＊ form a block-AFM structure. We suggest that all magnetic structures in iron-selenide compounds can be unified into an extended J1-J2-J3 model. Spin-wave excitations of the model are calculated and can be tested by future experiments on these two systems.     

The gauge theory of dislocations: Static solutions of screw and edge dislocations

We investigate the T(3)-gauge theory of static dislocations in continuous solids. We use the most general linear constitutive relations in terms of the elastic distortion tensor and dislocation density tensor for the force and pseudomoment stresses of an isotropic solid. The constitutive relations contain six material parameters. In this theory, both the force and pseudomoment stresses are asymmetric. The theory possesses four characteristic lengths ?₁, ?₂, ?₃ and ?₄, which are given explicitly. We first derive the three-dimensional Green tensor of the master equation for the force stresses in the translational gauge theory of dislocations. We then investigate the situation of generalized plane strain (anti-plane strain and plane strain). Using the stress function method, we find modified stress functions for screw and edge dislocations. The solution of the screw dislocation is given in terms of one independent length ?₁ = ?₄. For the problem of an edge dislocation, only two characteristic lengths ?₂ and ?₃ arise with one of them being the same ?₂ = ?₁ as for the screw dislocation. Thus, this theory possesses only two independent lengths for generalized plane strain. If the two lengths ?₂ and ?₃ of an edge dislocation are equal, we obtain an edge dislocation, which is the gauge theoretical version of a modified Volterra edge dislocation. In the case of symmetric stresses, we recover well-known results obtained earlier.     

五种SiO_2晶体原子和电子结构的第一性原理研究

α-石英、β-石英、α-方石英、β-方石英和超石英是自然界中SiO_2常见的五种晶型.本文利用第一性原理平面波赝势方法,系统计算了五种SiO_2晶体的体弹性模量、电荷密度、电子态密度,能带结构,并和可获得的实验进行了比较.α-英、β-石英和具有空间群F d3m结构的β-方石英显示间接的带隙,而α-方石英,超石英和具有空间群P2_13和I42d结构的β-方石英显示直接的带隙.五种晶型SiO_2的带隙宽度均大于5.5 eV,都是绝缘体.     

Observation of edge dislocations in helical smectic liquid crystals

We report the observation of elementary edge dislocations in smectic liquid crystals possessing helical structure. The dislocations were observed in the entire temperature range of helical phases, including ferroelectric, ferrielectric and antiferroelectric phases. The mechanism for visualizing the dislocations is based on the phenomenon of selective reflection of circularly polarised light. The performed observations of dislocations deliver not only information on the mechanisms of defect creation in various chiral smectic phases, but also on the structure and properties of the investigated smectics, often inaccessible using standard methods.     

Mobility of edge and screw dislocations in γ-irradiated LiF crystals

The free path lengths of ensembles of edge and screw dislocations in the stress field of a concentrated load are studied in γ-irradiated LiF crystals. The relative mobility of edge and screw dislocations is found to depend substantially on the irradiation dose and temperature. The results obtained are discussed in the context of additional retardation of screw dislocations with dislocation debris that appears during double cross slip.     

First-principles study of atomic and electronic structures of kaolinite in soft rock

Kaolinite is a kind of clay mineral which often causes large deformations in soft-rock tunnel engineering and thus causes safety issues. To deal with these engineering safety issues, the physical/chemical properties of the kaolinite should be studied from basic viewpoints. By using the density-functional theory, in this paper, the atomic and the electronic structures of the kaolinite are studied within the local-density approximation (LDA). It is found that the kaolinite has a large indirect band gap with the conduction band minimum (CBM) and the valence band maximum (VBM) being at the Γ and the B points, respectively. The chemical bonding between the cation and the oxygen anion in kaolinite is mainly ionic, accompanied by a minor covalent component. It is pointed that the VBM and the CBM of kaolinite consist of oxygen 2p and cation s states, respectively. The bond lengths between different cations and anions, as well as of the different OH groups, are also compared.     

First-principles study of atomic and electronic structures of kaolinite in soft rock

Kaolinite is a kind of clay mineral which often causes large deformations in soft-rock tunnel engineering and thus causes safety issues.To deal with these engineering safety issues,the physical/chemical properties of the kaolinite should be studied from basic viewpoints.By using the density-functional theory,in this paper,the atomic and the electronic structures of the kaolinite are studied within the local-density approximation(LDA).It is found that the kaolinite has a large indirect band gap with the conduction band minimum(CBM) and the valence band maximum(VBM) being at the Γ and the B points,respectively.The chemical bonding between the cation and the oxygen anion in kaolinite is mainly ionic,accompanied by a minor covalent component.It is pointed that the VBM and the CBM of kaolinite consist of oxygen 2p and cation s states,respectively.The bond lengths between different cations and anions,as well as of the different OH groups,are also compared.     

氧化锌掺钡的电子结构及其铁电性能研究

运用基于密度泛函理论的第一性原理方法计算了不同原子百分比含量的Ba掺杂Zn0半导体体材料超晶胞的能带结构、电子态密度、极化率和相对介电值.计算结果表明:Ba掺杂的Zn0体系为直接带隙半导体材料,其禁带宽度随着Ba原子掺杂百分比的增加呈现出逐渐增大的趋势.体系铁电性能的计算表明:与纯Zn0相比,Zn0掺入Ba原子后的极化率与相对介电值发生了较为明显的变化,其极化率随着Ba原子掺杂百分比的增加而增大,相对介电值随着Ba原子掺杂百分比的增加而减小.对角化后的极化率分量的数值结果表明:在电场作用下超胞中可能存在微畴结构,并且由于畴间电偶极矩的强相互作用,使得超胞宏观上表现为几乎具有各向同性的极化率特征.     

The fractional ionic character of alkali and silver halide crystals

The fractional ionic character of alkali and silver halide crystals is defined in terms of the deviations from the additivity rule for polarizabilities of ions. The electronic polarizabilities of ions are calculated using an empirical relationship according to which the electronic polarizability of an ion can be assumed to be directly proportional to the cube of its radius. The calculated ionicities indicate that the alkali halides are nearly or more than 90% ionic and silver halides are much less ionic which is also evident from the Phillips ionicity scale.     

Compressive energy of ions in ionic crystals

The possibility of writing the repulsive energy in the Born model of binary ionic crystals as a sum of two separate contributions from the two ions has been investigated. Such an approach leads to two identities, one connecting the lattice spacings of a family of ionic crystals and the other connecting their compressibilities. These identities have been tested on the alkali halide crystals over a range of pressures. The agreement is found to be quite satisfactory. Some further predictions with respect to crystals which exist as two polymorphs have also been tested. In all cases, the deviations of the experimental values from the exact identities can be traced to the fact that second neighbour repulsions in the crystals have been neglected. It is hence concluded that individual compressive energies for ions in ionic crystals is a very attractive possibility.     

The gauge theory of dislocations: A nonuniformly moving screw dislocation

We investigate the nonuniform motion of a straight screw dislocation in infinite media in the framework of the translational gauge theory of dislocations. The equations of motion are derived for an arbitrarily moving screw dislocation. The fields of the elastic velocity, elastic distortion, dislocation density and dislocation current surrounding the arbitrarily moving screw dislocation are derived explicitly in the form of integral representations. We calculate the radiation fields and the fields depending on the dislocation velocities.     

Analysis of contrasts and identifications of Burgers vectors for basal-plane dislocations and threading edge dislocations in 4H-SiC crystals observed by monochromatic synchrotron X-ray topography in grazing-incidence Bragg-case geometry

Contrasts of dislocations in the sub-surface region of the Si-face of a 4H-SiC wafer were observed by monochromatic synchrotron X-ray topography in grazing-incidence Bragg-case geometry. Basal-plane dislocations show very characteristic contrast depending on their Burgers vectors, running directions, and types of dislocations, whether they are screw dislocations, C-core edge dislocations, or Si-core edge dislocations. The rules for contrasts of basal-plane dislocations are summarized. It is shown that by observing those contrasts at fixed diffraction conditions, Burgers vectors of the basal-plane dislocation can be identified without performing a g?·?b analysis in some cases. Threading edge dislocations also have very characteristic contrasts depending on the angles between the projected g and their Burgers vectors. It is shown that Burgers vectors of threading edge dislocations can be determined uniquely by observing their characteristic contrasts without performing g?·?b analysis. Contrast mechanisms for these dislocations in grazing-incidence X-ray topography are discussed.     

Semi-quantitative theory of the structures of simple ionic crystals

A simple theory is developed which shows that the regions of stability of the CsCl, NaCl and ZnS structures can be demarcated in a two-dimensional plot of the radius ratio versus the strength of the van der Waals interaction. There is good agreement with experiment. The effect of pressure on these structures is explained qualitatively. The increased occurrence of the ZnS structure and the decreased stability of the CsCl structure in the A²⁺ B²⁻ crystals compared to the A⁺B⁻ crystals is also explained. Finally it is shown that the radius ratio and the polarizabilities of the ions are the important factors that determine the structures of AB₂ crystals.     

The F band of mixed ionic crystals as a function of temperature

Abstract

Experimental and theoretical study of the F absorption band were carried out in mixed ionic crystals as a function of temperature and molar composition. Both the point-ion and pseudopotential methods of Gourary–Adrian and Bartram–Stoneham–Gash, respectively, have been used to obtain the F band shift effect in KCl_1?x Br _x mixed crystals with molar fraction x and the temperature T of the crystal. The theoretical values were compared with the experimental ones, and a good agreement between experimental and theoretical calculations has been found.     

The influence of thermal processing on the relative mobility of edge and screw dislocations in NaCl crystals

The effect of thermal processing on the mobility of dislocations is investigated in NaCl crystals doped with impurities of various types—high-solubility impurities (Ca²⁺) and low-solubility impurities (Pb²⁺). The results obtained after aging and thermal processing indicate that the type of impurity and its state (e.g., level of aggregation) in the crystal have a strong effect on the relative mobility of edge and screw dislocations, and also on the parameters of double transverse slip. Fiz. Tverd. Tela (St. Petersburg) 41, 1041–1043 (June 1999)     

Smectic-A edge dislocations in a very thin cell

We study stable “bookshelf” smectic-A structures within a very thin plane-parallel cell of thickness L in which the mismatch between surface preferred (d _s) and intrinsic (d₀) smectic layer thicknesses occurs. The Landau-Ginzburg approach based on a complex smectic order parameter is used. For a weak enough smectic positional anchoring strength W smectic layers adopt the modified bookshelf profile. In a thick enough cell with increasing W a lattice of edge dislocations is continuously formed at the confining surfaces and then depinned from them. The structure with dislocations is formed when the condition d ₀/( d ₀/d _s - 1) ∼ 2 is fulfilled, where is the positional surface anchoring extrapolation length. If the cell is thin enough the dislocations formed at opposite cell plates annihilate and consequently the smectic layers adopt a locked bookshelf structure. This transition is discontinuous and takes place when d ₀/(L d ₀/d _s - 1) ∼ 5 is realized. To observe these transitions in a cell of thickness L∼ 1μm the conditions W∼ 10^-6 J/m ² and d ₀/d _s - 1∼ 5 ^. 10^-4 have to be fulfilled. All the three qualitatively different structures coexist at the triple point. Received 21 February 2002     

Cu,Fe掺杂LiNbO_3晶体电子结构和光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理对Cu,Fe单掺及共掺LiNbO_3晶体的电子结构和光学性质进行了计算.结果显示:Cu,Fe单掺杂LiNbO_3晶体禁带内均产生了杂质能级,主要由Cu3d,Fe3d轨道及O 2p轨道贡献;共掺LiNbO_3晶体禁带内出现了双能级结构,深能级由Cu3d和O2p轨道贡献,浅能级由Fe3d和O2p轨道贡献.Cu,Fe单掺和共掺LiNbO_3晶体带隙依次缩小,在可见光区的光吸收明显增强.共掺LiNbO_3在445和630nm左右分别表现出一个宽吸收峰,比单掺LiNbO_3晶体表现出更好的光吸收性质.研究表明,Fe占Nb位比Fe占Li位的双掺样品在双光存储应用中更有优势;同时,浓度比[Fe2+]/[Fe3+]值的适当降低有助于这种优势的形成.     

Effect of doping with divalent cationic impurities on relative mobility of edge and screw dislocations in NaCl crystals

In NaCl:Ca and NaCl:Pb crystals, we have investigated the effect of the impurity concentration and temperature on the parameter ξ=l _e /l _s where l _e and l _s are the path lengths traveled by edge and screw dislocations in the stress field of an indenter. The results obtained are explained taking into account additional inhibition of screw dislocations by debris. State Medical University, Moldavian Republic. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 98–102, May, 1996.     

Dipole of edge misfit dislocations and critical radius conditions for buried strained cylindrical inhomogeneity

A theoretical model is proposed for elastic stress relaxation of a buried strained cylindrical inhomogeneity, which assumes the edge misfit dislocation dipole formation in the soft matrix at some distance from the interface. The critical radius of the inhomogeneity for the formation of the edge misfit dislocation dipole is given and the influence of various parameters on the critical radius is evaluated. The result indicates that the critical radius decreases with increasing misfit strain and core radius of the misfit dislocation. It is also found that, compared to the edge misfit dislocation dipole formation in the interface, the critical radius of the inhomogeneity decreases when the location of an edge misfit dislocation dipole formation is in the soft matrix at some distance from the interface.