Effective pair interactions in transition metal carbides and nitrides

Effective pair interactions up to the 4-th nearest neighbors have been calculated for TiC_x, NbC_x, TiN_x and NbN_x as a function of the vacancy concentration using the KKR-CPA implementation of the generalized perturbation theory. While for the carbides good agreement with experimental data was found, for the nitrides the theory fails to predict the type of ordering presently accepted in the literature.     

Polarization contributions to the vacancy binding energy in doped oxides

An effective medium theory due to Onsager is generalized to yield the macroscopic dielectric constant of a random assembly of associated vacancy-dopant bound pairs embedded in a dielectric host. The model is then further developed to give the vacancy-dopant binding energy, employing concepts long-established in electrolyte theory. The results of this analysis are used to compute the activation enthalpy for electrical conduction in the ionic conduction regime. The values so found are in reasonable accord with experimental data from various sources on Ce_1?xCa_xO_2?x, with x?0.02, assuming a vacancy migration enthalpy of 0.71 eV. The latter is the only adjustable parameter in the theory here developed. With this same value, the predicted variation of the low-temperature conduction activation enthalpy in Ce_1?2xY_2xO_2?x and Ce_1?2xGd_2xO_2?x (again at small x) are acceptably reproduced, although more experimental data would be desirable. The dielectric constant of the yttria-doped material is also described by the present model, again with no adjustable parameters. Several different features of the theory lead to its loss of validity in more concentrated mixtures. These are examined in detail.     

Melting temperature variation with concentration of alkali halides in mixed crystal systems

Diffusion-vibration theory of melting (Sharmaet al 1991) has been extended to study the variation in the melting temperature of mixed ionic crystals with concentration. The melting temperature varies non-linearly with concentration in the KCl _x Br_1−x , RbCl _x Br_1−x , K _x Rb_1−x Br and NaCl _x Br_1−x mixed alkali halides and shows a sharp increase in melting temperature for values ofx>0.5 which is in good agreement with the experimental values. This behaviour has been explained on the basis of present propounded theory.     

高完整GexSi1-x/Si应变超晶格的X射线双晶衍射研究

本文用X射线双晶衍射技术对分子束外延生长的Ge_xSi_1-x/Si应变超晶格的结构参数进行研究,分别采用X射线运动学理论和动力学理论对超晶格的双晶摆动曲线进行计算模拟,得出超晶格的全部结构参数;并对这两种理论计算模拟的结果进行比较,发现这两种理论计算的结果基本一致,只是在细微结构上略有差别,对高完整Ge_xSi_1-x/Si超晶格,用动力学理论计算的曲线更接近于实验曲线。 关键词：     

Al和N共掺p型Zn1-xMgxO电子结构的第一性原理计算

采用密度泛函理论下的第一性原理平面波超软赝势方法,对Zn_1-xMg_xO超晶胞和掺杂Al,N后的Zn_1-xMg_xO超晶胞分别进行了优化计算.结合广义梯度近似计算了Al和N共掺杂后Zn_1-xMg_xO的能带结构、电子态密度和Mulliken电荷布居分布.计算表明：掺入N原子的2p态电子为Zn_1-xMg_xO价带顶提供空穴载流子,使Zn_1-xMg_xO价带顶向高能方向移动;掺入Al原子的3p态电子则与N原子的2p态电子在费米能级附近发生轨道杂化,使费米能级处价带能级展宽,Al和N共掺杂可获得p型Zn_1-xMg_xO. 关键词： 密度泛函理论 1-xMg_xO')" href="#">Zn_1-xMg_xO 电子结构 共掺杂     

Thermoelectric power of amorphous alloys NixZr1−x

Thermoelectric power studies of the paramagnetic amorphous alloys Ni_xZr_1?x (x = 0.36, 0.40, 0.60, 0.65) are in agreement with predictions of the extended Ziman theory and indicate the multi-stage crystallization of some alloys.     

Magnetic phase diagram of the Ca1−xMnxO systems

The magnetic properties of the Ca_1−xMn_xO systems in the range 0?x?1 have been studied by mean field theory and high-temperature series expansions (HTSEs). By using the first theory, we have evaluated the nearest neighbour and the next-neighbour super-exchange interaction J₁(x) and J₂(x) respectively, in the range 0.45?x?1. The corresponding classical exchange energy for magnetic structure is obtained for the Ca_1−xMn_xO systems. The HTSEs combined with the Padé approximants (PA) method is applied to the Ca_1−xMn_xO systems; we have obtained the magnetic phase diagrams (TN or TSG versus dilution x) in the range 0?x?1. The obtained theoretical results are in agreement with experimental ones obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) are deduced in the range 0?x?1.     

Raman spectroscopy of acoustic phonons in periodic and Fibonacci superlattices

Results of Raman scattering experiments on (a) periodic superlattices made up of GaAs/In_xGa_1−xAs layers with high indium concentrations, (b) GaAs/Ga_1−xAl_xAs Fibonacci superlattices, are presented. We discuss the observed peak positions and intensities using the continuum theory of acoustic wave propagation in layered media and the photo-elastic coupling model.     

Raman scattering and hot luminescence spectra of Zn1 − x Mn x Te quantum wires

The Raman scattering and luminescence spectra of Zn_{1 − x} Mn _x Te (0 ≤ x ≤ 0.6) quantum wires have been investigated. The quantum wires have been grown by molecular-beam epitaxy on the (100)GaAs substrate with Au used as a catalyst. The spectrum of optical phonons in ZnMnTe quantum wires varies with a variation in x in accordance with an intermediate (between one- and two-mode) type of transformation. The optical phonon spectrum has been analyzed in terms of the microscopic theory. It has been demonstrated that the experimental data can be brought in accord with the theory by properly modifying the calculated density of phonon states for ZnTe. The spatial confinement has been found to affect the electronic states in Zn_{1 − x} Mn _x Te quantum wires.     

固体宽带组合脉冲的一种设计方法

用量子力学微扰方法结合相干平均理论提出了固体宽带组合脉冲的理论,并用该理论设计了固体宽带组合π脉冲P_x(90°)P_y(90°)P_x(90°),实验表明效果较好。 关键词：     

Magnetic susceptibility studies of Zn1−xFexSe and Zn1−xFexS

The magnetic susceptibility of diluted magnetic semiconductors Zn_1−xFe_xSe and Zn_1−xFe_xS has been calculated within a crystal field model and by applying the extended nearest-neighbour pair approximation. The results are compared with available experimental data. It appears that a good agreement between theory and experiment is obtained for Zn_1−xFe_xSe taking the long-range exchange comprising several nearer interionic distances, whereas in Zn_1−xFe_xS the exchange interaction with nearest-neighbours alone gives a better solution.     

Electronic energy structure and X-ray spectra of GaN and BxGa1-x N crystals

The electronic energy structures of GaN wurtzite and zinc-blende modifications and B_xGa_1-x N solid solutions are calculated using the local coherent potential method and the cluster version of the muffin-tin approximation in the framework of the multiple-scattering theory. The electronic structures of binary GaN crystals and ternary B_xGa_1-x N solid solutions are compared, and their features are interpreted. The boron concentration dependences of the width of the upper subband of the valence band, the band gap, and the bulk modulus of B_xGa_1-x N solid solutions (x = 0.25, 0.50, 0.75) are studied, and these dependences are shown to be nonlinear.     

Hall effect in microscopically inhomogeneous magnetic alloys

A theory of the generalized conductivity for the normal component of the Hall effect is developed. It is shown that the normal Hall effect coefficient R ₀ of microscopically inhomogeneous magnetic alloys GdZn_xCu_1−x , which at low temperatures consist of ferro-, antiferro-, and paramagnetic phases, can be described satisfactorily on the basis of an effective-medium theory. The experimentally observed relationship between the coefficient R ₀(x) and the resistivity ρ(x) is obtained. Fiz. Tverd. Tela (St. Petersburg) 41, 98–102 (January 1999)     

First-principles study of structrural and corrected band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO systems

A first-principles method based on density functional theory(DFT),a generalized gradient approximation(GGA),and a projector-augmented wave(PAW) are used to study the structual and band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO(0 ≤x≤1) ternary alloys.By taking into account all of the possible structures,the band gaps of Zn1-xCdxO and Zn1-xMgxO alloys are corrected and compared with experimental data.     

Photoemission study of Pd Zr and Cu Zr alloys

The XPS valence bands and core levels of the alloys Pd_1?xZr_x (0<x<1) and Cu_1?xZr_x (0<x<1) have been measured. The alloys prepared by coevaporation are crystalline — but their valence band spectra are close to those of the metallic glasses of the same compositions. The large valence band and core level shifts observed for Pd can be explained by a simple theory, not necessitating the postulation of a new type of bonding in these systems.     

X-ray diffractometry and electrical conductivity of the ceramics LaSrAl(Cu,Ni)O4

A continuous series of solid solutions LaSrAL_1−x Me_xO₄ (Me = Cu, Ni) has been investigated by x-ray diffractometry and by electrical conductivity measurements. The results are of interest from the standpoint of the theory of close-packing, percolation theory, and the theory of the cooperative Jahn-Teller effect. Fiz. Tverd. Tela (St. Petersburg) 39, 985–990 (June 1997)     

Lattice dynamics and debye temperature of AlCu,AlSi and AlGe alloy systems

The lattice dynamics and specific heat at constant volume for AlCu alloy system is studied using our previous treatment based on the microscopic electronic theory. Considering the volume and electron density effect on the dynamical matrix of the pure constituent, we obtain the band and local mode frequencies at the temperature-dependent specific heat in the Al_1−x Cu_x solid solution. Then, using the data about the mean elastic wave and the specific heat extrapolated to the absolute zero temperature, the Debye temperature is presented for the Al_1−xCu_x, Al_1−xSi_x and Al_1−xGe_x alloy systems. The Debye temperature decreases as a function of the Cu atomic fraction x for Al_1−xCu_x alloy, and remarkably for Al_1−xSi_x and Al_1−xGe_x solid solutions.     

Nonlinear dielectric properties of amorphous Bi1.8Pb0.3Sr2Ca2Cu3−x KxOz (x = 0.2–0.3)

The dielectric constant of amorphous Bi_1.8Pb_0.3Sr₂Ca₂Cu_3?x K_xO_x (x = 0.2–0.3) is measured as a function of the static electric field strength. It is found that, as the field strength increases, the dielectric constant decreases following a quadratic law. This behavior of the dielectric constant in a static electric field indicates nonlinear dielectric properties, which can be described within the thermodynamic theory of ferroelectricity.     

The d-resonance scattering in amorphous Fe x Pd80−x Si20 alloy

The electrical resistivity of amorphous Fe _x Pd_80–x Si₂₀ alloy has been discussed in terms of extended Ziman theory. The increase in the electrical resistivity of Fe _x Pd_80–x Si₂₀ has been predicted due to the d-resonance scattering of the conduction electrons by Fe-atoms.Dedicated to Academician Vladimír Hajko on the occasion of his 65th birthday.     

Critical line broadening of the EPR in the two-dimensional antiferromagnet crystals Rb2MnCl4 and Rb2Mn0,8Cd0,2Cl4

The EPR spectrum of quasi-two-dimensional antiferromagnets Rb₂Mn_xCd_1?xCl₄ (x=1,0; 0,8) has been studied in a critical temperature range. Two theoretical approaches - the scaling theory and the soliton's theory - are used to explain experimental data on temperature of the EPR linewidth. In the first interpretation critical exponents are determined. For both crystal two temperature regions with different critical exponents are found. It is shown that the soliton's theory with an anomaleous great excitement energy describes the experimental data satisfactory.