Understanding of surface states in a correlated electron system

The aim of this work is to find a simple analytic model to explain some principal aspects of the behavior of surface states in correlated electron systems. We start from the analytic expression for the Green function of the semi-infinite tight binding linear chain. This Green function in case of modification of the center of gravity of the first atom and the change in coupling between the first and the second atom is evaluated as an exact analytic expression. Conditions for the existence and classification of surface states are given. The spectral weight of surface states and the local density of states are evaluated. The method is applied to a s.c. (100) surface of a local moment crystal. Conditions for the existence of surface states are derived and their locations in the Brillouin zone are predicted. It is shown that it is possible to include correlation effects within the framework of the discussed model. The comparison with former numerical results is performed. Received 8 March 2000     

Representation of density operators for optical fields having Gaussian quasiprobability functions

For radiation having Gaussian quasiprobability functions, a general method of representation of density operators in terms of outer products of coherent states with nonsingular weight function and only two real variables of integration for every mode is given. It is shown that the representation simplifies, if minimum uncertainty states are used.     

Fe和Ni共掺杂ZnO的电子结构和光学性质

基于密度泛函理论的第一性原理研究Fe,Ni单掺杂和(Fe,Ni)共掺杂纤锌矿型ZnO的能带结构、电子态密度分布、介电函数、光学吸收系数,分析了掺杂后电子结构与光学性质的变化.计算结果表明：掺杂体系的费米能级附近电子态密度主要来源于Fe 3d,Ni 3d态电子的贡献;与纯净ZnO相比,Fe,Ni单掺杂和(Fe,Ni)共掺杂ZnO的介电函数虚部均在0.46eV左右出现了一个新峰;Fe,Ni单掺杂和共掺杂ZnO的吸收光谱均发生明显的红移,并都在1.3eV处出现较强吸收峰.结合他人的计算和实验结果,给出了定性的讨 关键词： 氧化锌 掺杂 第一性原理 光学性质     

Density of states of the binary alloy in the coherent potential approximation

The density of states of an electron in a binary alloy in the tight binding model is calculated in the single site coherent potential approximation (CPA) as a function of the concentration and the site energy difference. The fluctuations in the site energies due to the random environment is taken into account approximately by giving width to the site energy probability distribution function, which is normally a sum of two delta functions with proper weight factor.     

Density of phonon states in amorphous germanium and silicon

The density of phonon states in amorphous germanium and silicon is calculated by statistically averaging the crystalline phonon density of states according to the radial distribution function. A simple rigid ion model is used to calculate the density of phonon states at various lattice spacings. The appropriate model parameters are obtained from the pressure dependent elastic constants and the Raman frequency. The calculated results compare favorably to experimental data obtained by infrared and Raman scattering and the results of other theoretical calculations.     

Impurity induced resonance states at the superconducting interface LaAlO3/SrTiO3

Motivated by the recent discovery of superconductivity on the heterointerface LaAlO₃/SrTiO₃, we theoretically investigate the impurity-induced resonance states with coexisting spin singlet s- and triplet p-wave pairing symmetries by considering the influence of Rashba-type spin-orbit interaction (RSOI). Due to the nodal structure of the mixed gap function, we find single nonmagnetic impurity-induced resonance peaks appearing in the local density of state. We also analyze the evolutions of density of states and local density of states with the weight of triplet pairing component determined by the strength of RSOI, which will be widely observed in thin films of superconductors with surface or interface-induced RSOI, or various noncentrosymmetric superconductors in terms of point contact tunneling and scanning tunneling microscopy, and thus shed light on the admixture of the spin singlet and RSOI-induced triplet superconducting states.     

Calculation of the density of states of NbNx by the recursion method

The density of states, n(E), of NbN_x has been calculated by means of the Recursion Method for x ? 1. The results have been compared with experiment, obtaining that the most important trends of the density of states are reproduced fairly well, even if the variation of the density of states at the Fermi level, as a function of the concentration of N-impurities cannot be predicted.     

Sc2O3电子结构和光学性质的第一性原理研究

本文基于第一性原理平面波赝势(PWP)和广义梯度近似(GGA)方法,研究了Sc2O3的电子结构、态密度和光学性质. 计算结果表明：Sc2O3是一种直接带隙半导体,其能带宽度为3.79eV,价带顶部主要由O的2p和Sc的3p3d杂化而成,导带主要由Sc的3d和O的2p构成. 同时,文中也分析了Sc2O3的介电函数、折射率、光电导率和吸收谱等光学性质. 计算得到静态介电常数 ,折射率n0=1.25,在紫外区有较大的吸收系数.     

Electronic properties of one-dimensional quasiperiodic lattices: Green's function renormalization group approach

Using the inflation-deflation symmetry, we have developed a new real-space decimation approach to study the electronic properties of one-dimensional quasiperiodic lattices. The key result is the construction of a compact renormalization group that allows simple calculation of the average Green's function and the average density of states to any degree. The Fibonacci and the generalized Fibonacci lattices are used to demonstrate the method. Numerical results for the average density of states of these lattices show a good agreement with the results obtained by other methods. This confirms the validity and the efficiency of the approach.     

Field induced interlayer interband coupled states in aperiodic InAs/GaSb heterostructures

The interlayer interband state coupling and the interfacial composition effect in aperiodic InAs/GaSb (001) heterostructures are studied with the scattering theoretic Green's function technique, which can handle interlayer multi-subband interaction under the external bias. The current density calculation shows that the interlayer interband coupled subbands enhance the peak current density by facilitating electron resonant tunneling. The calculated spectral local density of states of a heterostructure predicts that the GaAs interface shifts the energies of the quasibound states to lower energies than those of the InSb interface case, which agrees with experimental results.     

Spatial structure of an incompressible quantum Hall strip

The incompressible quantum Hall strip is sensitive to charging of localized states in the cyclotron gap. We study the effect of localized states by a density functional approach and find electron density and the strip width as a function of the density of states in the gap. Another important effect is electron exchange. By using a model density functional which accounts for negative compressibility of the QH state, we find electron density around the strip. At large exchange, the density profile becomes nonmonotonic. Both effects, localized states and exchange, lead to a substantial increase of the strip width.     

On the problem of zero-bias anomalies in the tunneling spectroscopy of superconductors

Taking into account nonmagnetic energy-loss processes in the barrier as a model for zero-bias anomalies, we derive an expression for the derivative of the tunneling current with respect to the applied voltage as a function of the superconducting density of states in a superconductor-normal metal junction. It is assumed that boson-like excitations are created in the barrier region. The investigations show, that the results of elastic tunneling calculations have to be corrected by a term proportional to an integral over the superconducting density of states.     

Point impurities remove degeneracy of the Landau levels in a two-dimensional electron gas

The density of states of a two-dimensional electron gas in a magnetic field has been studied taking into account the scattering on point impurities. It is demonstrated that allowance for the electron-impurity interaction completely removes degeneracy of the Landau levels even for a small volume density of these point defects. The density of states is calculated in a self-consistent approximation taking into account all diagrams without intersections of the impurity lines. The electron density of states ρ is determined by the contribution from a cut of the one-particle Green’s function rather than from a pole. In a wide range of the electron energies ω (measured from each Landau level), the value of ρ(ω) is inversely proportional to the energy |ω| and proportional to the impurity concentration. The obtained results are applicable to various two-dimensional electron systems such as inversion layers, heterostructures, and electrons on the surface of liquid helium.     

基于双激发态对稠密等离子体中双电子复合速率系数的研究

讨论了稠密等离子体中双电子复合速率系数的计算方法,推导出了在双激发态间跃迁过程和关于双激发态的碰撞电离和自电离过程的影响下双电子复合速率系数作为关于电子密度函数的计算公式,并以类氖镍离子为例进行了计算.计算结果展示了双电子复合速率系数随电子密度增大的具体变化趋势.此外,还给出了在不同原子过程影响下双电子复合速率系数的数据,并进行了分析.     

Local density of states of a finite-sized rectangular-lattice photonic crystal with separable profile of permittivity

A different approach in the calculation of two-dimensional local density of states has been presented for a two-dimensional finite rectangular-lattice photonic crystal with a separable profile of permittivity. Approximate staircase structures are already shown to be useful for their ability to reproduce actual properties of practical square lattice photonic crystals. Using the effective resonance approach in a Fabry–Perot resonator and transfer matrix method an analytical expression for calculating a two-dimensional local density of states can be derived for both polarisations in the structure. It is shown that for this geometry one can resolve the modes as a product of two separate functions each being a function of x- and y-coordinates. The results have been investigated for the two-dimensional local density of states in the ordered finite structure and in the disordered structure. The main advantage of this method in calculating the local density of states is its extremely efficient numerical evaluations. As an application we introduce a cavity-like defect in the vicinity of a waveguide-like defect and examine the variation of local density of states in the plane of the structure at band gap frequency.     

An alternative approach to calculate the density of states in nonextensive statistical mechanics

A relation between the generalized partition function (Tsallis) and density of states is established by using the method of integral transform which enables reducing some integral equations into the algebraic equations. Inverse Mellin transformation of this equation gives the density of states. Similar relation is also hold the for standard partition function (Boltzmann-Gibbs) and the density of states. Using these relations, we recover the density of states for the classical ideal gas within both statistics.     

第一性原理研究氧在Ni(111)表面上的吸附能及功函数

采用基于密度泛函理论（DFT）广义梯度近似（GGA）下的第一性原理方法系统地研究了不同覆盖度下O在Ni（111）表面的吸附特性.计算结果表明,O在Ni（111）表面的稳定吸附位为三重面心立方（fcc）洞位,吸附能随着覆盖度的增加而减小,O诱导Ni（111）表面功函数的变化量与覆盖度成近线性关系,并随着覆盖度的增加而增大.同时,通过对电子密度和分波态密度的分析发现：O在Ni（111）表面的吸附使得Ni表面电子向O原子转移,形成表面偶极矩,导致功函数增加;表面Ni原子的3d轨道和O的2p轨道通过耦合、杂化作用形成成键态和反键态,而反键态几乎不被占据,因而O—Ni键相互作用比较强,吸附能较大. 关键词： 表面吸附 密度泛函理论 吸附能 功函数     

立方型Ti-B-N的光学性质的计算

立方型TiBN材料的光学性质对于深刻理解TiBN薄膜材料的光学性质,以及监测和控制TiBN薄膜材料的生长过程起着重要作用。使用基于密度泛函理论的平面波超软赝势方法计算了立方型TiBN的电子结构和光学性质,给出了电子态密度、介电常数、吸收系数、反射力、折射系数、消光系数等计算结果,并对计算结果作了分析。态密度图显示,存在源于B的2p态的态密度峰,分析认为这对立方型TiBN的光学吸收造成了明显影响。同时还计算了立方型TiBN的色度坐标,并根据计算结果对工艺参量与TiBN薄膜材料色度坐标之间的联系作出了分析。     

Theory of the density of states of pure type-II superconductors in high magnetic fields

This paper deals with the theory of the density of states of pure type-II superconductors in high magnetic fields. An approximate expression of theGreen's function is derived by using the fact that it has the periodicity of the lattice of flux lines in the center-of-mass coordinates. In comparison to the normal stateGreen's function a correction term proportional to the absolute square of the order parameter arises. This is seen to act as a self-energy part depending strongly on the polar angleΘ of the “quasiparticle” momentum relative to the direction of the field. The corresponding density of states, as a function of the excitation energy, is found to vary from the gapless to the BCS type as the parameterΘ changes fromπ/2 to zero (orπ). The averaged state density varies gradually from a uniform to a BCS-like state density as the external field decreases sufficiently far below the upper critical field. The results of the present theory for the averaged density of states agree fairly well with the results due to a conjecture byMaki if the field is very close to the upper critical field.