Nb二维原子薄片中的Jahn-Teller效应

使用基于密度泛函理论的第一性原理平面波赝势法，研究了Nb二维单层原子薄片的结构稳定性和电子结构性质.对其所有的二维晶格结构的计算表明，由于Jahn-Teller效应，对称性较高的正方和六角晶格都是不稳定的二维结构.而稳定的二维结构是由对称性较高的六角晶格畸变后形成的对称性较差的斜方和中心长方结构.Nb单层原子薄片不能形成长方晶格结构.通过计算电子结构和Jahn-Teller效应，进一步讨论了这些结构的相对稳定性以及各二维晶格结构的电子能带和态密度等性质. 关键词： Nb原子薄片 Jahn-Teller效应 电子结构 从头计算     

ZnMgO/ZnO异质结构中二维电子气的研究

论文根据ZnMgO/ZnO异质结构二维电子气的能带结构及相关理论模型, 采用一维Poisson-Schrodinger方程的自洽求解, 模拟计算了ZnMgO/ZnO异质结构中二维电子气的分布及其对ZnMgO势垒层厚度及Mg组分的依赖关系. 研究发现该异质结构中ZnMgO势垒层厚度存在一最小临界值: 当垒层厚度小于该临界值时, 二维电子气消失, 当垒层厚度大于该临界值时, 其二维电子气密度随着该垒层厚度的增加而增大; 同时研究发现ZnMgO势垒层中Mg组分的增加将显著增强其二维电子气的行为, 导致二维电子气密度的明显增大; 论文对模拟计算获得的结果与相关文献报道的实验结果进行了比较, 并从极化效应和能带结构的角度进行了分析和讨论, 给出了合理的解释. 关键词： 氧化锌 二维电子气 异质结构 理论计算     

一种非轴对称磁绝缘电子鞘层边界的计算方法

长距离磁绝缘传输线内电极偏心、感应腔注入电流非均匀分布引起电子鞘层边界偏心等非对称磁绝缘特性.电子鞘层边界是研究非轴对称磁绝缘特性的重要参数.本文提出一种计算非轴对称磁绝缘电子鞘层边界的方法.通过引入角向非均匀分布的模数,将经典一维轴对称Creedon稳态磁绝缘理论推广应用于圆柱坐标系下二维(r,θ)平面.建立了感应电压叠加器次级非轴对称磁绝缘的二维Creedon物理模型,给出了非轴对称磁绝缘电子鞘层边界的数值计算方法和计算误差.当阴极角向磁场(阴极电流)角向分布满足余弦函数时,电子鞘层边界接近高斯分布.阴极电流角向不均匀程度越大,电子鞘层边界偏心程度越严重,计算误差越大.     

背势垒层结构对AlGaN/GaN双异质结载流子分布特性的影响

首先通过一维自洽求解薛定谔/泊松方程,研究了AlGaN/GaN双异质结构中AlGaN背势垒层Al组分和厚度对载流子分布特性的影响.其次利用低压MOCVD方法在蓝宝石衬底上生长出具有不同背势垒层的AlGaN/GaN双异质结构材料,通过汞探针CV测试验证了理论计算的正确性.理论计算和实验结果均表明,随着背势垒层Al组分的提高和厚度的增加,主沟道中的二维电子气面密度逐渐减小,寄生沟道的二维电子气密度逐渐增加;背势垒层Al组分的提高和厚度的增加能有效的增强主沟道的二维电子气限域性,但是却带来了较高的 关键词： AlGaN/GaN 双异质结构 限域性 寄生沟道     

姜泰勒扭曲和轨道序诱导的二维铁磁性

随着体系维度的降低,材料内部的量子限制效应和电子关联作用会相应地增强,往往可以衍生一些新奇的物理特性.在钙钛矿材料中,姜泰勒扭曲和轨道序通常会引起丰富的电子关联行为.本文通过第一性原理计算、对称性分析和蒙特卡罗模拟等方法,对比研究了钙钛矿氟化物KCuF₃及其单层结构,揭示了钙钛矿二维化引起的晶格动力学、结构、电子及磁性质等方面的变化.结果表明,KCuF₃块体中出现的协作姜泰勒扭曲和面内交错轨道序可以维持到单层极限,导致单层为二维铁磁绝缘体.与块体相不同的是,在单层中姜泰勒扭曲模式作为原型相的软模出现,且单层的绝缘性不依赖于姜泰勒扭曲的出现,而是与电子关联效应的增强有关.本文为研究二维钙钛矿材料以及设计基于钙钛矿的二维铁磁体提供了指导和借鉴.     

高温AlN插入层对AlGaN/GaN异质结材料和HEMTs器件电学特性的影响

研究了在GaN缓冲层中插入40 nm厚高温AlN层的GaN外延层和AlGaN/GaN异质结材料, AlN插入层可以增加GaN层的面内压应力并提高AlGaN/GaN高电子迁移率晶体管（HEMTs）的电学特性. 在精确测量布拉格衍射角的基础上定量计算了压应力的大小. 增加的压应力一方面通过增强GaN层的压电极化电场, 提高了AlGaN/GaN异质结二维电子气（2DEG）面密度, 另一方面使AlGaN势垒层对2DEG面密度产生的两方面影响相互抵消. 同时, 这种AlN插入层的采用降低了GaN与AlGaN层之间的 关键词： 高温AlN插入层 AlGaN/GaN异质结 二维电子气 应力     

二维原子层谷电子学材料和器件

人为操控电子的内禀自由度是现代电子器件的核心和关键.如今电子的电荷和自旋自由度已经被广泛地应用于逻辑计算与信息存储.以二维过渡金属硫属化合物为代表的二维原子层材料由于其具有独特的谷自由度和优异的物理性质,成为了新型谷电子学器件研究的优选材料体系.本文介绍了能谷的基本概念、谷材料的基本物理性质、谷效应的调控和谷电子学器件...     

双电子体系的简单自洽场计算

给出了双电子原子或离子基态的一种简单自洽场计算方法，其计算过程清晰地展示了自洽场方法的基本特征，因而有助于理解此类方法。     

AlGaN/GaN异质结构中极化与势垒层掺杂对二维电子气的影响

从Ⅲ族氮化物中压电极化对应变弛豫度的依赖关系出发，通过自洽求解薛定谔方程和泊松方程,分别研究了自发极化、压电极化和AlGaN势垒层掺杂对AlxGa1-xN/GaN异质结构二维电子气的浓度、分布、面密度以及子带分布等性质的影响.结果表明：二维电子气性质强烈依赖于极化效应,不考虑AlGaN势垒层掺杂，当Al组分为0.3时，由极化导致的二维电子气浓度达1.6×10--13cm-2,其中压电极化对二维电子气贡献为0.7×10-13cm-2，略小于自发极化的贡献(0.9×10-13cm-2)，但为同一数量级，因而通过控制AlGaN层应变而改变极化对于提高二维电子气浓度至关重要. AlGaN势垒层掺杂对二维电子气的影响较弱, 当掺杂浓度从1×10-17增加到1×10-18cm-3时，二维电子气面密度增加0.2×10-13cm-2. 关键词： AlxGa1-xN/GaN 异质结构 二维电子气 自发极化 压电极化     

估算等离子体效应对类氢离子束缚态能级能量影响的新公式

在均匀电子气模型下,通过求解Dirac方程,计算了类氢离子束缚态能级能量与等离子体密度的关系,得到了能近似估算能级能量随等离子体密度变化的新拟合公式,该公式同样可以用以估算不同束缚态能级发生压致电离时的临界电子密度.通过与自洽场离子球模型计算结果的比较,作为一种简单和快捷的计算方法,均匀电子汽模型在近似计算束缚态能级能量及压致电离临界电子密度方面是自洽场离子球模型得很好近似方法.     

On the exchange repulsion between beryllium atoms

Assuming the wave functions for free atoms in the form of the closed-shell SCF determinants, the first-order interaction energy for a system of three ground-state beryllium atoms has been computed. The decomposition of two and three-body interaction energies into individual, intershell contributions has been proposed. The results show that only the electrostatic energy is well approximated by the interaction of outer shells. For the two and three-body exchange energies this approximation is reliable only in the region of small orbital overlap. It has also been found that the three-body contribution to the interaction energy is considerably greater than in the case of the interaction of helium atoms.     

闭壳层原子相关效应的多体理论计算：Ne原子基态相关能

对coupled-pair many-electron theory(CPMET)展开中的非线性四体效应采取了近似处理,用角动量图技术和两体函数方法得到了适用于一般闭壳层原子的径向微分方程。然后以X_a模型作为零级近似,通过迭代求解成功地计算了Ne原子基态的任意阶相相关能,从而说明至少对于闭壳层原子体系而言,用X_a模型作零级近似同样可以进行成功的多体理论计算。     

Generalized Brillouin theorem and generalized forces in SCF and MC SCF theories and their Hartree-Fock limits

A formula for the change of energy is derived, valid for any infinitesimal change of the hamiltonian and the wave function in CI, SCF and MC SCF theories. A general form of MC Hartree-Fock equations is derived in a somewhat new form. A closed MC theory is defined and a condition of self-consistency is derived for this theory. The role of deorthogonalization of spin orbitals is discussed. The Hellmann-Feynman theorem and the generalized Brillouin theorem are rederived in a unique and simple way. It is shown that in the case of the closed MC SCF theory the Brillouin theorem derived by Levy and Berthier can be strengthened.

Formulae for generalized forces are derived, valid for closed-shell, openshell and multiconfigurational SCF theories also far from the Hartree-Fock limit.     

Study of the flavour violating (μ,e) conversion in nuclei

The muon number violating (μ⁻, e⁻) conversion in the presence of nuclei is investigated. The effective operators resulting from various mechanisms, both photonic and non-photonic, are expressed in terms of nuclear coordinates: (i) for intermediate neutrino models, and (ii) for supersymmetric theories. Using these operators the (A, Z) dependence of the corresponding widths is discussed throughout the periodic table. For the coherent process the dependence of the conversion rate on the nuclear parameters is given in terms of the elastic nuclear form factors, which are obtained in the context of shell model, taking into account finite nucleon-size effects. For the inclusive μ−e conversion process the total rates are calculated for closed-shell nuclei. The method of calculation utilizes microscopic non-energy-weighted sum rules (closure). The isospindependent part of the total (μ⁻, e⁻) rate is also related to that of the total (μ⁻, _vμ capture rate, the latter being calculated with Primakoff's phenomenological method. The agreement between the two provides a good test of the reliability of the shell-model closure calculations. For non-closed-shell nuclei the relevant rates can be obtained by interpolation techniques from the functions obtained in the present work for closed-shell nuclei. Finally the experimentally interesting ratio of the coherent to the total μ−e conversion rate is discussed.     

An E.S.R. study of the structure of phosphorus centred radicals in solution

Clementi and Veillard's analysis of the MC SCF theory of closed-shell systems is developed on a different basis. An equivalent Hartree-Fock operator is defined and its physical significance is discussed. A generalized Brillouin theorem is derived, different from that given by Levy and Berthier.     

On the MC SCF theory of closed-shell systems

A new formulation of the two-by-two rotation method valid for the MC SCF theory of closed-shell systems is given. The method is applied to H₂, LiH, BeH₂, CH₄, H₂O and HF.     

Polarizabilities of heavy closed-shell atoms from the relativistic local density approximation

Static polarizabilities of various closed-shell atoms are calculated using linear response theory in the relativistic local density approximation, with special attention to atoms of high nuclear charge. For atoms with closed outer np subshells the resulting values are found to be in good agreement with other determinations. For atoms with closed outer ns subshells the present values are considerably smaller than those from relativistic random-phase approximation calculations and in better agreement with recommended values.     

Ab initio intermolecular potentials of methane,nitrogen and methane + nitrogen and their use in Monte Carlo simulations of fluids and fluid mixtures

Intermolecular pair potentials of methane and of methane + nitrogen have been calculated by quantum chemical ab initio methods. The repulsive and electrostatic parts were determined pointwise for various distances and orientations of the dimers (supermolecule approach) by self-consistent field (SCF) calculations including the counterpoise correction. Gaussian basis functions of triple-zeta quality plus one set of polarization functions on all atoms were used. The dispersion energy, which cannot be calculated at the SCF level, has been added by a semi-empirical estimate. These potentials and the pair potential for nitrogen (of similar quality, taken from the literature) were fitted to analytical functions and used for NVT Monte Carlo simulations of thermodynamic properties of the fluids and their mixture over a wide temperature/density area. Comparison with measurements and with Monte Carlo results from the literature (pressure, internal energy, radial distribution function) obtained with other pair potentials indicates the quality of the present calculations.     

A general pseudopotential model for molecules with many valence electrons

Criteria are discussed for the practical application of ‘valence-electron only’ calculations aimed at simulating ab initio all-electron calculations on molecules containing heavy atoms with many core electrons. A theoretical model is outlined based on the use of pseudopotentials derived from atomic SCF calculations. The model has been used in calculations for NaH, Na₂, H₂S, S₂, H₂S₂, HCl, Cl₂ and NaCl, and has been tested by comparison with reference all-electron calculations. Most of these tests employ double-zeta basis sets and SCF wavefunctions, although polarization functions and complete valence-electron CI wavefunctions have been used in some cases. The model is generally successful for ground-state equilibrium structures, charge distributions and orbital energies.     

On the contribution of triply-excited and quadruply-excited configurations to electron correlation energies in closed-shell systems

The contribution made by triply-excited and quadruply-excited configurations to electron correlation energies in closed-shell systems is examined in detail. Many-body perturbation theory is used to calculate the contribution of these configurations to the correlation energies in 25 atoms and molecules. Calculations are performed for the nitrogen, carbon monoxide and carbon monosulphide molecules for a range of nuclear geometries.