一种由自由电子能带模型计算费米能级的方法

对面心立方（fcc)、体心立方（bcc)和六角密堆积（hcp)三种不同结构的晶体,在假设它们的原胞中包含8个价电子并将价电子近似为自由电子的情况下,采用“自由电子气理论”和“自由电子能带模型”,研究其根据费米球确定的费米能级EF与根据自由电子能带模型计算的平均键能Em。研究结果表明,由自由电子能带模型计算所得3种不同结构晶体（因而电子密度也不一样）的平均键能Em等于各自自由电子系统的费米能级EF。平均键能Em是我们在异质结带阶理论计算中建议的一种参考能级,研究结果在深化对平均键能Em物理实质认识的同时,提供了一种借助于自由电子能带模型计算自由电子系统费米能级EF的新方法。     

Electronic structure of rectangular HgTe quantum dots

We theoretically investigate the single- and few-electron ground-states properties of HgTe topological insulator quantum dots with rectangular hard-wall confining potential using configuration interaction method. For the case of single electron, the edge states is robust against the deformation from a square quantum dot to a rectangular ones, in contrast to the bulk states, the energy gap of the QDs increased due to the coupling of the opposite edge states; for the case of few electrons, the electrons first fill the edge states in the bulk band gap and the addition energy exhibit universal even-odd oscillation due to the shape-independent two-fold degeneracy of the edge states. The size of this edge shell can be controlled by tuning the dot size, shape or the bulk band gap via lateral or vertical electric gating respectively of the HgTe quantum dot.     

The study of optical properties of amorphous thin films of mixed oxides In<Subscript>2</Subscript>O<Subscript>3</Subscript>—SnO<Subscript>2</Subscript> system,deposited by co-evaporation

A discussion of optical properties of mixed oxides In₂O₃—SnO₂ system is presented. Film thickness, substrate temperature, composition (in molar %) and annealing have a profound effect on the structure and optical properties of these films. Initially the increase in band gap with the increase of SnO₂ content in In₂O₃ is due to the increase in carrier density as a result of donor electrons from tin. The decrease in band gap above the critical Sn content is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in band gap with film thickness is caused by the increase in free carrier density which is generated by (i) Sn atom substitution of In atom, giving out one extra electron and (ii) oxygen vacancy acting as two electrons donor. The decrease in band gap with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorate the film properties and reduce the mobility of the carriers, or to the formation of indium species of lower oxidation state (In²⁺).     

Effect of Parallel-Plate Geometry on Mode Transition Behavior in Argon Microplasmas: Two-Dimensional Simulation

A two-dimensional self-consistent fluid model is employed to investigate radio-frequency process parameters on the plasma properties in Ar microdischarges. The neutral gas density and temperature balance equations are taken into account. We mainly investigate the effect of the electrode gap on the spatial distribution of the electron density and electron temperature profiles, due to a mode transition from the regime(secondary electrons emission is responsible for the significant ionization) to the regime(sheath oscillations and bulk electrons are responsible for sustaining discharge) induced by a sudden decrease of electron density and electron temperature.The pressure, radio-frequency sources frequency and voltage effects on the electron density are also elaborately investigated.     

压力下替位氢对金属锂能带结构的影响

 利用第一原理的平面波-赝势密度泛函方法,研究了零温下体心立方（bcc）金属锂由于氢的替位掺杂和外界压力的改变所引起的电子结构变化。结果表明：掺杂体系仍然呈金属性,但是由于氢原子俘获了一个金属自由电子,具有了离子属性,使得费米面以下出现了孤立能带和带隙,最低金属价带偏离了自由电子带,形成鞍型带底,其它价带和导带均出现不同程度的简并解除和带型畸变。在压力的作用下,带隙加宽,费米面下的孤立带趋向于一条直线,成为一条能级。     

Effect of CDW and magnetic interactions on the electrons of the manganite systems

We address a model study which includes the co-existence of the charge density wave (CDW) and ferromagnetic interactions in order to explain the colossal magnetoresistance (CMR) in manganites. The Hamiltonian consists of the ferromagnetic Hund's rule exchange interaction between e_g and t_2g spins, Heisenberg core spin interactions and the CDW interaction present in the e_g band electrons. The core electron magnetization, induced e_g electron magnetization and the CDW gap are calculated using Zubarev's Green's function technique and determined self-consistently. The effect of core electron magnetization and the CDW interaction on the induced magnetization as well as on the occupation number in the different spin states of the e_g band electrons are investigated by varying the model parameters of the system like the CDW coupling, the exchange coupling, the Heisenberg coupling and the external field. It is observed that the induced magnetization exhibits re-entrant behaviour and exists within a narrow temperature range just below the Curie temperature. This unusual behaviour of the e_g band electrons will throw some new insights on the physical properties of the manganite systems.     

Atomic and molecular defects in solid He

The studies of defects formed by impurity particles (atoms, molecules, exciplexes, clusters, free electrons, and positive ions) embedded in liquid and solid ⁴He are reviewed. The properties of free electrons and neutral particles in condensed helium are described by the electron (atomic) bubble model, whereas for the positive ions a snowball structure is considered. We compare the properties of the defects in condensed helium with those of metal atoms isolated in heavier rare gas matrices.     

Influence of isoelectronic impurities on the electronic structure of BN nanotubes

Via the example of a (5, 5) boron-nitrogen armchair nanotube, the influence of isoelectronic substitutional impurities on the electronic structure of BN nanotubes has been investigated with the use of linear augmented cylindrical waves. The treatment is based on the local density approximation and the muffin-tin approximation for the electron potential. In this method, the electronic spectrum of a system is governed by the free motion of electrons in the interatomic space between cylindrical barriers and the electron scattering on atomic centers. It has been found that the substitution of one atom of N by P leads to the splitting of all twofold degenerate bands by 0.2 eV on average, a decrease in the energy gap from 3.5 to 2.8 eV, the separation of the s(P) band from the high-energy region of the s(B, N) band, as well as to the formation of the impurity π(P) and π*(P) bands, which form the valence-band top and conduction-band bottom in the doped system. The influence of an As atom on the electronic structure of (5, 5) BN nanotubes is qualitatively similar to the case of phosphorus, but the energy gap is smaller by 0.5 eV. The optical gap in the nanotubes is closed due to the effect of the Sb atom impurity. A substitution of one B atom by an Al atom results in the strong perturbation of the band structure and the energy gap in this case is only 1.6 eV in contrast to the weak indium-induced perturbation of the band structure of the BN nanotube. In the latter case, the energy gap is 2.9 eV. The above effects can be detected by the optical and photoelectron spectroscopy methods, as well as by measuring the electrical properties of the nanotubes. They can be used to create electronic devices based on boron-nitrogen nanotubes.     

Closed formulas for tunneling time in superlattices

New formulas for an exact and simple evaluation of the phase time associated with the passage of electrons or photons through a finite superlattice are derived. This time, named here the superlattice-tunneling time tau(n), exhibits a resonant-band structure or a superluminal phase time behavior depending on whether the particle's energy lies in a band or a gap. In the band, tau(n) remains larger than the free motion time tau(f), while in the gap it can be less than tau(f) (with strong substrate effects), but it is larger than the single-cell phase time tau(1). Extremely good agreements with optical-pulse and superluminal delay times measured by Spielmann et al. and Steinberg et al. are found, including the superlattice-tunneling-time limit and the substrate effects. Conditions for measurements of earlier electron arrival are also analyzed.     

Surface properties of a network model of electrons in solids

Surface properties of a network model of electrons in solids are discussed. In the model, electrons are restricted to move along lines connecting node points of a periodic network. Each node point on the network corresponds to an atom. A potential well is introduced for each atom or node point so that as the depth of the potential becomes large, one has the tight binding model and, as the potential vanishes and the number of node points increases, one has the Sommerfeld free electron model. Exact wave functions and energy levels have been obtained elsewhere for the model when periodic boundary conditions are applied.When free boundaries are introduced, a band of surface states develops. Its depth of penetration into the crystal as a function of wave number is investigated. The edges of the surface energy bands are located and their density of states is calculated.     

Nonlinear Transport of Three-Dimensional Electron Gases Under the Influenceo f an Intense Terahertz Radiation

We present a detailed theoretical and numerical investigation on nonlinear transport of a model three-dimensional electron gas driven by an intense terahertz (THz) radiation at lattice temperature T = 10,77,300 K using the conventional and recently developed balanceequation approach. Ionized-impurity, acoustic-phonon and polar optical-phonon scatterings were taken into account for electrons in a single parabolic band. The heating of electrons and the suppression of the dc electron mobility by the irradiation of the intense THz field are predicted. We find that the dc average mobility of electrons peaks around a certain value of the amplitude of the ac field at low lattice temperature.     

M- (Sm, Pr, Ga) 掺杂TiO2带隙及电子结构的第一原理研究

采用密度泛函理论对M-（Sm、Pr、Ga）掺杂锐钛矿型TiO2能带和电子性质进行了系统的理论研究. 计算结果表明,通过Sm和Pr的掺杂可以降低TiO2的带隙进而使其产生吸收边红移,通过Ga的掺杂能使带隙稍增加. 这主要是由于Sm和Pr的掺杂使Sm和Pr上的4f层电子与原子相邻O原子上的2p层电子相互作用,形成的杂质能级影响了Ti-O的能带结构,从而降低带隙,提高TiO2的可见光吸收性能.     

Electronic shell and supershell structure in graphene flakes

We use a simple tight-binding (TB) model to study electronic properties of free graphene flakes. Valence electrons of triangular graphene flakes show a shell and supershell structure which follows an analytical expression derived from the solution of the wave equation for triangular cavity. However, the solution has different selection rules for triangles with armchair and zigzag edges, and roughly 40?000 atoms are needed to see clearly the first supershell oscillation. In the case of spherical flakes, the edge states of the zigzag regions dominate the shell structure which is thus sensitive to the flake diameter and center. A potential well that is made with external gates cannot have true bound states in graphene due to the zero energy band gap. However, it can cause strong resonances in the conduction band.     

On the origin of photoluminescence in heavily-doped silicon

The origin of the photoluminescence in heavily-doped silicon is examined. Transient photoluminescence data for Si(P) are presented and used to identify the “Low Level” emission bands in terms of recombination of impurity band electrons with holes bound to acceptor sites. The “High Level” bands are attributed to recombination of impurity band electrons with free holes. The energies of the band gap and optical band gap in heavily-doped silicon are determined from the photoluminescence measurements.     

掺铁铌酸锂晶体第一性原理研究

利用第一性原理研究了Mn:LiNbO3晶体的电子结构和光学性质。结果表明,Mn掺杂产生了杂质能级,主要由Mn的d态轨道贡献。杂质能级与导带之间的带隙小于理想LiNbO3晶体导带与价带之间宽度,降低了电子跃迁所需能量。同时,掺杂也降低了各原子的电子轨道能量。晶体中最高占据轨道不再是O的2p轨道,而是Mn的d 轨道。掺杂离子在晶体中同时充当电子的施主与受主。静态介电常数在掺杂后有明显的增大。Mn的掺杂使晶体在可见光区域的光学性质产生变化,并使晶体的吸收谱在可见光区域产生吸收峰。     

掺锰铌酸锂晶体第一性原理研究

利用第一性原理研究了Mn:LiNbO3晶体的电子结构和光学性质。结果表明,Mn掺杂产生了杂质能级,主要由Mn的d态轨道贡献。杂质能级与导带之间的带隙小于理想LiNbO3晶体导带与价带之间宽度,降低了电子跃迁所需能量。同时,掺杂也降低了各原子的电子轨道能量。晶体中最高占据轨道不再是O的2p轨道,而是Mn的d 轨道。掺杂离子在晶体中同时充当电子的施主与受主。静态介电常数在掺杂后有明显的增大。Mn的掺杂使晶体在可见光区域的光学性质产生变化,并使晶体的吸收谱在可见光区域产生吸收峰。     

Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U _cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U _cr , pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam. The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation. The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.     

Introduction in the band theory of molten salts

The electrochemical properties of molten salts are considered in the frame of band theory for ideal crystalline dielectrics. It turned out that metal impurities at low concentration in them generate the local electron levels in the wide band gap (>4 eV) of molten salt divided by 50 meV and more. They are occupied by electrons when Fermi level is approached to them from below. Then, the minor metal impurities fall out the molten salt when the electron occupation of their energy levels in the band gap achieves their solubility (<0.1 ppm) in the molten salt. At the same time, the given RedOx-potential of molten salt can be maintained without chemical additives but forced shifting of Fermi level by the electrochemical cell with one electrode strongly polarized and the other in equilibrium with this molten salt.     

Thermoelectric power of mixed electronic-ionic conductors II. Case of titanium dioxide

The purpose of the present work is the determination of the thermopower components corresponding to different charge carriers (electrons, electron holes and ions) for TiO₂ and the use of these data for evaluation of the effect of symmetry between these two properties. The procedure of the determination of these components was based on the following two approximations:

The first approximation is based on a symmetrical model assuming a consistency between thermopower and electrical conductivity within the n-p transition (minimum of electronic component of the electrical conductivity corresponds to zero value of the electronic component of thermopower).

The second approximation is based on the apparent asymmetry between thermopower and electrical conductivity within the n-p transition as determined from the first approximation.

The analysis, based on the data of the electronic components of thermopower and electrical conductivity for TiO₂ single crystal, results in the band gap (using the Jonker formalism). The determined band gap is equal to 2.77 eV and 2.57 eV at the first and the second approximations, respectively, while the band gap determined from the experimentally measured data is equal 3.35 eV. These values are consistent with the band gap determined from the data of electrical conductivity corresponding to the n-p transition point (E_g=3.16 eV) and for the data measured experimentally and those free of the ionic conductivity component (E_g=2.79 eV). The obtained results indicate that thermopower and electrical conductivity most likely exhibit the effect of symmetry.