Model studies of the Tamm-like and field-sustained surface states of germanium

The effects of varying the intensity of an applied positive electric field on the Tamm-like and field-sustained surface states of a semi-infinite 6-well model of germanium are studied. The effects of a neon image gas atom on such states are also examined. It is found that there are ranges of the field intensity for which field-sustained surface states do not exist. The localization properties of electrons in the various surface states are discussed with the help of relative probability density functions. As is well-known, electrons in Tamm-like states in the first forbidden energy gap tend to be strongly localized near the surface and inside the crystal. However those in Tamm-like states in higher gaps do not exhibit any pronounced localization. Electrons in field-sustained surface states tend to be strongly localized outside the crystal.     

Widths of the Hall Conductance Plateaus

We study the charge transport of the noninteracting electron gas in a two-dimensional quantum Hall system with Anderson-type impurities at zero temperature. We prove that there exist localized states of the bulk order in the disordered-broadened Landau bands whose energies are smaller than a certain value determined by the strength of the uniform magnetic field. We also prove that, when the Fermi level lies in the localization regime, the Hall conductance is quantized to the desired integer and shows the plateau of the bulk order for varying the filling factor of the electrons rather than the Fermi level.     

Interface states on semiconductor/insulator surfaces

It is well known that impurities and defects in semiconductors are associated with energy levels in the forbidden gap. Similar states occur at the surface of a semiconductor where the crystal lattice and the symmetry are strongly disturbed. These states are called surface states. Owing to the two-dimensional nature of the surface, their density is measured per unit area, in contrast to bulk states, which are measured per unit volume. A third type of states, similar to surface states, occurs at the interface between two adjacent materials. These states are called interface states. Very often they are also simply called surface states.     

Diagnostics of many-particle electronic states: Non-stationary currents and residual charge dynamics

We propose the method for identifying many particle electronic states in the system of coupled quantum dots (impurities) with Coulomb correlations. We demonstrate that different electronic states can be distinguished by the complex analysis of localized charge dynamics and non-stationary characteristics. We show that localized charge time evolution strongly depends on the properties of initial state and analyze different time scales in charge kinetics for initially prepared singlet and triplet states. We reveal the conditions for existence of charge trapping effects governed by the selection rules for electron transitions between the states with different occupation numbers.     

Quasiparticle conductivities in disordered d-wave superconductors

We study the quasiparticle transport coefficients in disordered d-wave superconductors. We find that spin and charge excitations are generally localized unless magnetic impurities are present. If the system is close to a nesting point in the impurity-scattering unitary limit, the tendency towards localization is reduced while the quasiparticle density of states gets enhanced by disorder. We also show that the residual repulsive interaction among quasiparticles has a delocalizing effect and increases the density of states.     

Temperature dependence of surface magnetization in local-moment systems

We present a theory to study the temperature-dependent behavior of surface magnetization in a ferromagnetic semi-infinite crystal. Our approach is based on the single-site approximation for the s-f model. The effect of the semi-infinite nature of the crystal is taken into account by a localized perturbation method. Using the mean-field theory for the layer-dependent magnetization, the local density of states and the electron-spin polarization are investigated at different temperatures for ordinary and surface transition cases. The results show that the surface magnetic properties may differ strongly from those in the bulk and the coupling constant of atoms plays a decisive role in the degree of spin polarization. In particular, for the case in which the exchange coupling constant on the surface and between atoms in the first and second layer is higher than the corresponding in the bulk, an enhancement of surface Curie temperature and hence the spin polarization can be obtained.     

Electronic vacancy states in silicon by the chemical pseudopotential method

The electronic states of a neutral vacancy in Si are studied through the chemical pseudopotential method by creating a vacancy in a large crystal unit cell containing up to 54 atoms. A localized vacancy state is found in the forbidden gap and its energy is shown to be convergent with respect to the size of the cell. The density of states of the valence band is modified by the presence of the vacancy with additional peaks which give charge localization on the vacany nearest neighbour atoms.     

Topological response theory of doped topological insulators

We generalize the topological response theory of three-dimensional topological insulators (TI) to metallic systems-specifically, doped TI with finite bulk carrier density and a time-reversal symmetry breaking field near the surface. We show that there is an inhomogeneity-induced Berry phase contribution to the surface Hall conductivity that is completely determined by the occupied states and is independent of other details such as band dispersion and impurities. In the limit of zero bulk carrier density, this intrinsic surface Hall conductivity reduces to the half-integer quantized surface Hall conductivity of TI. Based on our theory we predict the behavior of the surface Hall conductivity for a doped topological insulator with a top gate, which can be directly compared with experiments.     

Relaxor properties and the mechanism of conduction in TlInS<Subscript>2</Subscript> crystals exposed to gamma irradiation

The density of states at the energy levels associated with radiation-induced defects, the localization length of a defect center, and the hopping distance of charge carriers are determined in a TlInS₂ crystal. It is demonstrated that, by varying the dose of gamma irradiation, it is possible to control the dielectric properties of ferroelectrics and to attain a stable relaxor state. In the temperature range of existence of this state, charge carriers execute tunneling from electron levels in the band gap through potential barriers created by an incommensurate superstructure of the TlInS₂ crystal.     

Correlation induced switching of the local spatial charge distribution in a two-level system

It was found that tunneling current through a nanometer scale structure with strongly coupled localized states causes spatial redistribution of localized charges induced by Coulomb correlations. We present here theoretical investigation of this effect by means of Heisenberg equations for localized states electron filling numbers. This method makes it possible to take into account pair correlations of local electron density exactly. It is shown that inverse occupation of the two-level system caused by Coulomb correlations appears in particular range of applied bias. Described effects can give a possibility of charge manipulation in the proposed system. We also expect that described results can be observed in tunneling structures with impurities or with small quantum dots.     

表面氧钝化碳化硅纳米团簇电子结构和光学性质的第一性原理研究

基于密度泛函理论(DFT)和广义梯度近似(GGA),对氧钝化条件下4H-SiC纳米团簇的电子结构和光学性质进行了研究。计算了不同直径的4H-SiC纳米球氧钝化后的能带结构、电子态密度和光学性质。团簇的尺度在0.4~0.9 nm之间,构建表面仅存在硅氧双键和表面仅存在碳氧双键的两种模型。研究表明硅氧双键和碳氧双键所引起的缺陷态位于原4H-SiC的价带和导带之间,并且缺陷态与价带顶的能量差随纳米团簇颗粒直径的增大而减小;缺陷态主要是由Si原子外层电子和氧原子外层电子轨道杂化引起的。同时,由于氧的存在,对碳化硅的结构产生一定的影响,这也是缺陷态形成的一个原因。另外,碳氧双键和硅氧双键钝化对4H-SiC纳米团簇的光学性质有着不同的影响。在表面仅存在C=O的情况下,4H-SiC纳米团簇表现出各向同性的性质。在表面仅存在Si=O的情况下,4H-SiC纳米团簇表现出各向异性的性质。     

Quantum-size states of the deformed nanosphere

The effect of shape deformation of a spherical nano-object on quantum states of a particle in it has been analyzed theoretically. For this purpose, a functional method of surface shape perturbation theory has been proposed. The method allows us to determine the effect of a wide range of deviations of a limited three-dimensional volume (nano-object) from the initial shape on the quantum characteristics of a charge localized inside the nano-object. An analysis has demonstrated that the probability density for a particle in quantum-size states is more sensitive to surface shape perturbations than the energy spectrum and the density of quantum states. Even small shape perturbations lead to a smearing of the probability density for the location of a particle inside a nano-object and lift the degeneracy in the magnetic quantum number. A mixing of pure states corresponding to a sphere has been observed. The degree of mixing and the splitting of the energy levels increase with increasing quantum numbers l and m.     

Electronic surface states in cleaved transition metals

The local densities of states (LDS) on clean low-index surfaces of fcc Ni and bcc Fe are presented. They are calculated within the tight-binding approximation using a moment method and a continued fraction analysis. The d band degeneracy and the charge rearrangement occurring near the surface when cleaving the crystal are fully taken into account by computing exactly a large number of moments (22–26). The surface LDS are found to be strongly dependent on the cleavage plane. Indeed, whereas the LDS on dense planes are rather bulk like, sharp peaks arising from states localized near the surface are obtained near the atomic level of the metal on non-dense planes of cleavage. In addition, the bandwidth is broadened by surface states for Ni. On the contrary, for bcc Fe the surface potential is not strong enough to modify the bandwidth. A comparison is made with spectroscopic observations.The method lends itself to other interesting applications in the surface physics field and, as an example, we present the results of calculations of the spatial distribution of the electronic charge near the surface and its deviation from spherical symmetry. The charge asphericity is slightly increased when going from a bulk to a surface site as the difference between the occupations of two surface orbitals can reach 0.34 ē instead of 0.15 ?c for a bulk site. Diagrams are shown for (100) and (110) cleaved Ni to illustrate the results. The consequences upon preferred adsorption sites according to the electronic affinity and size of the adsorbate are discussed.     

Bound states of 3He at the edge of a 4He drop on a cesium surface

We show that small amounts of 3He atoms, added to a 4He drop deposited on a flat cesium surface at zero temperature, populate bound states localized at the contact line. These edge states show up for drops large enough to develop well defined surface and bulk regions together with a contact line, and they are structurally different from the well-known Andreev states that appear at the free surface and at the liquid-solid interface of films. We illustrate the one-body density of 3He in a drop with 1000 4He atoms, and show that for a sufficiently large number of impurities the density profiles spread beyond the edge, coating both the curved drop surface and its flat base and eventually isolating it from the substrate.     

FeS2(100)表面原子几何与电子结构的理论研究

采用密度泛函理论研究了FeS2(100)表面原子几何与电子结构.理论计算结果表明:FeS2(100)表面无弛豫、无重构,是体相原子几何的自然终止.与体相电子结构相比,FeS2(100)表面电子特性明显不同,禁带中央产生新的表面态,且表面态局域性强,主要由Fe原子的3d分波贡献.配位场理论定性分析表明:FeS2(100)完整晶面表面态产生机制是Fe原子的配位数减少、局部对称性下降所致 关键词： 密度泛函理论 表面电子结构 FeS2     

Mobility edges and reentrant localization in one-dimensional dimerized non-Hermitian quasiperiodic lattice

The mobility edges and reentrant localization transitions are studied in one-dimensional dimerized lattice with nonHermitian either uniform or staggered quasiperiodic potentials.We find that the non-Hermitian uniform quasiperiodic disorder can induce an intermediate phase where the extended states coexist with the localized ones,which implies that the system has mobility edges.The localization transition is accompanied by the PT symmetry breaking transition.While if the non-Hermitian quasiperiodic disorder is staggered,we demonstrate the existence of multiple intermediate phases and multiple reentrant localization transitions based on the finite size scaling analysis.Interestingly,some already localized states will become extended states and can also be localized again for certain non-Hermitian parameters.The reentrant localization transitions are associated with the intermediate phases hosting mobility edges.Besides,we also find that the non-Hermiticity can break the reentrant localization transition where only one intermediate phase survives.More detailed information about the mobility edges and reentrant localization transitions are presented by analyzing the eigenenergy spectrum,inverse participation ratio,and normalized participation ratio.     

Doping in silicon nanocrystals: An ab initio study of the structural, electronic and optical properties

There are experimental evidences that doping control at the nanoscale can significantly modify the optical properties with respect to the pure systems. This is the case of silicon nanocrystals (Si-nc), for which it has been shown that the photoluminescence (PL) peak can be tuned also below the bulk Si band gap by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. In this work, we report on an ab initio study of impurity states in Si-nc. We consider B and P substitutional impurities for Si-nc with a diameter up to 2.2 nm. Formation energies (FEs), electronic, optical and structural properties have been determined as a function of the cluster dimension. For both B-doped and P-doped Si-nc the FE increases on decreasing the dimension, showing that the substitutional doping gets progressively more difficult for the smaller nanocrystals. Moreover, subsurface impurity positions result to be the most stable ones. The codoping reduces the FE strongly favoring this process with respect to the simple n-doping or p-doping. Such an effect can be attributed to charge compensation between the donor and the acceptor atoms. Moreover, smaller structural deformations, with respect to n-doped and p-doped cases, localized only around the impurity sites are observed. The band gap and the optical threshold are largely reduced with respect to the undoped Si-nc showing the possibility of an impurity-based engineering of the Si-nc PL properties.     

Analytic conditions for the existence of localized electron states in linear chains with two impurities

The negative eigenvalue theorem is used to obtain an analytic result for the integrated density of electron states in several types of low-dimensional lattices with two impurities in the tight binding approximation. From this expression, analytic conditions for the existence of localized electronic states are obtained. The result is directly applicable to the problem of surface states and that of chemisorption on a linear chain.The author would like to thank Prof. Dr. L.Valenta for suggesting the problem and for constant encouragement.     

Metal-to-insulator transition and electron-hole puddle formation in disordered graphene nanoribbons

The experimentally observed metal-to-insulator transition in hydrogenated graphene is numerically confirmed for actual sized graphene samples and realistic impurity concentrations. The eigenstates of our tight-binding model with substitutional disorder corroborate the formation of electron-hole?puddles with characteristic length scales comparable to the ones found in experiments. The puddles cause charge inhomogeneities and tend to suppress Anderson localization. Even though, monitoring the charge carrier quantum dynamics and performing a finite-size scaling of the local density of states distribution, we find strong evidence for the existence of localized states in graphene nanoribbons with short-range but also correlated long-range disorder.     

Localized states at zigzag edges of bilayer graphene

We report the existence of zero-energy surface states localized at zigzag edges of bilayer graphene. Working within the tight-binding approximation we derive the analytic solution for the wave functions of these peculiar surface states. It is shown that zero-energy edge states in bilayer graphene can be divided into two families: (i) states living only on a single plane, equivalent to surface states in monolayer graphene and (ii) states with a finite amplitude over the two layers, with an enhanced penetration into the bulk. The bulk and surface (edge) electronic structure of bilayer graphene nanoribbons is also studied, both in the absence and in the presence of a bias voltage between planes.