Effects of electron levels broadening and electron temperature in tunnel structures based on metal nanoclusters

We study the influence of energy levels broadening and electron subsystem overheating in island electrode (cluster) on current-voltage characteristics of three-electrode structure. A calculation scheme for broadening effect in one-dimensional case is suggested. Estimation of broadening is performed for electron levels in disc-like and spherical gold clusters. Within the two-temperature model of metallic cluster and by using a size dependence of the Debye frequency the effective electron temperature as a function of bias voltage is found approximately. For helium temperature of ion subsystem, the heating temperature of electrons in a quantum disc is almost one order of magnitude higher than that in a sphere; it achieves thousands of Kelvin. We suggest that the effects of broadening and electron overheating are responsible for the smoothing of current-voltage curves, which is observed experimentally at low temperatures in structures based on clusters consisting of accountable number of atoms. However, a role of the broadening is much more significant.     

Effects of single-electron charging in a tunnel structure with a metallic cluster

The effects of single-electron tunnel charging and Coulomb blockade in a cluster structure (molecular transistor) are studied theoretically with allowance for the quantization of the electronic levels in an island electrode. The electronic spectrum is calculated for small spherical and disk-shaped clusters. Under the assumption that the total energy of the system is conserved with inclusion of the contact potential difference, equations are derived for analyzing the current-voltage characteristic. Limitations associated with the Coulomb instability of a cluster and with electron relaxation are introduced into the theory. For single-electron transistors with small gold clusters, the current gap and the asymmetry in its position on the voltage axis are calculated. The current gap is shown to vary nonmonotonically with the cluster size.     

Electrophonon resonance in narrow band semiconductors

The theory of the electric current splashes on a current-voltage characteristic due to the phonon-induced-electron-transitions between Stark ladder levels is developed. Splitting and broadening of the current peaks because of an electric field vector deviation from the [ 111 ] direction is considered. The recent experimental results on current-voltage characteristic of the tellurium cluster superlattice in ceolite are accounted for by the electrophonon resonance studied in the present paper.     

Cu原子掺杂在Al团簇中的结构和电子性质

利用密度泛函理论,对Al_n(n=1～15)团簇中掺杂Cu原子后的双金属团簇进行了研究,在结构优化的基础上,同时计算了双金属团簇的电子性质,即电子亲和能、电离势、Cu原子的Mulliken分布、平均极化率、极化率的各向异性、偶极矩及HOMO-LUMO能隙随团簇尺寸增加时的变化情况. 结果表明,Cu掺杂Al团簇的双金属团簇中也存在幻数结构,团簇的电子性质随团簇尺寸大小出现不规则的奇偶振荡变化. n=13的团簇电子亲和能和电离势与毗邻团簇相比,其变化要大于0.3和0.6 eV.     

Silicon dioxide modification by an electron beam

The overheating temperature of a microvolume of silicon dioxide produced by bombardment by a high specific-power electron beam has been estimated. Calculations showed that the maximum temperature to which a microvolume of silicon dioxide is overheated can be as high as 1200°C for an electron beam current of 100 nA. The variation in the cathodoluminescence characteristics of amorphous silica with different contents of hydroxyl groups was studied for various electron beam specific-power levels. The impact of a high specific-power electron beam was shown to create additional lattice defects up to the formation of silicon clusters.     

Observation of electron localization in rough gold nanoclusters on the graphite surface

The results of scanning tunneling spectroscopy of the electronic states of Au nanoclusters on the graphite surface are presented. The tunneling current is found to be different at different points of a rough-surface nanocluster. The measured differential current-voltage curve of the clusters is nonmonotonic near the Fermi energy, and the tunneling conductance decreases by almost a factor of two as the cluster volume changes from 1 to 0.1 nm³. This decrease can be associated with the change in the density of the electronic states near the Fermi energy. The observed features are qualitatively described within the framework of the mechanism of electron localization in disordered systems.     

Experimental investigation of emission inhomogeneity of gyrotron cathodes basing on their current-voltage characteristics

A scheme for automatic measurement of current-voltage characteristic of gyroresonance devices has been developed. Efficient emission inhomogeneity of cathode in powerful gyrotrons has been studied basing on analysis of current-voltage characteristics. Experimental data on the parameter of efficient emission inhomogeneity for different regimes are presented, as well as the estimate for admissible efficient inhomogeneity of the cathode for powerful gyrotrons and the dependence of electron beam parameters on efficient inhomogeneity of the cathode. A method of complex measurements is proposed, which makes it possible to judge about the contribution of different physical mechanisms into emission inhomogeneity of the cathode.     

Transport properties of poly(GACT)-poly(CTGA) deoxyribonucleic acid: A ladder model approach

In this paper, based on the tight-binding Hamiltonian model and within the framework of a generalized Green’s function technique, the electronic conduction through the poly(GACT)-poly(CTGA) DNA molecule in SWNT/DNA/SWNT structure has been numerically investigated. In a ladder model, we consider DNA as a planar molecule containing M cells and four further sites (two base pair sites and two backbone sites) in each cell, sandwiched between two semi-infinite single-walled carbon nanotubes (SWNT) as the electrodes. Having relied on Landauer formalism, we focussed on studying the current-voltage characteristics of DNA, the effect of the coupling strength of SWNT/DNA interface and the role of tube radius of nanotube contacts on the electronic transmission through the foregoing structure. Finally, a characteristic time was calculated for the electron transmission, which measures the delay caused by the tunnelling through the SWNT/DNA interface. The results clearly show that the calculated characteristic time and also the conductance of the system are sensitive to the coupling strength between DNA molecule and nanotube contacts.     

Field-Induced Electron Emission from Artificially Produced Carbon Sites on Broad-Area Copper and Niobium Electrodes

It has been experimentally demonstrated that micrometersized particles of graphite deposited on the surface of copper and niobium electrodes can promote the field emission of electrons in the range of 5-20 MV ·m-1. From measurements of the current-voltage characteristic, electron spectrum, and emission image of such sites, it has been concluded that electrons arc emitted by a mechanism similar to that operating at the naturally occurring sites responsible for prebreakdown electron emission. A hot-electron-based metal-insulator-metal (MIM) model is considered.     

CsSi_n~u(n=2-12;u=±1)团簇结构与电子性质的密度泛函研究

运用卡利普索结构预测方法并结合密度泛函理论中的杂化密度泛函B3LYP方法对CsSi_n~u(n=2-12;u=±1)进行了系统的研究.结果发现:除了CsSi_7~(+1)与CsSi_(2,4,6,10)~(-1)之外,大多数CsSi_n~(±1)团簇的基态结构与对应中性CsSi_n团簇的结构不相同;稳定性分析显示得失电子明显提高了体系的稳定性,CsSi_(4,7,9)~(+1)与CsSi_(2,5)~(-1)分别在对应团簇中具有相对较高的稳定性;Cs原子总是占有正电荷.最后讨论了团簇的电离势、电子亲和能与结构之间的关系.     

Features of electron-phonon interaction in nanotubes with chiral symmetry placed in a magnetic field

The interaction of electrons with acoustic phonons is considered in a nanotube with chiral symmetry placed in a magnetic field parallel to the nanotube axis. It is shown that in such a system, the electronic energy spectrum is not invariant under electron wavevector reversal and, therefore, the electron-phonon interaction is different for identical phonons with oppositely directed wavevectors. This phenomenon leads to the occurrence of an electromotive force during spatially homogeneous heating of an electron gas and to the presence of a term quadratic in current in the current-voltage characteristic of a nanotube.     

Agn(n=2～10)团簇的几何结构和电子特性

应用密度泛函理论中B3LYP/LANL2DZ 方法优化计算并分析了Agn(n=2～10)团簇的基态几何结构及电子性质.同时计算和讨论了银团簇的原子化能、能级分布、能级间隙、电子亲和能和电离势,所得理论计算值与实验值符合较好.研究结果表明:银小团簇的结构不同于块体,且随团簇尺寸大小而相应变化,原子化能和电子亲和势随原子尺寸的增加而增加,团簇的费米能级、电子亲和势和电离势随团簇大小变化具有明显的奇偶振荡特性,并对此作了分析.团簇的电子性质和几何结构之间的密切关系及其随团簇尺寸大小变化的规律,可以从理论上确定团簇的最稳定结构,并可对实验观测结果做出解释.     

Negative differential conductivity of bigraphene controlled by an external voltage in a magnetic field

The current-voltage characteristic of a graphene bilayer has been calculated using the average electron method in the case of applied strong electric and magnetic fields. In the relaxation time approximation, it has been shown that, in bigraphene in the presence of a constant magnetic field applied in the direction perpendicular to the bigraphene layers, these can arise states with a negative differential conductivity. In addition, the generation of terahertz pulses can be performed in this system over a rather wide range of interlayer voltages.     

Nonuniform electron states near cleaved bismuth surface

Using scanning tunneling microscopy, we have detected nonuniform electron states on cleaved bismuth surfaces, which manifest themselves in the form of a random relief with a characteristic lateral size of 1–2 nm and a vertical size of a fraction of an angström. This relief is due to variations in current-voltage characteristics when the tip is moved over the sample surface. Features on the current-voltage characteristics associated with states near the surface have been observed in a voltage range of ?0.4 to +0.4 V. The nonuniform states are tentatively associated with defects generated in the process of crystal cleavage.     

Improved Self‐Energy Calculations for Pressure Broadening of Spectral Lines in Dense Plasmas

Pressure broadening of Lyman‐lines of hydrogen‐like lithium (Li²⁺) has been studied using a quantum statistical approach to the line shape in dense plasmas, for details see [1]. In this communication, we concentrate on the electronic self‐energy, which is a basic input to the theory of spectral line profiles. We discuss the effect of strong, i.e. close, collisions which have been neglected so far for Li²⁺ plasmas, but play generally an important role in dense plasmas, as has been shown in [2]. We present a method to calculate an improved electronic self‐energy including strong collisions based on a two‐body T‐matrix and an effective optical potential. The method is tested for level broadening of the ground state of hydrogen (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First-Principles Study of Geometries,Stabilities and Electronic Properties of Ca2Sin （n=1-11） Clusters

The equilibrium geometries, relative stabilities, and electronic properties of Ca2Sin （n = 1-11） clusters have been systematically investigated by using the density function theory at the 6-311G （d） level The optimized geometries indicate that the most stable isomers have three-dimensional structures for n = 3-11. The electronic properties of Ca2 Sin （n = 1-11） dusters axe obtained through the analysis of the natural charge population, natural electron configuration, vertical ionization potential, and vertical electron affinity. The results show that the charges in corresponding Ca2Sin clusters transfer from the Ca atoms to the Sin host. Based on the obtained lowest-energy geometries, the size dependence of cluster properties, such as averaged binding energies, fragmentation energies, second-order energy differences, HOMO- LUMO gaps and chemical hardness, are deeply discussed.     

Density of states of a two-dimensional electron gas measured by high-resolution photoelectron spectroscopy

We present high energy-resolution photoemission measurements of the spectral density at the discrete quantized electronic levels of a two-dimensional (2D) electron gas. The dynamical 2D electron gas has been obtained by generating a strong accumulation layer at the (110) surface of narrow-gap III–V semiconductors. Exploitation of a number of cases generating band bending (metallic chains or clusters, atomic structure, defects) demonstrates the generality of 2D electron gas formation at charge-accumulated semiconductor surfaces. A self-consistent solution of the Poisson and Schrödinger equations gives the potential well shape, the sub-band energy level position and the accumulated charge density, in excellent agreement with the present experimental data.     

TinMg (n=1—10)掺杂团簇的密度泛函研究

采用密度泛函(DFT)中的B3LYP方法,选择sto-3g基组,优化并得到了Ti_nMg(n=1—10)小团簇的基态平衡结构,计算出了掺杂团簇的基态结构的平均键长、对称性、原子化能、能级分布、能级间隙、束缚能、总能的二阶差分. 结果表明,随着团簇原子数的增加,镁原子更容易趋于团簇表面位置,镁-钛平均键长大于钛-钛平均键长,以对称性结构为最稳定的基态结构,且呈多个五角双锥结构. 其中Ti₅Mg和Ti₈Mg的结构更为稳定. 关键词： 密度泛函 钛-镁团簇 基态结构     

Influence of parameters of the potential barrier at the metal/polymer interface on the electronic switching in the metal/polymer/metal structure

The influence of the first-order phase transition on the parameters of the potential barrier at the indium/polymer interface has been investigated. It has been established that the phase transition occurring in the metal initiates switching of the polymer insulator into a high-conductivity state. Performed investigations have shown that the main charge transfer mechanism in the metal-polymer-metal structure at high temperatures is the current caused by the electron thermionic emission. The analysis of current-voltage characteristics has demonstrated that the first-order phase transition in indium leads to the variation in the potential barrier height at the metal/polymer interface by Δφ ≈ 0.18 eV. It is this phenomenon that is responsible for the electronic switching.