Composition and electric field effects on the transport properties of Bi doped chalcogenide glasses thin films

The temperature dependence of the DC electrical conductivity σ_DC was measured in the temperature range from 300–500 K. It was found that there are double activation energies, E_σ, for Ge₂₀Se_80−xBi_x (x=0, 2.5 and 5 at%) films, while there is single activation energy for Ge₂₀Se_72.5Bi_7.5. when incorporation of Bi=7.5 at%, the pre-exponential value σ₀ decreases by about six order of magnitude, the activation energy in the extended states E_σ decreases from 0.96 to 0.09 eV. Also the effect of applied electric field was studied and observed that, activation energy in high temperature region increases with increasing electric field; this behavior can be understood assuming that the contribution to the conductivity activation process is due to conduction in the extended states and also due to hopping in the localized states. With the increasing electric field, as former process, which is having high activation energy, becomes more predominant due to the dumping of the carriers in the extended states, the effective activation energy of the system increases, in spite of the fact that the activation energy of the extended states conduction may remain constant. Finally, the electrical data suggests that the addition of bismuth produces localized states near the conduction band edge so that the electrical transport is due to hopping of electrons after being excited into localized states at the conduction band edge.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Conductivity anisotropy and localization of charge carriers in TlInTe<Subscript>2</Subscript> single crystals

The electrical resistivity of TlInTe₂ chain-structure semiconductors in directions parallel and perpendicular to the chains is analyzed as a function of temperature. It is demonstrated that, in both cases, the temperature dependences of the electrical resistivity in the temperature range under investigation are characterized by two portions associated with different mechanisms of electrical conduction. In the high-temperature range, the electrical conduction is predominantly provided by thermally excited impurity charge carriers in the allowed band. In the low-temperature range, the conduction occurs through charge carrier hopping between localized states lying in a narrow energy band near the Fermi level. The activation energy for impurity conduction is determined. The localization lengths and the density of localized states near the Fermi level, the spread in energies of these states, and the average carrier-hopping distances are estimated for different temperatures.     

Conductivity anisotropy and localization of charge carriers in CuFeTe2 single crystals with a layered structure

The temperature dependences of the electrical resistivity of CuFeTe₂ semiconductor single crystals with a layered structure are investigated parallel and perpendicular to the plane of the crystal layers in the temperature range 5–300 K. It is demonstrated that, in both cases, the temperature dependences of the electrical resistivity in the temperature range studied are characterized by two portions associated with different mechanisms of electrical conduction. In the high-temperature range, the electrical conduction is predominantly provided by thermally excited impurity charge carriers in the allowed energy band. In the low-temperature range, the electrical conduction occurs through charge carrier hopping between localized states lying in a narrow energy band near the Fermi level. The activation energy for impurity charge carriers is determined. The density of localized states near the Fermi level, the spread in energies of these states, and the average carrier-hopping distances are estimated for different temperatures     

Magnetic and electrical properties of zinc selenide crystals containing disorder

The electrical and magnetic properties of ZnSe single crystals containing disorder have been studied between temperatures 290K and 900K. The study of the magnetic properties has been extended to low temperatures (100K). Paramagnetism has been found to appear at high temperatures (460–900K). From the fact that this paramagnetism is proportional to e^?δE/kT, it is suggested that localized states of single occupancy are created by thermal excitation. The study of the magnetic properties has been of help in ascertaining the nature of the transport (band conduction or hopping conduction) and in finding the hopping energy and excitation energy separately. It has also been shown from this that both band conduction and hopping conduction exist simultaneously in the sample. A study of the thermo electric power (t.e.p.) shows that below 450K current is carried by electrons in the conduction band and above by hopping of holes.     

Magnetic and transport properties of transition-metal implanted ZnO single crystals

ZnO single crystals were implanted with Mn, Co and Ni with fluences between 1 × 10¹⁶ cm^-2 and 1 × 10¹⁷ cm^-2 and energy of 200 keV. Results indicate that aggregation of transition metal ions in the as implanted state occurs only in the case of Ni. After an annealing stage to recover the ZnO structure aggregation occurs for the higher fluences of all implanted species. For lower concentrations paramagnetic behaviour with magnetic moments close to those of individual ions is observed. No polarised impurity band is formed as a result of the presence of transition metal ions and all samples show electrical conduction by carriers in extended states of ZnO. Significant values of magnetoresistance are measured at low temperatures, where electrical transport is described by hopping mechanisms between localized states. The sign of the magnetoresistance is dependent of the doping ion and is correlated with the observed aggregation.     

The Meyer-Neldel rule in the processes of thermal emission and hole capture in Ge/Si quantum dots

The hole thermal-emission rates and the cross sections for hole capture to the bound states in Ge quantum dots in Si are determined by admittance spectroscopy. The capture cross sections and the activation energies for emission rate are found to be related to each other by the Meyer-Neldel rule with a characteristic energy of 27±3 meV, which does not depend on the quantum-dot size. It is established that the capture cross section changes with temperature following the activation law. The experimental data are evidence of a unified multiphonon mechanism for the activation processes of hole transitions from the Ge quantum dots to the Si valence band and hole capture back into the quantum dots.     

扩展Su-Schrieffer-Heeger模型在周期性边界条件下的相变

文章计算了含有三种跃迁项(最近邻(NN)、次近邻(NNN)、次次近邻(NNNN)跃迁项)的扩展Su-Schrieffer-Heeger(SSH)模型(模型ⅰ)的热力学特性.对仅含有二种跃迁项(NN、NNN跃迁项)的扩展SSH模型(模型ⅱ)、仅含有一种跃迁项(NN跃迁项)的SSH模型(模型ⅲ)也作了类似的计算.在低温下,每个模型都有相变且跃迁项的存在会降低相变温度.由于NNN跃迁项的存在,在极低温条件下,关于热容量与温度比(HCOTR)随温度变化的单调性,模型i与其它两个模型存在差异.在低温下,模型ⅰ的HCOTR的单调性,会受NN或NNN或NNNN跃迁项的影响并且NN或NNN跃迁项会影响其相变.     

Molecule opacity study on low-lying states of CS

CS molecule, which plays a key role in atmospheric and astrophysical circumstances, has drawn great attention for long time. Owing to its large state density, the detailed information of the electronic structure of CS is still lacking. In this work, the high-level MRCI+Q method is used to compute the potential energy curves, dipole moments and transition dipole moments of singlet and triplet states correlated with the lowest dissociation limit of CS, based on which high accurate vibration—rotation levels and spectroscopic constants of bound states are evaluated. The opacity of CS relevant to atmospheric circumstance is computed at a pressure of 100 atms for different temperatures. With the increase of temperature, band systems from different transitions mingle with each other, and band boundaries become blurred, which are originated from the increased population on vibrational excited states and electronic excited states at high temperature.     

Electrical conduction properties of Si δ-doped GaAs grown by MBE

The temperature dependent Hall effect and resistivity measurements of Si δ-doped GaAs are performed in a temperature range of 25–300 K. The temperature dependence of carrier concentration shows a characteristic minimum at about 200 K, which indicates a transition from the conduction band conduction to the impurity band conduction. The temperature dependence of the conductivity results are in agreement with terms due to conduction band conduction and localized state hopping conduction in the impurity band. It is found that the transport properties of Si δ-doped GaAs are mainly governed by the dislocation scattering mechanism at high temperatures. On the other hand, the conductivity follows the Mott variable range hopping conduction (VRH) at low temperatures in the studied structures.     

氢化非晶碳薄膜的光致发光线型及其激发能量依赖关系

本文报道了氢化非晶碳薄膜在2.9-4.5eV光激发下的发光谱。它的光致发光谱是无结构的不对称宽带,半宽度约为0.8eV。在低于3.56eV的光激发下,谱带的峰值能量随激发能量的降低明显红移。在安德森带结构和指数分布的带尾态密度的基础上,考虑了尾态中粒子的两种跃迁过程,实验的PL谱就可得到解释。并用这个简单模型计算了这种材料的光致发光谱特征。     

THERMALLY ACTIVATED HOPPING IN Dy1-xPrxBa2Cu3O7-δ SYSTEM

The temperature dependence of resistivity in Dy_1-xPr_xBa₂Cu₃O_7-δ system with x>0.6 was measured. The experimental results show that Pr substitution leads to the localization of mobile holes and such a localization is enhanced with increasing Pr concentration. The gradually enhancing of localization induces Anderson transitions one by one in this system, including the transition from the conduction by excitation of holes to the one by thermal activation hopping between localized states, the so called Anderson transition type-I, and the transition from nearest neighbor hopping (NNH) to variable range hopping (VRH), the Anderson transition type-II, and the Anderson transition type-lI from 3D to 2D.     

Generalized adiabatic polaron hopping: Meyer-Neldel compensation and Poole-Frenkel behavior

The commonly employed adiabatic treatment of polaron hopping is extended to treat the continuous alteration of a carrier wave function with the atoms' movements and a carrier's long-range interaction with a polar surrounding. These features, respectively, introduce carrier-induced softening of the atoms' vibrations and a hopping activation energy that depends on hopping distance. The Meyer-Neldel compensation effect results from carrier-induced softening of vibrations. Poole-Frenkel behavior emerges for electric-field driven polaron hopping in ionic and polar media.     

On the theory of recoil implantation

Abstract

EPR experiments on photoholes in both AgCl :Cu and AgCl :Pd show the existence of an activation energy barrier, of height near 1.8 meV, in the transition to the localized self-trapped state. The self-trapped hole then migrates athermally via the small-polaron band, for temperatures below 30 K, and by phonon-assisted hopping for temperatures above 35 K. In the hopping régime, the activation energy is 61 meV, suggesting that the binding energy is about 0.1 eV. From the pre-exponential factor, one estimates that the electron transfer integral is about 1% of the energy of the pertinent acoustical phonon. Experiments on AgX:Cu in which 1% of the halide is Br^? show that all of the self-trapped holes ultimately migrate to sites with 2 bromide neighbors. Upon annealing, these disappear with complex kinetics and an activation energy of about 180 meV. In AgCl :Pd, Fe, the temperature-dependence of the efficiencies of production of trapped photocarrier centers is determined and correlated with the dynamics of migration of the self-trapped hole and the cation vacancy.     

Bulk,surface and thermal effects in inverse photoemission spectra from Cu(100), Cu(110) and Cu(111)

We have performed a study of empty electronic bulk and surface states on the three low indexed copper surfaces employing momentum resolved inverse photoemission. The bulk electronic features may be well understood in the frame work of the bulk direct transition model using state of the art band structure calculations. Surface states of both, the crystal derived and the image potential induced type have been identified and were found to agree with previous work. Several radiative transitions into unoccupied bands were also investigated at elevated temperatures. Characteristic temperatures of an exponential attenuation law are distinctly different between surface and bulk transitions. However, no systematic behaviour of bulk transitions at different points of the Brillouin zone could be established.     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带一个在平面内的悬挂键及相应的未配对电子,而通过C替换可以消除未配对的电子.系统研究了(CN)3VB缺陷的几何结构、电子结构以及光学性质,结果表明,缺陷可以由一个对称的亚稳态经过原子结构弛豫变成1个非对称的、3个C原子连在一起的基态结构.缺陷的形成在hBN中引入了一些由缺陷悬挂σ键及重构的π键贡献的局域缺陷态.这些缺陷态可以导致能量阈值在2.58 eV附近的可见光内部跃迁.本文的工作有助于进一步理解hBN中点缺陷的构成及光学性质,为实验上探讨发光点缺陷的原子结构起源及其性质提供理论依据.     

Intraexciton and Intracenter Terahertz Radiation from Doped Silicon under Interband Photoexcitation

Terahertz photoluminescence of boron- and phosphorus-doped silicon at low temperatures under interband photoexcitation is investigated. The lines of radiative transitions between free-exciton levels and between the levels of shallow impurity centers are observed. The intensities of these lines exhibit different dependences on temperature and excitation intensity. At temperatures near the temperature of liquid helium (T ~ 5 K), the terahertz radiation spectrum features a broad band (about 18–20 meV wide) with a peak at an energy of about 20–22 meV. This band is apparently associated with radiative transitions of nonequilibrium charge carriers from the states of the continuum to the state of an electron–hole liquid.     

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe₂ single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe₂ single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N _F=1.3×10¹⁹eV·cm³)^?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe₂ single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe₂ single crystals is investigated.     

具有p波超流的一维非公度晶格中迁移率边研究

研究了具有p波超流的一维非公度晶格中迁移率边的性质. 发现适当的p波超流可以增加体系中的迁移率边的数目, 并且通过多分形分析确定了迁移率边所在的位置.     

New Energy Spectra Structure of Polaron in Trans-Polyacetylene

In the weakly coupled electron-phonon systems, the existing theory pointed out that the energy spectra of polaron include four electronic bound states. Our work shows that, due to the non-nearest neighbor hopping interactions, the electron-hole symmetry of the energy band structure implied by SSH model is broken, and the numbers of the bound electronic states are changed. For a negative charged polaron, one new bound state is found near the bottom of conduction band, and the original two bound states below the bottom of the valence band and above the top of the conduction band disappear. For a positive charged polaron, five bound states have been found: one of them is an additional bound state at the top of the conduction band, the others are just the states found in the SSH model. Besides, the energy gap 2Δ is slightly shifted by turning on the long-range hopping interactions.