Electron-stimulated desorption of lithium atoms from oxidized molybdenum surface

The yield and energy distributions of lithium atoms upon electron-stimulated desorption from lithium layers adsorbed on the molybdenum surface coated with an oxygen monolayer have been measured as functions of the impact electron energy and lithium coverage. The measurements are performed using the time-of-flight technique and a surface ionization detector. The threshold of the electron-stimulated desorption of lithium atoms is equal to 25 eV, which is close to the ionization energy of the O 2s level. Above a threshold of 25 eV, the yield of lithium atoms linearly increases with an increase in the lithium coverage. In the coverage range from 0 to 0.45, an additional threshold is observed at an energy of 55 eV. This threshold can be associated with the ionization energy of the Li 1s level. At the electron energies above a threshold of 55 eV, as the coverage increases, the yield of lithium atoms passes through a maximum at a coverage of about 0.1. Additional thresholds for the electron-stimulated desorption of the lithium atoms are observed at electron energies of 40 and 70 eV for the coverages larger than 0.6 and 0.75, respectively. These thresholds correlate with the ionization energies of the Mo 4s and Mo 4p levels. Relatively broad peaks in the range of these thresholds indicate the resonance excitation of the bond and can be explained by the excitation of electrons toward the band of free states above the Fermi level. The mean kinetic energy of the lithium atoms is equal to several tenths of an electronvolt. At electron energies less than 55 eV, the energy distributions of lithium atoms involve one peak with a maximum at about 0.18 eV. For the lithium coverages less than 0.45 and electron energies higher than 55 eV, the second peak with a maximum at 0.25 eV appears in the energy distributions of the lithium atoms. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model, in which the adsorbed lithium ions are neutralized after filling holes inside inner shells of the substrate and lithium atoms.     

Influence of magnetic quantization on the effective electron mass in Kane-type semiconductors

Summary We study the effective electron mass at the Fermi level in Kane-type semiconductors on the basis of fourth order in effective mass theory and taking into account the interactions of the conduction electrons, heavy holes, light holes and split-off holes, respectively. The results obtained are then compared to those derived on the basis of the well-known three-band Kane model. It is found, takingn-Hg_1−x Cd _x Te as an example, that the effective electron mass at the Fermi level in accordance with fourth-order model depends on the Fermi energy, magnetic quantum number and the electron spin respectively due to the influence of band nonparabolicity only. The dependence of effective mass on electron spin is due to spin-orbit splitting parameter of the valence band in three-band Kane model and the Fermi energy due to band nonparabolicity in two-band Kane model. The same mass exhibits an oscillatory magnetic-field dependence for all the band models as expected since the origin of oscillations in the effective mass in nonparabolic compounds is the same as that of the Shubnikov-de Hass oscillations. In addition, the corresponding results for parabolic energy bands have been obtained from the generalized expressions under certain limiting conditions.     

Auger-spectroscopic appearance of electron correlation at the Fermi surface of graphite

It is shown that, in Auger-electron spectra of three-dimensional semimetal graphite and two-dimensional graphite (a zero band-gap semiconductor), an energy gap should be observed between the thresholds (edges) of the forward and inverse processes (threshold gap). In the one-electron approximation, this gap is zero, since the threshold for the Auger spectrum of the forward process is the minimum hole energy in the valence band, while the threshold for the spectrum of the inverse process is the minimum energy of conduction electrons. Inclusion of the electron correlation at the Fermi surface within the quantum-chemical approximation of a single open electron shell for multiplet structures of the restricted Hartree-Fock method makes it possible to determine the threshold gap as 1.5 eV for a 48-atom cyclic model of three-dimensional graphite and as 2.0 eV for a 24-atom model of two-dimensional graphite. The threshold gap does not contain the Fermi energy, in contrast to the Auger spectrum thresholds, where \(\frac{1}{2}(4.0 eV - \varepsilon _F )\) for the forward Auger spectrum (holes) and \(\frac{1}{2}( - 1.1 eV + \varepsilon _F )\) for the inverse spectrum (conduction electrons), the sum of which gives this gap. The results of calculations for the forward Auger spectra of three-dimensional graphite (including the conclusion that electron correlation of holes in the top valence bands is weak in the Auger process) are shown to agree with the experimental data.     

Impurity band conduction in CdHgTe crystals

Electrical conduction at 77 K in Cd_xHg_1−xTe, with the composition x ⩽ 0.2, is by electrons in the conduction band, by holes in the valence band and by holes in the impurity band. In samples with zero energy gap, x < 0.14, electrical conduction by holes in the valence band is comparable to electrical conduction by holes in the impurity band. In the open energy gap CdHgTe, electrical conduction by holes in the valence band is negligible in comparison to electrical conduction by holes in the impurity band. In CdHgTe samples, electrical conduction in the impurity band is described by the “Fermi Glass” model.     

Polarised hot electron luminescence in p-GaAs: Electric field effects

Electrons photo-excited to high-energy conduction band states of GaAs exhibit complex energy and momentum distributions determined by the anisotropic valence band structure and the optical matrix elements. In p-type GaAs a fraction of these hot electrons combine with localised acceptor states, producing a hot electron luminescence (HEL) spectrum with a cascade of peaks corresponding to discrete energy losses resulting from LO-phonon emission. The highest peak involves unscattered electrons, and their energy distribution is due to warping of the initial heavy-hole (HH) bands. We report measurements of the line shape of this 0-HH peak, and its polarisation profile which identifies emission from electrons along particular directions. An applied electric field of 1 kV cm⁻¹ distorts the hot electron momentum distribution, and this is reflected in the polarisation profile. These line shapes and profiles, with and without field, are calculated using a computer model incorporating a band structure and optical matrix elements, the effect of electric field being included using a k-broadening model. The data and model are in good quantitative agreement assuming an electron lifetime of 100 fs, and confirm the expected differences in the profiles for different excitation polarisation states and applied field directions.     

Optical orientation in p-doped semiconductor structures with a split valence band

The optical orientation of electron spins in heavily doped semiconductor structures with a valence band that is split as a result of size quantization or uniaxial deformation is investigated theoretically. It is shown that lowering the Fermi level by doping and by lowering the temperature should lead to sharp changes in the photon-energy-dependence of the average spin of the excited electrons in structures excited by circularly polarized light. This effect is due to an interchange of the dominant contribution of transitions from a light-hole subband and transitions from the heavy-hole subband in absorption. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 12, 867–871 (25 June 1997)     

Resonant photoemission at the 3s threshold of Ni

A resonant enhancement of valence band photoemission features in Ni near the 3s threshold is presented. The emission behavior with photon energy of the Ni-3d band is characteristic of a Fano-type resonance. In addition to the main 3d-band emission and 6eV binding energy satellite, a weak satellite is observed at 7.2eV below the Fermi level with photon energies in the vicinity of the 3s threshold.     

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。     

Relation between orbital binding energies and ionicities in alkali and alkaline earth flourides

The differences between the ionization potentials and binding energies for M^q+ (np⁶), F^?(1 s²), andF^?(2p⁶) orbital electrons, adjusted for the electrostatic self-potentials, in alkali and alkaline earth fluorides have been correlated with ionicites derived from the indices of refraction through optical dispersion theory of Phillips and van Vechten. The differences are linear in ionicities and are related to the covalent energies and the polarizabilities. The gap between the valence band and the Fermi level, determined with X-ray photoelectron spectroscopy, has been compared with the energy in the Phillips-van Vechten model. Whereas the former is a measure of the thermal activation energy for conduction, the latter is determined essentially by the valence electron density, the molar volume, and the polarizability.     

Electronic States near the Fermi Level in the Antiferromagnetic and Ferromagnetic Spinels of Zn 1− x Cu x Cr 2 Se 4 ; 0.0 ≤ x ≤ 1.0

The electronic states of normal-type spinels in Zn 1 m x Cu x Cr 2 Se 4 have been studied by X-ray absorption spectroscopy near the K-edge of Zn, Cu, Cr and Se ions. It is found that characteristic white lines occur near the X-ray absorption threshold for Zn and Se ions, but not for Cu and Cr ions. It is also found that the white lines show a slight energy shift, which depends on the Cu concentration. The energy shifts mean that Fermi level gradually approaches the top region of the valence band of Se 2 m , on increasing the Cu ions. The substitution of Zn 2+ with Cu 1+ ions increases the number of positive carriers, holes. The holes occupying the top region of the valence band strongly correlate with Cr 4+ , which forms an empty orbital in the 3d-band of Cr 3+ and exchanges the valence electrons with Cr 3+ . It is interpreted that the ferromagnetism in Zn 1 m x Cu x Cr 2 Se 4 is made by double-exchange interaction between Cr 3+ and Cr 4+ ions and that holes contribute to the electronic conductivity of Zn 1 m x Cu x Cr 2 Se 4 .     

GaAs/AlGaAs超晶格的光致发光

在室温下测量了GaAs/A l0.3Ga0.7As超晶格的光致发光,发现在波长λ=761 nm处存在一较强的发光光峰,此发光峰目前尚未见报道。经理论分析表明,此峰是量子阱中的第一激发态电子与受主空穴复合发光。实验还观测到在λ=786 nm处,λ=798 nm处和λ=824 nm处分别存在一发光峰,分析表明λ=786 nm处的发光峰为量子阱阱中费米能级附近的电子与轻空穴复合发光;λ=798 nm处的发光峰为量子阱内的基态电子到轻空穴的复合发光;λ=824 nm处的发光峰为阱中激子复合复合发光。理论计算与实验结果符合的很好。     

Electronic structure of LaNi and its hydride LaNi H

The electronic structure of LaNi₅ and its hydride LaNi₅H₇ are obtained using the self-consistent cluster-embedding calculation method. The Fermi level of LaNi₅H₇ is 5.172 eV higher than that of LaNi₅. In both materials, the La 5d electrons locate nearby the Fermi levels, and make only a small contribution to the density of states (DOS) of the valence bands. There is no significant charge transfer from La to Ni in LaNi₅. But for LaNi₅H₇, there is a charge transfer of 1.16 electrons from La to H, and H atoms are combined mainly with Ni to form hybridized orbitals in the energy regions far below the Fermi level. An explanation of hydrogenation of LaNi₅ is proposed: It is easy for hydrogens to take off some localized La 5 d electrons near the Fermi level, and combine with Ni to form hybridized orbitals in lower energy regions. This process is therefore in favor of energy, and forces a lattice expansion until the Fermi level rises to zero. Received 13 July 2001 / Received in final form 10 December 2001 Published online 17 September 2002     

锯齿型石墨烯纳米带的能带研究

运用π电子紧束缚模型,具体研究了锯齿型石墨烯纳米带(ZGNRs)的边界结构对能带,特别是费米面附近的导带和价带电子的影响.计算了七种不同边界结构的ZGNRs的能带色散关系及费米面附近价带电子在原胞中各原子上的分布情况.计算结果表明:两边界都无悬挂原子的NN-ZGNRs,只有一边界有悬挂原子的DN-ZGNRs,两边界都有五边形环的SPP-ZGNRs和ASPP-ZGNRs为金属性.两边界都有悬挂原子的DD-ZGNRs,一边界为五边形环另一边界无悬挂原子的PN-ZGNRs和一边界为五边形环另一边界有悬挂原子的P 关键词： 锯齿型石墨烯纳米带 紧束缚模型 电子密度分布 缺陷结构     

Electron and hole spectra of silicon quantum dots

In the framework of perturbation theory, the first several one-particle energies and wave functions for electrons and holes (six for each) in spherical silicon quantum dots are obtained in the envelope function approximation (kp method). It is shown that the model of an isotropic dispersion relation with the mean reciprocal effective mass is applicable for the ground state of holes in the valence band. Anisotropy of the dispersion relation, which takes place for bulk semiconductors, becomes significant for the electron ground state in the conduction band as well as for all excited (both electron and hole) states.     

Angle-resolved resonant photoemission of Ni(100) and Ni(110) single crystals

We have studied the valence band photoemission spectra of Ni(100) and Ni(110) single crystals near the excitation threshold for 3p core electrons. The resonant behavior of the 6 eV satellite does not depend on both the surface orientation and the polarization of the electric vector of an incident light for excitation. These results indicate that the 6 eV satellite should be under little influence of spatial symmetry of the valence band. In the angle-resolved photoemission spectra of Ni(100), we have observed another broad feature near the 6 eV satellite. It shows the large energy dispersion and is interpreted as due to the interband transition. In Ni(110), we have observed the weak valence band satellites at binding energies of about 9.3 eV and 13.4 eV. They do not show well-defined resonance around the 3p threshold.     

Impact ionization induced negative far-infrared photoconductivity inn-GaAs

Far-infrared photoconductivity ofn-GaAs epitaxial layers showing impact ionization breakdown has been investigated by molecular lasers at photon energies below the 1s-2p shallow donor transition energy. Negative photoconductivity was observed if a magnetic field was applied to the crystals and if impact ionization of donors by the electric bias field was the dominant electron excitation mechanism. The experimental results are qualitatively explained on the basis of the generation-recombination kinetics of electrons bound to donors. Negative photoconductivity is attributed to optically induced free to bound transitions of electrons from theN=0 Landau band to donor levels shifted by the magnetic field above the low energy edge of the conduction band.     

Cascade multiplication of strongly non-equilibrium charge carriers in metals by low energy electron excitation

The steady-state distribution function of the energies of non-equilibrium electrons and holes formed as a result of a cascade of electron-electron collisions in the presence of a primary electron flux is found by solving the linearized transport equation in the isotropic scattering approximation. The distributions obtained in this way include dependences on the energy of primary electrons and on the characteristics of the medium. Near the Fermi level they have a singularity and are close to a power law far from this level. A study is made of the possibility of deriving distributions for crystals with a complex energy band structure (in particular, of tungsten).     

B与N掺杂对单层石墨纳米带自旋极化输运的影响

通过第一性原理计算研究了具有锯齿状边沿并且具有反铁磁构型的单层石墨纳米带的自旋极化输运.研究发现,在中心散射区同一位置掺入单个B和N原子,尽管对整个体系磁矩的影响完全相同,但对两个自旋分量电流的影响却完全相反.掺B时,自旋向上的电流显著大于自旋向下的电流;而掺N时,自旋向下的电流显著大于自旋向上的电流.这是由于不管掺B还是掺N都将打破自旋简并,使得导带和价带中自旋向上的能级比自旋向下的能级更高.掺B引入空穴,使完全占据的价带变为部分占据,从而自旋向上的能级正好处于费米能级,使得电子透射能力更强、电流更大,而自旋向下的能级则离费米能级较远使电子透射的能力较弱.掺N则引入电子,使得原来全空的导带变为部分占据,从而费米能级穿过导带中自旋向下的能级,使得自旋向下的电子比自旋向上的电子透射能力更强. 关键词： 自旋极化输运 单层石墨纳米带 第一性原理 非平衡格林函数     

Two-photon photoionization by singlet–singlet annihilation in polycrystalline,thin perylene films

Two-photon photoionization processes in thin, polycrystalline, α- and β-crystalline perylene films have been investigated for photon energies at the optical absorption threshold. The kinetic energy of the emitted electrons is found to be essentially independent of the photon energy, indicating that geometrically relaxed excited states are involved. The high ionization yield at such low photon energies is therefore attributed rather to the annihilation of adiabatically relaxed singlet excitons than to the direct two-photon ionization process. This leads to hole final states different from those obtained in one-photon, vertical ionization processes.     

Subband states inn-inversion layers on small-gap semiconductors

Subband states inn-inversion layers on small-gap semiconductors are subject to the coupling between valence and conduction band (nonparabolicity effects). In order to account for this coupling we study different models: two of them are based on Kane's 6×6 and 8×8 bulk-Hamiltonians, the third one takes into account higher order terms of the electron momentum in a 2×2 conduction band Hamiltonian. We perform selfconsistent calculations for these models with parameters characteristic for InSb and HgCdTe and electron concentrationsN _S , for which up to two subbands are occupied. The calculated subband separations and Fermi energies are independent of the models only if the same energy band dispersion is used and depend strongly on the applied boundary conditions.Work supported in part by the Deutsche Forschungsgemeinschaft