Several Tenths Of eV Research Articles

Molecular-Dynamics Study of Dimer Formation on a GaAs (001) Surface at Low Temperatures

The molecular-dynamics simulation of the formation of dimers for the case of the low-temperature (001) reconstruction of a GaAs surface terminated in Ga or As atoms is employed using the analytical potential, which takes into account σ- and π-bonds between atoms. The values of the decrease in the potential energy of atoms during the formation of the surface of an isolated dimer are found, and it is discovered that the potential energy of an atom in the As dimer is lower by several tenths of eV than that in the Ga dimer. In the temperature range of 25–40 K, the kinetics of the initial stages of the formation of Ga dimers is studied, and it is obtained that the characteristic energy of thermal activation of the formation of single isolated Ga dimers is ~29 meV and lower than the same value for As dimers (~38 meV). In the temperature range of 28–37 K, the time constants characterizing the average rate of the conversion of a single dimer into a chain of two dimers are estimated. For double Ga and As dimers, the reciprocal values of these times turn out to be in the ranges of 1011–1012 and 109–1010 s–1, while similar parameters for the formation of single dimers lie in the range of 4 × 106–108 and 1.4 × 106–7.4 × 107 s–1.

Systematic development of ab initio tight-binding models for hexagonal metals

A systematic method for building an extensible tight-binding model from ab initio calculations has been developed and tested on two hexagonal metals: Zr and Mg. The errors introduced at each level of approximation are discussed and quantified. For bulk materials, using a limited basis set of $spd$ orbitals is shown to be sufficient to reproduce with high accuracy bulk energy versus volume curves for fcc, bcc, and hcp lattice structures, as well as the electronic density of states. However, the two-center approximation introduces errors of several tenths of eV in the pair potential, crystal-field terms, and hopping integrals. Environmentally dependent corrections to the former two have been implemented, significantly improving the accuracy. Two-center hopping integrals were corrected by taking many-center hopping integrals for a set of structures of interest, rotating them into the bond reference frame, and then fitting a smooth function through these values. Finally, a pair potential was fitted to correct remaining errors. However, this procedure is not sufficient to ensure transferability of the model, especially when point defects are introduced. In particular, it is shown to be problematic when interstitial elements are added to the model, as demonstrated in the case of octahedral self-interstitial atoms.

Capacitance-voltage characteristics of the electrolyte-n-InN surface and electron states at the interface

Dependences of differential capacitance of the system degenerate n-InN-electrolyte on the voltage are measured at a frequency of the probing voltage of 300 Hz. Qualitative analysis of these characteristics is performed based on a one-dimensional model of the metal-insulator-semiconductor structure in the region of bias voltages near the flat-band voltage and depletion. It is shown that the magnitude of capacitance in this voltage region is affected by electron states at the interface. The density and energy distribution of these states are evaluated. The form of the voltage dependence of capacitance in the region of accumulation also indicates the existence of the states at the interface, the energy of which exceeds the energy of the conduction band bottom by several tenths of eV. The density of these states increases as the energy increases.

Spin and orbital states inLa1.5Sr0.5CoO4studied by electronic structure calculations

Electronic structure of the layered perovskite ${\text{La}}_{1.5}{\text{Sr}}_{0.5}{\text{CoO}}_{4}$ with a checkerboard ${\text{Co}}^{2+}/{\text{Co}}^{3+}$ charge order is studied, using the local-spin-density approximation plus Hubbard $U$ calculations including also the spin-orbit coupling and multiplet effect. Our results show that the ${\text{Co}}^{2+}$ ion is in a high spin state (HS, ${t}_{2g}^{5}{e}_{g}^{2}$) and ${\text{Co}}^{3+}$ low spin state (LS, ${t}_{2g}^{6}$). Due to a small ${\text{Co}}^{2+}\text{ }{t}_{2g}$ crystal field splitting, the spin-orbit interaction produces an orbital moment of $0.26{\ensuremath{\mu}}_{B}$ and accounts for the observed easy in-plane magnetism. Moreover, we find that the ${\text{Co}}^{3+}$ intermediate spin state (IS, ${t}_{2g}^{5}{e}_{g}^{1}$) has a multiplet splitting of several tenths of eV and the lowest-lying one is still higher than the LS ground state by 120 meV, and that the ${\text{Co}}^{3+}$ HS state $({t}_{2g}^{4}{e}_{g}^{2})$ is more unstable by 310 meV. Either the IS or HS ${\text{Co}}^{3+}$ ions would give rise to a wrong magnetic order and anisotropy.

High-speed data acquisition system for neutron time-of-flight experiments

A high-speed data acquisition system comprising two parallel metering channels has been developed for neutron time-of-flight measurements. Each metering channel is equipped with a flash-ADC. The first channel is intended for measuring fast neutrons, whose energy ranges from tens of eV up to several keV, and uses a 40 MHz sampling rate. The second channel is intended for measuring slow neutrons, whose energy ranges down to several tenths of eV, and uses a 4 MHz sampling rate. Laboratory tests using the developed system have been performed and the system's efficiency dependence on the incoming counting rate has been obtained. As a result of this new design, the performance of the data acquisition system has increased by about an order of magnitude in comparison with existing systems. The designed data acquisition system is now used for the measurement of neutron capture cross-sections in a wide neutron energy range by a time-of-flight method.

Exciton dissociation and charge photogeneration in pristine and doped conjugated polymers

There is abundant experimental and theoretical evidence of the fact that the primary optical excitation in conjugated polymers is a singlet exciton with a binding energy of typically 0.5 eV. In the absence of excess photon energy, free or geminately bound charge carriers (polarons) have to be generated from relaxed excitons via a secondary dissociation process. This process requires further excitation of several tenths of eV in order to compensate for the exciton binding energy. Concomitantly, exciton breaking can occur either in a strong external electric field or via charge transfer to an electron or hole scavenger. Experimentally, these processes are revealed in field- and dopant-assisted photoluminescence quenching, photoinduced optical absorption, and photoconductivity. Experimental and theoretical aspects of these phenomena are reviewed in the present article. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Adsorbed Si on theSi(111)−(7×7)surface studied by scanning tunneling microscopic and molecular-orbital approaches: Stationary and diffusing Si adsorbates

In experiments of Si deposition onto the $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface using scanning tunneling microscopy, we observe ``diffusing'' Si adsorbates detected as a noise-shaped pattern in addition to the previously reported ``stationary'' ones that remain at the same positions. The stable positions and diffusion energy barriers of Si atoms on the surface are obtained using molecular-orbital calculations. For one Si atom, the diffusion barrier is only several tenths of eV, while it exceeds 1 eV for two Si atoms. We propose some assignments: the ``stationary'' adsorbate for the two Si atoms and ``diffusing'' one for the single Si atom.

Electron-stimulated desorption of lithium atoms from oxygen-covered molybdenum surfaces

We have measured electron-stimulated desorption (ESD) yields and energy distributions for lithium (Li) atoms from Li layers adsorbed at 300 K on an oxygen-monolayer-covered molybdenum surface as a function of electron energy and lithium coverage. The measurements have been carried out using a time-of-flight method and surface ionization detector. The appearance threshold for Li atoms is close to the O 2s level ionization energy of 25 eV and is independent of the lithium coverage. Above the 25 eV threshold the lithium atom ESD yield increases linearly with increasing lithium coverage. An additional threshold of 55 eV is observed in the lithium coverage range from 0 to 0.45; this appears to be associated with the Li 1s level ionization energy. For energies above the 55 eV threshold the incremental change of lithium atom ESD yield exhibits a maximum as the lithium coverage increases, and decreases to zero at lithium coverages greater than 0.45. Additional features for lithium atoms appear at about 40 and 70 eV, correlating with the molybdenum 4p and 4s ionization energies at Li coverages of 0.6 and 0.75, respectively. There are broad peaks at the latter energies indicating that desorption proceeds via a resonance process, in which the initial excitation may lead to a spectator electron in a quasi-bound state. The average kinetic energy of lithium atoms is several tenths of eV. It depends on the lithium coverage and electron energy. The ESD energy distribution for lithium atoms consists of a single peak with a maximum of around 0.18 eV for electron energies less than 55 eV. A second peak with a maximum of about 0.25 eV appears at electron energies greater than 55 eV. The data can be interpreted in terms of the Auger-stimulated desorption model, in which neutralization of adsorbed lithium ions occurs after filling of holes created by incident electrons in inner shells of substrate and lithium atoms.

Soliton defects in polyacetylene: Local‐density functional results

AbstractWe report local‐density functional results for the electronic structure of neutral soliton defects in polyacetylene. The results were obtained using a modified version of the discrete variational method Xα molecular cluster model. For defect‐free chains the calculated X‐ray and ultraviolet photoelectron spectra are in good agreement with experiment. For chains with low concentrations of solitons, we find the forbidden gap broadens slightly, and two nearly dispersionless bands, split by several tenths of eV, are introduced into the gap. The predicted splitting is in accord with recent experimental results for this defect.

Developments in non-destructive testing equipment: Welding and Metal Fabrication, Vol 35, No 11 (November 1967) pp448–458

A high-speed data acquisition system comprising two parallel metering channels has been developed for neutron time-of-flight measurements. Each metering channel is equipped with a flash-ADC. The first channel is intended for measuring fast neutrons, whose energy ranges from tens of eV up to several keV, and uses a 40 MHz sampling rate. The second channel is intended for measuring slow neutrons, whose energy ranges down to several tenths of eV, and uses a 4 MHz sampling rate. Laboratory tests using the developed system have been performed and the system's efficiency dependence on the incoming counting rate has been obtained. As a result of this new design, the performance of the data acquisition system has increased by about an order of magnitude in comparison with existing systems. The designed data acquisition system is now used for the measurement of neutron capture cross-sections in a wide neutron energy range by a time-of-flight method.

Dielectric materials for electro-optic, elasto-optic and ultrasonic device applications: Spencer, E.G., Lenzo, P.V. and Ballman, A.A. Proceedings of the Institute of Electrical and Electronics Engineers, Vol 55, No 12 (December 1967) pp2074 108

A high-speed data acquisition system comprising two parallel metering channels has been developed for neutron time-of-flight measurements. Each metering channel is equipped with a flash-ADC. The first channel is intended for measuring fast neutrons, whose energy ranges from tens of eV up to several keV, and uses a 40 MHz sampling rate. The second channel is intended for measuring slow neutrons, whose energy ranges down to several tenths of eV, and uses a 4 MHz sampling rate. Laboratory tests using the developed system have been performed and the system's efficiency dependence on the incoming counting rate has been obtained. As a result of this new design, the performance of the data acquisition system has increased by about an order of magnitude in comparison with existing systems. The designed data acquisition system is now used for the measurement of neutron capture cross-sections in a wide neutron energy range by a time-of-flight method.