bluuebunny/crossref_metadata_2025_split huggingface dataset:
Indirect exchange interaction via electrons in spin–orbit coupled bands in semiconductors
Authors: Ginter J. , Kossut J. , Świerkowski L. 
Published: July 2007

AbstractIndirect coupling between two localized magnetic moments in a semiconductor is considered. This coupling may be of two origins: due to intraband electron transitions (RKKY) and virtual interband transitions, the latter being of particular importance in materials considered. Strong spinorbit coupling between electronic bands leads to a complicated form of the effective Hamiltonian. It no longer contains a Heisenberg term only. The number of independent constants describing the interaction depends on the symmetry of the interacting pair. The special case of zincblende structure is considered. Several simple models are reviewed and found insufficient to describe the indirect interaction, at least for closely lying moments, a fact which stresses the necessity of numerical calculations.
Sintering and random structures
Authors: Lacour C. 
Published: July 2007

AbstractIt is shown that the initial state of a compressed powder, as a rule complex and unorganized, actually obeys the statistics laws which inspired the percolation. This makes it possible to calculate all the elements of the dimensional distribution of the interparticulate spaces of a compact powder, of which only the total porosity, volume, and mean particle size are known. The evolution of these distributions during sintering is then investigated in the case of a volume diffusion mechanism and in the case of creep. An essential practical implication emerges from this study: to arrive at the theoretical density of a material in which sintering is governed by a volume diffusion, it is far more important to destroy the initial structure of the compact by reducing the dimensions of the larger voids, even if this increases the number of smaller voids, than to lower the initial porosity globally.
On the existence of two‐interstitial species in platinum after thermal‐neutron irradiation
Authors: Piani C. S. B. , Aspeling J. 
Published: July 2007

AbstractThe prediction of the conversion‐two‐interstitial model that the crowdion in platinum converts at some temperature (≈︁ 150 K) in stage II to the dumbbell, and that this conversion will be seen as a change in the initial slopes of the production curves, is investigatd. A change in the initial slopes of the productoin curves as a function of temperature is observed in stage II between 150 and 170 K. However, both the one‐interstitial model and two‐interstitial model can explain the behaviour of the initial slopes as a function of temperature from the lowest temperature to the minimum in the production curve. Both models cannot satisfactorily explain the behaviour of the initial slopes after the minimum.
Reliability of Various Empirical Potential Methods
Authors: Esterling D. M. , Swaroop A. 
Published: July 2007

AbstractThe use of the Wedepohl potential or the use of the vacancy formation energy as auxiliary conditions for an empirical potential derived from phonon data are examined. A pair potential for aluminum, derived from a non‐local pseudopotential model, is used as a benchmark. These auxiliary conditions are found to be unreliable. Reasons for this are discussed within a more general framework.
Dielectric Properties of PbHPO4 in the Microwave Region and the Far‐Infrared
Authors: Kock E. J. , Happ H. 
Published: July 2007

AbstractThe temperature dependence of the microwave per mittivity of PbHPO4 is determined at 8.2, 9.6 and 37 GHz in the para‐ and ferroelectric phase. The data are not consistent with a Debye‐type relaxation. In addition the far‐infrared reflection spectra between 15 and 650 cm−1 are measured with radiation polarized along the crystallographic axis at several temperatures between 3.5 and 430 K. Kramers‐Kronig analysis is used to calculate approximately ϵ′, ϵ″ and to identify the external lattice vibrations.
EPR Measurements of Chromium Impurity Photoionization Transitions in CdTe
Authors: Godlewski M. , Baranowski J. M. 
Published: July 2007

AbstractThe spectral dependence of the intensity of the EPR signal allows to identify the absorption tail of the band‐to‐band transition as due to a transfer of an electron from the valence band into the d‐shell of Cr2+(d4) due to which the Cr+(d5) charge state is created. The temperature dependence of the Cr+(d5) EPR signal is investigated. It seems that the long lifetime of Cr+(d5) is connected with the influence of the exchange interaction on the recombination process of an electron from the Cr+(d5) state with a loosely bound hole.
Mobilité de l'ion fluor dans PbSnF4
Authors: Villeneuve G. , Echegut P. , Lucat C. , Reau J.‐M. , Hagenmuller P. 
Published: July 2007

AbstractAlors que les mesures de conductivité ne donnent qu'une idée macroscopique des mécanismes de conduction (diffusion à longue distance, énergie d'activation), la RMN fournit d'importantes informations sur les mouvements locaux dont la connaissance est fondamentale pour comprendre les mécanismes de diffusion. L'étude d'un nouveau conducteur ionique de structure fluorine distordue PbSnF4 montre la présence d'au moins deux et peut‐ětre trois différents mécanismes de sauts mettant en jeu les ions F− dans les sites normaux et dans les positions interstitieles.
Vibrational Spectra of TlGaTe2, TlInTe2, and TlInSe2 Layer Single Crystals
Authors: Gasanly N. M. , Goncharov A. F. , Dzhavadov B. M. , Melnik N. N. , Tagirov V. I. , Vinogradov E. A. 
Published: July 2007

AbstractPolarized IR‐reflection and Raman scattering spectra are obtained for TlGaTe2, TlInTe2, and TlInSe2 crystals (space group D). The IR‐reflection spectra of all the crystals show the presence of two (E ‖ c) and three (E ⟂ c) modes, which is in full agreement with the prediction of a group‐theoretical analysis. All seven Raman active modes are detected in TlInSe2. In TlGaTe2 and in TlInTe2 only three of seven Raman active modes can be observed. Normal coordinates, effective force constants, and effective charges are determined for the infra‐red active lattice vibrations.
The Semiempirical LCAO‐MO Method of Energy Spectrum and Electronic Structure Calculation for Open‐Shell Clusters
Authors: Shashkin S. Yu. , Nikiforov A. E. , Cherepanov V. I. 
Published: July 2007

AbstractA semiempirical method is proposed of open‐shell cluster calculation which is based upon Roothaan's rigorous theory and does not contain any variable parameters. It is shown that approximations used for Hamiltonian matrix elements in previous semiempirical calculations must be made more accurate. An estimation technique for Roothaan's coupling operator matrices is developed. Optical transition energies are calculated by a many‐electron way in the “frozen orbital” approximation with configuration interaction taking into account. Numerical computations are made for [NiF6]4− and [TiF6]3− complexes. Comparison of cluster spectra and also of the spin—orbit coupling constant λcr, unpaired spin densities ƒs, ƒσ, and neutron scattering form factor for [NiF6]4− with the experimental data proves the applicability of the method proposed.
The Adiabatic Approximation in Transient Donor Recombination
Authors: Pickin W. 
Published: July 2007

AbstractResults obtained for the steady‐state case in a previous paper are extended to the transient case for non‐equilibrium phenomena, such as photoconductivity and impact ionization, involving donors with excited states. The solution of the transient rate equations is in general so complicated as only to be feasible in small signal theory. However, it is shown that if NA is much less than a critical value ÑA (NA being the acceptor concentration of the sample), then one may apply the adiabatic approximation, and the solution reduces to a trivial extension of the steady‐state result. Numerical results for ÑA are given for various values of temperature, applied field, and electron concentration.
Dispersion and Damping of Phonon‐Like Excitations in Disordered Systems
Authors: Kleinert P. , Leihkauf R. 
Published: July 2007

AbstractModel systems are considered to investigate the dispersion and damping of phonon‐like elementary excitations in structural disordered chains. An approximate separation is carried out between the dynamical and structural disorder. For a rectangular distribution of the force constants the concept of quasistationary plane‐wave‐like excitations is inappropriate at the band edges. The influence of random equilibrium positions on the complex dispersion relation is also investigated. The quasi‐crystalline approximation is generalized to include the quasiparticle damping and the low‐temperature specific heat excess is briefly discussed.
Resonance Raman Scattering from Neutral and Doped Polyacetylene
Authors: Kuzmany H. 
Published: July 2007

AbstractA complete assignment for the modes obtained from resonance Raman scattering on cis‐ and trans‐polyacetylene is reported. The Raman lines in the cis‐isomer indicate a well expressed one dimensional behaviour of the chain. From the line profile and line position of the resonance enhanced CC stretch modes in the trans‐isomer a chain length distribution of modified Gaussian form with a maximum at 30 CC double bonds and a half width of 20 CC bonds is deduced. The experimental results confirm the alternating bond model and lead to a sine wave chain potential with amplitude 1.9 eV. The effective mass of the π‐electrons is determined for the first time experimentally to be equal to the free electron mass. Intercalating the cis‐polymer with AsF5 results in a cis‐trans‐transition and a pronounced chain shortening. It is concluded from the experiments that the intercalating molecules precipitate between the chains.
Demonstration of the Ghost Effect in the Presentation of the Orientation Distribution of Texturized Materials
Authors: Matthies S. , Pospiech J. 
Published: July 2007

AbstractThe orientation distribution function (ODF) reproduced from pole figures obtained by diffraction measurements is burdened with erroneous distribution densities (ghosts) which are coupled in a definite manner with the true distribution. This effect, being a consequence of the pole figure measurement symmetry, is demonstrated on a model of the rolling texture of brass. Possibilities of ghost corrections are outlined.
Cohesion and Cauchy Discrepancy in Stable Simple Cubic Metals
Authors: Rathore R. P. S. , Agarwal R. M. 
Published: July 2007

AbstractIt is shown that, within the framework of the electron gas central pair potential model, the bulk modulus and the pressure associated with the conduction electrons are responsible for the Cauchy discrepancy and cohesion exhibited by stable simple cubic metals. It is shown that the correlation part of the electron energy, along with the Fermi and exchange parts at the proper electron separation, effectively contribute towards the desired values of the bulk modulus and the pressure.
Effect of Nonmagnetic Impurities on the Néel Temperature of Chromium
Authors: Dǎdârlat D. , Gulacsi Z. 
Published: July 2007

AbstractThe effect of nonmagnetic impurities on the Néel temperature of chromium is investigated, using a Feders‐Martin model with an energy dependent density of states. The effect of the impurities on the Fermi energy is also considered. It is found that, in the case when the topology of the Fermi surface of chromium is changed by the impurities, the dependence of the critical temperature of chromium on the impurity concentration c is nonlinear and TN may increase or decrease with increasing c.
On the Electronic Structure of Transition Metal Carbonitrides, Carboxides, and Oxinitrides. II. A Study of the Electronic Structure of TiCxN1−x, TiCxO1−x, and TiNxO1−x in Terms of the Relativistic KKR‐ATA Method
Authors: Weinberger P. 
Published: July 2007

AbstractIn part II the relativistic KKR‐ATA method is employed to investigate the electronic structure of TiCxN1−x, TiCxO1−x, and TiNxO1−x. Two different models for the description of the electronic structure are discussed, the more appropriate one being used to calculate the Ti‐p3/2‐like local density of states and the Fermi energies for these systems with respect to the molar fraction. In particular the relevant terms for the effective lattices and the integral as well as the differential density of states are discussed in some detail. A comparison to X‐ray emission spectra proofs that the electronic structure calculated for these systems is at least qualitatively correct.
Estimation des corrections “non muffin‐tin” à l'équation d'état des métaux simples
Authors: Perrot F. 
Published: July 2007

AbstractOn établit, dans le cadre de la théorie des perturbations au second ordre, l'expression formelle de la correction à l'énergie d'un solide, associée aux effets “non muffin‐tin”. On applique cette formule au cas des métaux simples, ou la densité auto‐cohérente est obtenue par la théorie du potentiel modèle de Shaw. On obtient ainsi une évaluation approximative mais systématique de ces corrections qui n'ont jamais été calculées exactement à notre connaissance. Elles sont très faibles dans l'ensemble des métaux étudiés, n'entraǐnant pas une variation de plus de 3% sur l'énergie de cohésion et de 1% sur le volume d'équilibre.
A Study of Point and Planar Defects in Copper
Authors: Miller K. M. 
Published: July 2007

AbstractThe real‐space computer simulation technique is used to study point and planar defect properties in copper and the results are compared with a previous study on nickel. A new interatomic potential function to represent copper is produced which reproduces the most recent values of formation and migration energies for a vacancy and also the intrinsic stacking fault energy. Using this potential, the relaxation around a single vacancy, nine interstitial configurations, the intrinsic and extrinsic stacking faults and the twin boundary are simulated. The vacancy results are equivalent to those calculated previously using the lattice statics technique. The interstitial results show the < 110 > split configuration as having the lowest formation energy, 6.2 eV, and formation volume, 0.67 Ω. For all interstitial configurations, the elements of the defect strength tensor are calculated and compared with results from diffuse X‐ray scattering experiments on copper. The energies of the twin, intrinsic, and extrinsic faults are found to be 29, 55, and 58 mJ m−2, respectively.
Hot Electron Effects in Semiconductor Luminescence
Authors: Dean P. J. 
Published: July 2007

AbstractFine structure near the threshold for recombination of free holes at neutral donors in InP is attributed to the radiative decay of excitons weakly bound at ionized donors. The conditions for optimum development of this optical structure are discussed; dilute doping, low temperature, and optical excitation rates and photoexcitation energies close to the direct energy gap. It is argued that the separation of electrons and holes and the intense local generation of phonons which occurs when this latter criterion is strongly violated favours free particle rather than bound exciton recombination processes. Comparisons are drawn with the behaviour of an unusual green photo‐luminescence spectrum in ZnSe, where the transition probability seems strongly enhanced by promoting phonons although very weak coupling is exhibited to accepting phonons.
Theory of Double Spin Flips in Nuclear Double Resonance
Authors: Whitaker M. A. B. 
Published: July 2007

AbstractDouble spin flips in nuclear double resonance are studied using the Zwanzig method. A master equation is produced, and temperatures are assumed for each Zeeman reservoir and for a combined secular dipolar reservoir. A set of simultaneous differential equations is obtained for the development of the system, and these are solved for a number of cases.
The Role of Quantum Effects in Lattice Dynamics of Rubidium Halides
Authors: Singh R. P. 
Published: July 2007

AbstractThe generalized shell model, developed by incorporating the van der Waals interaction and the zero‐point energy in the framework of three‐body force shell model, is applied to study the dispersion of phonons, Debye temperature, and Raman spectra in rubidium halides. The study reveals the excellent agreement between theory and experiments.
Calculation of the Electronic Density of States in Liquid Metals
Authors: Halder N. C. 
Published: July 2007

AbstractAn expression for the electronic density of states in liquid metals is derived using nonlocal and energy dependent pseudopotentials. The electronic energies are expressed to second order in perturbation theory. Both the first order and second order energy terms show the characteristics of k‐dependence which were not observed previously in the local pseudopotential theory with no Fermi surface approximation. Especially the density of states expression is found to have more structures, in that it involves a number of additional terms due to the nonlocality of the pseudo‐potentials. The present theory is applied to liquid Hg and Cd and some numerical calculations are made. These results are compared and discussed with similar results from other calculations.
Additional Waves and Polariton Dispersion in CdS Crystals
Authors: Makarenko I. V. , Uraltsev I. N. , Kiselev V. A. 
Published: July 2007

AbstractA study of additional waves in the exciton region of CdS crystals is reported. Transmittivity and reflectivity of thin crystals is measured showing a structure due to the Fabry‐Perot interference and the interference of additional waves. This structure is a direct reflection of the polariton dispersion law. Using the formulas for a spatially dispersive platelet and Pekar's additional boundary conditions the theoretical reflectivity curves are obtained numerically reproducing the essential experimental results. Values for all physical parameters defining dispersion and damping of the polaritons are found. Some evident deviations between the experimental curves and the present computer results for the reflectivity are pointed in connection with a discussion of the role of the surface exciton potential.
Investigations of the Energy Gap of V3Si Monocrystals
Authors: Schumann J. 
Published: July 2007

AbstractOn the surface of single phase V3Si single crystals, tunnel junctions with reproducible and stable characteristics are prepared by an anodic oxidation method for the barrier preparation and by Pb and In top electrodes. Detailed energy gap investigations yield Δ0 = (2.52 ± 0.05) meV for V3Si. Based on this energy gap value the evaluation of the ratio 2Δ0/kTc is discussed and conclusions about the strength of electron‐phonon interaction in V3Si are given. The temperature dependence of the energy gap obtained by measurements in the temperature range 1.5 K. ≤ T ≤ 20 K can be described by the relation Δ(T)/Δ0 = tanh [Tc Δ(T)/TΔ0]. In order to exclude measuring errors, a numerical evaluation of the Δ(T) values is provided resulting in a Tc‐lowering in the surface range. Furtheron, at T = 1.5 K, the measurements show an additional gap structure, the cause of which is discussed. Investigations of the tunnel characteristics measured in different crystallographic orientations do'nt yield any measurable anisotropy of the energy gap.
On the Theory of Resonance Raman Scattering in Semiconductors. A Green's Function Approach
Authors: Quang Doan Nhat , Esser B. , Keiper K. 
Published: July 2007

AbstractA Green's function approach baaed on an S‐matrix expansion of the light induced current fluctuations is developed. Using a two‐band model the theory is applied to the one‐phonon resonance Raman scattering in degenerate semiconductors. The discrete (phonon line) and continuous parts of the scattering cross section are calculated. In the dispersion curve of the discrete part pronounced peaks at the energy of the Burstein‐Moss shift (and that plus phonon energy) are obtained. The conditions for single and double resonance of the continuous part are given. Plots illustrating the resonance behaviour are shown.
A Study of the Concentration Dependence of Hall and Verdet Coefficients in the Hubbard Model
Authors: Kogutyuk I. P. , Nitsovich V. M. , Skrypnik F. V. 
Published: July 2007

AbstractThe dependence of the Hall and Verdet coefficients on temperature and on the filling of the narrow energy band is investigated. The sign of the Hall and Verdet coefficients is shown to change three times as the band filling varies. Calculations are made by the Green function method on the basis of the Hubbard Hamiltonian.
Thermally Stimulated Depolarization of NCO− Centres in Alkali Halide Crystals
Authors: Wilbrand A. , Fischer F. 
Published: July 2007

AbstractIn single crystals of chlorides, bromides, and iodides of sodium, potassium, and rubidium a TSD (also named ITC) peak between 65 and 29 K is found which is due to NCO− centres. The up to 20% broader bandwidths, compared to the theoretical shape, are attributed to residual stresses of the probe. The experimentally determined reorientation enthalpies of the centres range between 0.192 and 0.080 eV and show a linear dependence on the lattice constant within the investigated sodium, potassium, or rubidium halides. The experimental data of the electric dipole moments are in reasonable relation to calculated values of the free NCO− ion.
Phonon Modes in a Disordered Lattice
Authors: Chakrabarti B. K. , Roy G. K. , Sinha S. K. 
Published: July 2007

AbstractThe vibrational excitations are studied using the lattice Green function expansion for a disordered simple cubic system with degenerate modes. Both mass and force constant disorders are considered. The resulting dispersion relation, including damping, of the long wavelength longitudinal phonon modes are obtained in the limit of small impurity concentrations. The results are obtained analytically and are exact up to the linear term in concentration. In a simple mass defect case, the results reduce to those obtained by Langer.
Band Structure of MgxZn1‐xTe Alloys
Authors: Montegu B. , Barbier D. , Chevallier J. , Guillaume J. C. , Somogyi K. , Laugier A. 
Published: July 2007

AbstractThe band structure of MgxZn1‐xTe alloys is studied using a double beam wavelength modulated system in first derivative mode. Modulated reflectivity measurements are made from 82 to 300 K within spectral range 2500 to 5400 Å. Structures corresponding to the E0, E0 + Δ0, E1, E1 + Δ1, e1 and e1 + Δ1 critical points are indexed on the basis of existing band calculations for ZnTe.
Crystal Stability and Three‐Body Interactions
Authors: Halicioǧlu T. 
Published: July 2007

AbstractStructural stabilities of simple crystals (h.c.p., f.c.c., and b.c.c.) are investigated using both, two‐body and three‐body interactions. A parametrical analysis is performed. The results emphasized the importance of multibody forces to explain structures such as b.c.c. Furthermore, calculations predict that, in general, the stability of crystals changes as h.c.p. → f.c.c. → b.c.c. with increasing intensity of three‐body interactions. For selected cases the possibility of structural transformations (such as b.c.c. → f.c.c.) are discussed.
The Energy Band Structure of Corundum
Authors: Evarestov R. A. , Ermoshkin A. N. , Lovchikov V. A. 
Published: July 2007

AbstractThe energy band structure of corundum is investigated theoretically using the semiempirical Mulliken‐Rüdenberg method. A crystal potential is constructed basing on the self‐consistent effective atomic charges and electronic populations obtained within the framework of the method used. The connection of the energy bands with the atomic states of the constituent atoms is discussed. The valence band widths and the electronic density distribution appear to be in good agreement with experimental data.