bluuebunny/crossref_metadata_2025_split huggingface dataset:
Charge Instabilities in Lateral Super‐Lattices under Conditions of Population Inversion
Authors: Ferry D. K. 
Published: July 2007

AbstractLateral super‐lattices along the surface of a semiconductor can generate mini‐band structure for distances of the order of 1000 Å or less. The detailed gap structure depends noticeably on the charge in the structure. Under conditions of strong population inversion, synergetic switching behavior and space‐charge waves can be expected to occur.
Structure de bande électronique de CdAl2Te4 dans les modifications avec les groupes spatiaux D, D, D et S
Authors: Chizhikov V. I. , Panyutin V. L. , Ponedelnikov B. E. , Rozenson A. E. 
Published: July 2007

AbstractLes structures de bande d'énergie sont calculées pour CdAl2Te4 dans quatre modifications par la méthode du pseudopotentiel empirique. Le pseudopotentiel du cristal ternaire utilisé dans ce calcul a été approximé par une superposition des pseudopotentiels atomiques des semi‐conducteurs binaires. La relation entre les structures de bande électroniques dans les modifications de CdAl2Te4 est discutée.
Lattice Mode Anharmonicity in Sodium Nitrate
Authors: Brehat F. , Wyncke B. , Hadni A. 
Published: July 2007

AbstractThe anharmonicity of the infrared active lattice modes of sodium nitrate is studied by measurements of the temperature dependence of the reflectivity. The infrared reflectivity is recorded from 10 to 600 cm−1, in the temperature range 23 to 300 °C, and analysed with the four‐parameter semi‐quantum model. The comparison between the results of the quantum theory and the observed dependence of damping shows that cubic anharmonicity drives the lattice dynamics of sodium nitrate in the low‐temperature phase. Damping divergence is observed for several modes in the vicinity of the phase transition temperature.
Emission Spectra of O and N Atoms and the Structure of Solid N2/Ne Mixtures
Authors: Sayer R. J. , Prince R. H. , Duley W. W. 
Published: July 2007

AbstractThe low temperature emission spectra of both the α(N2D−4S) and β(O1S−1D) transitions of N and O atoms trapped in solid nitrogen—neon mixtures are investigated using low energy electron excitation at a variety of N2 concentrations. It is determined that N2/Ne solids, formed fromed from gas phase deposition, solidify in one of three characteristic types of binary solids over the entire concentration region examined. It is shown that information on the crystal structure of these mixtures may be obtained from spectral data, extending information obtained from electron diffraction studies.
The Mechanism of Participation of Non‐Magnetic B‐Site Cations in the Superexchange in Spinels
Authors: Mayer I. , Angelov S. 
Published: July 2007

AbstractThe superexchange between magnetic cations at the A‐sites of spinels with non‐magnetic B‐site cations (e.g. CoAl2O4, Co3O4, CoRh2O4 etc.) is analysed by using a four‐electron, four‐orbital model. The problem is treated in the framework of the CI‐method up to the fourth‐order of the perturbation theory. In the case of Co(III) and Rh(III) in the low‐spin state as B‐site cations four contributions are found: two negative and two positive. One of the ferromagnetic terms is not connected with Hund's rule but has the same origin (delocalization) as the antiferromagnetic ones. The resulting superexchange is always antiferromagnetic. When B is Al3+ there is only one small antiferromagnetic term.
EPR Investigations on the Dynamic Behaviour of Hole Centres in CaWO4
Authors: Cord B. , Hofstaetter A. , Scharmann A. 
Published: July 2007

AbstractBy X‐irradiation holes are created in the p‐orbitals of oxygen ligands of the WO2–4 complexes of CaWO4. These holes are stably trapped at temperatures below 150 K and are characterized by a hyperfine interaction with the W183 nuclei of two neighboured WO4 tetrahedra. When raising the temperature from 4.2 to 77 K this interaction changes from an inequivalent to an equivalent one. This thermally activated change is accompanied by a reorientation of the g‐tensor and a maximum in the EPR linewidth.
Micromagnetic Theory of Phase Transitions in Inhomogeneous Ferromagnets IV. Dynamics of the Spin System
Authors: Fähnle M. 
Published: July 2007

AbstractA time‐dependent molecular field theory (Landau‐Ginzburg theory) with correlations in space and time is proposed to calculate the dynamical properties of a spin system near the ferromagnetic phase transition. This theory moreover is a mean field theory, but it takes into account the effect of the thermal and spatial spin fluctuations implicitly when calculating the molecular field. The critical slowing down of the relaxation of the magnetization near the transition temperature, Tc, is enhanced in comparison to the result of the conventional molecular field theory as an effect of the finite lifetime of the spin clusters, even in spatially homogeneous materials. A further enhancement of the critical slowing down is obtained for the case of amorphous ferromagnets with spatial fluctuations of the exchange integral due to the finite size of the spin clusters for temperatures T > Tc. The relation between this dynamic theory and the static spatially correlated molecular field theory recently proposed for the calculation of the static paramagnetic susceptibility for amorphous ferromagnets is discussed in full detail.
Orbit–Lattice Interaction Hamiltonian for Iron‐Group Ions in Non‐Cubic Sites
Authors: Barker G. C. , Tucker J. W. 
Published: July 2007

AbstractThe orbit–lattice Hamiltonian for 3d ions in tetragonal and trigonal XY4 and XY6 complexes with D2d, C3v, D4h, and D3d symmetry is derived. A point‐ion model is adopted and the Hamiltonian is expressed in terms of operator equivalents in a form that exhibits clearly the symmetry of each component. The results are applied to a calculation of the phonon‐induced transitions in the ground state of V3+ in Al2O3. The ratio of the strengths of the ΔMs′ = 2 and the ΔMs′ = 1 transitions can depend strongly on whether the correct symmetry of the magnetic ion is accounted for – in particular it is shown that the commonly invoked 'cubic approximation' for the vibrational modes can lead to substantial errors.
A Quantitative Estimation of the Admixture in the Ground State Wave Function of Cu2+ in Low‐Symmetry Crystal Fields
Authors: Bhaskar Rao T. , Narayana M. 
Published: July 2007

AbstractA method of calculating the ground state wave function for Cu2+ in rhombic crystal fields using the spin‐Hamiltonian parameters is discussed. The expressions for spin‐Hamiltonian parameters are deduced when the ground state contains comparable contributions from |x2 – y2 and |3z2 – r2 including the effect of covalency. The effectiveness of these expressions is illustrated by estimating the wave function for Cu2+ in four pseudo‐Jahn‐Teller systems, with varying degrees of admixture in the ground state.
Temperature Dependence of Second‐Order Spin‐Lattice Coefficients for Eu2+ in MeF2 Single Crystals
Authors: Kuriata J. , Guskos N. , Rewaj T. 
Published: July 2007

AbstractThe temperature dependence of the second‐order spin‐lattice coefficients is investigated for BaF2:Eu2+ single crystals. The dependence is interpreted in terms of the simple Einstein model for phonon interactions. An analysis of the phenomenological parameters derived for BaF2:Eu2+ together with those published earlier for other systems indicates the fact that good consistency of the experimentally determined parameters with the implications of the Newman‐Urban superposition model is obtained if the rigid‐lattice values are taken into account.
Pressure dependence of superconductivity in non‐transition metals
Authors: Jain S. C. , Kachhava C. M. 
Published: July 2007

AbstractAn expression for the volume dependence of electron‐phonon coupling strength is derived by considering the corresponding variation of Fermi momentum explicitly. Numerical data for the critical volumes deduced from the formulation compare fairly well with the existing theories. The calculated values of logarithmic volume derivative of effective interaction strength are in good accord with experiments. The volume dependence of superconducting transition temperature evaluated for three representative cases of weak, medium, and strong coupling superconductors leads to the conclusion that the theory reproduces the experimental data satisfactorily.
Density of states in amorphous Si:H
Authors: Overhof H. , Beyer W. 
Published: July 2007

AbstractThe density of states in the pseudogap of doped and undoped glow‐discharge prepared amorphous silicon is determined from transport data. Thermopower and conductivity are combined to calculate the position of the Fermi level as a function of temperature and doping rate. An effective density of states model is constructed that can account for these data. The true density of states is then determined under the assumption of a simple model for the correlation energy. It is shown that this model can account in detail for the doping dependence of the spin densities of various spin resonance lines recently reported.
Recombination kinetics of electrons with α‐ centers in colored alkali halides
Authors: Hoffmann H. J. 
Published: July 2007

AbstractThe recombination kinetics of conduction electrons with halogen‐ion vacancies (α‐centers) in additively colored alkali halides is calculated and discussed for the high‐temperature range, in which concurring F′‐centers decay thermally. The steady state concentrations of conduction electrons, α‐, and F′‐centers as well as the time constants are explicitly given under the assumption that the charge carriers recombine with α‐centers predominantly via the excited state F* of the vacancies. These results are compared with experimental data and former theoretical calculations for the case that recombination directly into the ground state is predominant. The results on the effective recombination coefficient can also be applied to other defects in photoconductors and semiconductors if the charge carriers recombine via an excited state.
Disorder‐induced broadening for free direct excitons in II–VI and III–V semiconducting mixed crystals
Authors: Hennig D. , Strehlow R. 
Published: July 2007

AbstractA theoretical model is presented which is a modified version of an earlier description of indirect excitons in semiconductor mixed crystals. The influence of short‐range disorder on the broadening of free direct exciton lines from the scattering of the exciton by this disorder potential is studied. This leads for II–VI mixed crystals to a remarkable contribution to the observed disorder‐induced broadening of the exciton lines while for the III–V mixed crystals this contribution is negligibly small.
Electronic band structure of pseudodirect chalcopyrite semiconductors: II. ZnGeP2 and ZnSiAs2
Authors: Heinrich A. , Cordts W. , Monecke J. 
Published: July 2007

AbstractThe band structures of the chalcopyrite semiconductors ZnGeP2 and ZnSiA2 are calculated using the empirical pseudopotential method as applied successfully already to ZnSiP2 and CdSiP2. The band structures are in good agreement with experimental data. The influences of all three tetragonal parts of the crystal potentials (antisymmetric cation potentials, tetragonal compressions, and distortions of the anion sublattices) on the band structures and on optical transition energies are investigated in detail.
The CNDO/BW study of lithium clusters. I. Two‐, three‐, and four‐atomic clusters
Authors: Skála L. 
Published: July 2007

AbstractThe CNDO/BW method is used for a detailed study of Li2, Li3, and Li4 clusters. The equilibrium distances between atoms, binding energies, ionization potentials, dipole moments, charges at atoms, and bond orders are investigated for 14 different geometries of the clusters. The most stable geometrical forms are found and compared with available theoretical and experimental results.
Excitation spectra of disordered systems. II. Two‐and three‐dimensional disordered systems
Authors: Sinai J. J. , Wu Shi‐Yu , Chen Y. C. 
Published: July 2007

AbstractThe approximate theory developed earlier for calculating the ensemble average of the Green's function for disordered systems is applied to the dynamics of a disordered square lattice and a disordered cubic lattice. Good agreement with numerical calculations is demonstrated.
Low‐temperature renormalization group study of the random spin systems
Authors: Li Mai Xuan , Duc Nguyen Manh 
Published: July 2007

AbstractThe second‐order phase transition of a weakly disordered system in 2 + ϵ dimensions is investigated by means of the low‐temperature renormalization group technique. It is shown that there are two fixed points: the Migdal‐Polyakov and the unphysical ones. As the Migdal‐Polyakov fixed point is always stable it is concluded that the disordered and the ordered systems have the same critical exponents. These results agree with the Harris hypothesis, which predicts that phase transitions with positive value of α do not occur in the presence of quenched disorder. The effects of the long‐range exchange interaction on the critical behaviour of the quenched system in σ + ϵ dimensions are also discussed. The criteria for the concentration of nonmagnetic impurities in the systems under consideration are established.
The model potential for charged centres in semiconductors
Authors: Olejníková B. , Hrivnák Ľ. , Kedro M. 
Published: July 2007

AbstractThe model potential of an electron U(r) = −e2/4πϵ0ϵsr [1 + (ϵs − 1) exp (‐βr)] is, appliedtodeep charged centres in semiconductors instead of the commonly used combination of Coulombic and δ‐function potentials. The ground‐state energy of such a centre, photoionization cross‐section, and ionization energy lowering due to an applied electric field are calculated for various values of the parameters β and the values m* and ϵs corresponding to GaAs.
Nonlinear parametric interaction in a piezosemiconducting medium
Authors: Sinha D. K. , Gupta M. 
Published: July 2007

AbstractAn attempt is made to investigate the nonlinear parametric interaction of acoustic waves in n‐type piezoelectric semiconductors, the nonlinearity arising from the nonlinear transport property of electrons. The equations of mechanical motion and of electricity supplemented by basic equations of piezoelectric media are used to solve the problem. In the present analysis the gain of energy of the signal wave is theoretically investigated for different amplitudes of the pump wave, assuming the interaction to be of degenerate type and the drift velocity of electrons exactly equal to the sound wave.
Planar channeling of relativistic protons in bent crystal
Authors: Taratin A. M. , Vorobiev S. A. 
Published: July 2007

AbstractA calculation of the dechanneling half‐length x1/2 for relativistic protons in a bent crystal is carried out with account of the valence electron density. The energetic dependence of particle channeling in curved channels is discussed and the influence of the beam divergence on the trapping of relativistic protons in channeling and their dechanneling in bent crystals are considered.
Photosensitive EPR spectra of Pb‐doped ZnS single crystals
Authors: Dziesiaty J. , Backs D. , Mantwitz D. 
Published: July 2007

AbstractPhotosensitive EPR investigations of lead‐doped ZnS single crystals have been made. The results of excitation and quenching measurements are discussed in terms of the A‐centre model, assuming a Zn vacancy associated with a neutral Pb2+ impurity on next cation site. A simple level model, involving the energetic positions of the photosensitive impurities in the band gap of ZnS, is proposed to explain the EPR excitation and quenching spectra.
Aspects of electrical conductivity in a moving base approach
Authors: Wagner M. 
Published: July 2007

AbstractThree types of unitary transformations (Fröhlich, adiabatic base, modified Bloch‐Whitfield) are contrasted. The adiabatic base approach improves the Fröhlich philosophy: the effective electron‐electron interaction turns out to be purely attractive, and it also is symmetric in the involved electronic wave vectors. The rise of superconductivity proves to be a purely nonadiabatic effect. Electrical resistivity is split into adiabatic and nonadiabatic contributions and depends both on the vibrational and electronic occupation. Moreover it turns out that one never can get rid of diagonal electron‐phonon interactions. These establish selfconsistent interrelations between lattice dynamics and electronic occupation, which most simply may be handled by the Bogolyubov inequality. The modified Bloch‐Whitfield transformation seems to offer a possibility to calculate Tc and electrical conductivity with a somewhat diminished knowledge about the atomic potentials. Perspectives for detailed calculations are given.
Binding energies and chemical shifts of least bound core electron excitations in cubic ANB8−N semiconductors
Authors: Bechstedt F. , Enderleln R. , Wischnewski R. 
Published: July 2007

AbstractCore electron binding energies EB with respect to the vacuum level and their chemical shifts are calculated for the least bound core levels of cations and anions of cubic ANB8−N semiconductors. Starting from the HF‐binding energy of the free atom absolute values of EB are obtained by adding core level shifts and relaxation energies. Core level shifts are calculated by means of an electro‐static model with ionic and bond charges according to Phillips' bond charge model. For the calculation of relaxation energies the linear dielectric theory of electronic polarization is applied. Valence and core electrons, and diagonal and non‐diagonal screening are taken into account. The theoretical results for chemical shifts of binding energies are compared with experimental values from XPS‐measurements corrected by work function data. Good agreement is obtained in all cases within the error limit of about one eV. Chemical and atomic trends of core level shifts, relaxation energies, and binding energies are discussed in terms of changes of atomic and solid state parameters. Chemical shifts and relaxation energies are predicted for various ternary ANB8−N compounds.
Kinetics of Defect Accumulation and Recombination: II. Diffusion‐Controlled Defect Annihilation
Authors: Kotomin E. , Kuzovkov V. 
Published: July 2007

AbstractIn the present part of the paper the kinetic equations, derived in its first part and taking into account the spatial correlations of both, the similar and dissimilar defects, are applied to both the theory of diffusion‐controlled defect annihilation, A + B → 0, and energy transfer, A + B → A. It is shown that only taking into account the correlation of the similar defects, one can describe the transition from bimolecular to monomolecular kinetics if the correlation within initially created Frenkel pairs varies from a random distribution to a very strong one. Even for the case of initially uncorrelated pairs of dissimilar defects an annihilation stimulates with time the creation of the similar defect correlation. In turn, it results in the deviation from the wellknown constant of quasi‐steady state recombination, k0 = 4πDRa. The present theory is compared with analogous theories by Waite and Antonov‐Romanovskii.
A High‐Order Magnetoelastie Hamiltonian for the Conduction Electron Contribution in Cubic Rare‐Earth Metals
Authors: Del Moral A. 
Published: July 2007

AbstractThe effect of the conduction electrons on the magnetoelastic interaction due to the crystal field in cubic rare‐earth metals is treated within a model similar to the so called deformable ion one. The resultant effective Hamiltonian has the proper dependence on the irreducible strains and angular momentum operators as the usual second‐order Hamiltonian, but it is now modulated by an operator proportional to the unperturbed cubic crystal field Hamiltonian. This Hamiltonian is particularized for a symmetry quantization axis. Expressions for the magnetostrictive equilibrium strains are provided to show their peculiar temperature dependence.
Theory of the Nuclear Magnetic Resonance Lineshape in Magnetically Diluted Crystals
Authors: Aliev M. N. 
Published: July 2007

AbstractThe lineshape of the nuclear magnetic resonance in diamagnetic crystals with paramagnetic impurities is calculated with the help of the Green's function method for the case when both, the nuclear and electron spins are ½. The linewidth and the line shift at slow and rapid fluctuations of the local field produced by the paramagnetic impurities on the nuclei of the diamagnetic atoms magnetic are calculated.
Enhancement of Polaron Effect in Thin Dielectric Films
Authors: Sherman A. V. 
Published: July 2007

AbstractThe binding energy and effective mass of a strong‐coupling large polaron in a polar crystal film are calculated. The film with a thickness less than the polaron radius and a high‐frequency dielectric constant larger than unity is assumed to be surrounded by vacuum. It is shown that the absolute value of the binding energy and the effective mass of such a polaron may considerably exceed the corresponding values for an infinite crystal.
Effect of Anharmonicity on the Second‐Order Doppler Shift for a Vacancy‐Trapped Mossbauer Impurity Atom in Metallic Solid Solution
Authors: Acharyya P. K. , Tewari A. K. , Roy S. K. , Bhattacharya D. L. 
Published: July 2007

AbstractThe anharmonic contributions to the second‐order Doppler shift are estimated theoretically for a simple lattice model containing impurities and vacancies at the lattice points by considering the double‐time Green's function technique in the high temperature limit. The cubic anharmonic terms in the expansion of the potential energy function of the proposed model are found to produce a significant change in the time dependence of the correlation functions and result in the appearance of additional terms proportional to the temperature in the expression for the energy shift. The estimated result thus shows that the effects of anharmonicity on the second‐order Doppler shift become important at high temperatures and it becomes possible to estimate the changes in the third‐order force constants of the defect lattice.
Thermally Stimulated Depolarization of NO−3, NO−2, and PO−2 Centres
Authors: Nierste G. , Fischer F. 
Published: July 2007

AbstractA TSD peak is found between 18 and 33 K in single crystals of sodium and potassium halides doped with NO−2 and NO−3 centres. The peak belongs to NO−2 in sodium halides and to NO−3 in potassium halides. The discrimination is possible by applying pressed pellets as normally used in IR absorption work. This new technique allows TSD measurements below 200 K. The electric dipole moment p of NO−3 centres is of a pure displacement type. In KCI:NO−3 the experimental value of p is 0.52 e Å. It reasonably agrees with that obtained from a model which explains the off‐centre position of the NO−3 mainly by the repulsive potential of the surrounding lattice ions. In case of PO−2 centres TSD peaks are found in potassium halides and in RbBr between 29 and 39 K.
Effect of Solid Dilution and Temperature on the Crystal Field Spectra of Chromium Ions in Single Crystals of Chrome Alum
Authors: Kesavamoorthy R. , Krishnaswamy R. 
Published: July 2007

AbstractThe optical absorption spectra of mixed single crystals of chrome alum and potash alum are studied for different concentrations at room temperature and for one concentration at different temperatures between 100 and 310 K. The cubic crystal field splitting parameter Dq, and the interelectronic repulsion parameter B are obtained using the observed values of the peak positions. It is found that Dq, B, and half width at half maxima Γ of the spectral bands associated with the paramagnetic chromium ion in chrome alum do not change on replacing varying amounts of chromium ions with the diamagnetic aluminium ions. The temperature dependence of Γ is explained on the basis of the vibronic transition and that of Dq on the basis of the results of crystal field theory. A simple model is proposed to explain the temperature dependence of B.