bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic magnetic moments of Fe, Co, Ni and hyperfine fields at impurities in ferromagnetic iron
Authors: Galperin F. M.
Published: July 2007

Magnetic moments of free atoms of the elements Fe, Co, and Ni of Mendeleev's periodic table (MPT) are calculated. The calculated values are equal to the measured (fractional) ones of the atomic magnetic moments of Fe, Co, and Ni ferromagnetic metals. Further, a hyperfine field is calculated at the nucleus of the impurity atom of the transition 3d element in ferromagnetic iron. It is shown that this field depends on the own field at the nucleus of the impurity atom.
Strong coupling of electrons to black body radiation at high temperatures
Authors: Hora H.
Published: July 2007

An evaluation of the Richardson equations for electron emission leads to the most general formulation including the branches of degeneracy, classical statistics, and that of strong photon interaction. The photoelectric yield constant is derived in general form. For black body temperatures above mec2, strong coupling is found generally, while for lower temperatures all three branches exist, depending on the electron density. A confirmation of the consistency of the theory is given by the fact that the quantum process of photoemission does not take more than 5% of the energy density of the black body radiation.
Room temperature FF′ conversion in KCl crystals of various purity and method of coloration
Authors: Nierzewski K. D. , Todorov T. , Georgiev M.
Published: July 2007

The FF′ conversion is initiated by flash‐light excitation at room temperature with the aim of obtaining data on the electron trapping and transition characteristics in KCl crystals of various purity and coloured in different ways. The purest, γ‐irradiated sample exhibits a relatively short F' lifetime and a proportionally large F centre‐to‐anion vacancy cross‐section ratio presumably resulting from an enhanced trapping ability of the F centre. In contrast, Na‐containing irradiated samples show unusually long F' lifetimes, not correlated to the corresponding cross‐section ratios. The F' yield in the pure samples is found to drop on repeated flashing which presumably reflects the drop in F centre ionization quantum efficiency as a result of clustering processes. Electrolytically coloured samples doped with divalent cations are also investigated.
Raman scattering of HgCr2Se4
Authors: Iliev M. N. , Anastassakis E. , Arai T.
Published: July 2007

The Raman spectrum of the ferromagnetic semiconductor HgCr2Se4 (TC = 106 K) and its dependence on temperature and incident photon energy is measured. Five Raman‐active modes are found at 65, 157, 168, 210, and 237 cm−1 and are assigned to certain ionic vibrations in the spinel unit cell. The temperature variations of the scattering cross‐section for different modes show no pronounced anomalies near the magnetic phase transition at TC but depend significantly on the wavelength of the exciting radiation. The resonance curve at 300 K exhibits two peaks at 1.9 and 2.35 eV superimposed by a broad structure which increases toward low energies.
Carrier mobility in ferromagnetic semiconductors
Authors: Patil C. G. , Krishnamurthy B. S.
Published: July 2007

The electron mobility in ferromagnetic semiconductors below the Curie temperature incorporating electron–two‐magnon scattering is calculated besides the usual non‐magnetic scattering using the variational procedure. It is found that the mobility due to two‐magnon process is proportional to T−1/2 (T lattice temperature). The calculation is compared with the experimental Hall mobility data in CdCr2Se4 (In doped) accepting ionized impurity scattering as the non‐magnetic scattering. The calculated mobility variation with temperature is in close agreement with the experimental data indicating the importance of electron–two‐magnon scattering mechanism.
Vacancy‐induced splitting of excited states and the structure of Sn2+V centres in alkali halides
Authors: Hizhnyakov V. , Zazubovich S.
Published: July 2007

The polarization characteristics are calculated for the A and C emissions of Sn2+V centres in alkali halides, taking into account the rearrangement in the electronic level system of the excited sp states in the course of relaxation. The light absorption processes are considered in the approximation of a weak crystal field. The emission processes are described by a model in which the interaction of the optical electrons of impurity ions with Eg lattice vibrations (JTE) is assumed to be the principal, and the spin–orbit interaction and the interaction with T2g vibrations are taken into account as additive corrections. It is also assumed that in Sn2+V centres the influence of the cation vacancy V is small as compared with JTE. Comparing the results of the calculation with the experimental data, the order of three tetragonal minima on the adiabatic potential energy surface (APES) of the 3P state of Sn2+V centres is established which is consistent with the order of the corresponding minima of 1P state. It is shown that in both excited states Z minima are the lowest ones. The vacancy‐induced splitting of the relaxed excited A state is found to be about 10−2 eV. The limit values are determined for the polarization degrees of A and C emissions at various temperatures, the parameters are found and the structure is discussed for the Sn2+V centres in alkali halides.
Long optical vibrations and elastic constants of hexagonal boron nitride
Authors: Ramani R. , Mani K. K. , Singh R. P.
Published: July 2007

The long optical vibrations of hexagonal BN are studied using the valence force constants of the borazine molecule. It is shown that the in‐plane interactions are adequately described by the transferred molecular force constants. The elastic constant (c11 + c12), calculated from the above model compares well with the experimentally deduced value. It is argued that the interplanar forces cannot be satisfactorily described by a simple Lennard‐Jones potential.
Impurity scattering of electrons in quantizing magnetic fields
Authors: Baranskii P. I. , Demidenko L. S. , Korolyuk S. L. , Samoilovich A. G.
Published: July 2007

The isotropic scattering of current carriers is investigated in quantizing magnetic fields at arbitrary value of the action radius of the scattering centre. A variational method is suggested allowing to obtain the phase functions of scattered electron, by means of which the electroconductivity is calculated. A comparison is made with earlier works, which are valid for the short‐range forces. In semiconductors, as a rule, the case of an intermediate action radius of scattering centre is found. It is shown, that in the case of short‐range forces the known results are received in first approximation. Numerical calculations are made for the parameters of GaAs.
Theory of Stark‐Ladder Currents
Authors: Kümmel R. , Rauh H. , Bangert E.
Published: July 2007

Current and relaxation time of electrons are calculated for a lattice where a sufficiently strong applied electric field forms a Stark ladder. The population of states with momenta ℏk⟂ normal to the applied field is described by a function which is either uniform or a Maxwellian distribution. Various temperatures between 0 and 300 K are considered. The current‐voltage characteristic exhibits negative differential conductivity. Current peaks due to resonant phonon transitions between Stark‐ladder levels do not appear except in the case, when all electrons occupy states very close to k⟂ = o, which is not a stationary Stark‐ladder situation. Therefore and because of the smallness of the relaxation time of Stark‐ladder electrons interacting with LO phonons in ZnS, the previously observed resonant‐type current anomalies in ZnS should not yet be considered as the final experimental proof of the existence of Stark ladders.
Effects of Core and Correlation Corrections on the Electrical Resistivity of Lithium
Authors: Ball M. A. , Islam M. M.
Published: July 2007

The effects of correlation and core corrections on the electrical resistivity of lithium are investigated; the form factor is evaluated using the modified screened uniform charge model. The calculation of the screening is discussed, and a new form for the correction factor G(q) in the dielectric function ϵ(p) valid for q ≦ 2kF is suggested. The improvement of the calculated resistivity due to the core corrections are more significant than due to the changing the form of G(q).
The “Quasidirect” Radiative Recombination of Free Holes at Neutral Shallow Donors in GaP
Authors: Bindemann R. , Schwabe R. , Hänsel T.
Published: July 2007

The luminescence band at 2.24 eV in n‐type GaP is investigated in the temperature range of T = 27 to 120 K. It consists of a no‐phonon line and several phonon replicas (TAX, LAX, TOX), which overlap with the donor–acceptor pair band. According to the analysis of the line shape and the energetic position of the intensity maximum in dependence of the temperature, the excitation intensity, and the time after excitation this band can be interpreted as being caused by free (hole)‐to‐bound (electron bound at the sulfur donor) transition. Taking into account the consequences of the GaP band structure on the symmetry properties of the bound electron states the free‐to‐bound transition is described as ‘quasidirect’. The calculated line shape is in good agreement with experiment using an indirect gap value which is 11 meV higher than accepted hitherto. The obtained indirect gap energy and free exciton binding energy are determined to Eg(T = 4.2 K) = (2.350 ± 0.002) eV and EFE = (22 ± 2) meV, respectively.
Study of Linewidth and Exciton–Impurity Scattering in Cu2O
Authors: Nikitine S. , Elkomoss S. G. , Schwab C.
Published: July 2007

The profile of the n = 1 line of the yellow exciton series in Cu2O, which was studied experimentally as a function of the concentration of silver impurity, is calculated by the Lorentz theory used in atomic spectroscopy. According to this theory, the broadening of the line as a function of the impurity concentration is due to exciton–impurity elastic scattering. The theory depends of two parameters, namely the maximum absorption Kmax and the width of the line Δv1s, the values of which are taken from experiment. The line in the perfect crystal is assumed to have a pure Gaussian profile with an extrapolated width ΔvG = 0.57 cm−1. The calculations are also carried out for the values ΔvG = 0.4 and 1 cm−1. In silver doped crystals, the n = 1 lineshape is a contribution of a Gaussian and a Lorentzian component. The width of the Lorentzian ΔvL increases monotonically with impurity concentration. A good agreement between the calculated and observed profiles is obtained. The mean value of the exciton‐impurity scattering cross‐section σ, calculated from ΔvL, is of the order of 10−14 cm2. Further discussion of these results leads to useful information.
Magnetic‐Resonant Modulation of Photoconductivity of Crystalline Charge Transfer Complexes. Anthracene‐Tettacyanbenzene
Authors: Frankevich E. L. , Tribel M. M. , Sokolik I. A. , Pristupa A. I.
Published: July 2007

A magnetic‐resonant modulation of the photoconductivity of polycrystalline samples of CT complex anthracene–TCNB is observed. The modulation is due to microwave induced transitions between Zeeman sublevels of the excited triplet CT state. The magnetic resonance spectrum of the CT triplet state is registered, the lifetime of the latter is estimated to be about 10−9 s. Switching‐on the resonant microwave field decreases the photoconductivity. The nonresonant effect of the permanent magnetic field on photocurrent and fluorescence of anthracene–TCNB crystals is investigated. A scheme is given representing the processes of the electronic excitation energy conversion in the anthracene–TCNB CT complex.
Critical Behaviour near the Intersection of Second‐Order Phase Transition Lines in a Random System
Authors: Izyumov Yu. A. , Skryabin Yu. N. , Laptev V. M.
Published: July 2007

The critical behaviour of inhomogeneous systems with the coupled order parameters is studied by means of Wilson's renormalization group theory. A disordered binary magnet, which is characterized by the mixed exchange interactions, is the example of those systems. It is shown that there is no stable fixed point in the system corresponding to the intersection of phase transition lines, and, hence, the second‐order phase transition cannot take place in it.
Lundqvist Three‐Body Interaction and Lattice Dynamics of Ionic Crystals
Authors: Sarkar S. K. , Sengupta S.
Published: July 2007

The Lundqvist three‐body interaction which was originally deduced for rigid ions is rederived in the framework of a shell model. Some discrepancies in a previous interpretation are pointed out. Application is considered for NaCl crystal. If the parameter of the three‐body interaction is determined from the overlap integrals it is found that the calculated dispersion curves, in general, deviate more from the experimental ones. Some static properties are also calculated.
Electronic Absorption of SrCl2:Fe2+ and SrCl2:Co2+
Authors: Droste R. , Gmeiner R. , Geick R.
Published: July 2007

SrCl2 single crystals doped with Fe2+ and Co2+ are grown and the absorption spectra in the far infrared (5 to 100 cm−1) and middle infrared (400 to 4000 cm−1) spectral ranges are investigated. Several lines are observed which are assigned to electronic transitions of the impurity ions based on their dependence on the impurity concentration and on the temperature. The crystal field Hamiltonian appropriate for Fe2+ and Co2+ in SrCl2 is diagonalized. The crystal field parameter Dq and the spin–orbit coupling parameter λ are obtained from an optimum fit of the calculated level spacing to the experimentally observed transitions. Application of a magnetic field yields for both kinds of impurity ions the g‐factor of the ground state.
Jahn‐Teller Coupling of Cr2+ Ion in ZnS, ZnSe, and ZnTe
Authors: Biernacki S. W.
Published: July 2007

Calculations for the T–e‐problem of Cr2+ ion in ZnS, ZnSe, and ZnTe are performed. The coupling with acoustical and optical phonons of the entire phonon spectrum is taken into consideration. The Jahn‐Teller energy and the displacements of the neighbors nearest to the impurity which result from the Jahn‐Teller interaction are estimated. Attention is paid to the estimation of the Jahn‐Teller coupling constant on the basis of the simple crystal field theory.
Validity Range of the Maxwell‐Garnett Theory
Authors: Ruppin R.
Published: July 2007

The various restrictions which have to be imposed on the particle size in order for the Maxwell‐Garnett theory to be valid are discussed. It is shown that the requirement that the particle size must be much smaller than the wavelength of the light in the surrounding medium is insufficient near resonances of the dielectric constant of the particles. The relative merits of the various validity criteria are illustrated by some numerical examples.
EPR Spectrum of the Oxygen Vacancy in Single Crystals V2O5
Authors: Vanhaelst M. , Clauws P.
Published: July 2007

A new EPR spectrum of V2O5 is reported which is related to the non‐stoichiometry of the crystals. Its intensity increases on reduction and decreases on reoxidation. The spectrum is labelled El. El can be described by an S = 1 spin Hamiltonian with parameters gx = 1.33, gy = 2.51, gz = 1.01 (y ‖ b, x and z at angles of 25° to c and a), and D = 200 × 10−20 erg, E = −40 × 10−20 erg. E1 and the near‐infrared spectrum F1 (absorption bands at 1.25 and 1.52 eV with E ‖ a) are assigned to the two donor electrons from the oxygen vacancy in V2O5. The electrons are localized on nearest vanadium neighbours of the vacancy.
Electronic Structure of Silicon with Non‐Spherical Potentials for the Two Basis Atoms
Authors: Kim C. K. , Kunz A. B.
Published: July 2007

The potential difference of two basis atoms, which arises from the tetrahedral bonding, is included in the calculation of the silicon band structure using the local empirical pseudopotential method (EPM). It is shown that the small antisymmetric form factor due to the potential distortion gives substantial contribution to high lying conduction bands.
Thermal Properties and Grüneisen Parameters of Alkali Metals Employing the Model Potential of Krasko and Gurskii
Authors: Prasad B. , Srivastava R. S.
Published: July 2007

A new form of the local model potential proposed by Krasko and Gurskii is used for computation of phonon frequencies, specific heat, and Grüneisen parameters for the two alkali metals, potassium and sodium. The Brillouin zone is divided into 1000 miniature cells. Corresponding to the centre of each miniature cell, phonon frequencies and Grüneisen parameters are computed and used for studying the variation of the mean value of the Grüneisen parameter with temperature. Variation of the thermal expansion coefficient with temperature is also studied in the manner of Wallace. The computed results are discussed in the light of available experimental and theoretical results. There is over all qualitative agreement between theory and experiment.
Interpretation of Compositional Modulation Spectra to Determine Some Optical Properties of Alloys
Authors: Enderlein R. , Hummel R. E. , Andrews J. B. , Nastasi‐Andrews R. J. , Shanley C. W.
Published: July 2007

A theory is developed for the line shape of compositional modulation spectra of alloys. Inter‐band transitions between d‐ and s‐band states just above the Fermi level are assumed. Rigid band shifts and changes of the Fermi energy with composition are considered. The calculated line shape reproduces remarkably well the experimental modulation spectra of copper based alloys in the vicinity of the fundamental absorption edge. From a line shape analysis of the experimental spectra the threshold energy for interband transitions and the lifetime broadening parameter can be obtained.
First‐Order Mott Transition of the Exciton Gas in GaP
Authors: Maaref H. , Barrau J. , Brousseau M. , Collet J. , Mazzaschi J. , Pugnet M.
Published: July 2007

The high‐density electron‐hole system in GaP is investigated. The luminescence spectra are discussed as function of the excitation level and temperature. The Mott transition of the exciton gas is observed and the conditions are found for the separation into two phases between 40 and 70 K. A low‐density phase is an insulator: a high‐density phase (np = 5 × 1018 cm−3) is a conductor.
Phonon Dispersion in Red HgI2
Authors: Prevot B. , Schwab C. , Dorner B.
Published: July 2007

Phonon dispersion curves of red HgI2 are measured by inelastic neutron scattering up to a frequency of 1.5 THz for waves propagating in the [100] and [001] symmetry directions. Indications of a two‐dimensional character are found through a slight anisotropy in the sound velocities and through the comparison between inter‐ and intralayer stretching forces. Further, the unusual low c66 value suggests that strong directional anisotropy may exist within the layer planes. A new assignment of the optical phonon frequencies is proposed on the basis of the observations.
Role of Electron—Phonon Interaction and Peripheral Phonons in the Lattice Thermal Conductivity of Doped Semiconductor at Low Temperatures. Application to Phosphorus‐Doped Germanium
Authors: Boghosian H. H. , Dubey K. S.
Published: July 2007

The role of the electron—phonon scattering and the peripheral phonons are studied by calculating the lattice thermal conductivity of phosphorous doped Ge in the temperature range 1 to 5 K. The temperature and the carrier concentration dependences of the reduced Fermi potential (η*) and of the density of states effective mass (m*) of electrons are also studied at low temperatures. The entire study is made using the Ziman expression of the electron—phonon scattering relaxation rate and the Callaway integral of the lattice thermal conductivity. The contribution of the peripheral phonons in the total phonon conductivity of the P‐doped Ge is also studied. A good agreement is found between the calculated and experimental value of the phonon conductivity for five different samples of P‐doped Ge having different carrier concentrations of 1.2 × 1017 to 1.1 × × 1018 cm−3 in the entire temperature range 1 to 5 K. The analytical expressions are also obtained for the phonon conductivity of the doped semiconductor in the low temperature approximation.
Electronic Transitions in the Raman Spectra of FeCl2 · 2 H2O and FeCl2 · 2 D2O1
Authors: Graf L.
Published: July 2007

Raman spectra in FeCl2 · 2 H2O (FC2) and FeCl2 · 2 D2O (FC2D) are observed at 2 K. The electronic transitions of the Fe2+−ion in a crystal field are identified after a thorough assignment of the phonon lines in the four isomorphic compounds to symmetry types, using backscattering and 90°‐scattering techniques. The lowest of these phonons is known to be coupled resonantly to the magnons below TN. This coupling and other important magnetic properties of the iron salts are determined by the influence of the crystal field. With the approach of a rhombic symmetry the crystal field parameters are evaluated directly from the observed electronic transitions and related to the single‐ion anisotropy and the g‐value of the iron salts. Also the reductions of spin—orbit couplings in FC2 and FC2D by covalency effects are discussed.
Spectroscopy of Mixed Alkali Halide Crystals Activated by Thallium
Authors: Kalnin A. E. , Trinkler M. F.
Published: July 2007

Spectral, polarizational, and kinetic characteristics of mixed alkali halides KBr · KI—Tl and KCl · KI—Tl with three different iodine concentrations are investigated. The conclusion of a paramount role of Jahn‐Teller effect in relaxed excited 3T1u‐state formation is made. It is shown that A‐absorption subbands and shortwave emission bands correspond the five possible variants of coordinate wave function overlapping with four anions of nearest environment, rather than iso‐structures. The effect of preferential orientation of emitting oscillators in mixed crystals quenched at uniaxial compression, confirming the correctness of proposed interpretation, is discovered.
Magnetic Behaviour of the s—f Two‐Band Hubbard Model
Authors: Frauenheim Th. , Röpke G.
Published: July 2007

The magnetic phase diagram of a two‐band Hubbard Hamiltonian as proposed by Jullien and Coqblin in actinide metals is constructed using the functional integral technique. The direct exchange interaction between the localized electron states and the indirect one via the conduction electrons is considered.
Theory of Cyclotron Resonance of Two‐Dimensional Electrons Interacting With Surface and Volume Phonons
Authors: Dykman M. I.
Published: July 2007

The interaction between nondegenerate two‐dimensional electrons in a quantizing transverse magnetic field and volume phonons is shown to be weak for the majority of systems. The coupling to surface phonons often appears to be strong. To separate the contribution of the interaction with volume phonons the diagram technique is developed. The influence of strong and weak interaction with surface phonons on the shape of the absorption peaks near the cyclotron frequency and its overtones and on the dc conductivity is analysed. The dependence of parameters on temperature and magnetic field strength is considered. The transverse electric field is shown to affect the electron—phonon coupling in the case of two‐dimensional electrons near semiconductor surfaces.
Theory and Experiment of the Light Emission of Surface Plasmons Induced by Fast Electrons on Modulated Surfaces
Authors: Heitmann D.
Published: July 2007

The light emission of surface plasmons induced by fast electrons on periodically modulated surfaces by using a first order scattering theory is calculated. Experiments are made on sinusoidally modulated Ag‐surfaces with 80 keV electrons. Good agreement of experiment and calculation is found for the shape of the angular‐ and wavelength spectra, for the influence of the angle of electron incidence, the polarization of the light emission, and — for weakly modulated gratings — for the absolute intensity, too.
Propagation of Electromagnetic Waves in a Direction Parallel to the Magnetic Field in Semiconductive Bi—Sb Alloys
Authors: Oelgart G. , Stegmann R. , Herrmann R.
Published: July 2007

Measurements are carried out on the small‐gap‐semiconductor Bi—Sb at temperatures of liquid helium and in a frequency range between 9 and 70 GHz. Special attention is paid to the investigation of spatial resonances in a magnetic‐field range in the neighbourhood of cyclotron resonance. The anisotropy of the Fermi surface and the redistribution of charge carriers over the parted Fermi surface in a quantizing magnetic field have a great influence on the dispersion behaviour of the waves. A comparison between the experimental and the theoretical dispersion behaviour provides information about the cyclotron masses and the dispersion relation of the charge carriers for the alloy system investigated.
Polarization Properties of Biexciton Luminescence and Two‐Photon Raman Emission in CuCl
Authors: Hönerlage B. , Phach Vu Duy , Grun J. B.
Published: July 2007

The polarization properties of biexciton luminescence and resonant two‐photon Raman emission are studied. For the first time, direct evidence on the biexciton distribution function in momentum space is obtained. Raman lines are found to be due to a four‐particle process where all momenta are well defined. The origin of the biexciton luminescence lines is, on the contrary, a cold gas of biexcitons, which is subjected to elastic and inelastic collisions. Its distribution function is wide spread but not yet isotropic in the momentum space.
The Effect of Oxygen Impurities on the Electron Tunneling
Authors: Otschik P.
Published: July 2007

The effect of dissolved oxygen ions in the vicinity of the interface between tunnel barrier and superconductor on the tunnel density of states and on the parameters of the Eliashberg equations is investigated. Because of existing solvent profiles averaged quantities are measured, as the tunneled quasiparticle averages over the properties in the range of mean penetration depth. Therefore, the theoretical tunnel density of states and the Eliashberg equations are modified. Assuming a solvent profile and a reasonable Coulomb pseudopotential the effective phonon density of states α2F(b) of bulk material is obtained. The set of equations is applied to niobium.
On the Positron Localization in Ordered and Disordered Metallic Alloys
Authors: Koenig C.
Published: July 2007

The positron relative affinity for one of the components of a disordered binary metallic alloy is estimated within an ideal solid solution model. The calculation of the probability of finding the positron on each atomic site is reduced to the determination of a pseudo particle density in a tight binding model; these densities are then obtained in the coherent potential approximation (CPA). In the ordered alloy FeAl, the positron wave function is a Bloch wave function, and its relative amplitude on each of the two sublattices gives the positron density on the corresponding metal. The numerical results show that in each case, the inhomogeneities or structural defects must have an important effect on the experimental results.
Localized Vibrations of Substitutional Atomic Hydrogen in KCl and RbCl Studied by ENDOR
Authors: Hoentzsch Ch. , Spaeth J. M.
Published: July 2007

The superhyperfine (shf) interactions of atomic hydrogen trapped on a cation vacancy next to a Ca++ ion in [110] are measured by ENDOR. The shf interactions increase with increasing temperature. This is explained as due to the occupation of higher modes of the localized hydrogen vibrations. The thermal excitation energy is (17 ± 2) meV for RbCl and (25 ± 2) meV for KCl. A theoretical interpretation of the shf interactions is given including vibrational effects and the lattice distortion of the nearest neighbours.
Thermalization of the Electron—Hole Plasma in GaAs.
Authors: Goebel E. O. , Hildebrand O.
Published: July 2007

The band‐to‐band luminescence lineshape of the electron—hole plasma emission in GaAs is analyzed as a function of the excess energy of the photocreated electrons and holes. This yields experimental data for the dependence of the effective carrier temperature of the electron—hole plasma on the excess excitation energy for high electron and hole densities. Carrier temperatures as low as 5 K can be obtained in the case of resonant excitation. A discussion of these data in terms of the possible energy dissipation processes shows that hole—phonon interaction gives an important contribution to the cooling of the carrier system.
Stimulated Emission, Absorption Spectra, and Recombination in Epitaxial PbTe
Authors: Galeskii F. , Yunovich A. E. , Herrmann K. H. , Kostial H. , Rechenberg I. , Schäfer P.
Published: July 2007

Stimulated emission at temperatures up to 220 K is detected in epitaxial PbTe layers by means of a Q‐switched Nd3+:YAG laser. From an analysis of simultaneous measurements of the absorption edge, the conclusion is drawn that the emission is due to direct interband transitions with k‐conservation. A unique quantitative description of generation and radiative recombination at high levels of excitation is given. The role of nonradiative processes is outlined.
A Result Not Dependent on Rationality for Bloch Electrons in a Magnetic Field
Authors: Wannier G. H.
Published: July 2007

There are some facts known about the wave functions of a Bloch electron in a magnetic field. A great deal more is known for a special model. All results are restricted to rational fields and depend heavily on the denominator of the representative fraction. A result is described below which is independent of rationality. It is the statistical weight of identifiable regions in an energy versus field plot. In fact if one goes further and plots instead of the energy the integrated density of state versus field a set of computable polygons results for these regions.
Exciton Luminescence in Semiconductors. Surface Recombination and Space‐Charge‐Layer Effects
Authors: Sachenko A. V. , Tyagai V. A. , Kundzich A. G.
Published: July 2007

The effect of surface conditions on excitonic luminescence of wide‐band gap semiconductors like CdS in a situation when excitons are created by the binding of electron—hole pairs is considered. The surface contribution is considered by the velocity of surface exciton annihilation, the effective rate of electron and hole surface recombination, and by surface band bending. The analysis of continuity equations for electrons, holes, and excitons in line with Poisson's equation is carried out. The above set of equations is found to have analytical solutions in the following simple situations: a) at low exciton concentrations when both transfer and annihilation of the excitation energy are determined by the motion and recombination of electrons and holes; b) intermediate exciton concentrations when excitation energy transfer is realized by electron—hole diffusion while its annihilation is due mainly to the excitonic channel; c) high exciton concentrations (in respect to the minor carrier concentrations) when the diffusion as well as the annihilation of a excitation energy is realized by the excitonic channel. A new physical picture of the appearence of “dead” layer in excitonic luminescence based on the enhanced electron—hole recombination near the semiconductor surface, particularly, in conditions of a strong tunneling ionisation of excitons in surface electric fields is proposed.
Theory of the Phononless High Frequency Hall Effect in Disordered Systems
Authors: Fishchuk I. I. , Rudko V. N.
Published: July 2007

The anti‐symmetrical phononless high frequency part of the conductivity tensor of disordered systems which determines the Hall effect in the low temperature region is calculated by Kubo formula. A general expression which takes into account the contribution to conductivity of both the diagonal and non‐diagonal (with respect to the site index) components of the dipole momentum operator of the electron is obtained. The frequency dependence of the anti‐symmetrical part of the phononless conductivity tensor is investigated for a particular form of the density curve of continuous distribution of electron levels of the atoms in the system. The calculations show that the relation between the noted contributions strongly influences the frequency dependence of the Hall effect.
Azbel‐Kaner Cyclotron Resonance as a Long‐Wavelength Limit of an Exact Expression for the Surface Impedance of a Metal in a Magnetic Field
Authors: Saermark K.
Published: July 2007

A recently derived, rigorous expression for the surface impedance of a metal placed in a magnetic field is considered in a long‐wavelength approximation. The result is shown to be equivalent to surface impedance expressions used in the Azbel‐Kaner theory.
Vibrational Properties of the 〈001〉 Split Interstitial in Silicon
Authors: Wruck D.
Published: July 2007

A theoretical investigation is made of the vibrational behaviour of the 〈001〉 split interstitial in silicon using the Green's function formalism. In particular, the cases of the Si‐Si, C–C, and C–Si dumbbell are discussed. For the numerical calculation of the vibrational frequencies a modified Keating force model is used, in which the coupling of the dumbbell stoms each with another and to the four nearest‐neighbour atoms as well as the changes in the force constants between these four neighbours are described in terms of the ratio d/x (2d separation of the dumbbell atoms,4a lattice constant). The calculations are performed with d/a as parameter using a perfect crystal Green's function determined from neutron scattering data. The results show for reasonable values of d/a (0.5 ≦ d/a ≦ 1) in the case of the equiatomic dumbbell (D2d symmetry) the existence of two high‐frequency localized vibrations (an A1 ‐ and a B2 ‐ mode) and of two doubly degenerate (E ‐ type) resonance modes or localized modes near the maximum lattice frequency. In the case of the mixed dumbbell (C2v symmetry) also two high‐frequency localized modes (both of A1 ‐ type) and four resonance or low‐frequency localized modes (two of B1 and two of B2 ‐ type) are obtained. On the basis of the above calculations, an interpretation of the C(I) infrared absorption bands observed in carbon‐doped electron‐irradiated silicon crystals as the two A1 ‐ modes of the C–Si〈001〉 dumbbell can be made on the basis of the absolute values of the frequencies and the relative line intensities but becomes difficult when considering also the separation of the two modes and its isotopic shift with respect to carbon.
The Influence of Internal Strain on Bound Hole–Phonon Interaction in p‐Type Semiconductors with Zincblende Structure
Authors: Singh Mahiradhwaj
Published: July 2007

Expressions for the resonance scattering of phonons by bound holes are obtained for semiconductors with zincblende structure for the situation when the degeneracy of the acceptor ground state belonging to J = 2/3 which is a fourfold degenerate state, is split into two doublets namely M1 = ± 2/3 and M1 = ± 2/1 besides the usual valley‐orbit splitting of the sixfold state into one doublet state belonging to J = 2/1 and one fourfold state belonging to J = 2/3. These expressions are used in the analysis of the unexplained asymmetric nature of the resonance dip in the phonon conductivity of Mn‐doped GaAs at low temperatures. The agreement between the theory and the experimental results is excellent.
A Quantum Mechanical Approach to the Velocity of Dislocations in Ice
Authors: Forouhi A. R. , Bloomer I.
Published: July 2007

A quantum mechanical formulation is used to calculate the velocity of a kink on an otherwise straight dislocation line in ice. Treating the kink as a quasi‐particle there exists the probability that it can tunnel through the potential barrier caused by unfavorable positions of the protons on the site jute ahead of it. By unfavorable it is meant that the protons arrre arranged such that point defects occur if the kinkk proceeds. When an external stress is applied to the system there will be a net drift of the kink which advances the dislocation. The velocity thus calculated is in agreement with experimental evidence.
Dipole Barriers at Structured Metal Surfaces
Authors: Fritsche L. , Gollisch H. , Noffke J.
Published: July 2007

Mattheiss' successful method of approximating the electronic charge distribution in crystals by superposing the charge densities of free atoms is used to calculate dipole barriers at metal surfaces. Low index surface planes are studied for five simple metals and eleven transition metals of either f.c.c. or b.c.c. structure. Some of these metals are also studied by other authors with different methods. The agreement with the results obtained is fairly good. In addition, the differences in the work function obtained from the calculated dipole barriers for different surface planes of a specific metal agree surprisingly favourable with the majority of the respective experimental data.
Polarization Dependence of the Photoelectron Energy Distribution Curves of Cesium
Authors: Humberg H. , Merz H.
Published: July 2007

The energy distribution curves of photoelectrons, emitted from compact cesium films by linearly polarized light in oblique incidence, are measured for photon energies between 3.0 and 4.9 eV. s‐polarization and p‐polarization give different results: the relative contribution of primary electrons is more prominent for p‐polarization than for s‐polarization; only for p‐polarization a weak maximum occurs 1.2 eV below the Fermi energy.
Drift Mobility and Photoconductivity in Amorphous Silicon
Authors: Fuhs W. , Milleville M. , Stuke J.
Published: July 2007

Drift mobility and photoconductivity are studied on amorphous silicon (glow discharge type) by conventional time‐of‐flight technique as well as measurements of the response time. The time‐of‐flight experiments yield dispersive transients which do not allow the definition of a transit time. The drift mobility obtained from steady‐state photoconductivity and response time is thermally activated with an energy of 0.13 to 0.16 eV. No indication for a change in transport mechanism is found in the investigated temperature range (100 K < T < 500 K). Longer exposure to light leads to a decrease of the response time, enhancing the recombination via deep lying centres.
Dynamic Dielectric Function in the Three‐Band Model for Zero‐Gap Semiconductors in a Magnetic Field
Authors: Kacman P. , Blinowski J. , Grynberg M.
Published: July 2007

The energies of the magnetic subbands associated with conduction and valence Γ8 bands in zero‐gap semiconductors are calculated analytically in the three‐band model corrected by first‐order perturbation theory to account for the effect of higher bands. Based on the obtained eigenvalues and eigenfunctions both, the imaginary and real parts of Γ8→Γ8 intra‐and interband contributions to the dynamic dielectric function (DDF) are evaluated in collisionless approximation. The influence of frequency and magnetic field dependence of DDF on the reflection coefficient in intrinsic and n‐type samples at T = 0 is presented.
An Exactly Soluble Model of Singlet‐Ground‐State Ferromagnetism
Authors: Kventsel G. F. , Katriel J.
Published: July 2007

A system of “atoms”, each one of which can be in one of two spin states, is considered. On the assumption that in one of the two atomic states a ferromagnetic interaction exists, the magnetic properties of the system are investigated. Various types of magnetic behaviour and phase transitions are found. Over a certain reasonable range of model parameters heat magnetization is obtained.
Positron thermalization as quantum statistical process
Authors: Mokrushin A. D. , Sabirov R. Kh.
Published: July 2007

The positron thermalization process is considered by the non‐equilibrium statistical operator method. Kinetic equations describing the slowing‐down of the fast positrons are given in the second order of the positron–medium interaction. Thermalization with account of the positron–phonon scattering is analyzed in detail in that case when the phonon subsystem may be in the non‐equilibrium state. Estimations show that in metals the positrons can be completely thermalize ed due to positron–phonon interaction even at very low temperatures – up to liquid helium temperature.
Dynamical screening and self‐energy of excitons in the electron–hole plasma
Authors: Zimmermann R. , Kilimann K. , Kraeft W. D. , Kremp D. , Röpke G.
Published: July 2007

The dependence of two‐particle energies on plasma density is investigated using Green's functions. An effective exciton equation is derived taking into account dynamically screened potential and one‐particle self‐energies. A variational calculation shows that the bound state levels (exciton energies) remain practically constant whereas the continuum edge (band gap) shifts to lower energies with increasing density, which is in accordance with experimental observations.
Kinetics of the intraband absorption and magnetoabsorption coefficients in mixed semiconductors with composition fluctuations
Authors: Flatau P. J.
Published: July 2007

The influence of short‐range ordering in a mixed semiconductor on the intraband absorption and magnetoabsorption coefficients is determined. The coefficients are shown to be temperature‐dependent. Their time‐evolution after changing the temperature is calculated. An experiment, permitting measurements of the degree of ordering (non‐ideality) in alloys is proposed, and a numerical example for HgTe–CdTe is given.
Dipolar relaxation induced by an AC field in various crystallographic directions of Me2+‐doped alkali halide crystals. II. Next‐nearest neighbour approach
Authors: Poźniak J.
Published: July 2007

Theoretical analysis of the dipolar relaxation in Me2+‐doped f.c.c. alkali halide crystals is performed by considering the action of an ac electric field acting parallel to 〈100〉, 〈110〉, and 〈111〉 crystallographic directions of the crystals. The possibility of the existence of impurity dipoles in both n.n. and n.n.n. states is taken into account and the difference explained in theoretical results obtained by Lidiard and Dreyfus. The possibility of evaluating the jump probabilities w1 (lattice cation → cation vacancy), w2 (Me2+ → cation vacancy), w3 (n.n.n. state → n.n. state), and w4 (n.n. state → n.n.n. state) on the basis of experimental tan δ = F(ω) curves is analyzed.
Consequences of the linear part of the specific heat of dilute magnetic alloys
Authors: Freudenhammer A.
Published: July 2007

For large but finite systems the density of energy states can be obtained uniquely from the specific heat as a function of temperature. In a dilute magnetic alloy with linear dependence of the specific heat on the temperature T (at low T) the density of energy states in the lower energy region is given by a modified Bessel function.
Calculation of the g‐tensor and 29Si hyperfine tensors of the E′1 centre in silicon dioxide
Authors: Gobsch G. , Haberlandt H. , Weckner H.‐J. , Reinhold J.
Published: July 2007

Both the g‐tensor and the tensors of 29Si hyperfine interactions are calculated for the model of an O− vacancy in α‐quartz with asymmetric lattice distortion proposed by Yip and Fowler to explain the E′1 centre in SiO2. The extended Hückel theory (EHT) is applied to an α‐quartz cluster. The dangling bonds of the outer atoms are saturated by hydrogen. Theoretical and experimental results agree quantitatively well. Similar calculations are performed for the analogous model in β‐cristobalite. Computations with and without consideration of Si3d functions are compared.
Influence of band‐tail parameters on electronic specific heat and transport coefficients in heavily doped semiconductors
Authors: van Cong H.
Published: July 2007

The high density and low temperature dependences of the electronic specific heat and of the electronic and thermal transport coefficients are investigated for a simplified model of heavily doped semiconductors. The theoretical treatments are based on a Green's function method which was developed in preceding paper. A brief comparison with existing experiments on n‐Ge, n‐Si, and n‐GaAs is made and discussed.
Polaron state of an electron near a structural instability
Authors: Batyev E. G. , Efanov A. V.
Published: July 2007

The formation of a Pekar polaron‐type self‐trapped state by an electron in ferro‐ and antiferroelectrics near the phase transition point is shown to be possible in the case when there is no strong coupling polaron far from the transition. This feature is due to the “soft mode” in the phonon spectrum. The properties of the polaron strongly depend on the proximity to the phase transition point.
On the interband absorption in lead chalcogenides
Authors: Genzow D. , Mironov A. G. , Ziep O.
Published: July 2007

The optical absorption coefficient K for direct allowed interband transitions between states L and L, respectively, is calculated on the basis of the six‐band model for lead chalcogenides, taking into account the k‐dependence of optical matrix elements. Comparisons are made between theoretical results of the six‐band and two‐band models and experiments. Good agreement is obtained between the calculated and experimental absorption spectrum in the vicinity of the absorption edges. For photon energies ħω > 1.5EG the experiments give higher K than theory. The disagreement is stronger for the six‐band model. The temperature‐ and composition‐dependence of K near the absorption edge are described better by the two‐band than by the six‐band model.
Nucleation phenomena in electron–hole drop formation in Ge and Si. III. Experimental results for Ge
Authors: Westervelt R. M. , Staehli J. L. , Haller E. E.
Published: July 2007

A systematic experimental study and theoretical analysis of hysteresis and threshold phenomena in the luminescence of electron–hole drops (EHD) and free excitons (FE) in volume‐excited crystals of ultrapure Ge is presented. Data are given between 1.2 and 2.0 K for 1) hysteresis curves for the EHD and FE luminescence intensity obtained by cycling the excitation; 2) metastable time development of the EHD signal for times ≈ 4 × 103 s; and 3) a calibrated phase diagram for the FE density thresholds n+ and n− for the creation and destruction of EHD vs. T. Excellent agreement between these data and nucleation theory is found. The principal experimental results are: 1) reproducible hysteresis curves for EHD exhibiting well‐defined excitation thresholds; 2) logarithmic variation of the EHD signal with time and direct evidence that individual EHD can exist for times ≈ 6 × 104 s; 3) the first measurement φ0 = (1.9 ± 0.2) meV of the EHD condensation energy from threshold data which correctly accounts for non‐equilibrium effects, resolving previous discrepancies with spectroscopic data; and 4) an accurate measurement of the EHD surface energy w = (2.6 ± 0.3) × 10−4 erg cm−2.
An optimized LCAO version for band structure calculations application to copper
Authors: Eschrig H. , Bergert I.
Published: July 2007

An LCAO version for band calculations is presented which is conceptional simple, highly effective, and satisfies any reasonable accuracy demand. Due to the very convenient way of representing the results for wave functions it can easily be carried out self‐consistently and e.g. can be used to calculate Wannier functions. For illustration, numerical results for copper are given.
The influence of interatomic potentials on the interaction of He with a \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{1}{2}\left\langle {111} \right\rangle $\end{document} {110} edge dislocation in molybdenum
Authors: van Heugten W. F. W. M. , van den Berg F. , Caspers L. M. , van Veen A. , de Hosson J. Th. M.
Published: July 2007

The binding of clusters of up to four helium atoms to a \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{1}{2}\left\langle {111} \right\rangle $\end{document} {110} edge dislocation in Mo is calculated, using two sets of interaction potentials. The first set contains MoMo, MoHe, and HeHe potentials derived by several authors between 1970 and 1974. The second set contains potentials recently fitted to phonon curves and helium desorption experiments. Both sets give binding energies between He and the dislocation of respectively 1.6 and 1.8 eV and binding energies between dislocation bound helium of 0.7/0.9 and ;0.2/+0.1 eV, respectively. The migration energy of helium along the dislocation is found to be equal to that in the bulk.