bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic Properties of Iron and Cobalt in a Combined Model of Itinerant Electrons and Localized Spins
Authors: Sakoh M. , Edwards D. M.
Published: July 2007

The temperature variations of the spontaneous magnetization and spin paramagnetic susceptibility are calculated numerically, using molecular field theory, for the combined model of localized spins and itinerant electrons proposed by Edwards for some 3d transition metals. A simple parabolic band is assumed for the itinerant electrons and results are given for those values of the effective mass and exchange parameters which give the best fit to the observed properties of iron and cobalt. A reasonable description of the magnetic properties of b.c.c. iron is obtained and the calculated curves have none of the instability against small changes in values of parameters which is characteristic of pure Stoner theory applied to iron and cobalt. Agreement with the observed temperature dependence of the susceptibility of cobalt could not be obtained and a more sophisticated theory may be required.
Far UV Photoemission Density of States of CuNi Alloys
Authors: Shevchik N. J. , Penchina C. M.
Published: July 2007

The photoemission density of states of CuNi alloys deposited by cosputtering have been measured for hv = 40.8 eV. In contrast to previously reported measurements, the new results of superior resolution (0.3eV) are free of structure due to emission of the photoelectron into the unfilled states and are in even better agreement with the CPA calculations.
Effect of an External Electric Field on Phase Transitions of Order‐Disorder Crystals
Authors: Merten L. , Andrade P. Da R. , Borstel G.
Published: July 2007

Recently a model theory was given by the authors for the coupling of long optical phonons and polaritons with a Debye‐Mason relaxation in order‐disorder crystals. This theory is extended to the case of applying an external electric field to the crystal. It is shown that the effective field entering into the dielectric function can be determined by an iteration method. As the relation between the effective field and the external field is known, the (complex) frequencies of polaritons, phonons, and relaxations can be calculated as functions of the external field. Numerical evaluations shall be given elsewhere.
Positronenannihilation in Ionenkristallen. II. Nachweis eines Positronium‐ähnlichen Zustandes in KCl
Authors: Brümmer O. , Brauer G. , Andrejtscheff V. , Käbler L.
Published: July 2007

Die Benutzung eines Ge(Li)‐Detektors bei Positronenlebensdauermessungen an KCl‐Einkristallen gestattet es, zusätzlich die Dopplerverbreiterung der 2γ‐Annihilationsstrah‐lung zu messen. Es wird gezeigt, daß die Dopplerverbreiterung der Annihilationslinie von der Lebensdauer des Positrons abhängig ist. Die vorgelegten Messungen stützen die Hypothese der Bildung eines Positronium‐ähnlichen Zustandes in ungefärbten Alkalihalogeniden.
Shubnikov‐de Haas Effect and Energy Relaxation of Hot Carriers in p‐Type Tellurium at 4.2 K
Authors: Kahlert H.
Published: July 2007

The influence of electric fields on the amplitude of the Shubnikov‐de Haas (SdH) oscillations of p‐type tellurium has been studied for the configuration j ⊥ B and the trigonal c‐axis oriented perpendicular to the magnetic field B and to the current direction. From a comparison of the temperature dependence of the amplitudes of the ohmic SdH data and the electric‐field dependence of the amplitudes at 4.2 K the relation between applied electric field and resulting hole temperature is deduced. The measured energy loss rate of the holes to the lattice is compared with calculated values assuming scattering by acoustic and polar optical phonons. Piezoelectric interaction is responsible for the energy transfer below 6 K. An estimation of the contribution of deformation‐potential scattering allows to derive an approximate value for the valence‐band deformation potential constant EV ≈︁ 1.2 eV. Above 6 K emission of polar optical phonons becomes the dominant energy dissipation mechanism.
A Mechanism for Reduced Radiation Damage
Authors: Yu S. , Bichsel H.
Published: July 2007

In highly anisotropic targets, radiation damage due to electron irradiation can be drastically reduced as electron energy increases. A theoretical study of this effect is made, and some experimental evidences of this effect are cited.
Electroreflectance Spectra of Heavily Doped Semiconductors. Field Inhomogeneity in the Low‐Field Limit
Authors: Humlíček J. , Lukeš F.
Published: July 2007

The influence of the electric field inhomogeneity on the electrorefleetance (ER) spectra has been included into the theory in the low‐field limit by means of the corrected Seraphin coefficients. One typical example of the ER spectra of heavily doped Ge (n‐type samples with donor concentration ND = 9 × 1017 cm−8) in the range of E1 and E1 + Δ1 transitions proves that the lineshape of the ER spectra may well be expressed by the microscopic model verified earlier for pure Ge with additional inclusion of the field inhomogeneity.
An Explanation for the Plateau in the Thermal Conductivity of Non‐Crystalline Solids
Authors: Zaitlin M. P. , Anderson A. C.
Published: July 2007

The tunnelling‐states model is applied to the thermal conductivity of non‐crystalline materials. It is shown that the presence of a quadratic term in the energy dependence of the density of tunneling states, as implied by specific heat and ultrasonic measurements, can give rise near 10 K to the plateau in the thermal conductivity which is characteristic of amorphous solids.
A Hydrodynamic Model of the Optical Generation of Bulk Plasma Waves
Authors: Boakdman A. D. , Parker M. R.
Published: July 2007

The possibility of using a hydrodynamic model of plasma wave generation in solids is investigated. The simplicity of the approach is in sharp contrast to earlier theoretical investigations based on an elaborate kinetic theory. The basic equations presented here apply to metals or semiconductors and an extension which includes the means of studying the influence of a magnetic field is provided. A thorough discussion of the boundary conditions at a dielectric–conductor/semiconductor interface is given and some numerical comparisons with published experimental data are included.
On the Theory of Conductivity of an Electronic Gas with Anisotropic Effective Mass in Quantizing Magnetic Fields
Authors: Bakanskii P. I. , Demidenko L. S. , Korolyuk S. L. , Samoilovich A. G.
Published: July 2007

The anisotropic impurity scattering of current carriers has been studied in quantizing magnetic fields for n‐Ge type semiconductors. General expressions for the conductivity for the cases of nondegenerate and degenerate electronic gas were obtained. The formulae for the mobility in the quantum limit for these two cases were analysed and a physical interpretation of the obtained results is given.
Effective Reduction of the Unit Cell for the Interpretation of the Optical Properties of Monoclinic Molecular Crystals with Four Molecules in the Unit Cell: 1,5‐Dichloroanthracene
Authors: Flakus H.
Published: July 2007

It is shown that symmetry considerations allow to interpret the spectral properties of the 1,5‐dichloroanthracene molecular crystal which contains four molecules in the unit cell by using an effective optical unit cell with only two molecules. The numerical computations performed for the models of vibronic coupling in the crystals with respectively two and four molecules in the unit cell confirm the equivalence of the models. For the crystals having the Cj site symmetry group and four molecules in the unit cell of the monoclinic system the equivalence of considerations of those crystals is demonstrated.
Galvanomagnetic Properties of Bi2Se3with Free Carrier Densities below 5 × 1017 cm−3
Authors: Köhler H. , Fabbicius A.
Published: July 2007

Bi2Se3, Bi2Te3, and Sb2Te3 tend to grow non‐stoichiometric, the defects causing high free carrier densities. Here it was achieved, in the case of Bi2Se3, to produce crystals of n‐and p‐type conductivity showing concentrations down to 4 × 1016 cm−3. The temperature and magnetic field dependences of their galvanomagnetic properties are investigated. At low magnetic fields and low temperatures negative magnetoresistance occurs, at high magnetic fields no saturation of the positive magnetoresistance is observed which is assumed to originate from the influence of an impurity conduction band. The temperature dependence of the Hall constant enables the evaluation of the energy gap for thermal excitations to (160 ± 10) meV for T  0. The temperature dependence of the mobility between about 50 and 200 K supports the assumption of dominating scattering of the free carriers by acoustical phonons in this temperature range.
Anisotropy of Electroreflectance Due to Orbital Degeneracy of Energy Bands
Authors: Enderlein R. , Renner P. , Scheele M.
Published: July 2007

A theory is developed for the electric field induced change, Δϵ, of the dielectric function for interband transitions between orbitallydegenerate bands like the E0−transition of tetra‐hedrally coordinated semiconductors. The strong dependence of oscillatorstrengths on the direction of quasimomentum and on the polarization of light for such transitions, together with the field influence on the quasimomentum direction results in strong polarization and field direction anisotropies of Δϵ which can explain the experimentally observed anisotropies of electroreflectance. Explicit results are given for the E0−transition and a valence band model with warping parameter C 0. The results are compared to those from an asymptotic approximation introduced by Keldysh et al. and by Aspnes. Lifetime broadening is taken into account and the first term of a power series for Δϵ with respect to the field strength is obtained. The low field expression of Δϵ consists of two contributions, the third‐derivative term being isotropic and a second term which depends on directions of light polarization and of field strength.
De Haas‐van Alphen‐Type Oscillations in the Interband Faraday Effect of Tellurium
Authors: Lutz M. , Bangert E. , Mizrah T. , Stolze H.
Published: July 2007

Measurements of the interband Faraday rotation were performed on samples with p  1014 to 1018 cm−3 in magnetic fields up to 6 T. Furthermore an oscillatory contribution to the interband rotation at 4 K in fields up to 15 T is observed. For a calculation of the interband rotation a band model is used which takes into account the strong coupling between the H4 and H5 valence bandsand the H61 and H62 conduction bands. The good agreement of theory and experiment gives evidence for the validity of the band model near point H of the Brillouin zone. The oscillations are explained in this model by the inversion asymmetry splitting of the H4 band.
Calculation of the Dislocation Damping Constant in NaCl
Authors: Paetsch B.
Published: July 2007

The temperature dependence of the dislocation damping constants B for NaCl is calculated, using Brailsford's theory of the anharmonic drag force. The calculation of B consists in the numerical evaluation of integrals over the first Brillouin zone (i.e. over the phonon coordinate space) for the energy dissipated in the dislocation motion. The integrand contains the generalised viscosity ξijkl(s, ω) as a factor, and the major part of the work is the evaluation of ξ.ξ itself is another integral ‐ a thermodynamic average ‐ over the Brill'ouin zone. Shell model calculations of the phonon dispersion curves and the mode Grüneisen parameters are used in evaluating these integrals. Separate calculations are made for the high temperature (T/θD > 1) and low temperature (T/θD)‐≪1) regimes. In each temperature regime the longitudinal and transverse phonon modes are assigned single lifetimes τL, τT. The temperature dependence of the lifetimes follows the theories of Herring (low temperature) and Klemens (high temperature), and the temperature coefficients are fitted to the experimental thermal conductivity data. Numerical results show that the optical modes contribute the major part of B. The calculated values are much larger than the experimental values; the agreement can be improved if smaller values of τL, τT are assumed than those which fit the thermal conductivity data.
Ultraviolet Photoemission Spectroscopy of Solid Nitrogen and Oxygen
Authors: Himpsel F.‐j. , Schwentner N. , Koch E. E.
Published: July 2007

Photoelectron energy distribution curves and constant final state spectra of polycrystalline films of solid N2 and O2 have been measured for photon energies 10 eV ≤ hω ≤ 30 eV by use of synchrotron radiation. Varying the photon energy and final state energy, respectively, the observed transitions originating from the valence bands could be attributed to the known molecular orbitals (MO's) of the gaseous samples. The binding energies of the various MO's are lowered by 1.5 eV for solid N2 and 1.6 eV for solid O2. This is explained by electronic relaxation via the dielectric polarization of the medium. Results on other nonpolar molecular crystals are included to check a simpleestimate of the polarization energy based on the Mott‐Littleton model.
Plasmon dispersion relation in zero‐gap semiconductors
Authors: Goettig S.
Published: July 2007

Frequency‐dependent, long‐wavelength random‐phase approximation (RPA) polarizabilities of a n‐type zero‐gap semiconductor are calculated for parabolic and spherical bands. The long wavelength plasmon dispersion relation is established. The dispersion of plasmons is mainly determined by the wave vector dependence of the intraband (free‐electron‐gas type) polarizability. For high electron concentrations, due to Γ8–Γ8 interband polarizability, in the standard plasmon dispersion relation, the concentration‐dependent background dielectric constant has to be used. For low electron concentrations the frequency and wave vector dependence of the Γ8–Γ8 interband polarizability makes the above procedure inaccurate. A more accurate procedure is derived.
Non‐adiabatic calculation of the binding energy of the excitonic molecule ion
Authors: Munschy G. , Stébé B.
Published: July 2007

A non‐adiabatic calculation is made of the binding energy of the excitonic molecule ion. In the limiting case of the hydrogen molecule ion good agreement is obtained with very accurate calculations. The results confirm the stability of the complex for all values of the electron‐to‐hole mass ratio.
Spectral representation of nonlinear susceptibilities of solids
Authors: van Hieu Nguyen
Published: July 2007

The analytical properties of nonlinear susceptibility of solids are investigated and its spectral representation is obtained. The nonlinear susceptibility is compared with the amplitudes of multiphoton processes. One‐dimensional dispersion relations and the threshold behaviour of the susceptibility are considered. On the basis of analyticity asymptotic relations are derived of nonlinear susceptibility at high frequencies.
Effective phonon energies in optical spectra of deep impurities in wurtzite‐type crystals
Authors: Boyn R.
Published: July 2007

Effects of electron–phonon interaction optical absorption bands due to electronic transitions in deep substitutional impurities in wurtzite‐type crystals are discussed, assuming that the electronic states are coupled only to the displacements of the impurity and the nearest‐neighbour atoms forming a regular tetrahedron. The question is considered under what conditions such effects in experimental spectra may be analysed as for “pure cubic” systems. Referring to the second moment 〈E2〉 of an A → T absorption band it is shown that this should be possible if experimentally 〈E2〉 is found to be not significantly polarisation dependent. Expressions are given for effective frequencies of the tetrahedral cluster vibrations of the various symmetry types.
Polariton‐ and directional dispersion of polar modes in orthorhombic KNbO3 investigated by light scattering
Authors: Winter F. X. , Wiesendanger E. , Claus R.
Published: July 2007

Experimental data on polaritons of type A1(TO), B1(TO), and B2(TO) in orthorhombic KNbO3 with wave vectors propagating in the xy‐, yz‐ and xz‐planes, respectively, have been compared with dispersion curves calculated on the basis of the phonon data. Data on mixed mode polaritons to type B1 + B2, A1, + B2, and A1 + B1 in the xy‐, yz‐and xz‐planes are shown to fit numerically calculated corresponding dispersion curves. Finally all directional dispersion branches of polaritons in the phonon limit (k → ∞) are calculated for the three symmetry planes. Experimental data on the 45°‐phonons are presented in order to check the theoretical curves.
EPR investigations on lithium manganate powders
Authors: Scharmann A. , Vitt B. , Hoppe R. , Meyer G.
Published: July 2007

EPR powder measurements above 77 K on two forms each of Li3MnO4 and Li2MnO3 are reported. Here the systems Mn5+/Td and Mn4+/Oh are presented. The new spin‐resonance data are compared with those of analogous systems, the g‐shift is discussed. The analysis of lineshapes indicates that exchange effects for Li3MnO4 as well as for Li2MnO3 must be taken into account. Magnetic susceptibility measurements down to 4 K show antiferromagnetism for the hexagonal β‐Li2MnO3 below 50 K. Examining the temperature dependence of intensity and linewidth of the EPR signal one can identify this magnetic behaviour already above TN. The antiferromagnetic coupling between the Mn4+‐ions (3d3) is realized by superexchange through O2‐‐ligand orbitals.
On the instability of paramagnetism in the extended Hubbard model
Authors: Vedyaev A. V. , Kondorskii E. I. , Mizie E.
Published: July 2007

The Hubbard Hamiltonian, including the interatomic exchange terms, is studied in the framework of the alloy analogy. The static paramagnetic susceptibility of the system is calculated and the criterion for instability of paramagnetic state is obtained.
Computer simulation of the phase transition in ammonium halides
Authors: Trikha S. K. , Jain S. C.
Published: July 2007

Computer simulation studies were made in ammonium chloride at low temperature. Using the Lennard‐Jones (6–12) potential as the representative interaction between NH and Cl−, the value of the rotational phase transition temperature is obtained which agrees with the experimental results of Schwartz.
Flux pinning by grain boundaries in anisotropic superconducting materials
Authors: Khanra B. C.
Published: July 2007

Flux pinning by grain boundaries in anisotropic superconducting materials is studied by assuming that pinning comes through the local change in the Ginzburg‐Landau free energy. The experimental results of Kes and others on the Nb sample, containing grain boundaries as lattice defects, are explained by Labusch's theory. The results are discussed.
The determination of X‐ray dispersion corrections from measurements of X‐ray linear absorption coefficients
Authors: Creagh D. C.
Published: July 2007

The X‐ray linear absorption coefficients have been measured for the alkali halides LiF, NaF, and NaCl using characteristic radiations ranging from SnKα1 to CrKα1. From these measurements values of the imaginary part Δf″ and the real part Δf′ of the X‐ray dispersion correction have been deduced for each atomic species. A comparison is made with the values of μ/ϱ, Δf′, and Δf″, which have been calculated by Cromer and Liberman using relativistic quantum mechanics.
On the scattering of conduction electrons by magnetic impurities in semiconductors of InSb‐Type and HgTe‐Type band structure
Authors: Kossut J.
Published: July 2007

The relaxation time and the mobility are calculated for the case of the conduction electrons scattered by impurities possessing localized magnetic moments. The Kane model is used to describe the energy bands. The perturbing potential is assumed to be of a short range. The numerical estimations are made for two special forms of perturbation: delta‐like and square‐well form. The estimations show that the contribution of the considered mechanism to the mobility is small compared to that of other scattering mechanisms. However, there exist transport phenomena which seem to be strongly influenced by magnetic impurity scattering.
Quantum size effect in SnTe observed by tunnel experiments
Authors: Altwein M. , Finkenrath H.
Published: July 2007

In SnTe–Al2O3–Al tunnel junctions with size quantization of the electron states in the SnTe electrode, oscillations of the 2nd derivative of the I(U) characteristics were observed at 4 K. From this it was possible to determine a series of points of the E(k) relation perpendicular to the film plane in a range of ±100 meV around the Fermi energy. The results are in agreement with values calculated from band structure data.
The influence of local‐field corrections on the collective behaviour of the homogeneous electron gas
Authors: Brosens F. , Lemmens L. F. , Devreese J. T.
Published: July 2007

The dynamic structure factor of the electron gas is studied using different static local‐field corrections, and a comparison is made with the Hartree approximation. The plasmon dispersion relation is rather insensitive to the approximations used. However, the relative contributions of the plasmon branch and the pair creation region change substantially from one approximation to the other. This point is investigated by a Kramers‐Kronig analysis of the different approximations.
Optical spectra and local order in amorphous TlCl and TlBr
Authors: Heidrich K. , Kramer B.
Published: July 2007

The optical absorption spectra of amorphous TlCl and TlBr films are investigated in the photon energy range near the absorption edge. Annealing the films and tracing the spectra with annealing temperature discontinuous changes in the shape of the spectra are observed at two different annealing temperatures. A continuous random network model is presented for the structure of the amorphous TlCl and TlBr films, which is in agreement with recent structural data. The annealing dependence of the spectra is understood by assuming the existence of two disordered modifications with different short‐range order.
Modulated spectroscopy of zincblende semiconductors. Calculation of piezomodulation parameters for indirect semiconductors
Authors: Mathieu H. , Auvergne D. , Merle P.
Published: July 2007

Relative modulation parameters are calculated for the absorption coefficient in the indirect absorption edge region for periodically stressed zincblende‐type semiconductors. The calculations are made for [001] and [111] uniaxial and coplanar stresses, for all possible phonons, for all independent polarizations of light, and through all relevant intermediate states (Γ1c, Γ15c and X5v or L3v).
Dispersion relation for cyclotron waves as a spatial condition for minimum collisionless damping
Authors: Saermark K. , Lebech J.
Published: July 2007

It is shown that the dispersion relation for cyclotron waves of the extraordinary type (ECW) propagating in a metal with a cylindrical Fermi surface may be found in a simple way from the zero's of certain Bessel functions. This follows as a consequence of a requirement of minimum damping of the waves by direct collisionless interaction between the electrons and the waves. A detailed comparison with calculations based on a realistic silver Fermi surface and a spherical Fermi surface, in both cases for a finite value of ωτ, is performed. The behavior of the dispersion relation in the limit of vanishing magnetic field is considered in some detail and a number of interesting features are pointed out.
Piezoresistance of tellurium and selenium–tellurium mixed crystals
Authors: Takagi T.
Published: July 2007

The piezoresistance coefficients of Te, Se5Te95, and Se10Te90 are measured in the temperature range from 220 to 410 K, and at liquid nitrogen temperature. They are analyzed by a simple deformation potential model. The stress coefficients of energy gap, ϵ1 and ϵ3, are obtained. The nonlinearity of ϵ1 for tellurium is restricted to the pressure range smaller than 106 dyn/cm2. The electron mass as well as the hole mass possibly increase with selenium content.
Theory of slow neutron scattering on parametrically excited polarization waves in hydrogen‐bonded ferroelectrics
Authors: Stamenković S.
Published: July 2007

The coherent neutron scattering of slow neutrons on polarization waves excited by a possible parametric resonance in hydrogen bonded ferroelectrics is considered. The expressions for the cross sections both in the case of the parallel and transversal “pumping” are obtained. The experimental possibilities to observe such a scattering and a scattering under an arbitrary oriented “pumping” are discussed.
Ionic size effect on the radiation‐induced dynamic motion of interstitial halogen in alkali halides
Authors: Saidoh M. , Itoh N.
Published: July 2007

By pulse radiolysis technique, the interaction volume between the dynamic replacement sequence and the H center in RbBr and CsBr has been obtained between liquid nitrogen and room temperatures. Similarly as the previous results on KBr, the temperature dependence of the interaction volume follows an Arrhenius equation, with an activation energy smaller than that for the H center migration in each salt. The activation energies for the interaction volume in RbBr and CsBr and also those in KCl and KBr, previously obtained, are compared with the ionic size of each salt and it is found that the activation energy is smaller in salts with smaller ratio of the halogen‐ion radius to the space between two neighboring alkali ions. The amount of the interaction volume at liquid helium temperature in KI and CsBr is also measured and found to be larger than that in KBr and KCl, which is in accordance with the fact that the ratio of the halogen ion radius to the space between two neighboring halogens is larger in the former salts.
F–FA(Na) conversion by repetitive light pulses in additively coloured KCl crystals
Authors: Mladenova M. , Yankov R. , Georgiev M.
Published: July 2007

The optical conversion of F centres to FA(Na) centres induced by repetitive light pulses is investigated above room temperature. The conversion kinetics is investigated as a function of the pulse intensity and repetition rate, as well as of the temperature. The dependence of the initial FA formation rate and the FA density at saturation are found to fit fairly the predictions of a F–FA conversion model in which the thermal dissociation of the ionized FA centre is rate determining. Data on the binding energy of the latter are obtained and subsequently discussed.
α‐Oxygen doublets. I. Theory
Authors: Gaididei Yu. B. , Loktev V. M. , Prikhotko A. F. , Shanskii L. I.
Published: July 2007

A group‐theoretical classification is made of the α‐O2 states. It is found that the bimolecular absorption lines must be polarized. It is shown that the bimolecular absorption lines split into doublets by taking into account the collective nature of excited crystal states. The electron‐vibrational structure of bimolecular transition bands is studied.
Crystal field levels of Pr3+ in PrFeO3 and PrGaO3 determined by inelastic neutron scattering
Authors: Feldmann K. , Hennig K. , Kaun L. , Lippold B. , Lukina M. M. , Matthies S. , Matz W. , Warming E.
Published: July 2007

The crystal field splitting of the 3H4 ground state of the Pr ion in PrFeO3 and PrGaO3 has been investigated by inelastic scattering of thermal neutrons. At several temperatures the transitions have been measured by TAS and TOF methods for polycrystalline PrFeO3 and by the TOF method for polycrystalline PrGaO3. Energy level schemes which are different for these materials are given.
Dynamical Theory of Neutron Diffraction by Magnetic Crystals for Laue Geometry
Authors: Schmidt H. H. , Deimel P.
Published: July 2007

The dynamical diffraction of thermal neutrons by magnetic crystals is being considered for the case of the symmetrical Laue‐geometry. It is shown that the results depend on the angle between BA, the average value of the magnetic field in the crystal, and BH, the Fourier‐transform of the magnetic field. The neutron wave function in the crystal consists of four wave‐fields, which have the form |Ψ> =[Ψ0|χ0> +ΨH|χH> · exp (i Hr)] · exp (ik′r), where |χ0> and |χH> are spin states. In the general case |X0> and |X)H> differ from each other. The ratio of the reflected to the incident intensity is derived for the incidence of neutrons polarized parallel or antiparallel to BA and for the incidence of unpolarized neutrons, for the case of pure magnetic diffraction, and for the case that BA is normal to BH. In both cases, for each of the two spin‐directions of the incident beam, there are up to two Braggpeaks. For unpolarized neutrons there are up to three peaks.
Theory of Resonance Raman Scattering near Critical Points
Authors: Bechstedt F. , Enderlein R. , Kolpakov A. , Peuker K.
Published: July 2007

The resonance behaviour of the Raman scattering cross section for allowed one‐phonon scattering is investigated as a function of incident frequency. Both the cases of uncorrelated and of interacting electron‐hole pairs as intermediate states are considered. The scattering amplitude is calculated for uncorrelated pairs for all types of critical points of the interband electronic energy. The dispersion of the scattering cross section is shown to be independent of the type of the critical point. The effect of Coulomb electron‐hole interaction is studied for M0 and M3s critical points. As a result of this interaction the resonance enhancement is much more stronger and the scattering cross section as a function of incident frequency shows more pronounced structures as compared to the case of uncorrelated pairs.
Theoretical Investigation of the Hopping Conductivity in Disordered Systems in a Large Frequency Range
Authors: Böttger H. , Bryksin V. V.
Published: July 2007

The frequency dependence of the hopping conductivity in disordered systems is investigated taking electron‐phonon interaction of arbitrary strength into account. The underlying theoretical concept is based on the density matrix formalism. The hopping conductivity is shown to be essentially governed by two parameters with a dimension of a time τ, the average time between two consecutive hops from site to site and δt, the average time of restoring the equilibrium on a site after a hop. The dimensionless parameter ωτ, governs the relative contribution of the diffusive and the Ohmic currents to the total current. The quantity ω δt is responsible for presence or absence of memory in hopping processes.
A Simple Approach to the Energy of Bound Exciton Complexes
Authors: Loose P. , Wöhlecke M.
Published: July 2007

From some basic properties of excitons bound to ionized impurities a rough estimate and an approximation is obtained of the energy of the complex. Especially the upper limit of the estimate reflects the available experimental data quite well. The critical σ = me/mh value, above which the complex becomes unstable, is found to be 0.462 ≦ σc ≦ 0.576. For the region σ ≦ 1 we compute both upper and lower limits and an approximation of the energy of excitons bound to neutral impurities. From the limiting cases σ → 0, ∞ the forbidden region 2.1 ≦ σF ≦ 12 is obtained where the upper value is certainly too large.
Determination of the Conduction Band Effective Masses of the Germanium—Silicon Alloy by Infrared Cyclotron Resonance
Authors: Fink D. , Braunsteih R.
Published: July 2007

Cyclotron resonance measurements have been performed on the conduction bands of germanium‐silicon alloys at 891 GHz (337 pm) using an infrared laser. This has enabled measurement of the longitudinal and transverse effective masses to within 4 at% of each side of the alloy composition at which the conduction band edge switches from the [111] to the [100] direction (15 at% Si in Ge). The masses do not appear to change from their values in the pure constituents, although only the transverse mass of the [111], Ge‐like, minimum was measured with good accuracy. Within 3 at% of the switch‐over concentration, intervalley scattering becomes predominant; throughout the rest of the alloy system, disorder scattering predominates and the scattering frequency obeys an x(1 − x) law.
Quantum Theory of Rosette‐Motion Channeling
Authors: Tamtura A. , Kawamura T.
Published: July 2007

A previous quantum theoretical study of rosette‐motion channeling is developed assuming a realistic string potential of a Coulombic form and using a proper boundary condition. The quantization of channeling electrons is performed on the basis of a certain hypothetical, two‐dimensional atom model. Wave functions of bound states around a string and their excitation probabilities are calculated in analytical forms. The calculated channeling profiles agree fairly well with experimental ones.
Anwendung gruppentheoretischer Methoden zur Lösung des Grundgleichungssystems der elastischen Elektronenstreuung in Einkristallen (Si)
Authors: Hansen‐Schmidt J.
Published: July 2007

Das Grundgleichungssystem der dynamischen Theorie der Elektronenstreuung wird mit Hilfe von Projektionsoperatoren in einer der Symmetrie des Problems angepaßten Art in zwei Spezialfällen (Primärstrahl in [lll]‐Richtung, Anregung des symmetrischen 6‐Strahlfalls) formuliert und exakt gelöst.
The Influence of the Electron–Hole Exchange Interaction on the Biexciton Ground State in CuCl and CuBr and Related Optical Transitions
Authors: Ekardt W. , Sheboul M. I.
Published: July 2007

The influence of the electron‐hole exchange interaction on the biexciton ground state is recalculated. Due to the analytical and nonanalytical parts of the electron‐hole exchange interaction of the single exciton there exist two analytical parts. Both can be quite different in the order of magnitude. General formulas for s‐type states in cubic crystals are derived and numerical results for the ground states in CuCl and CuBr are given. Finally the results are compared with experimental data.
Stability of an Electron–Hole Plasma
Authors: Kraeft W. D. , Fennel W.
Published: July 2007

The thermodynamic functions of a two component degenerate plasma are calculated using Sommerfeld's expansion. Especially the thermodynamic stability of an electron‐hole plasma is investigated and the phase diagram in the n‐T plane is discussed.
Effect of Damping on the Plasmon‐Phonon Coupling in CdS and Gap
Authors: Giehler M. , Jahne E.
Published: July 2007

The influence of dampings of the free carrier motion and polar lattice vibration on the coupled plasmon‐phonon states is studied both semi‐phenomenologically and experimentally. It turns out that the correct frequencies and damping rates of the normal modes are the roots of the dielectric function ε(ω) for complex ‐values and the dielectric function takes a factorized form. Scattering of the free carriers mainly counteracts the well known coupling mechanism by the electric fields of the two subsystems, and reduces the forbidden frequency gap. Infrared reflection spectra of differently doped CdS and GaP samples and Raman spectra of GaP are analyzed by fitting the optical spectra end calculating the complex roots of ε(). The analysis shows that the shifts of the normal mode frequencies due to the high scattering rates of free carriers in these materials is beyond the limits of experimental error and that the effect of coupling is completely suppressed in heavily doped Gap. An overall fit of the reflection spectra with a frequency‐independent free carrier damping rate fails in the frequency region of the coupled plasmon‐Phonon modes and at frequencies below them.
Cation Distribution and Magnetic Exchange Interactions in MgFe2O4 Studied by the Mössbauer Effect
Authors: De Grave E. , Dauwe C. , Govaert A. , De Sitter J.
Published: July 2007

The quenching temperature dependence of the magnetic hyperfine fields at the Fe57 nucleus in magnesium ferrite has been investigated by the Mössbauer effect technique. Since the outer absorption lines are broad and asymmetric, the average magnetic hyperfine field is calculated from the two most inner lines. Using a molecular field model and an equilibrium distribution law and taking into account the difference in covalency for the A‐ and B‐site Fe3+ ions, reliable results are obtained concerning the magnetic exchange interactions and the ionic distribution in MgFe2O4.
Electroreflectance and Wavelength Modulation Study of the Direct and Indirect Fundamental Transition Region of In1−xGaxP
Authors: Lange H. , Donecker J. , Friedrich H.
Published: July 2007

The direct Eo, Eo + Δo and indirect EΓ−X transition energies of In1−xGaxP crystals are studied by electromodulation and wavelength modulation methods at 77 K and room temperature. The Eo energy at 77 K and room temperature obeys a parabolic relation as a function of composition x with a bowing parameter of b = 0.764 and 0.76 eV, respectively. The EΓ−X energy at 77 K can be expressed by a linear x‐dependence with slope 0.072 eV. The crossover point at 77 K is determined as xc = 0.72 ± 0.015; Ec = (2.300 − 0.003) eV. The spin‐orbit splitting Δo shows an upward bowing with bowing parameter −0.060 eV.
Frequency‐Dependent Exchange Correction to the Dielectric Function of the Electron Gas
Authors: Brosens F. , Lemmens L. F. , Devreese J. T.
Published: July 2007

The equation of motion of the Wigner distribution function in the homogeneous electron gas is decoupled using the Hartree‐Fock prescription. After linearization, the variational procedure, used by Rajagopal and Jain and by Langreth is applied. The transverse and the longitudinal dielectric functions in the Hartree‐Fock approximation are derived, and the consistency of the equations for the charge density and the current density is examined. Several limits are calculated. The slope of the plasmon mode is substantially lowered by the exchange interactions, and the damped acoustic mode disappears for the densities that give a negative bulk modulus in the Hartree‐Fock approximation. The exchange interaction only slightly influences the dispersion of the transverse collective modes.
Über die Berechnung des elektrostatischen Potentials in Kristallgittern: I. Grundlagen der Berechnungsmethode
Authors: Matthies S.
Published: July 2007

Eine Methode zur Berechnung des elektrostatischen Potentials in Gittern punktförmiger Multipole wird vorgeschlagen. Die Grundformeln der Berechnungsmethode werden abgeleitet. Die Methode benutzt Größen vom Typ eines Formfaktors der Basis der Elementarzelle und bestimmte Gittersummen über die einfachen Bravaisgitter, die, wenn sie in tabellarisierter Form vorliegen, die Berechnung des Potentials mit vorgegebener Genauigkeit merklich vereinfachen.
A New Approach to Exciton‐Phonon Interaction and Some Consequences
Authors: Kapor D. V. , Stojanović S. D. , Čkrinjar M. J. , Tošić B. S.
Published: July 2007

In the present paper a new definition of the exciton‐phonon interaction is given, whose basic idea is that even the exciton operators are expanded in molecular displacements. The interaction Hamiltonian defined in this way gives the value of Urbach's coefficient which is in good agreement with the experimentally measured values. Another good agreement between theory and experiment is obtained if this new approach to the exciton‐phonon interaction is used in the calculationof the level shift and exciton absorption line broadening.
ESR Analysis of IIB‐IV‐V2 Semiconductors with Mn2+ as a Paramagnetic Probe
Authors: Kaufmann U. , Räuber A. , Schneider J.
Published: July 2007

The electron spin resonance spectra of Mn2+ impurities in the chalcopyrite semiconductors ZnSiP2, CdSiP2, and ZnSiAs2, have been analysed. These impurities are used as a paramagnetic probe to demonstrate the magnetic non‐equivalence of the two group IIB sites in the unit cell of a IIB‐IV‐V2 compound. The 55Mn2+ hyperfine constants A observed here are very close to those in the binary analogs GaAs and GaP indicating that bond ionicities in both classes of semiconductors are quite similar. The ESR line widths of Mn2+ in the ternary compounds are considerably smaller than those in GaP and GaAs and anion ligand hyperfine structure is resolved. These results suggest that cation ligand hyperfine interaction and/or strains are an important ESR line broadening mechanism in III‐V semiconductors.
Mean‐Square Displacements for NH4Cl and ND4Cl Crystals
Authors: Kurittu J. , Merisalo M.
Published: July 2007

The mean‐square displacements for NH4Cl and ND4Cl crystals at various temperatures between 85 K and 165 K have been calculated from the eigenfrequencies and eigenvectors of the phonon states as obtained from a rigid ion‐rigid molecule model of lattice dynamics, and from the internal vibrations of ammonium ions. The calculated results are discussed in relation to available experimental data.
Mössbauer Effect and Raman Studies of the Structural Phase Transitions in K2[SnCl6]
Authors: Winter J. , Rössler K. , Bolz J. , Pelzl J.
Published: July 2007

The 110Sn Mössbauer spectrum and the Raman phonons in polycrystalline samples of K2[SnCl5] have been investigated in the temperature range from 4 and 150 K, respectively, to 300 K. With decreasing temperature a discontinuous increase of the halfwidth of the Sn‐Mössbauer line and a concommitant change in the isomeric shift are observed. The former is attributed to the appearance of a quadrupole splitting at the first order crystallographic transition at T1 = 262 K. The Raman spectrum of the low‐symmetry phase exhibits an external lattice mode located at 33 cm−1 at 150 K. It shows a pronounced softening following the relation ω(T) ∼ (T0 − T)1/2 where T0 is close to T1. Two models are discussed in which alternatively a zone centre and a zone boundary rotary mode is considered to drive the crystallographic transition.
Dyson‐Maleev Transformation and Three‐Magnon Problem for an s = ½ Ferromagnet
Authors: Gochev I. G.
Published: July 2007

Non‐physical three‐magnon states at real energies of Dyson's Hamiltonian were found. Longitudinal anisotropy and interaction between nearest neighbours were taken into account.
The Critical Voltage Effect in Transmission Electron Microscopy: VI. Renormalized Perturbation Theory for the Treatment of Absorption
Authors: David M. , Gevers R. , Serneels R.
Published: July 2007

For a study of the critical voltage effect (CVE) with absorption, treating the latter as a perturbation, one has to apply degenerate perturbation theory to a non‐Hermitean perturbation. It is pointed out that the perturbation expansion may then become divergent under certain illumination conditions. A renormalization technique is developped to remove the divergence. The following conclusions result: In all centrosymmetric crystals where the CVE occurs, and in certain noncentrosymmetric ones, the dynamical matrix with absorption becomes degenerate at a voltage close to the usual CV and for two small misorientations which are symmetrical with respect to the exact Bragg orientation for the second order reflection. At the degeneracy the diagonalizing matrix is singular. Nevertheless the renormalized perturbation theory yields for the latter a simple and very accurate approximative expression which is valid even at the singularity.
Influence of Self‐Consistency on the Relaxation near a Vacancy or a Surface in Transition Metals
Authors: Allan G. , Lannoo M.
Published: July 2007

In the tight‐binding approximation a complete intra‐atomic screening of the charge is assumed to evaluate the self‐consistent potential near a defect in a transition metal. The importance of this potential on the relaxation near a defect is demonstrated. It can, for instance, invert the sign of the displacements for a nearly empty or full d band where the screening by the d electrons is less efficient. The agreement with the experimental results is notably improved for the vacancy formation volume of platinum.
The Tensor of the Drift Mobility of Electrons and Holes in the Anthracene‐1,3,5,‐Trinitrobenzene Charge Transfer Complex
Authors: Zboiński Z.
Published: July 2007

The mobilities of electrons and holes in differently orientated crystals of anthracene‐1,3,5‐trinitrobenzene are studied as a function of temperature over the range 275 to 355 K. The mobilities of both electrons and holes are found to increase linearly with temperature; the mobility of holes being markedly higher and its temperature dependence weaker than that of electrons. The most probable process for the migration of an excess electron occurs along infinite columns formed by donors and acceptors stacked alternatively plane to plane; the direction of the highest hole mobility deviates from that of electrons by an angle of about 33°.
On the Electronic Molecular Structure of the Organic Semiconductor Copper Phthalocyanine
Authors: Malter H.
Published: July 2007

The electronic eigenvalue problem of the molecule of copper phthalocyanine is approximated using the molecular orbital formalism. The symmetry of the molecule allows a group theoretical reduction of the secular equations and a classification of the eigenvalues and eigenfunction coefficients. The model statements are compared with the experimental molecular spectra in the vapour phase and in solution from the near UV to the near IR.
Crystal‐Field‐Induced Surface Anisotropy in Thin Magnetic Films
Authors: Duda A.
Published: July 2007

The general form of the spin Hamiltonian for the (001), (111), and (110) surfaces of cubic crystals is derived on the basis of symmetry requirements. The resulting angular dependence of the spin‐wave spectrum is considered. For rare‐earth ions, the surface anisotropy is shown to be related with the crystal field parameters.
Nucleation Phenomena in Electron‐Hole Drop Formation in Ge and Si: I. Nucleation Rates
Authors: Westervelt R. M.
Published: July 2007

A general nucleation theory for liquids possessing finite lifetimes, specifically for the condensation of electron‐hole drops (EHD) from a gas of free excitons (FE), is presented. The theory is developed for both homogeneous and inhomogeneous nucleation in Ge and Si. In Part I explicit expressions are derived for the steady‐state rates of formation J+ and breakup J− of macroscopic EHD, and for the EHD size distribution in dynamic equilibrium. It is found that the FE‐EHD system above T ≈ 1.3 K in Ge is characterized by metastable states for which the equilibration time is extremely long. The expressions for J+ and J− yield two sharp excitation thresholds. In Part II these results are applied to quantitatively predict for EHD the metastable time behavior, a procedure for measuring the surface tension, the phase diagram, and the EHD radius as a function of temperature. In Part III experimental luminescence data are presented for Ge which support this theory and accurately determine the surface tension.
Deformation‐Induced Exciton Luminescence of Alkali Halide Crystals
Authors: Markova‐Osorgina I. A. , Shmurak S. Z.
Published: July 2007

Luminescence due to deformation is found in ionic crystals coloured at 77 and 193 K and the spectral characteristics of the process are investigated. It is shown that the deformation is accompanied by electron and hole capture by moving dislocations. Electron recombination with impurity ions and VK centres results in impurity and autolocalized exciton luminescence, respectively. Hole transfer by dislocations gives rise to a change of the F‐stimulated luminescence after deformation. New previously unknown luminescence bands occurring seems to be associated with exciton recombination in the dislocation field.
Chemical Shifts in the K X‐Ray Absorption Discontinuity of Arsenic
Authors: Kondawar V. , Mande C.
Published: July 2007

The K X‐ray absorption discontinuity of arsenic has been studied in the pure element and in several transition metal arsenides. It is observed that the discontinuity shifts towards the low energy side in the arsenides, indicating charge transfer from the metal to the arsenic atoms. The magnitudes |ΔE| of the shifts are found to be smaller than those observed for the arsenic discontinuity in some other compounds, where the arsenic atom is incorporated as a cation.
On the Pressure Dependence of Electron Transport in Noble Metals
Authors: Bansal O. P.
Published: July 2007

From experiment, it is known that the logarithmic derivative of the electron‐phonon interaction parameter K with volume V is negative for the noble metals. A previous interpretation of this anomalous behaviour is based on the assumption that the Fermi surface distortions increase with increasing pressure. An alternative simpler explanation is presented in terms of the properties of the pseudopotential form factors.