bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spectral dependence of the photocurrent in semiconductors
Authors: Holzhütter H.‐G. , Mocker M. 
Published: July 2007

AbstractThe distribution function of optically excited carriers is calculated by solving the Boltzmann equation with the help of a weak‐field approximation method. A general expression for the photocurrent as a function of the excitation energy is derived. For low excitation energies the structure of the photocurrent strongly depends on the mutual influence of elastic scattering and recombination. For sufficiently high excitation energies impact ionization takes place. If the carriers are injected closely below the ionization threshold a steep dip in the photocurrent is predicted. An expression for the quantum efficiency is presented which enables the determination of the ionization probability from optical measurements.
Magnetic circular dichroism of indirect excitonic transitions in GaP and In1‐xGaxP
Authors: Donecker J. , Kluge J. 
Published: July 2007

AbstractMeasurements of the magnetic circular dichroism (MCD) of the indirect absorption edge of intrinsic GaP and In1‐xGaxP (x ≧ 0.77) at temperatures from 2.5 to 300 K and at magnetic fields up to 52 kOe are reported. The fine structure of the indirect absorption edge is well resolved. The MCD peaks corresponding to LA and TA assisted indirect transitions have the same shape as the corresponding peaks in the wavelength modulated spectra, but the latter have different signs. That means that the thresholds of the indirect absorption edge are rigidly shifted by the magnetic field and the direction of the shift is connected with the type of the phonons. Additionally an MCD peak appears in mixed crystals which corresponds to the no‐phonon transition. It has the same sign as the LA‐MCD peak. An attempt is made to interprete the experimental results by group theory within an exciton model. Further theoretical effort is necessary for a satisfactory explanation.
The importance of backscattering in high‐energy electron diffraction calculations
Authors: van Dyck D. 
Published: July 2007

AbstractThe Schrödinger equation, describing the motion of a high‐energy electron in a crystal foil, can be solved exactly with appropriate boundary conditions. The formal analytical solution in matrix form however is not suitable for numerical calculations but can be expanded in a series of terms proportional to subsequent powers of the electron wavelength. In the case of fast electron scattering, the electron wavelength decreases so that, to a good approximation, the series can be cut off after the low order terms, yielding a relatively simple expression. This expression reveals two corrections to the dynamical solution of Howie and Whelan, which was obtained by neglecting second order derivative terms in the Schrüdinger equation. From an analysis of these corrections, an investigation of the validity of the Howie‐Whelan solntion could be performed as function of the cqwtal thickness and an estimation for the thickness limit could be derived.
Deformation potential theory for optical and acoustic phonons in PbSe‐Like semiconductors
Authors: Szymański J. 
Published: July 2007

AbstractThe matrix elements of electron‐phonon nonpolar interaction in PbSe‐like band structures is given for optical and acoustic phonons in the deformation potential approximation. Nonparabolicity and nonsphericity of conduction and valence bands (two‐band Kane model) are taken into account giving nonvanishing electron‐optical‐phonon nonpolar interaction and changing considerably the standard electron‐acoustic‐phonon matrix element.
Energiebetrachtung zum Peierls‐Übergang
Authors: Salchow R. , Liebmann R. , Appel J. 
Published: July 2007

AbstractIn einem einfachen Modell einer linearen Kette von dreidimensionalen Atomen mit stark gebundenen Elektronen wird die Änderung der Gesamtenergie beim Peierls‐Übergang berechnet. Die Wechselwirkung zwischen den Elektronen wird in der Hartreenäherung berücksichtigt. Die Ergebnisse werden für ein halbvolles Band angegeben. Dabei zeigt sich, wie die Änderung der Elektron‐Ion‐Energie und der kinetischen Energie den Peierls‐Über‐gang begünstigt, während die Änderung der Elektron‐Elektron‐Energie und der Ion‐Ion‐ Energie dem Übergang entgegenwirkt. Die Gesamtenergie wird abgesenkt. Austausch‐ und Korrelationseffekte werden diskutiert.
The electron‐phonon enhancement factor on the Fermi surface of copper
Authors: Bosacchi B. , Franzosi P. 
Published: July 2007

AbstractThe Fourier series representation technique is extended to deal with the study of the electron‐phonon enhancement factor λ on the Fermi surface. The scheme is applied to the case of copper end a detailed map of λ(k) at any point of the surface is obtained. Finally the results are compared with the predictions of microscopic calculations of λ(k) and the meaning of discrepancies and agreement are briefly discussed.
The damping of Excitons by phonons
Authors: Klähn D. , Kilimann K. 
Published: July 2007

AbstractA theory of the exciton ionization by phonons is developed. A non static effective electron‐hole interaction is derived from the Bethe‐Salpeter equation in the ladder approximation. The imaginary part of the effective potential is related to the life time and to the level width of the excitons. An application of the theory to Ge shows, that the ionization of excitons by LO‐phonons in comparison with the recombination is the dominating damping process, where KBT is or order of the effective binding energy of the excitons.
On the theory of lattice vibrations in mixed crystals. The connection between the REI model, ATA, and CPA
Authors: Böttger H. 
Published: July 2007

AbstractFor an isotopically disordered one‐dimensional model of a mixed crystal a hierarchy of equations for conditionally averaged Green functions is derived. A definite single‐site approximation leads to the random element isodisplacement model (REI‐model). This approximation is shown to be equivalent to a somewhat modified average t‐matrix approximation (ATA) and, for a self‐consistent formulation, to a somewhat modified coherent potential approximation (CPA).
The critical field for the nucleation of n‐domains in type I superconductors
Authors: Kronmüller H. , Riedel H. 
Published: July 2007

AbstractIn type I superconductors of ellipsoidal shape and large demagnetization factors the maximum diamagnetic magnetization in general is larger than the value (‐)HC/4π. Furthermore the magnetic field where the intermediate state starts is shifted to higher values. These deviations from the macroscopic theory may be attributed to the effect of the s‐n‐wall energy as well as to the interaction energy of domain walls with lattice imperfections. An investigation is made of the combined effect of these two energy contributions on the mag‐netization curve of type I superconductors.
Calculation of the electrostatic energy of the laves phases C14, C15, and C36
Authors: Rennert P. , Radwan A. M. 
Published: July 2007

AbstractThe electrostatic energies of point ions in a uniform compensating background are evaluated for a number of crystal structures using a modified form of the Ewald‐Fuchs method. These are the first calculations for the Laves phases C15 and C36.
Noncritical fluctuations and the temperature dependence of the order parameter
Authors: Franck S. , Kühnei A. , Nattermann T. , Wendt S. 
Published: July 2007

AbstractAn effective Hamiltonian with anharmonic terms of fourth and sixth order is investigated by means of perturbation theory. Noncritical fluctuations of the order parameter lead to a temperature dependence of the coefficient of the quartic term. It turns out that this coefficient can vanish at a certain temperature T1. Depending on the parameter of the substance a first‐order phase transition, a tricritical point, or a second‐order phase transition may appear. The model describes the situation in ferroelectric TSCC.
Non‐empirical calculation of structural properties and valence force fields: Application to Se and Te
Authors: Bullett D. W. 
Published: July 2007

AbstractA local orbital method is used to describe the electronic structure and the static and lattice dynamical properties of a covalent solid on an equal footing, with present application to trigonal Se and Te. Special point sampling is utilised to provide accurate values for the total energy as a function of atomic positions. With the introduction of a single parameter to describe the van der Waals interaction between non‐bonded atoms good agreement is obtained with the observed crystal structures and with empirical valence force fields.
Local mode of H− substitutional ion in KCN
Authors: Castro J. C. , Basso H. C. , de Souza M. 
Published: July 2007

AbstractThe local mode of a substitutional H− ion in KCN single crystals, the U‐center, is measured in the three phases of the crystal. In the cubic phase the local mode peaks at 510 cm−1 with a linewidth of 120 cm−1. The higher energy phonon side band in the lower temperature phase is found to be displaced 80 cm−1 from the local mode peak. The low‐temperature linewidth of the H local mode is approximately twice that of the D localized vibration. Also in opposition to the behavior in the alkali halides the linewidth keeps a high value even a t temperatures as low as 2 K.
The state of dispersion of Pb2+ in highly doped KCl crystals
Authors: Marculescu L. , Popescu M. , Baltog I. , Ghita C. 
Published: July 2007

AbstractThe state of dispersion of Pb2+ in highly doped KCl crystals after different heat treatments is investigated. Optical and X‐ray diffraction measurements show that the new phase formed in KCl crystals by precipitation of the impurity is imperfectly ordered and composed of agglomerations of cation‐vacancy dipoles, different from the case of Pb‐doped NaCl crystals where a PbCl2, phase is obtained. X‐ray diffraction and luminescence data show that after both, strong precipitation and dissolution, the impurity evolves towards the same equilibrium state characterized by partial aggregation of Pb2+ cation‐vacancy dipoles.
Direct identification of the L′3 → L1 transition in Ge
Authors: Humlíček J. , Lukeš F. 
Published: July 2007

AbstractThe electroreflectance (ER) spectra of n‐type Ge with free carrier concentrations of 6.2 × 1018 cm−3 are studied within the energy range of the E1 and E1 + ΔI transitions. A detailed analysis of the spectra shows in addition to the Franz‐Keldysh (FK) effect an electron distribution (ED) effect, too. An appropriate electrode potential decreases the intensity of the electric field and increases the probability of the occupation of the bottom of the conduction band at the same time so that the ED effect predominates over the FK effect in the ER spectrum. The lineshapes of both the effects agree well with the theoretical curves. Therefore the transition L′3 → L1 clearly manifests itself in the E1, E1 + δ1 structure.
A model of the electrical transport phenomena in imperfect crystals of copper phthalocyanine IV. Concentration of charge carriers and current—voltage characteristics
Authors: Hamann C. , Tantzscher Ch. , Lehmann G. 
Published: July 2007

AbstractUsing the results of former papers the effective concentrations of carriers, which mainly determine the dependence of the conductivity on temperature, will be found theoretically for copper phthalocyanine crystals as well as for thin films having different real structure. The relations between the trap distribution parameter Tc as a function of temperature and the assumed trap spectra agree well with experimentally determined ones.
Hyperfine fields acting on diamagnetic impurities and the anisotropic exchange via the s‐ and d‐electrons in the rare‐earth metals
Authors: Godovikov S. K. , Kozin M. G. , Turovtsev V. V. , Shpinel V. S. 
Published: July 2007

AbstractUsing the Mössbauer effect the hyperfine fields for the tin impurity in the dysprosium host is measured over the temperature range 6 to 293 K. Two hyperfine fields are found in the ferromagnetic region, which differ one from another by ≈20%. In passing through the Curie point the shape of the spectrum drastically changes by smearing because of the superposition of many hyperfine fields in the antiferromagnetic region. The existence of a temperature anomaly appears in a marked distinction of the hyperfine field curve from that for magnetization in the antiferromagnetic region. These results cannot be explained within the framework of the isotropic s—f exchange interaction model and suggest the significant role of the d‐electron exchange and anisotropic effects.
Many‐body effects on the third‐order elastic constants and pressure derivatives of the second‐order elastic constants of CsCl structure solids
Authors: Puri D. S. , Garg V. K. , Verma M. P. 
Published: July 2007

AbstractLundqvist's potential for ionic solids is used to derive expressions for the second‐order elastic (SOE) constants and the third‐order elastic (TOE) constants. It is observed that the Cauchy relations among these constants do not hold even at 0 K and the discrepancies in this relation are directly expressible as function of the many‐body term in the crystal potential. It is demonstrated that all parameters except one appearing in the expressions for the TOE constants and pressure derivatives of the SOE constants can be calculated from the values of the SOE constants, the equilibrium condition, and an approximate assumption concerning the overlap repulsion. Determining the remaining parameter (a2∂2ƒ/∂r2)0 from the expression of the pressure derivative of the bulk modulus all the six TOE constants and the remaining pressure derivatives of the SOE constants can be calculated. The values so obtained approach quite close to the measured values as against those of other theoretical workers in all the cases in which experimental results are available.
The magnetic after‐effect spectrum in cobalt between 20 and 350 K after electron irradiation at 4.2 K
Authors: Schaefer H. E. , Dander W. 
Published: July 2007

AbstractAfter electron irradiation of cobalt single crystals a spectrum of magnetic after‐effect processes is observed where nearly all the relaxation peaks anneal out rapidly during the measurement. For the discussion of the experimental results a survey is given of the configurations of interstitial atoms and close Frenkel pairs which can be expected for the h.c.p. lattice. Taking into account resistivity annealing experiments the relaxation process at 325 K can be attributed to a diffusion after‐effect which is due to a freely migrating defect.
Dielectric function of hexagonal metals III. Magnesium
Authors: Taut M. , Ziesche P. 
Published: July 2007

AbstractThe longitudinal dielectric function ϵ(q, ω) of magnesium is calculated using model wave functions and realistic energy band values. Comparison with experiment is performed in the optical properties (including anisotropy), plasmon dispersion relation, and the half‐width of the plasmon peak.
Quantum chemical investigations on vacancy diffusion and vacancy lattices in crystallites
Authors: Müller H. , Opitz Ch. 
Published: July 2007

AbstractProblems of activation of vacancy diffusion in the volume and on the surface of nickel and iron are discussed with the two‐dimensional cluster model of the solid by means of semi‐empirical molecular‐orbital methods (EHT, HMO). In addition it is shown with the model of the two‐dimensional crystallite that, besides the predominant trend to vacancy condensation, a vacancy repulsion acts, which accounts for the existence of relatively stable vacancy lattices in metal crystals.
On the theory of electron localization in amorphous systems
Authors: Schreiber J. , John W. 
Published: July 2007

AbstractFor a generalized tight binding model of amorphous one‐component systems, where the energy levels and the surrounding overlap and hopping integrals are linearly coupled, the localization theory of Economou and Cohen is applied. In terms of several approximations the localization function L(E) is discussed. Thereby it is confirmed that the off‐diagonal randomness has a delocalization effect. An Anderson transition can only take place if diagonal disorder is strong enough. As an example, the influence of diagonal spin disorder due to electron correlation, treated in the alloy analogy for the Hubbard model, is studied.
Anharmonicity in the surface and size effects in the specific heat of linear chains
Authors: Ma S. K. S. , de Wette F. W. , Alldredge G. P. 
Published: July 2007

AbstractContributions of anharmonicity to the size and surface effects in the specific heat of the monatomic linear chain are investigated with anharmonic perturbation theory for small to intermediate number of atoms and for temperatures from 0 K up to θ0, the zero‐temperature Debye temperature. It is shown that the surface specific heat CS can reliably be extracted for chains of intermediate particle number, although size effects are still quite appreciable for chains of as many as 50 atoms. The main effects of anharmonicity in the surface specific heat (as compared to that of a harmonic chain) are a small peak at T ≈ 0.09θ0 and a linear increase with T for T ⪆ 0.7θ0.
Coexisting phases in an electron—hole plasma
Authors: Ebeling W. , Kraeft W. D. , Kremp D. , Kilimann K. 
Published: July 2007

AbstractThe thermodynamic stability of the optically excited electrons and holes in a semiconductor is investigated in a wide temperature—density range. In dependence on the position of the instability region with respect to the Mott curve the two coexisting phases (i), (ii) may have very different properties: e.g. (i) drops of degenerate electrons and holes and (ii) a partially ionized plasma of excitons, electrons, and holes. For Ge the region of instability is determined. For this purpose the chemical potential is calculated exactly at all temperatures in the ideal and Hartree‐Fock contributions and approximately in the correlation part, using exact formulae for the highly degenerate and the non‐degenerate cases. In the low density region the mass action law is taken into account.
High‐temperature conductivity of organic bilinear addition compounds
Authors: Phillips J. C. 
Published: July 2007

AbstractA specific model is proposed that correlates the power index n = − (d log σ/d log T), in fits to the longitudinal conductivity σ(T) at temperatures above the maximum in dσ/d(T−1), with structural characteristics of addition compounds of TCNQ and similar molecular crystals.
Shubnikov‐de Haas oscillations in HgSe at 77 K in pulsed magnetic fields
Authors: Byszewski P. , Kowalski J. , Gałązka R. R. 
Published: July 2007

AbstractResults are given of measurements of the Shubnikov‐de Haas effect at 77 K in pulsed magnetic fields up to 32 T. HgSe single crystal samples are measured in the electron concentration range from 2.2 × 1017 to 2 × 1018 cm−3. To explain the experimental data the curvature of the lowest Luttinger level versus kz has to be taken into account. The change of the Fermi energy with magnetic field is also computed. The results show good agreement between experimental data and theory.
Zero‐field splitting of 6S(d5) ions in tetragonal and rhombic symmetry. II. Application of complete d‐electron calculations to the zero‐field splitting in Mn:MnF2 and Mn:ZnF2
Authors: König E. , Schnakig R. 
Published: July 2007

AbstractThe results of a complete d‐electron calculation for configuration 3d5 of orthorhombic symmetry are applied to a study of zero‐field splitting in Mn2+:MnF2 and Mn2+:ZnF2. The parameters are determined from the optical spectra of the crystals by a least squares fit and adjusted by conditions governing the zero‐field splitting. The results for Mn2+:ZnF2, D = −180.0 × 10−4 cm−1, E = −42.2 × 10−4 cm−1 are in excellent agreement with the experimental values D = (−186 ± 7.5) × 10−4 cm−1, E = (−41 ± 12.5) × 10−4 cm−1. The data for Mn2+:MnF2 are used to estimate the unknown covalency contribution. All contributions fit into a regular and consistent pattern.
On the theory of the structural phase transition in squaric acid
Authors: Zinenko V. I. 
Published: July 2007

AbstractCrystals of the squaric acid C4H2O4(H2SQ) and C4D2O4(D2SQ) exhibit a structural phase transition at T0H = 370 K and T0D = 520 K. The phase transition is associated with the ordering of protons, moving above T0 in the symmetric double well potential of OHO bonds. The statistic of the phase transition in H2SQ and D2SQ is investigated in the cluster approximation with account of the tunneling of the protons. It is shown that the phase transition in the model is always of second order and that the increase of the ordering parameter with the decrease of temperature takes place in a large temperature range. The predictions of the theory are compared with experimental data at hand.
Electronic structure of BiI3
Authors: Schlüter M. , Cohen M. L. , Kohn S. E. , Fong C. Y. 
Published: July 2007

AbstractThe band structure of BiI3 is calculated using the empirical pseudopotential method (EPM). Measurements of the reflectivity and wavelength modulated reflectivity spectra at liquid helium temperatures are presented. For energies below 4.5 eV, fair agreement is obtained between the experimental and theoretical reflectivity spectra. The valence band density is compared to recent X‐ray photoemission (XPS) data. Charge density distributions of distinct groups of valence bands are calculated and combined with the density of states spectrum. These are used to explain the nature of the chemical bonding in BiI3.
A Green‐function theory of an anisotropic ferromagnet
Authors: Pandey R. B. , Bhandari C. M. 
Published: July 2007

AbstractGreen's function technique is used to investigate the magnetic properties of a wide range of anisotropic ferromagnets. A method of terminating the series of equations through the standard spin identity for spin‐1 systems is used to treat the Green functions without decoupling and to obtain a set of coupled equations. Magnetization 〈Sz〉 and 〈(Sz)2〉 are studied as a function of different anisotropic parameters, i.e. single‐ion crystal‐field anisotropy parameters D1 and D2, exchange‐anisotropy parameters η1 and η2, and temperature. Analytic expressions are derived in some limiting cases. In the absence of crystal‐field anisotropy expressions for magnetization and critical temperature as functions of exchange anisotropies are obtained by series expansion methods. All these physical properties are discussed in different models and are in limiting cases in agreement with some earlier results.
Volume dependence of the electronic structure of lanthanum and cerium
Authors: Glotzel D. , Fritsche L. 
Published: July 2007

AbstractResults of a band structure calculation for the f.c.c. lattices of lanthanum, γ‐and α‐cerium are presented. The computation is based on Altmann's rigorous cellular method. A local potential is used generated according to the standard method. Charge densities of the free atoms are obtained from a SCFXα‐integration of the Dirac equation. Earlier band calculations using the full Slater exchange led to f‐levels far too low in energy. Instead the more adequate Gaspar‐Kohn‐Sham value of 2/3 is used for the exchange scaling factor, which yields reasonable f‐band energies. The Permi level proves to be well below the f‐bands for both metals. As follows from an analysis of the occupied band states in terms of angular momenta, there are f‐admixtures of 0.3 and 0.6 electrons/atom for La and Ce, respectively, which increase slightly under pressure. As the pressure rises the Fermi level moves across three van Hove singularities in the electronic density of states at –15, 10, and at 60 kbar. This is also reflected in topological changes of the Fermi surface which can give rise to isostructural phase transitions. Recent Bremsstrahlung‐ and XPS‐spectra of La and Ce are in fair agreement with the results on the density of states.