bluuebunny/crossref_metadata_2025 huggingface dataset:
Comments on the theory of photoionization of transition metal impurities in semiconductors
Authors: Langer J. M.
Published: July 2007

The shape of photoionization spectrum connected with removal of an electron from the 3dn shell of a transition metal impurity into the conduction band has been calculated. The influence of configurational mixing between 4pn and 3dn states on the shape of the spectrum is discussed. The absorption coefficient for uniaxial crystals (α for E ‖ c and E ⊥ c, c crystal axis) is given.
Pressure effects on zero‐field resonance. II. Gd3+ in various zircon structure compounds
Authors: Nowicki P. , Hillmer W. , Urban W.
Published: July 2007

A systematic investigation of the influence of hydrostatic pressure on the zero‐field resonance of trivalent gadolinium has been made in a set of nine isomorphic crystals. The pressure coefficients of the rotational symmetric crystal field parameters (which are reliably determined by our experiment) do not only vary markedly in the absolute value with the compounds but also change sign.
Generalized phase shifts for a cluster of muffin‐tin potentials
Authors: John W. , Ziesche P.
Published: July 2007

Starting with the partial wave method generalized to non‐spherically symmetric potentials by Demkov and Rudakov a purely algebraic scheme is proposed how to calculate the generalized phase shifts for a cluster of non‐overlapping muffin‐tin potentials. These cluster phase shifts and the corresponding partial wave amplitudes are determined by a system of homogeneous linear equations, into which the phase shifts of each muffin‐tin potential and the structure enter separately, completely similar to the KKR method for the band structure of a crystal. By the proposed scheme bound states can also be calculated, the number of which is limited by the properties of cluster phase shifts. As a mathematical remedy an addition theorem about the spherical Neumann functions is derived.
Effect of Ni2+ on the EPR of Mn2+ Ions doped in single crystals of nickel salts
Authors: Janakiraman R. , Upreti G. C.
Published: July 2007

Electron paramagnetic resonance of Mn2+ has been studied in single crystals of magnesium sulfate heptahydrate (MgSO4 · 7 H2O) at 300°K. Mn2+ was found to substitute for Mg2+ and exhibit four inequivalent magnetic complexes corresponding to the tetramolecular unit cell of MgSO4 · 7 H2O. The results are compared with those for Mn2+ in isostructural NiSO4 · 7 H2O. The observed differences in the EPR of Mn2+ in crystals of the two salts are attributed to the magnetic interaction of Ni2+ and Mn2+.
Cyelotron absorption in germanium and silicon in crossed fields
Authors: Akopyan R. M.
Published: July 2007

This paper deals with the observability of the cyclotron resonance spectrum changes in Ge and Si in crossed magnetic and weak electric fields. It is shown that the displacement of the fundamental line in the electric field is of the same order of magnitude as the estimates made in [9] and the observability of the second harmonics is within the limits of experimental possibilities.
Energy band structure of vanadium, zirconium, and niobium monocarbides
Authors: Potorocha V. I. , Tskhai V. A. , Geld E. V.
Published: July 2007

The results of calculations of the electron energy spectrum and the contribution of atomic states to the formation of band structure of vanadium, niobium, and zirconium monocarbides based on the tight‐binding method arc reported. The calculations were performed in two‐centre approximation, taking into account the ns and (n – 1) d‐orbitals of metal atoms and 2s, 2p carbon atoms. The secular equation was solved in 214 points, 1/48th of the Brillouin zone. A discussion of results and their comparison with experimental data are presented.
Transient photocurrents induced by time‐dependent diffusion of excitons to the surface of anthracene and p‐chloranil single crystals
Authors: Killesreiter H. , Braun R.
Published: July 2007

Transient photocurrents produced by a light flash were used as a probe for the time‐dependent diffusion of excitons in anthracene and p‐chloranil single crystals. An analytical solution for the time‐dependent diffusion equation for excitons near a quenching phase boundary is presented. Inserting experimental data allows evaluation of the lifetime, τ = 4×10−6s, the diffusion coefficient, D = 5×10−6 cm2/s, and hence the diffusion length, L = 5×10−6 cm, for n‐π* triplet excitons diffusing perpendicular to (ab) planes in p‐chloranil. For triplet excitons in anthracene, the diffusion coefficient normal to (ab) plane, D ≈ 5 × 10−5 cm2/s, is estimated. The method allows to distinguish between different sorts of excitons diffusing in a crystal with different velocities. This is of importance for anthracene, where both singlet and triplet excitons are produced by strongly absorbed light.
Study of lH in paraelectric and ferroelectric K4Fe(CN)6 · 3 H2O by pulsed NMR
Authors: Avogadro A. , Cavelius E. , Müller D. , Petersson J.
Published: July 2007

The nuclear spin lattice relaxation time T1 of 1H in a K4Fe(CN)6 · 3 H2O single crystal was measured between room temperature and 70 °C by a 90°‐t‐90° sequence. At 15 and 8.48 MHz T1 is influenced by a translational diffusion of the water molecules and by a flip motion around the bisectrix of the water molecules. T1 has a discontinuity at the Curie point. In addition a dependence of T1 on the orientation of the crystal is observed and the absorption line shape is investigated. The results arc interpreted assuming that the intramolecular interactions of the protons of the water molecules give the main contribution to the spin lattice relaxation.
Excitation‐diffusion‐limited energy transfer in solids I. Theory of the dynamics
Authors: Heber J.
Published: July 2007

The dynamics of emcrgy transfer processes in solids are treated theoratically under the following assumptions: 1. All microscopic processes involved are incoherent. 2. The corresponding interactions are short. ranged. 3. The diffusion of the excited states in the system of the donors is the process limiting the energy transfer. The fluorescence decays of the donors and acceptors after a δ‐shaped excitation arc given, and discussed as a function of the two parameters of the theoretical model.
Untersuchungen an abgeschrecktem und elektronenbestrahltem Wolfram
Authors: Kunz W.
Published: July 2007

Hochreines, zonengeschmolzencs Wolfram wurde in superfliissigem Helium abgeschreckt. Rei der Bestimmung der Leerstellen‐Bildungsenergie E aus Messungen des elektrischen Restwiderstands ist der Dickeneffekt berüeksichtigt. Isochronc und isotherme Anlß‐kurven ergeben komplexes Verhalten, was auf die Bildung von Leerstellenkomplexen hin‐deutet. Ergänzende isochrone Erholungskurven nach Elektronenbestrahlung bei 4,5 °K zeigen im wesentlichen drei Stufen. In Proben, die vor der Bestrahlung abgcschreckt wur‐den, verstärkt sich die Erholung im Tieftemperaturbereich. Dies stiitzt die Auffassung, daß beim Abschrecken tatsächlich Leerstellen und nicht Verunreinigungen eingefroren werden.
Relativistic generalization of the Saxon‐Hutner theorem
Authors: Subramanian R. , Bhagwat K. V.
Published: July 2007

It is shown how a delta potential can be replaced by appropriate boundary conditions for the wave function for the one‐dimensional Dirac equation. This technique is used to derive en explicit expression for the transfer matrix in the case of a linear chain of such potentials. Luttinger's proof of the Saxon‐Hutner theorem on band gaps is extended to the relativistic case by introducing necessary modifications.
On the temperature dependence of the DC conductivity in disordered materials
Authors: Croitoru N. , Popescu C. , Telnic M. , Vescan L.
Published: July 2007

The general expression of the conductivity σ(F) = f μ(E, T, F) f0,(E, T, F) g(E) dE is discussed, in which μ(E, T, F) is a generalized mobility which may depend on energy, field, and temperature. It is shown that constant activation energies result for sharply limited energy or mobility gaps and that a continuously temperature dependent activation energy may appear for a tailed‐type product μ(E) g(E). Expressions of the type σ ∼ exp (— B T−1/n) may be obtaincd even if the temperature dependence of the mobility is negligible.
The dependence of the energy gap on the composition in the mixed crystals CdνHg1νSe
Authors: Slodowy P. A. , Giriat W.
Published: July 2007

The dependence of the energy gap on the composition of CdνHg1‐νSe solid solutions has been determined from the measurements of the fundamental absorption edge. It has been found that, within the experimental error, E0 = E0(ν) exhibits a linear dependence. The coefficient of the temperature dependence on the energy gap with composition has been determined. dE0/dT1 has been found to be positive for ν < 0.4, and negative for x > 0.4.
Classical theory of nonlinear oscillators interacting with a medium
Authors: Dykman M. I. , Krivoglaz M. A.
Published: July 2007

The correlation function Qx(t) of the normal coordinates of singled out nonlinear vibrations interacting with the vibrations of the continuous spectrum of a medium (e.g. of local or quasi‐local vibrations in crystals) as well as its spectral representation Qx(ω) were investigated. The calculations have been carried out for the case of high temperatures when the classical approach is applicable. The use of asymptotic methods of nonlinear mechanics and some results of the theory of random processes allowed to consider the case of an arbitrary ratio between the constants characterizing the non‐linearity of singled out oscillators and their energy of interaction with the medium (but with the assumption that both these constants are small). In this general case asymptotic explicit expressions were derived for Qx(t) and Qx(ω). Using the Fokker‐Planck equation for this problem we have also derived the difference equations which determine Qx(ω).
Calculation of density of states for crystalline and amorphous III‐V semiconductors
Authors: Kramer B. , Maschke K. , Thomas P.
Published: July 2007

The densities of states of crystalline and amorphous Gap, GaAs, GaSb, InP, InAs, and InSb have been calculated using the pseudopotential method and the complex band structure (CBS‐)method. The short range order of the amorphous structures has been taken into account by an approximative correlation function.
Faraday rotation and the effective g‐values of excitons in cuprous halides
Authors: Suga S. , Koda T. , Mitani T.
Published: July 2007

Faraday rotation spectra have been measured in the exciton absorption region on evaporated films of cuprous halides including CuCI, CuBr, CuI, and CuBr‐CuCl mixed crystals. From an analysis of the observed spectra, the exciton effective g‐ values are estimated and compared with a theory which takes account of the Zeeman, exchange, and spin‐orbit energies for the IS exciton states. Effects of the hybridization of the Cu 3d and halogen p orbitals in the valence bands in cuprous halides are quantitatively investigated.
Theory of singlet–triplet splitting of M‐ and Ft‐centres in ionic crystals
Authors: Berezin A. A.
Published: July 2007

The modified method of Gorkov‐Pitaevskii and Herring‐Flicker is used for the calculation of the value of singlet–triplet splitting in the system of two interacting F‐centres. The energy of singlet–triplet splitting (Δ) is expressed by a five‐dimensional integral in the configuration space of two optical electrons which for the case of 1s hydrogen‐like wave functions of F‐centres can be computed in explicit form. The numerical results are given for the M‐centre in alkali halides and the Ft‐centre (two electrons trapped by a linear trivacancy) in alkaline earth oxides (MgO and CaO). For these centres the calculated values of Δ are in good agreement with the experimental ones. It is shown that the theoretical values of Δ obey the Mollwo‐Ivery relation of the form Δ ∼ d−n (d lattice constant), where n ≈ 2.3 for the M‐centre and n ≈ 0.5 to 0.9 for the Ft‐centre.
Pressure and temperature dependence of the energy Gap in CdxHg1−xTe
Authors: Stankiewicz J. , Giriat W.
Published: July 2007

The investigation of the Hall coefficient in CdxHg1−xTe under hydrostatic pressure over the temperature range 77 to 300 K is reported. It is shown that within the experimental error, the energy gap is linear with x in the range 0 < x < 0.2. The pressure coefficient (dE0/dp)T and temperature coefficient (dE0/dT)p have been determined. The temperature coefficient of E0 has been found to be positive and to decrease with x. From the behaviour of RH = f(p) it has been established that the cross over Γ6, Γ8 is continuous.
On investigation of the structure of energy bands using quasielastic knock‐out of an electron by an electron (e, 2e)
Authors: Levin V. G. , Neudachin V. G. , Smirnov Yu. F.
Published: July 2007

The question of obtaining information on the band structure of solids using quasielastic knock‐out of an electron by an electron from a very thin crystalline film is discussed. It has been found that Van Hove singularities practically have no effect on the quasielastic knock‐out cross section. Form factors for the bands Γ1 Δ1 X1, Γ12 Δ1 X, Γ1 Δ1 L1, Γ λ1 L in a Cu single crystal have been calculated. The shape of the form factors is very sensitive to the type of electron binding in a crystal and to the hybridization of states in a single‐particle electron wave function. Form factors for a single crystal and a polycrystal of Al are also discussed.
ENDOR investigations on the superhyperfine interaction of Fe3+ in SnO2
Authors: Rhein W. , Rosinski Ch.
Published: July 2007

ENDOR measurements on Fe3+ at substitutional lattice sites of SnO2 show a well resolved superhyperfine interaction with the ten nearest Sn ligands and confirm a substitution of Sn4+ by Fe3+. The isotropic and anisotropic parts of the interaction are obtained. The spin Hamiltonian parameters of ESR are redetermined at 4.2 °K.
The temperature variation of magnetization of bulk amorphous Ni85P15: An amorphous ferrimagnet?
Authors: Simpson A. W. , Brambley D. R.
Published: July 2007

The variation of saturation magnetization with temperature of an Ni85P15 alloy has been measured with the sample firstly in the as‐deposited amorphous state and secondly in a crystalline metastable single phase condition. The Curie temperature of the amorphous sample was roughly three quarters that of the crystalline form, and the saturation magnetization at absolute zero of the amorphous sample roughly one third that of the ordered form. The corresponding states curve of the amorphous alloy was appreciably below that of its crystalline state which, in turn, was lower than that normally observed in ferromagnetic materials. The results are interpreted in terms of a large distribution in exchange interaction between neighbouring nickel atoms such that a significant proportion of the exchange coefficients are negative; an assumption supported by other radial distribution data. The moments of those atoms whose interactions are predominantly negative will tend to lie antiparallel to those of the bulk of the material thereby accounting for the low saturation moment at absolute zero of the amorphous sample. The name amorphous ferrimagnetism is suggested for such a phenomenon.
The precise evaluation of derivatives of the anisotropic elastic Green's functions
Authors: Barnett D. M.
Published: July 2007

Using Fourier transforms a simple scheme for numerically obtaining derivatives of the tensor Green's functions of elasticity with great precision is derived. The technique developed circumvents the need for solving an auxiliary eigenvalue problem and a related sextic algebraic equation common to previous treatments of this problem. Numerical results for the dilatation associated with a “dilatation center” in copper are given. The speed and accuracy of the present technique indicates that a previous suggestion by Lothe is indeed quite practical — namely, that internal stress problems (dislocations, point defects, inclusions, thermal stresses) in anisotropic media may be treated using tabulated data for the Green's functions and their derivatives.
Black–white contrast figures from small dislocation loops. I. Analytical first order perturbation solution
Authors: Wilkens M. , Rühle M.
Published: July 2007

The paper deals with a theoretical study of the black‐white contrast figures on electron micrographs produced by small dislocation loops under dynamical two‐beam conditions. The loops possess arbitrary orientations of their loop plane normals and Burgers vectors. The theory is based upon an analytical first order perturbation solution of the diffraction contrast differential equations of the Bloch wave type. An equation for the calculation of equal‐intensity contour diagrams is derived which can be applied easily without extended numerical computations.
On the theory of the infrared vibrational spectrum of a solid with structural disorder
Authors: Böttger H.
Published: July 2007

A formalism is presented for the calculation of the infrared vibrational spectrum of a solid with structural disorder. A perturbation theory that takes an averaged field model as the unperturbed system is used to discuss one‐phonon selection rules. In zeroth order there is a q = 0 selection rule. In higher orders all vibrations are allowed to take part in the absorption.
Infrared reflectivity studies of p‐type Pb1 − xSnxTe
Authors: Korn D. M. , Braunstein R.
Published: July 2007

Near normal incidence reflectivity spectra of p‐type Pb1 − xSnxTe have been obtained. The electric susceptibility effective mass m★, dielectric constant ε∞ and relaxation lifetime τ were determined by a computer least squares best fit of the data to a single band Drude free carrier model. A possible anomalous behavior of m★ has been observed.
Calcul de la conduetivité thermique du réseau par la formule de Kubo
Authors: Conan A.
Published: July 2007

A l'aide de la formule de Kubo et dans le cadre de l'approximation de Hartree‐Fock, nous calculons, à partir d'un modièle hamiltonien simple, la conductivité thermique du réseau. Le temps de relaxation est calculé à l'aide de la méthode de la moyenne harmonique et conduit à une expression de la conductivité thermique à basse température en bon accord avec celle obtenue par la méthode variationnelle.
Zur Quantentheorie der Ionenkristalle. Eine Analyse der Lowdinschen Näherung
Authors: Bauer R.
Published: July 2007

Es wird der Näherungscharakter der Löwdinschen Methode zur Losung der Hartree‐Fock‐Gleichungen für Ionenkristalle untersucht. Die Dichtematrix der Löwdinschen Nlherung (LN) geht über ein eingeschränktes Variationsverfahren aus dem Grundzustand der freien Ionen hervor. Der Einfluß des winkelabhängigen Anteils des kubischen Gitterpoten‐tials und der Austauschwechselwirkung der Nachbarn auf die Dichtematrix ist in der LN vernachlässigt. Die Auswirknngen der Löwdinschen Näherung auf die Beschreibung der Ionenbindung werden kurz diskutiert.
Photoconductivity associated with free excitons in GaAs
Authors: Richard C. , Dugue M.
Published: July 2007

A peak of photoconductivity associated with free excitons has been observed in GaAs. This peak has been investigated as a function of the free electron density, the temperature, and the excitation intensity. A model which takes into account the dissociation of free excitons on ionized acceptors is presented to explain the presence of this peak.
Green's function theory of the polaron energy‐momentum relation
Authors: von Baltz R.
Published: July 2007

The energy E(p) of a polaron is studied as a function of temperature and momentum p with particular attention focused on the region where the excitation energy E(p) – E(0) closely approaches the energy of a longitudinal optical phonon. It is shown that in this region a quasi‐particle description is not adequate, e.g., the horizontal tangent in E(p) does not give rise to a singularity in the density of electronic states. The shape of the spectral weight function indicates that even for small coupling strength there is an internal excited state of the polaron.
The phase transition in solid orthohydrogen
Authors: Moraal H.
Published: July 2007

A mean‐field theory for the phase transition in solid orthohydrogen is presented. For this particular form of the field it can be shown that the order‐disorder transition has to be accompanied by a change of crystal lattice. The transition temperatures found for o‐H2 and p‐D2 are 3.44 and 4.19 K, respectively, which values have to be compared with the experimental values of 2.91 and 3.99 K, respectively.
Density of states in the one‐dimensional Hubbard model
Authors: Elk K.
Published: July 2007

For the one‐dimensional Hubbard model with nearest‐neighbour interaction a functional equation is derived, which gives an exact solution for the density of states Dδ(ω). This functional equation is numerically solved for different values of the electron density nδ and the interaction strength U. From the resulting behaviour of the density of states, band splitting and an effective band narrowing is obtained. In the limit U → ∞ the band splits into several separate peaks.
Surface states in ionic crystals: Calculations for CsBr
Authors: Bertoni C. M. , Calandra C.
Published: July 2007

The Green's function approach has been applied to surface electron states together with a realistic model of bulk band structure of ionic crystals for the calculation of surface energy band structure of the (110) face of simple cubic CsBr. The results, which supply the first accurate test of the method, are in agreement both with the main trend of the experimental data for ionic crystals and with previous theoretical evaluations. Two main bands appear lying almost entirely inside the bulk band gap, the upper band has a metal‐like behaviour and the lower one is insulator‐like. This picture is confirmed also by the sketch of the electron density of states. It turns out from the calculation that the Green's function method is especially suitable for the description of electron states of not‐reconstructed surfaces.
Messungen der komplexen Dielektrizititszahl von Bariumtitanat‐Einkristallen bei Mikrowellenfrequenzen
Authors: Kaatze U.
Published: July 2007

Es wird über Messungen der komplexen Dielektrizitätszahl von Bariumtitanat‐Einkristallen im Frequenzbereich zwischen 0,5 und 25 GHz berichtet. Sie wurden im para‐elektrischen Bereich nahe oberhalb der Curie‐Temperatur und im ferroelektrischen Bereich in der Umgebung der Kristallumwandlungstemperatur 5 °C durchgefiihrt. Aus dem Fre‐quenzgang der dielektrischen Verluste im paraelektrischen Bereich wird auf Polarisations‐fluktuationen und auf die Dämpfung der ferroelektrischen Gitterschwingung geschlossen. Im ferroelektrischen Bereich scheint ein Dispersionsverhalten mit Resonanzcharakter vor‐zuliegen in tfbereinstimmung mit Modellen von Kittel und Devonshire.
The effect on the spin resonance of conduction electroils in magnesium of a surface layer of oxide or nitride
Authors: Bowring C. S. , Cousins J. E.
Published: July 2007

Measurements are reported of the conduction electron spin resonance linewidths of magnesium, contaminated with surface layers of Mg3N2 and MgO, as a function of temperature and at a frequency of 9.27 GHz. The samples all had dimensions which were large in comparison with the skin depth and a temperature range of 200 to 600 °K was covered. The results are interpreted in terms of a temperature‐dependent intrinsic relaxation mechanism, a temperature‐dependent surface relaxation mechanism, and a temperature‐independent impurity relaxation mechanism.
The vibrational spectrum and dielectric behaviour of sodium chlorate
Authors: Dawson P.
Published: July 2007

The optical phonons of NaClO3 have been studied over a wide temperature range by Raman spectroscopy. It has been shown that the observed dielectric behaviour of this crystal may be accounted for solely in terms of the contributions from the polar phonons and that it is not necessary to postulate the existence of other dielectric mechanisms in the region between the microwave and far infra‐red regions. No evidence has emerged to support the suggestion that sodium chlorate is a fcrroelectric.
Stress study of the Jahn‐Teller effect in the 4T2, state of Cr3+ in α‐Al2O3
Authors: Duval E. , Louat R. , Lacroix R.
Published: July 2007

The vibronic wavefunctions of the 4T2, relaxed excited state of Cr3+ in α‐Al2O3 are determined when there are a Jahn‐Teller coupling with the Eg‐mode and a trigonal potential which cannot be treated as a perturbation with respect to the electron‐vibration interaction. The interaction Hamiltonian with a uniaxial stress, parallel or perpendicular to the C3,‐axis, is obtained. The experimental results are interpreted by the existence of a Jahn‐Teller effect with the Eg‐mode. Choosing 200 cm−1 for the vibrational quantum energy, the Jahn‐Teller energy is calculated from the experimental data.
Monte Carlo simulation on the behavior of energetic ions in polyatomic targets
Authors: Ishitani T. , Shimizu R. , Murata K.
Published: July 2007

The behavior of energetic ions in polyatomic targets is simulated by the Monte Carlo technique, which is based on a single scattering model taking account of nuclear and electronic stopping. The results follow the experiments fairly well as to range and energy distribution of transmitted ions through thin films. The present simulation is, in principle, applicable to any combination of ion beam and target.
Interprétation des structures magnétiques des composés TCo3 (T = Pr, Nd, Tb, Ho, Er)
Authors: Yakinthos J. , Rossat‐Mignod J.
Published: July 2007

L'étude par diffraction neutronique des composés TCo3 montre que la direction des moments est fonction de l'ion terre‐rare considéré. Comme prevu par l'analyse des modes magnétiques on obtient des structures colinéaires, soit dans le plan de base (Nd, Tb, Ho), soit selon l'axe c (Pr, Er). La direction de ces moments est imposée par l'anisotropie du sous‐réseau TI, elle est par consequent une propriété intrinsèque de l'ion terre‐rare envisage et est déterminée seulement par le signe du coefficient a dans I'Hamiltonien de champ cristallin Hc = α V [3 Jz2 – (J + 1)].
Polarized luminescence of In++ and In++V centres in KCl:In crystals
Authors: Lukantsever N. , Osminin V. , Zazubovich S.
Published: July 2007

Polarized luminescence is observed for In++ (hvmI = 2.12 eV) and In++v (hvmI = 1.75eV) centres in X‐irradiated KCl:In. Spectral and polarization characteristics of the luminescence are investigated. The symmetry of excited In++ centres is established to be tetragonal. The energy level diagram and the possible models of the In++ centres in the ground and excited electronic states are considered. It is assumed that In++ and In++v centres in the excited state may be represented as In+Cl and In+vCl quasimolecules, respectively.
Optical Anisotropy of Anthracene Single Crystals for Photon Energies from 4.5 to 11.5 eV
Authors: Koch E. E. , Otto A.
Published: July 2007

Reflection spectra between 4.5 and 11.5 eV were measured for near normal incidence (7.5°) with s‐polarized light from the (001) and (010) plane of anthracene single crystals at room temperature. The strong molecular 1B2u transition causes a strong anisotropic reflection band from the (010) face. By a Kramers‐Kronig analysis ε2 spectra have been obtained for the most important polarization directions. The Davydov splitting for the prominent excitation at approximately 4.5 eV, taken for polarization directions where no longitudinal contributions to the interaction lattice sums are present, is smaller than 0.05 eV. There is evidence, that a transition starting at 7.05 eV is polarized perpendicular to the molecular plane.
Infrared Active Two‐Phonon Processes in Some Alkali Iodides
Authors: Srivastavas S. P. , Bist H. D.
Published: July 2007

The infrared transmission spectra of NaI and KI have been recorded on a Beckman IR‐11 spectrophotometer using nujol mull technique at room temperature. Besides the fundamental absorption bands at the centre of the Brillouin zone, other lattice bands involving the interaction of infrared radiation with more than one phonon in the range 33 to 300 cm−1 have been observed. These multi‐phonon combinational bands (sum or difference) are explained in terms of the four characteristic phonons at or near the zone boundary and are consistent with the selection rules.
On the Theory of Optical Absorption Spectra of Disordered Semiconductors
Authors: Burtsev E. V.
Published: July 2007

The low frequency tails of the intrinsic and impurity optical absorption coefficients of nondegenerate semiconductors are calculated by the Green's functions method. The tails are due to the random force fields present in the samples. The fields are supposed to be generally anisotropic, Gaussian, and slowly varying in space. The intrinsic absorption tail has a Gaussian shape and a classical origin if the random fields acting upon electrons and holes are different. If the field is independent of the band index the absorption tail has a purely quantum origin; its shape depends somewhat upon the type of the field anisotropy and is close to exponential. As an example a special problem is considered in which the random field is provided by an intense acoustic noise flux propagating through the sample (both the deformation and the piezoelectrical interactions are considered). The results obtained are applied to interpret some experiments in which a modulation of an intrinsic absorption edge of piezocrystals by acoustoelectric domains was studied.
NMR Study of Ferromagnetic Gd Intermetallic Compounds with CsCl Structure
Authors: Oppelt A. , Dormann E. , Buschow K. H. J.
Published: July 2007

By means of spin echo techniques the zero field resonance of Gd155, Gd157, Ag107, Ag109, and Zn67 nuclei could be detected in the ferromagnetically ordered CsCl type compounds GdAg1−xInx, GdAg1 ‐ xZnx, and GdZn1−xInx. In some cases well resolved lines could not be obtained due to broadening. Both, the hyperfine field of the magnetic and of the nonmagnetic ions are negative. No significant hyperfine field variation with conduction electron concentration is observed for the nonmagnetic ions while for the Gd ions it is appreciable. This cannot be explained by the simple RKKY approach, in which the interaction between the localized Gd‐moments and the conduction electrons is considered to be a δ‐function interaction and in which the Fermi surface is assumed to be spherical. The variation of the hyperfine fields was also studied in the systems GdCdZnx and Gd1no.5 (1−x)‐Ag0,5 (1−x)Znx, in which the number of conduction electrons remains constant. Essentially no change is found in GdCdZnx, where divalent Cd is replaced by divalent Zn. In Gd1n0,5 (1−x)Ag0,5 (1−x)Znx, where divalent Zn is replaced by monovalent Ag and trivalent In, the hyperfine fields become smaller with decreasing x. Apparently the reduction of the conduction electrons' mean free path due to potential scattering modifies their polarization.
Work Function and Surface Structure of Simple Metals
Authors: Paasch G. , Eschrig H. , John W.
Published: July 2007

The work function is calculated for ten simple metals in the framework of model potentials. The Shaw model potential is used. The surface structure is described by a model containing a decrease of the conduction electron density and a change of the lattice constant. The parameters of this model are determined from the requirement of a minimum surface energy. The calculated work functions agree well with the experimental ones. The calculated increase of the lattice constant at the surface is about 1%. It gives rise to a decrease of the total surface dipole potential. The latter is much smaller than that calculated for the usual model where the ions are replaced by a homogeneous background. The agreement of the work function with experiment justifies the use of the mean value of model potentials in the calculation of physical properties.
Effect of Crystalline Anisotropy on the Magnetostatic Spin Wave Modes in Ferromagnetic Plates. I. Theoretical Discussion for Infinite Plates
Authors: Schneider B.
Published: July 2007

The dispersion equations of magnetostatic spin wave modes in thin ferromagnetic plates are derived within the linear phenomenological theory for a general anisotropic tensor of the dynamic susceptibility (Polder tensor). The equations are discussed for the two special cases of the effective magnetic field being parallel or perpendicular to the surface of the plate. In the Appendix the Polder tensor is derived for crystals with an anisotropic gyromagnetic ratio.
Ginsburg‐Landau Theory of the Vortex Lattice in Type‐I1 Superconductors for All Values of ϰ and B
Authors: Brandt E. H.
Published: July 2007

A two‐dimensional variational method is established which permits the solution of the Ginsburg‐Landau equations for type‐II superconductors to any desired accuracy. Free energy, magnetization curves, GL function, microscopic magnetic field and current density are calculated for hexagonal and tetragonal lattice symmetries and for all values of the GL parameter ϰ and the induction B. The hexagonal lattice is found to be stable throughout. As a further result simple approximate expressions are found for the free energy (with en error <5 × 10−5) and for the magnetization as functions of ϰ and B or the external field He.
Raman Relaxation Times in Electronic Systems
Authors: Shrivastava K. N.
Published: July 2007

The theory of Raman relaxation is developed without the approximation of small wave vectors for the lattice phonons and it is shown that the accurate theory reduces into the known one if the expressions are expanded into infinite series and only the first term retained. The temperature dependence of electronic relaxation times is found to be more involved in the correct expressions than in the approximate ones. Eight different cases depending on the structure of energy levels are derived starting from a single formula. Four of these cases are new. Efforts are made to compare the theory with the experiments on Pr3+, Ce3+, Nd3+, Yb3+, and Mo5+ ions. A new T5 behaviour predicted for Kramers ions is apparently observed in Nd3+: LaCl3 and Mo5+:TiO2 and for the non‐Kramers Pr3+: LaCl3 system. Faster temperature dependence than the usual T9 one is predicted and probable indications of the same are noted on Ce3+, Nd3+, and Yb3+ in CaWO4.
Umorientierung von Hantel‐Zwischengitteratonien in verformtem Kupfer und Nickel
Authors: Wanger F. J.
Published: July 2007

Durch Verwendung genügend kleiner Frequenzen (f ≈ 10−1 Hz) konnte die versetzungsunabhängige Umorientierung der Hantel‐Zwischengitteratome (Hantel‐ZGA) getrennt von der versetzungsinduzierten Umorientierung beobachtet werden. Der versetzungsunabhängige Prozeß besitzt die von der Theorie vorhergesagte Orientierungsabhängigkeit und läßt sich nur mit dem Bild des Hantel‐ZGA mit 〈100〉‐Vorzugsrichtung verstehen. Beim Anlassen der Proben baut sich die versetzungsinduzierte Umorientierung der Hantel‐ZGA auf. Dieser Prozeß zeigt keine wesentliche Orientierungsabhängigkeit. Bei höheren Frequenzen (f ≈ 1 Hz) tritt er gleichzeitig mit der versetzungsunabhängigen Umorientierung der Hantel‐ZGA auf und maskiert dadurch teilweise deren Orientierungsabhängigkeit. Experimentelle Ergebnisse früherer Arbeiten finden durch die versetzungsinduzierte Umorientierung der Hantel‐ZGA ihre Erklärung.
Anharmonicity of Infrared Vibration Modes in Beryl
Authors: Gervais F. , Piriou B. , Cabannes F.
Published: July 2007

The anharmonicity of infrared vibration modes has been studied in beryl crystals by measurements of the temperature dependence of reflectivity. The infrared reflectivity data from 280 to 1500 cm−1 in the temperature range 300 to 1300 °K, were analyzed with the classical oscillator model. The comparison between the results of quantum theories and the dependence of observed damping on temperature shows that the cubic and quartic terms contribute to the anharmonicity of a set of modes, whereas another set of modes exhibits only a cubic anharmonic behaviour. In the latter case, a part of the modes are known as internal.
Analysis of the R2 Absorption Structures in LiF, KCl, and KBr
Authors: Giesecke P. , von der Osten W. , Röder U.
Published: July 2007

Starting from the linear and quadratic electron‐phonon coupling for defects in crystals, a method is described to obtain the one‐ and multiphonon contributions to the optical absorption structure for the case of intermediate coupling strength. It is based on the explicit knowledge of the absorption shapes of the zero‐phonon line and the vibrational sideband, which have to be measured with high spectral resolution as function of temperature. The analysis is performed for the R2, transition of the R‐centre in Lip, KCl, and KBr. The peaks of the spectral density of coupled modes resulting from the analysis are compared with characteristic phonon energies at the Brillouin zone boundary and to results of circular dichroism measurements of the R2, band. The linear coupling coefficient S is obtained (a) from the Fourier transform of the one‐phonon absorption spectrum, (b) from the ratio of the integrated absorption of the zero‐phonon line and the total band, and (c) from a calculation using the density of states obtained by the sideband analysis. Taking into account the quadratic coupling, from the spectral density of coupled modes both thermal shift and broadening of the R2, zero‐phonon line are predicted and prove to be in good agreement with the experimental data. A static contribution to the line shift as well as a residual line width at low temperature thereby have been taken into account.
On the Theory of Electroabsorption in Disordered Semiconductors
Authors: Ewer B.
Published: July 2007

The interband absorption coefficient is calculated for a disordered semiconductor subjected to an external electric field in the low‐temperature case. Two kinds of random fields are considered, the “smooth” random field possessing a correlation function which is continuous and differentiable as many times as needed and that of the Coulomb field produced, for example, by the randomly distributed charged centres (compensated samples are considered). If the external field is low enough, the change in the exponential tail (ħω < ϵg) may be expressed in the form of a shiftedband edge Δϵg. When the external field becomes strong enough, the absorption coefficient takes on the well‐known form deduced by Franz and Keldysh. In case of a “smooth” field the tail range in the electroabsorption coefficient is considered while in the Coulombic case graphs of the electroabsorption coefficient are plotted in the range of interest. The Redfield representation of an absorption coefficient of a disordered semiconductor is discussed.
EPR Studies of Impurities in Electron‐Irradiated CaO
Authors: Henderson B. , Tohver H. T.
Published: July 2007

The EPR spectrum of Mn4+ ions in CaO has been observed in single crystals containing Li+ or Na+ ions. Electron irradiation of such crystals results in Mn4+ being converted to Mn2+, a concomitant reaction being the formation of [Li]° and [Na]° trapped hole centres. Several other paramagnetic species are produced by the irradiation including atomic hydrogen, [Li]− and [Li+H]+ centres. The spin Hamiltonian parameters which describe the EPR spectra of these impurity centres are reported and discussed. Optical or thermal bleaching of the [Li]° or [Na]° centres above 77 K results in the annihilation of Mn2+ and [Li2+H]+ centres: atomic hydrogen and the [Li] centres are less readily bleached by these treatments.
Optical Spectra and Energy Band Structure of the Monoclinic Crystals ZnP2 and ZnAs2
Authors: Sobolev V. V. , Syrbu N. N.
Published: July 2007

The anisotropic spectra of edge absorption, reflection, and photoconductivity of the monoclinic ZnP2, and ZnA2, crystals have been experimentally investigated at 293 and 77 °K. The group‐theoretical calculations of selection rules for interband transitions and the dispersion relaxation are discussed. Energy band structure models at the band gap are proposed on the basis of known theoretical and obtained experimental data. Polarisation investigations allowed to determine the subband symmetry responsible for transitions in the energy range from 1 to 12.5 eV.
Faraday and Kerr Effects of Hot Electrons in n‐Type InSb in the Infrared (I)
Authors: Vorobev L. E. , Komissarov V. S. , Stafeev V. I.
Published: July 2007

The nonparabolicity of the conduction band and the anisotropy of the hot electron distribution function have been shown to lead to the anisotropy of the refraction index in strong electric fields (Kerr effect). Nonparabolicity causes also a decrease of the Faraday rotation angle θ when the electron gas is heated. A comparison of the calculated values of Δθ with the experimental ones allows to determine the temperature of hot electrons for n‐type InSb with concentrations of the electrons in the range of 3 × 1014 to 4.9 × 1015 cm−3 at T0 = 80 °K in longitudinal magnetic fields. The interaction constant with polar optical phonons was also determined. The influence of e‐e collisions on the distribution function of hot electrons is discussed.
Structural Phase Transitions in Langbeinites
Authors: Dvořák V.
Published: July 2007

Using Landau theory of phase transitions it is shown that in the langbeinite structure (space group T4) four spatially homogeneous structural phase transitions are possible. Theree of them are improper ferroelectric and result from a phonon instability at the M point of the Brillouin zone. The fourth one is very similar to structural phase transitions in hig temperature superconductors of the β‐tungsten structure. The free energies providing a basis for macroscopic description of these phase transitions are presented.
Theory of Disordered Systems.II. Functional Equation of tho Self‐Energy
Authors: Puff H.
Published: July 2007

The self‐energy of a disordered system, which is described as an ensemble average of non‐interacting electron systems of well‐defined configurations, is defined as a functional of the one‐particle funestion of the disordered systems as well as of the totality of its potential correlations by means of a functional‐differential equation. Relations to the theory developed in the first part of this series [1] are outlined.
Cluster Scatterings and Spectra. I. Electrons in Substitutional Alloys
Authors: Čàpek V.
Published: July 2007

A diagrammatical analysis of a previously reported set of approximations for the averaged electronic resolvent 〈[z ‐ H]−1〉 in substitutional alloys is made. The results are found to be in correspondence with applicability of the method revealed in a numerical analysis beyond the coherent potential approximation. A new modification of the method is suggested which is free of shortcomings of the original version. It enables relatively very easy and in principle almost exact computer calculations of spectra of substitutional alloys, at least for simplest model Hamiltonians. A simple numerical example is solved.
Core Levels of III‐V Semiconductors
Authors: Gudat W. , Koch E. E. , Yu P. Y. , Cardona M. , Penchina C. M.
Published: July 2007

The absorption and reflection spectra of GaP, GaAs, GaSb, InP, InAs, and InSb in the region from 15 to 40 eV are presented and compared with those obtained from characteristic electron energy losses. From these data, information about the density of conduction states, the matrix elements for transitions from the outermost core levels to the conduction bands, and the energy and spin‐orbit splittings of those core levels is obtained. X‐ray photoemission data (ESCA) in the region from 10 to 1487 eV are also presented and compared with the results of absorption and emission spectroscopy.
Optical Absorption Edge of Heavily Doped Semiconductors at Low Temperatures
Authors: Van Cong H. , Mesnard G.
Published: July 2007

Using the Green function methods in the Feynman‐path integral formulation, applied to disordered systems and developed already in previous papers, the optical absorption edge of heavily doped semiconductors in zero magnetic field at low temperatures is investigated. Two band‐structure models are used here and the absorption coefficeint is calculated directly from the Greenwood formula in the one‐electron approximation. Two examples concerning Ge and GaAs are discussed assuming that one excess electron is provided by each donor.
Dependence of the Phenomenological Energy Relaxation Time on Electric Field in n‐Si and n‐Ge at 77 °K
Authors: Dargys A. , Banys T.
Published: July 2007

A method to measure the dependence of energy relaxation time of hot carriers τϵ in semiconductors is proposed. The method is based on the dependence of the phase of the second harmonic ϑ on τϵ. The relationship between ϑ and τϵ was found from equations of the momentum balance and conductivity relaxation. The dependence of τϵ on the de electric field in n‐Go and n‐Si at a lattice temperature of 77°K was measured. In n‐Si the influence of redistribution of electrons among different valleys on the phase of thosecond harmonic was observed.
Electronic Structure and Spectrum of the (TmF8)6− Cluster
Authors: Anisimov F. , Dagys R.
Published: July 2007

The crystal field splitting and g values in SrF2:Tm2+ were calculated within the framework of the (TmF8)6− cluster using the Heitler‐London method. Two sorts of numerical Hartree‐Fock functions for Tm2+ electrons were used in the calculations. It is shown, that a better agreement between the values calculated and observed may be obtained using more expanded radial functions of the Tm2+ electrons.
The Electronic Structure of NaCl:Cu
Authors: Yip Kwok Leung , Fowler W. B.
Published: July 2007

The one‐electron energy levels of NaCl:Cu have been calculated by a charge‐consistent extended Hückel approach. The lowest transition is found to be electric‐dipole forbidden and its energy agrees with experiment when a Hückel constant of 2.0 is used. The ground state of the center is found to be partially covalent, with a net charge of +0.75 e on the Cu. Similar calculations on KCl:Cu tend to yield transitions too high in energy, although an estimate of the KCl valence band structure yields reasonable results. These results support the idea that the primary transition for on‐site Cu is the crystalline analogue of S → D, and that the cases of apparent allowed transitions arise from admixture of P into D through the electric field associated with a stable off‐center position of the Cu. It is suggested that the crystalline increase of the d10 → d9s transition energy arises in part from the penetration of d‐orbitals into the ligand cores, with consequent lowering of their energies.
Optical and Photoelectric Properties of the Lead Chalcogenides
Authors: Cardona M. , Penchina C. M. , Koch E. E. , Yu P. Y.
Published: July 2007

The reflection spectra of PbS, PbSe, and PbTe have been measured in the 14 to 26 eV region using as a source the DESY‐synchrotron. Three peaks are observed for PbSe and PbTe and two for PbS, corresponding to transitions from the outermost d‐levels of the Pb core to the conduction bands. In order to find out the relationship between these peaks and the spin–orbit splitting of the d‐core levels, X‐rays photoemission measurements (ESCA) have been performed. It is concluded that this spin‐orbit splitting of the d‐core levels does not appear directly in the optical spectra. A simple model is given to interpret these spectra. The valence band, and other core levels of binding energies in the range of the Kα line (1486.6 eV) aluminium‐X‐ray source have also been studied. The binding energies, splittings, and chemical shifts obtained are discussed and compared with theoretical calculations. The most intense ESCA lines exhibit satellites which can be associated with lines produced by the excitation of valence plasmons. The effects of adsorbed oxygen on the ESCA spectra are also discussed.
On the Exciton Binding to Neutral Impurities in Semiconductors
Authors: Munschy G.
Published: July 2007

It is shown that the complex consisting of an exciton bound to a neutral donor (or acceptor) is not always stable. There is an intermediate range of values of the electron to hole effective‐mass ratio σ for which the complex becomes unstable against the dissociation into an exciton and a neutral donor (or acceptor). Furthermore, in the so delimited H2‐like region of stability of the complex, the binding energy expressed in atomic units is shown to vary monotonically with the mass ratio σ up to a critical value àc (1 < σc < 2) which corresponds to the dissociation limit.