bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin waves in a deformed thin film
Authors: Dragila R.
Published: July 2007

The spin wave resonance and mode spectrum in a deformed thin film are calculated in the the frame of the nearest neighbour approximation. The perturbations of the exchange integrals are related to the displacements of atoms from their equilibrium positions in a crystal lattice near both a free surface and a contact of the film with the substrate.
Bandwidth anisotropy in metallic ferromagnets
Authors: Eagles D. M.
Published: July 2007

A simple model has been used to investigate what may be termed bandwidth anisotropy in ferromagnetic metals. This type of anisotropy arises in a tight binding model for energy bands when the bandwidths of the partial densities of states associated with particular atomic levels and a weighted mean width for occupied states depend on magnetisation direction. In the model used, the partial densities of states are assumed to be uniform over bandwidths centred on the atomic energy levels. General expressions for the bandwidth anisotropy subject to this assumption are found. Some insight into the dependence of bandwidths on energy overlap integrals is obtained by consideration of a linear chain system with two energy levels per atom. Very different dependencies are obtained according to whether atomic energy level separations are large or small compared with bandwidths. Plausible generalisations are made for the bandwidths in many level, three‐dimensional systems for the two cases. This allows simplifications to be made in the general expression for the bandwidth anisotropy energy in the case of relatively small atomic level separations.
Ion–ion potential and correlation functions in liquid Na
Authors: Cubiotti G. , Giuliano E. S. , Ruggeri R.
Published: July 2007

From recent accurate data on the static structure factor S(q) for liquid Na, quantities as the pair correlation function g(r) and the direct correlation function c(r) are computed. An attempt is made to extract some informations on the pair potential φ(r), deduced from the experimental data on S(q) by means of Percus‐Yevick and hyperchain approximations. The potential so derived is compared to that one given by the model potential approach.
Vibrational spectrum of an isotopically disordered diatomic chain with long‐range order in the coherent potential approximation
Authors: Braeter H. , Priezzhev V. B.
Published: July 2007

The vibrational spectrum of a linear diatomic chain with long‐range order is calculated in the coherent potential approximation. Defect bands are formed inside the gap between the acoustic and optical bands and outside the bottom of the optical band. The energy gaps disappear long before the chain becomes completely disordered. The density of states calculated in the coherent potential approximation is compared with the exact vibrational spectrum of the Markov chain for the same values of the short‐range order parameter.
Absorption edges for cyclotron wave propagation in silver
Authors: Saermark K. , Lebech J.
Published: July 2007

Experimental evidence for the existence of absorption edges for cyclotron waves propagating in metals, here silver, is given. The edges occur close to the ‘turning points’ of the infinite‐ωτ dispersion curve for (extraordinary) cyclotron waves. The experimental results are compared with numerical calculations based on a finite value of ωτ and on Halse's Fermi surface of silver. A physical mechanism accounting for the observations is presented.
Electric‐field‐induced gradient at the aluminium nucleus in corundum
Authors: Flyagin M. Ya. , Nikiforov A. E. , Men A. N.
Published: July 2007

In the ion shift model the influence of an external constant electric field on the quadrupole interaction of Al27 in Al2O3 is considered. The gradient changes induced by the electric field are calculated with overlap included. Four independent parameters are found defining the tensor of ion shift produced by the electric field. Using the found ion shift tensor an estimation of the metal‐ligand distance changes is performed. Tensor components of the dielectric constant are evaluated and agree with the measured values.
Experimental and theoretical effects of surface layers and spatial dispersion on the free exciton reflectance of ZnO
Authors: Lagois J. , Hümmer K.
Published: July 2007

The well‐known exciton reflectance spectra of ZnO at helium temperatures are changed drastically by surface layers. To explain the experimentally observed structure the reflectivity of a medium with harmonic oscillators (nonspatial dispersion) or with spatial dispersion and with an additional surface layer is calculated. The surface layer is either an intrinsic exciton‐free layer or an impurity layer in which multiple reflections and interference take place. From the comparison of experiment and calculation the following results are obtained: firstly, spatial dispersion of free excitons in ZnO has to be considered, secondly, the lineshapes of the closely spaced Al and Bl excitons are changed in a very different way, thirdly, the numerical evaluation yields the longitudinal resonance energies of the Bl and Cl excitons. By heavily doping a surface layer an enlarged exciton‐free layer could be produced which is demonstrated by the reflectance spectra.
Luminescence of high‐density excitons in CdS
Authors: Liebing H. , Mengel P. , Ruppel W.
Published: July 2007

The luminescence emission of CdS single crystals is investigated in the exciton spectral region for high one‐photon and two‐photon excitation at 1.8 K. The inelastic scattering of two excitons (P‐line) is resolved into three distinct lines, corresponding to the transition of one exciton from the 1s state into the states 2p, 3p, and n = ∞. The latter transition leads to photoconduction. The lines M and PM, discovered by Shionoya et al. and ascribed by him to the recombination and inelastic scattering of exciton molecules, are interpreted as Stokes acoustical phonon wings of the exciton–impurity complex lines I2 and I1. The M and PM‐lines are stimulated at high intensities. The onset of several laser modes is observed for the PM‐line. The mechanism of this two‐boson (photon and acoustic phonon) laser is discussed.
Generalising the Anderson model
Authors: Carroll C. E. , Lukes T. , Ringwood G. A.
Published: July 2007

An attempt is made to generalise the Anderson approach to localisation in order to handle other forms of disorder. After some discussion an approximate criterion for determining the mobility edge is adopted because it is simple and intuitive. The theory is applied to positional disorder at negative energies using a model derived from the Lloyd matrix expansion. The criterion is used on this model to compare localisation in s‐and p‐states.
On the energy band structure and the phase transitions in a cuprous chloride crystal
Authors: Rusakov A. P.
Published: July 2007

To explain the break‐down of the Lyddane‐Sachs‐Teller (LST) relation and the compressibility anomaly for CuCl a new energy band model is proposed to include a small indirect gap comparable to the binding energy of direct excitons. It is found for such a model that the condensation of excitons can result in the break‐down of the LST relation as well as a number of other anomalies of properties, i.e. CuCl is a high‐temperature excitonic dielectric with an effective mass ratio mh/mc ≈ 102 of electrons and holes. It is also shown that high pressure can induce a possible excitonic dielectric‐superconductor phase transition.
Photoconduction in CdS under high laser excitation
Authors: Mengel P. , Liebing H. , Ruppel W.
Published: July 2007

The photoconduction in CdS single crystals is investigated up to generation rates of 5 × 1028 cm−3 s−1 for one‐photon and two‐photon excitation by a giant pulse ruby laser. The initially generated hot carriers thermalize into excitons within about 10−13 s. At 1.8 K a strongly supralinear dependence of the photoconduction on excitation intensity (power 4 for two‐photon excitation and power 2 for one‐photon excitation) indicates that the free carriers observed by the photoconduction are generated by inelastic exciton–exciton scattering (P‐process). At higher temperatures (80 K, 300 K) the free electrons are due mainly to dissociation of the excitons. At generation rates higher than 1027 cm−3 s−1 up to the temperature of 80 K the condensation of the excitons into an electron‐hole plasma is observed. The phase transition is accompanied by a discontinuous increase in conductivity, strong fluctuations at the transition point, and a square‐root dependence of the photoconductivity at higher excitation intensities.
Pressure dependence of elastic constants and optical phonon frequencies in Se and Te. A study of the homological relationship between the vibrational modes in these materials
Authors: Fjeldly T. A. , Richter W.
Published: July 2007

The hydrostatic pressure coefficients for the elastic constants of Se and Te are shown to be large and positive in contrast to the negative mode Grüneisen parameters reported earlier for the optical vibrations. These results can be understood on the basis of the idea that the hydrostatic pressure promotes transfer of electrons from intrachain bonding orbitals to interchain bonding states. This strengthens the interchain “elastic” force constants while simultaneously weakening the intrachain “optic” force constants. The great similarity in the Se and Te lattices suggests that their vibrational spectra can be related by homology. Of the three lattice parameters describing each crystal, the intrachain bond angle is nearly identical and relatively pressure insensitive in the two lattices. Therefore, by a two‐step transformation in which the c/a‐ratio is first made equal by applying hydrostatic pressure, and secondly by properly scaling the mass and the unit cell, the two lattices can be made structurally identical. With this transformation and the available pressure data all zone centre phonons studied in the two materials could be related by a universal scaling factor for the “mode” force constants. Rough estimates for the higher‐order pressure coefficients can also be made.
Symmetries of the displacement field of a dislocation lying normal to a twofold axis
Authors: Blümel K. H. , Pongratz P. , Skalicky P.
Published: July 2007

It is shown that the displacements of a dislocation in an anisotropic crystal lying normal to a twofold axis and whose Burgers vector has components normal or parallel to it are even or odd functions in polar coordinates.
Thin film with anisotropically coupled internal layer defect. Localized states
Authors: Maciejewski W. , Duda A.
Published: July 2007

The model of a thin film with an internal anisotropically coupled layer defect is consider ed. Symmetric and antisymmetric defect coupling is assumed. An analytical method is proposed for the study of localized states. The conditions for the existence of acoustic and optical localized states are determined.
The cluster coherent potential approximation in the presence of off‐diagonal disorder
Authors: Herscovici C.
Published: July 2007

Off‐diagonal randomness is included in the cluster CPA using the “matrix‐locator” approach, suitably generalized for the many‐site approximations. At the limit of vanishing off‐diagonal disorder, the usual cluster CPA is reobtained; it is also shown that in a special case of off‐diagonal randomness, important simplifications are possible.
Magnetoplasma effects in semiconducting Bi1–xSbx alloys
Authors: Oelgart G. , Herrmann R.
Published: July 2007

The magnetic‐field‐dependent microwave absorption is measured in semiconducting Bi1–xSbx alloys in the frequency ranges of 36 and 70 GHz at helium temperatures. In addition to cyclotron resonance, tilted orbit resonance ist observed. A theoretical lineshape analysis is carried out. The comparison with the experiment shows good agreement. Moreover the hybrid resonance and the dielectric anomaly are discussed and their influence on the magnetic‐field‐dependent microwave absorption in the present material is investigated. The results can in effect be understood on the basis of a model of the isoenergy surface, which is composed of three ellipsoidal parts. Moreover there are indications of deviation from this model.
The electric conductivity and its anisotropy in the quasi‐one‐dimensional conductor K2Pt(CN)4Br0.3 · xH2O at variation of the water content
Authors: Drosdziok S. , Engbrodt M.
Published: July 2007

By varying the external water vapor partial pressure all three of the water molecules of the quasi‐one‐dimensional conductor K2Pt(CN)4Br0.3 · 3 H2O can be extracted and added again. This has been measured by gravimetrical means. The electric conductivity and its anisotropy vary over more than four orders of magnitude. This strong dependence under laboratory atmosphere conditions gives an explanation for the variety of results reported in the literature.
Asymptotic approximations of non‐linear mechanics in the dynamical theory of X‐ray scattering
Authors: Tikhonova E. A.
Published: July 2007

A dynamical scattering theory is developed in a crystal with distortions of second class. The method of movement separation is used to derive the equations determining the wave field in imperfect crystals. For a perfect crystal the obtained equations have an additional solution corresponding to an oscillation process of the overtone type with a frequency two times larger than the pendellösung one.
Energy–momentum relations in polaron problems with arbitrary coupling
Authors: Sheka V. I. , Khazan L. S. , Mozdor E. V.
Published: July 2007

A theory is derived suitable to obtain the energy‐momentum dependence for an arbitrarily coupling polaron. Using a variational principle general expressions are given which describe this dependence when an arbitrary shape of the electron surface of constant energy is considered and the polarization field is approximated by any number of fictitious particles. This method is able to yield the lowest band of electrons interacting with any mode of lattice vibrations. In particular, numerical results for the energy, depending on the total momentum, are obtained for isotropic optical and piezoelectric polarons in a wide range of a dimensionless coupling constant. Comparison of the results with known ones (only weak and strong‐coupling limits were investigated earlier) shows, that the energy values presented lie lower, though they represent themselves an upper bound for the exact polaron energymomentum relation.
Electron Paramagnetic Resonance of Cr3+ in Chalcogenide Glasses
Authors: Chepeleva I. V. , Zhilinskaya E. A. , Lazukin V. N. , Chernov A. P.
Published: July 2007

The EPR spectra of Cr3+ ions in chalcogenide glasses As2Se3, yM (100 – y) As2Se3 (M = = Cu, Hg, Pb) and TlAsSe2(S2) are observed at X‐, K‐, and Q‐band. It is found that Cr3+ ions may incorporate into the glass network with surroundings forming intermediate or strong crystalline fields. Considerable fluctuations may occur in the coordinational spheres of the Cr3+ ions. The phase transition from the ferromagnetic (or ferrimagnetic) state into the antiferromagnetic one is observed for the Cr3+ spin system in microcrystalline inclusions of CuAsSe glass by EPR and magnetic susceptibility measurements.
Untersuchungen zur anomalen Absorption von Elektronen in Si in der Nähe von Zonenachsen
Authors: Hansen‐Schmidt J.
Published: July 2007

Die Schwächung von Laue‐Reflexen aufgrund unelastischer Streuprozesse in Einkristallen wird im allgemeinen dadurch beschrieben, daß man zum reellen Kristallpotential V(r) ein imaginäres „Absorptionspotential”︁ iP(r) = i Ph exp (ihr) hinzufügt. Es wird die Frage untersucht, ob dieses Verfahren auch anwendbar ist bei Experimenten mit einer zweidimensionalen Anordnung von angeregten Reflexen. Experimentell ergibt sich eine Erhöhung der „Asorptionspotentiale”︁ Ph in der Nähe der Zonenachsen. Diese Abhängigkeit der Ph von der Kristallorientierung wird qualitativ durch Rechnungen bestätigt, denen die von Yoshioka eingeführten Parameter C zugrunde liegen. Absorptionsvorgänge können daher nicht mit Hilfe eines Potentials verstanden werden.
On a Structural Phase Transition in Ytterbium
Authors: Ausloos M.
Published: July 2007

Recent data on the electrical resistivity and the normal Hall coefficient of ytterbium are used in order to study the temperature dependence of the density of states at the Fermi level, away from as well as in the vicinity of the h.c.p.‐f.c.c. structural phase transition. Two recent theories of the Hall coefficient are discussed when the density of states presents a dip near EF. In the vicinity of the structural phase transition, the influence of the band gap temperature dependence is discussed. The theory based on a model valid in rare‐earth metals predicts the temperature dependence of the Hall coefficient near the critical temperature. The theoretical results compare favourably with the experimental data, thereby giving some insight into the mechanism of the transition.
Effect of Polarization on Charge Carrier Transfer in Molecular Crystals
Authors: Petelenz P.
Published: July 2007

The influence of the polarization of the molecular orbitals of a neutral molecule by the electric field of an ion on the charge carrier transfer integrals is investigated. A simplified crystal structure is assumed and the calculations are performed for a dimer model of the neighbouring molecules. The polarization effects are calculated in a scheme which does not involve the polarizability concept. Some essential differences with previous results are discussed.
Phonons and Quasi‐Periodic Static Fluctuations
Authors: Falicov L. M. , Hsu W. Y.
Published: July 2007

Electron wave‐packets moving in an external magnetic field experience a potential caused by quasi‐periodic static fluctuations which arise from electron‐lattice interactions. Such a potential is derived from first principles and the electron scattering probability is estimated. Zero‐point vibrations of the phonon modes are properly included. The importance and consequence of phase‐coherence are also emphasized. The calculation does not include effects due to electron‐electron interaction.
A New Type of Domain Wall NMR Spectrum in Canted Antiferromagnets: YFeO3 and LuFeO3 Crystals
Authors: Zalesskii A. V. , Zvezdin A. K. , Zheludev I. S. , Savvinov A. M. , Lebedev A. F.
Published: July 2007

Within the framework of the theory, specially developed for orthoferrites, the experimental results of NMR studies on Fe57 nuclei in domain walls of YFeO3 and LuFeO3 crystals are analysed. It is shown that the origin of two peaks in the wall NMR spectra for orthoferrites is quite different from that in a simple uniaxial ferromagnet and connected with a peculiar angular dependence of NMR frequency in a canted antiferromagnet. This analysis of wall NMR spectra is generally applicable to any other canted antiferromagnet with a similar magnetic structure.
X‐Ray Photoelectron Spectra of Quasi‐One‐Dimensional Conductors
Authors: Kaplunov M. G. , Shuwa Yu. M. , Pokhodnya K. I. , Borodko Yu. G.
Published: July 2007

X‐ray photoelectron spectra of several quasi‐one‐dimensional conductors, complexes of tetracyanoquinodimethane (TCNQ) and conductive complexes of iridium, are investigated. Highly conductive TCNQ complexes are characterized by splitting of the nitrogen Is line into two components, and conductive iridium complexes by widening of the metal 4f‐line. This splitting (widening) is possibly due to a variation of charge density along conducting chains.
Calculations of Optical Transitions of the F Center
Authors: Sati R.
Published: July 2007

All the low‐lying complete wave functions for the F center have been obtained in the molecular orbital approach. These functions are further improved by taking into account the interaction between different configurations. It is shown that the ground state is 1A1 and the first excited state is lE. The calculated transition energies are in reasonable agreement with the observed R+ transitions.
Temperature Dependence of the Magnetization of the Ferromagnetic Spinels CdCr2Se4 and CuCr2S4 and Its Calculation in the Spin‐Wave Approximation
Authors: Samokhvalov A. A. , Morozov Yu. N. , Karpenko B. V. , Simonova M. I.
Published: July 2007

The temperature dependence of the magnetization of the ferromagnetic spinels CdCr2Se4 and CuCr2S4 measured in the temperature range of 4.2 to 300 K is described by the expression Δσ/σ0 = C1T3/2 + C2T5/2, where σ0, is the magnetization at T = 0 and C1 and C2 the coefficients determined by the least‐square method. Expressions for C1 and C2 are calculated in a conventional method of the spin‐wave approximation for a ferromagnetic spinel with magnetic atoms in octahedral sites and 36 neighbours up to the fourth coordination sphere are taken into account as well as two exchange parameters I1 and I2. From the experimental values of C1 and C2 the following data are derived: I1 = (15.85±0.05) K, I2 = (0.01 ± 0.02) K for CdCr2Se4 and I1 = (−140 ± 40) K, I2 = (35 ± 10) K for CuCr2S4.
X‐Ray Scattering from Electrons in Random Semiclassical Potential Wells
Authors: Esser B.
Published: July 2007

The scattering of X‐rays is calculated which arises from the electron density fluctuations, due to localization in a semiclassical random field. For an exponential autocorrelation function of the random field the scattering is expressed by the density of states at the Fermi level, the correlation length, and the mean square of the fluctuations of the random field potential.
On the Theory of Electroabsorption in Polar Crystals
Authors: Röseler J. , Henneberger K. , Strehlow R.
Published: July 2007

The present paper is a first step in looking for polaron effects by changing the optical interband absorption due to the presence of an external electric field. It is shown that the dielectric function can be expressed by a generalized one‐particle Green's function GF(k, t). A hierarchy of equations of motion is derived for GF(k, t) and a generalized Dyson equation is found. Finally, an expression for GF(k, t) is obtained, which allows the explicit calculation of the absorption coefficient in the neighbourhood of the absorption edge.
Overlap and Covalency Contribution to the Zero‐Field Splitting of S‐State Ions
Authors: Novák P. , Veltruský I.
Published: July 2007

General formulation of overlap and covalency effects for parameters D, E in the spin‐Hamiltonian of S‐state ions using the independent bond approach is developed. The model may be applied to complexes with both arbitrary coordination number and positions of ligands. All local, nonlocal, and distant terms of spin‐spin and spin‐orbit interactions are computed and used both for comparison with previous works on the Mn2+ ion with fluorine ligands and for discussion of the origin of the parameter D of octahedral Mn2+ and Fe2+ ions in garnets. Some of the errors and improper approximations appearing in various papers are discussed.
Magnetic Circular Dichroism of Coe2+ ‐Doped Cubic Crystals
Authors: Douglas I. N.
Published: July 2007

The magnetic circular dichroism (MCD) and absorption spectra of Co2+ ions in MgO, CaO, SrO, and LiF crystals have been measured in the region from 12500 to 30000 cm−1. Most of the features in the MCD of MgO, CaO, and LiF crystals can be attributed to single Co2+ ions at sites of octahedral symmetry. In MgO some additional MCD features produced by Co2+Co2+ pairs are identified by their temperature independence. Although SrO has the same crystal structure as the other crystals, the Co2+ ions occupy trigonal sites (from EPR measurements) and the MCD and absorption spectra reflect this lowering of the symmetry.
Studies of the Band Structure of NbO by X‐Ray Photoelectróh Spectroscopy
Authors: Honig J. M. , Sinha A. P. B. , Wahnsiedler W. E. , Kuwamoto H.
Published: July 2007

X‐ray photoelectron spectra are reported for single crystals of NbO in the binding energy range of −5 to +540 eV with respect to the Fermi level; the results are correlated with band structure schemes reported in the literature. A substantial electron density at the Fermi level in a very wide conduction band reported here is consistent with the highly metallic characteristics of the compound. Observations of spectra of Ar ions imbedded in the NbO matrix yield a work function of 6.45 eV for NbO.
Construction of the Reciprocal Fourth‐Rank Tensors in Crystals
Authors: Iosilevskii Ya. A.
Published: July 2007

A simple covariant method is suggested to construct in a covariant form reciprocal quan‐ tities from the fourth‐rank tensors Xiklm, which describe arbitrary physical properties of single crystals invariant under the point symmetry group of the given crystal and the permutations i ⇄ k and l ⇄ m. The problem is solved for all crystal systems.
Polarizability of an Electron Trapped by an Interstitial Ion
Authors: Hattori K.
Published: July 2007

A variational method is used to determine the polarizability of an electron trapped by an interstitial ion in an ionic crystal. An analytic result for the polarizability is obtained and it reduces to 4.5 in at. units, the exact value of the polarizability for the hydrogen atom. The polarizabilities in alkali and silver halides are evaluated.
Effects of Three‐Body Interactions on Hyperfine Fields
Authors: Manohar C. , Kelkar V. K.
Published: July 2007

It is argued that three‐body interactions have to be introduced even to understand qualitatively the systematics of the measurements of hyperfine fields in Heusler and other related alloys. Formulae for spin polarization including three‐body interaction effects are derived and compared with the Ruderman‐Kittel‐Kasuya‐Yosida theory. As a special case we obtain the results of Blandin and Campbell. We conclude that for any meaningful quantitative comparison with experiments one should include both the three‐body effects and the effects discussed by Blandin and Campbell.
Free‐Carrier Generation via Exciton‐Phonon and Exciton‐Impurity Interaction in Ge Crystals
Authors: Novikov B. V. , Sokolov N. S. , Gastev S. V.
Published: July 2007

The dependence of free electron and hole concentrations on the incident photon energy near the indirect absorption edge in Ge crystals has been investigated by cyclotron resonance at T = 1.8 K. It is shown that such measurements give the possibility to determine the dominant mechanisms of free carrier generation via exciton‐phonon and exciton‐impurity interaction. It is found that in pure crystals free carriers are mainly generated as a result of exciton‐long‐wavelength acoustical phonon interaction. In doped crystals the free carriers are generated due to the interaction of excitons with ionized impurities.
Interaction between Excitons and Radiation‐Induced Radicals in Naphthalene Single Crystals
Authors: Chong T. , Itoh N.
Published: July 2007

Correlation between the fluorescence of a radiation‐induced radical and the quenching of the prompt and delayed fluorescence by the radicals has been studied in naphthalene single crystals. It is found that the fluoresceme of the I‐hydronaphthyl radical is induced by energy transfer from both the singlet and triplet excitons. The interaction rate constant ks between the singlet exciton and the radical is obtained as 6 × 10−9 and 8 × 10−9cm3 s−1 at 290 and 80 K, respectively. Quenching of the delayed fluorescence by the radical is explained by reaction kinetics which involves not only the triplet‐triplet annihilation and a direct collision of a triplet exciton with the radical but also a collision of a triplet‐radical complex or the radical at the excited state with another triplet exciton. The interaction rate constant kT between the triplet exciton and the radicals is obtained as 2 × 10−11 and 3 × 10−11 cm3 s−1 at 290 and 80 K, respectively. It is concluded that the interaction rate constant γ between the triplet exciton and the triplet‐radical complexes or the radical at the excited state is larger than kT which is larger than the rate constant γ for the triplet‐triplet annihilation. It is also shown that the temperature dependences of γ and kT are parallel.
Crystal–Impurity Energy Transport by Molecular Excitons in the Wave‐Like Limit: II. Diffusion in Crystals
Authors: Markvart T.
Published: July 2007

The effect of exciton transport by diffusion on the crystal‐impurity energy transfer is investigated. It is shown that the exciton is likely to be captured by the impurity only if its distance from it is of the order of the exciton mean free path or smaller. On the basis of this argument a modified diffusion equation is developed to account for the exciton motion towards an impurity prior to capture. The equation is solved approximately in the limit of low impurity concentrations.
The Binding Energy of an Impurity‐Vacancy Pair in Simple Metals
Authors: Yamamoto R.
Published: July 2007

The binding energy of an impurity‐vacancy pair in simple metals is treated by the pseudopotential method. It is shown that the total binding energy can be expressed as the sum of two terms, the electric interaction and the difference of the relaxation energies, that, is the elastic interaction.
Exciton Dynamics Study by Magnetic Resonance in TCNQ− · NH
Authors: Alizon J. , Bertret G. , Blanc J. P. , Gallice J. , Robert H.
Published: July 2007

The tetracyanoquinodimethan salt TCNQ− NH belongs to the low conductivity class. It is a simple salt, the two partly deuterated salts TCNQ− · ND and TCNQD · NH can be easily prepared. A substantial amount of information has been gathered concerning the magnetic resonance of these salts: electron spin resonance, dynamic nuclear polarization, and spin‐lattice relaxation. Thus it was possible to study the relaxation mechanisms in this particular case. The experimental results can be interpreted, above the transition temperature, in terms of the model of Frenkel spin excitons; this model introduced by Soos is in good agreement with paramagnetic susceptibility measurements. It is shown that the excitons are localized in the TCNQ chains and have the dynamics of a one dimensional triplet exciton system.
Thickness Dependence of Localized States in a Thin Film with Internal Layer Defect
Authors: Duda A. , Maciejewski W.
Published: July 2007

The thickness effect in the spin‐wave spectrum of a thin film withone internal layer defect is studied. The behaviour of the localized modes and their generation conditions under variation of the film thickness are described by strictly derived analytic formulac. The shape of the thickness effect is related to the pinning situations of the surface spins.
XPS Investigations for the Purpose of the Research of Positron Annihilation Effects in BixSb1−x Alloys Concerning Band Structure Modifications
Authors: Wagner N. , Brümmer O.
Published: July 2007

The reasons for the experimentally definable differences in the decay rate of thermalized positrons in BixSb1−x alloys with different compositions can be investigated with the help of the results of X‐ray photoemission spectroscopy. It is shown that the annihilation rates are influenced by the modification of the valence band properties in the region of p‐like states.
The Elementary Interaction between a Crystal Dislocation and the Flux Line Lattice of a Type II Superconductor
Authors: Schneider E. , Kronmüller H.
Published: July 2007

Analytical expressions for the interaction force between the flux line lattice (FLL) and parallel crystal dislocations are derived within the framework of the micromagnetic theory of superconductivity in the field range 0.6Hc2 < Hext < Hc2. The interaction force was calculated for the ΔV‐effect which results from the inhomogeneous distribution of the order parameter and of the magnetic induction in the periodic FLL. The force exerted per unit length of the FLL by an edge dislocation depends on the value of the applied external magnetic field Hext, and for niobium is of the order of magnitude 10−4dyn/cm, while it turns out to vanish for the case of a parallel screw dislocation. At the upper critical field Hc2 the pinning force due to one dislocation vanishes for a rigid FLL, however, for the volume pinning force a constant value is obtained.
Theory of Low‐Symmetry Effects in Paramagnetic Resonance
Authors: Grekhov A. M. , Roitsin A. B.
Published: July 2007

Expressions for the resonant value of a magnetic field and main characteristics of low symmetry effects have been received for the arbitrary local symmetry of a paramagnetic centre in two extremal cases of weak and strong magnetic fields (in comparison with the zero‐field splitting). All terms of the spin‐Hamiltonian allowed by the symmetry and describing the interaction with the external magnetic field and the internal electrical crystal field have been considered. Results of calculations are compared with experimental data.
Phonon‐Assisted Electroabsorption by Contact Excitons
Authors: Pribram J. K. , Penchina C. M.
Published: July 2007

The imaginary part of the dielectric constant for an indirect‐gap semiconductor in a uniform electric field is calculated in closed form using a one‐dimensional electron‐hole contact interaction model. Results are compared with previous calculations for direct transitions. Typical spectra are illustrated for direct and indirect transitions at various field strengths.
EPR Investigations on the (FeOF5)4− Cluster in KZnF3
Authors: Buzaré J. Y. , Fayet J. C.
Published: July 2007

The X band EPR spectrum of the (FeOF5)4− cluster in KZnF3 single crystals was analysed. We used a computer diagonalization for both the Zeeman interaction and the axial crystal field term and a perturbation treatment for the cubic field term and for the shf interaction. This cluster appears to be well adapted for EPR investigations of the structural properties of fluoperovskites.
Pressure Modulation of Exciton Fission in Tetracene
Authors: Whitten W. B. , Arnold S.
Published: July 2007

The fluorescence quantum efficiency of tetracene crystals is found to increase rapidly with pressure. This behavior is interpreted as a reduction in the singlet exciton fission rate due to an increase in the energy separation between the singlet exciton state and the triplet exciton pair state. A red shift in the energy of the S0 → T1 transition by 15 ± 7 cm−1/kbar is used to explain the experimental results. This shift is discussed in terms of the interactions which control the transition energy.
The Coherent Potential Approximation in the Theory of Paramagnetic Alloys
Authors: Kozarzewski B. , Nowak J. , Zieliński J.
Published: July 2007

It is shown how the coherent potential approximation (CPA) can be extended to describe the properties of paramagnetic alloys. The properties of conduction electrons are discussed. Even at very low concentrations of magnetic impurities, results presented here differ from those of the single impurity approximation. As an example the temperature dependence of the resistivity of alloys in the s–d exchange model is analysed. For T much lower than the Kondo temperature, T2 behaviour of the resistivity is derived, which then goes through a more or less pronounced linear region to logarithmic dependence for temperatures higher than TK.
CPA Treatment of the Hubbard Model with Intra‐Atomic Exchange Interaction
Authors: Mizia J.
Published: July 2007

The Hamiltonian for a completely degenerate d‐band including intra‐atomic Coulomb and exchange interactions is derived and studied using the alloy analogy method. Green's function, the magnetic susceptibility and a criterion for the appearance of ferromagnetism are obtained by this method. The dependence of the magnetization on the exchange constant and on the number of electrons per atom is calculated for strong Coulomb correlation and for the uncorrelated case. It is shown that Coulomb correlation of any strength leads to a decrease of the exchange interaction constant needed to produce ferromagnetism and additionally to a change in the shapes of the magnetic dependences for (and only for) weakly ferromagnetic metals.
Über die Berechnung des elektrostatischen Potentials in Kristallgittern: II. Kubisehe Glitter ‐ Berechnungsmethoden für die Summen σ
Authors: Matthies S.
Published: July 2007

Es werden auf der Grundlage einer Berechnungsmethode für das elektrostatische Potential in Kristallgittern, die im ersten Teil dieser Arbeit entwickelt wurde, der Fall kubischer Gitter und Berechnungsmethoden für die Summen σ für nichtkubische Gitter betrachtet. Im kubischen Fall werden die Summen σ für l = 4,6, …, 22 berechnet. Die Effektivität der Berechnungsmethode wird am Beispiel eines Gitters vom NaCl‐Typ gezeigt. Es wurde eine Beziehung zwischen den Summen σ für kleine und größere l gefunden.
Theory of Nondegenerate Two‐Dimensional Electrons Interacting with Phonons in Quantizing Magnetic Fields
Authors: Dykman M. I.
Published: July 2007

Under transverse magnetic fields the two‐dimensional electron energy spectrum transforms into a set of essentially discrete Landau levels. Taking into account the non‐parabolicity of the electron energy dispersion a quantum kinetic equation for the case of inelastic scattering by phonons is obtained. This equation is valid over a wide temperature range and at arbitrary ratios between the non‐equidistance of the Landau levels (which is connected with both the non‐parabolicity and the renormalization of the spectrum due to electron‐phonon interaction) and their widths. The shape of the cyclotron resonance peak is considered in the deformation potential approximation and also in the case of scattering by phonons whose frequency is determined by their two‐dimensional wave vector. In the latter case from the shape of the peak it is possible to obtain the phonon dispersion law.
A New Aspect of Jahn‐Teller Effect in the Low‐Spin Ferrihaemoglobin Azide
Authors: Bhattacharyya B. D.
Published: July 2007

Considering the more general molecular orbital method of Van Vleck, Stevens, and Bose et al. the ESR g‐values and the zero‐phonon line structure are discussed for the singlet‐doublet transition in the low‐spin ferrihaemoglobin azide having a predominant tetragonal axial distortion. The model takes into account the anisotropic covalency and the essential features of the Jahn‐Teller effect. Comparison is made with experimental results. The results of correlation between theoretical and experimental g‐values indicate : (i) there is an appreciable anisotropic covalency overlap of the ligand charges with the charge cloud of the central metal atom; (ii) the coupling between electronic and nuclear motions is moderately strong, and (iii) the g‐anisotropy (gx∼gy) has an oscillatory behavior with the degree of Jahn‐Teller coupling.
Optical Properties of MoRe Alloys
Authors: Bahl S. K. , Drew H. D.
Published: July 2007

Measurements of the reflectivity [R(pure) − R(alloy)] of 3000 Å thick Mo‐Re films for photon energies between 0.54 and 6 eV and Re concentrations around 4 at% are reported. The resistivity of the films is also measured and 2.2 μΩ cm/at% is found for the change in resistivity of the films upon alloying. The frequency dependence of the optical dielectric function, ϵ = ϵ1 + iϵ2, of the pure and alloy films is found from a Kramers‐Kronig analysis of the reflectivity data. The results for the differential optical conductivity ω[ϵ2(alloy) minus;ϵ2(pure)] are discussed in terms of the changes in the electronic structure of Mo upon alloying with Re. It is concluded that the rigid band model is inapplicable in this case. The results of the optical data and the resistivity are consistent with the formation of localized Re d‐levels 0.8 eV below the Fermi level where pure Mo has a low density of electronic states. However, this interpretation is not unique and further studies are suggested.
Zeeman Effect of Bound Exciton Complexes in ZnO
Authors: Loose P. , Rosenzweig M. , Wöhlecke M.
Published: July 2007

Emission and absorption spectra are measured of several ZnO‐crystals in the region of bound‐exciton complexes near the band gap. From Zeeman‐effect studies up to 8 T at several angles between the c‐axis of the crystal and the magnetic field most of the observed spectral lines are identified. The electron g‐factor is found to be ge = 1.8, if the exciton is formed with a hole from the A‐band, and ge = 2.1, if the exciton is formed with a hole from the B‐band. The mass of the B‐hole seems to be nearly equal to the mass of the A‐hole.
Electronic Properties and Relaxation Processes of Fe2+ in Iron Bisacetylacetonate Dihydrate from Mössbauer Spectroscopy
Authors: Mathieu J. P. , Chappert J.
Published: July 2007

57Fe Mössbauer spectra of Fe(acetylacetonate)2 · 2 H2O have been measured between 300 and 4.2 K and in external magnetic fields up to 60 kOe. Strong texture effects due to the shape of the microcrystals are observed. An analysis of the spectra shows that the Fe2+ ion is in a high‐spin state. Under the action of spin‐orbit coupling and external magnetic fields the ground state of the Fe2+ ion is a spin‐doublet somewhat split by about 1 cm−1. A magnetically induced reduction of the relaxation rate is observed.
The Self‐Damping of RKKY Interaction in Dilute Magnetic Alloys
Authors: Takanaka K. , Yamamoto B.
Published: July 2007

An analytical expression for the self‐damping of RKKY interaction is obtained using the temperature Green's function. This expression gives the expected damping factor, exp (– R/l), where l and R are the mean free path of an electron and the distance between two magnetic impurities, respectively. Some qualitative features of the BKKY interaction predicted by this expression are in good agreement with numerical results obtained by de Gennes.
Transport in Microscopically Inhomogeneous Materials
Authors: Pike G. E. , Seager C. H.
Published: July 2007

The solutions are discussed to several types of site and bond conductance networks. The site models include those which are used to describe diffusion in Si–Ge alloys and conductivity in such systems as metal‐ammonia solutions and sodium‐tungsten bronzes. The bond model discussed here is shown to provide a reasonable explanation of data obtained from He diffusion studies in SiO2. Several approaches to solving these conductance models are discussed including numerical solutions, percolation models, and effective medium theories. It is shown, that an extension of the site effective medium theory of Watson and Leath provides a good fit to the numerical data available on the site networks considered.
On the Coherence Properties of Parametric Magnon States
Authors: De Araujo C. B.
Published: July 2007

It is shown that photon‐counting maybe a useful technique to study the coherence properties of magnon states excited by strong microwave or ultrasonic fields.
Thermodynamical Properties of Isotropic Ferromagnets with Biquadratic Interactions
Authors: Biegala L.
Published: July 2007

The thermodynamical properties of a S = 1 system described by \documentclass{article}\pagestyle{empty}\begin{document}$ H = - \frac{1}{2}I\mathop \Sigma \limits_{i,j} {\rm S}_i {\rm S}_j - \frac{1}{2}K\mathop \Sigma \limits_{i,j} ({\rm S}_i {\rm S}_i)^2 $\end{document} are investigated by means of the Green functions method. Using the RPA decoupling the temperature dependence of the two order parameters are determined in the low‐temperature region and in the neighbourhood of the transition temperatures. The value of K for the tricritical point is found. The possibility of the first order transition from the quadrupolar to the disordered phase is analysed.
Correlation Factors and Isotope Effect for Self‐Diffusion by Mono‐ and Divacancies in H.C.P. Crystals
Authors: Steiner E. , Mehrer H. , Seeger A.
Published: July 2007

The tensor components of the Bardeen‐Herring correlation factor for a divacancy mechanism of self‐diffusion in an h.c.p. crystal are calculated and compared with Mullen's results on the monovacancy mechanism. The relationship between the correlation factors and the isotope effect parameters is discussed, in the monovacancy case making use of the work of Batra. For monovacancy diffusion parallel to the c‐direction the correlation factor provides an upper limit for the isotope effect parameter, whereas for diffusion perpendicular to the c‐axis the isotope effect parameter can be slightly larger than the correlation factor. For the divacancy case it is demonstrated analytically that the correlation factor parallel to the c‐axis provides an upper limit for the isotope effect parameter in this direction. Numerical calculations show that the upper limit of the isotope effect parameter perpendicular to the c‐axis exceeds the correlation factor at most by a few per cent.
Optical Relaxation Time and Effective Mass in Noble Metal Alloys
Authors: Brauwers M. , Brouers F.
Published: July 2007

The CPA expressions of optical relaxation time and effective mass in a two s–d hybridized band noble metal alloy model are investigated. This model considers the effect of the d‐diagonal and off‐diagonal and s‐diagonal disorders; comparison is made with experimental data on AuxAg1−x.
Measurement of Mössbauer Diffusion Broadening in α‐Iron Stabilized with Vanadium
Authors: Lindsey R.
Published: July 2007

The diffusion of iron 57 in α‐iron stabilized by small amounts of vanadium is studied by means of the Mössbauer effect. The Mössbauer line broadening is found to be proportional to the diffusivity. The broadening observed is found to be about half that predicted in uncorrelated sudden jump model of Sing wi and Sjölander. The observed lineshapes are Lorentzian to within experimental errors. The data indicates that the recoil‐free fraction falls off more rapidly than predicted by a Debye model at temperatures where diffusion broadening is observed. An effective Mossbauer characteristic temperature of (299 ± 13) °C represents the data taken between the Curie temperature and the onset region of diffusion broadening.
Magnetic Aftereffect Phenomena in Cobalt between 4.2 and 20 K after Low‐Temperature Electron Irradiation
Authors: Dander W. , Kronmüller H. , Schaefer H. E.
Published: July 2007

In Co single crystals irradiated by 3 MeV electrons at 4.2 K a complex magnetic aftereffect (MAE) spectrum between 4 and 350 K has been observed. This paper deals with the low‐temperature MAE processes at 5, 9, and 17 K as well as with their annealing after irradiation to different doses. Taking into consideration simultaneous measurements of the resistivity annealing and the initial susceptibility, a model will be suggested which attributes the 9K‐MAE and its complex annealing behaviour to the rotation and transformation of interstitials which may migrate freely at 65 K.
Second‐Order Raman Spectra of RbCl, RbBr, and RbI Crystals
Authors: Goel C. M. , Dayal B.
Published: July 2007

The polarized second order Raman spectra of three rubidium halide crystals, namely RbCl, RbBr, and RbI are computed on the basis of the theory proposed by Born and Brad‐burn, taking into account first as well as second neighbour polarizability coefficients. The three‐body force shell model is employed to determine the phonon frequencies, the phonon eigenvectors, and the polarizability tensor. The polarizability coefficients are assumed to be adjustable parameters. It is found that in addition to all the first neighbour coefficients, only eight second neighbour coefficients are sufficient to obtain good agreement between the experimental and the theoretical spectra.
The Theory of a Heisenberg Alloy
Authors: Kassan‐Ogly F. A. , Medvedev M. V.
Published: July 2007

Magnetic states of a disordered binary Heisenberg magnet, which is characterized (in the nearest neighbour approximation) by the exchange integrals: IAA > 0; IAB > 0; IBB < 0, are investigated. In the “mean” field approximation it is shown that in such an alloy below the transition temperature into the paramagnetic state ferromagnetic, antiferromagnetic, and noncollinear magnetic structures exist and that non‐trivial sequence of temperature phase transitions from one to another of these states may take place. Rather peculiar behaviour of spontaneous magnetization is obtained.