bluuebunny/crossref_metadata_2025 huggingface dataset:
EPR studies of VO2+ ions in rochelle salt crystal
Authors: Kulliński W. J. , Hoffmann S. K. , Więckowski A. B.
Published: July 2007

EPR results for vanadyl ion doped Rochelle salt are reported. Impurity sites in the unit cell, and the molecular structure of the VO(tartrate)2 complex in the crystal are found and the spin‐Hamiltonian parameters are determined at room temperature. A considerable non‐axiality of the g and A tensors and the MO bonding coefficients are discussed. The EPR spectra are not affected by the phase transitions.
On the shape of the absorption tail in amorphous solids
Authors: Szczyrbowski J.
Published: July 2007

A simple theoretical model is presented in which the exponential shape of the optical absorption tail of amorphous solids is attributed to elastic (or density) fluctuations in the band gap. The proposed mechanism ascribes this spectral dependence of the absorption coefficient to electronic transitions between states lying at different sites of the sample.
Model for local moments in a spin density wave host
Authors: Moyer C. A. , Kelly J. R. , Arajs S.
Published: July 2007

An effective spin Hamiltonian is obtained for a system of impurity spins embedded in an itinerant electron antiferromagnetic matrix. Each spin couples to the polarization field of the spin density wave (SDW) via a d–d exchange interaction with the conduction electrons. The impurity spins also interact among themselves via modulation of the conduction electron spin polarization. Using the two‐band model for antiferromagnetism in chromium, it is possible to obtain closed forms analogous to the RKKY result for the spin–spin and spin–matrix interactions. A general effect of SDW formation is to reduce the spin–spin coupling strength, since some conduction electrons are “tied up” in the SDW and are no longer free to mediate the interaction. The SDW is also responsible for the appearance of an antisymmetric interaction which favors canted spin arrangements.
Phonon‐assisted interband optical absorption in disordered semiconductors
Authors: Iskra V. D.
Published: July 2007

Optical transitions in disordered semiconductors are considered taking into account of many phonon and excitonic effects. Possible observable consequences of the results obtained are discussed.
Long‐range many‐body interaction and monovacancy heat of formation of rare‐gas crystals
Authors: Maity S. N. , Roy D. , Sengupta S.
Published: July 2007

The heat of formation of rare gas crystals is calculated using a relatively simple approach. As the difference in the values of heat of formation and cohesive energy depend entirely upon the relaxation and many‐body effects, the contribution of these terms to the heat of formation is examined critically. It is found that the relaxation effect is quite negligible. Though three‐body interaction other than triple‐dipole approximately cancels with the higher many‐body effects in the case of cohesive energy, the cancellation is not at all complete in the case of heat of formation, and the many‐body effects other than the triple‐dipole is found to contribute significantly to the difference between the energy of the perfect and imperfect crystal. The differences in the values of heat of formation and cohesive energy are calculated for the rare gas solids using a very simple approach and compared with experimental results and also with those obtained from more sophisticated calculation. The present calculation is found to reasonably reproduce the experimental results. It is emphasised that an accurate determination of this difference is necessary as this serves as a direct test of the role of many‐body forces in rare gas solids.
On Kinetic Exchange Theory
Authors: Eremin M. V. , Rakitin Yu. V.
Published: July 2007

The kinetic exchange in transition metal ion pairs is analyzed for the case of crystal fields of simply reducible symmetry groups. The effects of non‐orthogonality of molecular orbitals belonging to different centres are treated explicitly for the first time in the multi‐electron theory. The kinetic exchange parameters are related to the covalency parameters of the metal—metal “bond” and charge transfer energies. The particular cases of low and high charge transfer energies are considered. It is shown that with low charge transfer energies, strong ferromagnetic kinetic exchange may predominate. Certain differences in the operator forms of the direct and kinetic exchange Hamiltonians are discussed. The results obtained are applied to interpret experimental data.
Scattering of Photons by Channeled Particles
Authors: Bazylev V. A. , Zhevago N. K.
Published: July 2007

The theory of spontaneous and induced resonance scattering of photons by relativistic channeled electrons is developed. The coefficient of enhancement of the scattered radiation is calculated. It is shown that the population of the transverse energy levels of a channeled particle can be effectively changed with the help of modern lasers.
Investigation of the Crystal Field in PrNi5
Authors: Alekseev P. A. , Andreeff A. , Griessmann H. , Kaun L. P. , Lippold B. , Matz W. , Sadikov I. P. , Chistyakov O. D. , Markova I. A. , Savitskii E. M.
Published: July 2007

Inelastic slow‐neutron scattering experiments are made on polycrystalline samples of intermetallic compounds PrNi5 and LaNi5 over the temperature range 8 to 295 K. The peaks of the inelastic magnetic‐neutron—rare‐earth‐ion scattering resulting from the splitting of the ion ground state into a number of levels by the crystal electric field (CEF) of hexagonal symmetry are identified by temperature and angular dependence of resonance intensities and by comparing PrNi5 spectra with those of LaNi5. On the basis of an effective point charge model for the CEF Hamiltonian with four variable parameters, calculations are made and CEF level scheme is restored. Magnetic contributions to the temperature dependence of heat capacity and magnetic susceptibility of PrNi5 are calculated and compared with available experimental data.
EPR of Hydrogen and Deuterium Atoms in Alkali Halide Crystals
Authors: Badalyan A. G. , Baranov P. G.
Published: July 2007

In RbCl, KBr, and NaCl crystals EPR spectra of anion atoms of hydrogen (H) and deuterium (D) are obtained and studied after UV irradiation. Hyperfine structure constants and g‐factors of H and D centres are determined from these spectra. In RbCl resolved structure of the EPR lines of H and D is observed in the orientation H ‖ [100]. The atoms H and D are supposed to occupy off‐centre positions in crystals.
Radiative Recombination at Donors and Acceptors. Averaging over the Configuration of Majority Impurities
Authors: Paasch G. , Christke E. , Kausch M.
Published: July 2007

For the determination of the radiative recombination the configurational average for the majority impurities is calculated taking into account that the impurities fall on discrete sites. This gives only slight changes for the stationary behaviour in comparison with a completely random distribution while the kinetics is more affected. In addition the initial decay rate is examined.
ENDOR Study of the F‐Center in KMgF3
Authors: DuVarney R. C. , Niklas J. R. , Spaeth J. M.
Published: July 2007

With ENDOR the superhyperfine (shf) interactions of the F‐center electron in KMgF3 with neighboring nuclei are measured out to the tenth shell. Using ENDOR‐induced ESR it could be established that the shf interactions of the next‐nearest F‐neighbors along the tetragonal axis are bigger roughly by a factor of two than those in the perpendicular plane. The other shells also reflect the tetragonal symmetry of the site. The g‐factor, however, is isotropic with g = 1.9983 ± 0.0005.
Optical Properties of Molybdenum by Electron Energy Loss Spectroscopy
Authors: Manzke R.
Published: July 2007

The energy loss function Im (−1/ϵ(ΔE)) of polycrystalline Mo films is determined by electron energy loss experiments in transmission with 56 keV electrons. By means of Kramers‐Kronig analysis the complex dielectric function ϵ = ϵ1 + iϵ2 and the reflectivity R(ΔE) are calculated in the energy range from 1.5 to 38 eV. The results are compared with those obtained from optical reflectance data.
Activation Volume for Vacancy Motion in Calcium Fluoride
Authors: Fontanella J. J. , Wintersgill M. C. , Andeen C.
Published: July 2007

The audio frequency complex dielectric constant for sodium doped calcium fluoride is measured at pressures up to 0.4 GPa over the temperature range 240 to 270 K and at zero pressure over the temperature range 5.5 to 400 K. Consequently, the reorientation of a nearest‐neighbor vacancy around a substitutional sodium ion is studied. The activation volume for the motion is found to be (1.73 ± 0.1) cm3/mol. It is argued that this value should be similar to the migration volume for “free” vacancies. The compressibility of the activation volume is found to be forty times the compressibility of the host lattice. Finally, a large negative thermal expansion coefficient is found for the activation volume for vacancy motion.
The Scattering and Recombination of Coherent Excitons in Molecular Crystals
Authors: Ivanov G. K. , Kozhushner M. A.
Published: July 2007

The elastic and inelastic exciton—exciton scattering is regarded. Exact expressions for the scattering matrix are obtained in the long‐wave approximation for any molecular crystal. Calculations for the exciton—exciton scattering in anthracene are carried out and two possible values for the dynamical interaction between excitons are obtained by using the experimental data on the nonlinear fluorescence quenching of excitons.
EPR of Mn2+ and VO2+ in K2C2O4 · H2O Single Crystals
Authors: Jain V. K.
Published: July 2007

The electron paramagnetic resonance spectra of Mn2+ and VO2+ impurities in K2C2O4 · H2O are analysed in X‐band at 298 K. The ions do not appear to enter the lattice as substitutional impurites. The double separation of forbidden hyperfine transitions (Δm = ± 1) are studied in Mn2+ doped crystals. It is found that the different parts of the single crystals are characterized by different intensity ratios of two magnetically inequivalent complexes. This behaviour is prominent in VO2+ doped crystals.
The Entropy of Internal Stress Fields
Authors: Schoeck G.
Published: July 2007

In the strictly linear elasticity theory (no third order terms in the expansion of the free energy) or in the harmonic approximation for a crystal, there is no entropy change when an internal stress field is introduced into a body. When higher order terms are considered it is shown that in the expressions given by Zener and DiMelfi et al. terms are missing which in the hierarchy of perturbation are of the same order however due to the magnitude of the material parameters generally make a small contribution. In addition there exists an entropy of interaction between two internal stress sources.
Short‐Range Magnetic Order in a Disordered Ising Ferromagnet
Authors: Medvedev M. V. , Goryainova S. M.
Published: July 2007

In the molecular field approximation it is shown that in an Ising magnet with nearest‐neighbour random exchange bonds, both the long‐range ferromagnetic order and a short‐range magnetic one appear at the transition from the paramagnetic state into the random ferromagnetic one. The two‐point two‐spin correlation function of the short‐range magnetic order falls off like R−1on at large distances Ron ≫ a (a is the lattice spacing).
Calculation of Jahn‐Teller Coupling Constants of 3d Transition metal Ions in Crystals. I. General Theory
Authors: Nikiforov A. E. , Shashkin S. Yu. , Krotkii A. I.
Published: July 2007

A method is proposed for the calculation of the Jahn‐Teller coupling constants for orbitally degenerated states of ions in crystals based on the application of perturbation theory in adiabatic potential matrix calculation. The cluster approximation and LCAO‐MO scheme are used.
A General Pseudopotential Approach to Metallic Compressibilities
Authors: Kachhava C. M. , Sharma K. S.
Published: July 2007

A general theory for the binding energy and compressibility of metals applicable to any form of model pseudopotentials is presented. The usual contributions of the electrostatic, kinetic, exchange, correlation, and Hartree terms to the binding in metals are considered in the formulation. The band‐structure part is explicitly worked out using a general pseudopotential in conjunction with the Hartree dielectric screening. The formula so obtained for the compressibility is then specialized for the case of linear model potential and tested on eight metals. A good agreement between the theory and the experiment justifies the formulation based on the second order perturbation theory. Qualitative improvements are observed on using the self‐consistent screening.
Boundaries of Metastable States in type II Superconductors
Authors: Shehata L. N.
Published: July 2007

For a cylindrical type‐II superconductor in an external magnetic field H0 parallel to the sample surface the values are calculated for the critical exit field Hex(B) where a flux‐vacancy may be formed in the layer of vortices nearest to the surface and the critical entry field Hen(B) where a new flux layer may be formed. The interaction of vortex lines with large‐scale surface distortions is also considered and shows a decrease in the value of the penetrating field in agreement with experimental results.
Electrical Resistivity of Polyvalent Liquid Metals by Harmonic Model Potential
Authors: Sahay B. B. , Srivastava P. L. , Mitra N. R.
Published: July 2007

A parabolic form of potential in place of a constant potential (Heine‐Abarenkov type) inside the ion core region is considered. The form has the advantage of reducing the difference between the potential gradients at the model radius. Its effect on electrical resistivity of liquid Zn, Cd, Hg, In, Tl, and Pb is investigated. The results show positive influence in all cases except Hg.
Critical Temperatures of the Random Heisenberg‐Ising Magnets Amorphous Magnets and Spin Glasses in the Binary Mixtures
Authors: Katsura S. , Shimada K.
Published: July 2007

The Curie temperature of the amorphous Heisenberg‐Ising ferromagnets with random distribution of exchange integrals, and the Curie, the Neél, and the spin glass transition temperatures of binary mixture of the ferro‐ and the antiferromagnetic bonds in the Heisenberg‐Ising magnets are calculated. Similarity and difference between the Heisenberg‐Ising models and Ising models are clarified.
On the Direct Interaction Between Impurities in the Anderson Model
Authors: Zieliński J.
Published: July 2007

The method of functional derivatives is used in the problem of two interacting impurities. The symmetric Anderson model is considered and the prescription for generating diagrams for the selfenergy and the magnetic susceptibility of localized d‐electrons is given. Correspondence is found with the case of an isolated impurity. The temperature variation of the electrical resistivity and magnetic susceptibility is presented and the physical mechanism are discussed.
Energy Band Structure and Bloch Functions in the Planar Channeling Model with the Kronig‐Penney Potential
Authors: Babakhanyan E. A. , Kononets Yu. V.
Published: July 2007

The motion of a charged particle in a one‐dimensional periodic potential of the Kronig‐Penney type is considered. The energy band structure, Bloch wave functions (BWF) in coordinate and momentum representation are investigated in detail. Two sharply distinguished groups of states, i.e. below‐barrier and above‐barrier, are extracted and the role of both of them in the channeling of positively and negatively charged particles is explained. It is shown that only using a dispersion equation form one is able to obtain information on the symmetry properties of the BWF at the edges of energy bands. An estimate of the corresponding regions of the edge coherence in the Brillouin zone is given. In the above‐barrier case a nontrivial effect is found of parity interchange violation of the BWF at the edges of energy bands, connected with the nullification of the reflection coefficient either from the single barrier or well. An oscillatory behaviour of both allowed and forbidden band widths is revealed. The analytical results for different values of the parameters are illustrated by computer calculations.
Anisotropy of Spin Splitting and the Band Structure Parameters of HgSe from Shubnikov‐De Haas Experiments
Authors: Gałązka R. R. , Dobrowolski W. , Thuillier J. C.
Published: July 2007

Measurements are made of the Shubnikov‐de Haas (SdH) effect at 4.2 K up to 180 kOe in HgSe samples with electron concentration in the range 1.7 × 1017 to 2.2 × 1018 cm−3. The theoretical analysis is based on the Pidgeon and Brown model. The spin splitting of SdH oscillations shows an angular dependence. The anisotropy of the spin splitting exceeds 15% between < 100 > and < 111 > direction and depends on energy and the quantum number of the Landau levels. The theory gives satisfactory agreement with the experimental data. A new set of band structure parameters is established. Additional peaks in SdH oscillations, which are not predicted by the model, are also observed.
Spin Waves in Amorphous Antiferromagnets
Authors: Bogomaz I. V. , Ignatchenko V. A. , Iskhakov R. S.
Published: July 2007

The modifications of the dispersion relation of the spin waves are calculated using two methods: the discrete method for the lattice model with exchange integral fluctuations in the form of δ‐correlated “white noise” and the phenomenological method for the continuous model with arbitrary correlation ranges of the fluctuations of the exchange parameters 1/k0. A change in the character of the dispersion law ω(k) occurs in the vicinity of the point k ≈ k0/2. The results which are obtained by these methods coincide in the long‐waves limit (at k<π/a and k ≪ k0/2, respectively).
Transition Rates for Auger and Other Processes in Situations far Removed from Thermal Equilibrium
Authors: Pickin W.
Published: July 2007

Recombination rates into traps in semiconductors are usually calculated from the corresponding ionization rates by application of the principle of detailed balance; but this procedure is only valid if the carrier distribution function is Maxwellian. Results are presented which allow the calculation of Auger and other recombination rates for an arbitrary heated carrier distribution. They are shown to reduce to the detailed‐balance results when the distribution is Maxwellian.
Gain and Reflection Spectroscopy and the Present Understanding of the Electron–Hole Plasma in II–VI Compounds
Authors: Bohnert K. , Schmieder G. , Klingshirn C.
Published: July 2007

The reflection and gain spectra of CdS and ZnO are investigated as a function of excitation intensity and temperature. For the gain spectroscopy the two beam method and the variation of the excitation strip length are used. The results indicate, that an electron–hole plasma is formed under high excitation, preferentially if small excitation spots are used. The electron–hole plasma comes close to its thermal equilibrium only in especially favourable cases. Excitonic recombination processes are predominant, if larger excitation spots are used.
Micromagnetic Theory of Phase Transitions in Inhomogeneous Ferromagnets. II. Applications
Authors: Fähnle M. , Kronmüller H.
Published: July 2007

The influence is calculated of spatially random fluctuations of the Landau parameters A(r) and B(r) as well as the influence of inhomogeneous stress fields on the magnetic behaviour near the phase transition of inhomogeneous ferromagnets. In general this behaviour is determined by fluctuations of the Landau parameter A(r), whereas there is only a negligibly small contibution of spatially random fluctuations of B(r) and of inhomogeneous stress fields. The shifts of the ferromagnetic Curie temperatures due to the fluctuations of A(r) are of the order of magnitude of 1 to 10%. However, for actual material parameters only small curvatures are derived of the Arrott plots and the inverse paramagnetic susceptibility when considering spatially random fluctuations of A(r).
Manganese Centers in Low Temperature X‐Irradiated CaF2:Mn
Authors: Alcalá R. , Alonso P. J. , Lalinde G. , Carretero A.
Published: July 2007

The changes in the optical absorption and emission properties of CaF2:Mn after 15 and 80 K X‐irradiation are measured. A new absorption band is created at 450 nm. The same band appears in the excitation spectrum corresponding to Mn2+ emission. Optical and thermal bleaching experiments combined with emission measurements indicate that the 450 nm band is due to Mn+ created by X‐irradiation. The new band is tentatively assigned to the 7S(3d54s) → 7P(3d54p) electronic transition of Mn+ in CaF2.
The Magnetic Susceptibilities of Some Liquid Chalcogenides
Authors: Uemura O. , Satow T.
Published: July 2007

The magnetic susceptibilities for seven binary alloy systems (SnTe, SnSe, InTe, InSe, AgTe, AgSe and AgS) are measured as a funtion of temperature and composition. The diamagnetic susceptibilities of these systems fall rapidly on melting except for the InSe system, and decrease gradually with increasing temperature in the liquid state. The change of magnetic susceptibility at the melting point is considerably large in SnTe and SnSe, while rather small in Ag2X (X: Te, Se, and S). Diamagnetic maxima appeared around the stoichiometric composition (SnTe, SnSe, In2Te3, In2Se3, and Ag2X) and correspond well to minima in the electrical conductivity reported previously. These data suggest that electrons tend to localize at these compositions. Relationships between the degree of electron localization and the chemical bonding are discussed using the bond orbital approximation for the diamagnetism of semiconductors. The result indicates that the electron localization is stongly influenced by the bonding nature of anions.
Hall Effect of Magnetically Ordered MnAs1−xPxMixed Crystals
Authors: Bödecker G. , Bärner K. , Funke K.
Published: July 2007

The Hall resistivity ϱHof MnAs1−xPxpolyerystals is measured for three compositions:x= 0.11,x= 0.08,x= 0.05, for magnetic inductions up to 0.28 T and for temperatures between 80 and 370 K. Forx= 0.11, ϱHcould be separated into a normal and an anomalous contribution to the Hall effect. The total free d‐carrier density may be inferred at high temperatures (ndf= 7 × 1021cm×3at 300 K); also, d‐electron localization effects are detected at the high spin–low spin transition atThland at the Néel temperatureTN. The anomalous Hall effect below the Curietype temperatureT1is assigned to anisotropic scattering of free d‐carriers on localized cation d‐spins.
Lattice Dynamics of ZnS, GaAs, and GaSb
Authors: Kushwaha M. S.
Published: July 2007

Lattice dynamics of ZnS(II–VI), GaAs(III–V), and GaSb(III–V) compounds is investigated using an eight‐parameter bond‐bending force model (BBFM) recently developed for II–VI and III‐V compounds having zincblende (ZB) structure. The total potential energy of the system embodies three types of interactions which give rise to central (radial), bond‐bending, and long‐range forces. The model involves in total eight disposable parameters which are calculated by making use of six critical‐point phonon frequencies, two elastic constants, and the lattice equilibrium condition. The model is physically correct as it satisfies all the symmetry requirements of a perfect crystal. The phonon dispersion relations, phonon density of states, and temperature variation of the Debye characteristic temperature of the above‐mentioned crystals are calculated. Theoretical results are compared with the available experimental data. A fairly good agreement is observed between theory and experiments and thus lends support to the success of the dynamical calculations.
Plasma Reflection and Magnetoreflection in Epitaxial Graded‐Gap CdxHg1−xTe Layers
Authors: Dudziak E. , Jędral L. , Brzezińki J. , Becla P.
Published: July 2007

Measurements of reflectivity and magnetoreflectivity in the region of the plasma edge are performed at 300 K on n‐type graded‐gap epitaxial CdxHg1–xTe layers. From these measurements the position‐dependent effective masses and concentrations of free carriers are determined in the semimetallic region (x < 0.1) and for small composition gradient (≦ 10−3 μm−1). It is found that the position‐dependent effective masses determined by combined reflectivity and magnetoreflectivity measurements in the examined graded‐gap CdxHg1–xTe, at least within experimental error limits, are the same as in homogeneous CdxHg1–xTe crystals with the respective composition x. This result is discussed in the light of the recent effective mass theory for carriers in graded mixed semiconductors.
The Role of the Normal Phonon–Phonon Scattering in the Lattice Thermal Conductivity and Phonon Drag in Metals at Low Temperatures
Authors: Borchi E. , De Gennaro S. , Pelosi G. , Rettori A.
Published: July 2007

The problem of computing the lattice thermal conductivity and the phonon‐drag contribution to the electrical resistivity in presence of normal phonon–phonon processes is examined in the framework of the Boltzmann‐Callaway equation. A previous treatment for the lattice thermal conductivity of insulating crystals by Callaway and Krumhansl is appropriately extended in order to treat the properties of interest. The so‐obtained expressions for the lattice thermal conductivity and the phonon‐drag contribution are presented. Some inconsistences of recent works on the same arguments are discussed in reference to the results obtained.
Extended de Launay Model Study of Debye‐Waller Factors of B.C.C. Metals
Authors: Cavalheiro R. , Shukla M. M.
Published: July 2007

The temperature variation of the Debye‐Waller factor exponents for three body centered cubic metals, namely sodium, α‐iron, and chromium, are studied theoretically on the extended de Launay model. The theoretical computations show reasonable agreement with experimental observations.
Calculations of the Multiple Scattering of Low‐Energy Heavy Ions in a Finite‐Range Potential
Authors: Scanlon P. J. , Macarthur J. D.
Published: July 2007

The calculation is considered of the multiple scattering of heavy ions by a finite‐range potential that gives zero scattering angle at the range of the potential and is compared with the truncated cross‐section method used by Meyer. For a suitable choice of the range of the potential and the number of collision centres encountered by a projectile traversing a thin foil, substantial agreement can be obtained with experimental data on polycrystalline foils in the energy range of a few tens of keV. For higher energies, of the order of 500 keV, some of the trends in the experimental data are reproduced. To obtain agreement with experimental data for amorphous materials a change in the model describing the number of collisions is found to be necessary.
Light Scattering on Residual Sb Layers Formed After Surface Oxidization of Ga1−xAlxSb
Authors: Charfi F. , Zouaghi M. , Joullie A. , Balkanski M. , Hirlimann Ch.
Published: July 2007

The Raman spectra of Ga1–xAlxSb alloys (x > 0.2) exhibit at low frequencies two peaks: 118 and 150 cm−1 which are attributed to the presence of Sb in the transition region between surface oxide and bulk material. The evolution of this peaks with concentration x and temperature is investigated. The analogy between the experimental results obtained on Ga1–xAlxSb and those obtained on binary oxidized III–V compounds lead to the demonstration that the peaks 118 and 150 cm−1 are the vibrational modes of the Sb‐enriched layer resulting from the preferential oxidization of the group III element.
Interface Model of Impurity Incorporation into Vicinal Faces
Authors: Pfeiffer H.
Published: July 2007

The capture of solvent as impurity during the growth of vicinal faces from dilute solutions is treated analytically. Rate equations for the incorporation of the host and impurity into the steps on the vicinal face are derived within the SOS‐lattice model. In this approach the influence of the “immuring effect” on the impurity capture is automatically taken into account. Especially, analytical expressions for the impurity concentration in the crystal in dependence on the growth rate are given and discussed with special emphasis on the growth of garnets.
Elementary Electronic Excitations in Pure Sodium Silicate Glasses
Authors: Trukhin A. N. , Tolstoi M. N. , Glebov L. B. , Savelev V. L.
Published: July 2007

Electronic structure in high‐purity sodium silicate glasses is studied. The spectral dependence of the relative quantum yield of the external and internal photoemission, the luminescence excitation spectra, the luminescence decay, and the polarization degree of the luminescence are measured. The luminescence band at 3.4 eV is created by the intrinsic anisotropic luminescence centre labelled L, whose absorption causes the fundamental absorption tail of high‐purity silicate glasses, where the localized elementary excitations are created by photons. The phonon‐assistent or tunneling delocalization processes produce colour centres. An electron release from the L‐centre leads to the creation of the H‐centre. An electron capture of the L‐centre results in the E‐centre. A model of the structure Na+ ‐ O−‐ Si = is proposed for the L‐centre. Hence the structure for the E‐centre model will be Na0. O−  Si =, but for the H‐centre it will be Na+ O  Si =. The pair of E and H‐centres can probably recombine in a tunneling process.
Electroabsorption on the Indirect Gap of As2Se3
Authors: Althaus H. L. , Weiser G.
Published: July 2007

The electroabsorption spectra of As2Se3 single crystals are studied near 2 eV for polarization of light E o and E c. The low temperature spectra show fine structure related to indirect transitions with creation of phonons of an energy of 5, 11.5, 24.5, and 26.5 meV. This fine structure broadens rapidly above 50 K and disappears at 77 K. With rising temperature a low‐energy tail of the spectra develops which is attributed to indirect transitions with annihilation of phonons. The energy of the indirect gap is determined to 1.989 eV at 10 K. Above 100 K it decreases with a temperature coefficient of ‐6.25 ∧ 10−4 eV/K.
Superimposed Effects of Three Arrays of Scattering Planes on Electrical Conductivity
Authors: Pichard C. R. , Tellier C. R. , Tosser A. J.
Published: July 2007

Starting from a three‐dimensional conduction model a comparison is made between the initial equations and those derived by assuming that the three arrays of scatterers act independently. A sufficient fit is obtained, in good agreement with experiments. A tool is thus available for calculating strain coefficients.
Theory of Photoelastic Constants of Alkali Halide Crystals
Authors: Shanker J. , Verma T. S.
Published: July 2007

A theoretical model for evaluating the photoelastic constants of ionic crystals, is developed by considering the short range repulsive interactions between nearest and next nearest neighbours, the van der Waals potentials, and three body interactions as well as the electronic polarizabilities of cations and anions. The model is applied to calculate the photoelastic constants of alkali halide crystals with NaCI structure. The results obtained are in good agreement with experimental data. The importance of the short range polarizability effect is demonstrated in determining the photo‐elastic constants of the crystals under investigation.
Optical and Thermal Stimulation of Reactions in α‐A12O3 Containing V3+, V4+, Co3+, and Co2+
Authors: Delbecq C. J. , Marshall S. A. , Yuster P. H.
Published: July 2007

Optically or thermally stimulated reactions among the species V,V,Co, and Coin single crystals of α‐A1,O, and the location of the ground state energy levels of these species with respect to the valence or conduction band are studied. A previously unidentified anisotropic absorption band, peaking near 310 nm, is identified as a charge transfer transition of V.Excitation in this band results, with appreciable probability, in the formation of free positive holes and the consequent conversion of Vto Vand of Co to Co;this probability is found to increase as the excitation wavelength decreases. At temperatures above 400 oC an equilibrium is readily established among these four ions and the temperature dependence of this equilibrium is studied. It is shown that this equilibrium is arrived at by a mechanism involving the thermal formation of holes at Co and V.
Study of Energy Band Structure and X‐Ray Emission Properties of Some 5d Metals
Authors: Nemoshkalenko V. V. , Antonov VL. N. , Antonov V. N.
Published: July 2007

Energy band structure and partial densities of states are calculated for 5d metals with the method of relativistic augmented plane waves. The theoretical intensity distribution is obtained for NVI, NVII‐spectra of iridium, platinum, and gold taking into account the transition probability, the inner level finite width, and instrumental distortions.
AC Conductivity Tensor of Disordered Systems in the Presence of a Magnetic Field
Authors: Fishchuk I. I. , Rudko V. N.
Published: July 2007

In the presence of magnetic field the anti‐symmetrical part (s̀xy) of the ac conductivity tensor in disordered systems with localized electronic states is calculated by the Kubo formula. The large range of not too low frequencies, where the phononεess contribution to the conductivity predominates the phonon‐assisted contribution, is investigated. In calculating s̀xy(ω) the exact diagonalization of the used three‐site Hamiltonian is performed. In the same frequency range the symmetrical part (s̀ xx) of the conductivity tensor is calculated in the two‐site approximation. The results obtained show that in the frequency region where Re s̀xx(ω) has a maximum (the absorption band) the curve Re s̀xy(ω) undergoes a twofold sign inversion.
Surface Dynamics of Molecular Crystals. A Calculation for Naphthalene
Authors: Pawley G. S. , Chaplot S. L.
Published: July 2007

A model for calculating the statics and dynamics of a surface of a molecular crystal is presented. The (001) surface of naphthalene is chosen. The potential function used is the 6‐exp non‐bonded atom‐atom function, which has gained success in lattice dynamics. Surface layers of different thicknesses are investigated, all indicating that the third layer deep has a maximum binding energy. A slab of twelve layers thickness is used for the dynamics calculation, which shows clear evidence of surface modes. These are in general of lower frequencies than the bulk modes. The frequency distributions presented are divided into translational and librational parts, showing more clearly the nature of the surface modes.
Decay Model for AT and Ax Emissions in KBr:Tl
Authors: Benci S. , Manfredi M.
Published: July 2007

The decay signal of AT and Ax emissions in KBr: Tl is studied in the temperature range 80 to 300 K. A complex decay with three time constants is observed for both the emissions. The experimental results of the lifetimes and intensities versus temperature are explained with a single model for AT and Ax emissions.
Paramagnetic Properties of the Mott Isolator LaCoO3
Authors: Marx R.
Published: July 2007

The paramagnetic properties of LaCoO3 are investigated theoretically. LaCoO3 belongs to those perovskites which enjoy because of their catalytic qualities as exhaust catalyst increasing interest. The theoretical temperature dependence of the susceptibility is given and compared with experimental results. Good agreement between theory and experiment is achieved under the condition that only every second Co ion is excitable into the magnetic quintet state. The energy gap between low spin CoIII and high spin Co3+ is determined and compared with the gap resulting from the optical data.
A Structure Model and the Description of Structural Changes in Amorphous Metals
Authors: Hermann H. , Mattern N. , Kobe S.
Published: July 2007

The structure of an amorphous metal is assumed to consist of an arrangement of structural units which are generated from a basic unit by a coordination transformation. The parameters of the transformation matrix are distributed statistically. The basic unit is produced by energy relaxation of an assembly of dense random packed hard spheres and a dense arrangement of several icosahedral clusters of compressible spheres, respectively. The model provides realistic pair distribution functions. An interpretation of structural changes which occur during low‐temperature annealing and isothermal creep in amorphous substances is suggested.
Influence of 3d Ions on the Production Efficiency of Intrinsic Defects in CaF2
Authors: Alonso P. J. , Orera V. M. , Alcalá K.
Published: July 2007

The influence of 3d impurities on the production efficiency of VK and F centers is studied in low‐temperature X‐irradiated fluorite crystals. The optical absorption spectra of pure CaF2 and CaF2 doped with Mn, Fe, Co or Ni are compared before and after 15 K X‐irradiation. An increase in the efficiency for Vk‐centers production accompanied by the formation of M+ ions (M = Mn, Fe, Co, Ni) is observed at 15 K. A decrease of the F‐centers production efficiency is also detected.
Correlation of Zero‐Field Splittings and Site Distortions. II. Application of the Superposition Model to Mn2+ and Fe3+
Authors: Lehmann G.
Published: July 2007

Sign and size of b for Mn2+ and Fe3+ with O, F, or Cl as ligands are correlated with axial site distortions. Consistent values of b are obtained by application of the superposition model. These values are more than twice as large for Fe3+ than for Mn2+, but their sign may vary for different compounds and/or metal ion‐ligand pairs. Local charge compensation and effects of lattice relaxation are apparent in several cases. Unambiguous assignments of EPR spectra to specific lattice sites via these correlations are also demonstrated.
Electron Paramagnetic Resonance of Gold in Silicon. I. Single Atoms; Strong Nuclear Quadrupole Effect
Authors: Höhne M.
Published: July 2007

In Si: Au rapidly quenched two centres are observed by EPR, the first of which is investigated in this paper. The spectrum, is characterized by a dominating Zeeman interaction and by a nuclear quadrupole interaction large compared to the hyperfine interaction. Passage conditions depend on the direction of the magnetic field. The lattice site of the gold atom localized in a trigonal distortion is supposed to be an interstitial one.
Luminescence de l'ion Ce3+ dans le luminophore MgS
Authors: Asano S. , Yamashita N. , Ohnishi T.
Published: July 2007

Les spectres d'émission et d'excitation des phosphores MgS: Ce3+ ont été mesurés à 300,80 et à 6 K. Les bandes d' émission 2T2g(5d) → 2F7/2, 2F5/2(4f) et les bandes d'excitation 2F5/2(4f) → 2Eg, 2T2g(5d), attributées à transitions intraioniques dans Ce3+, ont été observées. On a trouvé, également, une bande d'excitation, qui peut ětre attribuée à une transition de transfert de charge. La connaissance de la position des raies à zéro‐phonon de ces bandes a été utilisée pour déterminer les paramètres de champ de cristal 10Dq, V, V et les constantes du couplage spin‐orbite ζf et ζd pour les orbites 4f et 5d.
Interstitial Hydrogen Centres in LiF Crystals
Authors: Kamikawa T.
Published: July 2007

In LiF:OH crystals neutron‐irradiated at low temperatures a new ESR absorption is observed after thermally annealed and aged at room temperature for some time. From the analysis of the ESR spectra the absorption is identified to be due to interstitial hydrogen atoms (H01centers) grown in LiF crystals during aging. This hydrogen center is unusually stable and decays very slowly even at room temperature. The ESR absorption lines of this center broadens at low temperatures. The (g‐factor, proton hf constant and shf constants are determined from the spectra obtained at room temperature measurements.
Some Investigations on the Description of Optical Absorption of Hole Centres in Oxide Crystals
Authors: Bernhardt H. J.
Published: July 2007

The common properties of colour centres in YAG, LiNbO3, and PbMoO4 crystals are discussed basing on the good applicability of the polaron model to these centres. A simplified Coulomb model enables one to describe the influence of melting point, lattice constant, and other properties of the host lattice on the properties of colour centres. Regarding the optical data of other oxide crystals known from literature the generalizability of colour centre properties in oxides is tested.
Quantum Numbers for Bloch Electrons in a Magnetic Field
Authors: Wannier G. H.
Published: July 2007

Landau functions characterize the motion of an electron in a plane perpendicular to a magnetic field. They have the degeneracy parameter ky. This parameter is resolved into four quantum numbers by a periodic crystalline potential. Two are phase angles; two are restricted to a finite set of values. Only one of the latter is a degeneracy parameter. A constructive proof is given for this proposition; it is valid under wide conditions, the only restriction being the famous ‚rationality condition’︁. The two discrete quantum numbers have as their range the numerator and denominator of the representative fraction. Group theory yields three of the numbers; the fourth labels the subbands, that is the equivalent representations within the spectrum. In the Peierls‐Onsager low field approach, the same four quantum numbers arise from one (two‐dimensional) Bloch band.
A Density‐Matrix Functional Approach to Bound Multiexciton Complexes
Authors: Wünsche H.‐J. , Henneberger K.
Published: July 2007

A general Hamiltonian is introduced describing multiexciton complexes bound to impurities in semiconductors. Band couplings by the impurity potential as, e.g., the valley—orbit or intervalence band couplings in cubic many‐valley semiconductors are involved and require a generalization of the conventional density‐functional formalism (DFF). Therefore, instead of the usual densities, the reduced one‐particle density matrices of the electrons and holes are introduced as basic quantities. Within the resulting density‐matrix functional formalism (DMFF) the inhomogeneous multiparticle systems are also described by a system of selfconsistent one‐particle equations. Some general properties of these equations are discussed, especially the consequences concerning the shell structure of bound multiexciton complexes. In case of uncoupled bands they are shown to transform into the conventional Kohn‐Sham equations.
Orthonormal Functions for Fermi Surfaces A Test of Convergence for Nb and Cu
Authors: Khan F. S. , Allen P. B.
Published: July 2007

The electronic mass enhancement λk on the Fermi surfaces of Cu and Nb is expanded in various basis sets. In particular, orthogonal polynomials (‚Fermi surface harmonics’︁) are constructed in the velocity vector components (FSH(v)), in the wave vector components (FSH(k)) and the cubic harmonics (CH). The results indicate that the set FSH(k) gives better convergence than the sets FSH(v) or CH.
The Most One‐Dimensional CTC Belonging to the TTF‐TCNQ Series The Trans DEDMTTF‐TCNQ Compound
Authors: Keryer G. , Delhaes P. , Amiell J. , Flandrois S. , Tissier B.
Published: July 2007

An experimental study of the most one‐dimensional compound of TTF‐TCNQ series, the transdiethyl‐dimethyl TTF‐TCNQ complex, is presented. In parallel with the X‐ray crystallographic structure different physical properties are investigated: magnetic susceptibility, electron paramagnetic resonance, specific heat, and dc electrical conductivity. In spite of the cation size the trans‐isomer only gives a TCNQ charge transfer complex which crystallizes with regular segregated stacks of acceptors and donors and thus is conducting. Because of the pronounced electronic one‐dimensionality, the disorder effect appears predominently at very low temperatures: the magnetism and specific heat temperature dependences are explained in terms of Bulaevskii's model used before for TCNQ anion‐radical salts only.
Ion‐Size Effects on the Energy Levels of F, FA, and FH Centers in Cesium Halides
Authors: Sivasankar V. S. , Sankar Chitra , Whippey P. W.
Published: July 2007

The energy levels of F centers in cesium halides are calculated variationally using the point‐ion lattice approximation with different sets of trial wavefunctions for the ground and the excited states. The effect of the finite size of the lattice ions on these energy levels is then evaluated self‐consistently within the framework of a local model pseudopotential approach. This method is then used to obtain the changes in the ion‐size effects induced by a substitutional monovalent cation impurity in a nearest neighbour position — the FA center — or a substitutional monovalent anion impurity in the second neighbor position — the FH center. In each case the ion‐size effects are calculated for all the trial functions considered. The calculated transition energies are compared with the observed positions of the corresponding optical absorption bands. It is concluded that the results of optical measurements on all the three centers can be accounted for, at least qualitatively, on the basis of a reasonably simple theory.
Effective Charges in Displacive Ferroelectrics
Authors: Gervais F.
Published: July 2007

The temperature dependence of the effective charges in ferroelectrics BaTiO3, KNbO3, and LiTaO3 is investigated in the vicinity of the ferroelectric—paraelectric phase transition. The results are understood in terms of a displacive‐type mechanism rather than order—disorder.
Hyperfine Interaction and Magnetic Phase Transition of BaVS3 Observed by Inelastic Spin‐Flip Scattering of Neutrons
Authors: Heidemann A. , Takano M.
Published: July 2007

High‐resolution inelastic neutron scattering experiments are performed to study the magnetic behaviour of stoichiometric BaVS3 at low temperatures. A magnetic phase transition is detected at Tc = (31 ± 1) K. The magnetic hyperfine field at a fraction of the vanadium nuclei has a value of (75 ± 10) kOe at T = 10 K. The results are compared with existing data of the magnetic susceptibility.
Effect of Lattice Distortion on Electron—Impurity Scattering Phase Shifts in the Distorted‐Wave Born Approximation
Authors: Sorbello R. S.
Published: July 2007

The effect of lattice distortion on electron—impurity scattering is estimated using the distorted‐wave Born approximation for phase shifts. For weak impurity scattering the phase shift for the scattering complex is the sum of the phase shifts from the point impurity and the lattice strain field. Model calculations show that there may be important corrections to this picture when the magnitudes of the impurity phase shifts are about π/10 or larger. These corrections arise from backscattering of electrons by the distortion field surrounding the impurity. It is concluded that one may incur appreciable error in calculating pseudopotentials or impurity phase shifts in the presence of lattice distortion if one does not include the lattice backscattering effect discussed in this paper.
Recombination Kinetics Involving Strongly Localized States Radiative Lifetime of a Chain with Light Emitting Ends
Authors: Esser B. , Gärtner B.
Published: July 2007

The problem of the motion of an excited electron (hole) over randomly distributed localized states to its recombination centre is investigated. The one‐dimensional case where the radiative recombination centres are placed at the ends of a chain of localized states is considered. With some simplifying assumptions the problem is shown to be equivalent to that of a Markov chain with absorbing states. The lifetime of the excited state is expressed by the transition probabilities along the chain. The general expression for the lifetime is applied to the special case of an ensemble of chains where the fluctuations of the transition probabilities are mainly due to the fluctuations of the distances between neighbouring states. For Gaussian distributed distances the dependence of the lifetime on the number of states in a chain is explicitly calculated.
Influence of Partial Long‐Range and Short‐Range Orders on the Electrical Resistivity of Substitutional Alloys
Authors: Christoph V. , Richter J. , Schiller W.
Published: July 2007

Starting from a resistivity formula the electrical resistivity of substitutional alloys with partial long and short‐range orders is calculated. Restrictions for the distance dependence of the Cowley short‐range order parameters are derived. In the weak scattering limit the resistivity contributions caused by precipitations and AB ordering effects are discussed.