bluuebunny/crossref_metadata_2025 huggingface dataset:
The magnetic properties of charge‐ordered crystals
Authors: Lubimov V. S. , Makarov E. F. , Ionov S. P.
Published: July 2007

The energy spectrum of the low‐lying collective triplet excitations is calculated. It is shown that the boundary frequency of such states is lower than that of a singlet collective mode taking into account the interband exchange interaction. Without the exchange both types of excitation have identical spectra. The susceptibility of the charge‐ordered state is calculated and some effects are discussed which are caused by the collective states.
On rate equations in the theory of diffusion‐controlled reactions
Authors: Kalnin Yu. H. , Pirogov F. V.
Published: July 2007

A connection between the two equations frequently used in the theory of diffusion‐controlled reactions, the Antonov‐Romanovskii and the Waite rate equations, is established. A new rate equation is proposed, which in the case of not very strong correlations between the reaction partners can substitute the more complicated Waite equation.
Spectroscopic investigation of optically excited doublet states of Ho3+ in magnetically ordered (Ho0.01, Gd0.99) VO4
Authors: Kasten A. , Becker P. J.
Published: July 2007

The splittings of optically excited doublets of Ho3+ in (Ho0.01, Gd0.99)VO4, caused by the antiferromagnetic ordering are studied, and exchange fields in the antiferromagnetic as well as in the saturated paramagnetic state are derived by means of Zeeman effect spectroscopy. The description of the superexchange interactions in terms of effective magnetic fields holds for most doublets, but at least for two states the Ho3+–Gd3+ exchange interaction is of a more complicated form.
Dynamic behaviour of the ising model in a transverse field
Authors: Kühnel A. , Wendt S. , Wesselinowa J.
Published: July 2007

The Ising model in a transverse field is treated by means of Green function technique to calculate the dynamic structure factor above and below Tc. The soft mode is overdamped in the disordered phase and near Tc, while it is nearly undamped at low temperatures. The coupling between the transverse soft mode and the relaxing longitudinal mode produces a central mode in the dynamic structure factor. The temperature dependence of the dynamic structure factor for zero wave vector is calculated numerically.
One‐particle spectrum of correlated electrons near the free‐electron limit
Authors: Monecke J.
Published: July 2007

An approximate solution of the Hubbard model is proposed which is correct in second order in I/Δ. In difference to the energy‐dependent Born approximation it preserves the presumed particle–hole symmetry of the Hamiltonian and is correct in the atomic limit, too. The one‐particle spectrum is calculated neglecting finite lifetime effects.
Singly, doubly, and multiply bottlenecked EPR based on the linear response theory
Authors: Sugawara K.
Published: July 2007

Based on the linear response theory, the spin–spin relaxation rate T2−1 of S‐state magnetic impurities in metallic hosts is investigated. It is found that the EPR linewidths of S‐state impurities in metals developed by Hasegawa (singly bottlenecked EPR) and by Hirst et al. (doubly bottlenecked EPR) can be naturally obtained using physical principles contained in the theory of Moriya and Obata which was originally developed for an insulator.
The kinetic exchange interaction in doubly degenerate narrow bands
Authors: Chao K. A. , Spałek J. , Oleś A. M.
Published: July 2007

A canonical transformation is applied to the doubly‐degenerate‐band Hubbard model with arbitrary n in the strong correlation regime. Projection operators are constructed to project out the splitted subbands. Only the inter‐subband electron hopping is treated as a perturbation which couples different subbands at the second order. In localized picture the coupling is of the Heisenberg type, corresponding to the virtual hopping of electrons and generating the spin waves, the orbital waves, and the coupled spin‐orbital waves. For n = 1 the ground state has ferromagnetic spin and antiferromagnetic orbital ordering. For n = 2 the spins in the ground state have ferromagnetic intra‐site but antiferromagnetic inter‐site ordering. These orderings are due to Hund's rule, the effect of which gets weakened by the intra‐subband electron hopping as n deviates from an integer. This canonical‐transformed effective Hamiltonian is suitable for investigating the phase diagram of the Hubbard model with orbital degeneracy.
Theory of the ESR spectra of Tl2+ ions in K2SO4 single crystals
Authors: Eremin M. V. , Silkin N. I.
Published: July 2007

The electron paramagnetic resonance spectrum of β‐K2SO4: Tl2+ is considered. The cluster model of the paramagnetic centres is used. It is suggested that the spectrum anisotropy is caused by the crystalline electric field which mixes the state 2P of the Tl2+ ion to the ground state 2S. Such an electric field is absent in the point charge model. An additional proof of the important role of the overlap of the Tl2+ and oxygen ion electron orbits is obtained when analysing the hyperfine structure of the ESR spectrum.
On the theory of resonant secondary radiation of excitons weakly interacting with phonons
Authors: Hizhnyakov V. V. , Sherman A. V.
Published: July 2007

Formulae are obtained for the partial spectra of the resonant secondary radiation of excitons weakly interacting with optical phonons, separated from the Rayleigh line by two or three phonon frequencies. The cases of the Frenkel exciton as well as that of the Wannier‐Mott one are considered. The obtained shapes of the spectra depend upon the type of the dispersion law of the exciton band and on the type of the dependence of the exciton—phonon interaction constants on the wave vector. A classification of these spectra according to the transformation mechanism from the primary photon into secondary is made. The criteria are obtained identifying the spectra under consideration as scattering, ordinary or hot luminescence.
Comments on the theory of hopping conduction
Authors: Zvyagin I. P.
Published: July 2007

Some aspects of the existing quantum‐mechanical theory of hopping conduction are discussed. Special attention is paid to the problems of the definition of current density in the site represetation, to the effects of variation of occupation numbers of the sites and to the approximations used in the quantum‐mechanical derivation of the rate equation. The applicability of the random resistor network model to the problem of hopping conduction is further confirmed.
On the use of generalized Langevin equations in phonon transport problems
Authors: Michel H. , Wagner M.
Published: July 2007

A system of phonons and local excitations is considered. The coupling of both subsystems is taken linear in the phonon coordinates. The generalized Langevin equations of the phonon density operator and of the subsequent mixed dipole operators are considered. The elimination of the dipole operators, a decomposition of the averaged second order occupations and specified further assumptions lead to the evolution equation of the phonon distribution function, which contains the local occupation numbers. In full analogy the time evolution of the local occupation numbers can be derived. A Markoff assumption, which is justified for realistic systems, and a restriction to slowly varying phonon densities lead to a rate equation type of a Peierls‐Boltzmann equation, which is coupled to the kinetic equations of the local occupations.
Lattice dynamics of the molecular crystals KCN and NaCN in their disordered cubic phase
Authors: Jex H. , Maetz C. J.
Published: July 2007

A model for the lattice dynamics of the disordered cubic high temperature phase of KCN and NaCN is proposed using a shell model for the translationnal motion, while the librational degrees of freedom of the CN− ‐ddumbbells are taken into account in the approximation of rigid molecular ions. The calculted dispersion curves are in adequate agreement with the experimental data from inelastic neutron scattring especially for the TA‐branches, which show an anomalous dispersion.
The phonon induced zero field splitting in vanadium doped alkali chlorides
Authors: Vanhaelst M. , Matthys P. , Boesman E.
Published: July 2007

The zero field splitting in V2+ doped NaCl, KCl, and RbCl is temperature dependent and has a shallow minimum near 50 K. Previously reported experiments are compared with theoretical results based on an anisotropic continuum phonon model. The qualitative behaviour of the spin‐Hamiltonian constant D as well as the quantitative agreement of the coefficients, which determine D as a function of temperature, are discussed.
Many special point scheme for noncubic lattices
Authors: Lin‐Chung P. J.
Published: July 2007

The sets of special points which are used in the evaluation of the integral of periodic functions over the Brillouin zone are presented for the tetragonal and trigonal lattices. The accuracy of this approximation is discussed.
On the crossing of dispersion curves of alkali metals in [ζ00] direction
Authors: Ramamurthy V. , Singh K. K.
Published: July 2007

The experimentally observed dispersion curves of lithium, sodium, and potassium in [ζ00] direction, especially the degeneracy of the zone boundary frequencies and the anomalous crossing in lithium, are reproduced completely by a lattice dynamical which model which takes into account the umklapp processes and the screening associated with the electrons. It is shown that the observed crossing in lithium is associated with the large positive value of the bulk modulus of the electron gas, together with the variation of umklapp contribution to these modes of vibration with the wave vector. The latter leads to the observed degeneracy at the zone boundary point. The role played by umklapp processes in producing effects is discussed.
On the properties of cobalt impurities in zinc selenide crystals II. The optical properties in near‐infrared. 4A2(4F) ↔ 4T2(4F) Transitions
Authors: Radlinski A. P.
Published: July 2007

Absorption and luminescence spectra due to the 4A2(4F) ↔ 4T2(4F) crystal field transitions in ZnSe:Co2+ are presented. It is shown that the structure of the absorption band is caused by the spin‐orbit interaction within the 4T2(4F) state and the coupling to the lattice modes of A1 symmetry. According to a model proposed for the thermal depopulation of the excited 4T2(4F) state, the ionization energy of the Co2+ impurity in ZnSe is estimated to be about 5600 cm−1 (0.69 eV).
Structural phase transitions of antiferroelectric and displacive modulated types caused by electron‐phonon interaction
Authors: Konsin P.
Published: July 2007

A generalization of the vibronic theory of ferroelectric phase transitions is made and the theory of antiferroelectric and structural modulated phase transitions, caused by the electron‐phonon interaction in dielectrics and semiconductors, is developed. The criteria for the occurrence of such transitions are obtained. The basic characteristics (spontaneous lattice distortion, dielectric susceptibility, frequencies of soft modes, etc.) are calculated. The dispersion of electronic states, corrections to the mean field approximation for the driving vibronic interaction, and phonon anharmonicity are taken into account. The relation between ferroelectric and antiferroelectric phase transitions is discussed.
Magnetic freeze‐out effect in noncrystalline semiconductors
Authors: Majerníková E. , Barta Š.
Published: July 2007

The density of states of an electron and an exciton gas in a strong magnetic field and in a Gaussian random potential is calculated. The effect of the magnetic freeze‐out of the tail states owing to the Landau shifts, the dependence of the „damping”︁ of the tails on the magnetic field, as well as the magnetic field dependent quantum shift of the deep tails are found. In some cases, the effect can provide means for a statement of the presence of excitons in non‐crystalline solids.
Some aspects of spin relaxation in ferromagnets in a magnetic field
Authors: Kashcheev V. N.
Published: July 2007

In the Vaks‐Larkin‐Pikin spin model the asymptotics of the pair spin correlators are derived in the right neighbourhood of the Curie point and at the non‐zero external magnetic field. Using this asymptotics the critical anomalies of the NMR line are found as well as the phonon damping and renormalization of the sound velocity in the isotropic Heisenberg ferromagnet.
An exact calculation of the average density of states in disordered systems
Authors: Zawisz S.
Published: July 2007

An exact non‐self‐consistent formula for the direct calculations (i.e. without solving the eigenvalue problem for the Hamiltonian) of the density‐of‐states function for a three‐dimensional disordered system is presented. An interpretation is given and an example of using the formula is discussed.
One‐ and two‐phonon states in tetragonal ZnP2 and CdP2 crystals
Authors: Gorban I. S. , Gorinya V. A. , Dashkovskaya R. A. , Lugovoi V. I. , Makovetskaya A. P. , Tichina I. I.
Published: July 2007

Second‐order Raman spectra are studied in tetragonal ZnP2 and CdP2 crystals. The main feature of the spectra is a strong dominance of the A1‐component over the E‐ and B2‐components. Whereas the former is due to overtones, the latter ones are associated with combination tones. Besides, the A1‐component reveals a number of bands associated with a high density of one‐phonon states having a strongly expressed anisotropy: the width of the phonon bands is large (≈︁ 20 cm−1) along the directions of phosphorus chains and is extremely small (about 1 to 2 cm−1) in other directions.
Influence of the position of the interface boundary on the existence of interface states
Authors: Kandilarov B. D. , Tashkova M. G. , Petrova P. C. , Detcheva V.
Published: July 2007

Necessary and sufficient conditions for the existence of interface states localized at the interface in two one‐dimensional models of deformed heterojunctions are derived in terms of a scattering theoretical method. The existence conditions thus obtained are compared with the corresponding expression for an ideal undeformed heterojunction. The impact of both symmetrical and asymmetrical deformations on the existence of interface states is discussed.
Electronic band structure of SnSe
Authors: Car R. , Ciucci G. , Quartapelle L.
Published: July 2007

The electronic bands of SnSe are calculated along all high symmetry lines of the Brillouin zone using the local empirical pseudopotential method. Spectralization of the involved large matrices is performed by means of a new approximate folding‐down method which locates the s‐states of valence bands much better than the Löwdin‐Brust perturbative technique. The parameters of the analytical form used to interpolate the pseudopotentials are roughly adjusted to obtain a band structure consistent with optical and photoelectric data. Satisfactory results for the general features are obtained, a better agreement requiring a more cautious tuning of pseudopotential values in the region of small wave vectors.
A pseudopotential study of the electronic properties of noble metals
Authors: Sharma O. P.
Published: July 2007

A local model pseudopotential, depending on ionic and core radii and which incorporates the sd hybridization effects phenomenologically, is employed to calculate the phonon frequencies, liquid metal resistivity, binding energy, and the effective ion‐ion interaction of the noble metals Cu, Ag, and Au in conjunction with a dielectric function in the non‐interacting band scheme. Reasonably good agreement is found between the computed and the experimental results.
Magnetic solutions of the Hubbard model
Authors: Schumacher W. , Lindner U. , Jauch R.
Published: July 2007

Existence and stability of antiferromagnetic and of ferromagnetic solutions of the Hubbard model are investigated in the framework of the Hartree‐Fock approximation. It is found that the Hubbard model for simple cubic lattice and certain parameters admits stable magnetic solutions in a finite temperature range, even in cases, where for these parameters at T = 0 no magnetic ordering exists (so‐called heat magnetization).
The theory of autolocalized electron states in ferromagnetic semiconductors at arbitrary relation between the band width and the spin–electron interaction
Authors: Egorov B. V. , Krivoglaz M. A.
Published: July 2007

The conduction electron energy is determined in the adiabatic approximation for ferromagnetic semiconductors with large atom spins and an arbitrary relation between the band width ΔE and the spin–electron interaction constant A. The thermodynamical potential of the system is found. The conditions are discussed when it is thermodynamically favourable to form large radius autolocalized electron states (fluctuons) and their characteristics are determined. A consideration is given for the temperature range where the spin‐wave approximation or that one of self‐consistent field are applicable. It is shown that the temperature range of fluctuonic states is the most wide in crystals with large |A|/kTC and with |A| S and ΔE of the same order.
F Centers in Alkaline‐Earth Fluorides. Inadequacy of the Muffin‐Tin Approximation
Authors: Oliveira L. E. , Oliveira P. Murilo , Maffeo B.
Published: July 2007

The SCF‐MSXα method is applied in the study of the electronic structure of F centers in CaF2, SrF2, and BaF2. The predicted optical transition energies are in disagreement with the experimental data. An explanation for the discrepancy is provided showing the inadequacy of the spherical averaging of the potential within the muffin‐tin approximation.
Exciton Molecule Creation by Pure and Phonon‐Assisted Two‐Photon Transitions in Solids
Authors: Hassan A. R.
Published: July 2007

Direct creation of bi‐exciton states, by direct and phonon‐assisted two‐photon absorption in direct‐ and indirect‐gap semiconductors, is theoretically investigated. A group‐theoretical analysis of the final direct (indirect) bi‐exciton symmetry is given for CuCl(Ge). Numerical applications to the case of CuCl and Si show that the direct and indirect two‐photon absorption coefficient for bi‐excitonic transitions are several orders of magnitude larger than those for the corresponding processes of interband and excitonic transitions. The essential contribution to this enhancement of the absorption coefficient for the transitions to the direct and indirect excitonic molecule states is shown to be from the resonance effect.
Third‐Order Contribution to the Effective Ion–Ion Interaction and Formation of Angular Forces in Crystalline Solids. I. General Formulation of the Expression for the Total Energy
Authors: Milchev A. , Unger K.
Published: July 2007

The importance of the third‐order correction to the effective interatomic potential is investigated making use of the Dirac density matrix formalism. Starting from the perturbation expansion for the one‐electron density matrix a general expression for the total energy up to third order of the perturbing pseudopotential is obtained. A simplified expression for the third‐order filter function is suggested and used in computing the appropriate corrections to the effective ion–ion potential. The building‐up of tetrahedral noncentral forces in the group IV elements is revealed and the connection with tight‐binding models is established.
Unpaired Elastic Force Model in the Lattice Dynamical Study of Rare Gas Solids
Authors: Kushwaha M. S. , Kushwaha S. S.
Published: July 2007

The lattice dynamics of rare gas solids (RGS) is investigated using an elastic force model which involves the central forces and the unpaired forces of the type of Gazis, Herman, and Wallis. The central forces are assumed to arise from the atomic displacement from their mean positions. The unpaired forces, on the other hand, arise from the deformation in certain angles made by the reference atom and two of its nearest and next nearest neighbours. The present model involves five parameters in total and satisfies the condition of invariance under rigid body rotation. As an application, the phonon dispersion relations of argon (Ar) and krypton (Kr) are calculated along all the three symmetry directions [100], [110], and [111]. The comparison of theoretical results reveals reasonably good agreement with the latest available experimental data.
The Crystal Field for Nd3+ in Garnets
Authors: Nekvasil V.
Published: July 2007

A crystal field analysis of the reported optical spectra of Nd3+ in the series of aluminium and gallium garnets LuAG, YAG, GGG, LuGG, and YGG is presented. The reliability of the crystal field parameters determined in this way is confirmed by comparison with ESR g‐values given in the literature. The results obtained represent a basis for a further study of the relation between the crystal field parameters of rare earths in garnets and the structure of the coordination dodecahedron.
Theory of Resonance Scattering for Electron–Phonon Interaction in Doped Semiconductors with Zincblende Structure
Authors: Singh Mahiradhwaj
Published: July 2007

Expressions for the resonance scattering of phonons by bound donor electrons are obtained in internally strained n‐type semiconductors with ZB structure for the situation, when, besides the usual spin–orbit splitting of the sixfold degenerate state, derived from the triplet, into a quartet and lower lying doublet, the quarted state further splits into two Kramers doublets owing to the internal and dynamic strains present in the crystal. Similarly expressions for the resonance scattering of phonons by bound donor electrons are obtained in presence of an external magnetic field for the most general case when the degeneracy of each Kramers doublet splits into two singlet states. The expressions obtained for the resonance scattering relaxation rates for internally strained semiconductors are used to explain the thermal conductivity of n‐type GaAs and n‐type GaSb. Excellent agreement between theory and experiment is obtained for both the samples.
On the Second‐ and Third‐Order Elastic Constants of ZnS Structure Solids
Authors: Garg V. K. , Puri D. S. , Verma M. P.
Published: July 2007

Expressions for the second‐ and the third‐ order elastic constants of solids crystallizing in the ZnS structure are derived from a general potential. As far as known, this study provides the first analysis of the third‐ order elastic constants of ZnS structure solids. It is observed that the internal strain, arising due to the sublattice displacement, affects only the elastic constants C44, C166, C144, and C456. Applications to a number of solids based on the Lundqvist potential lead to values of the third‐ order elastic constants and pressure derivatives of the second‐ order elastic constants which agree fairly well with the available measured values.
Third‐Order Contribution to the Effective Ion–Ion Interaction and Formation of Angular Forces in Crystalline Solids. II. Application to Tetrahedral Solids
Authors: Milchev A. , Unger K.
Published: July 2007

A third‐order correction to the structure dependent part of the total energy derived in Part I is used to demonstrate the building‐up of angular forces in tetrahedral solids and simple interpolation expressions are proposed, which reproduce the numerical data.
On the Screening of the Ions in Solids. Band Structure Calculations of Si with Several Screening Variants
Authors: Pasemann L.
Published: July 2007

By using the approach for the Dirac density matrix of a perturbed Fermi gas given by March et al. the complete screening matrix is derived. Furthermore, band calculations of Si are performed using the non‐diagonally screened pseudopotential in second order in the ionic potential, and the diagonally screened pseudopotential in third order in the ionic potential, respectively.
Calculation of Electrical Resistivity and Its Temperature Coefficient of Amorphous Cu60–Zr40 Alloy Using t‐Matrix
Authors: Waseda Y. , Chen H. S.
Published: July 2007

The electrical resistivity of amorphous Cu60–Zr40 alloy is calculated on the basis of the t‐matrix formulation of the Ziman nearly‐free‐electron theory. Experimental structure factors at different temperature are used for calculating the variation of the electrical resistivity with temperature. The calculated values of the electrical resistivity and its temperature coefficient show reasonable agreement with the available experimental data. The observed high value of the electrical resistivity and its negative temperature coefficient for amorphous Cu60–Zr40 alloy is interpreted as due to the proximity of the Fermi momentum to the position of the first peak of the structure factors.
Spin Waves in a Uniaxial Ferromagnet with Random Exchange Bonds of Different Signs
Authors: Medvedev M. V.
Published: July 2007

The spin wave dispersion law Ek and the spin wave damping Γk in a uniaxial ferromagnet with an easy axis anisotropy and random exchange bonds of different signs between the nearest magnetic neighbours are investigated. It is shown that in the long‐wavelength limit (ak ≪ 1) the spin wave dispersion law Ek has the form Ek = Δ + D̃(ak)2, where Δ is the spin‐wave gap and the damping Γk is proportional to ak. In the limit of the zero anisotropy Ek = ρak and Γk ∼ (ak)4 are obtained.
Magnetic Field Dependence of the Photoconductivity in Amorphous Silicon
Authors: Mell H. , Movaghar B. , Schweitzer L.
Published: July 2007

We report the first observation of magnetic field induced changes in the photoconductivity σp of a‐Si produced by the glow discharge technique. The relative change of σp in fields H0 up to 10 kG is of the order of 1%. The dependence of σp on the magnitude of H0 is similar to that of the dark conductivity in evaporated a‐Si and a‐Ge, suggesting that the same type of mechanism is responsible for both phenomena. We explain the change of σp in terms of a spin dependent pair recombination model in which the recombination rate of triplet pairs is correlated with the spin relaxation rate which depends on the applied magnetic field H0.
Crystal Optics of Surface Polaritons in Anisotropic Crystals
Authors: Puchkovskaya G. A. , Strizhevskii V. L. , Frolkov Yu. A. , Chepilko N. M. , Yashkir Yu. N.
Published: July 2007

A general theory of the surface polariton states in anisotropic crystals at arbitrary orientation of crystal axes with respect to the surface is developed. General formulae describing dispersive curves and polarization of surface polaritons (SP) are obtained. It is shown that (contrary to the case of SP in isotropic crystals in which they are H‐waves) the anisotropic SP polarization is in general of mixed (EH‐) type, and the spatial decay of SP is nonexponential. Attenuated total reflection spectra of anisotropic SP's in a crystal α‐LiIO3 are investigated. Experimental and theoretical results are in a good agreement.
Annihilation of Positrons in Electron‐Irradiated Silicon Crystals
Authors: Fuhs W. , Holzhauer U. , Mantl S. , Richter F. W. , Sturm R.
Published: July 2007

Lifetime and Doppler‐broadening measurements in positron annihilation are used to investigate defects in silicon crystals irradiated with electrons of 1 MeV energy. The lifetime in undoped defect‐free Si is found to be (218 ± 8) ps. Isochronal annealing studies in the temperature range 12 K < Ta < 600 K show that in undoped Si at low temperature positrons annihilate at neutral monovacancies and double vacancies with lifetimes of (266 ± 10) and (318 ± 15) ps, respectively. In boron‐doped crystals before and after irradiation no special defect reveals in the spectra up to 400 K. This result is explained by the different charge of the defect states in the doped Si. The mean lifetime in doped crystals amounts to (233 ± 10) ps.
Solitons in a One‐Dimensional Modified Hubbard Model
Authors: Lindner U. , Fedyanin V. K.
Published: July 2007

It is shown, that in a Hubbard chain modified by coupling to the lattice solitons exist. They are accompanied by solitary lattice distortions. The non‐lincarity is caused by the electron correlation as well as by the electron‐phonon interaction.
Excitation Spectra of Exciton Fission in Organic Crystals
Authors: Albrecht W. G. , Coufal H. , Haberkorn R. , Michel‐Beyerle M. E.
Published: July 2007

In a sensitive experimental arrangement the excitation spectra of singlet exciton fission into a pair of triplet excitons are measured via the magnetic field effect on the prompt fluorescence. For monomeric crystals as anthracene and β‐perylene the excitation spectra show similar structure which in the case of anthracene does not change remarkably upon perdeuteration. Excimer forming crystals as α‐perylene, however, exhibit a blue shift of the fission threshold of ≈ 3500 cm−1. This effect is explained by the competition between fission and excimer formation.
Calculation of Auger Coefficients for III–V Semiconductors with Emphasis on GaSb
Authors: Haug A. , Kerkhoff D. , Lochmann W.
Published: July 2007

In contrast to former approximate treatments an exact calculation of Auger coefficients for III–V compounds is given taking fully into account the restrictions given by the energy and wave vector conservation. In particular the valence band process involving the split‐off band is considered which is essential for semiconductors with nearly equal energy gap and valence band splitting like GaSb. The well‐known conduction band process follows as a special case. The result for GaSb is in reasonable agreement with experiment.
Nouveaux centres paramagnétiques dans les halogénures alcalins mixtes: Ag(Br1) dans KCl:KBr
Authors: Amanis I. K. , Kliava J. G. , Purãns J. J.
Published: July 2007

On a mis en évidence les spectres de RPE des centres paramagnétiques Ag(Br1) dans KCl. Ces centres sont les atomes d'argent neutres situés dans les sites cationiques de réseau où l'un des ions Cl− de la première sphère de coordination est remplacé par Br−. En comparant les spectres expérimentaux à ceux calculés à l'aide d'un ordinateur on a déterminé les valeurs des paramètres du hamiltonien de spin des centres Ag(Br1) pour le cas H ‖ [111]. Les valeus du facteur g, (1,996 ± 0,001), de la constante de structure shf, (—656 ± 10) G pour l'isotope 109Ag à 285 K et de la constante de structure shf résultant du chlore, (12,8 ± 0,1) G à 285 K sont proches des valeurs correspondantes pour les centres Ag dans KCl. La valeur de la constante shf résultant du noyau de brome est considérablement plus grande dans le cas des centres Ag(Br1) ((95,5 ± 0,3) G pour 81Br à 285 K) que dans le cas des centres Ag dans KBr. Ce désaccord démontre que le réseau de KCl est notablement déformé aux environs du centre Ag(Br1).
Frequency Spectra and Thermal Properties of Rare Gas Solids (Argon and Krypton)
Authors: Kushwaha M. S. , Kushwaha S. S.
Published: July 2007

The unpaired elastic force model developed by the present authors is extended for studying the phonon frequencies of solid argon and krypton at all the irreducible points of the first Brillouin zone with finer subdivisions (1000). This is used to calculate the lattice heat capacities and Debye characteristic temperatures at different temperatures in the range 3 to 50 K. The comparison of theoretical values with the recently available experimental data reveals a fairly good agreement.
Luminescence of the Pb2+‐Ion Dimer Center in CaS and CaSe Phosphors
Authors: Asano S. , Yamashita N. , Nakao Y.
Published: July 2007

Reflection, emission, and excitation spectra of CaS:Pb2+ and CaSe:Pb2+ are measured at 300, 80, and 6K. At higher concentrations of Pb2+ ions, the emission and exeitation bands due to the Pb2+ dimer are observed besides those of the Pb2+ monomer. Theoretical calculation of energy levels of the dimer of Tl+‐type ions is made on the basis of the localized electron model. The values of the spectroscopic parameters appearing in the energy matrices are determined so that the electronic levels obtained experimentally and theoretically may coincide with each other.
The KLL Auger spectra of Na and Mg metal and their plasmon structure
Authors: Steiner P. , Reiter F. J. , Höchst H. , Hüfner S.
Published: July 2007

The KLL‐Auger electron spectra of clean Na and Mg metal are measured. A quantitative analysis of these spectra is performed taking into account the core hole–conduction electron coupling and the creation of plasmons by electron scattering. The plasmon intensities are found to be equal to those observed in XPS experiments. The relative Auger line intensities differ from those in the vapor phase.
The exciton dispersion in polar semiconductors
Authors: Behnke G. , Büttner H.
Published: July 2007

The influence of optical phonons on the exciton dispersion is studied for various coupling strengths as a function of the exciton radius aex. The variational method yields an effective exciton mass which for not too large radii aex (smaller than the polaron‐radii) is reasonably approximated by the bare mass. Only for very large aex, the exciton mass is that of two independent polarons. Corrections to the quadratic dispersion in higher order of the centre of mass momentum are found to be negligible small. The dispersion curves for the materials TlCl and PbI2 are calculated.
Magneto‐electric splitting of the crystal field Γ2‐level of Cr3+ ions in MgCr2O4 studied by neutron spectroscopy
Authors: Ligenza S. , Ptasiewicz‐Bąk H.
Published: July 2007

Neutron spectroscopic studies are made on polycrystalline MgCr2O4 spinel at temperatures between 5 and 293 K. Neutron spectra connected with local spin interaction are observed and interpreted basing on the splitting of Γ2‐level belonging to the 4F3/2‐term of Cr3+ ions situated at octahedral B‐sites. The fourfold Γ2‐level splitting and the neutron excited transitions are analysed in terms of a spin‐Hamiltonian assuming a magneto‐electric interaction of the 3d3 electron shells. In spectroscopic measurements the set of peaks connected with two kinds of B‐sites are observed. According to neutron diffraction measurements the local magnetic‐molecular field at those sites makes the angles θx = 90° (B(x)) and θv = 35° (B(v)) with a trigonal axis of electric field. From spectral analysis the energies of Γ2‐level splitting are determined for Cr3+ ions at B(x) and B(v) sites, and the respective interaction parameters are found. Basing on interaction parameters the magnetic moments of Cr3+ at B(x) and B(v) sites are calculated to be μ(x) = (2.64 ± 0.02) and μ(v) = (1.80 ± 0.02) BM, respectively. From neutron diffraction measurements the magnetic moments at B(x) and B(v) sites are found to be μd(x) = (2.70 ± 0.10) and μd(v) = (1.83 ± 0.10) BM, in good agreement with moments calculated from spectroscopic data.
The influence of an electromagnetic wave on the electronic spectrum of crystals
Authors: Quang Nguyen Vinh
Published: July 2007

The variation of the crystal electronic spectrum under the influence of both monochromatic and nonmonochromatic, both lineraly and circularly polarized electromagnetic wave is studied using a simple method. The difference between the cases of parabolic and nonparabolic, spherical and nonspherical dispersion relations is analysed in detail. The one‐band approximation is justified when the photon energy is much less than the gap energy.
Spectral and time decay analysis of photoluminescence excited in the A band region in KCl:Pb
Authors: Benci S. , Capelletti R. , Fermi F. , Manfredi M. , Damm J. Z. , Mugeński E.
Published: July 2007

The A band spectral region in KCl:Pb is studied from the point of view of the optical absorption spectra, photoluminescence (emission and excitation) spectra, and the luminescence lifetime measurements in the temperature range 15 to 320 K. The sensitive ITC technique is exploited as a parallel tool to monitor and check the solution state of the impurity in order to rule out the additional and unwanted complications arising from the presence of aggregates. As a consequence only very dilute (4 to 6 ppm) and quenched samples are chosen to perform the detailed and systematic analysis of the photoluminescence spectra. The emission and excitation spectra do not depend significantly on the wavelength of the exciting and emitted light respectively, while their shape depends strongly on the temperature. The careful analysis of the emission spectrum, which looks asymmetric and apparently structureless, suggests that two Gaussians are contributing to it (main band and red side band, with associated average phonons of 1.25 × 10−2 and 1.6 × 10−2 eV, respectively) for T < 120 K, while a third weaker component seems to increase on the high energy side (violet side band) for T > 120 K. The luminescence time decay is complex exhibiting three distinct lifetimes: the length of two of them (τ1 = 30 and τ2 = 160 ns) is temperature independent, while the third one τ3 is strongly temperature‐dependent. The comparison between the intensity behaviour versus temperature both, of the time decay components and of the emission spectral components brings to a tentative attribution of τ1 to the main band, τ2 to the violet side band, and τ3 to the red side band. A scheme for the excited state energy levels, from which the emission excited by A light takes place is discussed both, by taking into account the dynamical Jahn‐Teller effect and the role of the neighbouring cation vacancy.
Contribution of the Fe2+ ion to the magnetic anisotropy constant in ferrites
Authors: Watanabe Y. , Urade K. , Saito S.
Published: July 2007

The magnetic anisotropy constant K1 originating from the presence of Fe2+ ions in ferrites is calculated, using the theory of Wolf, for a case that the electron hopping between Fe2+ and Fe3+ ions is allowed within a cluster of iron ions around some lattice defects. The calculated value of K1 exhibits a characteristic temperature dependence, i.e. the change of the sign from positive at lower temperatures to negative at higher temperatures. It is shown that the result well agrees with the Fe2+ contribution to K1 estimated from the available data of the anisotropy constant for the Ni1−xFe2+xO4 system.
Characteristic features of the electronic spectrum and the stability of ternary diamond‐like semiconductors
Authors: Altshuler A. M. , Vekilov Yu. Kh. , Umarov G. R.
Published: July 2007

The electronic spectrum and the assessment of parameters and trends of the 31‐st ternary diamond‐like compound band structure are presented and conditions for possible structural change are defined on their basis. Areas of stability of narrow band semiconductors and semimetals are established. The characteristic features of the electronic compound spectrum in various fields of application are considered. Composition and structure of some compounds having anomalously high melting temperatures are predicted.
Non‐isodisplacement of P atoms in long‐wavelength optical phonons in In1–xGaxP
Authors: Jahne E. , Pilz W. , Giehler M. , Hildisch L.
Published: July 2007

A study on long‐wavelength optical phonon modes in In1–xGaxP is carried out by infrared reflection and Raman scattering. The oscillator strengths of the single reststrahlen band (one‐mode behaviour) and of the additional band between the TO and LO mode are determined by classical dispersion analysis. The origin of the additional structure is explained by non‐isodisplacement motions of the P atoms in the long‐wavelength phonon modes arising from changes in the nearest‐neighbour force constants with the kind of surrounding atoms. Compositional dependences of the mode frequencies and oscillator strengths are calculated using a virtual crystal model with five types of basic units.
On the electronic structure of hexagonal boron nitride
Authors: Tegeler E. , Kosuch N. , Wiech G. , Faessler A.
Published: July 2007

The anisotropic emission of the K‐X‐radiation of boron and nitrogen in hexagonal boron nitride BN permits the determination of the π‐ and s̀‐subbands of the two X‐ray spectra. Together with the X‐ray photoelectron spectrum a consistent picture of the valence band of BN is obtained. Position and width of the s‐, s̀‐, and π‐bands are determined. None of the band structure and density‐of‐states calculations known so far is in satisfactory agreement with the experimental observations.
Temperature dependence of the electrical resistivity of disordered metallic alloys
Authors: Christoph V.
Published: July 2007

The electrical resistivity of disordered substitutional alloys is calculated starting from a resistivity formula containing force‐correlation functions. Potential and electron–phonon scattering are taken into account. The correlation functions are calculated by solving approximatively the equations of motion of corresponding Green functions. For a simple model the concentration and temperature dependences of the resistivity are discussed.
Band structure and cluster calculations of FeAl systems
Authors: Müller Ch. , Seifert G. , Lautenschläger G. , Wonn H. , Ziesche P. , Mrosan E.
Published: July 2007

The electronic band structure and the density of states of the ordered B2‐type intermetallic compound FeAl are compared with the density of states profiles obtained from scattered wave‐Xα‐ calculations for four different clusters of the system FeAl and with results obtained for localized substitutional Fe impurities in an Al host. Comparing band structure calculations and cluster calculations the influence of long range order and short range order on the electronic structure is discussed. Band structure calculation, cluster calculations, and also the calculation of the additional screening charge around localized substitutional Fe impurities in Al give a charge transfer from the Al atoms to the Fe atoms in the system FeAl.
Angular dispersion of the internal phonon modes in ferroelectric sodium nitrite
Authors: Castellucci E. , Schettino V.
Published: July 2007

The angular dispersion of the frequencies of the internal modes in ferroelectric sodium nitrite are determined by measuring the Raman spectra with observation of phonon propagating in the ab, ac and bc crystallographic planes. Using the exciton model and a resonant dipole–dipole interaction mechanism the angular dispersion is calculated resulting in good agreement with the experiment. The k dispersion curves are also calculated for points along the [100], [010], and [001] directions.
Nuclear flip‐flop transitions induced by paramagnetic impurities
Authors: Sabirov K. Kh.
Published: July 2007

The general formulas for the nuclear flip‐flop transition probability induced by paramagnetic impurities are given based on the correlation function of the electronic subsystem. The calculation does not deal with the exact electronic spin fluctuation mechanism. The expressions for the spin‐diffusion coefficients associated with the modulation of the nuclear dipole–dipole interactions and indirect nuclear–nuclear interactions are derived.
Modulation of Mössbauer radiation by coherent ultrasonic excitation in crystals
Authors: Mketchyan A. R. , Arutyunyan G. A. , Arakelyan A. R. , Gabrielyan R. G.
Published: July 2007

For the first time experimental observations are made of the oscillation in the intensity of the unshifted line and in the intensities of the US satellites when the γ‐resonance radiation is modulated by the coherent US excitation. It is stated that the coherent US can be excited in the investigated sample if the US amplitude is transmitted homogeneously from the generator to the sample. A special sample‐unit is described which provides a homogeneous transmission, and it is shown hat the lifetime of the US phonons is not substantial and does not affect the oscillation of the absorption line intensities.
Equilibrium vacancy concentration measurements on tin single crystals
Authors: Balzer R. , Sigvaldason H.
Published: July 2007

Simultaneous length and lattice parameter change measurements are performed on tetragonal tin single crystals in a‐ and c‐direction from room temperature to melting temperature. Length and lattice parameter change measurements agree within experimental error for all temperatures. From this it can be concluded that the maximum vacancy concentration is less than 3 × 10−5, which is the maximum experimental uncertainty. Using the energy of formation E = 0.50 eV from other experiments an upper limit for the entropy of formation of s/k = 1.1 is calculated.
The influence of two‐particle states (excitons) on the dielectric function of the electron—hole plasma
Authors: Röpke G. , Der R.
Published: July 2007

Improvements of the RPA‐dielectric function of a plasma are investigated using the framework of Green's‐function technique which are of importance especially if bound states (excitons) are formed. It is shown that beyond the contribution of free carriers to the dielectric function a contribution due to the polarizability of the bound states arises.
The electro‐absorption edge spectra of 2H‐MoS2
Authors: Murray R. , Evans B. L.
Published: July 2007

Measurements are described of the effects of applied electric fields on the delocalised exciton spectra which comprise the direct absorption edge in 2H‐MoS2 (77 K). The field‐modulated transmission spectrum is predominantly due to thermal modulation of the illuminated crystal, but when this is reduced, field ionisation of the exciton states is observed at predicted field strengths.
Theory of Inelastic Reflection of Particles by Rayleigh Waves
Authors: van Trong Nguyen
Published: July 2007

A quantum mechanical theory of particle reflection by Rayleigh waves is presented. The semiinfinite target is considered as a continuous elastic medium and its interaction with incident particles is treated as fluctuations of the boundary surface on which the particle wave function must vanish. The cross section is found to be expressed in terms of the correlation function for surface displacements of reflecting medium. The spectral distribution of correlation function is then related with the medium response function, in spirit of the fluctuation–dissipation theorem. That response function is obtained in the framework of the elastic theory. It is suggested that the spectral distribution of fluctuations shows the presence of Rayleigh waves.
Surface Waves in Low‐Dimensional Crystals
Authors: Chebotarev L. V.
Published: July 2007

The acoustic surface waves are investigated propagating along the free boundary of highly anisotropic crystals having layer‐ or chain‐structure. It is shown that due to the anomalous smallness of some of the elasticity moduli very slow surface waves can exist in such crystals penetrating deeply into the bulk of the crystal.
Theory of Exciton Migration in a Critical Region
Authors: Gaididei Yu. B. , Trofimov A. S.
Published: July 2007

The migration of exciton excitations in a crystal undergoing a second‐order phase transition is considered using the density matrix theory. Near the phase transition temperatures in a crystal appear large‐scale fluctuations of the order parameter at which, as shown, an exciton may be scattered additionally. An anomalous temperature dependence of the exciton diffusion coefficient in the critical region is predicted.
The Lattice Dynamics of Quantum Crystals of F.C.C. 4He
Authors: Kushwaha M. S. , Kushwaha S. S.
Published: July 2007

The lattice dynamics of solid helium in the f.c.c. phase is investigated using an unpaired elastic force model (EFM). The potential energy of the crystal is assumed to be made up of two parts: (1) the potential energy, giving rise to the central (pair‐wise) forces and 2. that one, giving rise to the unpaired forces. The model parameters are calculated using five experimental phonon frequencies. The frequency spectruy calculated at all the irreducible points of the first Brillouin zone is further used to compute the phonon density of states, lattice heat capacities, and Debye characteristic temperatures. The use of the model parameters is also made to predict the experimentally unavailable elastic constants in the f.c.c. phase. The theoretical results for the phonon frequencies along [100], [110], and [111] symmetry directions compare well with recent available experimental data.