bluuebunny/crossref_metadata_2025 huggingface dataset:
Raman scattering in mixed crystals
Authors: Martin T. P.
Published: July 2007

A simple model is used to describe the relative intensity of Raman scattering in mixed crystals with two mode behaviour. The local susceptibility, both linear and nonlinear, is assumed to be a sum of concentration independent atomic contributions. The four independent nonlinear model parameters are determined by the electro‐optic and second harmonic generation coefficients of the pure component materials. The concentration dependence of the Raman intensity of a longitudinal mode can demonstrate an antiresonance. Explicit calculations are made for CdS1‐xSex.
Elastic, optical, and dielectric properties and their pressure derivatives of rutile‐structure oxides in a modified rigid ion approximation
Authors: Striefler M. E. , Barsch G. R.
Published: July 2007

The elastic and static dielectric constants and their first pressure derivatives, and the optical mode frequencies and mode gammas at zero wave vector have been calculated for GeO2, TiO2, and SnO2 from a modified rigid ion model, in which first and second nearest neighbor central force repulsive interaction of Born‐Mayer type and oxygen–cation–oxygen angle bending forces are included. The nine free parameters of the model are determined from a least squares fit to the zero pressure values of the optical frequencies and elastic constants. The agreement with the experimental optical and elastic data at zero pressure is very good, and fair to moderate for the pressure derivatives.
Luminescence spectra of nitrogen in solid argon I. Experiment
Authors: Jodl H.‐J. , Bruno R.
Published: July 2007

The X‐ray stimulated luminescence spectra of Ar crystals having 0.01% and 5% N2 impurities were studied at 4.2 and 77 K, in the region of the Vegard‐Kaplan (A3Σ → X1ΣΣ) bands. Each band of the 5% N2/Ar system, at 4.2 K, was composed of three, almost equispaced, peaks. The fine structure was defined by a two‐peak asymmetric sideband, between 25 and 180 cm−1, to the red of the N2 electronic‐vibrational transition. At other impurity concentrations and crystal temperatures, different relaxation rates were found but no fine structure was observed. The whole Vegard‐Kaplan band system for N2/Ar at 4.2 K was found to be shifted by (128 ± 15) cm−1, to the red of V00,. the free molecule transition.
Observation of CESR in high‐purity aluminium by a reflection spectrometer
Authors: Janssens L. , Stesmans A. , Cousins J. , Witters J.
Published: July 2007

The observation of CESR in a 6N aluminium sample using a reflection spectrometer operating at 21 GHz is reported. The main characteristics of the signal are a g‐factor close to 2 and a linewidth varying from 52 G at liquid He temperature to 190 G at 150 K. A minimum of 38 G is found near 20 K. The results are compared with the many body effect theory of Fredkin and Freedman. Good quantitative agreement is not found with the results of Lubzens et al. The results also indicate a g‐value variation in the phonon dominated regime.
Hydrogen vibrations in strontium chloride
Authors: Hayes W. , Wiltshire M. C. K. , Jtjmeatj D. , Taurel L.
Published: July 2007

The sideband structure of the local mode of H− in SrCl2 has been measured. A simple lattice dynamical model including a change of mass at the impurity site and a change of nearest‐neighbour central force constant has been used to calculate this structure. Lattice Green functions computed from the shell‐model parameters of Kuhner et al. were used in the calculation. With this model, agreement with the H− and D− local mode frequency was obtained for central force constant reductions of 53% and 56%, respectively. The sideband structure contains a resonance at 50 cm−1. A reduction of 32% in the central force constant was required to obtain agreement between the calculated and observed frequencies.
The role of a double oxygenous bridge in radiation‐induced centre formations in glasses from EPR data
Authors: Cherenda N. G. , Yudin D. M.
Published: July 2007

Experimental data for effects of various penetrating radiations on oxidic glasses and of changes in the isotopic composition of the glasses on induced electron paramagnetic resonance (EPR) spectra are presented. It is found that the present interpretation of investigation data of radiation centres in irradiated glasses is inadequate. A mechanism of paramagnetic centre formation which is different from an electron–hole mechanism and which allows to interpret the experimental data correctly is proposed.
A general formulation of the dynamical theory of electron diffraction including absorption
Authors: Andrew J. W. , Sheinin S. S.
Published: July 2007

Generalized perturbation theory has been used to derive an exact solution to the matrix form of the equations of the dynamical theory of electron diffraction including absorption. It has been shown that the introduction of the imaginary potential leads to a mixing of the unperturbed Bloch waves characteristic of the system without absorption. Numerical calculations have been presented which show that it is important to take the mixing of Bloch waves into account in high atomic number materials and when quasi degeneracy of the kinetic energies of the unperturbed Bloch waves is obtained.
The role of the spin‐spin interaction reservoir in nuclear relaxation
Authors: Skrebnev V. A.
Published: July 2007

The role of the dipole–dipole interaction reservoir (DDR) of impurity ions in the nuclear relaxation is considered based on the idea of the interaction temperature between the nuclear and the dipole–dipole systems. The condition in which the DDR can be heated and the effect of this heating on nuclear relaxation are investigated. The expression has been obtained which takes into account the role of transverse components of the magnetic moment of impurity ions in nuclear relaxation.
Effect of impurities on magnetic properties of ferromagnetic and almost ferromagnetic metals
Authors: Strauss M. , Ron A.
Published: July 2007

An investigation is made of the effect of the mean free path due to static impurities on the dynamic susceptibility of ferromagnetic and almost ferromagnetic metals. Using the itinerant electron model, in the random phase approximation the equation of motion of the density matrix – the quantum kinetic equation – is derived in the presence of an external spatial and time‐dependent magnetic field. The impurities are introduced by generalization of the relaxation time approximation and application of the conservation laws to the collision term of the kinetic equation. The linear response of the system to the small external field is calculated by linearizing the kinetic equation, and the dynamic susceptibility is obtained. The dynamic susceptibility, which depends now on the mean free path, enables the investigation of the magnetic properties of the system.
Calculation of EPR spectra of Fe3+ with high zero‐field splitting in polycrystalline materials
Authors: Shcherbakova M. Ya. , Istomin V. E.
Published: July 2007

Expressions of the shape of EPR spectrvim in a polycrystal for a paramagnetic centre with an effective spin S = 1/2 and an anisotropic g‐factor are obtained. Based on these expressions the model spectra of Fe3+ (6S5/2) with a large zero field splitting are calculated as derivative curves of the absorption and the results are analysed.
Binding energies and wave functions of Wannier excitons in uniaxial crystals—A modified perturbation approach II. Applications
Authors: Gerlach B. , Pollmann J.
Published: July 2007

A modified perturbation approach is used to describe anisotropic Wannier excitons. The influence of small external fields on the energy levels is discussed. The whole scheme is examplified in the case of GaSe and CdSe; anisotropy parameter and masses can be calculated in a consistent way.
Properties of a 5T2 in iron (II) under the action of arbitrary‐symmetry ligand fields and spin‐orbit coupling I. On the quadrupole splitting
Authors: Zimmermann R. , Spiering H.
Published: July 2007

The quadrupole splitting of the ground state 5T2 of high‐spin iron(II), split by a ligand field and spin–orbit coupling, has been treated. It is shown to depend up to a constant factor only on the energy splittings Δ1 and Δ2 of the orbital states, the spin–orbit coupling constant λ, and the temperature, but not on the orbital eigenstates of the ligand field. The well‐known method of fitting ligand field parameters from the temperature dependence of the quadrupole splitting now can be applied even in the case of low symmetry ligand fields without having trouble with too many parameters. From the temperature dependence of the quadrupole splitting it can be only differentiated between cubic (Oh, Td, …), axial (D3, D4, D2d, …) or lower symmetry (D2, C2v, C2, …, C1) ligand fields. A formula for calculating the components of the EFG in low symmetry ligand fields is derived.
The effects of a variable field of differential frequency on spin‐system dynamics
Authors: Skrebnev V. A.
Published: July 2007

Effects are investigated arising from the superposition of a field with a frequency equal to the difference of the resonance frequencies of two kinds of magnetic centres in a sample. The external variable field is regarded as a sub‐system with infinite temperature. The cross‐relaxation between the two kinds of magnetic centres is considered as to be sufficiently pronounced. The conditions have been found under which it is possible to observe resonance effects, and their role in the spin‐system dynamics is considered.
Luminescence spectra of nitrogen in solid argon. II. Theory
Authors: Bruno R. , Jodl H. J.
Published: July 2007

A theoretical investigation of the X‐ray stimulated spectra of N2/Ar has been made. The fine structure of the N2/Ar sideband is identified as the combined result of host‐lattice vibrations, an impurity induced local mode, and multiphonon processes. The width of the peaks is attributed to hindered rotations of the trapped molecule. The details of the matrix red shift are explained via Buckingham's formulation and the effects of hindered rotations.
A theory of DC‐hopping conductivity accounting for local electrical fields and electron‐phonon interaction of arbitrary strengths
Authors: Böttger H. , Bryksin V. V.
Published: July 2007

Starting from first principles the dc‐hopping conductivity in a disordered one‐band tight‐binding model is investigated. Equations are derived for the current which represent a generalization of Kirchhoff's equations for a steady state random network problem. These equations hold for local electrical fields of arbitrary strength and for arbitrary strong electron–phonon interaction.
The g‐factor of the conduction electrons in Bi2Se3
Authors: Köhler H. , Wöchner E.
Published: July 2007

The oscillatory part of the magneto‐resistance (Shubnikov‐de Haas effect) was investigated on n‐Bi2Se3 single crystals with carrier concentrations between 5 × 1017 and 1 × 1018cm−3 in the temperature range from 1.6 to 4.2 K in pulsed magnetic fields up to 35 T (350 kG) magnetic induction. Most experiments were performed for the magnetic induction B parallel to the trigonal axis c of the crystals, where the smallest cyclotron mass mc and a parabolic ε(κ) relation is present. No spin‐splitting of the Shubnikov‐de Haas resistivity extrema is observed at high quantum numbers. At low quantum numbers the influence of spin‐splitting causes a shift of the resistivity extrema and an additional maximum, which should not be present for M = gmc/(2m0) ≪ 1. A calculation of the magnetoresistance in quantizing magnetic fields based on Kubo's theory for a field‐independent carrier concentration enables the determination of the expression M = gmc/(2m0) = 2 with ±5% uncertainty for B‖ c. The absolute value of the g‐factor is evaluated hence as (32 ± 3) for B‖ c. For arbitrary crystal orientation in the magnetic field M may be assumed to be constant within the error margin, the g‐factor being consequently smaller for B ⟂ c: g ⟂ = (23 ± 3).
Thermopower calculation for variable range hopping — application to α‐Si
Authors: Overhof H.
Published: July 2007

An expression for the thermopower S in the variable range hopping regime near the Fermi energy is derived. This allows the calculation of S for an arbitrary density of states, provided the localisation length, α−1, is kept constant. An application is made to estimate the density of states function of amorphous Si from conductivity and thermopower data.
Method of calculation of a typo of coherent electronic fluctuations connected with the viscosity of the electronic gas of a solid
Authors: Haymann P.
Published: July 2007

In the frame of optical potential the interaction of a fast electron beam with crystalline targets is considered in second order approximation. It is supposed that only the electrons produced during electron–hole interactions can be scattered. These electrons possess a behaviour of a quasi‐particle with a definite momentum. Then a calculation of the imaginary part of the crystalline pseudopotentiel coming from the electronic transitions is given.
Optical absorption edge of CdAs2‐based glasses
Authors: Voelïček V.
Published: July 2007

The results of the absorption edge study of some glasses based on CdAs2 are discussed in terms of internal potential fluctuations. Both the electrostatic and deformation fluctuations are present in a‐CdGeAs2 whereas in remaining glasses only the former were observed. A qualitative description of the electronic properties of these glasses offers Boer's real‐glass model. Its applicability limits are discussed. At elevated temperatures, the edges formally obey the Urbach rule. Possible reasons for this behaviour are considered.
Additional long‐ and short‐range potentials for direct excitons in zincblende structure solids: I. General correction formula
Authors: Ekardt W.
Published: July 2007

Starting from the general equation derived for the evaluation of corrections a calculation is made within the ladder approximation and with restriction t o the particle‐hole part of the exciton amplitude, of all correction potentials which arise as a result of the expansion of the effective Bethe‐Salpeter equation (BSE) in terms of three independent translation vectors e.g. K – K′, K′ and q up to the order |wavevector|°. This means a,n expansion up to EB/Egap or EB/hωLO taking into account spatial dispersion, local field corrections, dynamical corrections, exchange interaction etc. The results are evaluated for eleven III–V or II–VI zincblende semiconductors using calculated dielectric constant values and interband oscillator strengths. Comparison with experiment is quite satisfactory.
Nuclear spin dynamics in the presence of electronic relaxation
Authors: Bosse J. , Gabriel H. , Vollmann W.
Published: July 2007

Redfield's relaxation theory, which essentially includes all treatments published so far. has been expressed in a form which is independent of the explicit representation. The relaxation operator is given as an expression sufficiently general to deal with various types of experiments sensitive to the dynamical behaviour of a small subsystem coupled to its environment. e.g. NMR, PAD, ME. Special attention has been paid to the treatment, of a spin system of cylindrical symmetry which relaxes via magnetic dipole interaction with the surroundings. The fast and slow relaxation regime as well as the transition between them is considered. As an application the Mössbauer relaxation spectra of FeNH4(SO4)2·12H2O are discussed.
Lattice dynamics of covalent ionic compounds (I)
Authors: Altshuler A. M. , Vekilov Yu. Kh. , Izotov A. D.
Published: July 2007

The dynamics of covalent ionic crystals in terms of the “dielectric model” is analysed. Non‐diagonal elements of the screening matrix correspond to both the bonding charge and the anion localized charge. The dynamic matrix reflects the screened Coulomb interaction between the ions and the point bonding charges, the short‐range part being described by two parameters. The phonon dispersion in high symmetry directions of the Brillouin zone and the elastic constants are calculated for Si, GaAs, ZnS, and CuCl. It is shown that the interaction between the localized charges should not be taken into consideration in calculations of the crystal energy.
The role of annealing on the optical absorption edge of CdAs2‐based glasses
Authors: Vorlíček V.
Published: July 2007

The influence of annealing on the absorption edge of some glasses based on CdAs2 was studied. Samples annealed at t<tg (tg, being the softening temperature) behave similarly as non‐annealed samples. Their properties are discussed in terms of the CS concept. Samples annealed at t ≈︁ tg recrystallize. The influence of disordering on the band structure of compounds CdAs2, and CdGeAs2 is discussed.
A high pressure search for K′‐bands in KBr and KI
Authors: Mamola K. C.
Published: July 2007

The optical F‐band in KBr has been studied at pressures up to 11 kbar and in KI up to 9 kbar. No new absorption bands were found to appear nor was any other anomalous behavior observed. It is thus shown that so‐called K′‐bands observed by previous investigators and the K′‐center to which they were attributed do not in fact exist in crystals subjected to hydrostatic pressure.
On the theory of the Wannier‐Mott excitons in disordered semiconductors
Authors: Bonch‐Bruevich V. L. , Iskra V. D.
Published: July 2007

Two problems of the Wannier‐Mott exciton theory in disordered semiconductors are considered: a) limitation of the exciton life‐time due to the second kind collisions with the random field and b) influence of the excitonic effects upon the interband light absorption and emission. The first factor is shown to be of interest in some photoconductivity experiments while being relatively unimportant in interband optics. The role of the second factor depends upon the relation between the exciton binding energy and the characteristic energy of a charge carrier in a random field: the slope of an exponential optical tail depends upon which of the two energies is dominating.
Effect of the trigonal field and a uniaxial stress on the vibronic triplet states of (3d)n ions in α‐Al2O3
Authors: Lacroix R. , Duval E. , Champagnon B. , Louat R.
Published: July 2007

The local strain around the cation is expressed in function of the ion displacements in the cluster. It is shown that this local strain is related to the applied uniaxial stress by “local elastic compliances” which can be different from the crystal elastic compliances. The strain Hamiltonian which represents the ion‐strain interaction is given for any direction of pressure. Experimental parameters are defined, allowing to interpret the effect of the uniaxial stress on the transition between the fundamental vibronic singlet and the three lower vibronic states originating from T1 or T2 states in cubic symmetry.
Double reflection dips from grating ruled semiconductors
Authors: Teng L. F. , Alexander R. W. , Bell R. J. , Fischer B.
Published: July 2007

The double reflectivity dips, previously observed by Fischer et al. and Anderson et al., which appeared in the reflection spectra of grating surfaces on, the Te‐doped semiconductors GaAs and InSb around both the plasmon and phonon frequencies have been measured in more detail. In the plasmon region, several possible explanations of the phenomenon are discussed, but the favored explanation involves surface damage. A simple two‐region reflectivity equation checked with a rigorous grating theory is proposed and is shown to fit the data well.
The optical properties of LiNbO3
Authors: Bärner K. , Braunstein R. , Weakliem H. A.
Published: July 2007

The optical constants of LiNbO3 from 0.3 to 14.5 eV are presented from a Kramers‐ Kronig analysis of the near normal incidence reflectivity spectrum. Comparison with the cubic perovskites suggests that the electronic transitions from 3.5 to 12 eV are between the O2‐2p valence band and Nb4+ d‐like conduction band states. Higher transitions are most likely into Li+ 2s states. Sum rules are applied to calculate the effective number of electrons involved; this number is found to be consistent with the above assignment. Also, the combined density of states function is found to be very similar to that calculated for the analog compound KTaO3.
Temperature dependence of Pb3+ EPR spectrum in irradiated calcite
Authors: Popescu F. F. , Grecu V. V.
Published: July 2007

The spin‐Hamiltonian parameters and the line widths of Pb3+ in CaCO3 have been measured over the temperature range 77 to 300 °K. The hyperfine interaction is very large so that the intermediate field approximation was used. The hyperfine interaction decreases as the temperature increases, in agreement with the effect of lattice‐vibrations on the ground state. The hypothesis of a large‐amplitude resonance mode is used for explaining the qualitative temperature dependence. Extra line widths is described to the presence of hyperfine interaction of the 207Pb isotope. A dynamic process is assumed which is related to the change of spin−lattice relaxation via phonon modulation of the hyperfine interaction.
A contribution to the discussion on the relaxed excited F centre
Authors: Bosi L. , Cova S. , Spinolo G.
Published: July 2007

An interpretation is given of the behaviour of the relaxed excited F centre in terms of a hydrogenic model with broad wave functions. In particular the dependence is considered of the lifetime τ on the host crystalline matrix at a fixed low temperature, and on the temperature T for a given matrix. Precision experimental data are given for τ(T) in NaCl, RbCl, and KBr.
Parallel pumping of magnetoelastic waves
Authors: Goncalves L. L. , Zagury N.
Published: July 2007

The magnetoelastic wave excitations driven by a microwave field applied parallel to the strong dc field in the region of the cross over are investigated. The power absorption arid the critical field are obtained by analyzing the direct excitation and damping of the eigenstates of the system.
Magnetic properties of superconductors with uniaxial symmetry
Authors: Takanaka K.
Published: July 2007

In type‐II superconductors with uniaxial symmetry the three critical fields (Hc1, Hc2, Hc3) are calculated near the transition temperature Tc using the anisotropic Ginzburg‐Landau equations. It is shown that in general an induced magnetic field δH⊥ perpendicular to the applied magnetic field exists and that the anisotropy removes the degeneracy in the direction of the flux‐line lattice. The pattern and the orientataion of the flux‐line lattice are in good agreement with the experimental results of technetium.
Electroabsorption and electroreflectance of PbI2
Authors: Daunois A. , Deiss J. L. , Nikitine S.
Published: July 2007

Electroabsorption and electroreflectance spectra of thin and thick single crystals of PbI2 have been investigated at 85 and 4.2 °K. In the vicinity of the fundamental edge, the main excitonic series is well resolved in both spectra. An important temperature dependence is observed for this series. Different electroabsorption or electroreflectance structures are also studied beyond the fundamental edge.
Lattice dynamics of rare‐gas solids by Kihara's potential: II. Specific heat and thermal expansion
Authors: Agrawal T. N. , Gupta R. K. , Gupta G. L.
Published: July 2007

The specific heat and thermal expansion of rare‐gas solids have been computed by studying the lattice vibrations of these solids using Kihara hard core potential. The theoretical calculations have been compared with the other theoretical results and available experimental data.
Many‐phonon vibronic spectra of molecular crystals: II. Combination frequencies
Authors: Lalov I. J.
Published: July 2007

Vibronic spectra of molecular crystals with two phonons of different types (compound tones) are investigated in the weak coupling approximation. The foIIowing cases are considered: a) one of the phonons corresponds to a totally symmetrical vibration with linear vibronic coupling constant η2= 1; b) the purely electronic transition in the molecule is forbidden and one of the phonons corresponds t o a non‐totally symmetrical vibration; c) a two‐phonon vibronic spectrum with one totally symmetrical phonon and one non‐totally symmetrical phonon. In a) and b) the problem is reduced to a two‐particle problem with a known solution, while in c) the bands of the one‐particle states are found and the absorption taking into account the role of anharmonicity of intramolecular vibrations.
The influence of an electric field on the auger recombination in semiconductors
Authors: Gebranzig U. , Haug A. , Rosenthal W.
Published: July 2007

Auger recombination in semiconductors is sometimes affected by an electric field. Proceeding from Bloch electrons in such a field the transition probability in this case is calculated. The result shows that electric fields of the order of 103 V/cm or 105 V/cm enhance the recombination probability in III−V‐compounds remarkably; the first value holds for low temperatures (T ≈ 10 K), the second one for high temperatures (T ≈ 300 K).
ESR study of the paramagnetic 10 centre in an α‐LiIO3 crystal
Authors: Martirosyan V. O. , Meilman M. L. , Marov I. N. , Shukov V. V.
Published: July 2007

In crystals of a polar phase of α‐LiIO3 an ESR spectrum at a frequency of v = 34.633 GHz and T = 290 °K is observed. The spectrum consists of two sextets. Each line of the sextet splits into a doublet and a multiplet. The spectrum is attributed to two centres of IO with an equivalent structure. It is shown that a 27‐electron centre, IO, is formed from 10 ions under γ‐irradiation. The charge compensation is based on the oxidation of Fe2+ present in crystals as an impurity. Experimental values of g‐tensor, hf splitting and shf splitting are determined. The values of spin‐Hamiltonian parameters and the mechanism of the centre formation and its location in the crystal are discussed.
Impurity‐induced transitions in differential spectroscopy
Authors: Mathieu H. , Camassel J. , Auvergne D.
Published: July 2007

Careful investigation of differential spectra superimposed on interference phenomena permits one to obtain original information on impurity states localized in the gap. The piezomodulation technique used to obtain homogeneous modulation is a complementary test of luminescence experiments in identifying impurity states. Two examples of investigations on GaSb and ZnTe, are given.
Far‐infrared absorption of electron–hole drops in pure and doped germanium
Authors: Timusk T. , Silin A.
Published: July 2007

Electron–hole drops in doped germanium are found to give an absorption at long wavelengths. A study of the neutral donor absorption, the near‐infrared luminescence, and the movement of electron–hole in a magnetic field gradient shows that in the doped materials the drops are very small and localized on the neutral donors.
Polarization dependence of indirect two‐photon transitions in AgCl
Authors: Popova M. N. , Pelant I.
Published: July 2007

The polarization dependence has been calculated for the allowed, indirect, phonon assisted two‐photon transitions L3 → Γ1 and Σ4 → Γ1 which are most important in the case of AgCl. The polarization dependence of the two‐photon absorption of ruby laser light in AgCl single crystals has been measured and found to belong to the Eg symmetry of the point group Oh. Comparison with the presented theory leads to the conclusion that the Σ4 → Γ1 transition with participation of the Σ1 phonon takes place.
Magneto‐optical investigations of the two‐dimensional ferromagnet (CH3NH3)2CuCl4
Authors: Arend H. , Schoenes J. , Wachter P.
Published: July 2007

The optical absorption of (CH3NH3)2CuCl4 has been investigated at 300 K in the spectral range between 0.2 and 40 μm and down to 4.2 K between the optical absorption edge and 2.5 μm. Absorption has also been measured with polarized light, parallel or perpendicular to the crystal axes or a magnetic field. In addition the magnetic linear dichroism and the Faraday rotation has been investigated in function of frequency, magnetic field strength and temperature, as well above as below TC = 8.9 K. From the Faraday rotation one can evaluate the magnetization for T < TC and the susceptibility for T > TC. The discussion is made in terms of critical exponents of the susceptibility. A cross‐over region could be determined above, but close to TC. The system changes from three dimensionality to two dimensional Ising. At higher temperatures only molecular field behavior is observed.
Effect of electric field on mixing of Frenkel excitons and charge transfer states of a linear molecular crystal
Authors: Petelenz P. , Zgierski M. Z.
Published: July 2007

The mixing between Frenkel excitons and charge‐transfer states of a linear molecular crystal in an electric field is investigated. The Merrifield model is adopted. It is shown that the electric field increases the above mentioned mixing, leading therefore to an increase of the light absorption with simultaneous creation of an electron–hole pair. It also can change slightly the position of the Frenkel exciton absorption line. Semiempirical calculations of anthracene dimers in an electric field show that this effect should be observable at electric fields with strengths higher than 106 V/cm, the direction of the field being along the long c‐axis of the anthracene crystal.
Modification of phonon distribution in degenerate semiconductors by high electric fields: Effect on transport coefficients
Authors: Sharma S. K. , Agarwal D. B.
Published: July 2007

Drift mobility, Hall mobility, and Hall coefficient of a degenerate semiconductor at very low temperatures and high electric fields are calculated. Following Paranjape's approach the modified distribution of phonons are taken into account in the calculation of the transport coefficients for arbitrary degeneracy of simple model semiconductors. The electron scattering due to ionized impurities as well as due to acoustic phonons have been considered. The calculations show that the inclusion of the modified phonon distribution in the theory reduces the theoretical values of drift and Hall mobilities of degenerate semiconductors by a factor of two or more; its effect on Hall coefficients is to decrease the theoretical value slightly (20%).
Zone‐centered phonons in ternary compounds of chalcopyrite structure
Authors: Bettini M.
Published: July 2007

A Keating model calculation of the zone‐centered phonons in ternary compounds of the chalcopyrite (ch) structure is presented. The calculations are restricted to phonons of the phosphor(P) ch‐family. The model describes reasonalby well the phonons of high energy. The spectra are determined preponderantly by the masses of the cations and the force constants between nearest neighbours. The influence of the force constants between second nearest neighbours and the effective charges is small. The dependence of the individual phonons upon changes in the model parameters is unravelled. The variation of the ir‐oscillator strength of the same mode in different compounds demonstrates the importance of changes in the eigenvectors. The force constants of the chalcopyrite (ch) bonds are compared to those of corresponding zincblende (zb) bonds and can be explained by changes in the bond length. The model calculations yield a symmetry assignment of the experimental spectra in CdSnP2 and ZnSnP2.
Theory of the absorption of electromagnetic radiation by hopping in n‐type Si and Ge considering all polarizations of acoustic phonons
Authors: Banaszkiewicz M.
Published: July 2007

The theory of the absorption of electromagnetic radiation by hopping in n‐type Si and Ge is investigated. Photon wavelengths from 100 to 800 μm for Si and from 500 μm to 2.5 cm for Ge are considered. Both longitudinal and transversal acoustic phonons are taken into account. It is shown that the absorption coefficient α(1) for absorption of a photon with emission of a phonon can be neglected in comparison with the coefficient α(0) describing the absorption of a photon without emission of a phonon. A new averaging method in the computations of α(0) is used. The agreement between the numerical results and the experimental data is good except of the region of short wavelengths.
Determination of the electron excitation spectrum in scandium and yttrium by means of characteristic energy loss measurements
Authors: Brousseau‐Lahaye B. , Colliex C. , Frandon J. , Gasgnier M. , Trebbia P.
Published: July 2007

Energy loss spectra of high energy electrons transmitted through thin foils of scandium and yttrium have been registered using two different experimental techniques. A detailed comparison of the results allows to distinguish between the spectra of metals, hydrides, and oxides and to calculate the optical constants in the pure metals. It is shown that the electron excitation spectra of these elements contain two main contributions in the energy range between 5 and 60 eV: collective excitations of the conduction electrons at 14 and 12 eV (plasmon peaks) and a strong absorption band due to the excitation of the inner 3p and 4p electrons between 30 and 50 eV. At higher energies, these spectra display well‐defined lines corresponding to the L2−3 edge in scandium and to the M4−5 and M2−3 edges in yttrium which are characteristic of the atom.
Recombination luminescence in alkali halides doped with divalent cations: (Vk + e)z‐type emission
Authors: Ikeya M.
Published: July 2007

Recombination luminescence of alkali halides doped with divalent cations has been studied during X‐irradiation at several temperatures. Besides the emission bands from the excited states of the impurity, four types of additional bands were observed. These emission bands were tentatively assigned to (X2− + e) recombination perturbed by divalent cations on the basis of a model of (X2− + e) in which nearby cation orbitals are involved.
On the pressure dependence of the electrical conductivity of aluminium
Authors: Claesson A. , Larsson† R.
Published: July 2007

The electrical conductivity of metals depends on the shape of the Fermi surface. This surface for nearly‐free‐electron‐like substances like aluminium is spherical almost everywhere. However, the lattice structure gives rise to changes in curvature at the zone planes. The effect of hydrostatic pressure up to 100 kbar on the electrical conductivity of aluminium is investigated taking the detailed shape of the Fermi surface into account.
Investigation of the conduction band fermi surface in Bi2Se3 at high electron concentrations
Authors: Köhler H. , Fischer H.
Published: July 2007

An investigation is made of non‐parabolic terms in the ϵ(k) relation up to the order of [k4] at high electron densities up to 4 × 1019 cm−3. At low electron densities (<1018 cm−3) the Fermi surface is almost an ellipsoid of revolution with its symmetry axis along the trigonal kc axis. Besides, with increasing Fermi energies, a flattening of the Fermi surface in the direction of the kc axis was found, caused by [k4] or higher terms in the ϵ(k) relation. Furthermore the influence of trigonal warping of the Fermi surface was already observed at a concentration of 1.7 × 1018 cm−3 from Shubnikov‐de Haas effect. Here by means of the Shubnikov‐de Haas effect the extremal cross‐sectional areas of the Fermi surface were determined by application of stationary magnetic fields up to 15 T magnetic induction. The parameters of the assumed ϵ(k) model up to [k4] terms were fitted by numerical calculations of the extremal cross‐sectional areas of the Fermi surface perpendicular to arbitrary magnetic field directions and by comparison with the corresponding reciprocal Shubnikov‐de Haas periods.
Multiple s‐scattering in an ideal random medium. I. The coherent field in the low‐density limit
Authors: Lenk R.
Published: July 2007

Starting from the multiple scattering hierarchy the low‐density solution for point‐like scatterers is constructed. A given scatterer interacts with itself via the medium consisting of all the other scatterers. All pair effects are neglected. A new and simple result has been found for the attenuation of the wave by elastic scattering. The damping mechanism in the total reflection regime represents an unsolved problem. The relation of the results obtained to the behaviour of ultracold neutrons is discussed.
On the hyperfine structure of the Mn2+ EPR line with g = 4.3 in chalcogenide glasses of some ternary systems
Authors: Lazukin V. N. , Chepeleva I. V. , Zhilinskaya E. A. , Chernov A. P.
Published: July 2007

EPR of Mn2+ ions in glasses of ternary systems MAsSe (M = Cu, Zn, Ge, Ag, Hg, Tl, Pb, Bi), PGeSe, IAsX (X = S, Se, Te) of different compositions and in glassy As2Se3 are studied. The dependence of the Mn2+ EPR spectra on frequency, temperature, impurity concentration, and glass composition is observed. The complex HFS structure of the g = 4.3 line is explained by superposition of two sextets with a shift of the isotropic g‐factor of ≈︁0.08. The model of the Mn2+ incorporation into the glass network is proposed. A high regularity of Mn2+ surroundings is explained by a high degree of short range order in the glasses investigated.
Phase diagram for the conventional Hubbard model system
Authors: Röpke G. , Albani B. , Schiller W.
Published: July 2007

For the conventional Hubbard model system, the static magnetic susceptibility and the local magnetic moments are studied by means of the functional integral technique. Expanding functional integrals in the lattice space, the magnetic susceptibility, and the local moment are represented by graph series. Looking for the dependence of magnetic properties on the parameters of the model Hamiltonian, a phase diagram is obtained. The stability of the phases is considered. For special parameter values of the model system the phase diagram can be compared qualitatively with the diagram obtained experimentally for V2O3.
Diffusion drift paths in the core region of an edge dislocation
Authors: De Hosson J. Th. M. , Sleeswyk A. W.
Published: July 2007

Using the Johnson potential, the atomic configurations around perfect dislocations with 〈100〉Burgers vector in Ni and γ‐Fe were calculated using the relaxation procedure of Bullough and Perrin. Johnson's carbon–metal potential was used to calculate paths of maximum energy gain between octahedral sites. Four rows of sinks were found, in regions where the shear stress, the compressional and dilational stress predominate. Around each sink is a region draining into it. The binding energy of the C‐atom is strongest for the dilational sink: 0.21 and 0.11 eV for Ni and γ‐Fe, respectively. The imperfect 1/2 〈100〉 dislocation, which is a Frank partial, was also investigated for Ni. The binding energy of the C‐atom to the stacking fault sites is 0.74 eV (Suzuki effect).
The Lβ2valence band spectrum of niobium in the metal and in the α,β two‐phase region of the NbH system
Authors: Gilberg E.
Published: July 2007

The Lβ2valence band spectrum of niobium in the metal and in the α,β two‐phase region of the niobium‐hydrogen system was investigated using a high resolving bent crystal spectrograph with photoelectric registration. The spectrum was excited in fluorescence. The atomic ratio [H]/[Nb] varied between 0.45 and 0.70. The spectrum of the niobium metal has the shape of an asymmetric line with long tailing on the low energy side. In the NbH system a new emission band arises within the low energy tailing; in addition the line is slightly shifted towards higher energies. It is suggested that the new emission band is due to transitions from an energy band which is responsible for the NbH bonding.
On the theory of Hc2 anisotropy in cubic superconductors
Authors: Teichler H.
Published: July 2007

The anisotropy of the upper critical magnet field Hc2 of cubic type‐II superconductors with anisotropic Fermi surface and anisotropic attractive interaction is investigated. For a separable model interaction explicit results are given on the temperature and impurity dependence of the Hc2 anisotropy of impure metals in the „quasilocal”︁ regime and of clean metals for arbitrary temperatures below Tc. The clean‐limit results agree well with experimental data on Hc2 anisotropy of high‐purity Nb.
On the oscillator strength of F centers in alkali halides
Authors: Bosi L. , Fantola‐Lazzarini A. L. , Lazzarini E. , Ramaglia V. Marigliano , Tagliacozzo A.
Published: July 2007

A radiochemical method, previously applied to NaCl, KCl, and KBr crystals has been used to determine the oscillator strength ƒ of F‐centers in RbCl. By extending theoretical calculations carried out by Smith and Spinolo, and by Iadonisi and Preziosi, estimates of ƒ for several alkali halides, including those previously mentioned, have been computed. The experimental results agree very closely with those obtained by Rauch and Heer and are in good accord with the theoretical estimates (the theoretical results are reported in brackets). ƒNaCl= 0.64 ± 0.02 (0.57); ƒKCl= 0.68 ± 0.02 (0.61); ƒKBr= 0.72 ± 0.02(0.57); ƒRbCl= 0.51 ± 0.05 (0.57). The experimental and theoretical results seemed to bear each other out and led to the conclusion that the ƒ values given in the literature, except those by Rauch and Heer, are overestimated.
The critical voltage effect in transmission electron microscopy. III. Influence of weak beams on degeneracy
Authors: Gevers R. , David M. , Serneels R.
Published: July 2007

In previous papers a theory of the critical voltage effect has been proposed based on a three‐beam treatment. The influence of further supplementary beams is considered using a general perturbation treatment. Two cases are considered: (i) the supplementary beams are all weak; (ii) there is a single non‐systematic strong perturbing beam. It is shown that the degeneracy may be retained, provided one changes in a proper way the illumination conditions, i.e. the accelerating voltage and possibly the crystal orientation. Perturbation calculus enables to obtain the shift of the critical voltage and of the excitation errors, as the sums of series in ascending powers of the Fourier coefficients (of the crystal potential) associated with the supplementary beams.
Model‐pseudopotential calculations of Grüneisen parameters, specific heat, and binding energy of sodium and potassium
Authors: Kushwaha S. S. , Rajput J. S.
Published: July 2007

Calculations of the specific heat, Grüneisen parameter, binding energy, and compressibility of sodium and potassium are made using a two‐parameter model potential. The theoretical results are in qualitatively good agreement with available experimental results.
Hyperfine interactions in high‐silicate and quartz glasses
Authors: Cherenda N. G. , Shendrik A. V. , Yudin D. M.
Published: July 2007

EPR measurements of HFS of nuclei 1H, 2D in spectra of quartz glasses and 29Si in spectra of high‐silicate glasses have been made and analysed. A stochastic model for the γ‐radiation‐induced paramagnetic centres are suggested. Calculations of the electronic structure of the unit AB4 have been carried out and a correlation diagram between the orbitals possible for tetrahedral (Td) and pyramidal (C3v) configurations has been obtained. This enables one to make comments on the EPR spectra of the units SiO43− and ROSiO32−.
A Quantum Spin Wave Theory for an Ising Ferromagnet in a Transverse Magnetic Field
Authors: Filatova L. D. , Tsukernik V. M.
Published: July 2007

With consideration of the Ising model a single‐axis ferromagnet is described to which a steady magnetic field is applied perpendicular to the anisotropy axis. The dispersion law for elementary excitations of such a system are derived in the extreme cases of weak and strong magnetic fields. It is shown that in the case of weak magnetic field an essential difference between quantum and classical results exists contrary to the case of strong magnetic field.
Ruby‐Laser‐Excited Delayed Fluorescence of Crystalline Tetracene. Combined Effects of Trap Saturation and Bimolecular Processes
Authors: Delannoy P. , Schott M.
Published: July 2007

The temporal decay of ruby‐laser‐excited delayed fluorescence of tetracene crystals is discussed. It is shown to be sensitive to minute amounts (≪ 1 ppm) of trapping centers, probably pentacene, whose lifetime may have been mistaken for the free triplet lifetime. General equations for the decay of the host delayed fluorescence in the presence of traps, trap saturation, and bimolecular processes, are given; their application to tetracene yields trap parameters and a lower limit of the free triplet lifetime, in the crystals studied. The two‐photon generation rate constant of singlets at 6943 Å is about 1.5 × 10−27cm s, a value comparable to those found in naphthalene and anthracene at corresponding energies.
The Effect of Uniaxial Stress on the Mutual Diffusion in the AgAu System
Authors: Gorskii V. V. , Dekhtyar M. I. , Yadekhtyar I.
Published: July 2007

Considerable decrease of the diffusion rate in the binary AgAu system was observed on applying pressure in the direction perpendicular to the contact zone. Making use of thermodynamic relations the above mentioned effect is shown to be connected with the increased degree of the localization of electrons near the atoms of each kind. A tentative explanation of this phenomenon is suggested.
The Generation of Individual Phonons and Photons under the Influence of Raman Paramagnetic Relaxation
Authors: Alekseev A. V. , Kopvillem U. Kh.
Published: July 2007

The relaxation of isolated spins in phonon‐photon fields is discussed using a two‐quantum incoherent relaxation process. It is shown that under the conditions of acoustic pumping the spins spontaneously generate photons and that electromagnetic pumping supports the phonon generation in the spontaneous regime in a controlled manner. Single quanta are urgently needed for testing the sensitivity of many quantum devices and masers and are also of interest for modelling the cosmic space. The processes discussed here may be applicable to the construction of two‐quantum masers and quantum counters.
Herzberg‐Teller Interaction and Vibronic Coupling in Molecular Crystals IV. Interaction of Two Exciton States via a Totally Symmetric Vibration
Authors: Zgierski M. Z.
Published: July 2007

The three‐level model of a molecular crystal with two exciton bands coupled by a totally symmetric vibration according to the Herzberg‐Teller interaction mechanism is discussed. The absorption spectrum to a lower, narrow exciton band is investigated. The formula for distribution of intensity among different vibronic lines is obtained and discussed. It is shown that this distribution depends not only on the Herzberg‐Teller coupling constant, the distortion parameter in the lower band, and the ratio of the intensities of the two coupled bands, but it is also very sensitive to the difference between distortion parameters in the two bands, as well as to the angle between the transition dipole moments in the molecule. The one‐phonon region of the spectrum is discussed with respect to the existence of vibronic exciton and dissociated exciton—phonon states. General conditions for the existence of different numbers of vibronic excitons in this region of a spectrum are obtained. The changes introduced by the Herzberg‐Teller coupling in the one‐phonon region of the spectrum are illustrated by an example. The general formulas obtained reduce for zero distortion parameters to the formulas describing the coupling of two exciton states by a non‐totally symmetric coordinate, and for zero Herzberg‐Teller coupling constant they reduce to the formulas describing the simple vibronic coupling in a molecular crystal.
Propriétés thermodynamiques et optiques des oxydes alcalino‐terreux
Authors: Galtier M. , Montaner A.
Published: July 2007

Un modèle dynamique décrivant les forces à long rayon d'action sous forme d'interactions dipôles‐dipôles, et les forces à court rayon d'action à l'aide d'intéractions à trois corps, est appliqué aux oxydes alcalino‐terreux de structure NaCl. Les résultats expérimentaux existants (chaleurs spécifiques, absorption infrarouge, diffusion Raman et diffraction des neutrons) sont ensuite comparés aux prévisions théoriques déduites des densités d'état à un et deux phonons. L'accord assez satisfaisant permet de conclure sur la validité du modèle et d'interpréter les spectres optiques.