bluuebunny/crossref_metadata_2025 huggingface dataset:
On fermi resonance theory
Authors: Chisler E. V. , Davydov V. Yu. , Goncharuk I. N. , Ivanova E. A.
Published: July 2007

Studying the resonance coupling between fundamental vibration of the molecule or crystal and one of the combinational tones the number of equations for determining the resonance parameters for nondegenerate vibrations is always less than the number of unknown variables. A method is considered which allows on the basis of one of the vibrational spectra (Raman or infrared) to reveal the resonance coupling and to obtain quantitative information about its parameters in the form of two different sets of numerical data. The choice between these sets demands additional physical information. If both Raman and Infrared spectra are available, it is possible to obtain exact quantitative information about the resonance coupling. The method is based on the study of interaction between a fundamental vibration and a system of combinational tones of the form v ± ω.
Phase transitions in the bilinear organic compounds (TTF, HMTSF)‐TCNQ
Authors: Phillips J. C.
Published: July 2007

A variety of lattice phase transitions and conductivity anomalies are observed in the archetypical bilinear organic conductors TTF‐TCNQ and HMTSF‐TCNQ. The elastic microdomain model (EMM) explains these transitions in terms of alignments of coexisting conducting and insulating domains.
Resonant scattering of exciton‐polaritons in CdS crystals
Authors: Permogorov S. , Travnikov V.
Published: July 2007

The secondary emission of CdS single crystals under monochromatic excitation in the region of exciton resonance is investigated at 77 K. Rather distinct emission lines, 1LO and 2LO, shifted from the excitation energy by the energy of one and two longitudinal (LO) phonons are observed. It is shown that these lines are caused by inelastic scattering of exciton polaritons by optical and acoustical phonons. Possible mechanisms of polariton scattering and the composite character of the observed emission are discussed. The energy dependence of secondary emission efficiency, connected with two‐step scattering of primary polaritons by the exciton‐like states is compared with theoretical calculations.
One‐centre exchange potentials in a crystal (TiC)
Authors: Mokhracheva L. P. , Tskhai V. A. , Geld P. V.
Published: July 2007

A method of constructing the exchange potential in a crystal as a superposition of effective one‐centre exchange potentials is considered. Avaraging is used over atomic wave functions located on different lattice sites. The results obtained are compared with atomic exchange potentials calculated by the Slater approximation and by averaging the exchange term in the atomic Hartree‐Fock equation.
Mixing of frenkel excitons and ionic excited states of a linear molecular crystal with two molecules in the unit cell I. General model
Authors: Petelenz P.
Published: July 2007

The Merrifield theory of the interaction between the neutral and ionic excited states of a linear molecular crystal is generalized for crystals with two molecules in the unit cell. It is demonstrated that the Hamiltonian can be split into three communting terms. For two of them the original Merrifield theory is directly applicable and its conclusions are valid. The third term describes the mixing between the (+) Davydov component of the Frenkel exciton, the (+) intra‐cell CT exciton, and the (+) lengthwise and (+) crosswise ionic pair states. It can no more be treated within the frame of the original Merrifield theory. However it can be further transformed to a renormalized form which consists of two decoupled Hamiltonians. Both of them are formally analogous to the Merrifield Hamiltonian but with renormalized effective parameters. In the result some ionic states become active in the absorption spectrum.
Optical phonons in AgGaSe2
Authors: Miller A. , Holah G. D. , Dunnett W. D. , Iseler G. W.
Published: July 2007

Polarised infrared reflectivity measurements are made on a single crystal of the chalcopyrite compound, AgGaSe2. This is the only selenium I—III—VI2 compound so far investigated for zone centre phonon properties. They are compared with previous studies of lattice dynamics in the sulphide family and its binary zinc blende analogues. Four modes having E irreducible representation are observed between 35 and 75 μm with the light polarised perpendicular to the c‐axis, while two B2 modes are observed in the range 35 to 65 μm with the light polarised parallel to the c‐axis. A classical dispersion harmonic oscillator computer fit is made to the data to give the oscillator parameters.
Localization theory for the random weaire model
Authors: John W. , Schreiber J.
Published: July 2007

The localization theory of Economou and Cohen is applied for the random two‐band Weaire model of amorphous covalent semiconductors. Several approximations for the localization function L(E) are calculated in the case of Lorentzian distribution potential parameters. The behaviour of the mobility edges in dependence on the strength of fluctuations are studied in comparison with the shape of the density of states.
A study of the band structure of graphite based on the rigorous cellular method
Authors: Wegener W. , Fritsche L.
Published: July 2007

A band structure calculation for a single layer of graphite is presented. The computation is carried out from first principles and requires four basic informations, i.e. the lattice constant and the lattice symmetry of the layer, the stacking distance of consecutive layers, and the atomic number of carbon. Altmann's rigorous cellular method is used to obtain the band states and the energy bands. It is shown that this method can be simply modified to include non‐muffin tin parts of the potential to a large extent. Hence, the results may be compared with earlier calculations based on the discrete variational method. As to the agreement with experimental data on the band structure of graphite, both methods seem to work equally well. It is assumed however, that the cellular method has some advantages as regards the accuracy of the wave functions obtained and its wider range of applicability. The results of the present work demonstrate that this method is competitive also for open and highly anisotropic structures and is not restricted to close‐packed or nearly close‐packed lattices, as has often been suspected.
X‐ray spectroscopic investigation of the electronic structure of MgCu alloys
Authors: Neddermeyer H.
Published: July 2007

The Mg Kβ, Mg Kα1,2, and Cu Lα1,2 emission spectra are measured of Mg90Cu10, Mg80Cu20, Mg2Cu, MgCu2, and of the pure metals. In the MgKβ spectra of the Cu‐rich alloys on the low energy side of the main maximum a second maximum is observed, which can be related to the Cu d‐electrons. The position of the MgKα1,2 doublet shows a very small shift to higher energies with increasing Cu concentration. The Cu Lα1,2 spectra show a narrowing with decreasing Cu concentration. The shape of the valence band spectra can be qualitatively explained by recent calculations.
Relationships between the nonradiative multiphonon carrier‐capture properties of deep charged and neutral centres in semiconductors
Authors: Pässler R.
Published: July 2007

On the basis of a certain set of empirical trap‐structure parameters, an analytical description is given of nonradiative multiphonon capture and ejection processes (Shockley‐Read processes) occurring at deep traps in direct and indirect semiconductors. The mechanism‐specific connections between the capture properties of charged and neutral centres are represented in terms of suitably defined temperature‐averaged Sommerfeld factors. These factors are, within the approximation of an isotropic effective free‐carrier mass, calculated explicitly both for attractive and repulsive centres.
Shubnikov‐de Haas investigations on n‐type Bi2(SexTe1−x)3
Authors: Köhler H. , Haigis W. , von Middendorff A.
Published: July 2007

Shubnikov‐de Haas magnetoresistance oscillations are observed in the mixed crystal system Bi2(SexTe1−x)3 for 0 < x ≦ 0.1 and 0.8 ≦ x < 1 at temperatures between 1.7 and 4 2 K in magnetic fields up to 10.5 T flux density. The electron concentrations ranged from 5 × 1017 to 6.5 × 1019 cm−3. In both the separate ranges (Te‐rich and Se‐rich composition) the electronic mass parameters are essentially unchanged compared to pure Bi2Te3 and Bi2Se3, respectively. For the Te‐rich side the energy gap between the lowest conduction band and the postulated second type of conduction band valleys decreases with increasing selenium content. The evaluated g‐tensor ellipsoids correspond to the results for n‐Bi2Te3 in the region 0 < x ≦ 0.1.
Entropy production and choice of heat flux for degenerate semiconductors and metals
Authors: van Vliet K. M.
Published: July 2007

The concept of heat flux in multi‐band solids, having built‐in fields, is definable in various ways, depending on which energies are included in the transport. First the macroscopic entropy balance is examined, though more insight is gained from a microscopic picture. To this purpose the entropy flux and production function are computed for a degenerate nonuniform electron gas from the Boltzmann equation, using the streaming‐vector method, up to first order terms. From this a consistent picture of total energy, internal energy, and entropy transport emerges. The Boltzmann H‐theorem for electron—phonon interaction is also briefly discussed.
Lattice dynamics of AgBr at 295 K
Authors: Singh R. K. , Singh K. P.
Published: July 2007

The lattice dynamics of silver bromide at 295 K is studied with the three‐body‐force shell model which is adequately capable to account for the Cauchy violation and interatomic interactions in ionic crystals. The results calculated from this model are found to agree reasonably well with the measured data on the dispersion of phonons, two‐phonon Raman‐scattering, and infrared‐absorption spectra. The model reveals the physical properties equally well both at 80 and 295 K.
Influence of uniaxial stress and electron—electron scattering on the phenomenological conductivity relaxation time in n‐type germanium
Authors: Dienys V. , Kancleris Ž.
Published: July 2007

The dependence of the phenomenological conductivity relaxation time τσ on the compressive uniaxial stress F applied in 〈111〉 direction is investigated in n‐type Ge. The measurements at 80 K are made for two groups of samples with free carrier concentrations of 5 × 1012 cm−3 and 2.4 × 1014 cm−3. At intermediate stresses (0 to 5 kbar) a nonmonotonous dependence of τσ on F is observed. It is shown that this effect is mainly caused by the influence of stress on the inertia of the electric field‐induced electron redistribution process. At stresses F > 5 kbar τσ asymptotically approaches a saturation value which increases with electron concentration. The qualitative analysis of these data leads to the conclusion that the coupling constants for the electron scattering by the electrons of the same and of the other valleys are of the same order.
Raman scattering by KI containing F centres
Authors: Buisson J. P. , Lefrant S. , Sadoc A. , Taurel L. , Billardon M.
Published: July 2007

Raman spectra of KI (F centres) are described and calculated in the “off resonance” approximation neglecting the spin of the F centre electron. The theory predicts a very intense Γ resonant mode at 96 cm−1 in agreement with the experiment but is unable to account for the observed depolarization of this mode. Application of Henry's theory shows that this effect can be partially due to the spin—orbit coupling of the F centre in its first excited state.
The electronic states in molybdenum
Authors: de Hosson J. Th. M.
Published: July 2007

A method is given for calculating the electronic states in molybdenum. The formalism of the multiple scattering theory (SCF‐Xα‐SW) forms the basis of this method. The charge densities in each point of a cluster of nine Mo atoms are determined by superimposing atomic densities calculated by the Dirac‐Slater formalism. For the purpose of comparison similar results are obtained from the relativistic and non‐relativistic Hartree‐Fock model. From photo‐emission and optical reflectance measurements of Mo it is concluded that the energy levels are at −0.037, −0.118, −0.287 Ryd below and 0.290 Ryd above the Fermi level. The corresponding values calculated are −0.041, −0.128, −0.238 Ryd below and 0.209 Ryd above it. The results are in good agreement with those obtained using APW technique.
Non‐linear absorption of electromagnetic waves by band charge carriers in a laser field
Authors: Fomin V. M. , Pokatilov E. P.
Published: July 2007

The absorption of a probe electromagnetic wave by charge carriers is discussed leading to intra‐band transitions in a laser field in the presence of a magnetic field. The absorption coefficient is calculated for different scattering mechanisms at arbitrary temperatures using the path‐integral method. A stationary combination of the phases of the probe and laser waves is shown to cause an additional absorption. The dependence of the absorption coefficient on the magnetic field applied is studied. The influence of scattering mechanism, type, and reciprocal orientation of polarizations of both the waves on the absorption non‐linearity pattern is analyzed. The change of the probe wave polarization due to the anisotropy of the nonlinear absorption is found. The effects discussed can be used for an effective identification of the main scattering mechanism as well as for an investigation of the charge carrier plasma state.
Tunnelling effect between superconductors in an alternating field
Authors: Yubovskii Yu. P.
Published: July 2007

An expression for the Josephson current is obtained which is not based on the assumption that the voltage applied to the contact is small and varies at a slow rate. The Josephson superconducting tunnelling current at T = 0 is considered.
Paramagnetic domain tips in quasi‐one‐dimensional conductors
Authors: Phillips J. C.
Published: July 2007

Fits to the paramagnetic susceptibility of three mixed valency planar (MVP) compounds with columnar structure and one‐dimensional metallic conduction properties reveal discontinuities in Curie constants for which no explanation is available. This discontinuity corresponds to a third‐order phase transition, which is explained here as intrinsic and as associated with the lateral alignment of paramagnetic domain tips.
On the domain structure near the penetration field of the intermediate state in type‐I superconductors
Authors: Fahnle M. , Kronmller H.
Published: July 2007

The radius r and the distance a of the flux tubes in the triangular flux‐tube lattice of type‐I superconductors are determined within the framework of the Ginzburg‐Landau theory as functions of the applied magnetic field, the surface energy parameter, Δ, and the thickness, L, of a superconducting layer in the limit N → 1 (N demagnetizing coefficient of the sample, 0 ≤ N≤ 1). It is shown that near the penetration field of the intermediate state the Gibbs free energy of the flux‐tube structure is lower than that of a laminar domain structure.
Energies and fermi level of electrons in thin size‐quantized metal films
Authors: Sculte F. K.
Published: July 2007

The energies and the Fermi levels of electrons in thin size‐quantized metal films are investigated. Frequently a model with a constant potential is used and it is demanded that the wave functions or their derivatives vanish on planes parallel to the film surfaces. The position of these planes relative to the ion lattice is determined from the condition that the Fermi energy does not include a term inversely proportional to the film thickness. A comparison with numerical results of a fully self‐consistent calculation reveals that the results for the case that the wave functions vanish on the bounding planes (Dirichlet boundary conditions) form a satisfactory approximation.
Surface polariton effects in spontaneous emission
Authors: Agarwal G. S. , Vollmer H. D.
Published: July 2007

Analytic expressions of a previous paper, for the width of the excited state of an atom in presence of a dielectric interface are integrated numerically for two different types of material medium viz. a metal and the one involving phonorus such as GaP. In each case there is a very large transition probability that the atom decays with the emission of an appropriate surface polariton. It is also found that the diffuse contribution to the width closely related to the irradiated power is much smaller than the surface polariton contribution. The influence of the damping of the dielectric function on the width of the atomic states as well as the surface plasmon‐polariton contribution to frequency shifts for a metal are studied.
On the theory of exciton absorption in amorphous semiconductors
Authors: Majerníková E.
Published: July 2007

An investigation is made of the exciton effects, including the effect of the deformation of the internal state due to “collisions” with the random barriers in the case when ϱ ∼ L ‐ De, on the absorption coefficients of amorphous solids characterized by a Gaussian random potential. The factor exp [η2(1  αnl) β2] in the density matrix due to the random potential, appearing in this case, is shown to be responsible for an approximately Gaussian shape of the absorption edge.
The charge‐ordered state and collective excitations
Authors: Ionov S. P. , Lubimov V. S. , Manakova L. A. , Mironov V. S. , Makarov E. F.
Published: July 2007

The spectrum of collective excitations is investigated for the charge‐ordered state of the first type, when the constants of the intra‐ and inter‐site interactions correspond to electronic repulsion. Taking the interband exchange interaction into account the lower boundary of the spectrum is calculated and the conditions are determined when the collective mode is within the energy gap. It is shown that the considered excitations cause instability of the charge‐ordered state with respect to the transition into the Mott state, and the conditions of existence of such an instability are found. The collective spectrum of a one‐dimensional crystal at T = 0 is studied.
Acceptor ground states in nearly cubic semiconductors
Authors: Schmidt Martin
Published: July 2007

The splitting of acceptor ground states and their energy shift under the influence of uniaxial stress or noncubic crystal fields are calculated on the basis of a spherical or cubic valence band structure with large spin‐orbit splitting. The knowledge of the band parameters and the perturbation induced valence band splitting is sufficient to estimate the energies of the ground and first excited acceptor states. The experimental determination of the deformation potentials of three shallow acceptors in GaAs as well as the quadratic dependence of the acceptor ground states on the uniaxial stress provide a direct check of the calculations.
Spherical effective impurity electron mass in Si
Authors: Sarker A. Q.
Published: July 2007

The spherical effective electron mass for a point charge impurity in Si is determined such that it gives the same energy as the single valley effective mass theory. A first principle screened impurity potential is used and the appropriate ellipsoidal symmetry is incorporated into the Hamiltonian by the measured transverse and longitudinal effective masses and into the pancake‐type wave function by two variational parameters.
Multiphoton absorption by charge carriers in semiconductors in a magnetic field
Authors: Fomin V. M. , Pokatilov E. P.
Published: July 2007

A theory of absorption of strong electromagnetic radiation by charge carriers in semiconductors in a static uniform magnetic field is developed. By using the density matrix method in path‐integral representation the multiphonon absorption coefficient in semiconductors is calculated at arbitrary temperatures for different scattering mechanisms such as ionized impurities, polar and nonpolar interactions with optical phonons, and deformation and piezoelectric interactions with acoustical phonons. The coefficients of magnetic non‐linearity in weak magnetic fields are studied. The structure of the absorption spectrum in an arbitrary magnetic field is investigated, the existence of multiphoton absorption resonances revealed and the dependence of their parameters on the order of resonance and on the conditions of absorption is obtained. The asymptotic properties of the absorption coefficient in a strong magnetic field are analyzed. The anisotropy of absorption due to a magnetic field is discussed in each case.
Evidence for zero‐field fluctuations in Cr3+ near the phase transition in NH4Al(SO4)2 12 H20
Authors: Owens F. J.
Published: July 2007

The effect of the phase transition of NH4Al(SO4)2 12 H2O on the EPR of the Cr3+ ion is studied. The axial zero‐field tensor shows a strong linear temperature dependence with a change in slope occurring at −160°C which correlates to the onset of a marked broadening of the high and low field resonances as well as with an unusual increase in the intensity of the center resonance. The effects are attributed to fluctuations in the local symmetry of the H2O molecules about the Cr3+ ion. The strong temperature dependence of D is argued to be due to an unusually large coupling of the paramagnetic ion to local vibrations of the complex.
A molecular model for the emission process of FA(II) ‐centres in alkali halides
Authors: Kung A. Y. S. , Vail J. M.
Published: July 2007

Emission by FA(II)‐centres in alkali halides is investigated by treating the centre as a molecule immersed in a dielectric medium. The molecule consists of a single electron in the field of two unit positive charges and a central anion of effective charge (‐Ze). Its ground and first excited states are estimated variationally as a function of interionic separation and of Z. Lattice relaxation is treated in the rigid point‐ion model with Coulomb and nearest‐neighbour Born‐Mayer interaction. Only displacements of the saddle‐point ion and its two nearest neighbours are considered. These displacements determine Z, which gives a single‐parameter characterization of the defect. Three new FA(II)‐CentreS are postulated, namely in NaF:Li, RbF:Li, and RbF:Na, and their emission energies are estimated from a semi‐empirical scheme to be 0.915, 0.513, and 0.591 eV, respectively.
The influence of dipolar energy in nuclear double‐resonance
Authors: Whitaker M. A. U.
Published: July 2007

Nuclear double‐resonance is discussed in terms of cross‐relaxation in the rotating frame between Zeeman and secular dipolar reservoirs for each species. Temperatures are assumed for each reservoir, and the treatment limited to the case where interactions between reservoirs are less effective than those inside reservoirs. A set of four first‐order linear simultaneous differential equations is obtained for the development in time of the properties of the system.
A theoretical study of the absorption of photoelectrons created by X‐rays in semi‐infinite targets
Authors: Lantto V.
Published: July 2007

With the use of the diffusion model developed by Bethe et al. under conditions where the distribution of electron velocities is almost random, the absorption of photoelectrons, due to energy loss and scattering, is discussed quantitatively. An exponentially decreasing isotropic source is assumed for the photoelectrons created by X‐rays in a semi‐infinite target. Different boundary conditions are used for the total density of electrons on the plane surface. As in a work of Meister for monoenergetic β‐rays, the analysis is also carried out for a model where a straight motion of electrons changes into diffusion by a direct transition after a fixed value of the path length. Several expressions are introduced to describe the energy distribution and the total yield of the photoelectrons emerging from the target. These expressions are applied to the determination of the energy distributions and total yields of the photoelectrons for some practical cases.
Helicon damping in semiconductors in the quantum limit
Authors: Aronzoh B. A. , Metlikhov E. Z.
Published: July 2007

Results are given of theoretical and detailed experimental studies of alternating magnetic field regions with weak and strong helicon damping in semiconductors in the quantum limit for InSb at 1.7 ≦ T ≦ 78 K. The positions of strong and weak helicon damping region boundaries are investigated as functions of the temperature and dimension of the sample and also wave frequency and power. The agreement of the theory with experimental data is satisfactory.
Extra‐atomic relaxation and X‐ray spectra of narrow‐band metals. II. Results
Authors: Grebennikov V. I. , Babanov Yu. A. , Sokolov O. B.
Published: July 2007

The results of a numerical analysis of the effect of the hole potential on the form of the X‐ray spectra of narrow band metals are given. The dependence of the spectra on the magnitude of conduction band filling as well as on the magnitude of intratomic interaction between conduction band electrons and the hole is investigated. It is shown that when the band is filled, the emission and absorption spectra vary from a form of the spectra, which occurs if the hole potential is engaged adiabatically, to a form where the interaction with the hole is not taken into account. In the first case the spectra reflect the local impurity density, and in the second one the density of states of the unperturbed matrix. If the hole potential is strong enough, then the formation of a local level is possible. The most interesting consequence of this is the presence of two lines in core XPS. The systematic change is recorded of the intensities of these lines as the band is filled. The results of the investigation are drawn into a qualitative interpretation of the width of emission bands and of the shift of the inner lines of d‐metals.
The influence of plastic deformation of the flux‐line lattice on flux transport in hard superconductors
Authors: Schmucker R.
Published: July 2007

In hard type‐II superconductors flux lines may overcome pinning obstacles by plastical flow of the flux‐line lattice around the pinning centres. The conditions for this process to occur are investigated. The results for the volume pinning force obey a scaling law and show a specific dependence on the anisotropy of the pinning obstacles and on the Ginsburg‐Landau parameter.
Frequency‐dependent exchange correction to the dielectric function of the electron gas (II)
Authors: Brosens F. , Devreese J. T. , Lemmens L. F.
Published: July 2007

The influence of the exchange interaction on the dielectric response of the homogeneous electron gas is investigated. Starting from an expression for the dielectric function obtained earlier, the function G(q, ω) that describes the effect of exchange, is analysed. It is shown that G(q,ω) has a logarithmic singularity if ω = h (qkF ± q2/2), which makes the full frequency dependence of this function very significant. The dielectric function with exchange differs from the Hartree approximation by a very peaked frequency dependent structure. It even has a pole of first order. Furthermore, due to exchange, the plasmon branch is substantially changed and does not penetrate in the pair creation region. Finally, the exchange interaction induces a zero in the dynamic structure factor, where in the Hartree approximation only a discontinuous derivative with respect to frequency occurs.
Optical investigation of the electrical anisotropy of (SN)x single crystals
Authors: Geserich H. P. , Möller W. , Scheiber G. , Pintschovius L.
Published: July 2007

Polarized reflectance spectra of (SN)x single crystals are investigated in the range from 0.15 to 4 eV. For light polarized parallel to the polymeric chains the Drude behaviour found previously is confirmed. The optical properties perpendicular to the chains, however, are explained by a modified Maxwell‐Garnett model. This model takes into account the fibrous structure of (SN)x crystals and allows to separate the intrinsic and extrinsic transverse transport properties.
Lattice dynamics of lithium. A theoretical microscopic calculation
Authors: Weilacher K. H. , Roth‐Seefrid H. , Bross H.
Published: July 2007

The microscopic theory of lattice dynamics is employed to calculate the phonon dispersion curves of lithium. Instead of pseudo wavefunctions, Bloch wavefunctions are used that are solutions of the Schrödinger equation with a realistic crystal potential. The requirement of translation invariance leads up to a sum rule for the density‐density correlation function (q + Q, q + Q') with q=0. For arbitrary vectors q any approximation could be avoided for by means of interpolating the density of the conduction electrons. The resulting phonon spectrum is in good agreement with experiment.
Theory of allowed resonance Raman scattering of first and second order in a magnetic field
Authors: Enderlein R. , Bechstedt F.
Published: July 2007

The effect of a magnetic field on first‐ and second‐order resonance Raman scattering from nonpolar optical phonons is investigated theoretically. Both the Landau and spin quantizations are considered within a simple spheric and parabolic two‐band model. Magnetic field effects on the dispersion of the scattering amplitude can be observed which are similar to those of the dielectric susceptibility in magneto‐optics. New effects appear which have no counterpart in magnetooptics if the ratio of phonon frequency ω0 and cyclotron frequency ωc is an integer. For ωc = ωo e.g., in first‐order scattering all Landau resonances of the scattering amplitude, except one, compensate each other. The magnetic field dependence of the scattering amplitude is similar to its dependence on the incident frequency, so that resonance Raman scattering can be investigated experimentally by changing the magnetic field strength instead of the laser frequency. The two processes of second‐order Raman scattering show essential different dispersions in the presence of a magnetic field. The magnetic field induced circular anisotropy of the scattering cross section is considered and the polarization of the scattering light is investigated.
Determination of the velocity of mobile dislocations by nuclear spin relaxation measurements
Authors: Hackelöer H. J. , Selbach H. , Kanert O. , Sleeswyk A. W. , Hut G.
Published: July 2007

Wolf's general theory of rotating‐frame nuclear spin‐lattice relaxation is applied to the effect caused by dislocations moving at an average velocity v. Equations are presented for the dependence of the NMR relaxation rate R on the mean waiting time τ of a dislocation before an obstacle, valid in both the weak‐and the strong‐collision regions. In the latter the Slichter‐Ailion‐Rowland‐Fradin theory applies. Nuclear spin relaxation tests on 23NaCl and 23NaF single crystals deformed at constant strain rates ϵ confirm the theory. The experimental data for the predicted maximum in R (reached only for NaCl at ϵm = 20 s―1) yield directly τm = 2.8 × 10―5 s, the mobile fraction of dislocations Qm/Qt = 0.26. The value of ϵm, together with the mean distance between obstacles of ≈ 2 × 10―4 cm, derived from the slope of the curve in the strong‐collision region, leads to vm = 7.1 cm s―1, corresponding to a mobile dislocation density ϱm = 1.4 × 108 cm−2.
Thermal vibration amplitudes of TlCl between 296 and 534 K
Authors: Haav A. A. , Peljo E. , Suortti P.
Published: July 2007

The thermal motion parameters, B, of the Tl+ and Cl― ions of TICI are determined by X‐ray diffraction from powder samples between 296 and 534 K. The B‐values increase linearly with temperature from BTI = 3.39 Å2 and BCI = 2.09 Å2 at 296 K to BTI = 5.68 Å2 and BCI = 3.38 Å at 483 K. This corresponds to a constant Debye temperature of ΘM = 95 K. The results suggest strong second‐neighbour interactions and a mutual cancelation of the anharmonic effects.
Dynamical properties of no impurity molecules in alkali halides. I. Theory
Authors: Zavt G. S.
Published: July 2007

The effect of an impurity molecule on the lattice vibrations is studied. The translational and librational motion of the molecule and lattice vibrations are considered from a unified point of view. Green's functions describing all types of motions as well as their mixing are calculated in the harmonic approximation. Optical response functions (far infrared absorption coefficient and Raman scattering tensor) are found. It is shown that, unlike the case of point impurities, the spectral functions can display interference features, such as asymmetric peaks or antiresonances.
Vibrations of Isotopic Impurities in Crystals Built up by Complex Ions
Authors: Belousov M. V. , Pogarev D. E. , Shultin A. A.
Published: July 2007

The IR reflection and Raman spectra of NaNO3, Ba(NO3)2, KNO3, and NH4Cl crystals containing isotopic impurities of 15N and 18O are studied. The experimental data on local vibration modes are compared with those calculated by the method of Green's functions, the density of vibration states being determined from two‐phonon spectra, as well as those following from the model of additive refraction (AR). The latter is found to yield satisfying results provided the isotopic shift exceeds the vibrational bandwidth. A nomograph is provided by means of which the deviation of frequency and intensity of a local mode calculated in AR approximation can be estimated. The absence of isotopic structure in vibration spectra of some crystals studied by other authors is also explained.
The Optical Properties of MnAs and MnAs0.85P0.15
Authors: Bärner K. , Braunstein R. , Chock E.
Published: July 2007

The reflectivity spectra of MnAs (hexagonal NiAs structure) and MnAs0.85P0.15 (orthorhombic lattice) are measured in the energy range 0.3 to 14 eV at room temperature and a Kramers‐Kronig analysis is performed. It is found that below 1 eV plasma effects due to free d‐electrons dominate. At energies from 3 to 8 eV plasma resonances due to bound d‐electrons are observed. In the intermediate region, the imaginary part of the dielectric constant ϵ2 reveals structure due to transitions between d‐states and from the valence band into d‐ and sp states. Analysis of the combined density‐of‐states function ϵ2E2 yields 3.0 eV for the energy gap between bonding and antibonding sp states for both compounds. Also, from interpretation of ϵ2 together with ϵ2E2 the energy separation between the Fermi level EF and the bottom of the minority spin e‐band and the distance of the valence band edge from EF are obtained.
Spin‐Dependent Luminescence of Donor‐Acceptor Pairs in Semiconductors. I. General Theory
Authors: Thuselt F.
Published: July 2007

By use of orientation parameters of electrons (spin 1/2) and holes (angular momentum 3/2) expressions are given for the angular dependence of both the intensity and polarization of the luminescence of donor‐acceptor pairs in semiconductors with diamond and zincblende structure. The luminescence light may be polarized either linear or circular depending on the observation direction. Its degree of polarization depends on internal or external fields as well as on the eventual polarization of the excitation light.
Electrical Conductivity in the Hubbard Model Including Electron‐Phonon Interaction
Authors: Ihle D.
Published: July 2007

Using the Hubbard model including weak electron‐phonon coupling the electrical conductivity σ is calculated for U ≫ t (U intra‐atomic Coulomb energy, —t nearest‐neighbour hopping integral) and low frequencies. Especially the dc conductivity is investigated for the half‐filled band and T ≫ TD (TD Debye temperature). It is compared with the dc conductivity in the phononless Hubbard model calculated in the approximation lim (σ/t2). The inclusion of phonons results in t → 0 a finite transport relaxation time the temperature dependence of which leads to a shift of the conductivity maximum temperature to a lower value, the sharpening of the maximum, and to the decrease of the conductivity for high temperatures according to T−2.
Nuclear Acoustic Resonance Absorption and Dispersion in Vanadium via Coupling to the Magnetic Dipole Moment
Authors: Müller V. , Schanz G. , Fischer E. , Unterhorst E. J.
Published: July 2007

The first observation of both NAR absorption and NAR dispersion in a single crystal vanadium due to the coupling of longitudinal acoustic waves to the magnetic dipole moment of the V51 nuclei is reported. The experiment is performed in the temperature range of 5 to 294 K. The observed angular dependence and NAR line shapes are in good agreement with the formulas derived in this paper.
The Temperature Dependence of the Absorption Spectrum of NiO
Authors: Nasu T.
Published: July 2007

The temperature dependence of the spin‐dependent absorption of NiO single crystals is measured in the infrared region in a wider range of the temperature from 20 to 650 K. The experimental results are analyzed by a molecular field model proposed by Mizuno and Koide.
Exciton Dissociation in the Electric Field of the Charged Defect
Authors: Petelenz P. , Petelenz B.
Published: July 2007

Dissociation of Frenkel excitons in the neighbourhood of a charged defect in anthracene crystals is investigated in relation to the recent experimental results of Fort and Coret. The trimer model is adopted, and the charge‐transfer integral relevant to the exciton decay is calculated in a simple semiempirical scheme. The dissociation probability is found to be enhanced due to the interaction with a cationic defect. An alternative interpretation of the experimental results in terms of the mixing of the exciton and charge‐transfer states is proposed. A possible role of both the mechanisms is discussed.
Spin‐Dependent Luminescence of Donor–Acceptor Pairs in Semiconductors. II. Application to Inner Strains
Authors: Thuselt F. , Unger K.
Published: July 2007

The influence of inner strains in semiconductor crystals caused i) by impurity misfit and ii) by bending of epitaxial layers on the polarization properties of the emitted pair radiation is theoretically studied with the aid of the general theory developed recently. It is shown that anisotropic pair distributions in the crystal can be detected in this manner. The predictions of the theory for bended epitaxial layers fits well the observations of Vekua et al.
Energy Bands in Cubic Ice. Ab Initio Calculation Using the Method of Linear Combination of Molecular Orbitals
Authors: Resca L. , Resta R.
Published: July 2007

An LCMO calculation is made of the energy bands of cubic ice with a suitable model structure. With respect to a previous tight‐binding calculation important terms are included consistently with an ab initio derivation and the matrix elements are obtained at any k vector in the Brillouin zone. The problem of the excitonic levels is discussed. The influences of crystal structure features on the electronic states are evaluated. A comparison with very recent experimental VUV, XPS, UPS data is performed and a better understanding of the electronic properties of ice is achieved.
Scattering of DC Electrons by a Spherical Macroscopic Potential Including Screening
Authors: Gerlach E. , Rautenberg M.
Published: July 2007

Using a transport theory based on the equivalence of energy loss and Joule's heat the scattering of electrons by a spherical macroscopic potential is studied which serves as a simple model for radiation damage, inhomogeneities, or precipitations. It is shown that screening may strongly effect the resistivity if the potentials are spatially slowly varying. Further the model calculations predict the possibility of geometrical resonances for a degenerate carrier distribution.
Optical Side Bands in a Cooperative Jahn‐Teller Material: TmAsO4
Authors: Laugsch J. , Gehring G. A.
Published: July 2007

Optical side bands are observed in the spectrum of TmAsO4. Measurements of their polarisation and intensity are made as a function of magnetic field. These results are interpreted using a tight binding theory developed earlier. It is found that the most intense side band is caused by off‐diagonal exchange (Γ3 − Γ1 and Γ4 − Γ1) between the ground doublet and the excited singlet arising from virtual phonon exchange. The less intense side bands are caused by a large number of exchange interactions and the analysis is less clear.
Absorption Spectra of S2 Molecules Embedded in van der Waals Solids
Authors: Arai T. , Martin T. P.
Published: July 2007

The \documentclass{article}\pagestyle{empty}\begin{document}$ {}^3\sum\nolimits_{\rm u}^ - {} \leftarrow {}^3\sum\nolimits_{\rm g}^ - {} $\end{document} transitions are investigated of sulphur dimers, matrix‐isolated in Ar and Kr. Each vibrational subband of the electronic absorption appears to consist of several superimposed narrow bands. The separation between the narrow bands is determined to be 60 cm−1 in Ar and 50 cm−1 in Kr. The complex structure is interpreted to be caused by phonons of the matrix materials. The potential parameters of sulphur dimers in Ar and Kr are also calculated.
Ideal Thermal Resistivity of Lithium, Potassium, and Rubidium
Authors: Gupta O. P. , Kharoo H. L. , Hemkar M. P.
Published: July 2007

The temperature variations of the thermal resistivity of lithium, potassium, and rubidium is calculated within the free‐electron approximation using the anisotropic continuum dispersive model proposed by Sharma and Joshi for the lattice dynamics of metals. The theoretical results are compared with available X‐ray measurements. Except for lithium, there is reasonable good agreement between theory and experimental data.
Piezoelectric Coupled Soft Mode in CsH2AsO4 and CsD2AsO4 Studied by Brillouin Scattering
Authors: Azoulay J. , Gerlich D. , Wiener‐Avnear E. , Pelah I.
Published: July 2007

The Brillouin scattering from shorted and free single crystals of CsH2AsO4 and CsD2AsO4 is studied as a function of temperature. Utilizing the (z + x)[y, (z + x)] (z − x) scattering geometry, the softening of the elastic constant C is observed on approaching the Curie temperature from above. The results, which are also compared with similar data on KH2PO4 and KD2PO4, indicate a strong piezoelectric coupling between the acoustic shear mode and the soft optical ferroelectric mode. The temperature of the Brillouin shifted frequencies for all the KDP isomorphous crystals can be computer fitted with a Curie‐Weiss law for the soft mode. The difference between the extrapolated temperature of the shorted and free crystals are also explained on the basis of the thermodynamic relations of the coupled modes and are thus similar to those obtained for the clamped and free dielectric responses.
Plasma Wave Damping in a Layered Electron Gas with Random Inhomogeneities
Authors: Abramovich B. S. , Ignatov A. A.
Published: July 2007

The propagation of plasma excitations in a layered electron gas with random inhomogeneities of the conductivity of the separate layers is investigated. An expression is obtained for the effective dielectric permittivity of the given system at weak fluctuations of the equilibrium carrier concentration. The mechanism is studied of plasma wave damping associated with the energy transformation of the mean field into chaotic self‐oscillations of the system. Decrements of the mean field damping both in the centre and on the boundary of the Brillouin zone are calculated. A strong dependence of the damping character on the degree of fluctuation correlation between different layers is found.
Host and Impurity Excitations in Antiferromagnetic and Paramagnetic FeCO3 in the Far Infrared
Authors: Eremenko V. V. , Maslennikov A. I. , Naumenko V. M.
Published: July 2007

The host and impurity excitations in FeCO3 containing 3% Mn2+ are studied in the range from 12.5 to 500 cm−1. The host excitations are observed both in the antiferromagnetic and the paramagnetic temperature range and are well described for a wide temperature range with a single ion Hamiltonian, including the interactions between all sublevels coming from the ground term 5D of the Fe2+ ion: the temperature dependent trigonal field, the spin–orbit interaction, and the Ising molecular field. For the first time in an Ising antiferromagnet a pair mode (host excitation plus impurity excitation) characterized by the absence of magnon‐magnon interaction is observed. The g‐factors of the observed lines are measured. A sharp drop of the g‐factors near TN cannot be explained by an existent theory of the g‐factor renormalization.
Reduction Symmetry Method in Crystal Field Theory (III)
Authors: Kustov E. F.
Published: July 2007

The commutators of tensor operators of the point groups G are introduced and tensor operators {kk′} are regarded as infinitesimal operators of the unitary groups U[γ]γ. The classification of the eigenfunction and symmetry properties of tensor operators are developed by using the following reduction series U4l+2 ⊃ U × U × … ⊃ (SU2 × U) × (SU2 × U) × … ⊃ (SU2 × G) (SU2 × G) × … ⊃ SU2 × G. The main formulas of this analysis are given for the matrix elements of the tensor operators.
Molecular Exciton Cluster States and Spectra with Application to Benzene
Authors: Hoshen J. , Kopelman R.
Published: July 2007

A cluster expansion is introduced for the determination of the optical absorption properties of cluster states in substitutionally disordered molecular crystals. This method is based on the Green's function expansion in terms of the localized cluster states in the energy region of the guest sub‐band. The general features of the method are illustrated by calculations for a simple two‐dimensional molecular crystal. The physical parameters for the calculation are based on the v11 benzene vibrational exciton. Both polarized and unpolarized cluster optical absorptions display strong asymmetries, with respect to the isolated guest spectral line. Calculations are performed on the monomer, dimer, trimer, and tetramers (three kinds) of the benzene crystal ac‐plane.
A Method to Derive Interatomic Potentials in Metals from Experimental Phonon Spectra
Authors: Brosens F. , Cornelis J. , Wallace D. C. , Devreese J. T.
Published: July 2007

In the central potential approximation, the first and second derivative of the two‐body potential for metals in the atom positions can be calculated by inverting the eigenvalue equations for the phonons. The potential is obtained from a straightforward integration procedure. Explicit results for Na are reported, which fully reproduce the experimental phonon spectrum.
On the Excited States of the T1+ Monomer Centre in NaI
Authors: Benci S. , Fontana M. P. , Manfredi M.
Published: July 2007

The Ax emission band in NaI crystals containing the TI monomer centre is studied in the temperature range 25 to 295 K using a lifetime technique after pulsed excitation in A, B, C, and D absorption bands. The results show that the decay signal is complex and consists of three different slopes better resolved with A and D excitation than with B and C excitation. It is possible to conclude that at least two radiative levels contribute to the Ax emission. The measured behaviour of the intensity of the different components is discussed taking into account also the available data of integrated emission.
A Theory of Dislocation Dynamics with Internal Stresses
Authors: Gradowczyk M. H. , Rojas F. A.
Published: July 2007

The present theory completes a previous work by extending Kröner's concept of incompatibility of crystal deformation to dislocation sources. For a continuous distribution of moving dislocations, differential equations are derived that describe crystal flow in terms of the total stress tensor, particle velocity vector, flux‐density vector, a plastic strain tensor produced by the dislocations created by dislocation sources, and the necessary constitutive equations. This new theory allows for the explicit computation of the internal stress field produced by the motion and creation of dislocation fields when some additional assumptions are introduced.
The Electronic Spectra of Charge‐Ordered Crystals
Authors: Ionov S. P. , Lubimov V. S. , Manakova L. A. , Makarov E. F.
Published: July 2007

The electronic excitation spectra for an isotropic charge‐ordered crystal are analysed on the basis of a band model. It is found that the charge‐ordered state is unstable relative to a transition into a state with pair correlation when the intra‐band attraction among the electrons exceeds a certain critical value which is a function of the kinetic energy and interaction parameters. In this situation a state is realized in which the charge order and the pair correlation are coexistent and which is possible at a finite value of the band width.