bluuebunny/crossref_metadata_2025_split huggingface dataset:
Alloys with Off‐Diagonal Disorder
Authors: Mertsching J. 
Published: July 2007

AbstractThe tight‐binding alloy with some off‐diagonal disorder is exactly equivalent to an alloy with diagonal disorder only. Therefore cluster approximations for diagonal disorder also apply to off‐diagonal disorder. As an example, the pair CPA is numerically studied for the one‐dimensional tight‐binding model. Other proposed cluster approximations preserving the translational symmetry fail in this case.
Dispersion of Surface and Bulk Phonon‐Polaritons on α‐Quartz Measured by Attenuated Total Reflection
Authors: Falge H. J. , Otto A. , Sohler W. 
Published: July 2007

AbstractIn a continuation of the study of surface polaritons with “stop points” on α‐quartz, experimental ATR spectra for various thicknesses of the air gap between prism and α‐quartz sample were compared to theoretical ones. The relations between surface polaritons, stop points, anisotropic reststrahlen bands, longitudinal phonons, and bulk polaritons were studied. It is shown that the dispersion of bulk polaritons may be measured by ATR as well as the dispersion of surface polaritons. Results are presented for the dispersion of two bulk polaritons, one of which is Raman‐inactive.
Raman Scattering Studies on (NH4)Cl1−xBrx Mixed Crystals in the Tetragonal Phase
Authors: Bauhofer W. , Genzel L. , Perry C. H. , Jahn I. R. 
Published: July 2007

AbstractRaman measurements have been performed on the mixed system (NH4)C11−xBrx, in the tetragonal phase. For intermediate x‐values, new peaks are found in the acoustical frequency region of the spectra, whose mode strengths are proportional to x and (1 ‐ x), respectively. Adopting the assumption that the concept of a Brillouin zone is still useful in a mixed crystal, the Raman active modes of the tetragonal phase of the mixed crystals are identified with the M‐point modes of the cubic β‐phase. The experimental data are compared with a simple calculation based on the REI‐model.
Green Function Theory of Short‐Range and Long‐Range Order in Magnetite
Authors: Ihle D. , Lorenz B. 
Published: July 2007

AbstractFor the treatment of short‐range order effects of the extra B‐site electrons in magnetite on the basis of the lattice‐gas model a cluster approximation for Green functions is presented. The short‐range and long‐range (Verwey ordering) order parameters, various correlation functions, configuration probabilities, and thermodynamic functions are calculated self‐consistently as functions of temperature T. At Tv a first‐order phase transition to the disordered phase takes place. The system is found to be nearly completely ordered for T < Tv and strongly short‐range ordered for T ⪆ Tv. For U2/U1 ≈ 0.02 to 0.05 (U1, U2 Coulomb energies of nearest and further distant neighbours, respectively) both Tv and \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop f\limits_{0.9T_v }^{1.05T_v } c_v dT $\end{document} (cv specific heat) have nearly the values measured experimentally.
Electron Momentum Distribution in Vanadium and Vanadium Hydride
Authors: McIntire W. R. , Batterman B. W. 
Published: July 2007

AbstractThe Compton profiles of vanadium and vanadium hydride have been measured. The two profiles are in agreement for values of the electron moment, um greater than 1.5 at. units, indicating that the vanadium core electron momentum distribution is unchanged by addition of the hydrogen. The difference between the profiles is in the valence electron region, and it supports the model of the hydrogen atom giving its electron to the vanadium Fermi sea.
The Role of the Cut‐off Energy in Tests of the X‐Ray Threshold Theory
Authors: Dow J. D. , Smith D. L. 
Published: July 2007

AbstractIt is shown that the cut‐off energy ξ in the Mahan‐Nozière‐de Dominieis (MND) theory of X‐ray absorption thresholds, ϵ2(ω) = A2[(hω ‐ ET)/ξ]−α, is constrained by existing data to be almost implausible small for Li: ξ < 0.5 eV. If ξ is significantly smaller, ξ < 0.08 eV, then inelastic scattering experiments designed to convert the rounded Li threshold into a peaked edge may not be sucessful and may not test the MND theory.
A Study on Effective Charge in the Dipole Theory of Ferroelectrics
Authors: Pak K. N. , Yokoyama S. , Kinase W. 
Published: July 2007

AbstractThe effective charge used in dipole theory of ferroelectrics has been investigated theoretically. Particular emphasis is placed on its temperature dependence. Employing the concept of the statistical theory of ferroelectricity, it is shown that in the dipole theory the effective charge corresponds to the temperature, and the relation between them has been established in a statistical way within the framework of dipole theory. The temperature dependence of the effective charge has been determined in detail by carrying out the numerical calculations and the results obtained have been discussed. As a result, it is found that in the ferroelectric phase the effective charge depends strongly on temperature as the long‐range dipole interaction becomes large and this inclination is more remarkable in the vicinity of the transition point, whereas in the paraelectric phase the effective charge is nearly independent of temperature over a wide range of it, except near the transition point.
Band Structure of Layer Crystals NbSe2 and MoS2 and an Interpretation of Their Optical Spectra
Authors: Leveque G. , Robin‐Kandare S. , Martin L. 
Published: July 2007

AbstractThe optical functions a and ε2ω2 of NbSe2 and MoS2 are deduced from reflectivity measurements between 2 and 70 eV. Basing on the results of photoemission studies and band structure calculations, an experimental model is proposed of the density of states for both the valence and conduction bands. As expected, the optical behaviours of NbSe2 and MoS2 are very similar. The shape of the ε;2ω2 curve in the high energy range (30 to 50 eV) corresponding to deep level excitations describes the density of states of the conduction band in spite of some broadening of the structure. This broadening is not of experimental origin.
Relationship between the Composition of Binary FeMn Alloys and the Shape and Energy of FeKα1 and MnKα1 X‐Ray Emission Lines
Authors: Pessa V. M. , Uusitalo R. R. O. 
Published: July 2007

AbstractFeKα1 and MnKα1 (Is −2p3/2) X‐ray emission lines from Fe‐Mn binary systems th rough out the entire range of composition are studied. The half widths r of the lines are shown to depend linearly on the composition of the alloys, except for the alloy containing 2.2 at% Mn in an α‐iron matrix where exceptionally narrow Γ for both components is observed. It is suggested that changes in Γ are generally associated with changes in the atomic magnetic moments of Fe and Mn in these alloys. On the basis of the smallness of the energy shifts △ (≦0.09 eV for all the alloys studied) of Kα1 it is further concluded that there is only a slight, if any, dsp‐electron transfer from Fe to Mn atoms, and that △ cannot be clearly related to the magnetic moments.
Temperature Dependence of the Impurity Electric Quadrupole Interaction: Cadmium and Tantalum in Titanium
Authors: Kaufmann E. N. , Raghavan P. , Raghavan R. S. , Krien K. , Naumann R. A. 
Published: July 2007

AbstractThe electric quadrupole interaction at Cd and Ta impurity nuclei implanted in trace amounts in a titanium host lattice has been measured as a function of temperature by means of the time‐differential perturbed angular correlation technique. The results show a striking difference in the temperature dependences for the two impurity species. This implies that, in addition to the previously observed temperature dependence arising from lattice expansion and conduction electron redistribution of the host, there exist significant local temperature dependent effects associated with the specific characteristics of the impurity.
State of Conduction Electron Interacting with Frenkel Excitons
Authors: Nagaev E. L. , Sokolova E. B. 
Published: July 2007

AbstractA special type of interaction between conduction electrons and Frenkel excitons in magnetic or organic semiconductors different from the usual polarization interaction is described. It consists in that the transitions of the conduction electron from one atom to another are accompanied by the creation or annihilation of an exciton at one of these atoms. Furthermore, transitions of the electron from non‐excited to excited atoms are possible which are accompanied by exciton transitions in the opposite direction. As a result of the interaction, specific states of charge carriers called transferons arise which differ strongly from excitonic polarons. An expression for self‐energy and damping of the transferon is obtained in the limit of weak electron‐exciton coupling.
Local Vibrational Modes of Boron in Garnets
Authors: Andlauer B. , Tolrsdorf W. 
Published: July 2007

AbstractLocal vibrational modes of boron impurities have been detected in single crystals of Y3Fe5O12, Gd3Fe5O12, Y3Ga5O12, and Gd3Ga5O12, which were grown from a melt containing B2O3 flux. The B3+ ion is incorporated on a heavily distorted defect site, in threefold oxygen coordination. From the energetic position of the spectra, five different boron‐oxygen complexes may be distinguished. As the analysis shows, the (BO3)3− groups must be associated with a further lattice defect of complex and multiform structure.
Electron Energy Loss Spectra and Optical Constants for the First Transition Series from 2 to 120 eV
Authors: Werenkel C. , Gauthé B. 
Published: July 2007

AbstractEnergy loss spectra in the first series transition metals (Ti, V, Cr, Mn, Fe, Co, Ni) have been performed by transmission of 35 keV electrons through thin films (200 to 400 Å) in the spectral range 2 to 120 eV. Experimental data processing enables to distinguish between single scattering volume losses and surface losses. Characteristic functions, such as the energy loss function, the complex dielectric constant, and the optical absorption coefficient are computed and compared with results obtained by different methods.
First Observation of Landau Levels by Electroreflectance at the E1 and E1 + Δ1 Edges of InSb
Authors: Joullie A. M. , Alibert C. , Ranvaud R. , Cardona M. 
Published: July 2007

AbstractLandau oscillations are observed associated with the E1 and E1 + Δ1, edges of InSb with a heterojunction surface barrier method. The periods of the first oscillations are deformed by exciton effects. Those of higher order oscillations agree with predictions of the one‐electron theory.
Symmetry Restoring Modes and Soft Modes in Continuous Phase Transitions in Crystals
Authors: Shigenari T. 
Published: July 2007

AbstractThe mechanism which restores the symmetry broken by continuous (second order) phase transitions in crystals is discussed. It is a kind of extension of the concept of Goldstone's mode to the case of discrete broken symmetries. Relations between a symmetry restoring mode and a soft mode are given. It is shown that they are not necessarily identical, in particular if a soft mode is degenerate, the symmetry restoring mechanism gives rise to an additional mode centered at zero frequency. Qualitative behaviour of this mode near the critical temperature is derived using a specific model analogous to molecular rotation in liquids. It is essentially a reorientational diffusion motion originating from the anisotropic potential for the fluctuation of the eigenvectors of the soft mode. Phase transitions in several types of crystals are discussed from the symmetry restoring point of view.
Molecular Field Theory of the Order Parameter in NaNO2
Authors: Bonilla I. R. , Holz A. 
Published: July 2007

AbstractThe long range order parameter of NaNO2 is evaluated in the molecular field approximation. A self‐consistent method is developed to calculate the induced ionic polarization energy of the lattice. Lattice sums are evaluated by means of the method of Bertaut. A second order phase transition is obtained in contrast to the experiment.
The Extended Polarizable Ion Lattice Model
Authors: Abarenkov I. V. , Antonova I. M. 
Published: July 2007

AbstractThe influence of the electronic part of the polarization of an ionic crystal on the state of an extra electron is considered. The extended polarizable ion lattice model is proposed to account for this influence. In the case of the F‐center in alkali halides the r‐dependent dielectric constant ε(γ) corresponding to the proposed model is shown to differ but little from ε∞ if γ is greater than the nearest neighbours distance.
On the Determination of Parameters of Mössbauer Absorption Spectra at Ultrasonic Excitation
Authors: Aivazyan T. M. , Aivazyan I. M. , Mkrtchyan A. R. , Kocharyan L. A. 
Published: July 2007

AbstractThe influence of acoustic oscillations produced in an absorber on the shape of Mössbauer absorption lines is discussed. It is shown that the absorption lineshape strongly depends on the excited acoustic wave parameters in the absorber i.e. on the ultrasonic wave amplitude, its frequency, and standing wave coefficient. Some possible applications, especically the possibility of acoustic modulation of the Mössbauer emission passing the absorber are examined.
Phenomenological Force Model for a Lattice from Experimental Phonon Spectra Appropriate to Lattice Distortion by Point Defects
Authors: Chevychelov A. D. , Lifshits I. L. 
Published: July 2007

AbstractA general formal force model is constructed for a lattice with pair interactions between the atoms and appropriate to the problem of the point defects using group theory analysis. The pair interactions include bend and mixed parts. The concrete analysis is performed for cubic lattices. Numerical constants are chosen from experimental phonon dispersion curves. As an example of the application of the model the lattice distortion around a vacancy is calculated for copper.
The Transferability of Raman Tensors of Isolated Molecules to Crystals – Application to the Computation of Relative Intensities of the Internal Modes of SO4–– in K2SO4 Crystal
Authors: Montero S. 
Published: July 2007

AbstractA general group theoretical method for the construction of the Raman tensors of crystals as a function of the crystallographic data and the Raman tensors of the ‘free’ molecules or ions of the crystal is presented. In a first application of this method, the Raman tensors of all internal modes of K2SO4 were calculated. The relative intensities derived from the computed tensors showed a satisfactory agreement with the measured integrated intensities of the Raman spectra as obtained on oriented K2SO4 crystals.
The Contribution of Magnetic Dipolar Interactions to Nuclear Spin Relaxation by Vacancy‐Type Self‐Diffusion Mechanisms in Metals
Authors: Cavelius E. 
Published: July 2007

AbstractThe spectral densities determining the dipolar contribution to the nuclear spin relaxation times are calculated for a combined monovacancy–divacancy mechanism of self‐diffusion in solids. The treatment is based on Wolf's encounter model for monovacancies and isotopically pure crystals. Numerical results are given for isotopically pure f.c.c. and b.c.c. crystals. The extension to more than one spin species is indicated.
A Note on the Polarization Dependence of Two‐Photon Absorption Coefficient in Crystals
Authors: Doni E. , Girlanda R. , Parravicini G. Pastori 
Published: July 2007

AbstractThe expression of the two‐photon absorption coefficient in crystals is discussed using explicitly the Wigner‐Eckart theorem for finite symmetry groups. It is discussed how to separate the dynamical part, which contains appropriate reduced matrix elements of the momentum operator, from the geometrical part, which depends on the polarization vectors of both the radiation beams. For the geometrical part the standard results of the literature are recovered; detailed new information for the two‐photon absorption rate is supplied by the dynamical part. To illustrate the procedure, the case of TICI is considered as a specific example.
The Dynamic Contribution to the Surface Energy
Authors: Paasch G. 
Published: July 2007

AbstractThe dynamic or surface‐plasmon contribution to the surface energy of the electron gas derived by Craig is evaluated taking into account correctly the electron–hole excitations, dispersion of the plasmon and surface‐plasmon modes, and the damping of these modes by electron–hole excitations. The resulting surface energy vs. the density parameter is close to the earlier approximation which was criticized since all these influences were neglected there. The success of the simple approximation is explained using the simplified theory of electron correlations by Overhauser.
Spin–Plasmon Relaxation in Solids
Authors: Deigen M. F. , Suslin L. A. 
Published: July 2007

AbstractThe paramagnetic relaxation of impurity centres in solids due to the interaction between these centres and Langmuir's oscillations of the current carrier plasma is investigated. The transition probabilities of direct and combined relaxation processes as well as of the process accompanied by the resonance fluorescence of plasmons are calculated. The dependences of the transition probabilities on transition frequency, carrier concentration, and temperature are obtained. Comparison with experimental data shows agreeement between theory and the observed pattern of the paramagnetic relaxation in semiconductors.
Theory of Plasmon Excitation in the Case of Electron Diffuse Reflection at the Surface
Authors: Libenson B. N. , Rumyantsev V. V. 
Published: July 2007

AbstractA theory is given of the excitation of plasmons by fast electrons in solids with a spatially inhomogeneous surface. The properties of the surface are described similarly as in the theory of the anomalous skin effect. The probability of the excitation of plasmons is expressed through the retarded Green's function of the electric field. The Dyson equation for this Green's function is solved by the Fock method. It is shown that the surface roughness influences the probability of the excitation of the bulk as well as of the surface plasmons. The latter exhibit additional damping due to the surface roughness.
F‐Centre in KCl with the Extended Polarizable Ion Lattice Model
Authors: Abarenkov I. V. , Antonova I. M. 
Published: July 2007

AbstractThe F‐centre in KCl has been calculated within the same approximations as those used in calculations of the bottom of the conduction band in KCl. It is found that if the electronic polarization of the crystal is neglected, the calculated values of the optical excitation and optical ionization energies are 1.5 and 2.5 times the corresponding experimental values. The difference between the calculated and the experimental values is reduced considerably when the polarization potential is incorporated to account for the electronic polarization.
Semi‐Empirical LCAO Calculations for Tellurium Crystals with Vacancies
Authors: Weigel C. , Landwehr G. , Corbett J. W. 
Published: July 2007

AbstractThe results of semi‐empirical LCAO calculations based on the extended Hückel theory are presented for trigonal tellurium model crystals with vacancies. It is shown that an empty gap‐level close to the valence band is associated with a vacancy. The migration of the vacancy takes place preferentially in the c‐direction. The double vacancy is less mobile than the single vacancy, for which the migration energy is of the order of several hundred meV. The results are compared with experimental findings.
Effect of Exciton–Exciton Interaction on the Free Exciton Absorption and Luminescence Spectra of Si and CdS Crystals under High Intensity Excitation
Authors: Lelyakov A. V. 
Published: July 2007

AbstractLorentz shapes of the excitonic absorption and luminescence bands are obtained by Matsubara's Green's function technique in second order of the perturbation theory. The shapes of the excitonic absorption and luminescence bands are studied for the gaseous phase of free excitons in Si and CdS crystals under high intensity excitation. It is demonstrated that the elastic collisions of excitons lead to a broadening and shift of excitonic absorption and luminescence bands even when phonons are absent. The effect of phonons in the present case has not been taken into account. The possibility of smoothly varying the frequency of laser emission by changing the pumping level is pointed out.
Electronic Contribution to the Dynamical Screening in the Theory of Excitons in III–V and II–VI Semiconductors with Zincblende Structure
Authors: Ekardt W. 
Published: July 2007

AbstractStarting from the Bethe‐Salpeter equation formulation of Sham and Rice the influence of the dynamical screening in the theory of direct excitons in III–V and II–VI zincblende structure semiconductors is computed taking into account the wave vector dependence of the dielectric constant and higher reciprocal lattice vectors. With the aid of published results of the dielectric constant calculations in the R.P.A. for zincblende semiconductors the influence of the dynamical screening is calculated exactly within the ladder approximation up to the order of magnitude EB/Egap. The results agree with the general statement by Zimmermann that the binding energy should increase due to dynamical screening. Comparison with experiments becomes possible only through the inclusion of some further corrections.
Thermodynamics of a Two‐Band Hubbard Model
Authors: Steeb W.‐H. , Marsch E. 
Published: July 2007

AbstractUsing a variational principle the approximate free energy of a simple two‐band Hubbard model is calculated. Numerical results are presented for the simple cubic lattice and these are given for the free energy, chemical potential, and energy gap as function of temperature. The values are compared with those of Dichtel et al. By introduction of a second band a lower transition temperature TN is obtained. The system shows a second‐order phase transition.
Introduction to an Analysis of Ferromagnets with Dipole–Dipole Interactions
Authors: Nikin B. , Tošić B. S. , Zeković V. M. 
Published: July 2007

AbstractThe harmonic spectrum of a ferromagnet with dipole–dipole interactions is investigated. It is shown that the dipole–dipole interaction can be treated as a perturbation only after preparing of the Hamiltonian of the system. This preparation consists in elimination of linear terms of the Hamiltonian and in evaluation of the correct harmonic spectrum with respect to the fact that the system does not conserve the total number of quasi‐particles. The approach suggested is valid only in the presence of strong external electromagnetic fields.
Band Structure of K2[Pt(CN)4]Br0.30·3H2O
Authors: Zeller H. R. , Brüesch P. 
Published: July 2007

AbstractExperiments show that except for the Peierls instability the conduction electrons in K2[Pt(CN)4]Br0.30·3H2O behave as nearly free electrons in the c‐direction. This is concluded from the optically deduced effective mass m* ≈︁ me, from the oscillator strength sum rule and from energy and oscillator strength of the first interband transition. It is shown that this result although unexpected at first sight is a natural consequence of the one‐dimensionality of the system.
Spin‐Wave Spectra below Saturation Magnetization and at Various Orientations
Authors: Trautvetter C.‐U. 
Published: July 2007

AbstractSpin‐waves have been excited in a 2480 Å permalloy film at various orientations between the external magnetic field and the normal of the film. Absorption signals were measured at microwave frequencies between 0.9 and 17 GHz covering a magnetic field range up to 16 kG. Soohoo's formula is shown to be valid for magnetic fields even below saturation magnetization. Observation of spin‐waves in the limit of zero magnetic field leads to an unambiguous labeling of the modes. From these experiments the exchange coupling constant is determined to A = 2.03 × 10−6 erg/cm. No critical angle of orientation where the spin‐wave spectra should collapse to a single peak was found.