bluuebunny/crossref_metadata_2025 huggingface dataset:
Alloys with Off‐Diagonal Disorder
Authors: Mertsching J.
Published: July 2007

The 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

In 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

Raman 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.
The Hartree Energy of Simple Metals
Authors: Mahan G. D.
Published: July 2007

The Hartree energy is the energy eigenvalue of an electron in the potential field of an ion and the other electrons. This eigenvalue is computed numerically for seventeen metals. For divalent and monovalent metals, the eigenvalue is very small, of the order of a few hundreths of a Rydberg.
Green Function Theory of Short‐Range and Long‐Range Order in Magnetite
Authors: Ihle D. , Lorenz B.
Published: July 2007

For 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

The 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

It 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

The 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

The 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

FeKα1 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

The 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

A 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

Local 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

Energy 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.
Inversion Methods in Infrared Spectroscopy
Authors: Galtier M.
Published: July 2007

Several relations between the real and imaginary part of an analytic function are derived from causality conditions. These relations are applied to the calculation of the complex index of a crystal from transmission or reflection measurements. It is shown by some examples how it is possible to use at best experimental data.
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

Landau 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

The 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

The 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

The 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

The 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

A 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

A 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

The 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

The 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

The 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

The 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

A 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

The 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

The 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

Lorentz 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

Starting 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

Using 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

The 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

Experiments 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

Spin‐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.
Spin Wave Amplification in Ferromagnetic Semiconductors: Plasma Effects and s–d Interaction
Authors: Filho M. D. Coutinho , Miranda L. C. M. , Rezende S. M.
Published: July 2007

We evaluated the effects of the s–d interaction and of plasmon‐assisted processes on the spin‐wave amplification by drifting carriers in ferromagnetic semiconductors. Under the experimental conditions in CdCr2Se4 the contributions of these processes to the amplification factor are negligible compared with that one due to interaction of the current previously analyzed.
Some Applications of the Bond Orbital Model
Authors: Ciraci S. , Tiller W. A.
Published: July 2007

The ‘bond orbital’ model, which is based on the tight‐binding approximation, is summarized and utilized to calculate the cohesive energy of tetrahedrally coordinated solids. Several physical properties of such semiconductors are successfuly correlated with the bonding energy of the bond‐orbital. These are the band gap, the melting temperature, and the vacancy formation energy. In addition, the hardness is found to vary linearly with polarity.
Induced Scattering of Neutrons by an Electromagnetic Field
Authors: Benoit C. , Solal G. W. Cohen
Published: July 2007

An investigation is made of the modification of the one‐phonon scattering cross section of neutrons when the crystal is irradiated by an infrared monochromatic beam. It is found that under particular conditions an additional peak appears. The magnitude and the position of the peak depend on the intensity and the frequency of the IR beam.
Ohmic and Non‐Ohmic Transport in Undeformed and Plastically Deformed Tellurium
Authors: Thalmayr E. , Kahlert H. , Kuzmany H.
Published: July 2007

In order to study the influence of plastic deformation on hole transport in tellurium, the ohmic conductivity and Hall effect as well as the conductivity by warm and hot carriers have been investigated for various degrees of deformation. The temperature dependence of the mobility can well be explained by polar optical scattering, if additional scattering contributions due to lattice imperfections are taken into account which are particularly important for deformed samples. For warm carriers, positive deviations from Ohm's law are found which can be represented by σ = σ0(1 + βE2). The coefficient β turns out to increase strongly with the degree of plastic deformation. This may be caused by the variation of both the mobility and the carrier concentration with electric field strength. The two effects can be separated according to their different time‐dependent behaviour.
A Two‐Multiplet Tunneling Model
Authors: Bridges F.
Published: July 2007

The tunneling model is extended to include two multiplets of energy levels. The inclusion of tunneling matrix elements between the multiplets is found to modify the ground state level considerably and also gives rise to allowed paraelectric transitions similar to the “backward” lines observed in KI:OH−.
Separation of the Electronic and Lattice Thermal Conductivities in Bismuth Crystals
Authors: Uher C. , Goldsmid H. J.
Published: July 2007

The thermal conductivity of pure bismuth in a very high magnetic field has a contribution from the transverse thermomagnetic effects that is not negligible compared with the lattice contribution. It is demonstrated, however, that the lattice component in the binary direction can be determined accurately from measurements of the total thermal conductivity with a high magnetic field in the bisectrix and trigonal directions. Such measurements have shown that the lattice conductivity of bismuth is strictly inversely proportional to the absolute temperature in the range 35 to 140 K, and have allowed the electronic contribution to be evaluated.
Singlet Exciton–Charge Carrier Interaction in Anthracene Crystals
Authors: Kalinowski J. , Godlewski J.
Published: July 2007

Prompt fluorescence in anthracene crystals is shown to be modified by the application of an external voltage to the illuminated crystal–water contact. The singlet exciton quenching due to the interaction with the injected holes is investigated. The rate constant for the interaction Kst = (3 ± 1.5) × 10−9 cm3 s−1 is determined. The results indicate that singlet exciton motion is a diffusion process and that the quenching is caused by hole‐modified crystal defects. The efficiency of detrapping of holes by singlet excitons is found to be about 30%.
Photo‐Induced Electron Spin Resonance (Photo‐ESR) in β‐Rhombohedral Boron
Authors: Nadolny A. J.
Published: July 2007

Photo‐induced ESR has been investigated in the temperature range 150 to 300 K using nearly homogeneous optical excitation (hv ≈︁ 1.3 eV). The relaxation of the photo‐ESR shows, similarly to that of photoconductivity, two components which are approximately described by a logarithmical function of time, decay rate being enhanced with increasing temperature. The time and temperature dependences of the photo‐ESR as well as the relationship between the photo‐ESR and photoconductivity are interpreted on the basis of a model, with traps for electrons having various values of emptying energy continuously distributed in a certain energy range and two kinds of localized valence states.
Scattering of Phonons by Bound Holes and Phonon Conductivity of p‐GaSb
Authors: Singh M. , Verma G. S.
Published: July 2007

The effect of uniaxial stress along the [001] direction on the scattering of phonons by bound acceptor holes in doped semiconductors has been investigated for the situations ω < ωr and ω > ωr where ωr is the resonance frequency. The relaxation rates for the off‐resonance frequency regions are used in explaining anomalous resonance dips of the thermal conductivity curves of p‐GaSb. Two conductivity integrals, K0 < ω < ωr and K where ωD is the Debye frequency, are considered. Assuming that the phonon resonances are of very narrow width, the total conductivity is expressed as the sum of the above integrals and appropriate relaxation rates for ω < ωr and ω > ωr are used. The present approach is quite successful in explaining the experimental results of thermal conductivity of p‐type GaSb except at low temperatures.
Resonant Scattering of Phonons by Two‐Level Impurities
Authors: Joshi Y. P.
Published: July 2007

Application of the thermodynamic Green's functions is made to the study of phonen scattering by impurities that have two distinct integral configurations corresponding to different energy eigenvalues. Operators are defined which do the function of transferring the impurities from one state to another, and the Hamiltonian of the system is phenomenologically expressed in terms of these. Imaginary phonon Green's functious are calculated, using finite‐temperature S‐matrix expansion and diagrammatic representation. It is found that the phonon relaxation time, which is obtained from these Green's functions, explicitly involves a temperature dependent factor.
The Pseudopotential Method and the Problem of Ordering in Alloys. I. Pairwise Interaction
Authors: Krasko G. L. , Makhnovetskii A. B.
Published: July 2007

The thermodynamic statistical theory of ordering by Khachaturyan is formulated in terms of the pseudopotential method of second order perturbation theory. The general quantity of the theory, V(k) – the Fourier transform of alloying potential – is expressed through the pseudopotentials of the ions constituting the alloy. As an example the CaBa system is considered and an a‐priori procedure of determining the equilibrium superstructure and its thermodynamic characteristics is demonstrated. The difficulties and limitations of the theory are discussed.
On the Spectral Intensity Distribution of Donor–Acceptor Pair Recombination in GaP
Authors: Bindemann R. , Unger K.
Published: July 2007

The spectral intensity distribution of donor–acceptor pair spectra in GaP (donors at P sites) is theoretically investigated by use of a model recently developed for the evaluation of photon energies, and is compared with experimental data. For high excitation levels the intensity distribution is governed by the overlap integral of electron and hole wave functions and is found to depend on the impurity concentration, but hardly on the chemical nature of the impurities involved, i.e. not on the central cell correction. The implications of these observations are discussed.
Wave Vector Dependence of the Electron Energy Loss Functions of Graphite
Authors: Venghaus H.
Published: July 2007

Electron energy loss measurements were performed on graphite single crystals with 55 keV electrons. From spectra taken with transferred momentum hq parallel to the c‐axis and recorded for different absolute values of |q| the wave vector dependent energy loss function Im [−l/ϵ‖(E, q)] was determined for 0<E ⪅ 8 eV and 0.25 Å−1<|q|< < 0.75 Å−1. Apart from the dispersion of the energy loss maximum in the 5 to 6 eV region the main feature of Im [−1/ϵ‖(E, q)] is the appearence of a band gap Eg, increasing from Eg ≪ 1 eV for |q| = 0 up to Eg ≈︁ 1 eV for |q| as 0.7 Å−1. Additional measurements made with transferred momentum ħq perpendicular to the c‐axis confirmed the dispersion of the π‐plasmon, exhibiting an anisotropy for large wave vectors and different orientation of the latter within the layer planes of graphite.
Electronic Excitations (3 to 35 eV Range) in Some Layered III–VI Compounds: GaS, GaSe, and InSe
Authors: Soukiassian P. , Cazaux J. , Perrin J.
Published: July 2007

The energy losses suffered by a 40 keV electron beam through some monocrystalline layered III–VI compounds have been measured. By means of a Kramers Kronig analysis, the complex dielectric constant, in the 3 to 35 eV energy range, is computed for GaS (∥ and ⊥ to c‐axis), GaSe (∥ and ⊥ to c‐axis) and InSe (⊥ to c‐axis) and the e2 sum rule is applied. Attempts of interpretation are suggested with a special attention for the outer shell core electron excitations.
Localized Plasmon Modes in Lattice Defects of Metals
Authors: Nieminen R. M.
Published: July 2007

The localized modes of plasma oscillations in defects of free‐electron metals are discussed within the random phase and hydrodynamic approximations. The trapped plasmon frequency is calculated as a function of the defect size for spherical voids in several metals. The relevance of the defect modes to experimental detection techniques is discussed.
Raman Spectra and Optical Absorption Edge of Li3P04 Single Crystals
Authors: Harbach F. , Fischer F.
Published: July 2007

Large single crystals of the high‐temperature form of lithium phosphate have been grown from the melt. The Raman spectrum of these single crystals is reported in detail at room temperature and at liquid nitrogen temperature. The frequencies can be divided into groups of internal modes, librations and translational modes of the PO tetrahedra and into two groups of vibrations of the lithium ions against the oxygen lattice, split in consequence of the different site symmetries of the lithium atoms. The optical absorption edge in Li3PO4 (HT) single crystals is studied additionally. Li3PO4 single crystals are transparent up to 8.16 eV at 20°C or 8.37 eV at 78 K referred to an absorption constant K = 100 cm−1.
Anisotropic Hyperfine Interaction of Pb3+ Ions in 2S1/2‐State EPR
Authors: Born G. , Hofstaetter A. , Scharmann A. , Vitt B.
Published: July 2007

From the angular dependences of EPR‐spectra of the 2S1/2‐state system Pb2+ in CaWO4 an anisotropic hyperfine interaction with the 207 Pb nucleus is inferred. The weak anisotropy (≈︁10/00) of the hyperfine A‐tensor is caused by the admixture of covalent ligand orbitals to the central s‐function in D2d symmetry.
The Contribution of Electric Quadrupolar Interactions to Nuclear Spin Relaxation by Vacancy‐Type Self‐Diffusion Mechanisms in Metals
Authors: Cavelius E.
Published: July 2007

The spectral densities determining the quadrupolar contribution to the NMR relaxation times are calculated for randomly diffusing vacancies in cubic metals. For the computation of the field gradients due to the vacancies Friedel's oscillating potential for the screening of a vacancy by conduction electrons is used. It is shown that the quadrupolar relaxation rate can be used to determine separately the energies of migration and formation of a vacancy. Detailed numerical results and estimates are given for the metals aluminium and rubidium.
Cluster Theory of Spin Excitations of Mixed Antiferromagnets. Application to MncCo1−cF2 and KMnCCo1−cF3
Authors: Dzyub I. P.
Published: July 2007

The cluster theory which takes into account the fluctuations in the longitudinal and transverse parts of the exchange interaction is generalized for studying mixed antiferromagnets. It is shown that the spin‐excitation widths reach their maximum values for the wave vector at the zone boundary. It is demonstrated that the coherent wave approximation generalized for a mixed antiferromagnet describes rather well the wave‐vector dependence of the spin‐excitation frequencies and their structure factors measured in mixed fluorides by means of the slow neutron scattering.
Indirect Transitions in Spectra of BiI3 Crystals at Low Temperatures
Authors: Shekhmametev R. I.
Published: July 2007

Absorption and luminescence spectra of BiI3 crystals have been studied in a temperature interval of 1.6 to 77°K. In the vicinity of the intrinsic absorption edge steps in the absorption contour have been detected (steps A, B, C, AT, BT), having a positive coefficient of temperature shift. These steps can be related to indirect exciton transitions with phonon emission and absorption. Emission of the indirect exciton has been observed, which belongs to the spectral region where the inverse hydrogen‐like series lines of absorption and luminescence are located. The energy of the indirect exciton formation (Eik = 16205 cm−1) and of phonons taking part in indirect transitions have been determined.
The s–d Bond and the Hyperfine Field on the Nucleus of Iron Atoms in FecPd1−c Alloys
Authors: Galperin F. M. , Kizhaev F. G.
Published: July 2007

It is shown t at in consequence of s–d bond oscillations of the charge (spin) density of 3d–electrons appear in disordered ferromagnetic alloys of 3d and 4d transition metals. The origin of these oscillations is different from those in the RKKY and Friedel theories. The contribution of these oscillations to the hyperfine field on the nucleus of iron atoms in FecPd1−c alloys is calculated. The concentration of iron changes from c = 0 to c = 0.25.
The Effective Mass of Holes in p‐Type AIIIBV Compounds
Authors: Filipchbnko A. S. , Lang I. G. , Naslbdov D. N. , Pavlov S. T. , Radaikina L. N.
Published: July 2007

A rigorous calculation is made of the concentration dependence of the effective masses of holes in a series of p‐type AIIIBv compounds. The density‐of‐states effective masses and the optical effective masses (conductivity masses) in the compounds investigated are shown to differ essentially from each other both by their absolute value and by the character of their concentration dependences. These discrepancies are accounted for by the complexity of the valence band structures in the AIIIBv compounds.
Diffuse X‐Ray Determination of the Energy of Mixing and Elastic Constants of GeSi Solid Solutions
Authors: Bublik V. T. , Gorelik S. S. , Zaitsev A. A. , Polyakov A. Y.
Published: July 2007

The short‐range order and elastic properties of GeSi solid solutions of 28, 54, and 64% Si are studied by means of X‐ray diffuse scattering measurements. The mixing energy of the system taking into account the existence of two f.c.c. sublattices in the diamond structure is determined by extrapolating the long‐wave Fourier transforms of the concentration fluctuations Cq to q = 0 points corresponding to the node (400) and equals for q = 0 to W(q=o) = (2.5 ± 0.2) kcal/mol. The elastic constants of the GeSi solid solutions are determined and their positive deviations obtained from the additive values which are in good agreement with the data of the lattice parameter of the system.
Self‐Consistent Phonon Width Calculations in Alkali Halides
Authors: Fischer K.
Published: July 2007

The cubic anharmonic phonon decay process is reinvestigated. The δ‐functions which describe energy conservation in the harmonic approximation are replaced by Lorentzians in the self‐consistent theory, whose widths are given by the sum of the line widths of the interacting phonons. For KI at room temperature, the critical points in the damping function are “washed out” almost completely by this line broadening effect and the shift function is reduced considerably, as compared to conventional calculations with constant width parameters.
Magneto‐Thermoelectric Power of Bismuth in Weak Magnetic Fields
Authors: Hansen O. P. , Nielsen H.
Published: July 2007

The element α33(H) of the thermoelectric power tensor for bismuth has been measured at five temperatures between 15 and 37 K, and in magnetic fields up to 40 G. The analysis of the experimental data is based on the expansion of 4aL33(H) to second order in H. The coefficients in this expansion are calculated from the theory of Korenblit. Disagreement with the experimental results is found.
Shallow Donor States in Semiconducting CdF2
Authors: Langer J. M. , Langer T. , Pearson G. L. , Krukowska‐Fulde B. , Piekara U.
Published: July 2007

The origin of near infrared absorption spectra in semiconducting CdF2 crystals is reviewed. Good agreement between theoretical calculations and experimental data for CdF2:Gd or Ga gives strong support to the photoionization origin of the near infrared absorption. Lack of an evident “chemical shift” for the different shallow impurities studied indicates the intrinsic nature of the shallow donors responsible for both the infrared absorption and conductivity of CdF2. Arguments are presented for attributing the line spectrum observed in lightly doped CdF2 with phonon replicas of zero‐phonon ls–2p absorption masked by reststrahlen absorption of the host. It is proposed that for systems like semiconducting CdF2 such states should be described in terms of Fano's theory of discrete states degenerate with the continuum.
Optical Absorption of Defects in V205 Single Crystals: As‐Grown and Reduced V2O5
Authors: Clauws P. , Vennik J.
Published: July 2007

Absorption spectra are presented of V2O5 single crystals in the range 0.5 to 2.1 eV. From the behaviour of the optical absorption upon reduction, reoxidation, and annealing it follows that two different centres are active. The bands observed at 1.25 and 1.52 eV for a polarization parallel to the a‐axis (E‖a) and at 1.0 eV for E‖c increase after homogeneous reduction and are attributed to oxygen vacancies. The remaining structure in the range 0.6 to 1.1 eV is influenced by annealing and quenching and is probably due to a centre related with stacking faults. A possible centre is the V–V pair proposed by Perlstein.
Strong Coupling of the Proton–Phonon System in KDP. I. Isotope Effect on the Transition Temperature TC
Authors: Godzik K. D. , Blumen A.
Published: July 2007

A mixed mode model is proposed for KDP (KH2PO4), which includes explicitely the coupling of the proton to the O…︁O vibrations of its hydrogen bond. The Hamiltonian is formulated in the framework of the tunnelling model using delocalized states of single H‐bonds as a basis set. The linear transverse coupling term is handled by a canonical transformation in infinite order perturbation theory. Uncoupling the protonic and vibrational motion a Tokunaga‐Matsubara type Hamiltonian is obtained with a renormalized tunnelling frequency and a renormalized interaction between different hydrogen bonds, both effects being mass dependent. This mass dependence explains fully the isotope effect of the transition temperature.
The Lattice Vacancy in Si and Ge
Authors: Yip Kwok Leung
Published: July 2007

The extended Hückel theory is used to study the electronic structure of the vacancy in Si and Ge. Localized levels associated with the vacancy are found in the forbidden gap near the valence‐band edge. The equilibrium positions of the four neighboring atoms around the vacancy after symmetric relaxation are predicted for the four different charged states of the vacancy. Jahn‐Teller effects are found to he small. The migration energies for vacancies in Si and Ge are also estimated. Results are compared with other calculations and experimental data.
A Study of Spin–Phonon Interactions in Ferromagnetic Insulators. I. The Spin Wave Spectrum
Authors: Jones M. J. , Cottam M. G.
Published: July 2007

A field‐theoretical perturbation method is used to study the effects of Spin–phonon interactions in a Heisenberg ferromagnet. The calculations are carried out by employing an expansion in powers of the high‐density parameter 1/z, where z is the number of spins interacting with any given spin. In lowest order (1/z)0 this leads to molecular field theory, and the effects of spin fluctuations and Spin–phonon interactions are then introduced in higher orders of the perturbation expansion. The renormalised energy and damping of the spin wave excitations are evaluated to second order in the Spin–phonon coupling, and the dependence of these quantities on temperature, wave vector, and magnetic field is obtained. The calculations are valid over a wide range of temperature below Tc.
Electrical Properties of Tellurium under Hydrostatic Pressure. I. Effects on Its Effective Mass and Energy Gap
Authors: Koma A. , Tani T. , Tanaka S.
Published: July 2007

Conductivity and Hall coefficient have been measured in undoped and Sb‐doped tellurium at 77 and 273 K under hydrostatic pressure up to 7000 bar. It is found that the hole mobility increases with pressure and that the sample with its c‐axis perpendicular to the current direction shows a more rapid mobility increase than the parallel one. The mobility increase under pressure in undoped tellurium is interpreted as resulting from the decrease in the hole effective mass under pressure, and the pressure coefficients of the hole effective mass are derived from the experimental results. The pressure coefficients of the energy gap including the second order one are also obtained from the precise measurement of the pressure dependence of the Hall coefficient of undoped tellurium at 273 K, where it is intrinsic.
Point Defects in Lithium Hydride Crystals
Authors: Ermoshkin A. N. , Evarestov R. A.
Published: July 2007

The results of the calculations of the electronic structure of the F‐centre and the impurity centre LiH(Mg) by means of the molecular orbital method in the framework of the so‐called cluster model are reported. An attempt is made to estimate the positions of the local defect levels relative to the crystal bands. The results of the calculations are in satisfactory agreement with the experimental data.
The Semiempirical LCAO Treatment of Co2+ in ZnS, ZnSe, and ZnTe
Authors: Biernacki S. W.
Published: July 2007

The Semiempirical LCAO method is used to describe the electronic 3d levels of the paramagnetic impurity Co2+ as well as the band states in II–VI compounds. The effective one‐electron Hamiltonian taken as a sum of the kinetic energy, the interaction energy with point nuclei, and the interaction energy with spatial distribution of the electrons contains one adjustable parameter. The cubic field splitting parameter Δ and the positions of impurity levels with respect to valence band are calculated and compared with experiments.
Zur Theorie der ½〈111〉‐Schraubenversetzung in Alkalimetallen
Authors: Hübner U.
Published: July 2007

Die Energie der ½〈111〉‐Schraubenversetzung in den Alkalimetallen wurde mit Hilfe der linienweisen Summation für das Modell konstanter Ladungsdiehte gewonnen und minimalisiert. Die Gleichgewichtskonfigurationen stimmen mit Konfigurationen von Vitek et al. im wesentlichen überein. Eine metastabile Konfiguration konnte nicht ge‐funden werden. Die Winkelverteilung der kritischen Scherungen wurde für homogene Scherungen berechnet. Die Ergebnisse sind in guter Übereinstimmung mit aus experimen‐tellen Werten zu T =0 K extrapolierten Daten für Kalium.