bluuebunny/crossref_metadata_2025 huggingface dataset:
An Estimation of Force Constant Change of Mössbauer 119Sn Impurity Nuclei in Palladium Host
Authors: Puri R. K. , Gupta L. R.
Published: July 2007

Precision measurements are made of the second order Doppler shift and absorption areas of the Mössbauer line of 119Sn, present as a dilute impurity in palladium lattice, over a range from liquid helium temperature to about 750 °K. The weighted mean temperatures have been determined from the moments ω(−2), ω(−l), ω(+l), and ω(+2) of the phonon frequency distribution in the palladium lattice as seen by the Mössbauer 119Sn impurity atoms. Assuming a Debye spectrum, ratios of the impurity‐host to host‐host spring force constants have been estimated from the characteristic temperatures on the basis of Mannheim's model of nearest neighbour central forces. The Sn‐Pd force constant has been found weaker than the equivalent Pd‐Pd force constant indicating the presence of resonance band modes without any evidence of localised impurity modes.
Multiple s‐Scattering in an Ideal Random Medium. II. Incoherent Transport and the Density of States
Authors: Lenk R. , Füger M.
Published: July 2007

For a disordered system of point‐like scatterers the density of states is calculated in two ways: firstly as usual with the Green's function method and then based on a general relation between the density of states and the normal “particle” density. This density is determined in two steps: firstly its coherent part as the absolutely squared coherent field and secondly its incoherent part as the solution of a classical transport problem. Velocity and mean free path of the incoherent radiation are identical with the corresponding quantities of the coherent field. This opens the way to get information on the transport behaviour of the disordered material if only the properties of the coherent field are known.
Born Model Calculations of the Properties of the Alkali Azides. II. Azide Librational Frequencies and Elastic Moduli
Authors: Welch D. O. , Danemar Å. , Royce B. S. H.
Published: July 2007

Second derivatives of the lattice potential energy, derived on the basis of a Born model of cohesion that has been modified to account for ionic asphericity, have been examined by calculating zero wavenumber azide librational mode frequencies and two elastic moduli for the tetragonal alkali azides. Comparison of the results with experimental data favors a dispherical representation of the closed‐shell repulsive character of azide ions over ellipsoidal representations.
Tensor Components of the Linear Magnetoconductivity of Warm Electrons in Many‐Valley Semiconductors
Authors: Asche M. , Sarbei O. G.
Published: July 2007

The tensor components for the linear magnetoconductivity and the square law deviations of the conductivity from Ohm's law are calculated assuming the existence of one relaxation time only. The results are discussed for the case of τ depending on energy only.
Interband Faraday Rotation and Ellipticity in Silicon Measured by Polarization Modulation
Authors: Metzdorf J. , Kessler F. R.
Published: July 2007

Interband Faraday rotation has been measured in Si up to the first reversal in sign at 3.33 eV photon energy corresponding to the beginning of direct transitions at the zone centre. The direct gap is found to be (3.33 ± 0.02) eV at 300 K. The samples, very thin, self‐supporting films (d ≧ 0.1 μm), were obtained from epitaxial Si wafers. Faraday ellipticity as well as rotation has been measured to an accuracy of ≦ 10−5 using polarization modulation, which corresponds to a hundred percent magnetic field modulation. Faraday ellipticity, measured below 2.3 eV, is only associated with indirect transitions. But the Faraday rotation due to these indirect transitions calculated from ellipticity according to the Kramers‐Kronig relationship is at the maximum 2% of the total rotation. The influence of the direct and indirect transitions is discussed.
Processes Due to Optical Excitation of Impurities with One s‐EIectron (Ag0, Cu0, TP+, In2+, Ga2+) in KCI Crystals
Authors: Osminin V. , Zazubovich S.
Published: July 2007

At optical excitation of the electronic (A0) and hole (A2+) centres with 2S1/2 ground state (2S1/2 centres) in KCI crystals, polarized intra‐centreluminescence and destruction of the centres are observed due to the delocalization of electrons or holes. Localization of holes in the crystal, or interaction of electrons or holes with various crystal defects results in the formation of new centres (e.g. A0, Vk, Vk−T10 and Vk−T1+ pairs, H, A2+v−c). Luminescence is found to occur at the formation of new centres. Spectral, polarization, temperature, and concentration dependences are investigated for the phenomena observed.
On the Theory of Bloch Electrons in a Homogeneous Electric Field
Authors: Fiddicke J. , Enderlein R.
Published: July 2007

A general expression for the solution of the time dependent Schrödinger equation of a Bloch electron in a homogeneous electric field is derived. For a special choice of the initial conditions Wannier's solution is obtained. An iterative solution procedure for the Wannier equation is given. A general formula of the density of states is discussed and applied tothe case of strong electric fields and weak periodic potentials.
Numerical Calculation of the Drift Diffusion Current for Elastically Interacting Point Defects
Authors: Pbofant M. , Wollenberger H.
Published: July 2007

For long range interactions the probability of defect encounter is determined by the drift diffusion current of one defect to the other. This current is calculated numerically for iso‐tropic point defects in an anisotropio elastic continuum. The diffusion current is found to be a fixed fraction of that caused by a spherical potential for which the current can bewritten by a simple analytical expression.
Optical Absorption of VI2
Authors: Van erk W. , Haas C.
Published: July 2007

The absorption spectra of single crystals of VI2 in the region6000 to 26000 cm−1 are reported. The spectra show a large number of narrow absorption lines which disappear above the magnetic orderingtemperature TN = 15 K. The absorption bands are assigned to electronic transitions of the d electrons of vanadium. From the spectra the ligand field splitting 10Dq = 7870 cm−1 and the Racah parameters B = 540 cm−1 and C = 2322 cm−1 are calculated. The results are compared with data for VC12 and VBr2.
Zubarev Green's Function of the s‐f Model for Magnetic Semiconductors and Its Relation to the CPA Solution
Authors: Rangette A.
Published: July 2007

The s‐f model for magnetic 'semiconductors is solved approximately with Zubarev's formalism without any decoupling. The solution is shown to correspond to the CPA solution for the self‐energy. Diagrams are introduced and a complete analysis of the problem is given. It is shown that the CPA solution is obtained by neglecting all diagrams which contain crossing interaction lines.
Atomic Configuration of 1/2 <111> {110} Edge Dislocations in Pure V, W, Mo, and Fe and in Fe Containing C Interstitials
Authors: De hosson J. Th. M. , Sleeswyk A. W.
Published: July 2007

The atomic configuration around a perfect edge dislocation with ‐1/2‐ <111> Burgers vector lying on a {110} slip plane is calculated using the Wilson and Johnson potentials for V, W, Mo, and Fe. The boundary conditions are given by anisotropic elasticity theory. A narrow dislocation without any stacking fault results but in all these metals somedisplacement occurs in the core parallel to the dislocation line. Johnson's iron‐carbon potential is used to calculate the positions with maximum energy gain for carbon atoms near these dislocations. The strongestbinding energy of the carbon atom of ‐0.70 eV is found for the di‐latational sink close to the dislocation line.
Perturbative Valence Charge Density of Trigonal Se and Te
Authors: Bertoni C. M. , Bisi O. , Calandra C. , Nizzoli F.
Published: July 2007

The electron valence charge density in Se and Te is calculated using perturbation theory to second order in an average form factor which accounts for the lion‐locality of the pseudo‐potential. The convergence of the perturbative expansion is tested and discussed, and a comparison is made with previous calculations derived from band structures. The importance of the second order term is related to the covalence of the chemical bond, lattice dynamics, and bonding charge models.
Magnetic Behaviour of the Dihydrate Formates M(HCOO)2 2 H20 of the Transition Metals M  Mn, Fe, Co, Ni
Authors: Burlet Paulette , Burlet P. , Rossat‐mraxod J. , De combarieu A. , Bedin E.
Published: July 2007

Magnetic structures of the dihydrate formates of the transition metals (Mn, Fe, Co, Ni) are reported which are determined by means of neutron diffraction experiments. The magnetic moment directions are interpreted in terms of crystal field anisotropy. In order to determine the influence of anisotropy on the magnetic ordering some mixed compounds of manganese and iron formates are examined. For all these salts the metallic A ions (000, 01‐21‐2) linked together by formic groups show, below TN, a canted antiferromagnetic ordering. The metallic B ions (‐1‐2‐ 0, −1‐2− 0 ‐1‐2‐) not linked together show an ordered moment atlow temperature with respect to TN. In the manganeseand nickel formates the observed directions of the magnetic moments are notrelated to the axis of the oxygen octahedron and are due to a competition between the dipolar anisotropy and a weak single ion anisotropy. In the cobalt and iron counterparts the crystal field effects are strongly dominant and responsible for the observed moment arrangement. In iron formate the Fe2+ ions behave like a pure Ising model. The magnetic behaviour of the Mn formate is strongly affected by iron impurities. For iron concentrations higher than 0.1 the properties of the solid solutions are those of the pure iron salt since in the range of iron concentration 0 < x< 0.1 the solid solutions show a complex behaviour which can be interpreted by a variation of exchange and anisotropy energies.
Crystal Field Splitting in Octahedral and Tetrahedral Symmetry for Fe3+ Ions in Y3Fe5012
Authors: Vien Tran Khanh , Dormann J. L. , Le gall H.
Published: July 2007

The crystal field splitting of the energy levels of Fe3+ ions in octahedral and tetrahedral sites of Y3Fe5O12 have been computed between 10000 and 32000 cm−1. The calculation is performed using, first, the cubic field approximation, then the effect of a trigonal distortion, and finally the spin‐orbit interaction in the two magnetic sites. It is shown that the contributionof thetrigonal distortion to the absorption spectrum can be neglected. Thespin‐orbit coupling does not disturb the value of the calculated level energies but induces a coupling between some multiplets. With the use of the spin‐orbit coupling a very good fit between the computed and theexperimental spectra is obtained leading to an assignment of the observed absorption lines which is slightly different from that obtained without the spin‐orbit coupling effects.
Normal Inclusions in Type II Superconductors
Authors: Timms W. E. , Walmsley D. G.
Published: July 2007

A modified local model of an isolated fluxline has been used to calculate the energy of a system of fluxlines and a magnetically reversible cylindrical normal inclusion. Strict attention to quantisation of fluxoid leads to forces which are comparable to those derived from simple considerations of condensation energy; they can extend to distances much greater than the coherence length. It is found that an analytical treatment of the same problem by Mkrtchyan and Shmidt, when generalised, can give the energy of the system to better than 10% in most circumstances.
Total Energy of Simple, Transition, and Rare Earth Metals and the Ionic Radii
Authors: Solt G. , Kollár J.
Published: July 2007

A simple model is presented that gives an overall, semi‐quantitative description of the binding energy, equilibrium atomic radius, and bulkmodulus for simple, rare earth and IIIB‐VIB transition metals as well, by making use only of the valency (column number in the Periodic Table) and the value of the ionic (Pauling) radius. The two radius parameters of the modified empty‐core potential are linearly connected in the model, and both are correlated with the experimental ionic radius. The model, besides giving reasonable results for most of the simple metals, reproduces fairly well the cohesive parameters of the rare earth elements and the results are rather satisfactory also for transition metals up to the middle of each series.
The Theory of Sensitized Fluorescence in Solids
Authors: Golubov S. I. , Konobeev Yu. V.
Published: July 2007

The widely used Förster‐Galanin‐Dexter's (FGD) procedure of representation of the electronic excitation energy‐transfer rate between donor and acceptor impurity molecules, imbedded in small concentration in molecular crystals, is discussed by the overlap, integral of their light emission and absorption spectra. It is shown that this representation is only an approximative one, therefore a correction factor to the optical spectra overlap integral is given. This correction factor depends on the distance between donor and acceptor molecules and has a longer range in the case of the interaction of the donor and acceptor electronic excitations with acoustic phonons as compared to the interaction with optical phonons. The validity conditions of the representation mentioned above are discussed in detail.
On the theory of surface waves in a uniaxial semiconductor slab
Authors: Tarkhanian R. G.
Published: July 2007

The coupled optical‐phonon–plasmon surface waves in a uniaxial polar semiconductor slab are investigated. A number of new surface modes are found which do not occur in cubic materials. Some properties of surface magnetoplasmons due to finite size are investigated.
Rate‐equation approach to stage II damage curves of metals in terms of the two‐interstitial model
Authors: Frank W.
Published: July 2007

A theoretical expression describing the increase of the electrical resistivity of metals as a result of irradiations at temperatures in the recovery stage II is derived within the frame‐work of the two‐interstitial model of radiation damage. The derivation is based on the use of rate equations taking into account the following processes: simultaneous production of mobile crowdions or immobile dumbbell interstitials, and of an equal number of immobile vacancies; trapping or athermal conversion of both off‐line and on‐line crowdions at impurities, dumbbells, trapped crowdions or nearby vacancies; annihilation of crowdions at vacancies. It is shown that, as a matter of principle, the information which may be obtained from stage II damage curves cannot suffice to exclude the two‐interstitial model. On the other hand, under favourable circumstances stage II damage curves may enable us to demonstrate the validity of the two‐interstitial model unambiguously and thus to rule out the one‐interstitial model.
The path integral formalism as a new description for the diffraction of high‐energy electrons in crystals
Authors: van Dyck D.
Published: July 2007

It is shown that the Feynman path integral formalism provides an elegant way of describing the elastic scattering of electrons in crystals. Transformation of the formulae into momentum space permits the use of the Fourier transform of the periodic crystal potential so that the evaluation of the path integral results in a multiple sum. In the case of high‐energy electron diffraction, the introduction of the boundary conditions can be performed in a “forward scattering approximation”. It can be shown that the result obtained is equivalent with the solution of the system of Howie and Whelan, derived from the Schrödinger equation under a similar approximation. Applying a parabolic approximation to the multiple sum leads to the multi‐slice formulae of Cowley and Moodie. The kinematic and phase grating expressions can also be obtained using appropriate approximations.
Superconducting properties of lanthanum selenide
Authors: Sosnowski J.
Published: July 2007

Superconductivity phenomena in lanthanum selenide crystals with different electron concentration are observed. The parameters describing the superconducting state, in particular the critical temperature Tc and the lower critical field Hcl, are determined from measurements of the resistivity transition, the magnetic susceptibility and the Meissner effect on samples with various electron concentrations. From the experimental results the Ginzburg‐Landau parameter χ, the penetration depth λd, coherence length ξd, the Gorkov impurity parameter ϱ, etc. are deduced. The dependence of the critical temperature Tc on electron concentration is calculated on the basis of McMillan's theory and compared with experiment.
Use of representative points of the Brillouin zone for the self‐consistent calculations of solids in the large unit cell approach
Authors: Evarestov R. A.
Published: July 2007

For the self‐consistent (SC) calculation of solids an approach is suggested combining the “large unit cell” method with the theory of representative (special) pointsin the Brillouin zone. In the framework of the approach suggested the SC calculation of a solid can be realized by the self‐consistent methods developed for molecules. As an example cubic LiH, MgO, and KCl crystals are considered having face‐centered Bravais lattice. The SCCC (self‐consistent charge configuration) procedure used in molecular calculations is modified to be applicable for solids. Basing on the results obtained the character of the chemical bonding in the crystals considered is discussed.
Theory of free‐carrier absorption in n‐PbSe
Authors: Szymáski J.
Published: July 2007

Free‐carrier absorption is calculated for n‐type PbSe using the model of a nonparabolic, nonspherical, and multivalley conduction band. New scattering mechanisms of the electron in PbSe‐like structure (two‐band deformation potential for optical phonons and twooptical‐phonon deformation potential) are considered in order to explain the recent experimental data on n‐PbSe.
Theory of interference between electronic and phonon Raman Scattering
Authors: Bechstedt F. , Peuker K.
Published: July 2007

A theoretical treatment is given of continuous electronic and discrete one‐phonon Raman scattering for the case that the two scattering processes give rise to nearly the same Raman shifts. By using a Green's function formalism the final state interaction due to the interband electron–phonon coupling is described and it is found that the resulting line shape is similar to that of Fano's theory. A discussion is given of the frequency and the concentration dependence of the characteristic parameters.
Non‐Parabolic E(k) Relation of the Lowest Conduction Band in Bi2 Te3
Authors: Köhler H.
Published: July 2007

Electronic maw parameters of the lowest conduction band in Bi2Te3 are determined from Shubnikov‐de Haas investigations in stationary magnetic fields up to B = 10.5 T induction. The electron concentrations ranged from about 2 × 1017 to 2 × 1019 cm−3. In contradiction to earlier reported results the cyclotron masses of the six‐valley ellipsoidal energy surfaces depend on the Fermi energy at various doping levels of the samples, though the ellipsoidal shape of the Fermi surfaces (with invariable ratio of the main axis) remains unchanged. The extrapolated band edge cyclotron mass for B parallel to the trigonal axis is 0.06m0 ± 5%, the density of states mass for one single valley at the conduction band edge amounts to 0.082m0 ± 10%, which corresponds to an integral density of states mass of 0.27m0 ± 10% at the band edge.
Electron and Phonon Spectra of Calcium
Authors: Taut M. , Eschrig H.
Published: July 2007

A calculation of energy bands, electron density of states, and Fermi surface as well as phonon spectra, phonon density of states, Debye temperature, and elastic constants of calcium is presented and comparison is made with experiments. Shaw's model potential is used, however the parameter A1(ε′F) is fitted to an appropriate de Haas‐van Alphen‐frequency.
A Theory for Electron Transport Due to Phonon Scattering Including Screening
Authors: Doran N. J. , Gerlach E.
Published: July 2007

A new theory of resistivity is presented based on the equivalence of energy loss and Joule heat assuming acoustic deformation potential and piezoelectric scattering. Screening is explicitely taken into account. This leads to a reduction of the resistivity.
Optical Studies of Free and Bound Excitonic States in GaSb Evidence for Deep A+‐Complexes
Authors: Rühle W. , Jakowetz W. , Wölk C. , Linnebach R. , Pilkuhn M.
Published: July 2007

The energies of the free exciton and of four excitons bound to neutral acceptors (A°, X) are determined by either reflection, absorption, or emission with high purity p‐type GaSb. The data give evidence that bound excitons may be described alternatively as an electron bound by a donor‐like A+‐centre. For one particular deep A+‐centre, transitions from neutral donor states to that A+‐centre are observed besides the bound exciton decay. p‐GaSb is especially favourable for observing deep A+‐centres, because of the position of the Fermilevel. Conditions under which they may be observable for other III–V compounds are discussed.
The Effects of Surface Layers on Spin‐Wave Spectra in YIG Films
Authors: Ramer O. G. , Wilts C. H.
Published: July 2007

Experimental spin‐wave spectra for thin magnetic (YIG) films have been matched by computations in which the surface boundary conditions were provided by thin surface layers with magnetic properties different from the bulk. In the case of annealed films, a diffusion layer between the film and its non‐magnetic substrate (GGG) is assumed to result from the thermal annealing. Thislayer is shown to produce a surface anisotropy field similar to the tensorial field postulated by Yu, Turk, and Wigen. This model accurately predicts the experimental spin‐wave spectra for all angular orientations of the applied field and qualitatively predicts the temperature dependence of the critical angle, the angle at which only the uniform mode is excited. Expressions for the resonance field and the critical angle are derived. The properties of the surface layers are discussed and the model is compared with experimental data from annealed and unannealed YIG films.
The Determination of the Symmetry Group of Molecular Centres in Crystals
Authors: Ledovskaya E. M. , Petrashen M. I.
Published: July 2007

A simple method of determination of the symmetry group of molecular centres in crystals is developed. The knowledge of this symmetry group may be used for the interpretation of the vibronic and rotovibronic spectra of these centres.
Multiphoton Absorption by Free Charge Carriers in Semiconductors
Authors: Pokatilov E. P. , Fomin V. M.
Published: July 2007

The absorption coefficient for intensive electromagnetic radiation is calculated for different scattering mechanisms in semiconductors: ionized impurities, polar and nonpolar optical interaction, deformation and piezoelectric interaction with acoustic phonons — at any temperature using the path‐integral method. The dependence on the intensity of partial multiphoton absorption coefficients as well as that of the total coefficient is investigated. In the cases of low and high temperatures the dependence of the absorption nonlinearity form on the scattering mechanism is studied. The possibility isrevealed to identify the dominant scattering mechanism on the base of experimental data on the nonlinearity of absorption.
Effective Electron‐Hole Interaction in Molecular Crystals (I)
Authors: Petelenz P.
Published: July 2007

An effective electron‐hole interaction is investigated which arises due to the one‐particle exciton dissociation matrix elements of the Hamiltonian. Fröhlich‐type unitary transformation is adopted and the transformed Hamiltonian is discussed. The exciton energy is renormalized. The sign and absolute value of the effective electron‐hole interaction is revealed to depend on the energy difference between the neutral exciton and charge transfer states. The influence of the vibronic coupling is also investigated.
Polytypism and Lattice Vibrations of PbI2
Authors: Grisel A. , Schmid Ph.
Published: July 2007

Raman scattering and infrared reflectivity results are reported for the 2H and 4H polytypes of PbI2. The two polytypes can be clearly distinguished by their vibrational properties. The infrared reflectivity shows a broad reststrahlen band, pointing to the significantly ionic character of this compound.
Phonon Dispersion in the Layered Compound PbI2
Authors: Dorner B. , Ghosh R. E. , Harbeke G.
Published: July 2007

Phonon dispersion curves up to a frequency of 2.5 THz have been measured by inelastic neutron scattering in a sample of Pbl2 with D structure for the three principal symmetry directions. No appreciable two‐dimensional character has been detected in the dispersion of the acoustic modes, and the elastic constants derived from the measurements show only slight anisotropy. Comparisons are made of the present optic mode determination with previous measurements.
Double γ‐Acoustic Resonance in Crystals
Authors: Gabbielyan R. G. , Kocharyan L. A. , Mkrtchyan A. R. , Tumanyan S. S.
Published: July 2007

A theoretical analysis is made of nuclear transition probability from the ground state to the sublevels of the first excited state under the influence of γ‐quanta and external acoustic excitation. When the electric field possesses an axial symmetry, Heff = 0 and the ultrasonic oscillation frequency Ω satisfies the resonance condition between the nuclear sublevels, therearetwo possible cases of transition if the energy of the monochromatic γ‐quanta satisfies the resonance. The two cases are analyzed depending on the ultrasonic oscillation power and the angle between γ‐bunch and the direction of ultrasonic excitation. It is shown that for A0/λ≪ 1 and ψ = π/2 the 1 → 2 → 3 transition is more plausible. When Ao/λ ≧ 1, θ = 0, φ = 0, the 1 → 3 transition is more probable.
Crystal–Impurity Energy Transport by Molecular Excitons in the Wave‐Like Limit I. Capture Mechanism
Authors: Markvart T.
Published: July 2007

The treatment, by conventional diffusion methods, of exciton transport from the host crystal to impurities is proved to be valid only in the limit of strong exciton–phonon interaction. I n the present work a possible solution to the problem of crystal–impurity exciton transport is presented in the opposite limit. The assumption of rapid exciton transport within the crystal is used to explain the capture mechanism. Other assumptions are used to bring the final result to a form similar to Forster's for the electronic energy transfer between two molecules.
Far‐Infrared Interband Magnetoabsorption and Band Structure of Hg1−xCdxTe Alloys
Authors: Kim R. S. , Narita S.
Published: July 2007

Magnetoabsorption experiments on n‐type Hg1−xCdxTe alloy crystals (x = 0, 0.07, 0.126, 0.148) have been performed in the Faraday geometry in the spectral region 4 to 40 meV at 4.2 K and with magnetic fields up to 50 kOe. In the analysis of the spectra we have used the formulae collectively including the exact interactions among the T8,T6, and T7, bands. Our experimental results and the computed energies indicate that „the modified Luttinger parameters”︁ of the T8 bands are common to samples with various x‐values (x < 0.158).
To the Theory of the ESR Lineshape of Triplet Excitons in Molecular Crystals: Influence of the Quasi‐Incoherent Exciton Motion and Comparison with Experimental Data
Authors: Reineker P.
Published: July 2007

In a recent paper, on the basis of the Haken‐Strobl model for the coupled coherent and incoherent motion of excitons in molecular crystals, an equation of motion for correlation functions has been derived, describing the influence of the quasi‐incoherent motion of triplet excitons on the ESR lineshape. From this equation analytic expressions for the positions and widths of the ESR lines have been derived by second order perturbation theory. In this paper it is shown that these expressions are in excellent agreement with results obtained from exact computer calculations. The dependence of the positions and the widths of the ESR lines on the orientation of the magnetic high field in the crystal and their extreme values are discussed. From the comparison with experimental data of Haarer and Wolf for the line positions averaged values for the fine structure parameters in anthracene are derived as D = 765 G and E = −98.8 G. An effective hopping rate of T1 Y 4.4 × 1010 s−1 has been estimated from the line widths. Finally it is argued that the influence of librations on the ESR linewidth may be as important as that of the motion of the excitons between inequivalent molecules.
Magneto‐Optical Measurements on Two‐Dimensional Magnetic Model Systems
Authors: Arend H. , Von Känel H. , Wachter P.
Published: July 2007

The optical absorption of the two‐dimensional antiferromagnets (CH3NH3)2MnCl4 and (C2H5NH3)MnCl4 has been measured between 0.5 and 5.4 eV in the temperature range 4.2 to 300 K. An absorption maximum at 4.96 eV which only appears at low temperatures is proposed to originate from an inter‐3d transition between antiferromagnetically aligned Mn ions. The temperature variation of the absorption power of this maximum follows the spin correlation function for the two‐dimensional Heisenberg antiferrormagnet. Furthermore the energy of one of the intra‐3d transitions is determined as a function of temperature. However, a separation of the spectral shift of the absorption line into a Zeeman‐ and a crystal field component is not unambiguous.
The Electron and Magnon Spectra of Ferrimagnetic Semiconductors
Authors: Kozintseva M. B. , Zilbervarg V. E.
Published: July 2007

A calculation of the magnon spectrum is carried out for a degenerate ferrimagnetic semiconductor. The spin‐polarized conduction electrons indirectly effect the exchange between the local magnetic moments. As a result, in the case of antiferromagnetic direct exchange, the acoustic branch becomes harder with increase in the carrier concentration, whereas the optical branch becomes softer, if the signs of the s‐d exchange integrals are the same in both sublattices. With a certain concentration reached, the initial ferromagnetic ordering becomes absolutely unstable with respect to the long‐wave optical magnons. Also a study is made of the electron spectrum at the Néel point. In spite of the absence of the long‐range order, the short‐range order in the paramagnetic proximity still remains. Therefore, the conduction electrons move in such a manner that their spin follows the direction of the local magnetic moment slowly changing in space. This enables the electron to obtain a considerable gain in energy. It follows that a gigantic conduction band edge shift should also occur above the Néel point.
Dislocation Symmetries in the Integral Theory
Authors: Kirchner H. O. K.
Published: July 2007

The symmetry of the displacement field is investigated for the case that a dislocation runs normal to an axis of twofold symmetry. The three components of the displacement field are found to be even and odd functions of the three components of the Burgers vector. Calculations are done in the “integral theory”. The forms of the basic matrices Q, B, and S for the symmetry considered are found as well.
Role of the Jahn‐Teller Effect in the Excited States of KBr: Tl‐Type Crystals
Authors: Giorgianni U. , Mondio G. , Saitta G. , Vermiglio G.
Published: July 2007

Particular attention is paid to the role of the magnetic and thermal perturbations which allow for proper assignment of the B band and quantitative evaluation of degenerate energy peak separations. On the basis of experimental magneto‐optical rotation results an analysis of the effects of the electron‐lattice coupling upon the electronic impurity states is carried out by determining a more comprehensive expression for the excited states of Tl+ in KBr.
Vibrational Spectra, Hydrogen Bonding, and Ferroelectricity in the PbHPO4 Family
Authors: Blinc R. , Arend H. , Kandušer A.
Published: July 2007

The infrared and Raman spectra of polycrystalline PbHPO4, PbHAsO4, CaHPO4, and BaHPO4 were measured at room temperature in the range between 4000 and 30 cm−1, and the temperature dependence of the dielectric constant of these materials was studied. The OHO hydrogen bonds in PbHPO4 and PbHAsO4 are found to be very similar to those in KH2PO4 and KH2AsO4 and the XO4 groups (X = P, As) are – in contrast to CaHPO4 and BaHPO4 – as well close to being tetrahedral. The symmetry of the soft mode in PbHPO4 is deduced and it is found that the ferroelectric transition could be of second order as far as group theory is concerned. A microscopic model of the ferroelectric orderdisorder phase transition based on the interaction between the chains of OHO bonds linking the PO4 groups is proposed.
Fermi Surface Anisotropy and Properties of Tungsten Crystals
Authors: Dekhtyar I. Ya. , Silantev V. I. , Sakharova S. G. , Fedchenko R. G. , Patoka V. I. , Kolesnik V. N.
Published: July 2007

A study is made of the anisotropy of the properties of tungsten single crystals for equivalent crystallographic orientations relative to the growth axis. An essential difference in the interplanar spacings for samples with equivalent faces perpendicular and parallel to the growth axis was found. For the compared equivalent planes differences in the positron annihilation parameters were observed which point to an anisotropy of the Fermi surface caused by carbon atoms with an anisotropic electron configuration. The sublimation energy for equivalent faces is also shown to be anisotropic.
Non‐Parabolicity of the Highest Valence Band of Bi2Te3 from Shubnikov‐de Haas Effect
Authors: Köhler H.
Published: July 2007

The valence band mass parameters are evaluated from the Shubnikov‐de Haas effect in stationary fields up to B = 10T magnetic induction. The hole concentrations of the samples ranged from 5 × 1017 to 1.1 × 1019 cm−3. The ellipsoidal six‐valley model is confirmed, but in contradiction to earlier reported results the cyclotron masses depend on energy, though the shape of the Fermi surfaces does not vary with increasing oarrier concentration. The extrapolated band edge cyclotron mass for B parallel to the trigonal axis is 0.08%m0 ± 5%, for the density‐of‐states mass of a single valley at the valence band top 0.106%m0 ± 10% is obtained, which corresponds to the integral density‐of‐states mass at the band edge of 0.35%m0 ± 10%. The tilt angle ν = 31.5° ± 10% of the ellipsoidal main axes relative to the crystal axes is higher than published earlier.
Phonon Frequencies of InP Derived from Two‐Phonon Absorption
Authors: Ulrici B. , Jahne E.
Published: July 2007

Infrared absorption measurements have been performed on InP at various temperatures from 20 to 300 K in the spectral range from 370 to 700 cm−1 in which the absorption is due to two‐phonon absorption processes. The ir absorption peaks as well as the second‐order Raman scattering data by Alfrey and Borcherds are analyzed in terms of critical point phonon frequencies. The consistency of the derived phonon frequencies is checked from different points of view, above all by using the temperature dependence of the peaks. Especially the LA frequencies turn out to be larger than those of earlier assignments indicating a smaller frequency gap between the acoustic and optical phonons than that calculated by Talwar and Agrawal. Recent neutron scattering data support the assignment given here.
Size and Shape Effects on the Broadening of the Plasma Resonance Absorption in Metals
Authors: Ruppin R. , Yatom H.
Published: July 2007

Size and shape effects on the broadening of the plasma resonance absorption of light in metallic samples are investigated. The relation between three different quantum mechanical methods which have been used in the past to calculate this broadening, is clarified. The general theory is applied to various specific geometries, and some physical implications are discussed.
Symmetry Properties of the Planar Antiferrornagnet K2NiF4
Authors: Deonarine S. , Joshua S. J.
Published: July 2007

Using well established group theoretical methods a symmetry analysis is made of magaons and paramagnetic spin waves in antiferromagnetic K2NiF4. Excitons and single ion transitions are also considered and Selection rules for first‐order transitions are derived.
SpinLattice Coefficients of Eu2+ in SrF2
Authors: Kuriata J. , Guskos N. , Rewaj T. , Szumowski J. , Falkowski K.
Published: July 2007

EPR spectra of Eu2+ in SrF2 under uniaxial stress are observed. The full set of second‐and fourth‐order spin–lattice coefficients is determined. Line shifts of 15 different hyperfine transitions under uniaxial stress are measured. Five coefficients are evaluated by solving a set of 15 equations, some of them are compared with results of other authors. The differences are explained as being due to the introduction of fourth‐order terms in the spin‐Hamiltonian. The results are found to support Stedman and Newman's model. The correlation with the temperature dependence of b is discussed.
The Correlation between Optical Absorption Spectra, Carrier Mobility, and Phase Transition Temperature in Some Ferroelectrics
Authors: Bursian E. V. , Girshberg Ya. G. , Ruzhnikov A. V.
Published: July 2007

A relation between absorption band frequencies connected with free carrier absorption the IR‐region, mobility values, and phase transition temperatures in titanates, tantalates, and niobates is discovered. The existence of a single mechanism determining both phase transition and carrier kinetics in the conduction band is supposed.
Generalised Viscosity and Dislocation Damping Constant in Diatomic Crystalline Solids
Authors: Martin J. W. , Paetsch R.
Published: July 2007

Using Brailsford's model for the anharmonic dislocation drag, generalized viscosities and the damping constant have been calculated for a diatomic solid, represented by a modified Debye‐Einstein type phonon spectrum. The generalized viscosity and its temperature dependence have been examined in detail. It is shown that the effect of phonon dispersion is to increase the region in which the thermoelastic effect makes an appreciable contribution to the viscosity. Consequently, the dislocation drag constant for the Debye‐Einstein model is found to be enhanced by a factor \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{1}{2}q_D l_A $\end{document} compared with the Debye model (qD is the Debye wave number, lA the mean free path of the acoustic phonons). It is also shown that in general the damping constant is very sensitive to temperature, shape of phonon spectrum and phonon lifetimes and it is difficult to predict its temperature dependence. The Debye‐Einstein model gives B α Tm, where m < 1 for intermediate temperatures and tends to one for high temperatures.
On the Existence of Spin Waves in a Model Transition Metal with Finite Anderson Mixing Potential
Authors: Zieliński J.
Published: July 2007

Interband s–d exchange interaction is added to the transition metal model Hamiltonian involving the Anderson mixing term. Spin waves are shown to exist even when the Anderson mixing potential remains finite. The value of the exchange coupling constant which makes this possible depends on the mutual localization of s‐and d‐like bands and on the shape of the density of states function of the s‐like band.
Intraband Conductivity and Thermopower of Semiconductors with Slowly Varying Gaussian Random Field
Authors: Zhumatii P. G.
Published: July 2007

Intraband conductivity and thermopower of a unipolar semiconductor with a slowly varying Gaussian random field are computed. The calculations are made on the basis of the Kubo formula and a semiclassical theory of the particle motion in a random field. Explicit expressions are obtained for the do conductivity, thermopower and mobility of the charge carriers under various conditions related to different physical situations. Comparison is made with the available experimental data.
Neutron Spectroscopy of Naphthalene Crystal Internal Phonon Modes
Authors: Bokhenkov E. L. , Natkaniec I. , Sheka E. F.
Published: July 2007

Spectra of the incoherent inelastic neutron scattering from naphthalene crystals in the region of internal phonon modes (170 to 1300 cm−1) are studied at the temperatures 296, 80, and 4.7 K. The spectra obtained are quantitatively processed by means of a direct spectral procedure. The ambiguity present before in the frequency spectrum of the internal phonon modes of the crystal in the region from 170 to 1300 cm−1 is eliminated basing on these experimental results together with results on Kaman‐Mandelstam scattering spectra and infrared absorption. The au mode frequencies inactive in optical spectra are determined. The possibility is indicated to use the displacement amplitude of the free molekule hydrogen atoms in order to describe the investigated phonon modes in the crystal.
The Valence Band of Os Studied by Electron Spectroscopy
Authors: Nilsson R. , Berndtsson A. , Mårtensson N. , Nyholm R. , Hedman J.
Published: July 2007

The valence band of osmium is investigated using an HP5950A ESCA spectrometer. The electron spectrum is compared with density of states functions from the literature, one obtained from a linear muffin tin orbital calculation for osmium and one from an APW calculation for rhenium.
Surface Plasmon‐Polaritons in a Spatially Dispersive Inhomogeneous Medium
Authors: Boardman A. D. , Paranjape B. V. , Nakamura Y. O.
Published: July 2007

The dispersion of surface plasmon‐polaritons and their associated fluctuation densities and electric fields are calculated for a spatially dispersive semi‐infinite electron plasma, with, parameters appropriate to the metallic state. The model is hydrodynamic and the electron density is assumed to be uniform and equal to the bulk value, except for a small region near the surface. In this surface neighbourhood the electron density is assumed to fall in a step‐function manner as in previously reported successful models of surface profile properties. The momentum range covers that which is experimentally available and includes both the retardation and electrostatic regimes. It is emphasised that the inclusion of spatial dispersion, by the introduction of a pressure gradient, is an essential feature. This is because the distance over which the surface density changes is such that the transit time of the carriers is the same order of magnitude as the inverse plasma frequency. Resonances are possible, therefore, which are entirely absent in pure retardation models which ignore spatial dispersion. Fluctuation densities, electric fields, and polarisation plots are presented. The previously reported assertion that the charge distributions can be rigorously classified into multipoles is investigated numerically.
Transformation of Incoherent Phonon Flux into a Coherent Acoustical Signal as the Result of the Nonlinear Amplification of Acoustical Phonons in Semiconductors
Authors: Gantsevich S. V. , Gurevich V. L. , Kagan V. D. , Katilius R.
Published: July 2007

A theory of nonlinear interaction of sound waves in a noise beam is developed. The case is considered where the intensity of the beam grows along a semiconducting crystal in a do electric field. It is demonstrated that if the nonlinear contribution to the amplification constant is negative the transformation of a noise beam into a coherent sound signal with fixed amplitude is possible during a single transit through the crystal. This occurs if the linear amplification constant has a sufficiently sharp maximum while the nonlinear attenuation constant is a smooth function of the wave vector.
The Formation Energy of Vacancies in Aluminium and Magnesium
Authors: Tzanetakis P. , Hillairet J. , Revel G.
Published: July 2007

The formation energy of vacancies is measured under conditions of very low vacancy concentrations for ultrahigh purity aluminium, after quenching down to 4 K. A value of (0.69 ± 0.03) eV is found. Similar experiments made using pure magnesium yield a formation energy of (0.79 ± 0.03) eV. It is shown that in magnesium vacancies are mobile well below room temperature.
The g‐Factor of the Holes in the Highest Valence Band of Bi2Te3
Authors: Köhler H.
Published: July 2007

The Shubnikov‐de Haas effect is investigated on p‐type Bi2Te3 in the concentration range from 5 × 1017 to 1.1 × 1019 cm−3, in magnetic fields up to 10 T flux density, between 1.6 and 4.2 K. From spin‐splitting of the magnetoresistance maxima the product M = gmc/(2m0) is found to be strongly anisotropic in contrast to the results on other semiconductors. Hence the g‐factors may be determined, by inserting the known cyclotron masses mc, yielding a considerable energy dependence and anisotropy of the second rank tensor components of g. From values for |M| ≷ 1 a positive sign for the g‐factor (g > 0) is deduced. Assuming ϑo = 31.5° ± 10% (ϑ0 tilt angle of ellipsoidal principal axes of the Fermi surfaces relative to the crystal axes) to fit also to the corresponding g‐tensor ellipsoids, the principal values g′1 (parallel to the binary axis) and g′2 (in the mirror plane, tilted by ϑ0 relative to the bisectrix axis) are of about one order of magnitude larger than g′3 ≈ 2 (within the regarded mirror plane, tilted by ϑ0 relative to the trigonal axis). The results correspond to the data of the lowest conduction band of Bi2Te3 in every respect, and are discussed qualitatively in a six‐band model.
Displacement of Impurities through Interaction with Radiation‐Induced Interstitials in Alkali Halides
Authors: Matsunami N. , Yokoyama Y. , Itoh N.
Published: July 2007

Optical absorption and channeling studies of irradiated Pb++‐doped KCl are made. It is found by channeling experiment that Pb++ is displaced nearly to the face center by irradiation at room temperature. Optical absorption studies indicate that the position of the peak of the V‐type centers produced in Pb++‐doped KCl is different from the V centers produced in pure KCl and the A band for Pb++ becomes broad by irradiation. It is concluded that the interstitials are associated with the impurities and cause the displacement of the latter. A possible model for the complex of the interstitials and the impurity is suggested.
Non‐Thermal Stimulation of Three‐Level Impurities by Phonons
Authors: Michel H. , Wagner M.
Published: July 2007

A phonon system is considered, which is coupled to three‐level‐impurities (e.g. tunneling levels of rotator molecules in alkali halides). The kinetic equations for both the phonons (Peierls‐Boltzmann equation) and the occupations of the impurity levels are established. They are solved in a self‐consistent manner for the stationary case. Computer calculations yield the non‐thermal (i.e. finite flux) heat conductivity as well as the scattering and absorption of monochromatic phonons. Finite flux heat conductivity measurements are proposed and the expected deviations in the heat conductivity curves are drawn.
Theory of Electron–Hole Plasma in CdS
Authors: Zimmekmakn R. , Rösler M.
Published: July 2007

A finite temperature theory of the electron‐hole plasma in highly excited direct semiconductors is presented using Green's functions. The self‐energy is calculated within the single‐plasmon pole approximation. Results are given for CdS with respect to modified energy bands, free energy, and emission intensity. Special attention is paid to band anisotropy and the participation of two split valence bands.
Adsorption‐Induced Changes of Electron Levels
Authors: Brodskii A. M. , Urbakh M. I.
Published: July 2007

The electron spectrum of a crystal with an adsorbed centre is considered using the mathematical techniques of the quantum mechanical theory of scattering. A special pole approximation is developed which is substantiated by specific features of the problem, Formulas are derived for the adsorption‐caused changes of thermodynamical characteristics and compared with the experimental data.
Self‐Consistent Phonon Calculation for Hexamethylenetetramine Molecular Crystals
Authors: Wasiutyński T.
Published: July 2007

The self‐consistent phonon approximation is formulated for crystals with orientational degrees of freedom. The lattice dynamics regarded as a function of temperature and pressure is calculated for hexamethylenetetramine in the “6‐exp” atom‐atom approximation. Good agreement of the calculated lattice constant, phonon frequencies, and elastic moduli with experimental data is obtained.