bluuebunny/crossref_metadata_2025 huggingface dataset:
Valence bands of amorphous and crystalline GeTe determined by X‐ray and UV photoemission
Authors: Shevchik N. J. , Tejeda J. , Langer D. W. , Cardona M.
Published: July 2007

The density of valence states, the energies of the core levels, and the characteristic energy loss spectra of crystalline and amorphous GeTe have been measured by X‐ray (1486.6 eV) and UV (21.2 eV) photoemission spectroscopy. The valence band densities of states of the two GeTe phases are in general similar, but exhibit distinct differences in their detailed structure. That of the amorphous form of GeTe may be approximated by a sum of the densities of states of the elemental forms of amorphous Ge and Te. A model for the valence band density of states is proposed which may be generally applicable to random covalent networks. No difference in the separation of the core level of the Ge and Te atoms is found between the amorphous and crystalline phases, thus indicating that there is no significant ionicity difference in the bonding character of these two phases. In light of the current structural models for the amorphous form, the similarities between the photoemission from the two phases are surprising. The plasma frequencies of the amorphous and crystalline forms are found to be 16.4 and 16.9 eV, respectively.
Large transverse spin component around impurities in an almost saturated ferromagnet
Authors: Muggli J.
Published: July 2007

Around a nonmagnetic impurity in a ferromagnet as e.g. Nd in iron the spin system is strongly disturbed. The magnetic exchange is weakened and the spins interact with the strain through magnetoelastic coupling. It is shown in a simple continuum model that a strong enough strain can cause an instability of the solution with parallel average spin and produce thus a long range disturbed region with a net transverse spin component. The hyperfine field at the defect site makes then an angle to the total magnetization in accordance to experimental results.
A method for computation of dislocation loops in a pile‐up
Authors: Maurissen Y. , Minari F. , Capella L.
Published: July 2007

Analytical methods cannot be applied to problems of dislocation pile‐ups when the number of dislocations is large or when the stress field involved is not uniform. A general method is given of computation based on the decomposition of a dislocation loop into elementary loops. By these one can determine the configuration and the number of dislocations in a pile‐up in the case of every sort of stress field or barrier.
Resonance interband interaction and vibronic coupling in molecular crystals
Authors: Zgierski M. Z.
Published: July 2007

A two‐exciton‐band model of a molecular crystal with linear vibronic coupling of different strength in each band is investigated. The possibility of resonance interaction between two exciton bands is taken into account. A formula for the optical absorption coefficient of the lower, less intense band is derived and discussed for the first singlet exciton state of the anthracene crystal. It is shown that the resonance interband interaction is completely responsible for the difference between the relative intensities of the vibronic lines in the absorption spectrum of the crystal and the free anthracene molecule.
The Electronic Structure and Magnetic Moment of Fe, Co, and Ni Atoms
Authors: Galperin F. M.
Published: July 2007

A previously suggested new theoretical model for the extension of the charge density of 3d electrons of atoms of ferromagnetic metals of the iron group (in comparison to that in a free atom) and a concept of a neutral atom in metals is considered and on the same basis a calculation has been made for the electronic structure and magnetic moment of Fe, Co, and Ni atoms.
Magnetic Field Effects on the Spin Wave Spectra and Magnon Specific Heat of Antiferromagnetic NtF2
Authors: Deonarine S. , Joshua S. J.
Published: July 2007

The effects of magnetic field on the spin wave spectra, magnon density of states, and the magnon contribution to the specific heat have been calculated for the antiferromagnet NiF2. The results show that the Davydov splitting of the k= 0 magnons decreases with increasing field whilst the zone boundary magnons remain degenerate. The spin wave contribution to the specific heat decreases with increasing field as expected.
Computer‐Generated Structures of Amorphous Ge
Authors: Shevchik N. J.
Published: July 2007

Several structures for amorphous Ge have been generated on a computer with procedures that are believed to approximate the true deposition process. All of the structures are characterized by rigid bond lengths and 8 to 10° bond angle distortions. The primary differences among the structures are in the distribution of the dihedral angles. For the most physically reasonable assumed deposition process, in which the dihedral angle are unconstrained, the resulting RDF of the cluster agrees best with the experimental RDF. It is shown that the continuous random network model for amorphous Ge can fit even the higher features of the experimental RDF.
Scattering Probabilities for Holes I. Deformation Potential and Ionized Impurity Scattering Mechanisms
Authors: Costato M. , Reggiani L.
Published: July 2007

The integrated scattering probabilities per unit time for the hole‐deformation potential (acoustic and optical) scattering mechanism, and for the hole‐ionized impurity scattering mechanism have been evaluated for a typical two‐band (Ev1 and Ev2)valence band structure, spherical and parabolic. The wave functions of the two bands are mostly p‐like, the first (P3/2, M±1/2), the second (P3/2, M±3/2). The symmetry properties of the valence band wave functions are shown to introduce the following effects: (1) To scale down of about a factor two the transition rate for the acoustic and optical deformation potential scattering mechanisms with respect to the case in which an s‐like symmetry is assumed for the band wave functions. (2) To correct in a non‐trivial manner the transition rate for the ionized impurity scattering mechanism, leading to a decoupling effect of the two bands by increasing the hole energy. This decoupling effect opposes the otherwise strong coupling for the transitions from Ev2 to Ev1 found for the acoustic and optical deformation potential scattering mechanisms.
Study of the Excitation of 4d Electrons in Rare‐Earth Metals by Inelastic Scattering of a High Energy Electron Beam
Authors: Trebbia P. , Colliex C.
Published: July 2007

4d electron excitations have been investigated in several rare‐earth metals (Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) by means of electron energy losses of a high energy (75 keV) electron beam. Experiments were made with a transmission electron microscope modified by the adjunction of a Castaing‐Henry dispersive system. The results of the measurements lying in the energy range between 120 and 190 eV agree rather well with X‐ray photoabsorption spectra and oscillator strengths calculated in an atomic model.
EPR Study of Impurity—Vacancy Aggregates in NaCl Containing Both Mn2+ and Other Divalent Cations
Authors: Ikeya M. , Crawford J. H.
Published: July 2007

Several aggregation products involving divalent cation—cation vacancy complexes in NaCl have been studied using the EPR of Mn2+ which is present as a minor constituent. As the probability that the aggregation product contains more than a single Mn2+ is small, the EPR spectrum of the Mn2+ should be indicative of the symmetry of the aggregates. The principal axes of aggregates produced by an anneal were detected with EPR and compared with those calculated for possible models. Three spectra newly found were identified tentatively as a dimer with 〈100〉 symmetry, large aggregates probably in {111} planes, and a Mn2+ occupying a cubic site in a second‐phase precipitate.
Analytical derivation of spin configuration and intrinsic coercive field of a narrow domain wall
Authors: Hilzinger H. R. , Kronmüller H.
Published: July 2007

In ferromagnetic materials with narrow domain walls the wall energy depends periodically on the position of the wall centre with respect to the crystal lattice. This gives rise to an intrinsic coercive field. The spatial variation of the wall energy is determined by methods previously applied by Seeger and Schiller to the Peierls potential of dislocations. It is found that the oscillations of the wall energy depend sensitively on the precise spin structure of the domain wall. Therefore an improved micromagnetic treatment of the domain wall was required taking into account fourth‐order terms in the exchange energy.
First and second order Raman scattering of monocrystalline CuCl
Authors: Prevot B. , Sieskind M.
Published: July 2007

A detailed study of the first and second order Raman effect in CuCl is presented using either He–Ne or Ar‐ion laser beam. The spectrum is observed at room and liquid N2 temperature. For the first time, at room temperature a pronounced polarization effect on the 65 cm−1 line was observed when looked either at the Γ15 component of the spectrum or at the Γ1 and Γ12 ones. In the lower part of the Raman shift, the Γ15 spectrum is the strongest, whereas the Γ1, Γ12 contributions become more visible beyond 200 cm−1 but weaker by an order of magnitude. Using recent neutron scattering data (slightly corrected for optical phonons) the major peaks are explained by superimposed lines rather than by isolated processes. The most parts of the features are well interpreted. The lines at 162 and 199 cm−1 are shown to have a similar temperature shift while the 123 cm−1 one behaves quite different.
Mössbauer frequency shift in spin‐ordered systems
Authors: Shukla G. C.
Published: July 2007

Considering the displacement of magnetic ions up to cubic terms the temperature‐dependent Mössbauer shift has been calculated for a crystal of monoatomic lattice containing exchange coupled magnetic ions. The expression for the second order Doppler shift (SOD) is derived in the low‐ and high‐temperature limits.
The role of spatial dispersion in a quadrupole light absorption
Authors: Davydov A. S. , Eremko A. A.
Published: July 2007

The phenomenological Maxwell equations are used to investigate the passage of light through a crystal in a frequency range that corresponds to an isolated band of exciton states of the quadrupole type. The conditions are found under which the singularities connected with the wave‐vector dependence of the exciton frequency and an oscillating dependence of the light transmission coefficient on the crystal width may occur (when the width is small). The calculated results are compared with the Gorban‐Timofeev experiment with Cu2O in the quadrupole exciton absorption region when λ = 6125 Å.
Fine structure details in the LMM Auger spectra of germanium
Authors: Lindau I. , Ribbing C. G.
Published: July 2007

High resolution LMM Auger spectra of germanium are reported showing fine structure not previously observed and interpreted. The Auger spectra were obtained by X‐ray excitation and analyzed under high resolution with an electrostatic analyzer. The observed fine structure for the LMM transitions L3M23M45, L3M45M45, and L2M23M45 can not be interpreted within the j‐j coupling model but fits better into the pure L‐S coupling scheme of the final vacancies of the Ge atom.
Antiferromagnetic properties of NiO and CoO. II. Paramagnon symmetries and magnon dispersion relations
Authors: Joshua S. J. , Morgan F. D.
Published: July 2007

An investigation is made of paramagnons and mognons in NiO and CoO. In particular, the possible paramagnon symmetries for NiO and CoO are derived and the magnon dispersion relations calculated along all the important symmetry directions of the antiferro‐magnetic Brillouin zone of NiO and CoO. The latter are found to agree well with the group theoretical predictions reviewed in paper I.
Lattice vibrations of lithium on model pseudopotential
Authors: Kushwaha S. S.
Published: July 2007

A two parameter model pseudopotential is used to calculate the phonon frequencies along the symmetry directions for lithium. The calculated frequencies are in good qualitative agreement with the experiment, except the observed crossing of the longitudinal and transverse branches along the [100] direction. Grüneisen parameter, compressibility, and binding energy have also been calculated and compared with available experimental results.
Variational calculation of the three‐phonon umklapp resistivity
Authors: Singh D. P. , Verma G. S.
Published: July 2007

A variational method is used to calculate the three‐phonon umklapp resistivity in Ge, Si, NaCl, and KCl where the expressions are used for the three‐phonon scattering strengths, derived previously for a simple cubic crystal. The temperature ranges, considered for Ge, Si, NaCl, and KCl are 400 to 1000 °K, 400 to 1600 °K, 30 to 80 °K, and 40 to 100 °K, respectively. The present calculations show that for the semiconductors Si and Ge the calculated umklapp resistivities are half as great as the experimentally observed values. This suggests that the three‐phonon normal processes are quite important for Si and Ge even for temperatures greater than 400 °K. For alkali halides the agreement between theory and experiment is reasonably good. It is also found that the contribution due to T + T ⇌ L processes towards the umklapp resistivity is greater than that of the T + L ⇌ L processes for all the materials at all temperatures.
On the theory of metamagnet transitions
Authors: Mitsek A. I.
Published: July 2007

The domain structure in the region of the metamagnet transition is considered and the magnetic susceptibility, magnetostriction, and ΔE‐effect are calculated. It is shown that the appearing domain walls may be of two types: discrete walls having the smaller energy, where an activation energy is necessary to displace them and continuous walls whose energy is greater, but which can be displaced elastically. Two nucleation mechanisms are considered.
Minority carrier magnetophonon resonance in p‐InSb and in p‐GaSb–InSb mixed crystals
Authors: Byszewski P. , Walukiewicz W.
Published: July 2007

Measurements of longitudinal magnetoresistance have been made on p‐InSb and p‐GaxIn1−xSb over the range of temperature from 77 to 300 K. The magnetophonon effect on the minority carriers was observed. Theoretical discussion of the position of observed minima enables the determination of transitions contributing to the oscillations and parameters of the conduction band of mixed compounds.
Dielectric relaxation of Ca++–vacancy dipoles in KCl + KBr mixed crystals
Authors: Robert R. , Barboza R. , Ferreira G. F. L. , de Souza M. Ferreira
Published: July 2007

Using dielectric loss and ionic thermo‐current (I.T.C.) techniques the dielectric relaxation of KCl + KBr mixed crystals doped with Ca++ was measured. It was found that the activation energy, H, for dipole reorientation decreases as the Br− minority anion concentration increases in the KCl + KBr:Ca++ mixed crystal. In KBr + KCl:Ca++ (Cl− as a minority anion) the activation energy for dipole reorientation increases as the Cl− minority anion increases. In addition to the relaxation peak that displaces with the minority Cl− anion concentration, another one was found in KBr + KCl:Ca++, but not in the KCI + KBr:Ca++ system where Br− is the minority anion. The experimental results are explained, for minority anion concentration not higher than 10%, in terms of lattice parameter change and of the local effect of the minority anion.
The surface density of the acoustical phonon states
Authors: Iosilevskii Ya. A.
Published: July 2007

The density of the acoustical phonon states of an elastically isotropic solid is found with allowance for its surface. The difference is discussed between the surface “thermodynamic” density of states and the local one describing the spectral distribution of vibrations of the individual atoms. The surface lattice heat capacity and the Kapitza thermal resistance are considered as the characteristic applications of the formalism developed.
Note on the debye temperature of palladium
Authors: Brown J. S.
Published: July 2007

The results of the calculation of the 0 °K equivalent Debye temperature of f.c.c. Pd are given, based on an elegant iterative approach due to Fedorov. The results of Fedorov's first and second approximations are presented for Pd along with their correlation with earlier theoretical results and with elastic and calorimetric experimental determinations.
ESCA studies of some AIIIBv compounds with Ga and As
Authors: Leonhardt G. , Berndtsson A. , Hedman J. , Klasson M. , Nilsson R. , Nordling C.
Published: July 2007

Electron spectra have been recorded from the AIIIBV compounds GaP, GaAs, GaSb, and InAs as well as from Ga203, As2O3, Ga, and As. Binding energy values, determined b; careful energy calibrations, are reported for the Ga and As 3d electrons. The bindin energy shifts for the AIIIBV compounds suggest a charge transfer from A to B.
Resonance raman scattering in group Vb semimetals: As, Sb, and Bi
Authors: Renucci J. B. , Richter W. , Cardona M. , SchÖstherr E.
Published: July 2007

The dependence of the Raman scattering cross section of As, Sb, and Bi on the energy of the exciting laser was measured in the region from 1.6 to 2.7 eV. The shape of the resonance curve agrees reasonably well with the squared magnitude of the derivative of the dielectric constant with respect to frequency. Discrepancies are only seen at low frequencies for Sb and As. They are attributed to variations of the electron‐phonon interaction through the Brillouin zone. From these measurements, relative values of several average electron–phonon interaction constants are derived.
Anisotropic electron scattering by interstitial hydrogen impurities in copper
Authors: Dye D. , Lowndes D. H. , Lengeler B.
Published: July 2007

Maps of the scattering rate anisotropy for Cu(H) have been obtained using two different representations, one based upon ß phase shift analysis, and the other using a symmetrized Fourier series. The partial wave analysis indicates that p wave scattering is heavily dominant, although the analysis does not account entirely for the large scattering rates observed on the Neck and Dogsbone orbits. The dominance of p wave scattering suggests that the screening of the interstitial proton is carried out primarily by electrons in conduction states derived from orbitals with p‐like symmetry.
Interface and surface polaritons of semiinfinite biaxial crystals
Authors: Borstel G.
Published: July 2007

Interface and surface polaritons of an arbitrary semiinfinite crystal cut of biaxial symmetry are investigated. It is shown that regular interface modes can occur only for certain orientations of the propagation vector or special crystal cuts. From energy flow considerations it is found that ordinary interface modes can exist only if the two‐dimensional propagation vector (kx, ky) is along a principal axis of the dielectric tensor. In uniaxial crystals two different types of interface polaritons are found to occur. The first is a regular and real interface mode whose dispersion relation depends neither on the crystal cut nor on the direction of propagation. The second is a generalized interface mode which may have real or virtual character.
The phonon drag in wurtzite‐type semiconductors
Authors: Bulat L. P.
Published: July 2007

The formula for the phonon drag part of the thermoelectric power tensor α in wurtzite structure semiconductors has been received. The comparison between theory and experiment as well as the analysis of various possible mechanisms of phonon relaxation enable to conclude that the phonon drag effect is associated with the piezoelectric interaction between electrons and acoustic phonons, with the phonon relaxation stemming from the phonon‐phonon scattering. Calculation of Kα = α/α for a number of compounds has been performed, where the piezoelectric potential and the dielectric and phonon spectrum anisotropy are taken into account.
On the theory oi "heating" a cousto‐electric nonlinearity in semiconductors
Authors: Bugaev A. S. , Gulyaev Yu. V.
Published: July 2007

A nonlinear theory of acousto‐electric effects for the sound waves of large amplitude is developed for semiconductors with small effective masses and large energy relaxation times of electrons whore the dominant type of acousto‐electric nonlinearity is due to the stationary heating of the electron gas by sound waves and to the dependence of the kinetic coefficients on the electron temperature. It is shown that the dependence of the electron absorption coefficient, acousto‐electric current, acousto‐electric field etc. on the sound intensity is quite different for various electron momentum and energy scattering mechanisms and is also different from corresponding dependencies for the case of the usual concentrational acousto‐electric nonlinearity. In particular, the dependence of the acousto‐electric current on the acousto‐elcctric field may be superlinear ‐ contrary to the case of the concentrational nonlinearity. It is also shown that the appearance of this "heating" nonlinearity moves off the onset of concentrational nonlinearity. The influence of a strong electric field on the nonlinear acousto‐electric phenomena is studied. It is shown that by heating the electron gas the electric field reduces both the concentrational and the "heating" nonlinear acousto‐electric effect and thus widens the dynamic range of the ultrasonic amplifier.
Electromodulation Spectroscopy of Excitons: Molybdenum Disulphide
Authors: Bordas J. , Davis E. A.
Published: July 2007

In order to clarify the role of excitons in electromodulation spectroscopy, detailed electroabsorption experiments on single crystals of 2H‐MoS2 have been undertaken. Measurements were made at 80 °K using a longitudinal field configuration, i.e. a sandwich cell, to which a square wave modulating voltage was applied. Simultaneous application of a dc bias voltage allowed the effects of incomplete field penetration to be elucidated. Modulation was obtained from the flat band condition. Detection of signals associated with both the discrete states of the exciton and the continuum has been achieved.
A Phase Shift Analysis of the Scattering of Carriers by Ionised Impurities in Non‐Degenerate Semiconductors
Authors: Boardman A. D. , Henry D. W.
Published: July 2007

The reliability of using the first Born approximation in the study of singly‐ionised electron‐impurity scattering has been investigated. A model, Brooks‐Herring, potential with a screening radius derived from the first Born approximation has been used in a partial wave analysis to determine violations of the Friedel sum rule. New screening radii and electron‐donor/acceptor cross‐sections, consistent with the sum rule, have been computed for a non‐degenerate system.
Optical Properties of Rhodium
Authors: Pierce D. T. , Spicer W. E.
Published: July 2007

The normal incidence reflectance of an electron‐gun‐evaporated Rh film measured over the photon energy range 0.5 ≦ ħ ω ≦ 11.7 eV is presented. The optical functions ϵ1(ω), ϵ2(ω), σ(ω), and the volume and surface loss functions have been calculated by a Kramers‐Kronig analysis of the appropriately extrapolated reflectance data. The structure in the optical conductivity σ(ω) at 1.1, 2.5, and 5.6 eV is compared to the predictions of a recent band structure calculation; the optical transitions giving rise to this structure are discussed. The volume and surface loss functions, which have peaks at 8.8 and 8.1 eV, respectively, are compared to characteristic electron energy loss measurements.
Vibration Spectra, Structure, and Angular Dispersion of Phonons in Crystalline Iodic Acid (α‐HIO3)
Authors: Krauzman M. , Le Postollec M. , Mathieu J. P.
Published: July 2007

Raman and infrared spectra of oriented single crystals of α iodic acid allowed to determine the whole set of fundamental frequencies with their irreducible representations. Important static and correlation field effects did not allow to apply approximate methods to give a picture of the fundamental vibration modes. A study of the angular dispersion of polar Raman vibrations gave results in good agreement with the theoretical formulae.
Diffusion of Electron—Hole Drops and Excitons in Germanium
Authors: Martin R. W.
Published: July 2007

The diffusion of electron—hole drops and of excitons has been investigated in pure and doped germanium. It was found that the diffusion lengths depend on doping level, temperature, and excitation intensity. Qualitative explanations of the results based on simultaneous motion of excitons and electron—hole drops are presented.
Two‐Photon Photoconductivity Measurements in a Hexagonal ZnO Crystal
Authors: Koren G.
Published: July 2007

Two‐photon‐photoconductivity (TPPC) measurements in ZnO, using two different dye lasers as sources, are reported. Photocurrents produced by each laser beam separately were determined and subtracted. Linear dependencies of TPPC as a function of the intensity of one source while the other source intensity was kept constant, were found forvarious source frequencies. The spectral measurements of TPPC as a function of the frequency of one source while the other source frequency is constant show a square root dependent rise of TPPC at the crystal energy gap, at 3.38 eV, and a second very strong and approximately linear rise at 4.25 eV. This second rise is due to a forbidden one‐photonbut allowed two‐photon transition from a deeper valence band to the first conduction band in the ZnO crystal, and was also seen in ZnO crystals. A theoretical calculation, predicting qualitatively the behaviours of TPPC as a function of the sum of the energy ofthe two photons, is also given.
Calculation of Binding Energy and Elastic Constants of Simple Metals
Authors: Shimada K.
Published: July 2007

By use of the simple model potential which is a smooth function and whose parameters depend on the lattice constant, the total energy expression is obtained by the second order perturbation theory. One calculates the binding energy, the lattice constant, and elastic constants of simple metal with cubic structure and finds the numerical values of parameters which give good agreement with the experiment in the case of alkali metals except Li.
Localized Magnetic Vibrations of Quadrupole Impurity Strongly Coupled with the Matrix
Authors: Ivanov M. A. , Mitrofanov V. Ya. , Fishman V. Ya.
Published: July 2007

The spectrum of localized magnetic vibrations of an impurity atom situated symmetrically with respect to the magnetic sublattices of an antiferromagnetic crystal is considered. For an arbitrary exchange interaction of the impurity atom with the matrix the effective Hamiltonian of a spin cluster containing the impurity atom and its nearest neighbours is found. The spectrum of the low‐lying excited states for a weakly and strongly coupled impurity has quadrupole character (with Kramers degeneracy) and lies in the spin‐wave zone of the unperturbed crystal. In the case of a weakly coupled impurity the obtained results coincide with the data of exact calculation. The removal of the Kramers degeneracy in an external magnetic field and the angular dependence of the g‐factor are investigated.
Absorption, Photoluminescence, and Resonant Raman Scattering in Bil3
Authors: Petroff Y. , Yu P. Y. , Shen Y. R.
Published: July 2007

Absorption, photoluminescence, and resonant Raman scattering have been measured in BiI3 at low temperatures with a tunable dye laser. The results do not support the “bielectron” model of Gross et al.
On the Theory of the Interband Conductivity for Forbidden Transitions and Indirect Transitions
Authors: Henneberger K. , Strehlow R. , Wünsche H.‐J.
Published: July 2007

Following an approach of Enderlein and Peuker Kubo's formula serves to calculate the differential tunneling conductivity in the cases of direct forbidden transitions and indirect (phonon‐assisted) allowed transitions. Whereas the forbidden transitions lead to a conductivity roughly hundred times smaller than for allowed transitions the contribution from the indirect transitions depends on the coupling constant of the electron‐phonon interaction and may have the same order of magnitude as for the direct allowed transitions.
Optically Active Lattice Vibrations in Bi2Se3
Authors: Köhler H. , Becker C. R.
Published: July 2007

The preparation of Bi2Se3 single crystals with low free carrier densities allowed an investigation of the lattice vibrations to be carried out from the reflectivity of cleavage planes at nearly normal incidence of the radiation (E ⟂ c). The experimental results can be explained with two classical oscillators, whose eigenfrequencies occur at 92 and 69.5 cm−1. The static dielectric constant was determined to be 100 ± 10 for E ⟂ c.
Theory of the Residual Resistivity of Bloch Walls I. Paramagnetic Effects
Authors: Cabrera G. G. , Falicov L. M.
Published: July 2007

The contribution of Bloch‐wall electron‐scattering to the electrical resistivity of a ferromagnetic metal is calculated. Calculations are carried out for various cases which include small as well as large exchange splittings. In all cases, spherical Fermi surfaces are considered, the wall width is assumed to be small compared to the intrinsic electron mean‐free‐path and diamagnetic effects are neglected. It is found that the experimental results (large negative magnetoresistance between 0 and 100 G) could be explained, even when diamagnetic effects are not included, if the majority and minority spins have sizeably different density‐of‐states at the Fermi level.
Microscopic Theory of Lattice Dynamics in Nonconducting Crystals
Authors: Mitskevich V.
Published: July 2007

A consistent quantum‐mechanical theory of lattice dynamics in nonconducting crystals is proposed. Using the density matrix a general expression for the energy of the molecular system is derived. Taking into account the dipole moments of the electron clouds of atoms the equations of motion for the crystal lattice are obtained and the contributions in the dynamical matrix from the long‐ and short‐range interactions are derived. The influence of the overlap of the electron clouds on the terms in the dynamical matrix expression is considered.
On the Usability of Debye Theory in Mössbauer Spectroscopic Studies
Authors: Mahesh K.
Published: July 2007

The usability of the Debye theory of vibrational frequency spectra of solids for evaluating the Mössbauer recoilless fraction is discussed. Mössbauer effect in solid krypton is particularly considered for this purpose. The study of the effect of pressure on recoilless fraction of the Fe57 impurity in vanadium supports the Debye model. Further improvement is outlined using the concept of centro‐frequency of the vibrational spectra.
ENDOR Investigation of Cu++ Centres in NH4Cl. II. Theoretical Interpretation
Authors: Boettcher F. , Spaeth J. M.
Published: July 2007

A theoretical interpretation is given of the superhyperfine (shf) interactions of two Cu++ centres in NH4Cl described in Part I. In the approximation of an orthogonalised envelope function including covalency admixtures and using free ion and free molecule wave functions the centres can be understood as complex centres: [CuCl2(H2O)2] (centre I) and [CuCl4(NH3)2]2− (centre II). The shf data of the NH3 and H2O molecules are understood only qualitatively in this approximation, whereas the higher shell lattice nuclei are quantitatively understood as well as those in simple impurity defects in alkali halides. The analysis gives further support for the models proposed for the two centres in Part I.
Infrared Studies of Hydrogenated SrCl2:RE3+ Crystals
Authors: Sinnema S. G. , Den Hartog H. W.
Published: July 2007

Localized vibrations of hydride ions in hydrogenated SrCl2:RE3+ single crystals are studied with IR spectroscopy. In moderately hydrogenated crystals doped with La3+, Ce3+, or Pr3+ two doublets are observed at about 1050 and 700 cm−1. The position and the splitting of the high frequency doublet depend upon the dopant whereas only the position of the low frequency doublet is RE‐dependent. The high frequency doublet is ascribed to a complex consisting of a substitutional trivalent lanthanide ion and a substitutional hydride ion in nearest neighbor positions (C3v oscillator). The low frequency pair is due to a localized vibration of an interstitial hydride ion neighboring a substitutional RE3+ ion (C4v oscillator). The positions of the individual lines in the C4v doublet are calculated for SrCl2:La, H and are in good agreement with experiment.
Theory of Exciton Dissociation in Molecular Crystals at the Interface of a Metal
Authors: Singh J. , Baessler H.
Published: July 2007

The phenomenon of dissociation of anthracene singlet excitons at the interface of the crystal and an aluminium metal layer has been treated quantitatively applying the configuration interaction scheme. HMOs are used as the wave functions of anthracene molecule. Calculated values of the rate constants are two orders of magnitude lower than the experimental ones. This discrepancy has mainly been attributed to the use of HMOs in the calculations. It has been suggested, on the basis of the results obtained for a different set of MOs, that the agreement between theory and experiment would improve by using proper molecular wave functions.
Optical Properties and Collective Excitations in GaSe in the 2 to 80 eV Range
Authors: Mamy R. , Martix L. , Leveque G. , Raisin C.
Published: July 2007

Optical functions of GaSc are derived from the Kramers‐Kronig analysis of reflectivity data performed at quasi‐normal incidence and at room temperature in the 2 to 80 eV range. Attempts of interpretation are suggested according to recent band structure calculations. In the experimental range, all the valence electrons are excited. On the high energy side, some structure corresponds to the excitation of 3d‐electrons of Ga and Se, respectively. The calculated position of the plasmon peak is in good agreement with energy loss experiments.
Multiphonon Processes in the Spin–Lattice Relaxation of Paramagnetic Ion Nuclei
Authors: Vaisfeld M. P.
Published: July 2007

It is shown on the example of paramagnetic ions with a triplet ground electronic state that alongside with the two‐step Orbach‐Aminov processes multiphonon processes of the resonant type, in particular three‐ and four‐step ones, can contribute greatly to the relaxation of paramagnetic ion nuclei. 141Pr3+ ions in yttrium ethyl sulphate have been taken as an example.
On the Theory of an Impurity Molecule. I. Kinetic Energy Operator
Authors: Kiselev A. A. , Liapzev A. V.
Published: July 2007

For an impurity molecule the translational, rotational, and vibrational coordinates are introduced by means of the Eckart conditions. The kinetic energy operator is transformed from Cartesian to new coordinates mentioned with the help of the Frobenius theorem and the Gaussian algorithm of matrix algebra.
Dispersion Relations of Mode Grüneisen Parameters in Rubidium Halides
Authors: Jex H.
Published: July 2007

Dispersion relations of microscopic mode Grüneisen parameters have been calculated from an evaluation of a shell model to third order. Leading terms such as the anharmonic shell‐shell interaction between first and second neighbours and Coulomb contributions are taken into account. In the phonon dispersion curves breathing terms are included. The thermal expansion and mean Grüneisen constants of RbF, RbCl. RbBr, and RbI are analysed with this model.
Reconstructed Three‐Dimensional Valence Electron Momentum Density of Si from Compton Profile Measurements
Authors: Schülke W.
Published: July 2007

Recently measured anisotropies of Si valence electron Compton profiles with the scattering vector in the [100], [110], and [111] direction are analyzed together with calculated anisotropies of the corresponding Compton profiles from a modified free electron model. From both sets of profiles the momentum density ϱ(p) is reconstructed. In comparison with the modified free electron model the experimental momentum density is strongly reduced in the vicinity of p =0.6 to 1.0 at. units along the [100] direction and enhanced mainly in the vicinity of p =1.1 at. units along the [110] direction. These main features of the reconstructed experimental momentum density are well reproduced by a pseudopotential band calculation of valence electron momentum densities along the [100], [110], and [111] directions. Also the influence of the orthogonalization term on the momentum density is estimated.
Raman Spectra of BaZnF4
Authors: Quilichini M. , Poulet H.
Published: July 2007

Raman spectra of a single crystal of BaZnF4 have been obtained. Frequencies and symmetries of the Raman active optical phonons are given. The temperature dependence of the spectra have been examined. The directional dispersion of each of the oblique optical phonons has been measured from the Raman data and calculated. The dimensionless oscillator strengths are calculated.
Graphical Technique Applied to the Calculation of Excitonic Processes
Authors: Iskra V. D.
Published: July 2007

A modified diagrammatic technique is developed which is convenient to excitons in presence of external fields. The exciton Green's function is calculated as an illustration of the method.
Spin–Lattice Coupling Constants of Divalent Manganese in a Tetragonal Lattice
Authors: Owens F. J.
Published: July 2007

The shift of the fine structure resonances of Mn2+ in KN3 due to uniaxial stress are measured and the spin‐lattice Coefficients, C13 and C33 are measured t o be 3.53 × 10−13 and ‐7.06 × 10−13 cm/dyn respectively. The Blume‐Orbach model and the point ion representation of the lattice potential are used to calculate the zero‐field splitting yielding a value of +395.8 G compared to the experimental value of 534 G measured at room temperature. The theory is also used to calculate the zero‐field splitting as a function of uniaxial stress and spin‐lattice coefficients in excellent agreement with the measured values are obtained. A calculation of the implicit temperature dependence of the zero‐field splitting predicts a linear dependence on temperature with a slope of ‐0.14 G/deg which is markedly smaller than the measured value of ‐0.76 G/deg. The difference is attributed to the fact that the dominant influence in the temperature dependence is a coupling to the lattice vibrations. The effect of the relaxation of the nearest neighbor azide ions about the Mn2+ on the calculated zero‐field splitting is also considered.
Burstein Shift of the Absorption Edge of nBi2Se3
Authors: Köhler H. , Hartmann J.
Published: July 2007

The spectral position of the absorption edge of n‐type Bi2Se3, single crystals was determined from reflectivity measurements on cleavage planes at nearly normal incidence of the radiation (E ⊥ c). A small reflectivity peak yields a maximum of the real part n2 ‐ k2 of the dielectric constant, which is correlated to the critical photon energy of the direct absorption edge. A direct optical gap of 160 me V at 77 K is extrapolated for an empty conduction band. The temperature coefficient between 77 and 300 K is ‐2 × 10−4 eV/K. Assuming a linear temperature dependence down to T = 0 a gap of 175 me V at T = 0 and of 115 me V at 300 K is derived. For E ⊥ c a parabolic ϵ(k) relation of the valence band of Bi2Se3 is deduced with m = 0.125 m0 ± 10%.
Bloch Electrons in Irrational Magnetic Fields
Authors: Rauh A. , Wannier G. H. , Obermair G.
Published: July 2007

A group‐theoretical analysis is given of the eigenstates of a charged particle when a magnetic field and a periodic potential are simultaneously present. The field is assumed totally or partially irrational with respect to the crystalline structure. The representations of the magnetic translation group are generated from free electron Landau states for convenience, but no discrepancies with the Frobenius method arise. The results are as follows: if H is parallel to a lattice direction, k · H is a good quantum number; otherwise, k · H may be assigned a fixed value for all members of a representation but is not numerically unique. According to group theory, there is a fixed finite δk⊥ which separates members of the same representation, and hence, equal energy. However, consideration of the energy matrix shows that there is complete degeneracy in ky for totally irrational fields; this case is thus close to the case of free electrons. In the cases intermediate between zero and totally irrational field boundary conditions may be crucial for the energy spectrum, as illustrated by an example.
Influence of Clusters on the Thermal Conductivity of GaAs
Authors: Vuillermoz P. L. , Laugier A. , Pinard P.
Published: July 2007

The influence of a heat treatment on the thermal conductivity of GaAs in the range 5 to 77°K has been investigated. Below 20 °K the results are interpreted on the basis of scattering of phonons by extended defects (clusters). Using Callaway's analysis the size and the number of precipitates after each quenching are calculated. In the low temperature region the effects appear also in virgin crystals and give following typical values: diameter 30 Å, concentration 1015 cm−3.
Normal Electromagnetic Waves and the Propagation of Light through Crystals
Authors: Davydov A. S. , Myasnikov E. N.
Published: July 2007

The conditions under which monochromatic light propagates through a crystal in the region of exciton absorption frequencies are investigated. It is shown that the plane electromagnetic wave passing through a crystal with a broad exciton band is, in the general case, a superposition of two normal waves with different refractive indices and absorption coefficients. The relative contribution of these waves is defined by the crystal temperature and the frequency of light passing through a crystal. At temperatures above a critical one only one of the normal waves gives an essential contribution. At temperatures below a critical one both the waves play a comparable role. At frequencies less the resonance one the dominant contribution is made by one of the waves, and at frequencies greater than the resonance one by the other wave. In the resonance region both the waves with different refractive indices make comparable contributions. Therefore, the attenuation of the intensity of a plane wave is different from the exponential one and the concept of a refractive index in this frequency range gives no sense.
Les spectres RPE des ions V4+ dans les monocristaux VO2
Authors: Grunin V. S. , Ioffe V. A. , Patrina I. B.
Published: July 2007

Les spectres de la RPE des monocristaux VO2, étaient étudiés dans les diapasons X et Q à la température 77°K. Dans tous les échantillons on a observé le spectre des ions V4+ qui se décrit par le spin‐Hamiltonian avec les constantes: gx, = 1,895 ± 0,002, Ax, = 47 × 10−4 cm−1, gy, = 1,930 ± 0,005, Ay, = 44 × 10−4 cm−1, gz, = 1,925 ± 0,001, Az = 147 × × 10−4 cm−1. L'état des ions isolé V4+ dans VO2, est analogue à celui dans les structures de type routile. L'apparitions des electrons célibataires est probablement due à la présence des ions M4+ n'ayant pas des électrons 3d dans les sites de V du réseau de VO2.
Optical Properties and Energy Band Structure of Zn3P2and Cd3P2 Crystals
Authors: Sobolev V. V. , Syrbu N. N.
Published: July 2007

Edge absorption, photoconductivity, and reflectivity spectra are investigated of Zn3P2 crystals in the range from 1 to 12.5 eV and reflectivity spectra of Cd3P2 crystals in the range from 1 to 12.5 eV. The Eg value of Zn3P2 has been experimentally determined at 293 and 77 OK. Its nature has been found to be due to direct transitions. Three peaks of exciton and interband character have been observed in the photoconductivity spectrum of Zn3P2 single crystals. Complex structures (about 12 peaks) have been observed in the reflectivity spectra of both compounds. Most of these structures may be accounted for in terms of the well‐known theoretical calculations of their bands. Several peaks have been observed in the most thoroughly studied spectrum of Zn3P2. These peaks cannot be explained by the theoretical band scheme and are probably caused by the peculiarities of the bands of real crystals of the Zn3P2 type, which have been lost in the well‐known theoretical model because of the accepted simplifications of the crystal lattice.
Optical Intraband Absorption in Disordered Systems in the Presence of Electron‐Phonon Interaction
Authors: Böttger H. , Bryksin V. V.
Published: July 2007

Using Kubo's formula the optical intraband absorption is studied in two models of disordered systems. The model A is characterized by some few types of atoms and by strong electron‐phonon interaction, the model B by a continuous distribution of atomic levels and by an arbitrary strength of the electron‐phonon interaction. The frequency and temperature dependences of the optical absorption are discussed for both models. For model A the dependence of absorption on the concentrations of the different types of atoms is also investigated.
The Magnetic Field of Pure Superconductors with Non‐Isolated Flux Lines at Arbitrary Temperature
Authors: Brandt E. H.
Published: July 2007

The magnetic field distribution of pure type‐II superconductors with small magnetization is derived from the non‐local version of Gorkov's theory. At zero temperature Delrieu's result is reproduced: at the vortex centre the field shows a cusp and exceeds the external field. The form factors have alternating sign and decrease only as K−3. At larger magnetization all form factors become positive. At T = 0, even close to Hc2, a gap function is expected deviating considerably from the Hc2‐solution. This means a pronounced narrowing of the vortex core at low temperatures.
Influence of Spatial Dispersion on the Transmission Spectra of CdS Single Crystals
Authors: Voigt J.
Published: July 2007

Transmission spectra are measured on extremly thin CdS single crystals with high quality in the energy range of exciton transitions from 1.8 °K to room temperature. Analysing the characteristics of the exciton absorption lines such as lineshapes, halfwidths, and oscillator strenghts it is found that the line An=1, E ⊥ c shows at low temperatures some “anomalies”, which cannot be explained in a model with weak exciton‐photon coupling but are qualitatively well understandable in a polariton model with spatial dispersion. This is caused by the fact that at low temperatures the exciton‐photon interaction prevails over the exciton‐phonon interaction resulting in a weakly‐damped polariton.
Interaction of Interstitial Cu Atoms with One Another and with Substitutional Impurities in a F.C.C. Cu Lattice
Authors: Ivanov L. I. , Platov Yu. M. , Pletnev M. N.
Published: July 2007

Using the technique of lattice statics, the interaction energies are calculated for octahedral interstitials interacting with one another and with substitutional impurities in the f.c.c. Cu lattice. The existence of a sufficiently strong attractive energy between a pair of octahedral interstitials has been established. It is shown that the character of interaction between an interstitial and substitutional impurity trap depends to a considerable extent on the sign of the dilatation volume of the impurity. On the basis of the results obtained, the structure of lower‐order interstitial clusters is examined, formed from substitutional impurity atoms under the influence of irradiation.
Optical Absorption of Ferroelectrical Copper and Chromium Boracites
Authors: Nesterova N. N. , Pisarev R. V. , Andreeva G. T.
Published: July 2007

Optical absorption spectra of ferroelectrical boracites Cr3B7O13Cl, Cu3B7O13Cl, Cu3B7O13Cl, Cu3B7O13Br, and also borate CuB2O4 were studied in the spectral range from 2000 to 30000 cm−1 in the temperature interval 77 to 450 °K. The interpretation of the spectra are given on the basis of the crystal field theory including tetragonal distortion of the local environment of 3d‐ions in the lattice. The temperature dependence of the oscillator strength of the absorption band and absorption coefficient at a fixed frequency were investigated around the temperature of the ferroelectric phase transition in the copper boracites. The results show that the absorption is very sensitive to the displacement of the ions in the crystal lattice at the phase transition. For CuCl boracite a change of the slope of the temperature dependence of the absorption near the fundamental absorption edge was observed.