bluuebunny/crossref_metadata_2025 huggingface dataset:
A quantitative study of excitonic polariton reflectance in CdS II. B‐exciton and κ‐linear term
Authors: Broser I. , Rosenzweig M.
Published: July 2007

The influence of κ‐linear terms on the reflectivity spectra of excitonic polaritons is discussed for the case of the Γ5‐states of the B‐exciton of CdS. Using Pekar's abc and a dispersion relation consisting of three transverse and two longitudinal modes an effective index of refraction n* for oblique incidence of light is calculated. Spatial dispersion, the damping constant, the surface layer thickness, and the κ‐linear term are considered simultaneously. The energy parameters of the B‐exciton, especially the κ‐linear term, are deduced from the lineshape analysis of carefully measured reflectivity spectra. The structured spectra of the double resonance allow the more distinct investigation of the small discrepancies between theory and experiment still existing in the model of excitonic polaritons.
Reststrahlen spectra of PbSe1−xTex)
Authors: Finkenrath H. , Franz G. , Uhle N.
Published: July 2007

The TO‐ and LO‐modes of PbSe1−xTex mixed crystals are determined from optical measurements in the frequency range 660 to 10 cm−1 at 300, 100, and 8 K. The results for polycrystalline bulk samples as well as for thin films, prepared by hot‐wall epitaxy, show a two‐mode behaviour. This conclusion is in contrast to a number of criteria which permit a prediction of the mode behaviour.
Effect of second‐order spin–orbit coupling λ′ (L · S)2 on electronic spin relaxation of paramagnetic Fe2+ in CaCO3 and CdCO3
Authors: Srivastava K. K. P. , Singh H. C.
Published: July 2007

At low temperatures the Mössbauer spectra of paramagnetic Fe2+ (3d6, 5D) ions in CaCO3 and CdCO3 show magnetic hyperfine structure due to slow spin relaxation only in the presence of an applied magnetic field whereas no magnetic field is needed to observe the same in rhombohedral MgCO3 and ZnCO3. This field dependent magnetic relaxation behaviour of Fe2+ ions in CaCO3 and CdCO3 has earlier been explained by assuming a small rhombic distortion of Fe2+ substitutional sites from the trigonal symmetry of host cation sites. It is shown how the presence of second order spin–orbit coupling of the type λ′(L · S)2 for Fe2+ ions in CaCO3 and CdCO3 produces new ground states which can explain the observed magnetic relaxation properties and there is no need to artificially lower down the trigonal symmetry around Fe2+ ions. It appears that the perturbations due to the rhombic crystalline electric field and the second order spin–orbit coupling are very similar in nature.
On the X‐ray plasmon scattering from small particles of graphite at κ =0
Authors: Priftis G. D. , Boviatsis J. , Vradis A. , Kountouris K.
Published: July 2007

The inelastically scattered X‐ray radiation from small particles of graphite is investigated at zero scattering angle. It is established that there is no measurable intensity corresponding to either surface or volume plasmon excitation in agreement with the theory.
Effect of controlled defects on the microwave transport and dielectric constant of tetrathiofulvalene–tetracyanoquinodimethane
Authors: Gunning W. J. , Heeger A. J.
Published: July 2007

The introduction of controlled disorder in TTF‐TCNQ through irradiation provides information on the transport mechanism. The large changes in the magnitude and temperature dependence of the conductivity in the conducting regime are consistent with either 1d localization or pinning of CDW fluctuations by impurities. However, studies at low temperatures appear to be inconsistent with the localization ideas. Furthermore, the frequency dependence in the conducting regime at microwave frequencies suggests a substantial contribution of fluctuating CDW's to the low frequency transport. The introduction of controlled defects allows a separation of the low temperature conductivity into an intrinsic contribution plus a second contribution of impurity origin. Information on the frequency dependence of the intrinsic conductivity is obtained from comparison of the dc and microwave data. The results are interpreted in terms of soliton transport inhibited by the Larkin‐Lee tunneling.
Band structure of rhodium and photoemission from its low index surfaces
Authors: Borstel G. , Braun W. , Neumann M. , Seitz G.
Published: July 2007

Photoelectron spectra are given of the low index surfaces of rhodium together with band structure calculations which are extended up to 40 eV above the Fermi level. The photoelectron spectra are strongly dependent upon photon energy and crystal orientation. The spectra can be explained reasonably well within the direct model in terms of the band structure calculation using nonrelativistic dipole selection rules. Some features observed in the spectra of the Rh(110) surface indicate that relativistic dipole selection rules may have to be considered.
Magnitude of localized magnetic moments in metals
Authors: Kiwi M. , Pestana E. , Ramírez R.
Published: July 2007

The magnitude of the localized magnetic moment of a transition or rare earth element impurity in a metal is evaluated within the framework of the Anderson model. Rotational invariance is preserved throughout. Graphs of the magnitude of the magnetization as a function of the relevant darameters of the model are provided and discussed.
Density of valence states of the superionic conductor AgI
Authors: Ostrow M. , Goldmann A.
Published: July 2007

Temperature dependent photoelectron spectra of AgI are measured between room temperature and 251°C for photon energies hω = 21.2 and 40.8 eV. The spectra are decomposed into approximate partial p and d densities of valence states by using atomic cross sections for the corresponding partial photoexcitation probabilities. Significant abrupt reversible changes are observed in the partial p density of states at Tc = 147°C, where the crystal structure changes from the wurtzite (β) phase to the superionic “disordered” α‐phase. In contrast, the partial d density of states, except for a slow continuous broadening with temperature, shows no sudden change at Tc. The results strongly support the model, that in the α‐phase the Ag+ ions occupy lattice sites with tetrahedral symmetry for a large fraction of the time.
Electronic properties of n‐type (Bi1−xSbx)2Te3 from magneto‐quantum oscillations
Authors: Köhler H. , Weidner R.
Published: July 2007

The magnetoresistance of n‐type (Bi1−xSbx)2Te3 mixed crystals is investigated in quantizing magnetic fields (B ≦ 10 T) between 1.6 and 4.2 K. Up to x = 0.3 Shubnikov‐de Haas oscillations can be observed on samples with electron concentrations between 7.8 × 1017 and 4.4 × 1018 cm−3, allowing the evaluation of electronic mass parameters and g‐factors. The six‐ellipsoid non‐parabolic model for the lowest conduction band of Bi2Te3 can be shown to apply up to about 15% Sb content with unvariable mass parameters but a decreasing tilt angle ζ0(x) of their principal axes relative to the crystal axes within the mirror planes of the Brillouin zone. Starting from x = 0.2 the Shubnikov‐de Haas patterns are changed drastically due to a cross‐over of two different types of conduction band minima.
Elliptical spin precession correction factor in the theory of spin‐wave resonance
Authors: Puszkarski H.
Published: July 2007

Ellipticity of spin precession is analysed for its relevance to the microscopic theory of spin‐wave resonance. Applying the surface inhomogeneity model of a thin film with surface anisotropy described in terms of an effective pinning field, it is shown that elliptical spin precession gives rise to a correction factor in the resonance line intensity formula dependent only on the frequency of the microwave field and the orientation of the static field with respect to the film surface.
Phonon diffusion with frequency down‐conversion
Authors: Kazakovtsev D. V. , Levinson Y. B.
Published: July 2007

In most of the experiments with heat pulses the frequency spectrum of the nonequilibrium phonons does not differ very much from the equilibrium one. In this case nonequilibrity evolves only due to propagation in space (ballistic propagation, diffusion, second sound). But if the injected non‐equilibrium phonons are superthermal (i.e. have frequencies much higher than the bath temperature T), nonequilibrity also evolves in the frequency domain – the spectral distribution due to the phonon decay shifts toward lower frequencies. The frequency down‐conversion influences the propagation, since all the scattering rates strongly depend on frequency. A theory of phonon diffusion accompanied by spontaneous phonon decay is presented. The phonon occupation numbers are supposed to be small and the bath temperature is supposed to be zero.
Optical properties of manganese ferrites
Authors: Šimša Z. , Široký P. , Lukeš F. , Schmidt E.
Published: July 2007

The near normal reflectance spectrum together with the data on the index of refraction and the extinction coefficient are combined in a Kramers‐Kronig analysis to obtain the spectral dependences of the real and the imaginary parts of the diagonal elements of the dielectric function of the single crystal series MnxFe3−xO4 for 0 ≦ x ≦ 1.6. One‐oscillator fit to optical constants is used to show that the main rise in absorption at about 2 eV is due to the 3d‐crystal field transitions of the Fe3+ and/or Mn2+ ions in tetrahedral sites. The anomalous behaviour of optical constants below 1 eV in iron rich Mn‐ferrites is ascribed to the presence of charge carriers (Fe2+) in octahedral sites.
Fine structure of P excitons in CuBr
Authors: Mattausch H. J. , Uihlein Ch.
Published: July 2007

A multiplet structure of the 2P, 3P, and 4P exciton of the Z1,2‐series is observed in the two‐photon absorption spectrum of CuBr. It is shown that the P‐exciton fine structure results from a coupling between the angular momentum of the P exciton and the hole quasi‐spin associated with the Γ8 valence band. For the first time the energies of higher excited P excitons (3P) are compared with the theory of Baldereschi and Lipari. Therefore, the binding energies of the 3P excitons for a representative set of parameters μ and δ is calculated. Numerical calculations are performed using the “finite‐element method” (FEM). From the energy positions of nine excitons the effective Rydberg, the gap, and the parameters μ and δ of CuBr are determined with high accuracy. These data and the static dielectric constant are used to evaluate the reduced mass parameter γ1 and the Γ8 valence band parameters γ2 and γ3. The 3P exciton wave functions, calculated also with the FEM, are shown in addition.
Excitation and recombination of donor–acceptor pairs in ZnTe
Authors: Nakashima S. , Yasuda S.
Published: July 2007

The photoluminescence spectra and its excitation spectra of the donor‐acceptor pairs are observed in ZnTe crystals doped with Li and As in the region below the bandgap energy. The relaxation of electrons and holes into the first excited state of d‐a paris is studied for the three excitation processes: (1) bound‐to‐bound transitions, (2) bound‐to‐free transitions, and (3) free‐to‐free transitions. It is concluded that most of the electrons and holes at the excited states of each impurity level are relaxed rapidly into their ground states before the occurrence of the recombination involving the excited states. For the excitation process (2), conduction electrons are preferentially trapped by positively charged pairs. The redistribution of bound holes by hopping is suggested to explain the broad d– a emission band observed for the bound‐to‐free excitation for very distant pairs.
Diamagnetic band structure up to second order in the crystal potential
Authors: Neumann H.‐W. , Rauh A.
Published: July 2007

The energy spectrum is calculated of a crystal electron in the presence of a magnetic field by using the Landau functions as basis with the periodic potential treated perturbatively. In contrast to the prevailing opinion which is based on first‐order effects only, our second‐order calculation predicts an observable broadening of the lower Landau levels. Furthermore, our quantum mechanical treatment gives a similar magnetic breakdown condition as the well‐known semiclassical considerations. Also the optical transition probability is determined approximately and from this an expression for the cyclotron mass is derived.
Chemical shifts of the K‐Absorption edge of Mn in some mixed oxidic spinels
Authors: Nayak R. M. , Padalia B. D.
Published: July 2007

Chemical shifts of the K‐absorption edge of Mn in some mixed oxidic spinels of composition (AMn2O4)1−x(MgAl2O4)x, where A refers to Mg, Mn, Zn, or Cd and 0 < x < 1, are measured. Valence states of Mn in these mixed spinels are determined. Effective charges on Mn ions are estimated using Gianturco and Coulson's method. An attempt is made to interpret the chemical shifts in the light of the available data from other studies on the same samples.
On the low‐temperature resistivity of ferromagnetic transition metal alloys
Authors: Goedsche F. , Richter A.
Published: July 2007

The electrical resistivity of a disordered ferromagnetic alloy is estimated in terms of the two‐current model. Besides the residual resistivity due to spin dependent potential scattering a spin‐disorder residual resistivity is obtained. A significantT3/2term associated with incoherent electron magnon collisions and representing large deviations from Matthiessen's rule dominates the temperature dependence of the resistivity below about 10 K. Deviations from Matthiessen's rule arising from the two‐current conduction are discussed.
Debye‐waller factors for rare gas solids
Authors: Kushwaha M. S.
Published: July 2007

Mean‐square displacements of the rare gas solids (Ne, Ar, Kr, and Xe) are calculated as a function of temperature, using eigenvalues as obtained from the unpaired elastic force model, recently developed. The present results are compared with the theoretical results of Goldman, because of the paucity of experimental results. Lindemann's melting criterion is tested by evaluating the ratio of the root‐mean‐square displacement to the nearest neighbour distance. It is observed that this ratio is almost constant for all rare gas solids.
Non‐linear optical properties of band charge carriers due to scattering by lattice defects
Authors: Fomin V. M.
Published: July 2007

The influence functionals of point, linear, and plane defects in an electron density matrix represented by functional integrals are obtained. A non‐liner polarization due to the interaction of band carriers with the lattice defects is found. The dependence of the shape of the absorption coefficient non‐linearity on the dimension and nature of scatterers is analyzed. Orientations of the laser field polarization are ascertained, which are optimum for evidencing the non‐linearity of the absorption coefficient. The possibility of effectively identifying the deformation interaction with dislocations from the dependence of the laser radiation absorption coefficient on its intensity is established.
The optically detected magnetic resonance of nickel‐doped silver chloride
Authors: Marchetti A. P. , Scozzafava M.
Published: July 2007

Optically detected magnetic resonance is observed in nickel‐doped AgCl at liquid‐helium temperatures. The species found is thought to be [(NiCl6)5−Ag+vac], indicating that [(NiCl6)4−Ag+vac] traps electrons at low temperatures. The signals due to [(NiCl6)5−Ag+vac] are weak and represent a decrease in light emission upon microwave absorption in accord with a simple model for radiative recombination.
On surface density of states of more complex tight‐binding systems
Authors: Kolář M.
Published: July 2007

The previously formulated approach to the investigation of the surface density of states that includes deviations from the bulk density of states in the whole sample is shown to be applicable to a larger class of systems. A concrete example of the semi‐infinite linear regular chain with atoms of two kinds is investigated.
Model pseudopotential for elementary semiconductors
Authors: Mašović D. R. , Zeković S.
Published: July 2007

A simple model pseudopotential, particularly suitable for energy band calculations in elementary semiconductors, is presented. It includes the Veljković‐Slavić's general model pseudopotential and phenomenological correction. This correction represents a sum of spherical Bessel functions of higher order with the coefficients obtained by fitting the form factor of the pseudopotential to the experimental values of the transition energies around the energy gap. The band structures of Si, Ge, and α‐Sn are calculated and agree well with the experimental data. This pseudopotential is tested by calculating the resistivity of liquid Si, Ge, and Sn.
Excitonic magnetoabsorption in Cd1−xMnxTe mixed crystals
Authors: Twardowski A. , Nawrocki M. , Ginter J.
Published: July 2007

Results of magnetoabsorption measurements for CdMnTe mixed crystals on extremely thin samples (≦ 1 μm) are presented. The complex structure of the exciton transition is observed. The qualitative interpretation of the full spectrum and quantitative theory of the ground state splitting is presented.
The calculation of the electronic structure of a vacancy by the green's function theory
Authors: Reifman S. P. , Shulichenko B. V.
Published: July 2007

The Green's function theory in a non‐orthogonal basis of atomic orbitals is used to consider the problem of the electronic structure of the vacancy in non‐polar crystals. With certain assumptions relative to the perturbation a simple expression is obtained for the Green's functions of a crystal with a vacancy. A numerical calculation is carried out for the influence of the vacancy on the valence band of Ne and Ar crystals.
On the lattice dynamical background of optical activity in crystals
Authors: Claus R.
Published: July 2007

The lattice dynamical theory of crystal optical activity developed by Born is discussed in the view of polariton dispersion. Three major difficulties occur: 1) For large wave vectors k no continuous connection with the phonon dispersion branches as recorded by neutron scattering is found. 2) There are spectral regions in which two left‐ or two right‐circularly polarized waves with different k are expected. 3) Polariton group velocities in the Γ‐point remain undefined when time reversal is required. An alternative model omitting these difficulties is proposed. It is based on the assumption of the existence of anharmonic resonance coupling between orthogonally polarized polariton waves with degenerate dispersion branches.
Electron–phonon versus coulomb interaction effects at the verwey transition of Fe3O4
Authors: Lorenz B. , Ihle D.
Published: July 2007

The Verwey transition in Fe3O4 is treated as the condensation of a coupled charge density‐phonon mode with q = (0, 0, 2π/a) (a lattice constant of the cubic unit cell) and Δ(1)5 symmetry on the basis of a model for the extra electrons at the Fe ions taking into account the electron correlation due to the electron‐phonon and the Coulomb interaction between nearest (U1) and next‐nearest neighbours (U2). Within the model three charge ordered phases are possible: besides the Verwey order and the Δ(1)5 Yamada order (along the orthorhombic a‐axis), a new ordering scheme, the ab‐order (combination of Δ(1)5 order and charge order along the b‐axis due to U2) accompanied by a doubling of the primitive cell along the c‐axis, is obtained. In the phase diagram the various phases are separated by first‐order transition lines.
Indirect exchange interaction via electrons in spin–orbit coupled bands in semiconductors
Authors: Ginter J. , Kossut J. , Świerkowski L.
Published: July 2007

Indirect coupling between two localized magnetic moments in a semiconductor is considered. This coupling may be of two origins: due to intraband electron transitions (RKKY) and virtual interband transitions, the latter being of particular importance in materials considered. Strong spinorbit coupling between electronic bands leads to a complicated form of the effective Hamiltonian. It no longer contains a Heisenberg term only. The number of independent constants describing the interaction depends on the symmetry of the interacting pair. The special case of zincblende structure is considered. Several simple models are reviewed and found insufficient to describe the indirect interaction, at least for closely lying moments, a fact which stresses the necessity of numerical calculations.
Sintering and random structures
Authors: Lacour C.
Published: July 2007

It is shown that the initial state of a compressed powder, as a rule complex and unorganized, actually obeys the statistics laws which inspired the percolation. This makes it possible to calculate all the elements of the dimensional distribution of the interparticulate spaces of a compact powder, of which only the total porosity, volume, and mean particle size are known. The evolution of these distributions during sintering is then investigated in the case of a volume diffusion mechanism and in the case of creep. An essential practical implication emerges from this study: to arrive at the theoretical density of a material in which sintering is governed by a volume diffusion, it is far more important to destroy the initial structure of the compact by reducing the dimensions of the larger voids, even if this increases the number of smaller voids, than to lower the initial porosity globally.
On the existence of two‐interstitial species in platinum after thermal‐neutron irradiation
Authors: Piani C. S. B. , Aspeling J.
Published: July 2007

The prediction of the conversion‐two‐interstitial model that the crowdion in platinum converts at some temperature (≈︁ 150 K) in stage II to the dumbbell, and that this conversion will be seen as a change in the initial slopes of the production curves, is investigatd. A change in the initial slopes of the productoin curves as a function of temperature is observed in stage II between 150 and 170 K. However, both the one‐interstitial model and two‐interstitial model can explain the behaviour of the initial slopes as a function of temperature from the lowest temperature to the minimum in the production curve. Both models cannot satisfactorily explain the behaviour of the initial slopes after the minimum.
Reliability of Various Empirical Potential Methods
Authors: Esterling D. M. , Swaroop A.
Published: July 2007

The use of the Wedepohl potential or the use of the vacancy formation energy as auxiliary conditions for an empirical potential derived from phonon data are examined. A pair potential for aluminum, derived from a non‐local pseudopotential model, is used as a benchmark. These auxiliary conditions are found to be unreliable. Reasons for this are discussed within a more general framework.
Dielectric Properties of PbHPO4 in the Microwave Region and the Far‐Infrared
Authors: Kock E. J. , Happ H.
Published: July 2007

The temperature dependence of the microwave per mittivity of PbHPO4 is determined at 8.2, 9.6 and 37 GHz in the para‐ and ferroelectric phase. The data are not consistent with a Debye‐type relaxation. In addition the far‐infrared reflection spectra between 15 and 650 cm−1 are measured with radiation polarized along the crystallographic axis at several temperatures between 3.5 and 430 K. Kramers‐Kronig analysis is used to calculate approximately ϵ′, ϵ″ and to identify the external lattice vibrations.
EPR Measurements of Chromium Impurity Photoionization Transitions in CdTe
Authors: Godlewski M. , Baranowski J. M.
Published: July 2007

The spectral dependence of the intensity of the EPR signal allows to identify the absorption tail of the band‐to‐band transition as due to a transfer of an electron from the valence band into the d‐shell of Cr2+(d4) due to which the Cr+(d5) charge state is created. The temperature dependence of the Cr+(d5) EPR signal is investigated. It seems that the long lifetime of Cr+(d5) is connected with the influence of the exchange interaction on the recombination process of an electron from the Cr+(d5) state with a loosely bound hole.
Mobilité de l'ion fluor dans PbSnF4
Authors: Villeneuve G. , Echegut P. , Lucat C. , Reau J.‐M. , Hagenmuller P.
Published: July 2007

Alors que les mesures de conductivité ne donnent qu'une idée macroscopique des mécanismes de conduction (diffusion à longue distance, énergie d'activation), la RMN fournit d'importantes informations sur les mouvements locaux dont la connaissance est fondamentale pour comprendre les mécanismes de diffusion. L'étude d'un nouveau conducteur ionique de structure fluorine distordue PbSnF4 montre la présence d'au moins deux et peut‐ětre trois différents mécanismes de sauts mettant en jeu les ions F− dans les sites normaux et dans les positions interstitieles.
Vibrational Spectra of TlGaTe2, TlInTe2, and TlInSe2 Layer Single Crystals
Authors: Gasanly N. M. , Goncharov A. F. , Dzhavadov B. M. , Melnik N. N. , Tagirov V. I. , Vinogradov E. A.
Published: July 2007

Polarized IR‐reflection and Raman scattering spectra are obtained for TlGaTe2, TlInTe2, and TlInSe2 crystals (space group D). The IR‐reflection spectra of all the crystals show the presence of two (E ‖ c) and three (E ⟂ c) modes, which is in full agreement with the prediction of a group‐theoretical analysis. All seven Raman active modes are detected in TlInSe2. In TlGaTe2 and in TlInTe2 only three of seven Raman active modes can be observed. Normal coordinates, effective force constants, and effective charges are determined for the infra‐red active lattice vibrations.
The Semiempirical LCAO‐MO Method of Energy Spectrum and Electronic Structure Calculation for Open‐Shell Clusters
Authors: Shashkin S. Yu. , Nikiforov A. E. , Cherepanov V. I.
Published: July 2007

A semiempirical method is proposed of open‐shell cluster calculation which is based upon Roothaan's rigorous theory and does not contain any variable parameters. It is shown that approximations used for Hamiltonian matrix elements in previous semiempirical calculations must be made more accurate. An estimation technique for Roothaan's coupling operator matrices is developed. Optical transition energies are calculated by a many‐electron way in the “frozen orbital” approximation with configuration interaction taking into account. Numerical computations are made for [NiF6]4− and [TiF6]3− complexes. Comparison of cluster spectra and also of the spin—orbit coupling constant λcr, unpaired spin densities ƒs, ƒσ, and neutron scattering form factor for [NiF6]4− with the experimental data proves the applicability of the method proposed.
The Adiabatic Approximation in Transient Donor Recombination
Authors: Pickin W.
Published: July 2007

Results obtained for the steady‐state case in a previous paper are extended to the transient case for non‐equilibrium phenomena, such as photoconductivity and impact ionization, involving donors with excited states. The solution of the transient rate equations is in general so complicated as only to be feasible in small signal theory. However, it is shown that if NA is much less than a critical value ÑA (NA being the acceptor concentration of the sample), then one may apply the adiabatic approximation, and the solution reduces to a trivial extension of the steady‐state result. Numerical results for ÑA are given for various values of temperature, applied field, and electron concentration.
Dispersion and Damping of Phonon‐Like Excitations in Disordered Systems
Authors: Kleinert P. , Leihkauf R.
Published: July 2007

Model systems are considered to investigate the dispersion and damping of phonon‐like elementary excitations in structural disordered chains. An approximate separation is carried out between the dynamical and structural disorder. For a rectangular distribution of the force constants the concept of quasistationary plane‐wave‐like excitations is inappropriate at the band edges. The influence of random equilibrium positions on the complex dispersion relation is also investigated. The quasi‐crystalline approximation is generalized to include the quasiparticle damping and the low‐temperature specific heat excess is briefly discussed.
Resonance Raman Scattering from Neutral and Doped Polyacetylene
Authors: Kuzmany H.
Published: July 2007

A complete assignment for the modes obtained from resonance Raman scattering on cis‐ and trans‐polyacetylene is reported. The Raman lines in the cis‐isomer indicate a well expressed one dimensional behaviour of the chain. From the line profile and line position of the resonance enhanced CC stretch modes in the trans‐isomer a chain length distribution of modified Gaussian form with a maximum at 30 CC double bonds and a half width of 20 CC bonds is deduced. The experimental results confirm the alternating bond model and lead to a sine wave chain potential with amplitude 1.9 eV. The effective mass of the π‐electrons is determined for the first time experimentally to be equal to the free electron mass. Intercalating the cis‐polymer with AsF5 results in a cis‐trans‐transition and a pronounced chain shortening. It is concluded from the experiments that the intercalating molecules precipitate between the chains.
Demonstration of the Ghost Effect in the Presentation of the Orientation Distribution of Texturized Materials
Authors: Matthies S. , Pospiech J.
Published: July 2007

The orientation distribution function (ODF) reproduced from pole figures obtained by diffraction measurements is burdened with erroneous distribution densities (ghosts) which are coupled in a definite manner with the true distribution. This effect, being a consequence of the pole figure measurement symmetry, is demonstrated on a model of the rolling texture of brass. Possibilities of ghost corrections are outlined.
Cohesion and Cauchy Discrepancy in Stable Simple Cubic Metals
Authors: Rathore R. P. S. , Agarwal R. M.
Published: July 2007

It is shown that, within the framework of the electron gas central pair potential model, the bulk modulus and the pressure associated with the conduction electrons are responsible for the Cauchy discrepancy and cohesion exhibited by stable simple cubic metals. It is shown that the correlation part of the electron energy, along with the Fermi and exchange parts at the proper electron separation, effectively contribute towards the desired values of the bulk modulus and the pressure.
Effect of Nonmagnetic Impurities on the Néel Temperature of Chromium
Authors: Dǎdârlat D. , Gulacsi Z.
Published: July 2007

The effect of nonmagnetic impurities on the Néel temperature of chromium is investigated, using a Feders‐Martin model with an energy dependent density of states. The effect of the impurities on the Fermi energy is also considered. It is found that, in the case when the topology of the Fermi surface of chromium is changed by the impurities, the dependence of the critical temperature of chromium on the impurity concentration c is nonlinear and TN may increase or decrease with increasing c.
On the Electronic Structure of Transition Metal Carbonitrides, Carboxides, and Oxinitrides. II. A Study of the Electronic Structure of TiCxN1−x, TiCxO1−x, and TiNxO1−x in Terms of the Relativistic KKR‐ATA Method
Authors: Weinberger P.
Published: July 2007

In part II the relativistic KKR‐ATA method is employed to investigate the electronic structure of TiCxN1−x, TiCxO1−x, and TiNxO1−x. Two different models for the description of the electronic structure are discussed, the more appropriate one being used to calculate the Ti‐p3/2‐like local density of states and the Fermi energies for these systems with respect to the molar fraction. In particular the relevant terms for the effective lattices and the integral as well as the differential density of states are discussed in some detail. A comparison to X‐ray emission spectra proofs that the electronic structure calculated for these systems is at least qualitatively correct.
The Critical Voltage Effect in Zone Axis Patterns. A Theoretical Study
Authors: Gevers R. , David M. , Herremans W.
Published: July 2007

The critical voltage effect in zone axis patterns with four‐ or sixfold symmetry is investigated using symmetry considerations and analytical approximations for the Bloch waves, obtained from degenerate perturbation theory.
Estimation des corrections “non muffin‐tin” à l'équation d'état des métaux simples
Authors: Perrot F.
Published: July 2007

On établit, dans le cadre de la théorie des perturbations au second ordre, l'expression formelle de la correction à l'énergie d'un solide, associée aux effets “non muffin‐tin”. On applique cette formule au cas des métaux simples, ou la densité auto‐cohérente est obtenue par la théorie du potentiel modèle de Shaw. On obtient ainsi une évaluation approximative mais systématique de ces corrections qui n'ont jamais été calculées exactement à notre connaissance. Elles sont très faibles dans l'ensemble des métaux étudiés, n'entraǐnant pas une variation de plus de 3% sur l'énergie de cohésion et de 1% sur le volume d'équilibre.
A Study of Point and Planar Defects in Copper
Authors: Miller K. M.
Published: July 2007

The real‐space computer simulation technique is used to study point and planar defect properties in copper and the results are compared with a previous study on nickel. A new interatomic potential function to represent copper is produced which reproduces the most recent values of formation and migration energies for a vacancy and also the intrinsic stacking fault energy. Using this potential, the relaxation around a single vacancy, nine interstitial configurations, the intrinsic and extrinsic stacking faults and the twin boundary are simulated. The vacancy results are equivalent to those calculated previously using the lattice statics technique. The interstitial results show the < 110 > split configuration as having the lowest formation energy, 6.2 eV, and formation volume, 0.67 Ω. For all interstitial configurations, the elements of the defect strength tensor are calculated and compared with results from diffuse X‐ray scattering experiments on copper. The energies of the twin, intrinsic, and extrinsic faults are found to be 29, 55, and 58 mJ m−2, respectively.
Hot Electron Effects in Semiconductor Luminescence
Authors: Dean P. J.
Published: July 2007

Fine structure near the threshold for recombination of free holes at neutral donors in InP is attributed to the radiative decay of excitons weakly bound at ionized donors. The conditions for optimum development of this optical structure are discussed; dilute doping, low temperature, and optical excitation rates and photoexcitation energies close to the direct energy gap. It is argued that the separation of electrons and holes and the intense local generation of phonons which occurs when this latter criterion is strongly violated favours free particle rather than bound exciton recombination processes. Comparisons are drawn with the behaviour of an unusual green photo‐luminescence spectrum in ZnSe, where the transition probability seems strongly enhanced by promoting phonons although very weak coupling is exhibited to accepting phonons.
Theory of Double Spin Flips in Nuclear Double Resonance
Authors: Whitaker M. A. B.
Published: July 2007

Double spin flips in nuclear double resonance are studied using the Zwanzig method. A master equation is produced, and temperatures are assumed for each Zeeman reservoir and for a combined secular dipolar reservoir. A set of simultaneous differential equations is obtained for the development of the system, and these are solved for a number of cases.
The Role of Quantum Effects in Lattice Dynamics of Rubidium Halides
Authors: Singh R. P.
Published: July 2007

The generalized shell model, developed by incorporating the van der Waals interaction and the zero‐point energy in the framework of three‐body force shell model, is applied to study the dispersion of phonons, Debye temperature, and Raman spectra in rubidium halides. The study reveals the excellent agreement between theory and experiments.
Calculation of the Electronic Density of States in Liquid Metals
Authors: Halder N. C.
Published: July 2007

An expression for the electronic density of states in liquid metals is derived using nonlocal and energy dependent pseudopotentials. The electronic energies are expressed to second order in perturbation theory. Both the first order and second order energy terms show the characteristics of k‐dependence which were not observed previously in the local pseudopotential theory with no Fermi surface approximation. Especially the density of states expression is found to have more structures, in that it involves a number of additional terms due to the nonlocality of the pseudo‐potentials. The present theory is applied to liquid Hg and Cd and some numerical calculations are made. These results are compared and discussed with similar results from other calculations.
Additional Waves and Polariton Dispersion in CdS Crystals
Authors: Makarenko I. V. , Uraltsev I. N. , Kiselev V. A.
Published: July 2007

A study of additional waves in the exciton region of CdS crystals is reported. Transmittivity and reflectivity of thin crystals is measured showing a structure due to the Fabry‐Perot interference and the interference of additional waves. This structure is a direct reflection of the polariton dispersion law. Using the formulas for a spatially dispersive platelet and Pekar's additional boundary conditions the theoretical reflectivity curves are obtained numerically reproducing the essential experimental results. Values for all physical parameters defining dispersion and damping of the polaritons are found. Some evident deviations between the experimental curves and the present computer results for the reflectivity are pointed in connection with a discussion of the role of the surface exciton potential.
Investigations of the Energy Gap of V3Si Monocrystals
Authors: Schumann J.
Published: July 2007

On the surface of single phase V3Si single crystals, tunnel junctions with reproducible and stable characteristics are prepared by an anodic oxidation method for the barrier preparation and by Pb and In top electrodes. Detailed energy gap investigations yield Δ0 = (2.52 ± 0.05) meV for V3Si. Based on this energy gap value the evaluation of the ratio 2Δ0/kTc is discussed and conclusions about the strength of electron‐phonon interaction in V3Si are given. The temperature dependence of the energy gap obtained by measurements in the temperature range 1.5 K. ≤ T ≤ 20 K can be described by the relation Δ(T)/Δ0 = tanh [Tc Δ(T)/TΔ0]. In order to exclude measuring errors, a numerical evaluation of the Δ(T) values is provided resulting in a Tc‐lowering in the surface range. Furtheron, at T = 1.5 K, the measurements show an additional gap structure, the cause of which is discussed. Investigations of the tunnel characteristics measured in different crystallographic orientations do'nt yield any measurable anisotropy of the energy gap.
On the Theory of Resonance Raman Scattering in Semiconductors. A Green's Function Approach
Authors: Quang Doan Nhat , Esser B. , Keiper K.
Published: July 2007

A Green's function approach baaed on an S‐matrix expansion of the light induced current fluctuations is developed. Using a two‐band model the theory is applied to the one‐phonon resonance Raman scattering in degenerate semiconductors. The discrete (phonon line) and continuous parts of the scattering cross section are calculated. In the dispersion curve of the discrete part pronounced peaks at the energy of the Burstein‐Moss shift (and that plus phonon energy) are obtained. The conditions for single and double resonance of the continuous part are given. Plots illustrating the resonance behaviour are shown.
A Study of the Concentration Dependence of Hall and Verdet Coefficients in the Hubbard Model
Authors: Kogutyuk I. P. , Nitsovich V. M. , Skrypnik F. V.
Published: July 2007

The dependence of the Hall and Verdet coefficients on temperature and on the filling of the narrow energy band is investigated. The sign of the Hall and Verdet coefficients is shown to change three times as the band filling varies. Calculations are made by the Green function method on the basis of the Hubbard Hamiltonian.
Thermally Stimulated Depolarization of NCO− Centres in Alkali Halide Crystals
Authors: Wilbrand A. , Fischer F.
Published: July 2007

In single crystals of chlorides, bromides, and iodides of sodium, potassium, and rubidium a TSD (also named ITC) peak between 65 and 29 K is found which is due to NCO− centres. The up to 20% broader bandwidths, compared to the theoretical shape, are attributed to residual stresses of the probe. The experimentally determined reorientation enthalpies of the centres range between 0.192 and 0.080 eV and show a linear dependence on the lattice constant within the investigated sodium, potassium, or rubidium halides. The experimental data of the electric dipole moments are in reasonable relation to calculated values of the free NCO− ion.
Phonon Modes in a Disordered Lattice
Authors: Chakrabarti B. K. , Roy G. K. , Sinha S. K.
Published: July 2007

The vibrational excitations are studied using the lattice Green function expansion for a disordered simple cubic system with degenerate modes. Both mass and force constant disorders are considered. The resulting dispersion relation, including damping, of the long wavelength longitudinal phonon modes are obtained in the limit of small impurity concentrations. The results are obtained analytically and are exact up to the linear term in concentration. In a simple mass defect case, the results reduce to those obtained by Langer.
Band Structure of MgxZn1‐xTe Alloys
Authors: Montegu B. , Barbier D. , Chevallier J. , Guillaume J. C. , Somogyi K. , Laugier A.
Published: July 2007

The band structure of MgxZn1‐xTe alloys is studied using a double beam wavelength modulated system in first derivative mode. Modulated reflectivity measurements are made from 82 to 300 K within spectral range 2500 to 5400 Å. Structures corresponding to the E0, E0 + Δ0, E1, E1 + Δ1, e1 and e1 + Δ1 critical points are indexed on the basis of existing band calculations for ZnTe.
Crystal Stability and Three‐Body Interactions
Authors: Halicioǧlu T.
Published: July 2007

Structural stabilities of simple crystals (h.c.p., f.c.c., and b.c.c.) are investigated using both, two‐body and three‐body interactions. A parametrical analysis is performed. The results emphasized the importance of multibody forces to explain structures such as b.c.c. Furthermore, calculations predict that, in general, the stability of crystals changes as h.c.p. → f.c.c. → b.c.c. with increasing intensity of three‐body interactions. For selected cases the possibility of structural transformations (such as b.c.c. → f.c.c.) are discussed.
The Energy Band Structure of Corundum
Authors: Evarestov R. A. , Ermoshkin A. N. , Lovchikov V. A.
Published: July 2007

The energy band structure of corundum is investigated theoretically using the semiempirical Mulliken‐Rüdenberg method. A crystal potential is constructed basing on the self‐consistent effective atomic charges and electronic populations obtained within the framework of the method used. The connection of the energy bands with the atomic states of the constituent atoms is discussed. The valence band widths and the electronic density distribution appear to be in good agreement with experimental data.
Remarks on the Ferromagnetic Solutions of the Extended Hubbard Model
Authors: El‐Jaick Léa J. , Troper A. , Gomes A. A.
Published: July 2007

The extended Hubbard Hamiltonian is studied using the method of the equations of motion. The decoupling approximations, suitable for the strong intra‐atomic repulsion limit are discussed. One of the approximations (mean field approach to deal with next neighbour interactions) is worked out numerically. The magnetization is obtained as a function of the strength of the next neighbour couplings and of the number of electrons per atom.
Mutual Annihilation of Triplet Excitons in 4,4′‐Dichlorobenzophenone Single Crystals at Low Temperatures
Authors: Avdeenko A. A. , Dobrovolskaya T. L. , Kultchitskii V. A. , Naboikin Yu. V.
Published: July 2007

Relative quantum yields and decay kinetics of exciton phosphorescence (O–O band at 412.46 nm) and those of X1‐traps (O–O band at 413.54 nm) are measured versus the exciting light intensity in 4,4′‐dichlorobenzophenone single crystals at 1.5 and 4.2 K. The phenomena observed are treated from the point of view of mutual annihilation of triplet excitons, whose rate constant, equal to (4.2 ± 2.0) < 10−13 cm3 s−1, is unchangeable in the temperature range studied. Equivalent free excitons are shown to be responsible for mutual annihilation. The decay kinetics of X1‐trap phosphorescence is explained by optical spin alignment in the excited triplet state.
Mixing of Frenkel Excitons and Ionic Excited States in a Linear Crystal of Donor‐Acceptor Complex. II. ADA‐Type Complexes
Authors: Petelenz P.
Published: July 2007

The previously proposed Merrifield‐type model of the coupling between neutral and ionic excited states of a linear crystal of a weak AD‐type donor‐acceptor complex is generalized for ADA‐type complexes. It is demonstrated that CT interactions involving the locally excited Frenkel states can be adequately described in terms of a Hamiltonian entirely analogous to that used for AD‐type complexes but with renormalized parameters. The spectrum of ADA‐type complexes is however richer due to the appearance of a new system of absorption bands due to the (–) CT states which do not couple with the Frenkel states.
Theoretical Investigation of the Critical Josephson Current of Niobium‐Lead Tunnel Junctions
Authors: Seidel P. , Richter J.
Published: July 2007

In order to calculate the critical Josephson current Ic of niobium‐lead tunnel junctions the proximity‐effect model previously presented by the authors is used to describe the niobium electrodes. The resulting Ic.‐dependence on temperature differs from the commonly used Ambegaokar‐Baratoff result in accordance with many experimental observations. The theoretical model allows to explain these differences and offers good possibilities to describe the experiments.
Variational Approach for the Anderson Model: Impurity Pair
Authors: Oleś A. M. , Chao K. A.
Published: July 2007

With a Variational method the effect is studied of impurity‐impurity interaction on the formation of local moments within the framework of the Anderson model. Compared to the similar calculation on. a single impurity, the local moment is enhanced, especially when the two impurities are nearest neighbours of each other with a ferromagnetic direct coupling.
Lattice Vibrations of MnxCdi1‐xTe Mixed Crystals through Raman Scattering Measurements
Authors: Picquart M. , Amzallag E. , Balkanski M. , Julien Ch. , Gębicki W. , Nazaeewicz W.
Published: July 2007

Raman scattering measurements are performed on Mn‐xCd1‐xTe mixed crystals, in the concentration range x = 0 to x = 0.5. The first‐order spectra show a “two‐mode” behaviour for the long‐wavelength optical phonons, which can be explained using the theoretical REI model. The rest of the spectra, confirmed by infrared absorption measurements, is tentatively interpreted in terms of two‐phonon processes and presumably one‐acoustic‐phonon disorder‐induced modes, resulting from mass disorder.
Electrical Resistivity in a System of Coupled Donor and Acceptor Chains
Authors: Bulka B. R.
Published: July 2007

A system of weakly coupled donor and acceptor chains possesses a sink‐type energy band structure. The resistivity due to the inside sink and between sinks electron scattering is calculated. For acoustic phonon scattering the resistivity is described by Q = ATλ where λ. > 2. A case of electron scattering against optical phonons is considered as well.
d‐Electrons and Dispersion Relations in B.C.C. Zirconium
Authors: Rathore R. P. S.
Published: July 2007

A new scheme, comprising of all the possible interactions coupling various constituents of the b.c.c. transition metal is developed. It is inferred that the electrons partially filling the d‐levels (called d‐electrons) of these metals contribute significantly towards the Cauchy discrepancy. The crystal equilibrium is considered under the combined influence of cores d‐electrons and s‐electrons. Deformation parameters associated with these electrons are evaluated on the consideration of their respective energies. The dispersion relations, thus derived, are employed to predict the phonon frequencies in b.c.c. zirconium. Recently reported data on the phonon‐energy of this metal are compared and a promising good agreement is obtained.