bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Pressure on the Raman Scattering Efficiency of GaP
Authors: Aoki K. , Cardona M.
Published: July 2007

The effect of pressures up to 10 GPa on the efficiencies for Raman scattering by TO and LO phonons is measured in GaP for the 1.92 and 2.18 eV laser lines. These scattering efficiencies show a logarithmic decrease with pressure. From the measured efficiencies, the lattice and electro‐optic contributions to the Raman polarizability are obtained as functions of pressure. The decrease of STO with increasing pressure is interpreted as a preresonant process induced by the increase of the E0‐gap with pressure. The second order susceptibility is also discussed on the bases of a simple dispersion model.
Charge Transfer and Energy of Formation in Binary Alloys: CsCl Intermetallic Compounds
Authors: Chakraborty S. , Manna A. , Ghosh A. K.
Published: July 2007

The density functional obtained through Löwdin's α‐expansion method is utilized to study equi‐volume charge transfer and energy of formation of equiatomic ordered transition intermetallic compounds having CsCl structure. Electronic charge is transferred, by and large, in the direction as expected from the Pauling electronegativity series. The computed formation energies predict the stability of compounds satisfactorily. It is concluded that the α‐expansion method may be applied with confidence to study ground state properties of metals and alloys.
Two‐Phonon Resonance Raman Scattering and Quantum Interference Effects in Polar Semiconductors in High Magnetic Fields
Authors: Belitsky V. I. , Goltsev A. V. , Lang I. G. , Pavlov S. T.
Published: July 2007

The EHP contribution to the two‐phonon resonance Raman scattering (RRS) cross‐section is calculated for a polar semiconductor located in a high magnetic field H. This contribution is of order α, where α is the Fröhlich electron (hole)‐LO phonon coupling constant (in the case H = 0 the cross section is of order α2). This α‐dependence results from the quasi one‐dimensional motion of the electron (hole) in a high magnetic field. For such a polar semiconductor as InSb (α ≈ 0.013 ≪ ≪ 1) a high magnetic field leads to a sharp increase of the secondary radiation intensity. The two‐phonon RRS cross section is an oscillating function of the magnetic field H and (or) of the incident light frequency ω1. The oscillations exist due to the increase of the density of states near the Landau zone bottom. It is shown that in the vicinity of a number of values of the magnetic field H = H0/L, where H0 = mecωLO/|e|, (me(e) = effective mass (charge) of electron, L = 1/2, 1, 3/2, 2, …, there is a number of additional contributions of interferential character to the cross section, which lead to a resonance decrease of the secondary emission intensity.
Effect of Magnetic Field on the F‐Band Absorption in NaCl and KCl Single Crystals Irradiated with X‐Rays under DC Fields
Authors: Chaudhary R. K. , KrishnaRao A. V. , Rao K. V.
Published: July 2007

Optical absorption is measured in the F‐band region of NaCl and KCl single crystals X‐ray irradiated under de fields and also later subjected to a magnetic field perpendicular to the (100) faces. The presence of de fields during irradiation seems to produce an absorption band overlapping the F‐band on the long wavelength side. The magnetic field tends to resolve this band from the F‐band of these crystals. An attempt is made to understand these results.
Effect of Forced Vibrations on the Scattering of X‐Rays and Mössbauer Radiation by a Crystal (I)
Authors: Dzyublik A. Ya.
Published: July 2007

A general scattering theory of photons (particles) by field targets, periodically depending on time is developed. On the basis of this formalism the scattering of γ‐quanta is considered by a crystal, which performs enforced vibrations under the influence of alternating fields. The Raman scattering amplitudes and corresponding cross‐sections for scattering and absorption of γ‐quanta by the crystals are calculated. The possibility of compensation of the recoil energy for γ‐resonance at energies ≈200 keV by infrared lasers is analysed.
Magnetization and Spin‐Flip Raman Scattering in Cd1−xMnxS
Authors: Nawrocki M. , Planel R. , Mollot F. , Kozielski M. J.
Published: July 2007

High‐magnetic field magnetization data are reported on Cd1−xMnxS alloys, with x = 3% and 15%. Effective parameters describing the Mn—Mn interaction are deduced. Spin‐flip Raman scattering from donors provides a measure of the s—d exchange interaction.
Low‐Temperature FMR and FMAR Measurements of Metal Single Crystals. II. Saturation Magnetization, Magnetocrystalline Anisotropy, and g‐Factor Determination
Authors: Půst L.
Published: July 2007

FMR and FMAR fields measured on Fe—3.16 wt% Si single crystals between 3.5 and 300 K at microwave frequencies between 35 and 96 GHz are used to evaluate the saturation magnetization μ0Ms, the first magnetocrystalline anisotropy constant K1, and the g‐factor temperature dependences with accuracies of ±0.5 mT, ±0.15 kJ/m3, and ±0.0004, respectively. The g‐factor is found to be temperature independent and the temperature dependence of μ0Ms is described according to the spin‐wave theory.
On the Thermoelectric Power of Amorphous Ferromagnets
Authors: Richter R. , Goedsche F.
Published: July 2007

The spin‐disorder model is adopted to calculate the electrical resistivity and thermoelectric power of ferromagnetic metallic galsses within the molecular field approximation. The resistivity expression interpolates correctly between the low (T = 0 K) and the high temperature (T ≧ TC) results derived previously. Using Mott's formula a magnetic contribution Sm(T) to the thermopower is calculated which exhibits a nonlinear temperature dependence. Both, sign and curvature of Sm(T) are correlated with the actual position of 2kF in the structure factor. For a combined spin and potential scattering the total thermopower is evaluated according to the Nordheim‐Gorte rule. The results reproduce qualitatively sign and curvature of the thermopower observed experimentally in ferromagnetic metallic glasses.
First‐Principles Calculation of Deformation Potentials of Copper Halides
Authors: Christensen N. E.
Published: July 2007

Self‐consistent, relativistic LMTO band structure calculations, using the local‐density approximation, are performed for strained CuCl and CuI crystals in the zinc‐blende structure. Equilibrium volumes and bulk moduli are derived. Calculations for uniaxially strained crystals are used to deduce deformation potentials for splitting of the Γ15‐state (top of the valence band) with and without spin—orbit coupling (SO). In case of trigonal strain, these quantities are calculated as a function of the internal‐strain parameter ζ. It is shown that the deformation potentials can be evaluated by moving the potentials obtained by one self‐consistent calculation for the unstrained crystal to the lattice positions of the strained crystal. The inclusion of SO is important in the calculation of the trigonal‐shear deformation potentials but affects only slightly those for tetragonal strain.
Visible Luminescence of Colour Centres in Sapphire
Authors: Springis M. J. , Valbis J. A.
Published: July 2007

The colour centres introduced in sapphire crystals by neutron irradiation and by thermochemical treatment exhibit polarized luminescence. In neutron‐irradiated crystals an absorption band at 4.1 eV and luminescence bands at 2.45 and 3.8 eV are due to one type of centres, most probably interstitial aluminium ions. Other donor‐type centres give rise to an absorption band at 2.75 eV and a luminescence band at 2.2 eV. In magnesium containing crystals similar centres absorbing at 2.93 eV and emitting at 2.35 eV can be introduced by thermochemical treatment in reducing atmosphere. The preliminary study of the red luminescence indicates its connection with the absorption bands at 3.45 and 3.8 eV.
Investigation of the Equation of State and the Grüneisen Parameters for Transition Metal Oxides
Authors: Agrawal S. K. , Narain J. , Shanker J.
Published: July 2007

The equations of state of the four transition metal oxides MnO, FeO, CoO, and NiO are investigated from the knowledge of interionic potentials. Recently determined values of the Van der Waals dipole—dipole and dipole—quadrupole interaction energies are used in the present paper. The repulsive interaction energies for unlike ion pairs as well as like ion pairs are taken into account upto second neighbours. Different values of repulsive hardness parameters are used for different pair interactions. The compression values corresponding upto about 250 kbar are calculated and reported. Good agreement between theoretical and experimental values is obtained. The second neighbour interactions between the oxide ions are found to be quite significant and much stronger than the interaction between transition metal ions. Values of the Grüneisen parameter and its volume derivative are also estimated for the crystals under study.
New Results on the Decay Properties of Perturbed and Unperturbed M‐Centers in NaCl:CaCl2
Authors: Bosi L. , Gagliardelli P. , Nimis M.
Published: July 2007

Lifetime measurements concerning the relaxed excited state of the perturbed and the unperturbed M‐center in X‐ray colored NaCl:CaCl2 samples, give the following results: a) the decay spectra are characterized by two components, due to the perturbed ((18.2 ± 0.5) ns at 9 K)) and to the unperturbed ((26.5 ± 0.5) ns at 9 K) M‐center respectively, in the temperature range 9 to 320 K: the component due to the perturbed M‐center disappears at higher temperatures; b) the lifetime of the M‐perturbed center is a decreasing function of the temperature contrary to what happens in KCl samples doped with divalent impurities, where it is constant up to room temperature; c) no difference from the data obtained in undoped NaCl samples is noted for the unperturbed M‐center and it is shown, for the first time, that its lifetime decreases above 310 K; d) the simple scheme of a two‐channel decay process is not valid for the perturbed M‐center whereas it enables to fit with accuracy the data concerning the unperturbed M‐center (by assuming a de‐excitation energy value equal to (0.46 ± 0.02) eV). An analysis of the optical properties of the M‐perturbed center in NaCl:CaCl2 samples leads to the assumption that the perturbative effect is low compared to that observed in KCl doped with divalent impurities: in particular, it seems to be inefficient as regards the absorption properties; this hypothesis also enables to explain the absence of formation of Z4‐centers in the same samples.
Thermal and Magnetic Properties of CrxMn1−xAs Compounds
Authors: Bärner K. , Santandrea C. , Neitzel U. , Gmelin E.
Published: July 2007

The specific heat of magnetically ordered metallic CrxMn1−xAs crystals is determined in the temperature range 100 K < T < 500 K for nine compositions. Several anomalies in the specific heat are observed and assigned to carrier localization effects, spin fluctuations, and long range spin ordering. For the compositions x = 0.5, 0.4, 0.2, and 0.1 a sequence of two magnetic specific heat peaks are observed and interpreted in terms of a “magnetic alloy” model.
Solitary Clusters of Spin Deviations and Lattice Deformation in an Anharmonic Ferromagnetic Chain
Authors: Pushkarov Kh. I. , Primatarowa M. T.
Published: July 2007

An anisotropic Heisenberg ferromagnetic chain is considered where the lattice deformation caused by the spin deviations is taken into account both, in harmonic and anharmonic approximation. The possibility of solitary bound formation between spin clusters and lattice deformation is investigated. It is shown that such a solitary “domain” can exist and propagate along the chain with constant velocity and without “smearing”. In this way a magnetic moment can be transferred without dissipation.
Polarizability Studies of Amorphous Silicon
Authors: Ravindra N. M. , Morhange J. F. , Thomas P. A. , Ance C.
Published: July 2007

An attempt is made to evaluate the polarizability of amorphous silicon. These evaluations are carried out for samples prepared by chemical vapour deposition (CVD) and glow‐discharge (GD). Spectroscopic models like that of Wemple‐Didomenico and Ance are employed for the purposes of calculation. This study may serve as a bridge between experimental results that follow from Raman or infra‐red spectroscopy and those that are obtained from absorptance/reflectance studies.
A Calculation of Absorption Potentials for Low Energy Electrons in MgO, 2H‐MoS2, and 2H‐NbSe2
Authors: Smith A. E.
Published: July 2007

A simple calculation scheme for determining the absorption potential for low energy electron scattering in crystalline materials, based on experimental values of the dielectric loss function, is outlined. In particular calculations are performed for the semiconductors magnesium oxide and molybdenum disulphide together with the metal niobium diselenide. A larger value of the mean absorption potential for the metallic compound is found and its implication for low energy total current measurements is commented upon.
Resonant Energy Transfer in Ruby under Magnetic Field
Authors: Ferrari M. , Montagna M. , Gonzo L. , Viliani G. , Pilla O. , Farace G.
Published: July 2007

The effect of a magnetic field on the single ion‐single ion resonant transfer in ruby is investigated theoretically by using a simple model. It is found that the field induces a quenching of the resonant transfer. The results are compared with recent experimental data.
Quantum Freeze‐Out of Carriers in Semimetallic and Semiconducting Thin Wires
Authors: Arora V. K. , Spector H. N.
Published: July 2007

A semimetal—semiconductor transition is predicted to take place due to the quantum freeze‐out of carriers in semimetallic thin wires. These wires are assumed to be cylinders of radius R0 although this quantum freeze‐out effect is also predicted to take place in thin wires of rectangular cross section as well. In cylindrical wires, the onset of semiconducting behavior is expected to occur at a critical radius R*0 = α01ħ/(2M Δ)1/2, where α01 = 2.405 is the first zero of the Bessel function of the zeroth order, M = memh/(me + mh) is the reduced mass of the electron—hole system, and Δ is the overlap of the conduction and valence bands. For wires of radii smaller than R*0, the electrical conductivity of the thin wire will exponentially decrease with temperature because of the presence of an effective band gap Eg = (αħ2/2 M R) ‐ Δ > 0.
Ultrasonic Attenuation in Quasi‐One‐Dimensional Ferroelectrics
Authors: Aksenov V. L. , Yu. Didyk A. , Plakida N. M.
Published: July 2007

The ultrasonic attenuation in uniaxial ferroelectrics with the stronglyanisotropic spectrum of order‐parameter fluctuations is considered. It is shown that quasi‐one‐dimensional character of fluctuations dispersion leads to qualitatively new effects in the behaviour of ultrasonic velocity and attenuation. The theory allows one to explain qualitatively recent experiments on ferroelectrics CsH2PO4.
Multiphoton Absorption in Solids Non‐Perturbative Approach in the Weak‐Field Limit
Authors: Brandi H. S. , Jalbert Ginette , Malta O. L.
Published: July 2007

A closed analytical expression for multiphoton transition rate, in direct gap crystals, is obtained from a non‐perturbative approach in theweak‐field limit. This scheme is related to an approximate high order perturbation theory and compared with tunneling‐like theories as proposed by Keldysh. The accuracy and the limitations of the use of the saddle‐point in this type of methods are discussed.
Screening in a System of Many Particles Composed of Two Charged Fermions Application to Excitons
Authors: Stolz H. , Zimmermann R.
Published: July 2007

A many‐particle system of composite particles consisting of two different fermions each and interacting by the Coulomb potential is considered. For the case that the composites are present in bound states only the constituent self‐energy is calculated to linear order in the densitytaking all symmetries of the many‐body wavefunctions into account. The correlational part (screening part) of the selfenergy is calculated in the Born collision approximation. Numerical values for the shift of the continuum‐edge of the spectrum of excitons in GaAs are presented.
Dipole Forbidden Transmission of Magnetopolaritons in CdSe
Authors: Kochereshko V. P. , Röseler J. , Uraltsev I. N. , Henneberger K.
Published: July 2007

Transmission spectra of the well‐known dipole forbidden excitonic lines AF and AL are studied in magnetic fields up to 18T in Faraday and Voigt configuration. Magnetic field induced shifts are observed and carefully interpreted within a theoretical model including mixingof A‐ and B‐valence bands. The unusual shift of line AF in the Voigt configuration is explained as a polariton effect. Furthermore, the absence of any doublet structure in both geometries is confirmed by theoretical investigations.
Diffusion and Localisation in Random Systems
Authors: Elyutin P. V. , Hickey B. , Morgan G. J. , Weir G. F.
Published: July 2007

Direct computer experiments are conducted on electron diffusion in a simple cubic lattice where the disorder corresponds to the Anderson model. Results for the onset of total localisation are in good agreement with other numerical estimates and it is found that the critical value of the parameterW/ZV is (17.7/6). A random structurally disordered system is also investigated with hopping integrals given by V(r) = ‐V0 exp (‐r/a) and found that localisation occurs at the critical value a = 3 ± 0.3. Thisis in agreement with the only other numerical estimate but is substantiallydifferent from theoretical calculations.
Ground and Excited States of Tl+ Impurity in KCl Crystal by the Use of the Green's Function Method
Authors: Moretti P. , Ranfagni A. , Agresti A.
Published: July 2007

The problem of determining the position of impurity levels in ionic crystals is considered in the framework of the Green's function method which allows the location of these levels with respect to the valence and conduction bands of the host crystal. The case of KCl:Tl+ is examined both, in the ground (a21g) and first‐excited(a1gt1u) state configuration, in order to make possible acomparison between the theoretical predictions and the experimental absorption energies. The calculation is first performed in the one‐electronscheme; the Coulomb (G ≈︁ 0.3 eV) and the spin—orbit (ζ ≈︁ 0.7 eV) interactions are considered later perturbatively since the one‐electron energy difference (W0 ≈︁ 6 eV)is sufficiently high. In such a way a more plausible energy‐level scheme is obtained with respect to molecular‐orbital computations.
Vibrational Relaxation and Hot Luminescence of Ne*2 Centers in Solid Neon
Authors: Gaethke R. , Gürtler P. , Kink R. , Roick E. , Zimmerer G.
Published: July 2007

Hot luminescence of Ne*2 centers (molecular type self‐trapped excitons) in solid Ne is measured under state selectiveexcitation using synchrotron radiation. The luminescence spectra are analysed with the aid of a new theory on vibrational relaxation of the Ne*2 centers. It is possible to show that the rate of relaxation is slower for centers located at the surface than for centers in the bulkof the Ne solid. In combination with a direct measurement of the lifetimes (1 ns for 1∑+u; 5 μs for 3∑+u), absolute values for the relaxation rates are obtained. They range between 6.6 × 104 s−1 (bulk) and 6.6 × 103 s−1 (surface).
Electric Field Gradient at Gd in Gadolinium and Rare Earth Trifluoride Single Crystals
Authors: Budzynski M. , Goremychkin E. A. , Kochetov O. I. , Latuszynski A. , Mikolajczak P. , Mühle E. , Muminov A. I. , Subotowicz M.
Published: July 2007

The electric field gradient at the Gd nucleus in gadolinium and rare earth trifluoride single crystals, LaF3, CeF3, PrF3, NdF3, SmF3, EuF3, and GdF3 are measured by the method of integral perturbed angular correlation. The experimental data are compared with those of point charge model calculations and withthe results of other experiments such as Mössbauer spectroscopy, electron paramagnetic resonance, nuclear quadrupole resonance, and optical absorption. The results obtained for Gd in trifluorides of the light rare earth elements allow to conclude that the origin of the field gradient cannot be explained by the point charge model.
Spectral Diffusion and Triplet Exciton Localization in an Organic Glass
Authors: Jankowiak R. , Ries B. , Bässler H.
Published: July 2007

Phosphorescence spectra of glassy 2‐bromonaphthalene films prepared by vapor deposition at 4.2 K are recorded under site‐selective excitation. The Stockes shift between absorption and emission origins gradually vanishes as the excitation energy is scanned across the inhomogeneouslybroadened profile of the T1 ← S0 OO transitionindicating a continuous decrease of the exciton diffusivity with exciton energy. The effect is rationalized in terms of stochastic hopping transport across a Gaussian density of states. A Monte Carlo computer simulation provides the basis for quantitative data interpretation. A criterion for excitonimmobilization is established.
Exact Density Functionals for Ground‐State Energies II. Details and Remarks
Authors: Englisch H. , Englisch R.
Published: July 2007

Statements utilized or mentioned in the Part I are proved or discussed in detail. Especially it is shown that for finite‐dimensional state spaces the strictly positive densities being strictly less than two are E‐V representable and that the density functional by Lieb is differentiable at the E‐V densities.
Dependence of the Total Mobility in a One‐Band Model Applicationto n‐Type MoTe2
Authors: Conan A. , Bonnet A. , Zoaeter M. , Ramoul D.
Published: July 2007

In a one‐band model, the total carrier mobility relations, deduced from the Debye‐Conwell formula and Mathiessen's rule, are compared. With the help of these two laws the resultant mobility and the thermoelectric power (TEP) kinetic term variations are given. For n‐type MoTe2 single crystals, it is shown that a classical compensated semiconductor model can explain all the experimental results (electrical conductivity, TEP). A good accuracy is obtained by the use of Mathiessen'srule with exponents departing not much from the theoretical values. The model adopted is in good agreement with dichalcogenides band structure calculations. A donor level arising from the presence of bromine impurities, is located at 9.5 meV from the conduction band.
Two‐Spin Cluster Approximation in the Blume‐Capel Model
Authors: Lock S. L. , Lee B. S.
Published: July 2007

A two‐spin cluster approximation based on a perturbation expansion of the free energy in powers of the order parameter is applied to calculate the properties of the Blume‐Capel model. The results obtained for the tricritical point are comparable to those of other two‐spin‐type cluster approximations.
Molecular Cluster‐INDO Calculations of Substitutional Defects inAlkali Halides
Authors: Matos Maria , Maffeo B.
Published: July 2007

The one electron level structure of LiF, NaF, LiCl, and NaCl crystals containing F and U centers and of NaCl crystal containing U3 centers are calculated using the INDO method and the molecular cluster (MC) approximation, with special emphasis in parameterisation. Spin polarisation is considered in the unrestricted Hartree Fock formulation of the method. The Fand U band transition energies are estimated using a limited configuration interaction (CI) calculation, and the electronic mechanism involved in the U3 absorption band are also examined. A rough estimation is made of the spin density at the substitutional H atom nucleus in NaCl.
Non‐Equilibrium Population of Fe3+ Electronic States after the Electron Capture I. 57Co in LiNbO3 and LiTaO3
Authors: Doerfler R. , Nagy D. L. , Pfannes H.‐D. , Putzka A. , Ritter G. , Zeman N.
Published: July 2007

All Zeeman levels of Fe3+ ions contribute intense and narrow lines to the Mössbauer emission spectra of LiNbO3:57Co and LiTaO3:57Co in 2 T ≤ Hext ≤ 7 T at 4.2 K. To explain the electronic spin and magnetic field dependence of the line intensities irreversible processes like the decoupling of electrons from the lattice or a field‐dependent slow relaxation should be involved. The existence of a long‐lived heated localized mode is ruled out.
Superconducting Properties of the Hexagonal‐Close‐Packed Alloy System TcZr
Authors: Chatterjee Parsathi
Published: July 2007

The theoretical formula of McMillan, modified via the multiple‐scattering theory of Gomersall and Gyorffy, is very successful in computing the electron—phonon coupling constant (λ) and the superconducting transition temperature (Tc) of elements and compounds from quantities readily obtainable from band structure work or approaches based on the scattering theory. However, for disordered solids this theory fails because of the breakdown of the translational symmetry used in the multiple scattering theory. In the particular case of substitutional alloys, the problem can still be solved however if, at each lattice point, the t‐matrix of an individual scatterer is replaced by a configurational average of the t‐matrices of the alloying materials (average t‐matrix approximation). This modified theory, which has already been successfully applied to some cubic substitutional alloys, is herein used to predict λ and Tc for the h.c.p. TcZr alloy system. The results indicate that this system has good superconducting properties.
Two Kinds of Self‐Oscillations and the Flip‐Flop Effect in a Semiconductor with Shallow Donors
Authors: Voronin I. N.
Published: July 2007

Some non‐equilibrium electronic processes are considered which may take place in a homogeneous semiconductor at low temperatures. Conditions are found of the excitation of two kinds of self‐oscillations—those of electron temperature and electric field strength and of electrontemperature, density, and electric field strength — as well as the conditions under which a flip‐flop effect is to be expected. An adiabatic approximation is used assuming the energy and electric field relaxationtimes to be far smaller than the recombination time. The concentration of the free charge carriers (electrons) is regulated by an extrinsic illumination and impurity breakdown. Frequencies and amplitudes of the self‐oscillations as well as a flip‐flop time are estimated.
Angular Dependence of SSWR in Thin Metallic Films Approximate Analytical Solution
Authors: Jirsa M.
Published: July 2007

The spin‐wave resonance spectrum characteristics, namely the mode position, linewidth, and intensity, for a thin homogeneous conducting film with an uniaxial surface anisotropy and an arbitrarily oriented external magnetic field are determined analytically using the spin‐wave approximation. Comparison with the numerical data computed in the “full” model shows good agreement for all spin‐wave modes including the lowest‐order ones.
Kink Lattice Solution in a Quasi‐One‐Dimensional Peierls‐Fröhlich System with a Nearly One‐Third‐Filled Band. II. Thermodynamic Considerations
Authors: Fischbeck H. J.
Published: July 2007

The thermodynamic meaning of the kink lattice solution derived in the first part of this work is analyzed. To this end the grand potential of this solution is calculated at zero temperature. Its minimum is self‐consistent only if a special relation between the external parameters is realized. In general this solution describes the incommensurate phase only approximately. The phase transition into this approximate phase is for the most part discontinuous but in a certain region of the parameter plane continuous. This picture is expected to be essentially correct.
ESR Study of Cu2+ Diffused into Sodium β″‐Alumina
Authors: Abello L. , Schwerdtfeger S. J. , Schwerdtfeger C. F.
Published: July 2007

ESR spectra due to Cu2+ ions diffused into sodium β″‐alumina are studied. The symmetry of the spectra place the Cu2+ ions at the mid‐oxygen sites of the conduction planes. The complex spectra are a superposition from several different configurations of ions around this site which change as a function of the amount of annealing time. The angular variations of the spectra can be divided into two main types; an anisotropic site symmetry with spin‐Hamiltonian parameters g∥ ≈ 2.355, g⟂ ≈ 2.087, A∥ ≈ 12.0 mT, A⟂ ≈ 2.0 to 4.0 mT, and a more isotropic site symmetry with g∥ ≈ 2.003, g⟂ ≈ 2.240, A∥ ≈ 9.9 mT, A⟂ ≈ 8.8 mT. These spectra are only visible at liquid‐helium temperatures. In addition an anisotropic, random‐type spectrum is observed to replace the Cu2+ spectra at the temperature of annealed crystals is increased. This is attributed to thermal motion of the Cu2+ ions due to their weak binding to the site.
Multimode Pseudo Jahn‐Teller Effect for Off‐Centre Impurities in Crystals
Authors: Bersuker G. I. , Polinger V. Z.
Published: July 2007

Systems with off‐centre impurity in the orbitally nondegenerate ground state are discussed. The off‐centre position of the impurity is shown to result from the mixing of the ground state and one of the excited electronic states by odd crystalline vibrations, displacing the impurity from the lattice point (pseudo Jahn‐Teller effect, PJTE). At high enough temperatures transitions of the impurity between the equivalent off‐centre positions occur due to the coupling with the PJTE active phonons. The qualitative estimation of the transition probability obtained for this process is in agreement with the line broadening data for the Li+ :KCl.
Dielectric Relaxation at the Glass Transition as a Free Volume Process. II. A Continuous Distribution of Relaxation Times
Authors: Alegráa A. , Barandiarán J. M. , Colmenero J.
Published: July 2007

A previous work describing the α dielectric relaxation in glasses by means of a single, free volumedependent, relaxation time, is generalized by allowing for a distribution of relaxation times. Such a distribution is assumed to arise from differences in free volume within the glass. A phenomenological parameter is introduced for describing the interaction among the different zones. By using a temperature independent, symmetric, distribution in the inverse of the free volume expansion coefficient the experimentally found, asymmetric and temperature‐ as well as thermal‐story dependent distribution of dielectric relaxation times is reproduced. The physical meaning of the assumed distribution is discussed together with some implications on the dielectric response of a glass. Comparison with measurements performed on PVC shows good agreement.
Generation of Hot Free Electrons Due to the Interaction between Charge‐Transfer Excitons and Thermalized Free Electrons in Molecular Crystals
Authors: Andreev V. , Pope M.
Published: July 2007

The interaction of a free, thermalized electron with a CT exciton (nearest‐neighbor hole–electron pair) in an anthracene crystal is discussed theoretically. The rate constant for the annihilation of the CT exciton by the electron, in which an Auger electron is produced in a plane‐wave crystal state by the decay of an intermediate excitonic ion state is estimated to be about 10−10 to 10−9 cm3 S−1.
Determination of Valence Band Parameters in ZnTe
Authors: Fröhlich D. , Nöthe A. , Reimann K.
Published: July 2007

The fine structure of the 2P exciton in ZnTe is studied by two‐photon‐absorption. The energy splitting into four states, caused by the envelope–hole coupling, allows the determination of the Luttinger parameters γ2 and γ3, which describe the complex valence band. γ2, 0.8 and γ3 = 1.7 are obtained. These values will be compared to results of other experiments mainly on 1S excitons.
Exciton Drops in One‐Dimensional Molecular Chains. New Method of Solving of Gap Equation
Authors: Tošić B. S. , Šetrajčić J. P.
Published: July 2007

A new method of solving the gap equation for exciton drops is proposed. By the use of this method the elementary excitation spectrum is found. It is shown that this spectrum satisfies the superfluidity condition.
Excitation Spectra of Photoluminescence Bands in Cd1–xMnx Te
Authors: Ambrazevičius G. , Babonas G. , Rud Yu. V.
Published: July 2007

The excitation spectra of photoluminescence bands B (2.0 eV), C (1.2 eV), and D (1.4 to 1.6 eV) are investigated at 5 and 78 K in Cd1–xMnxTe single crystals for 0.40 ≦ x ≦ 0.70. The data for the B‐band are interpreted in terms of the model of Mn2+ intra‐ion d–d transitions. The disagreement of some experimental results with this model is discussed. The photoluminescence band C, in contrast to B, is excited by several different temperature‐dependent processes. The models based on the common excitation mechanism for the B and C photoluminescence bands are shown to be insufficient to interpret the experimental results.
Zero‐Phonon Lines in the Excitation Band Centred at 23200 cm−1 Due to Mn2+ in Polymorphic ZnS
Authors: Pohl U. W. , Gumlich H.‐E. , Busse W.
Published: July 2007

A system of zero phonon lines is observed within the 23200 cm−1 excitation band of Mn2+ in ZnS. By site selecting laser spectroscopy four zero phonon lines are identified due to Mn2+ at cubic lattice site (AN) and also at the main polymorphic lattice sites (AS, PN). A similar structure is registered due to the nearly hexagonal site (PS). The zero phonon lines of Mn2+ at polymorphic sites are shifted in energy with respect to those of Mn2+ at the cubic site. The energy shift is correlated to the shift of zero phonon lines observed within the three lower energy excitation bands.
Positron Age–Momentum Correlation in Ultrafine Particles of Silver and Gold
Authors: Matsuoka Y. , Terakado S. , Tanigawa S.
Published: July 2007

Annihilation characteristics of positrons in ultrafine particles of silver and gold are studied by the dual parameter measurements between the positron age and the momentum of annihilating pairs. The momentum dependence of the mean lifetime in the fine particles examined shows a maximum at zero momentum and minima at intermediate values of the momentum. This momentum dependence can be attributed to the positronium formation and the conversion of ortho‐positronium at the surfaces.
Temperature Dependences of Third‐Order Elastic Constants of CsCl, CsBr, and CsI Crsystals
Authors: Varshney R. K. , Shanker J.
Published: July 2007

Expressions for third‐order elastic constants (TOEC) of CsCl‐structure solids are derived taking the thermal contribution into account. Six distinct values of TOEC for CsCl, CsBr, and CsI crystals are calculated at 0 K and at room temperature (300 K). Values of pressure derivatives of second‐order elastic constants are also calculated corresponding to both the temperatures. The phonon‐pressure contribution is incorporated within the framework of the three‐body potential model. The results are discussed and compared with other investigations.
Calculation of Grüneisen Constants in Si
Authors: Mayer A. P. , Wehner R. K.
Published: July 2007

Lattice dynamical calculations of the Grüneisen constants for general strain components are performed using the bond charge model of Weber with additional assumptions on anharmonic forces. The model is adjusted to fit data available at the Γ, X, and L points from various experimental techniques. The results along the main symmetry axes indicate that the Grüneisen constants depend sensitively on those model parameters which were originally determined from the phonon dispersion in the harmonic approximation. It is concluded that for a consistent description of phonon dispersion and anharmonic properties the model has to be revised.
Measurement of Phonon Energies in Silicon under Uniaxial Stress
Authors: Estel J. , Kalus J. , Pintschovius L.
Published: July 2007

Stress‐induced changes in phonon energies of the TA (ξ, ξ, ξ) branch are investigated in silicon by using inelastic neutron scattering. With stress in the [111] direction of 1.01 GPa the energy shift of the phonons changes from 0.5% near the Γ‐point to −1.2% at the zone boundary. Comparison with theoretical calculations of Mayer and Wehner [1] shows good agreement.
Critical Study of the Static Functional Integral Method in the Hubbard Model
Authors: Lorenz B. , Fehske H.
Published: July 2007

The effect of the static approximation in the functional integral representation of the paramagnetic Hubbard model is critically investigated. The exactly solvable two‐site model is treated within the one‐ and two‐field static functional integral method and some thermodynamic properties (local magnetic moment, specific heat cv) are compared with the exact solution for various interaction strengths and electron densities. It is shown that the static functional integral theory yields unphysical results for cv in the strong interacting system at small densities. The two‐field and one‐field methods are preferable in the low‐ and high‐temperature regions, respectively.
Electronic Band Structure of CdF2. Photoemission Experiment and Pseudopotential Calculations
Authors: Orlowski B. A. , Plenkiewicz P.
Published: July 2007

Energy distribution curves (EDC) of electrons photoemitted (hv1 = 21.22 eV and hv2 = 40.8 eV) from the valence band (F− 2p) and the Cd 4d band of a CdF2 crystal are measured and the valence and conduction band structures are calculated using the local empirical pseudopotential method (EPM). Comparison of the measured and calculated data show that the three main maxima measured in the valence band correspond well to the maxima in X5, L3, and X5 obtained in the calculated histogram of the valence band density of states. The influence of the final‐state structure on the position of peaks in the valence band is not observed on EDCs obtained for both, hv1 and hv2 photoemission exciting energies. For Cd 4d band the wide splitting of 1.46 eV is obtained only for hv1 = 21.22 eV while it is not obtained on EDC for hv2 = 40.8 eV. This Cd 4d band splitting may be caused by the influence of the final density‐of‐states maximum obtained in conduction band in X1‐ point. The results obtained are compared with reflectivity data available in the literature.
The Spin Density and Optical Properties of Amorphous Thin Arsenic Silicate Films
Authors: Al‐Ani S. K. J. , Hogarth C. A.
Published: July 2007

The spin density of amorphous As2O5/SiO thin films is found to be lower than that of SiO films deposited under similar conditions. The measurements show a decrease in the spin density of an amorphous As2O5/SiO film with increasing temperature which can be explained in terms of some annealing of dangling bonds. The fundamental absorption edge of these films at different temperatures is investigated and analysed assuming optical absorption by indirect transitions. The infrared absorption spectra indicate some chemical bonding between the two oxides. Experimental data on the wavelength dependence of the refractive index are presented. The dispersion of the refractive index follows a single‐oscillator model. Some effects of room temperature aging of the films are also presented.
Temperature Dependence of the Exciton Lineshape of GeO2
Authors: Fröhlich D. , Weber H.‐J. , Wille R. , Francini R.
Published: July 2007

Two‐photon measurements are presented of the IS exciton lineshape in GeO2 between 4.5 and 150 K. With a frequency‐doubled dye laser and a Raman‐shifted Nd:YAG laser the excitonenergy around 4.65 eV can be scanned. The temperature dependence of the total line width can be explained by a theoretical model of exciton‐phonon coupling. Furthermore the results reveal an unexpected asymmetry, which seems to originate from a non‐uniform contribution of optical phonons.
Optically Induced EPR of Conduction and and Donor Band Electrons in Zinc Sulphide
Authors: Backs D. , Hertrampf A.
Published: July 2007

Conventional EPR of mobile electrons as well as optical EPR excitation and quenching spectra of variously doped ZnS single crystals are reported. The different kinetic behaviour and the EPR parameters of the resonance lines are discussed involving donor and conduction band electrons. From the optical EPR excitation and quenching spectra and from ac‐microwave photoconductivity measurements a simple band model is proposed for the optical transitions.
Thermoluminescence Governed by Simultaneous Thermal Stimulation of Electrons and Holes
Authors: Chen R. , Mathur V. K. , Rhodes Joanne F. , Brown M. D. , McKeever S. W. , Bull R. K.
Published: July 2007

The possibility of a simultaneous thermal release of electrons and holes in the thermoluminescence (TL) process was suggested in the early stages of the development of this discipline. However, most investigators utilized a model in which carriers of only one kind are thermally released from their “traps”. In this model these carriers then radiatively recombine with carriers of opposite charge in “luminescence centers”. This paper presents the results of a theoretical investigation into a more complex model, which allows the thermal release of electrons and holes in the same temperature range. The solution of a set of four simultaneous differential equations is required in this investigation. The equations are not amenable to analytical solution, so the equations are solved numerically for given values of the model parameters. Conclusions are drawn about the shapes of the TL curves, and the expected stability of the corresponding traps at given temperatures, and the apparent effective parameters namely, the activation energy, the frequency factor, and the order of kinetics, are obtained using the “general order” formalism. In all cases examined, there is a good three parameter fit to the TL peak generated by the examined model. This shows that even a rather general TL peak can be described quite accurately by the three parameter model. The activation energy determined using the three parameter analysis is found to agree rather well with the input activation energy of trapped electrons which is used in the original peak generation for the particular cases examined. In cases where the activation energy for holes is small, the glow peaks occur at relatively low temperatures, and are characterized by a very high effective frequency factor in the three parameter analysis. Furthermore, several cases are studied in which the calculated kinetics order is found to be less than 1.0.
Decay Kinetics and Polarization of the AT Emission of Pb2+ Centres of Different Structure in KBr:PbBr2
Authors: Egemberdiev Zh. , Nagirnyi V. , Soovik T. , Zazubovich S.
Published: July 2007

Three types of Pbc [001] centres) are found in a KBr:PbBr2 crystal. The luminescence of all these centres is caused by electronic transitions from tetragonal minima of the adiabatic potential energy surface (APES) of the triplet excited state of the Pb2+ ion. A comparison of the spectral, polarization and kinetic characteristics of their emission is performed. The position and structure of the defect associated with the Pb2+ ion are shown to have strong influence on the luminescence properties of KBr:PbBr22+v dipole and interstitial bromine atom associates (Hz(Pb2+) centres) as well as a Pb2+ ion associated with the next‐nearest‐neighbouring cation vacancy (Pb2+v− crystal.
Charge Transfer and Electron‐Molecular Vibration Coupling in Tetramerized Quasi‐lD Semiconductors
Authors: Yartsev V. M.
Published: July 2007

Electron‐molecular vibration coupling in tetramerized organic ion‐radical semiconductors is considered. Eigenvalues and eigenfunctions of the molecular tetramer with two electrons are calculated in terms of extended Hubbard model parameters. The energy inequivalence of the monomer sites is introduced explicitly. The complex frequency dependent conductivity of the tetramerized compounds is found to produce a number of sharp maxima due to indirect excitation of the totally symmetric (ag) internal molecular modes of vibrations. These peaks are shown to have a fine structure if the charge density is not shared equally by the constituent molecules. The calculated reflectance is compared with the measured one for TEA (TCNQ)2 single crystals.
The Effect of Many‐Electron Correlation on Photothresholds of Semiconductors and Valence Band Discontinuities at Heterojunctions
Authors: Bechstedt F. , Enderlein R. , Heinrich O.
Published: July 2007

Photothresholds of semiconductors are known to deviate by 3 to 4 eV from the predictions of one‐electron band theory. It is shown that these discrepancies are due to many‐electron correlation effects, in particular, to electronic relaxation. The corrections to photothresholds due to electronic relaxation are calculated by means of Green's function technique applying the linear shielded potential approximation to the exchange‐correlation self‐energy. Within the uncertainties of LCAO theories the calculated photothresholds of group IV, III‐V, and II‐VI semiconductors agree with experimental data. A simple physical picture of the correlation effect is provided. Relaxation corrections to valence band discontinuities from one‐electron band theories are also calculated.
Relativistic Theory of Photoemission from Solids. I. Key Aspects of the Model
Authors: Thörner G. , Borstel G.
Published: July 2007

The key aspects known to be important in photoemission: matrix elements, scattering of the outgoing wave, band structure, and surface effects are formulated within a relativistic singleparticle theory with the objective of developing a workable relativistic model for photoemission from magnetic and nonmagnetic solids.
On the Theory of Ion‐Induced Electron Emission
Authors: Rösler M. , Brauer W.
Published: July 2007

A microscopic theory of H ion‐induced electron emission is developed in close analogy to the theory of secondary electron emission and applied to polycrystalline aluminum. The formation of excited electrons by screened Coulomb interaction with H+ ions of primary energy E0 between 20 and 800 keV is considered. Both, elastic and inelastic collisions of these electrons are taken into account. As far as possible the results calculated are compared with recent experimental data. Energy and angular distributions of ejected electrons prove to be in accordance with measurements. The calculated electronic yield γ shows a maximum in the primary energy dependence at E0 ≈ 55 keV with a maximum value γmax = 1.1. With increasing primary energy there are larger deviations from the measured yield values.
Magnetic Field Dependence of the Spin Polarization of the F Center
Authors: Iwahana K.
Published: July 2007

A simple model for the magnetic field dependence of the spin polarization of the F center is developed, in which the spin mixing due to the hyperfine interaction both in the ground state and in the relaxed excited state is taken into account. It is shown that the spin mixing due to the hyperfine interaction plays an essential role in the interpretation of the spin polarization at low fields less than 0.5 T. Quantitative fitting of the experimental data of the magnetic field dependence of the spin polarization is performed for KBr and RbBr.
Effective Medium Approach for the Residual Resistivity of Alloys Containing Guinier‐Preston Zones
Authors: Gläser U. H. , Löffler H.
Published: July 2007

Different models for describing the residual resistivity of alloys with precipitates are briefly reviewed. The microscopic theories are able to reproduce the initial increase in the resistivity appearing during the ageing in some alloys forming Guinier‐Preston zones but fail in the largezone limit, whereas just the opposite situation is known for the macroscopic effective medium theories. The classical Maxwell‐Garnett theory for inhomogeneous materials is extended to particle sizes comparable or smaller than the electron mean free path. Regarding only the inner part of a GP zone as a macroscopic inclusion and treating the outer part microscopically as a part of the matrix one obtains a model for the resistivity containing only two essential parameters: The extra resistivity due to coherent scattering and the mean free path of the electrons inside the zones. The values of the parameters are derived from the electronic structure of the alloy. The model is found to be in a good agreement with experimental data for an Al‐5 at% Zn alloy.