bluuebunny/crossref_metadata_2025 huggingface dataset:
Luminescence of RbI above Liquid Nitrogen Temperature
Authors: Mitsushima Y. , Itoh N.
Published: July 2007

Luminescence spectra of RbI induced by bombardments with pulsed Ne+ and electron beams and X‐rays are measured above liquid nitrogen temperature. The luminescence bands at 3.8 and 2.4 eV are studied in detail. The former shows the usual quenching character, while the latter shows following characters. (1) The intensity of the 2.4 eV band obtained by pulsed Ne+ beams at liquid nitrogen temperature increases superlinearly as the current density increases. (2) The change of the luminescence after an electron pulse bombardment around room temperature shows an exponential decay superposed by a growth followed by another exponential decay. The time constants which characterize the luminescence change after an electron pulse bombardment are obtained between 260 and 300 K and are found to follow the Arrhenius equation with an activation energy of about 0.39 eV. To explain these experimental results, it is proposed that the 2.4 eV luminescence band is caused by the radiative recombination of a complex center formed by the interaction between the mobile VK center and a quasi‐stable electron trapped center, the concentration of which increases linearly with the radiation dose.
Kinetics of Flash‐Stimulated Changes of F‐Centre Absorption in γ‐Irradiated KCl
Authors: Werner B. , Stradowski C. , Sugier H.
Published: July 2007

The decay of F−‐centres, produced by flashing the γ‐irradiated KCl crystals is studied in the microsecond regime at room temperature. The kinetics of the decay of F−‐ and recovery of F‐centre absorption bands are compared and a scheme of reactions in the system investigated is proposed. The decay of flash produced F− ‐centres leads initially mainly to the recovery of the F‐band, probably via tunnelling of an electron to an anion vacancy on relatively long distances. The excess energy, released after tunnelling, allows F‐centres subsequently to diffuse and form aggregates. Another possibility is the migration of F‐centres in electronically excited 2p state, leading also to the formation of aggregate centres.
Short‐Range Effects in Germanium‐Silicon
Authors: Martín‐Rodero A. , Vergés J. A. , Tejedor C.
Published: July 2007

In order to analyze simple approximations used in the study of Ge‐Si alloys, a self‐consistent calculation of the Ge‐Si perfect crystal is presented. The description in terms of localized functions allows to discuss a few‐neighbours approach. Short‐range effects are dominant for valence bands, but this does not apply for conduction electrons.
Studies of the Fine Structure of the Direct Gap of As2Se3 Single Crystals by Electroreflectance
Authors: Althaus H. L. , Weiser G.
Published: July 2007

Electroreflectance spectra of As2Se3 single crystals in the energy range between 2 and 2.6 eV are presented for polarization of light parallel and perpendicular to the layer plane. Comparison of the spectra with band structure calculations and bond models reveal a very complicated structure of the energy bands near the direct gap. The direct gap close to 2.20 eV is located near Γ. The spinorbit splitting of the uppermost valence bands of 40 meV is similar to the splitting in trigonal selenium. Optical selection rules and the dependence of the spectra on the orientation of the electric field indicate that the uppermost filled bands arise mainly from the lone pair p‐states of the Se atoms.
Molecular Field Theory and Landau‐Ginzburg Theory of Phase Transitions in Disordered Ferromagnets
Authors: Fähnle M.
Published: July 2007

The equivalence is demonstrated of molecular field theory and Landau‐Ginzburg theory for the description of the ferromagnetic phase transition. In analogy to the original (inhomogeneous) molecular field theory of disordered materials and its homogeneous version (S = Sz in the expression for the molecular field) an inhomogeneous and a homogeneous version of the Landau‐Ginzburg theory are derived. Within the framework of the local versions of these theories the influence of statistically random structure fluctuations on the critical temperature, Tc, is investigated. It is shown that Tc is either unchanged (homogeneous theory) or increased (inhomogeneous theory) in comparison to a corresponding homogeneous system, if correlations between site disorder and bond disorder are neglected. A possible way is described to decide which version of the theory is more appropriate for the description of the phase transition in disordered materials.
On the Interaction between a Vacancy and a Dislocation in Metals
Authors: Masuda K.
Published: July 2007

The interaction ΔEvd between a vacancy and a (screw or an edge) dislocation in metals is investigated using a tight‐binding‐type electronic theory. The repulsive core‐core interaction energies at short distances are simulated by the Born‐Mayer potential. It is shown that there are systematic differences in the ΔEvd between the present results and those obtained by using a pair potential method. Furthermore, it is found that the atomic configuration around a vacancy in the dislocation core region is essentially different from that in the unperturbed lattice.
Electronic Raman Scattering of Polarized Light on Donor Levels in Semiconductors with Direct Band Gaps
Authors: Ba An Nguyen , Van Hieu Nguyen , Toan Thang Nguyen , Ai Viet Nguyen
Published: July 2007

Explicite approximate analytical expressions are derived for the differential cross sections of the electronic Raman scattering of circularly polarized light on donor levels with the electronic transition 1s → 2s in semiconductors with zincblende structure and direct band gaps. The photon energies are assumed to be nearly equal to the band gap and therefore the intermediate states in the valence bands give dominant contributions. The real symmetry properties of the degenerate valence bands are taken into account. The absolute values of the cross sections are calculated for GaAs.
Many‐Particle Effects at the E1‐Transition in Silicon
Authors: Jungk G.
Published: July 2007

The ellipsometric determination of the dielectric function of Si around the controversial 3.4 eV optical structure yields at least three critical points at 3.34, 3.41, and 3.58 eV with types M0, M1 and M0, respectively. The exact lineshape fit of the energy‐derivative spectra shows that the M1‐type singularity is modified by many‐particle effects, which can be accounted for by an admixture of M2‐type lineshape.
Infrared and Raman Spectra of Non‐Stoichiometric Cerium Hydrides
Authors: Fujimori A. , Ishii M. , Tsuda N.
Published: July 2007

Infrared reflectivity and Raman spectra of CeHx; (2 < x < 3) are reported. Some of phonon features in the spectra shift with the hydrogen concentration. The spectra are analyzed in terms of lattice vibrations of supercells which simulate non‐stoichiometric CeHx. Force constants thus obtained show that the hydrogen atoms at octahedral sites are more weakly bound than those at tetrahedral sites. Anomalous changes in the force constants are suggested to account for the observed shifts.
Local Density of States for an Arbitrary Array of Muffin‐Tin Potentials
Authors: d'Albuquerque J. , Castro E. , Muniz B. Bechara , Jacobs R. L.
Published: July 2007

New formulae are presented for the local density of states in both the spherically symmetric regions and the interstitial region of an arbitrary array of muffin‐tin potentials. No assumption is made about periodicity, there need be no underlying lattice, the muffin‐tins may be of arbitrary sizes (provided that they do not overlap) and the spherically symmetric potentials are arbitrary. The formula at a point in the spheres is site‐diagonal and reduces in spherical average to that of Chen and Fromhold. A similar formula is obtained for points in part of the interstitial region. The formulae when integrated over all space reproduce the result for the total density of states due to Lloyd.
Itinerant and Localized d‐Electrons in Transition Metals from Positron Annihilation Data
Authors: Johnson O.
Published: July 2007

The positron annihilation data for non‐transition metals are treated as a combination of annihilation by conduction electrons, ion‐core electrons, and d‐electrons localized on the ion core. It is possible to account for the high positron annihilation rate of transition metals and the gradation across the transition series by the further partition of the d‐electrons into localized and itinerant d‐electrons. All d‐electrons are itinerant for Sc, Ti, and V, and only a fraction of a d‐electron is itinerant in Fe, Co, and Ni; there are two and five localized d‐electrons for Cr and Mn, respectively.
On the Phonon Induced Hopping between Stark Levels
Authors: Foltin J.
Published: July 2007

A method for calculating the current due to the phonon induced hopping between Wannier‐Stark states, taking into account the space charge field resulting from the localization of these states, is suggested.
Infrared Spectrum and Structure of Hydrogenated Amorphous Silicon
Authors: Shanks H. , Fang C. J. , Ley L. , Cardona M. , Demond F. J. , Kalbitzer S.
Published: July 2007

The ir absorption spectrum of hydrogenated amorphous silicon is discussed in the context of structural models for this material. It is shown that the integrated strength of the bond stretching bands in hydrogenated amorphous silicon cannot be used to determine the hydrogen concentration because the local effective charge for ir absorption is a function of hydrogen concentration and sample preparation. The effective charge for the wagging—rocking—rolling vibrations at 640 cm−1, however, is independent of hydrogen concentration and sample preparation. Hence the integrated strength of this band can be used to measure the hydrogen concentration provided the proportionality constant is determined empirically. Changes in the ir absorption bands as a function of isochronal annealing temperature show that two different types of bonds contribute to the bond stretching band at 2100 cm−1. The bond bending bands at 890 and 840 cm−1 are associated with one of the bonds producing the 2100 cm−1 stretching band. The relative strengths of the 890 and 840 cm−1 bands to the 2100 cm−1 band depend strongly on the film preparation method: the glow discharge produced films show much stronger bending bands than those produced by rf sputtering. A structural model for hydrogenated amorphous silicon is presented in which a Maxwell‐Garnett‐type local field correction, which is dependent on the dipole location within a microvoid and the shape of the microvoid, is used to explain the data.
A Full Single‐Quantum Radiative Recombination of Biexcitons Due to Exciton—Biexciton Interaction
Authors: Petrashku K. G. , Russu S. S. , Khadzhi P. I.
Published: July 2007

The single‐quantum annihilation process of the biexciton as a whole at the inelastic collision of the biexciton with an exciton is studied. Analytic expressions for the spectral and temperature dependence of the transition probability are obtained for allowed interband optical transition for various temperatures of the exciton and biexciton subsystems. It is shown that the single‐photon radiative recombination process of the biexciton as a whole in the presence of the exciton is characterized by a transition probability different from zero in the crystal with the symmetry centre in the dipole approximation, while the biexciton radiative recombination itself is impossible.
On the Core Structure of the Glide‐Set 90° and 30° Partial Dislocations in Silicon
Authors: Marklund S.
Published: July 2007

By considering valence force potentials the cores of the glide‐set 30° and 90° partial dislocations in silicon are investigated. It is shown that the 30° partial probably does not contain dangling bonds, while on the other hand dangling bonds at the 90° partial are more likely to exist as pairing of bonds across its core is opposed by a strong shear field. The implications of the results on the electron states of the partials are briefly discussed.
Ultraviolet Photoelectron Spectroscopy (UPS) and LEED Studies of BaTiO3 (001) and SrTiO3 (100) Surfaces
Authors: Courths R.
Published: July 2007

UPS studies (21.2 and 40.8 eV) are performed on BaTiO3 (001) and SrTiO3 (100) surfaces. In both the compounds there is an enhanced emission at the top of the valence band for hv = 21.2 eV, which cannot be explained with the existing theoretical bulk densities of states. From the results of angle‐dependent UPS measurements it is concluded that the top of the valence band emission is due to an oxygen surface state. This interpretation is supported by band structure calculations for thin films of BaTiO3 of Marathe et al. A band gap state at 0.9 eV (BaTiO3) and 1.2 eV (SrTiO3) below the bottom of the conduction band can be detected on oxygen deficient surfaces and is interpreted as arising from Ti3+ species. In UHV annealed surfaces are 1 × 1 ordered. They show a 2 × 2 or two mutually perpendicular 2 × 1 LEED patterns in case of a higher concentration of Ti3+—O‐vacancy defects due to defect ordering.
Photoelectron and Optical Spectra Derived from Self‐Consistent Charge MO and Band Calculations for Bulk and Surface of BaTiO3
Authors: Marathe V. R. , Lauer S. , Trautwein A.
Published: July 2007

Both MO and band structure calculations are performed for the cubic phase of BaTiO3 based on a self‐consistent charge procedure following the semi‐empirical Cusachs scheme. The photoelectron spectra derived from the MO calculations are considerably cluster dependent. The experimental XPS valence band (VB) spectrum is well represented by the band structure calculations for the bulk. Comparing band structure calculations for three hypothetical thin films of BaTiO3 with experimental UPS VB results from the preceding paper of Courths it is concluded (i) that the surface of BaTiO3 consists of a TiO2 layer, and (ii) that the experimental UPS VB spectra may be interpreted in terms of contributions from the surface TiO2 layer plus those from the inner BaO and TiO2 layers. Calculated optical transitions in the range 0 to 15 eV are in rough agreement with experimental reflectivity spectra of Bäuerle et al. and Cardona. The calculations show that selfconsistency has significant effects on ionization and Madelung potentials and therefore on the electronic structure of BaTiO3.
Scattering by Clusters in Compensated Semiconductors
Authors: Gerlach E. , Harbecke B.
Published: July 2007

The energy‐loss method is applied to calculate an expression for the dc resistivity of compensated semiconductors assuming scattering by clusters of the ionized impurities. A numerical example is given in which the effects of interference and screening become apparent.
Influence of Exciton Gas and Electron—Hole Plasma on Exciton Energy Levels
Authors: Röpke G. , Seifert T. , Stolz H. , Zimmermann R.
Published: July 2007

Corrections to the energy levels of ground state excitons embedded in a gas of excitons, electrons, and holes are obtained within the framework of the Green's function technique. Contributions of the interaction with free carriers and excitons are considered in the first Born approximation, and plasmon effects are taken into account. Numerical values are given for the exciton energy shift linear in the densities at different temperatures and different electron—hole mass ratios.
The Dissociation Energy of the Surface Biexciton
Authors: Bobrysheva A. I. , Beryl S. I. , Moskalenko S. A. , Pokatylov E. P.
Published: July 2007

The effective Hamiltonian of two electrons and two holes located near the boundary of the polar crystal and the vacuum, interacting via the Coulomb law and through exchange by virtual surface longitudinal optical phonons is derived. On ground of the approximate form of this Hamiltonian the binding energies Wm(σ) of the surface biexciton are estimated in the whole range of mass ratio 0 ≦ σ = me/mh ≦ 1 (me and mh is the effective mass of the electron and of the hole, respectively). The surface biexciton is stable at all σ and the dissociation energy is greater than that of bulk biexciton.
New Model Exhibiting Metamagnetic Transitions in Absence of Magnetic Field
Authors: Onyszkiewicz Z.
Published: July 2007

A new model is discussed, defined through a Hamiltonian with anisotropic fourth order terms in spins. It turns out that such a term when added to the ferromagnetic Heisenberg coupling, stabilizes the antiferromagnetic phase at low temperatures. With increasing temperature the meta‐magnetic transition to the ferromagnetic state occurs even when the external magnetic field is absent.
Work Function in Alkali and Noble Metals
Authors: Khanna K. N.
Published: July 2007

The pseudopotential approach is applied for the calculation of the work function of alkali and noble metals. The results indicate that the effective mass contributes significantly to the work function. The results agree well with the experimental value.
The Mobility of Electrons and Holes in HgTe in the Temperature Range of Intrinsic Conduction
Authors: Dziuba Z. , Wróbel J.
Published: July 2007

HgTe samples containing no acceptors are studied. The results of the investigation of Hall coefficient and conductivity versus magnetic field up to 20 kG in the temperature range 4 up to 100 K are presented. Concentration and mobility of electrons and holes in the intrinsic range of the conductivity are calculated. Density state effective mass of holes mh = 0.3m0 is determined. It is shown that acoustic and optical phonon scattering are most important modes limiting the hole mobility.
On the Nonstationary Processes in Exciton Systems
Authors: Hadžiselimović E. A. , Marinković M. M. , Tošić B. S.
Published: July 2007

In consequence of the nonconservation of excitons, the excitation number operator as well as the exciton pair creation and annihilation operators are expressed as functions of time and of the corresponding stationary operators. The nonstationary pair correlation processes and the nonstationary excitation density fluctuation processes are analysed using the nonstationary operators. It is found that the nonstationary pair correlations are characterized by a specific kind of dissipation that is linearly proportional with time. In contradistinction to stationary processes, the corresponding nonstationary ones, lead to elementary excitations having a double energy: E (nonstationary) = 2E (stationary). It is shown that in contrast to exciton density fluctuations, pair correlations have a tendency of localization related to a lattice point. It is also shown that the exciton non‐statinoary pair correlations are in many respect similar to the density fluctuations that take place in the crystal due to a near slow neutron passage. Finally, a general method for the evaluation of the correlation function, applicable even in the case when the Green's function has a n‐th order pole, is developed in the Appendix.
CNDO/2 Calculations of α‐Quartz Electronic Properties of the Perfect Cluster and EPR Parameters of the O−; Vacancy
Authors: Haberlandt H. , Ritschl F.
Published: July 2007

The CNDO/2 method is applied is applied to a cluster cut out from a perfect α‐quartz crystal. The result is compared with that of an extended Hückel theory (EHT) calculation of the same cluster. The dominant character of valence band states in terms of atomic orbitals and the width of the energy gap are discussed. The formal charges of central cluster atoms are given. Furthermore EPR parameters of a relaxed O−; vacancy in α‐quartz are calculated on the basis of CNDO‐RHF and ‐ UHF approximations. The results are compared with those of an EHT calculation of the same model and with the experimental result of the E′1 centre in SiO2.
Theory of the Anomalous Temperature Dependence of C12 in NaCl‐Like Structure Solids Direct Observation of Phonon—Lattice Interactions
Authors: Shrivastava U. C.
Published: July 2007

The basic quantum theory is presented for the temperature dependence of the elastic constants of solids. The phonon—lattice interaction is invoked to explain the anomalous temperature dependence of C12 in NaCl‐like structure alkali halides. The calculated values of dC12/dT for lithium and sodium halides are negative and dC12/dT for potassium and rubidium halides (except KF) are positive (showing an anomalous temperature dependence of C12). These results are in excellent agreement with available experimental data.
Modulational Instability of an Electromagnetic Wave in Magnetized Plasmas
Authors: Guha S. , Namjoshi P.
Published: July 2007

A modulational instability is investigated in a piezoelectric semiconductor plasma when the amplitude modulation of an electromagnetic wave by an acoustic wave is considered. Analytical expressions for threshold electric field amplitude as well as the growth rate of the unstable mode are obtained. A numerical analysis is made for n‐InSb crystals.
Cascade Capture of Non‐Equilibrium Carriers
Authors: Bhattacharya D. P.
Published: July 2007

The low‐temperature galvanomagnetic effects in non‐polar semiconductors are investigated under the condition that the non‐equilibrium carriers are captured by shallow attractive traps. The calculations are carried out assuming spherical constant energy surfaces in the momentum space. Numerical calculations are made for n‐Ge at 4.2 K. The possibility of using the results for the determination of some of the capture parameters is discussed.
EPR Study of Impurity Colour Centres in P‐State in Alkali Halides
Authors: Baranov P. G. , Khramtsov V. A.
Published: July 2007

Several types of (Ga°), (In°), and (Tl°) centres in the P‐state are produced in NaCl and KCl crystals doped with gallium, indium, and thallium by X‐irradiation, followed by suitable thermal and optical treatments. The EPR spectra of these centres are studied and the parameters of the spin Hamiltonian are determined from these spectra. The hyperfine interaction constants of (Ga°) and (In°) centres in NaCl are found to be nearly equal to those of free gallium and indium atoms. The structure of the centers investigated is discussed. In NaCl: Sn and KCl: Pb crystals X‐irradiated Sn+ and Pb+ centres in the P‐state are observed. The 2S1/2 state Pb3+ centres are found to have anisotropic hyperfine interaction constants.
Coherent Potential Approximation of Short‐Range Order in Cluster Models of Alloys
Authors: Batirev I. G. , Katsnelson A. A. , Kertész L. , Szász A.
Published: July 2007

In cluster approximation of coherent potential method the density of electron states of transitional element alloys with short‐range order is obtained. On the base of these calculations the configurational part of the total energy is determined and the equilibrium value calculated of the short‐range order parameter on the first co‐ordination sphere. The obtained parameter of orderliness in most of the considered systems agrees by sign with the experimental data and qualitatively they depend on the concentration on the right way.
Zeeman Effect at the 3T1 → 3T1(P) Absorption Transitions of CdS:Ni2+
Authors: Broser I. , Germer R. , Schulz H.‐J.
Published: July 2007

With magnetic flux densities up to 10 T, Zeeman experiments are carried out on CdS: Ni2+ crystals at low temperatures. Shifts and splittings of the 3T1(F) → 3T1(P) absorption components are studied in polarized light for H ‖ c and H ⟂ c orientation. The static crystal field theory is not a sufficient theoretical concept to interpret the findings, dynamic interactions must rather be considered.
Polarons and Wannier Excitons in Anisotropic Polar Crystals
Authors: Pertzsch B. , Rössler U.
Published: July 2007

For the case of anisotropic band masses and anisotropic electron–phonon coupling the polaron problem is treated by the method of Lee, Low, and Pines. In the same case the phonon‐induced effective interaction between electron and hole in a Wannier exciton is an anisotropic generalization of the well‐known Haken potential. Self‐energy and mass renormalization of the polaron as well as the binding energy of the exciton and the influence of the different terms of the effective potential are discussed as a function of the parameters of anisotropy.
Adiabatic Variational Calculation of the Lattice Relaxation at Metal Surfaces
Authors: Hietschold M. , Paasch G. , Baktoš I.
Published: July 2007

The relaxation is considered of the top lattice plane on surfaces of simple metals which is determined by a minimum surface energy. A general formulation of the problem is given which allows clear classification of previous theoretical attempts and their comparison with the present model. The latter is based on a variational procedure in terms of the density functional formalism using a parametrized surface model with extended ion cores (Ashcroft model potentials). Especially stressed should be the adiabatic properties of the model consisting in the fact that the electron density changes when the ionic positions are shifted. Numerical results for (100)‐Li, (110)‐Al, and (111)‐Al surfaces are presented. The relaxations have different signs, and they are smaller than 2% of the interlayer spacing even for the case of Al where some LEED interpretations indicate large contractions.
Analysis of the Dielectric Behaviour of Alkali Halide Crystals
Authors: Shanker J. , Sundaraj R.
Published: July 2007

An analysis of the dielectric data at low temperature (2 K) and at room temperature (290 K) is performed for the entire family of NaCl and CsCl structure alkali halide crystals. The recent data on dielectric constants and their pressure and temperature derivatives are used to study the nature of some important physical quantities like the energy gap between bonding and anti‐bonding states, the Phillips‐van Vechten electronegativity parameter and the Szigeti effective charge parameter. Some interesting results are obtained using the Penn model for the electronic dielectric constant and the Lawaetz relation between electronegativity and effective charge parameters.
Dynamic Spin Susceptibility of an Interacting Electron Gas
Authors: Choudhury S. K. , Mohanty S. P.
Published: July 2007

A simple theory of dynamic spin susceptibility χ(q, ω) of an interacting electron gas is presented by using the equation of motion method for the double time retarded Green's function. A suitable moment conserving decoupling approximation is used and an expression for χ(q, ω) is obtained which is quite general and results of all previous authors can be obtained from it in appropriate limits. The enhancement function I(q), in the theory shows a peak at q ≈ 2qF. By using a screened Coulomb potential and an effective mass approximation, the exchange enhancement of the spin susceptibility of alkali metals is calculated and the results agree quite well with experiments.
Tl− Centres in KCl Crystals
Authors: Ioan A. , Topa V.
Published: July 2007

Electrolytic colouring at about 300 °C and 8000 V/cm, in KCl:T1+ single crystals with a 10−4 mol% concentration of T1+‐ions, gives rise to the appearance of two new absorption bands peaking at 4.35 and 3.35 eV. The modifications of the absorption spectra of the coloured crystals induced by different colouring conditions, by the recording temperature and by the heat treatments in argon, hydrogen, and water vapour atmospheres are investigated. All experimental data support the substitutional T1−‐ion model, proposed in this paper as the colour centre responsible for these two new bands.
On the Optical Properties and Electronic Structure of AgSbTe2
Authors: Baleva M.
Published: July 2007

p‐type polycrystalline samples of AgSbTe2 are investigated. The reflectance and thermoreflectance spectra are measured and the results obtained are quantitatively interpreted. The absolute maximum in the reflectance spectra is found to be shifted to smaller energies in comparison to that of PbTe and the other lead compounds and AIVBVI tellurides. Two additional structures are observed in the thermoreflectance spectra of AgSbTe2 in comparison with PbTe. These structures and the shift of the maximum in the reflectance spectra are explained with imbedding the band structure of its binary analog PbTe into the AgSbTe2 zone.
Magnetic Field Effect on the Decay Times of KCl:Sn
Authors: Hizhnyakov V. , Kalder K. , Mihkelsoo V. , Niedrais H.
Published: July 2007

The effect of pulsed magnetic fields up to 15 T on the decay times of KC1:Sn AT luminescence is investigated at 4.2 K. The decay consists of two exponential components. The decay time of the fast component decreases from 3.3 × 10−3 s at H = 0 to 2 × 10−5 s at H = 15 T. Magnetic field dependences of the decay times are discussed in terms of a model with a strong interaction between the excited electronic states of Sn2+ centres and Eg vibrations.
Short‐Time Carrier Transport in Amorphous Pentacene
Authors: Hesse R. , Bässler H.
Published: July 2007

Motion of holes generated in an amorphous pentacene layer by a N2 laser flash is examined within a time range 10 ns to 1 p. Pentacene layers are prepared by vapour deposition onto a substrate held a t 140 K. An initial current spike is observed which is attributed to motion of holes with an short‐time mobility of the order of 0.1 cm2/Vs. Subsequently, carrier trapping occurs a t a rate constant > 107 s−1. The temperature dependence of the mobility is interpreted in terms of hopping through an array of hopping sites distributed in energy according to a Gaussian distri‐bution function.
Boundary Conditions for Stimulated Raman Effect of a Semi‐Infinite Piezoelectric Cubic Crystal
Authors: Merten L. , Wenk J.
Published: July 2007

The boundary conditions on the surface of a semi‐infinite crystal are formulated and solved for the so‐called two‐beam method on condition of normal incidence. The solutions are three “reflected” and three coupled “transmitted” waves. The electric amplitudes of the “reflected” and “transmitted” waves can be represented by explicit formulae. For continuously decreasing electric amplitude EL of the pump wave (EL → 0) the solutions change continuously into the well‐known Fresnel formulae of classical crystal optics.
Coherence Properties of Dynamically Diffracted X‐Rays
Authors: Holý V.
Published: July 2007

The description of dynamical diffraction of X‐rays by means of the mutual coherence functions of primary and diffracted beams is presented. The mutual coherence functions of a beam diffracted from a plane‐parallel crystal in Laue geometry are computed and the influence of the crystal thickness on the coherence width is investigated. The possible method is shown for measurement of a mutual coherence function of an X‐ray.
Structure and Energy Levels of the Glide 60° Partial in Silicon
Authors: Jones R. , Marklund S.
Published: July 2007

Deformation induced edge dislocations in Si are known to be dissociated into pairs of 60° partials and act as electrical recombination centres. The structure of this partial is investigated using isotropic elasticity theory and a simple core reconstruction is suggested. The effect of this reconstruction is to eliminate all dangling bonds leaving only an empty band in the upper part of the band gap.
Electron‐Phonon Interaction and Superconductivity in Metals
Authors: Jain S. C. , Kachhava C. M.
Published: July 2007

A first principle derivation, in terms of electron‐ion pseudopotential and the phonon spectrum, is presented for the electron‐phonon coupling constant λ. The phonon parameters, needed in the calculation, are characterised by a single average square frequency, which is evaluated after Allen and Cohen. The computed λ, along with the values of Coulomb pseudopotential μ* and Debye temperature θD, leads to an over‐all satisfactory result for the transition temperature and isotope effect exponent according to the formulation developed by McMillan. Further, the calculations demonstrate unambiguously the existence of Matthias regularities.
Pseudocubic Angular Dependence of the M = − 1/2 to + 1/2 EPR Transitions for Ions with S = 3/2 Produced by Small Crystal Fields of Arbitrary Symmetry and Random Strains
Authors: Gehlhoff W.
Published: July 2007

Previous investigations of the angular dependence of the −1/2 ⟷ +1/2 transition which turned out to be pseudocubic in spite of lower centre symmetry are extended to the case of arbitrary symmetry. Moreover the influence of random strains is calculated. The result is that also in both these cases the centre of gravity of the line groups exhibits a cubic angular dependence. Thus in the case of unresolved structures and non‐detectable ± 1/2 ⟷ ± 3/2 transitions the exact symmetry of the centre cannot be directly deduced from the angular dependence. The contributions from the local lattice distortions to the cubic angular dependence vary essentially inversely with the square of the magnetic field and can be determined by measuring at different microwave frequencies.
Low‐Temperature Optical Properties of Impurity Aggregates in Pb2+‐Doped KCl Crystals
Authors: Mugeński E. , Damm J. Z.
Published: July 2007

Low‐temperature absorption, emission, and excitation spectra of substitutional Pb2+ ions (2 to 100 ppm) in KCl crystals subjected to various thermal pretreatments are examined. The existence of two quite different aggregate types composed of Pb2+‐cation vacancy dipoles is ascertained. The aggregates of the first type, thermally stable below 80 °C, appear in samples stored e. g. at room temperature. Their optical properties (small changes in the original absorption spectrum, characteristic emission bands partially overlap that ascribed to the isolated dipoles) suggest a rather weak interaction between component impurity dipoles. The aggregates of the second type appear in samples annealed at higher temperatures up to 250 °C. Their optical properties (new distinct absorption bands, characteristic emission separated from that ascribed to isolated dipoles) suggest a strong interaction between component impurity dipoles. The optical properties (15 K) related with the above aggregates as well as with isolated dipoles and some unidentified aggregation products are compiled.
X‐Ray Determination of Bond Charges in Silicon
Authors: Pietsch U.
Published: July 2007

A bond charge model is introduced for the reevaluation of X‐ray data for silicon with respect to the electron density. Spherical silicon atoms and bond charges are adjusted separatly. In contrast to the model of deformable valence orbitals the size and shape of the bond charge can be investigated directly. The results are compared with calculations from empirical pseudopotentials and from the Dawson model of deformable orbitals.
Phase Boundaries between the Paramagnetic and the Antiferromagnetic Phases and between the Paramagnetic and the Spin‐Flop Phases of the Heisenberg‐Ising Model
Authors: Katsura S. , Shimada K.
Published: July 2007

The phase boundary between the paramagnetic and antiferromagnetic phases, and that between the paramagnetic and spin‐flop phases of the Heisenberg‐Ising model with anisotropic exchange energy are calculated by the method of pair approximation. The critical field curves are satisfactory except at the very low temperature. The reliability of the critical field in the low temperature region is closely related to the problem of the appearance of the anti‐Néel point, and is discussed by calculating the values of the Néel point and the anti‐Néel point by the method of the square approximation and comparing them with those by the pair approximation.
Dielectric Properties of Bi1−xSbx Alloys at the Semiconductor‐Semimetal Transition
Authors: Rudolph R. , Krüger H. , Fellmuth B. , Herrmann R.
Published: July 2007

The composition dependence of the lattice dielectric constant of semiconducting Bi1−xSbx alloys is investigated by means of helicon wave propagation for B∥C1 in high magnetic fields in the neighbourhood of the semiconductor‐semimetal transition. In contrast to the smooth variation of the parameters of the isoenergy surface of the electrons, in the neighbourhood of the transition first a rapid increase of the dielectric constant is observed, followed by a discontinuous drop when the band overlap begins. The comparison with theoretical calculations allows the conclusion that the lattice dielectric constant must be correlated with the indirect band gap of the alloy – the parameter of the semiconductor‐semimetal transition.
Electronic, Structural, and Magnetic Properties of 57Fe Atoms Created by 57 Co Electron Capture Decay in CoF2 and in CoCl2− and FeCl2− Bispyridine Complexes
Authors: Friedt J. M. , De Jesus Filho M. F. , Sanchez J. P.
Published: July 2007

A comparative investigation of the 57Fe absorption and emission spectroscopy data in hosts of CoF2, CoCl22py, and FeCl22py is reported in both the paramagnetic and antiferromagnetic phases of these compounds. The electron capture decay products are observed to coexist in the Fe2+ and Fe3+ ionic configurations in CoF2, while only the Fe2+ charge state is stabilized inFeCl22py and CoCl22py hosts. Distributions of hyperfine interactions are observed in the complex ligand compounds FeCl22py and CoCl22py, whereas there is no indication for such effects in the monoatomic ligand compound CoF2. This indicates unique unperturbed substitutional rest sites in CoF2, whereas in the other hosts, part of the decay products are observed in a distribution of defect sites.
The Kinetics of Stable Heterogeneous Binary Systems
Authors: Püschl W. , Aubauer H. P.
Published: July 2007

Among thermodynamically stable systems homogeneous one‐phase states are compared with two‐phase heterogeneous ones, while no restriction is made about the physical origin of stability. In the case of transitions from a one‐phase state to another one‐phase state or a two‐phase system, a continuous spectrum of relaxation times appears as a difference to a transition from a two‐phase to another two‐phase state: in this latter case one finds two processes of distinctly different rates in good agreement, with experiment. With the aid of a stochastic theory, containing elements of classical nucleation and coarsening theory, the early, as well as the late kinetic change of the size distribution function as well as of momenta, describing the system, are derived. This theory also allows for a rise of the number of zones of a phase during annealing as a contrary effect to coarsening.
Infared Spectra of the Neutron Irradiated Quasi‐One‐Dimensional Charge Transfer Salts TEA (TCNQ)2 and QN (TCNQ)2
Authors: Kamarás K. , Holczer K. , Jánossy A.
Published: July 2007

The infrared vibrational spectra of the organic quasi‐one‐dimensional charge transfer salts quinolinium (TCNQ)2, and triethylammonium (TCNQ)2 change markedly upon irradiation by high energy neutrons. A reduction of the intensity of some totally symmetric modes and an increase of visibility of other modes are observed in the irradiated samples. The results are discussed in the framework of the electron‐vibrational interaction theory. The doses required to obtain observable changes are relatively high and point rather to a model of simple electron coupled vibrational modes extended over a few molecules than to phase oscillations of a collective state with a long coherence length.
Temperature Dependence of Bond charge Vibration in Silicon
Authors: Pietsch U. , Unger K.
Published: July 2007

With help of a bond charge model introduced in a previous paper the thermal motion of the silicon atoms and that of the bond charge (BC) is separated. This analysis is made with structure factor data from silicon of Aldret and Hart, measured at two temperatures. The observed temperature dependence of the BC is caused mainly by the larger vibration amplitudes of the BC's in comparison with that of the Si atoms. Experimentally, a change of the BC amount is not detectable. The mean square displacement of the BCs is 4.2 times larger than that of the silicon atoms for 92.2 and 293.2 K, respectively. This fact is explained on the basis of the changing amount of the BC in the vibrating lattice, which corresponds to the results of calculations by Maschke and Baldereschi.
Radiation‐Induced Emission of F2 Centres in KCl
Authors: Chandratillake M. R. , Newton G. W. A. , Robinson V. J. , Rodgers M. A. J.
Published: July 2007

An emission has been observed in KCl single crystals subjected to 12 MeV electron pulses. The emission spectrum peaks at 1200 nm (1.03 eV) and has a temperature‐independent lifetime of 29.7 ns up to room temperature. Experiments with crystals previously irradiated to produce F and F2 centres showed that the emission mainly arose from direct excitation by the electron pulse of previously existing F2 centres. The lifetime data, but not the spectrum, are in good agreement with published data.
Magnetic Behavior of van Vleck Ions and an Electron Gas Interacting by Exchange
Authors: Palermo L. , Da Silva X. A.
Published: July 2007

The magnetic behavior of a model in which van Vleck ions, under the action of a crystal field, interacting by exchange with an electron gas is investigated. The condition of onset of ferromagnetism and the behavior of the critical temperature, band and ionic magnetizations (and susceptibilities) versus temperature, as a function of the band width, exchange interaction, and the crystal field splitting energy parameters are obtained within an approximation equivalent to a molecular field formulation.
Calculations of Solution Energies of Alkaline‐Earth Metal Ions in Alkali Halide Crystals
Authors: Urusov V. S. , Dudnikova V. B. , Garanin A. V.
Published: July 2007

The Born‐Mayer ionic theory of lattice energy is used to solve the problem of solution of B2+ ions in AX crystals with NaCl structure (A alkali metal, B alkaline‐earth metal, X halogen). About fifty combinations of initial parameters are obtained, by varying the ionic radii, polarizabilities, repulsion parameters, and pure AX and BX2 lattice energies. For example, there are four variants of the repulsion parameters: average ones and those determined from the elastic, di‐electric, and atomic spectral properties. The model is sufficiently accurate to yield reasonable values of solution energies for the most ionic impurity lattices (all fluorides, barium and strontium chlorides). Both, the “polarization catastrophe” (for large impurity ions on small cation sites) and lack of an ionic model for the Mg2+ impurities (except for fluoride) and all iodides are considered to be the main factors which reduce the calculation validity.
Lattice Sums on Noncentrosymmetric Cubic Ionic Crystals
Authors: Jain A. K. , Verma M. P.
Published: July 2007

General connections between lattice sums of a definite kind, corresponding to all the cubic ionic crystals are derived and discussed. Tables of values of certain sums are given which together with the tables earlier published and the relations in the present paper can give any of the lattice sums of the class considered dor all the cubic ionic crystals.
Raman Scattering of Light in Antiferromagnetic Chromium
Authors: Cuden C. B. , Mota R.
Published: July 2007

Raman scattering of light in antiferromagnetic chromium is discussed in terms of conventional second‐order time‐dependent perturbation theory. The scattering cross‐section as a function of frequency and temperature is calculated for the scattering of light by one‐electron, one‐hole, and electron‐hole pair excitations. It is shown that the unphysical divergence of the scattering crosssection for the photon energy transfer equal twice the antiferromagnetic energy gap found by Kwok, Woo, and Jha is removed by taking properly into account the statistical coherence factors.
Role of Magnetization in the Electron Quasiparticle Energy in the s–d Model of Ferromagnetic Semiconductors
Authors: Čápek V.
Published: July 2007

It is shown how the method of momenta based on fitting the lowest momenta of the spectral function may be reconciled with general arguments requiring that the magnetization can enter neither the position nor the form of quasiparticle peaks in the spectral function. The magnetization then enters just the weights of quasiparticle poles. Therefore, the temperature dependence of the quasiparticle energies strongly suppressed, in contrast to most of the contemporary theories.
Calculation of Vibrational State Density and Long‐Wavelength Spectral Density of Compositionally Disordered Chains
Authors: Böttger H. , Kleinert P.
Published: July 2007

The problem of a possible disorder induced reflection of the vibrational state density in infrared and Raman spectra is studied for simple mass disordered chains. The state density and the long‐wavelength spectral density are calculated exactly for the case of infinite mass impurity atoms and approximately by an embedded cluster method, which is applicable to systems with arbitrary impurity mass. A remarkable resemblance between state density and spectral function can occur.
AC Properties of a Random Barrier Network
Authors: Sinkkonen J.
Published: July 2007

The ac properties of random barrier network are analyzed using the effective medium theory. At low frequencies σm ∼ ωs and at high frequencies it saturates to the value determined by the conductivity of the bulk material. Below the percolation threshold s ≈ 0.8, at the threshold s ≈ 0.4, and above the threshold s still decreases. The effective dielectric constant εm decreases with increasing frequency saturating to the bulk value at high frequencies. At the percolation threshold εm shows a pronounced maximum exceeding by several orders of magnitude the bulk value.
ENDOR Study of Cubic Fe3+ Centres in KMgF3
Authors: Duvarney R. C. , Niklas J. R. , Spaeth J. M.
Published: July 2007

The superhyperfine (shf) interactions of the Fe3+ impurity ion in KMgF3 with its nearest neighbour (n.n.) and next‐nearest neighbours (n.n.n.) F− ions are determined. Using ENDOR‐induced ESR it could be shown that the cubic field splitting parameter has the same (positive) sign as the n.n. neighbour shf interaction. Weak satellites in the ESR and ENDOR‐induced ESR spectra are tentatively assigned to a hf interaction with the 2.2% abundant 57Fe nucleus. The interpretation of the shf data in terms of a covalency and overlap model shows that the n.n.n. shf interaction can be explained as a transferred shf interaction via F−–F− ion core overlaps.
Study of Cohesion and Allied Properties of Silver Halide Crystals
Authors: Singh R. K. , Khare P.
Published: July 2007

An interionic potential model is developed by introducing modifications of the Hafemeister and Flygare (HF) potential to include the effects of three‐body and van der Waals interactions. This modified HF potential is applied to investigate the cohesion and associated physical properties of silver halides and found to give better descriptions than those obtained by earlier workers. The importance of the significant role of the three‐body and van der Waals interactions is also emphasised.
Mobility of a Small Polaron at Low Temperatures
Authors: Gogolin A. A.
Published: July 2007

The mobility of a small polaron (SP) at low and intermediate temperatures is calculated. The interaction of SP with acoustic and optic phonons is considered. The contribution of multiphonon scattering processes to the SP mobility is taken into account.
Density Excitation Spectrum of the One‐Dimensional Fermi Gas
Authors: Apostol M. , Corciovei A. , Stoica S.
Published: July 2007

The collective excitations of the particle‐density fluctuations are studied in the one‐dimensional Fermi gas by means of the generator coordinate method. The dispersion relation are obtained both, for the forward and backward scattering interaction. It is shown that for repulsive interaction a gap occurs in the frequency of the backscattering excitations at wavevectors equal to twice the Fermi momentum.
Induced Magnetism via Conduction Electron Exchange Application to PrAl2
Authors: Palermo L. , Da Silva X. A.
Published: July 2007

The magnetization, susceptibility, and magnetic specific heat of PrAl2 versus temperature are studied within a model where the onset of magnetism is due to the mutual magnetization of rareerth ions and conduction electrons, and the ionic magnetization process is van Vleck‐like. The crystal field, bandwidth, and exchange parameters of the model which fit the available experimental data are 2.5, 7000, and 341 meV, respectively.