bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetooptical Investigation of Hg1−xCdxTe Mixed Crystals. I. Semimetallic Configuration
Authors: Guldner Y. , Rigaux C. , Mycielski A. , Couder Y.
Published: July 2007

Submillimeter resonance spectroscopy and interband magneto‐absorption are applied to the investigation of the Hg1−xCdx Te alloys system in the range of composition x < 0.3. Magneto‐transmission measurements are performed at 4.2 K up to 60 kG in both the spectral regions (ħω < 16 meV and ħω > 120 meV), on identical unoriented samples, in the Faraday and Voigt geometries. Magnetooptical data are analyzed using the virtual crystal approximation. All interband and intraband magnetooptical transitions are interpreted in terms of Γ6 and Γ8 Landau levels described by the Pidgeon and Brown model. The Kane matrix element, the interaction gap, and the Luttinger parameters are determined for each alloy composition from the comparison between theory and experiments. Magnetooptics also provide information on the evolution of the resonant acceptor states with the composition.
Vibrational and Electronic Structure of Hydrogen‐Related Defects in Silicon Calculated by the Extended Hückel Theory
Authors: Singh Vijay A. , Weigel C. , Corbett J. W. , Roth L. M.
Published: July 2007

Extended Hückel theory calculations are carried out for interstitial hydrogen atoms in silicon model crystals without and with vacancies. The energetically stable positions for the hydrogens appear to be the tetrahedral interstitial site in crystals without vacancies; in the case of vacancies the hydrogens favor positions in the dangling bonds of the vacancies 0.35 bond lengths away from the vacancy nearest neighbors. The vibrational frequencies for all defect types considered are in very close agreement with infrared bands observed after proton irradiation of silicon. The frequencies increase with the number of hydrogens in a vacancy. The hydrogen‐related impurity levels are very close to the valence band edge, both above or below.
Solitons in One‐Dimensional Ferromagnetic Systems
Authors: Pushkarov D. I. , Pushkarov Kh. I.
Published: July 2007

It is shown that the magnons in a ferromagnetic chain can turn into “solitary magnons” propagating without smearing along the chain with constant velocity smaller than the sound velocity in such media.
Positron Annihihtion in Colour Centres
Authors: Nagarajan T. , Ramasamy P. , Ramasamy S.
Published: July 2007

The angular correlation of positron annihilation in NaCl and KCl is compared for various concentrations of F‐centres produced by γ‐irradiation as well as additive doping. The intensity and the “full width at half‐maximum (FWHM)“ ‐Γ1/2 of the narrow component are slightly different for each of the X‐irradiated, electron‐irradiated, γ‐irradiated (light and heavy doses), and addi‐tively doped crystals. The possible role of additional defect centres in trapping positrons is discussed. Also angular correlation measurements of positrons annihilating in the reversible F ⇌ F' conversion centres in equilibrium in additively coloured KCI at −100 °C are given.
On the Theory of Additional Light Waves and Boundary Conditions in Anisotropic Crystals
Authors: Pekar S. I.
Published: July 2007

It is shown that the exciton contribution to the crystal polarization is not quite correctly separated in some papers. Therefore the additional boundary conditions derived there as well as formulas for the reflection coefficients are correct only if the waves in the crystal are strictly transversal. A general method is developed of separating the exciton contribution to the polarization for arbitrary angles between the wave vector and the electric field strength. The case of uniaxial crystals is considered in detail for the frequency region containing two near exciton transitions, both of them contributing to the dispersion. In this case the incident monochromatic ray generates in the crystal two ordinary rays and three nonordinary rays. The boundary conditions are derived for these rays. An expression is obtained for the light reflection coefficient of the crystal.
On Magnon‐Acoustic Phonon Interaction in Magnetic Semiconductors
Authors: Balkarei Yu. I. , Nikitov V. A.
Published: July 2007

The electronic mechanism of spin‐acoustic phonon interaction is investigated in a wide band ferromagnetic semiconductor with high concentration of charge carriers at temperatures when the magnon system is highly excited. The dispersion equation for longitudinal acoustic phonons is obtained and is studied in the region of their interaction with the spectrum of the magnon branch. It is shown that the dimensionless constant of magnon‐acoustic phonon coupling in degenerate semiconductors can be larger than the ordinary magnetoelastic interaction constant and is very close to it in non‐ degenerate ones. A principal possibility to single out experimentally the electronic part in the spin‐acoustic phonon coupling constant is discussed.
The Application of CPA to Calculations of the Mean Magnetic Moment in the Gd1‐x‐,Nix, Gd1‐x ‐,Fex, Gd1‐x ‐,Cox, and Y1‐x ‐,Cox Intermetallic Compounds
Authors: Szpunar B. , Kozarzewski K.
Published: July 2007

Calculations are made of the mean magnetic moment per atom of the transition metal and the rare‐earth metal in the intermetallic compounds, Gd1‐x,Nix, Gd1‐x Fex, Gd1‐x Cox, and Y1‐x Cox. A simple model of the disordered alloy consisting of spins localized on the rare‐earth atoms and interacting with a narrow d‐band is considered. The magnetic moment of the alloy at zero temperature is calculated within the molecular field and Hartree‐Fock approximations. Disorder is treated in the coherent potential approximation. Results are in good agreement with the experimental data obtained for the crystalline and amorphous intermetallic compounds.
Size Effect on the Thermal Conductivity of Small Dielectric Crystals
Authors: Nonnenmacher Th. F. , Wunderle R.
Published: July 2007

The influence of the size effect on the thermal conductivity of dielectric crystals is discussed and on the basis of the Callaway‐Debye theory a simple formula is derived which incorporates both, a surface and a curvature term.
Transformations of the Hubbard Model
Authors: Mertsching J.
Published: July 2007

The significance of some transformations of the Hubbard model with an arbitrarily occupied symmetric band in a magnetic field is studied. The repulsion‐attraction transformation yields nontrivial relations between solutions of the Hubbard model with a positive and a negative correlation energy, respectively. As an example, the connection between an antiferromagnet in a magnetic field and a superconductor with a non‐half‐filled band is discussed.
The Derivation of a Direct Formula for the Hall Coefficient
Authors: Schiller W. , Röke G. , Buhr G. , Christoph V.
Published: July 2007

A direct formula for the Hall coefficient is derived by using the non‐equilibrium statistical operator formalism of Zubarev‐McLennan. The formula for the Hall coefficient expressed by correlation functions is discussed in the weak scattering limit, and the equivalence to the Kubo expression for the Hall coefficient is shown.
Many‐Body Effects on the Fourth‐Order Elastic Constants of Ionic Solids
Authors: Garg V.K. , Puri D. S. , Verma M. P.
Published: July 2007

Expressions for the fourth‐order elastic (FOE) constants are derived from the Lundqvist potential for ionic solids along the line adopted in the derivation of the expression for the second‐order elastic (SOE) and third‐order elastic (TOE) constants. As in the case of the SOE and the TOE constants it is observed that the Cauchy relations among the FOE constants are broken even at O K and discrepancies in these relations are directly expressible as functions of the many‐body term in the crystal potential. The expressions for the FOE constants suggest that they must satisfy four identities so that the number of independent constants is only seven. It is suggested that measurements of fourth‐order elastic constants at O K in conjunction with these identities can provide an useful test for the validity of the Lundqvist potential in ionic solids.
Subnanosecond Time‐Resolved Spectroscopy of the Biexciton Luminescence in CuCl
Authors: Ostertag E. , Grun J. B.
Published: July 2007

The biexciton luminescence of CuCl at 4.2 K is studied under intense UV picosecond excitation, and its time dependence is investigated in the range of 0 to 800 ps by means of an ultrafast optical sampling gate. The time‐resolved spectra of the luminescence due to the radiative recombination of biexcitons show a change in shape of this emission band during the first 100 ps, which may be due to the relaxation time of the biexcitons by means of acoustical phonons. A dynamical model, taking into account the collisions between biexcitons and excitons, is proposed, which provides an acceptable fit for the time dependence of the luminescence if a value TM= 1 ns of the spontaneous radiative decay time of the biexcitons is assumed.
Phenomenological “Dilution Model” for Kondo‐Type Effects in Concentrated Alloy Systems
Authors: Yao Y. D. , Arajs S. , Anderson E. E.
Published: July 2007

Possible phenomenological treatments of Kondo‐like behavior in non‐dilute metallic systems are described. The approach is a “dilution” treatment of non‐dilute alloys in which moment clusters are considered as approximate single impurity magnetic moments. Possible consequences of this phenomenological approach are briefly discussed.
Determination of Tb2Ni17, Er2Ni17, and Tm2Ni17 Magnetic Structure by Neutron Diffraction
Authors: Yakinthos J. K.
Published: July 2007

Neutron diffraction measurements on the intermetallic compounds Tb2Ni17, Er2Ni17, and Tm2Ni17 show that these materials are ferrimagnet,ic below their Curie temperatures 178, 166, and 152 K, respectively. This magnetic property is in agreement with previous susceptibility data. Tb2Ni17 and Er2Ni17 have collinear magnetic structure with magnetic moments which are perpendicular and parallel to the c‐axis of the crystals. Tm2Ni17 has a canted magnetic structure. The coupling between rare earth and nickel moments is negative, whereas the coupling between atoms of the same kind is positive. The measured mean value of the rare earth moment in the above compounds is 8.6, 8.0, and 6.0μB. respectively. The nickel moment is 0.3μB. Bertaut's macroscopic theory gives magnetic modes compatible with the magnetic structure experimentally observed. Crystal field calculations give two different anisotropy directions for the two rare earth sites.
Semi‐Empirical Pseudopotential Calculation for the Electronic Structure of Polymeric Sulfur Nitride
Authors: Chelikowsky J. R. , Schlüter M. , Cohen M. L.
Published: July 2007

A three‐dimensional band structure calculation is performed for polymeric sulfur nitride. Using the semi‐empirical pseudopotential method, we calculate the electronic density of states with and without pressure, the optical spectrum, the valence charge density, and the Fermi surface. The bonding properties of (SN)x are explored via an analysis of the valence charge density as a function of energy and by means of an angular momentum projection of the electronic density of states. The shape of the Fermi surface is investigated. Estimates for the electron‐phonon coupling constant, λ, are given, and the increase of Tc with pressure is discussed.
Effect of the Next‐Nearest Neighbour Coupling in a Semi‐Infinite Ferrimagnet
Authors: Diep‐The‐Hung
Published: July 2007

The surface spin‐wave energy in a semi‐infinite Heisenberg ferrimagnet is calculated. The study is based on a body‐centered‐cubic structure with a (100) surface as the outermost layer and where the exchange interactions between nearest‐neighbours,J1and between next‐nearest neighbours,J2, are taken into account. The conditions under which the uniform spin configuration becomes unstable are derived by considering a surface “soft” mode. Surface spin rearrangement is also discussed.
Plasma Oscillations in a Superlattice
Authors: Shmelev G. M. , Ciiaikovskii I. A. , Pavlovich V. V.
Published: July 2007

Tho spectrum of plasma oscillations, propagating along the superlattice (SL) axis is calculated in the random phase approximation (RPA) taking the umklapp processes into account. In the case of strong screening the plasmon frequency is determined by SL parameters and temperature and found to do not depend on wave number. At weak screening quasineutral electron oscillations occur. In the case of small wave numbers the oscillations follow the acoustic law of dispersion and propagate with a frequency of the same order as the electronic ones along the SL axis. The characteristic feature of plasmons propagating along the SL axis, is the absence of Landau damping since the decay of these plasmons in pairs of quasi‐particles is forbidden by energy‐momentum conservation laws. The existence of plasmons, propagating along the axis, may be an indication of electron tunnelling between layers.
Kinetics of Dilute Solid Paramagnets under “Bottleneck” Conditions
Authors: Grinberg E. S. , Nicmatullin R. R.
Published: July 2007

Kinetic equations, describing a dilute paramagnet in “bottleneck” (BN) conditions, are obtained in the case of spin‐lattice mechanism of longitudinal and transverse relaxation of electron spins (direct processes). The spectral distribution function of phonons, obtained in stationary conditions, is compared with the corresponding function for a concentrated paramagnet. The influence of the resonance phonons on the spin‐system is considered. The possibility of studying the phonon avalanche in dilute paramagnets by pulse techniques is mentioned.
Multiple Scattering of Electrons by an Infinite Lattice Plane II. On the Properties of the Planar Scattering Matri
Authors: Solbrig H.
Published: July 2007

The symmetry properties of both the angular momentum representation and the plane wave representation of the planar scattering matrix are derived and discussed. The generalized optical theorem is obtained from the current balance of the muffin‐tin layer. For the case without absorbing medium the absorption matrix of the muffin‐tin layer is calculated in terms of the partial absorption cross sections of the muffin‐tins.
Optical Properties of Cu and Ag in the Intermediate Region between Pure Drude and Interband Absorption
Authors: Hollstein T. , Kreibig U. , Leis F.
Published: July 2007

The complex dielectric constant ϵ of thick Cu and Ag films on fused quartz substrates is determined at 300 and 1.5 K by a modified v. Fragstein‐Kampermann method in the spectral region between 0.41 and 0.71 m̈n. The relaxation frequency of the conduction electrons is determined and the contributions of electron‐phonon, electron‐surface, electron‐lattice, and electron‐electron scattering are separated. No further absorption mechanisms have to he taken into consideration to explain the experimental results. By decomposing ϵ into interband and conduction electron contributions, the temperature dependence of the interband transition edge of Cu is evaluated. The edge is shifted from 2.18 eV (1.5 K) to 2.15 eV (300 K). There are indications of indirect transitions, but a quantitative determination is not possible.
A New Group‐Theoretical Approach to Cell‐Preserving Phase Transitions in Crystals
Authors: Aoyei A. K. , Birman J. L.
Published: July 2007

In a previous paper the crystal space group representations associated with cell‐preserving phase transitions and the independence of sublattice transitions were investigated. Based on that paper, a simple method for determining the space group and microscopic structure of the daughter crystal arising in any such transition is developed. In the process a new, “site” subduction criterion is derived, which is more restrictive than the Birman subduction criterion. The theory is applied to the structural transition that occurs in the A‐15 compounds. The daughter‐crystal space group of V3Si and Nb3Sn is found to be C74V. This group and the predicted microscopic daughter‐crystal structure are consistent with the available experimental data.
Influence of a DC Field on the Free Polaron Infrared Absorptio
Authors: Huybrechts W. J.
Published: July 2007

The optical absorption by polarons in weakly polar semiconductors is studied in the presence of an external dc field as function of temperature. The LO‐phonon field is treated by perturbation theory. Firstly, a comparison is made between the absorption curves obtained for different orientations of the dc field with respect to the polarization vector of the infrared radiation, and secondly, the temperature‐dependent approximation is compared with the well‐known Gurevich‐Lang‐Firsov perturbational result calculated for zero external electric field.
Exchange Energy of a Strongly Inhomogeneous Electron Gas
Authors: El‐Mamoun E. , Ropke G.
Published: July 2007

The problem of exchange energy of a strong inhomogeneous electron gas is treated within a special model. An explicite expression for the density of exchange energy is given in dependence on the electron density as well as the gradient of the electron density, where the electron density and its gradient independently can take any arbitrary value. As a result, the relative difference between this expression and the local density approximation for the exchange energy strongly depend on the fractional change in the electron density over the local Fermi wavelength. The deformation of the Fermi‐hole in a strong inhomogeneous electron gas is considered.
Theory of Absorption and Dispersion Profiles of Molecular Transitions in Dense Media
Authors: Paeeschke H. , Süsse K.‐E. , Welsch D.‐G. , Wilhelmi B.
Published: July 2007

Starting from quantum theoretical principles the phase relaxation is described of a molecule localized in a dense medium by an integro‐differential equation. Using this equation, which leads to a non‐exponential phase decay, general formulas are derived for calculating the absorption and dispersion profiles of vibronic and vibrational transitions of the guest molecule. In detail the calculations are performed for the case that the acoustic phonons of the medium are responsible for the phase relaxation. Especially it is shown, that in comparison with the Lorentz profile the wings of a real absorption profile become steeper with increasing ratio between dephasing rate y and Debye frequency wD‐ Only for γ/wD ≪ 1 the absorption profiles are expected to be Lorentz profiles. The dispersion is found to increase with increasing ratio γ/wD. Both the real absorption and dispersion curves are asymmetric ones.
Theory of Surf‐Riding Electrons on the Wake Potential of Ion Beams in a Metal
Authors: Suzuki K. , Ohtsuki Y. H. , Kitagawa M.
Published: July 2007

The surface plasmon excitation and the quantum‐mechanical effect are taken into account in the surf‐riding electrons on the wake with ion beams in the metal. It is concluded that the surface plasmon excitations play an important role when the glancing angle of the incident ion beams to the crystal surface becomes smaller. The binding energy levels and the angular distribution of the surf‐riding electrons are calculated in detail.
Formation Energy of Spherical Cavities in Metals
Authors: Gyémánt I. , Solt G.
Published: July 2007

The formation energy of a spherical cavity in a metal is calculated by means of the density functional formalism, with gradient expansion for the kinetic energy. The method adds to earlier work by studying the energy as a function of the cavity radius S at a given density of the metal. Further, combining the statistical formalism with a pseudopotential description of an embedded ion, the heat of solving one (like or unlike) atom in the metal is also obtained. Despite the crude simplifications in the method, i.e. the statistical treatment of the density functional problem and the inherent limitations of the jellium model on the one hand, and the low flexibility of the simple trial density function on the other hand, the cavity formation energies and the trends in the energies of solution are realistic in the majority of cases.
Effect of Pressure on the Spectrum of Praseodymium‐Doped Single Crystals
Authors: Voloshin V. A. , Krimnus M. G.
Published: July 2007

The absorption spectrum of praseodymium‐doped single crystals at 78 K and P = 22 kbar is analyzed. It is found that the shift of the LSJ levels under pressure is due to the change of the electrostatic interaction of 4f electrons.
A Pseudopotential Approach to the Electronic Theory of Short‐Range Order
Authors: Khvaya F. A. , Katsnelson A. A. , Silonov V. M. , Khrushchov M. M.
Published: July 2007

Expressions for the local ordering energy and short‐range order parameter of the substitional solid solution with components having different valencies are obtained by using the pseudopotential method in second order perturbation theory. The calculations give a satisfactory agreement with experiment for the Al‐Zn system.
Adsorption of oxygen at room temperature on polycrystalline titanium studied by ultraviolet and X‐ray photoelectron spectroscopy
Authors: Hagsteöm A. L. , Platau A. , Kaelsson S.‐E.
Published: July 2007

Titanium films, freshly evaporated and exposed to varying amounts of oxygen, are studied by ultraviolet and X‐ray photoelectron spectroscopy (UPS and ESCA), using HeI, HeII and AlKα1,2 radiation. The valence band results are compared with previous experimental results and with reported densities of states. Contrary to previous results, no pronounced decrease in the emission from Ti 3d states is observed in the 21.2 eV spectrum at exposures of ≦ 3 × 10−4 Torr s. Information about the reaction kinetics is extracted from ESCA measurements. The intensity of the O Is photoelectron line is found to increase with exposure according to a logarithmic relation below about 10−6 Torr s and to saturate above about 10−4 Torr s. In the saturated region recordings of the Ti 2p doublet revealed presence of a TiO2 layer with a thickness of one to two monolayers.

Titanschichten, die unmittelbar nach dem Aufdampfen unterschiedlicher Sauerstoffexponierung ausgesetzt waren, werden mit Hilfe von Ultraviolett‐ und Röntgen‐Photoelektronenspektroskopie (UPS und ESCA) untersucht. Dabei wird HeI, HeII‐ und AlKα1,2‐Strahlung angewendet. Die Ergebnisse, die für das Valenzband erhalten werden, werden mit früheren experimentellen Ergebnissen und mit Zustandsdichten aus der Literafcur verglichen. Im Gegensatz zu früheren Ergebnissen wird keine deutliche Abnahme der Emission aus Ti 3d‐Zustanden im 21,2 eV‐Spek‐trum bei Exponierungen ≦ 3 × 10−4 Torr s beobachtet. Aus den ESCA‐Messungen können Informationen über die Reaktionskinetik gewonnen werden. Die Intensität der O Is‐Photoelektronen‐Linie steigt bei Exponierung unter ca. 10−5 Torr s gemäß einer logarithmischen Beziehung und geht oberhalb von ca. 10−4 Torr s in eine Sättigung über. Im Sättigungsgebiet zeigt das Auftreten des Ti 2p‐Dubletts, daß eine TiO2‐Schicht mit einer Dicke von ein bis zwei Monoschichten vor‐handen ist.
Temperature and hydrostatic pressure dependence of vibrational modes in PbTi1–xZrxO3
Authors: Bäuerle D. , Holzapfel W. B. , Pinczuk A. , Yacoby Y.
Published: July 2007

The room temperature Raman spectra of PbTi0.1Zr0.9O3 at hydrostatic pressures up to 68.5 kbar indicate three phase transitions. At 5.71 kbar a new phase appears, most probably the rhombo‐hedral high‐temperature phase, FR(HT) which is in coexistence with the rhombohedral low‐temperature phase FR(HT). Between 8 and 9.1 kbar the relative intensities of all lines change and two additional structures appear. It is likely that this change is due to a transition into the orthorhombic antiferroelectric phase. A further transition between 39.7 and 42 kbar cannot be positively interpreted on the basis of the present data. The temperature dependence of the Raman spectra gives evidence that the additional structure in the FR(LT) phase is due to the doubling of the unit cell, i.e. PbTi1–xZrxO3 seems to show one‐mode behaviour.
On the concept of cyclotron resonance in metals
Authors: Saermark K. , Lebech J. , Johansen H.
Published: July 2007

Experimental evidence is presented, indicating that an impinging, high frequency electromagnetic wave penetrates to a considerable depth beneath the surface of a monocrystalline, metallic sample. It is shown, on the basis of a rigorous dispersion equation for cyclotron waves, that the attenuation length for these waves is γi = vgτ where vg is the group velocity and τ the relaxation time. The attenuation length varies in a characteristic way with the magnetic field and may become as large as the mean free path of the charge carriers. A possible relation to Azbel‐Kaner cyclotron resonance is pointed out.
Theory of resonance Raman scattering in disordered solids
Authors: Bechstedt F. , Enderlein R.
Published: July 2007

A theory of the resonance Raman scattering from optical phonons is developed for disordered solids which can be described by the smooth random field (SRF)model. The disorder influences only the electronic subsystem but not the vibronic one. Despite the Q = 0 selection rule is broken, practically no change of the scattering spectrum due to disorder results within the SRF‐ model. The dispersion of the scattering cross‐section is calculated for the two cases of randomly fluctuating energy gap and of random electric field. The scattering cross‐section of the disordered system can be related to that of a crystal by means of an integral representation similar to the Redfield representation of the dielectric function.
Heterofission in pentacene‐doped tetracene single crystals
Authors: Burgos J. , Pope M. , Swenberg Ch. E. , Alfano R. R.
Published: July 2007

The magnetic field and the temperature dependence of both guest and host fluorescence in penta‐eene‐doped tetracene single crystals is studied. From an analysis of the experimental results it is concluded that pentacene enters substitutionally into the host lattice sites and that thermally activated heterofission is site‐dependent with activation energies ≈︁ (0.13 ± 0.01) e V and ≦ 0.06 eV. From a kinetic fit to the temperature dependence of the green and red fluorescence, a thermally activated host singlet exciton diffusion coefficient ≈︁ 2 × 10−2 cm2/s is inferred; its activation energy is estimated to be (0.018 ± 0.001) eV. The heterofission rate of ≈︁(4.7 ± 0.5) × l0−10 cm3 s−1 is determined and location of the pentacene triplet level is found to be at (0.86 ± 0.03) eV.
Electronic structure investigation of metallic vanadium by XS and XPS
Authors: Werfel F. , Dräger G. , Brümmer O.
Published: July 2007

The valence band of vanadium is investigated using X‐ray and X‐ray photoelectron spectro‐scopy. By XPS core level measurements the several spectra are correlated and drawn in unitary energy scale for comparison purpose. The spectra are compared with density‐of‐states functions from the literature and discussed in terms of electron states of different symmetry (s‐, p‐, d‐like). From the experimental spectra the widths of the occupied d‐band and s‐d band are estimated to 4.0 and 6.8 eV, respectively. The experimental results confirm the general accepted picture of the electron structure of 3d metals.
Specific features of many‐photon photoemission from metals and semiconductors in strong fields
Authors: Brodskii A. M.
Published: July 2007

Equations are derived describing photoemission from metals and semiconductors in strong fields. A perturbation theory is advanced which, is valid even in the presence of resonance transitions. Threshold and resonance expressions for photocurrent are obtained. New strong field effects, dependent on band structure and collective excitation spectrum, are considered. A number of experimentally observed phenomena are explained in terms of the theory developed.
The elementary interaction force between the flux‐line lattice and precipitates
Authors: Fähnle M.
Published: July 2007

The elementary interaction force p between a precipitate and the flux‐line lattice is calculated by means of the Ginzburg‐Landau theory. It is shown that p depends on the shape and the extensions of the precipitate in a complicated manner. This gives rise to a modification of the field dependence of the interaction force, which had not been taken into account so far. As a consequence it is impossible to describe the field dependence of the volume pinning force by a simple scaling law.
The energy–angle distribution of heavy particles penetrating solids
Authors: Meyer L. , Klein M. , Wedell R.
Published: July 2007

Particles of definite initial energy and direction penetrating into a solid have, in a given depth, a certain distribution with respect to energy loss and scattering angle. This energy‐angle distribution is calculated in small angle approximation for heavy low energy particles, taking scattering and nuclear stopping into account on the basis of a Thomas‐Fermi potential and electronic stopping with a definite dependence on scattering angle. Introducing redviced values of energy loss, scattering angle, and thickness of the penetrated layer, it is found, that it is possible to get a general solution independent of particle and target type and of energy. The result is given by a combination of two functions which are tabulated. The most probable energy losses of the total energy distribution and of the distribution of the particles scattered in forward direction as well as the half‐widths of these distributions are given. The results show, that stopping and scattering cannot be considered independently for heavy low energy particles. In particular the conception of ‘stopping cross section’ at low energies essentially looses its meaning.
Radiative electron capture by backscattered ions from solid surfaces
Authors: Koyama K. , Kaneko T.
Published: July 2007

Radiative electron capture by heavy ions scattered at grazing incidence on solid surfaces is investigated. Remarkable oscillations in cross‐sections of the process versus the distance of ions from the surface are found. This behaviour is ascribed to the correlation between the ion velocity and electronic states near the surface. The dependence of the cross‐sections on the incidence angle is calculated for the several primary energies. The possibility of the surface structure analysis by a measurement of the radiative electron capture is discussed.
The lattice dielectric constant in Bi100–xSbx as a function of x
Authors: Oelgart G. , Herrmann R. , Meschter U.
Published: July 2007

The dielectric constant of the lattice, ε1, is determined in semiconductive bismuth–antimony alloys (7 at% < x < 22 at%). For measuring ε1 the effect of the displacement current on the wave propagation in a direction parallel and perpendicular to the magnetic field is utilized. The measurements are carried out at helium temperatures and within a frequency range from 9 to 70 GHz. Thus the effect of the displacement current can be experimentally separated. The dielectric constant exhibits a strong dependence on x, which indicates a pole for the disappearing gap.
The phonon spectra of β‐CuI. II. The numerical evalutaion
Authors: Haxton M. B.
Published: July 2007

The phonon spectra of β‐CuI is numerically evaluated using the second neighbor ionic model of Banerjee and Varshni. The short‐range parameters are evaluated by correcting the parameters of the ideal wurtzite lattice, found by transformation of zinc blende parameters previously calculated, for the nonideal nature of β‐CuI. Dispersion curves are drawn in the [100], [110], and [001] directions and a crude estimate of the density of states is found.
Low‐temperature thermal conductivity of pure and impure niobium and tantalum single crystals
Authors: Wasserbäch W.
Published: July 2007

The thermal conductivities of niobium and tantalum single crystals of different impurity contents are measured between 0.3 and 20 K. Below about 1 K the thermal conductivities of all specimens vary with temperatures as T3, but the magnitude of the thermal conductivity decreases with increasing impurity content. A careful investigation of the defect structure of the specimens leads to the conclusion that interstitial and substitutional impurities give rise to a thermal resistance even at low temperatures.
The influence of fast neutron irradiation on the critical current densities in Nb3Sn I. Low neutron doses (Φ < 2 × 1018 neutrons/cm
Authors: Fähnle M.
Published: July 2007

By irradiation with fast neutrons the critical current densities Jc of Nb3Sn are modified in a characteristic manner depending on the irradiation dose. A short survey is given of the experimental work on this subject. By a model calculation it is shown that the modification of the critical current densities after low‐dose irradiation (Φ < 2 × 1018 neutrons/cm2) may be explained by a modification of the density of pinning centres and not by an influence on the intrinsic superconducting parameters, as assumed by other authors.
Electron spin–lattice relaxation measurements in a two‐dimensional crystal
Authors: Giordano M. , Martinelli M. , Pardi L. , Santucci S.
Published: July 2007

The spin relaxation times T1 and T2 are studied in a two‐dimensional magnetic system by varying the angle between the anisotropy axis and the external magnetic field. Measurements are done on the Mn2+ ions in a single crystal of (C2H5NH3)2MnCl4, using the new technique of longitudinal detection of electron spin resonance (LODESR). The angular dependence of T1 is explained by taking into account an isotropic interaction besides the exchange modulated dipolar interaction. The theoretical curves are in fair agreement with experiments.
Electrical conductivity of two‐band models for disordered binary substitutional alloys theory
Authors: Kudrnovský J. , Petrů J.
Published: July 2007

The static electrical conductivity for a two band S–D matrix generalization of the Shiba model is determined for a disordered binary substitutional alloy within the single‐site approximation. Unlikely the previous works, the conductivity is expressed in an analytic form allowing for an interpretation of the Boltzmann equation limit in terms of the scatterings in complex, hybridized bands.
The magnetic susceptibility of antimony and bismuth chalcogenides in the liquid state
Authors: Uemura O. , Satow T.
Published: July 2007

The magnetic susceptibilities of liquid SbTe, BiTe, SbSe, and BiSe systems are measured by the Faraday method using a torsion balance. The decrease in diamagnetic susceptibility at the melting point is relatively large in liquid Bi2Te3 and Sb2Te3. On the other hand, it is considerably small in liquid Sb2Se3. This corresponds to the temperature change in the electrical conductivity reported previously. The value of susceptibility for liquid SbTe alloys is considerably different from the data measured by a previous author. It is clarified from the composition dependence of susceptibility that both liquid Te systems behave like metals rather than semiconductors in a wide range of composition. The composition dependence of susceptibilities in the liquid Se systems showes that in the Se‐rich region the short‐range bonding remains unchanged when melting, and these alloys take rapidly a metallic character with increasing metal content beyond the composition of Sb2Se3 or Bi2Se3. These experimental results are discussed using the bond orbital approximation.
Reaction yield detected magnetic resonance spectra of intermediate pairs of triplet excitions evolution of spectra during degeneracy of energy levels
Authors: Lesin V. I. , Sakun V. P. , Pristupa A. I. , Frankevich E. L.
Published: July 2007

The changes in reaction yield for the reaction of equivalent triplet excition annihilation under the action of a resonant microwave power are studied both theoretically and experimentally. The resonant effect is shown to vanish when energy levels of the intermediate double–triplet complex are degenerated.
Auger lifetimes for excitons bound to neutral donors and acceptors in Si
Authors: Schmid W.
Published: July 2007

The lifetimes of excitons bound to the neutral donors Li, P, and As and neutral acceptors B, Al, Ga, and In in silicon are determined by time‐resolved photoluminescence measurements at 4.2 K and a strong dependence on the binding energy of the donor or acceptor is found. This dependence can be understood assuming a localized phononless Auger‐process to be the main recombination mechanism. A theoretical calculation using approximate wavefunctions is presented and leads to a very good agreement with the experimental results.
Evidence of coulomb interaction at the e1 interband transition of hexagonal CdSe
Authors: Lange H. , Iliev I. A.
Published: July 2007

Transmission and reflection spectra are measured on CdSe single crystals in the region 3.6 to 5.9 eV at 109 K and RT and the absorption coefficient, refractive index, and real and imaginary part of the DF are calculated from these data. The line shape of ϵ2 around the E1(B) peak indicates a strong influence of electron–hole interaction. A comparison with Kane's theory gives a binding energy of the two‐dimensional exciton of 0.23 eV. In thermally modulated transmission spectra a new structure is observed which is attributed to the M1 edge of the E1(A2) exciton yielding a binding energy of the same magnitude.
Dispersion relation of phonon‐like excitations in a simple model of a noncrystalline solid
Authors: Leihkauf R. , Böttger H.
Published: July 2007

The dispersion relation of phonon‐like excitations in a solid with structural disorder is calculated using a simple model system. The dispersion curve for the disordered system shows a disorder induced roton‐like minimum. The frequency gap connected with this minimum increases with increasing disorder. In the frequency spectrum of the disordered system are also frequencies above the maximum frequency of the corresponding crystalline system.
On the density of states at the fermi level for normal systems
Authors: Behn U. , Zinke H.
Published: July 2007

In the single‐site and the nearest‐neighbour approximation, respectively, the density of electronic states at the Fermi level for normal systems at T = 0 is considered as a functional of the unperturbed density of states. For electron–hole symmetric systems, e.g. the half‐filled‐band Hubbard model, the density of states at the Fermi level in a normal single‐site approximation is either equal to the unperturbed value or vanishes. The expected gradual change in the density of states can be obtained in the nearest‐neighbour approximation.
The multimode Jahn‐Teller coupling for an orbital triplet in cubic crystals
Authors: Biernacki S. W.
Published: July 2007

An investigation is made of the properties of an iron group ion which has an orbital triplet ground state and which interacts with the lattice vibrations of its nearest neighbours. The interaction with e‐type vibrations is considered only. For the phonon spectrum of the host crystal the Debye and Einstein approximations are used. Explicit formulae are obtained for the Jahn‐Teller energy, the displacements of the neighbours nearest to an impurity, and for the first‐order reduction factor of orbital angular momentum in terms of the phonon spectrum parameters. All cubic coordinations are considered: fourfold, sixfold, and eightfold.
Indirect exchange in europium chalcogenides
Authors: Mauger A.
Published: July 2007

The indirect exchange in heavily doped europium chalcogenides is calculated in the mean field approximation. The finite width of the conduction band and carrier statistics (splitting of spin subbands, non‐thermal degeneracy) are taken into account. The model is applied to Gd‐doped EuO and EuS for which magnetic properties are derived. The predictions agree with experimental data both in the para‐ and ferromagnetic configurations. Statistical effects lead to a renormalization of the paramagnetic Curie temperature. Its variations with electron concentration are in good agreement with experiments. Moreover, the model accounts for some particular properties, such as a Curie constant a few per cent below the theoretical one and the systematic deviation of the magnetization curve with respect to the classical Brillouin law in the ferromagnetic configuration. The stability of the ferromagnetic order is also discussed.
Response of metal electrons to local fields of holes arising in X‐ray processes
Authors: Grebennikov V. I. , Sokolov O. B. , Turov E. A.
Published: July 2007

X‐ray spectra computation methods are developed enabling allowance for metal valence band readjustment (extra‐atomic relaxation) caused by the field of a suddenly appearing finite‐lifetime hole. Allowance for the finite lifetime of the hole necessitates the introduction of a non‐hermitian perturbation Hamiltonian into the theory and, as a consequence, modification of Keldysh's technique of calculating nonequilibrium processes. The technique developed is illustrated with reference to an example of calculating the variation in time of the number of valence electrons on the site after the local field has switched on suddenly. Studying the asymptotic behaviour of that quantity allows one to determine the intensity of the satellite structure of inner‐level X‐ray photoelectron spectra.
A study of the kinetics of recombination radiation of CdS and CdSe crystals
Authors: Boiko G. A. , Dneprovskii V. S. , Kraevskii M. V. , Marinova K. , Oak S. M. , Silina E. K. , Fokin V. S.
Published: July 2007

The characteristic decay times of recombination radiation (P‐band) in CdS (2 × 10−11 to 2.7 × 10−9 s) and CdSe (1.7 × 10−10 to 3.7 × 10−9 s) monocrystals under different exitation levels, obtained by powerful picosecond light pulses, are measured with the help of a special laser spectrochronograph with a time resolution of 2 × 10−11 s (sample temperature 77 K). A comparison of the curves obtained for the rise and decay of the intensity of luminescence with the solutions of the system of rate equations for the exciton—exciton scattering processes makes it possible to estimate the values of rate constants of exciton—exciton interaction and of binding of carriers to excitons.
On the thermal broadening of core levels in metals due to many‐electron effects
Authors: Almbladh C.‐O. , Minnhagen P.
Published: July 2007

Ferrell's expression for the XPS lineshape is derived in a formalism which explicitly takes the electron—electron interaction into account. Also the corresponding expression for X‐ray absorption is given. In the presence of other broadenings, which are comparable or larger than kT, the electronic part of the core‐level broadening in XPS is indistinguishable from a Gaussian. This Gaussian broadening is of the same order as the expected phonon broadening for Al at room temperature and above, but it is of little importance for Li and Na. In X‐ray absorption, the effect seems in some cases to be important when extracting phonon broadenings from experimental data.
Transient photoconduction in a polydiacetylene single crystal determination of transit time, free lifetime, and recombination time of charge carriers
Authors: Reimer B. , Bässler H.
Published: July 2007

Photocurrent transients in polydiacetylene‐bis(toluenesulfonate) single crystals are investigated under 1060, 694, and 347 nm laser excitation and different sample geometry. With thick samples pulse decay is controlled by electron—hole recombination, the recombination coefficient being γ = (4 ± 1) X 10−6 cm3 s−1. Under 1060 nm irradiation silver‐contacted sandwich samples, typically 100 μm thick, show transit signals originating from injected carriers. The mobility is (4.8 ± 1.5) cm2/Vs. It varies as T0.5 as expected for carrier transport in a one‐dimensional band if scattering is by acoustic phonons. The carrier free lifetime depends on crystal preparation. For sandwich samples furnished by polishing monomer crystals it can be as low as 35 ns at 298 K.
Rayleigh wing scattering in disordered sodium nitrate
Authors: Neumann G. , Vogt H.
Published: July 2007

The temperature dependence of the Raman spectrum of NaNO3 is measured with special emphasis on those scattering configurations neglected in previous works. A Rayleigh wing of exponential lineshape is found arising from the increasing mobility of the NO3− ions in approaching the order—disorder phase transition and the nearby melting point.
Temperature dependences of the nonradiative multiphonon carrier capture and ejection properties of deep traps in semiconductors. I. Theoretical results
Authors: Pässler R.
Published: July 2007

Proceeding from the recently developed semiempirical theory of Shockley‐Read processes a short survey of theoretical predictions is given on the temperature dependences of the nonradiative multiphonon carrier capture ejection properties of deep traps in semiconductors. At low temperatures the corresponding capture cross‐sections are but weakly dependent on temperature and vary in the cases of attraction, neutrality, and repulsion as T−1, T−1/2, and T−7/6× × exp {−3[θ/T]1/3}, respectively, where the parameter θ depends on the magnitude of the centre charge. At high temperatures, activation‐type dependences exp {− U/kT} play an important part where the activation energies U depend on the actual structure of the given traps. On the basis of the complete theory the temperature dependences of the capture cross‐sections and ejection frequency factors in the range of intermediate temperatures are visualized graphically.
Deformation effects on exciton excitation spectra in CdS
Authors: Goede O. , Krug H.‐J. , Horst A.
Published: July 2007

The deformation effects for free excitons in CdS were studied by measuring the excitation spectra of the edge emission at 77 K for uniaxial stress parallel or perpendicular to the c‐axis. The stress‐induced shift of the A and B exciton peaks is in good agreement with the corresponding results of previous piezoreflection and piezoemission experiments. Moreover, the C exciton shift could be measured which enables a more precise determination of the deformation potential constants. For the first time a piezospectroscopy of excited states of free excitons could be carried out showing the exciton binding energy to be stress‐independent. The observed stress dependence of the excitation probability ratios for various excitons can be attributed to the stress‐controlled thermal exciton dissociation.
Electronic structure of dilute α‐phase AgCd alloys from thermoreflectance and XPS measurements
Authors: Rosei R. , Colavita E. , Modesti S. , Nisticò N.
Published: July 2007

The position of the s, p, and d‐like levels (at L) in four different silver—cadmium dilute α‐phase alloys is determined with respect to the Fermi level, using thermoreflectance and XPS measurements. The use of an analytical model, previously developed, to fit the thermoreflectance data, allows to determine, with substantially increased accuracy over previous alloy work, the dependence of the relevant optical gaps versus solute concentration. Of particular relevance is the behaviour of the L3L′2(EF) gap which shows at first a slight decrease versus solute concentration. XPS measurements on the same samples are consistent with this behaviour which is the opposite of what is predicted by the rigid band model.
Bound states of electrons and optical phonons under conditions of one‐dimensional quantizing
Authors: Sardaryan V. S. , Arakelyan M. M.
Published: July 2007

It is shown that there exist bound states of electrons and optical phonons in a film stipulated by the electron‐phonon interaction, quadratic in the displacement of the lattice. The full solution of the problem for the model Hamiltonian, in which the interaction is determined only by the momentum transfer is obtained. The existence of the bound states is also proved for a more realistic description of the polarization electron‐phonon interaction.