bluuebunny/crossref_metadata_2025 huggingface dataset:
Metallic systems containing CEF split impurities
Authors: Ahluwalia P. K. , Sharma K. C.
Published: July 2007

The thermoelectric power Q of dilute magnetic alloys in the presence of both, crystal electric field (CEF) and the external magnetic field H, is investigated for the case of metals containing magnetic impurities with crystal field split energy levels. It is found that the so called ‘anomalous’ thermoelectric power gets modified by H such that the maximum of Q gets shifted towards the lower side, i.e. Tm < 0.3δ, increasing its height for a positive aspherical Coulomb potential. Various limiting cases also suggest the change in the behaviour of Q with temperature and H.
The influence of anharmonicity on the electrical resistivity in TTF‐TCNQ
Authors: Schiller W. , Chilla E. , Aksenov V.
Published: July 2007

Using the method of force‐correlation functions the electrical resistivity is calculated for a system consisting of linear chains with quadratic electron‐phonon interaction above the Peierls‐transition temperature. Calculating the resistivity the theory of anharmonic crystals is applied to evaluate the phonon‐correlation functions. The temperature dependence of the electrical resistivity at constant pressure and at constant volume is calculated. The results are compared with experimenta data for TTF‐TCNQ.
Fluorinated Hydrogenated Amorphous Silicon Alloys. Vibrational Excitations of Chains
Authors: Ghosh B. K. , Agrawal Bal K.
Published: July 2007

The vibrational excitations of the various fluorine hydrogen complexes, and the polysilane‐like conformations in amorphous silicon are studied after employing a valence force field mode1 (VFFM) in a cluster Bethe lattice formulation. The calculated high frequencies are seen to be in excellent agreement with the infrared and Raman data. Some calculated peaks appearing at 214 and near 600 cm−1 have not been detected so far. More experimental measurements on a‐Si : F : H alloys need to be performed especially in the low‐frequency region ≈600 cm−1 to detect the predicted structure.
Electron‐Spin‐Resonance Study of Pb+(l) Centers of the Laser‐Active Structure in KCl and RbCl2)
Authors: Heyndebickx I. , Goovaerts E. , Nistor S. V. , Schoemaker D.
Published: July 2007

Several Pb+ defects possessing the so‐called laser‐active Pb+(l)‐type structure are examined by ESR in X‐irradiated KCl:PbCl2 and RbCl:PbCl2 crystals. The essential core of these centers consists of a substitutional Pb+ ion associated with a nearest neighbor anion vacancy along a 〈100〉 direction. Small perturbations in the vicinity of this core in the form of a charge compensating cation vacancy or a neighboring Pb2+‐cation vacancy complex generate a large number of Pb+(l)‐type defects, which are similar in KCl and RbCl. Different for both hosts, however, is the influence of oxygen on the production of some centers. In the RbCl crystals the presence of the oxygen ions modifies the structure of one of the defects and induces the production of two supplementary centers, while no effect of the O2− ions is observed on the Pb+(1)‐type defects in KCl crystals.
Formation and Migration Energies of an Interstitial Atom in Si and Ge
Authors: Soma T. , Matsuoka T. , Kagaya H.‐Matsuo
Published: July 2007

The formation and migration energies of an interstitial atom in Si and Ge are investigated using the perturbational formalism and the local Heine‐Abarenkov model potential presented earlier. The interstitial atom is most stable at the tetrahedral site for Si and at the hexagonal site for Ge. The calculated migration energy of the interstitial atom is about 0.4 eV for Si and Ge.
Magnetic Order of Singlet‐Singlet Ions Coupled to Conduction Electrons. Band Structure Effects
Authors: Palermo L. , da Silva X. A.
Published: July 2007

The onset of magnetic order of a system consisting of conduction electrons interacting with localized f‐electrons having only two singlet levels is analysed. The effects of band structure, crystal field and exchange parameters are deduced from a parametric study of the model. It is suggested that the combined role of these effects particularly the shape of energy density of states and number of electrons per band, may be of importance in explaining drastic differences in the magnetic properties of light rare‐earth intermetallics.
Frequency Dependent Susceptibility of Rare Earth Ternary Superconductors
Authors: Kakani S. L. , Upadhyaya U. N.
Published: July 2007

A theoretical study of the frequency dependent susceptibility of ternary superconductors is presented using the microscopic theory developed earlier by the authors. The theoretical results obtained are in good agreement with available experimental observations.
Variable Valence of Ce and Pr in Liquid Alloys with Fe, Co, and Ni
Authors: Geld P. V. , Singer V. V. , Radovskii I. Z. , Konovalov S. V.
Published: July 2007

The paramagnetic susceptibility and the electrical resistivity of liquid alloys Ce, Pr‐Fe, Co, Ni are measured and compared with calculations based on Edwards expressions and on the Ziman theory in t‐matrix approximation. In rare‐earth‐rich alloys the valence of Ce and Pr is found to increase monotonously with increasing concentration of the 3d metal. Above a critical concentration the valence remains constant. In this concentration range the 4f electrons of Ce are completely delocalized while Pr is in an intermediate valence state.
On a Certain Model Leading to a Fractal Dimension in the Schrödinger Problem
Authors: Bonch‐Bruevich V. L.
Published: July 2007

The types of solutions are studied of the non‐linear one‐dimensional problem of an electron interacting with acoustic phonons. Conditions are established under which the set of solutions is a fractal.
Induced Magnetic Form Factors for 3d Transition Metals and 3d‐3d Alloys. II. Induced Spin Form Factors
Authors: Pabst J. , Röll H.
Published: July 2007

Induced spin scattering of neutrons on transition metals and transition‐metal alloys is discussed. For a quantitative description of structured stoichiometric alloys, a conception based on atom form factors and spin susceptibilities is proposed. It is shown that spin susceptibilities and form factors of some alloys differ considerably from those of separate constituent atoms. As far as alloys of the chromium group (isoelectronic to chromium) are concerned, this is due to marked destruction of d‐states at the Fermi level. This leads to a reduction of susceptibilties, a rise of sp portions that oscillate more strongly in the form factors and a reduction of enhancement factors. Spin scattering is diminished by one order of magnitude in the alloys. Spin form factors are indicated by their Fourier transforms for all transition metals and alloys calculated.
The Influence of Quantization on the Space‐Charge Layer Capacitance of Si in Strong Accumulation
Authors: Frantsuzov A. A. , Okhonin S. A. , Pogosov A. G.
Published: July 2007

A numerical calculation is made of the space‐charge layer characteristics in terms of quantization of carriers. The results are used in processing the experimental capacitance‐voltage characteristics, measured on the MOS‐structures with different oxide thickness. It is shown, that quantization noticeably decreases the space‐charge layer capacitance in strong accumulation.
Zur Theorie des durch das äußere Magnetfeld induzierten Jahn‐Teller‐Effektes in orthorhombischen J = 1‐Paramagneten
Authors: Ulner J.
Published: July 2007

Es wird der durch das äußere Magnetfeld induzierte Jahn‐Teller‐Effekt in einem System von paramagnetischen Ionen mit orthorhombischer Symmetrie und dem Gesamtdrehimpuls J = 1 untersucht. Es wird vorausgesetzt, daß die magnetoelastischen Eigenschaften des Systems durch die Wechselwirkung der lokalisierten 4f‐Elektronen mit dem elektrostatischen Kristallfeld bestimmt werden. Mit Hilfe einer Greenfunktionsmethode, die die Eigenschaften des Drehimpulses exakt berücksichtigt, werden die magnetoelastischen Anregungen für bestimmte Richtungen des äußeren Magnetfeldes ermittelt. Es wird die Temperatur‐ und Feldabhängigkeit der Schallgeschwindigkeiten, die mit den elastischen Konstanten c44, c55 und c66 korrespondieren, bestimmt und die sich ergebenden Phasenumwandlungen diskutiert. Darüber hinaus wird die Wechselwirkung der einzelnen elastischen Wellen mit dem magnetischen Untersystem berechnet.
Spin Relaxation in the Presence of Coherent Averaging Pulse Sequences
Authors: Ursu I. , Balibanu F. , Demco D. E. , Bogdan M.
Published: July 2007

Theoretical calculations of the effective transversal relaxation time for homo‐ and heteronuclea dipole‐dipole interaction in the presence of the MW, WHH, and MREV sequences are performed in the theoretical frame of Gründer, Schmiedel, and Freude. The relaxation times are calculated as functions of the correlation time of the dipole‐dipole interactions, the time period, and the pulse length. The possibility of selective cancellation of the dipolar relaxation mechanisms by using multiple pulse sequences is discussed.
Exchange Coupling Model in Neutron‐Irradiated Graphite
Authors: Kazumata Y. , Nakano Y. , Yugo S. , Kimura T.
Published: July 2007

The temperature dependence of the ESR spectrum, in pyrolytic graphite irradiated by neutrons is measured from 4 to 300 K. The experimental result is qualitatively explained by the exchange coupling model proposed by Mrozowski. The validity of the model definitely confirms the presence of localized spins in neutron‐irradiated graphite.
Method of Generalized Bethe Lattices in Chemisorption Theory
Authors: Karpushin A. A. , Sorokin A. N. , Semenova I. Yu. , Tomášek M.
Published: July 2007

The method of cluster Bethe lattices is generalized and this way of treating surface electronic structure of clean surfaces and surfaces covered by the chemisorbate is further improved. The idea consists in better respecting the coordination of bonds in the crystal. Silicon (111) surface and disordered hydrogen chemisorption on it are used as examples to illustrate the proposed method.
Coexistence of Superconductivity and Sinusoidal (Helical) Magnetic Order
Authors: Schmeltzer D.
Published: July 2007

The free energy of a superconductor in the presence of a sinusoidal (helical) magnetic order is considered. The method of projection is used to diagonalyze the electronic Hamiltonian. The wavelength 2π/Q of the sinusoidal (helical) phase of amplitude M is computed self‐consistently. A reduction of the cooper pair, M2/Q, is found which stabilizes the sinusoidal phase.
Metal‐Insulator Transition in Titanium Disulphide Crystals
Authors: Das B. , Bartwal K. S. , Srivastava O. N.
Published: July 2007

A temperature induced metal (M) → insulator (I) transition is investigated which is possibly first of its kind in the titanium rich titanium disulphide crystals confirming to the composition Ti1.17S2. This transition is established by monitoring the temperature dependence of the electrical conductivity and the thermoelectric power. The structural characteristics at room temperature and at high temperatures (≈ 573 K) also are explored through electron diffraction. The M → I transition is believed tooccur due to the increase of disorder which occurs at high temperature where the short range order state of excess Ti atoms in the van der Waals gapgives rise to a highly disordered amorphous like state. This may lead toshifting of the mobility edge from below the Fermi energy to above the Fermienergy, thus leading to the observed M → I transition.
The Effect of Phonon‐Phonon Scattering in the Substrate on Temperature Dynamics of a Phonon Film Injector
Authors: Kazakovtsev D. V. , Levinson Y. B.
Published: July 2007

It is shown how the mode of cooling of the metal film injector of phonons can be determined from the heating pulse energy and duration. It can be of a ballistic, diffusion, or heat conduction character. The film temperature and its cooling time are estimated.
Polarizabilities, Energies, and Pressure Derivatives of Bulk Moduli of Some Rutile‐Type Oxide Crystals
Authors: Islam A. K. M. A. , Islam K. , Islam M. N. , Mortuza G.
Published: July 2007

An analysis of the dipole polarizabilities, cohesive energies, and pressure derivatives of bulk moduli of some rutile‐type oxide crystals is presented. Two interionic potentials, within the framework of the ionic model, are considered and the results obtained are compared with available experimental data and other recent theoretical calculations. A discussion of the controversy over the bonding character of these compounds is included. The suitability of a new interionic potential due to Islam as applied to these systems is also shown.
Invariant Harmonic Oscillator Functions. II. An Alternative Method for the Groups Td, 0, and Oh
Authors: Wöhlecke M. , Jovanović A.
Published: July 2007

Tunneling states of off‐center impurities and Jahn‐Teller systems are examples of an application of excited states of quantum‐mechanical three‐dimensional harmonic oscillators. A method is developed, which allows the calculation of symmetrized bases with minor computing expense. Results are given for the cubic groups O, Td, and Oh.
On the Theory of Atomic Migration in Monatomic Crystals
Authors: Buchbinder G. L.
Published: July 2007

The frequency of the overbarrier atomic jumps into a neighbour vacant site is calculated at high temperatures. The atom adjacent to a vacancy is in a two‐well potential constructed by its neighbours. The jump into a vacant site is considered as a transition of the atom from some quantum mechanical state of this potential into another one caused by interaction with its neighbours. It is assumed that the necessary condition for a jump is the transition of the atom from some state, localized in a lattice point, to a overbarrier state. Being at a overbarrier state the atom vibrates in a region whose size is compared with the lattice constant.
Spin Wave Resonance Approach to Interdiffusion in Coupled Magnetic Layers. Theoretical Study
Authors: Maciejewski W.
Published: July 2007

A nondestructive method for the investigation of interdiffusion process, based on spin wave resonance measurements is proposed. For the diffusion by the vacancy mechanism a procedure for determination of the approximate concentration profiles is introduced. The modified virtual crystal approximation and the exact dynamical boundary conditions are used to obtain the spin wave energies, amplitudes, and resonance line intensities. The numerical calculations show the characteristic influence of the relative movement of the two species on the shape of spin wave resonance spectra.
Surface Magnetic Properties of the Ising Ferromagnet with a Site‐Bond Diluted Anisotropie Free Surface
Authors: Yang C. Z. , Li Z. Y.
Published: July 2007

A semi‐infinite ferromagnetic simple cubic Ising lattice which has non‐magnetic impurities substituted only at the surface with anisotropic exchange interactions is investigated by the use of a new type of effective‐field theory with correlation. Some characteristic behaviours of magnetism at the site‐bond diluted free surface are obtained.
Optical Properties of Multi‐Layer Structures. II. Reflection and Transmission
Authors: Fomin V. M. , Pokatilov E. P.
Published: July 2007

Optical properties of multi‐layer structures with, an arbitrary number of layers are considered on the basis of the polariton theory. The reflectivity, transmissivity, and absorptivity of periodic structures are found in terms of elementary functions. The optical characteristics are calculated for superlattices GaAs/Ga1−xAlxAs with different number of periods.
Simple Description of Hydrogen‐Like Systems in Arbitrary Magnetic Fields
Authors: Chen Y. , Gil B.
Published: July 2007

A simple description is given of an hydrogen‐like system in an arbitrary magnetic field. A new expansion of the wave function is used in terms of hydrogenic states factorized by a simple exponential factor characteristic of Landau levels for free electrons. By using parabolic coordinates, such a choice for the basic function allows an analytical calculation for both the ground state and the excited states of the system under large range of field strength.
Comparison of Optical Activity and Faraday Rotation in Crystalline Si02
Authors: van den Keybus P. , Grevendonk W.
Published: July 2007

Optical activity (OA) and Faraday rotation (FR) in SiO2 are compared from both a theoretical and an experimental points of view. A simultaneous derivation of the one‐oscillator formulas of FR and OA is presented. For OA this formula is extended to a semi‐classical expression, in analogy with earlier derivations of FR formulae. Starting from dispersion relations for crystals without inversion symmetry, it is possible to derive the OA formula, obtained earlier by Natori with the Kubo‐Greenwood formalism. Within the limits of accuracy and in the wavelength range 300 to 700 nm the experimental curves of OA and FR differ by a numerical factor only. Nevertheless the gap energies, obtained by fitting these curves with OA and FR formulas are different. The energy E = 9 eV is believed to correspond to the real direct band gap.
Influence of Nonparabolicity on the Superlattice Band Structure
Authors: Milanović V.
Published: July 2007

The energy spectrum is analysed of the superlattice taking into account the nonparabolicity effect in the host materials. Starting from the Kane theory the dispersion relation is derived for the symmetric superlattice. Moreover the expression for the concentration is given. The expressions derived are general and can be applied even to cases when a potential dependence exists. An extended effective mass is introduced depending both, on the energy and the energy gap as well as on the potential. The numerical results are given for the doping superlattice, assuming a triangular dependence of the potential. It is shown that the model, neglecting the nonparabolicity effects, yields good results for smaller energies and smaller values of the transverse component of the wave vector; in the opposite case, the nonparabolicity effects must be taken into consideration.
A Dynamical Theory for a Disordered Hubbard Chain far from Equilibrium. III. The High‐Field Case
Authors: Heiner E.
Published: July 2007

For the high‐field limit of a ID Hubbard chain (eF0a/u) ≫ 1 the expectation values for single and double occupancy are calculated. The equation for double occupancy does not contain a diffusion term. In contrast to the low‐field case the current density is not splitted in two terms.
Cluster Embedding in an Effective Shell and Calculation of the Density of States for Amorphous Copper
Authors: Blaudeck P.
Published: July 2007

13‐atom clusters of amorphous copper are embedded in an effective shell of second neighbours. It is shown that the resulting cluster density of states approximates the density of states of amorphous copper better than the embedding in an effective medium.
A Theoretical Study of the Semiinsulating Behaviour of Fe in InP
Authors: Khowash P. K. , Singh Vijay A. , Khan D. C.
Published: July 2007

The transition metal impurities introduce levels deep in the band gap of the host semiconductor. For neutral Fe in InP the impurity level is exactly in the mid gap and this provides a clue to its semiinsulating behaviour. A detailed MS Xα SCF calculation of the different charge states of Fe in InP is presented. The agreement with the experimental results is satisfactory and shows the promise of the method for the analysis of impurities in semiconductors. The Mössbauer isomer shift for the different charge states of Fe in InP are also calculated.
Cluster‐Bethe‐Lattice Approach to the Electronic Structure of Intrinsic Defects in v‐SiO2
Authors: Bethkenhagen Viola , Hübner K.
Published: July 2007

A cluster‐Bethe‐lattice calculation of the local electronic densities of states of some important intrinsic defects of vitreous SiO2 is presented. Defect‐induced gap states are correlated with known optical transitions. The optical absorption at 2 eV and the luminescence at 1.9 eV can be ascribed to a non‐bridging oxygen atom. The calculations support also the experimentally found relationship of the absorption at 5.8 eV with the Ei centre. Furthermore, a threefold coordinated Si atom, the peroxy radical, and the peroxy bridge are investigated and possible optical transitions are discussed.
ENDOR Investigation of Cr3+ Centres in the Tetragonal Phase of RbCdF3
Authors: Studzinski P. , Spaeth J.‐M.
Published: July 2007

Cr3+ centres in the perovskite RbCdF2 are studied with ENDOR at 17 K. The spin‐Hamiltonian parameters obtained for 53Cr and the first, second, and third nearest set of 19F neighbours show the influence of the structural phase transition from the cubic to tetragonal phase at Tc = 124 K. The results indicate that the influence of the lattice relaxation around the impurity ion is seen even in the third shell of 19F neighbours, which limits the use of Cr3+ as a probe to study the structural phase transition.
Helicon Wave Propagation in Semiconductor Superlattices
Authors: Kushwaha Manvir S.
Published: July 2007

A theoretical investigation is made of the collision‐induced instabilities associated with helicon wave propagation in (type‐I) semiconductor superlattices. The dispersion relation which presents a rich structure of collective excitations in the quasi 2‐dimensional systems is used to study the convective and absolute instabilities. Numerical solutions of the dispersion relation are performed including the effect of retardation. The theoretical results in the strong‐coupling limit clearly indicate the parabolic nature of the modes at small wave vectors and thus lend support to the prediction of Quinn and coworkers. The analytical diagnosis, however, leads to some paradoxial conclusions and hence keeps one in dilemma to characterize these modes precisely as helicons.
Low Temperature Resistivity Anomaly in Glassy Metals. Two‐Level Systems and Incipient Localization versus Correlations
Authors: Kaiser A. B.
Published: July 2007

Two‐level systems in disordered metals can contribute to the \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt T $\end{document} resistivity anomaly at low temperatures, by reducing the resistivity increment due to weak localization. A comparison is made with the generally larger effects expected from electron‐electron correlations, and with data for Mg‐Zn glassy metals.
On the Theory of Quantum Hall Effect
Authors: Ziep O. , Keiper R.
Published: July 2007

One‐particle wave functions suitable for calculating the quantum Hall conductivity are discussed with respect to their local density of states. The new electron configurations connected with the quantum Hall effect indicate a spatial structuring in form of a Wigner‐like crystal or liquid. The calculated diagonal resistivity shows definite peaks in dependence on the magnetic field, the values of which are in good agreement with experimental data. The relationship between the filling factor and a two‐particle Green function is discussed.
Structures and Energies of Symmetrical 〈011〉 Tilt Grain Boundaries in Silicon
Authors: Kohyama M. , Yamamoto R. , Doyama M.
Published: July 2007

The structures and energies of a series of symmetrical 〈011〉 tilt grain boundaries in silicon are calculated for the first time using the tight‐binding type electronic theory (the bond orbital model). The boundary energies are highly dependent on the types and arrangements of the structural units which consist of 5‐, 6‐, 7‐, and 8‐membered rings of atoms. The boundary structures in the range 0° ≦ 0 ≦ 70.53° are analyzed in detail by the structural unit model, and it is shown that the comparatively simple structures of special boundaries, such as the Σ = 9 (122) boundary, are relatively stable.
Strain Derivatives of Dielectric Constants of MgO and Thallous Halides
Authors: Srinivasan K. , Raghavan S. Lakshmi
Published: July 2007

Calculations are made to determine strain derivatives of static and high‐frequency dielectric constants of MgO, TlCl, and TlBr, using the two‐ion polarizable shell model. The calculated values of pressure derivatives of the static dielectric constant of MgO, TlCl, and TlBr are in good agreement with the experimental values. A reasonable agreement is seen between the calculated values and the measured values of strain‐optical constants for MgO. Strain‐optical constants are also calculated for thallous halides. Anisotropy in the polarizability of ions arising from strain is fairly accounted for by the shell model through the large contributions from overlap forces.
On the Calculation of Self‐Diffusion and Impurity Diffusion Coefficients with the Aid of Elastic Constants
Authors: Neumann G.
Published: July 2007

It is demonstrated that the diffusion model proposed by Varotsos and Alexopoulos implicitely includes a correlation between diffusion energy H and entropy S analogous that deduced by Wert and Zener. For the impurity diffusion via vacancies this correlation between H and S is not correct. Thus, the model of Varotsos and Alexopoulos is not applicable to the impurity diffusion. For self‐diffusion the model roughly approximates the temperature function of the diffusion coefficient D0, however, it is unable to describe the curvature of the Arrhenius plot of D0.
On the Diagram Technique for Green Functions of the Heisenberg Ferromagnet
Authors: Kühnel A.
Published: July 2007

The Green function approach to the Heisenberg ferromagnet is confronted with the difficulties connected with the commutation rules of spin operators. Recently, Baryakhtar, Krivoruchko, and Yablonskii (BKY) proposed a new diagram technique for Green functions containing spin operators based on the corresponding technique by Izyumov and Kassan‐Ogly. The new approach is compared with that proposed earlier by Haberlandt and Kühnel. Calculating some crucial terms it is shown why the diagrams of BKY do not represent the corresponding analytical terms properly if kinematic interaction due to the peculiar commutation relations is involved. The results for the low temperature spin wave energy and magnetization are discussed.
The Three‐Spin Box Product Interaction
Authors: Brown H. A.
Published: July 2007

The three‐spin interaction in the form of a box product, σ1 × σ2 · σ3, is investigated including additional interaction terms in the Hamiltonian in the forms \documentclass{article}\pagestyle{empty}\begin{document}$ \sum\limits_i {\sigma _{iz},} \sum\limits_{ij} {\sigma _{iz} \sigma _{jz}} $\end{document}, and πiσiz. With the relative strengths considered as parameters, the thermodynamic properties are evaluated as a function of temperature and show considerable variety. Both second‐ and first‐order phase transitions are found.
Comments on Levin's Modifications in the Phillips and Van Vechten Theory. d‐Electron Effect on Ionicity
Authors: Singh O. P. , Gupta V. P.
Published: July 2007

Based on theoretical grounds some serious objections against Levin's modifications are discussed and an alternate approach to calculate the ionic gap Ec is presented. An attempt is made to see the validity of the theory in a wide range of structures. Calculated data show that the number of d‐electrons taking part in the bond formation depends on the ionicity which is in accordance with the earlier predictions.
On the Thermodynamics of Strongly Degenerate Two‐Component Plasmas
Authors: Rother T. , Kraeft W. D.
Published: July 2007

A generalisation of Gell‐Mann‐Brueckner's formula is derived for two‐component systems of charged particles with any mass ratio and multi‐valley degeneracy. Higher order rs‐contribution are determined in the random phase approximation. Numerical values are given especially for hydrogen plasmas and for optically excited electron—hole plasmas in Ge and GaAs.
Interface Plasmon Modes as Collective Electronic Excitations of Semiconductor Superlattices
Authors: Bechstedt F. , Enderlein R.
Published: July 2007

The frequency spectrum of polar interface modes of superlattices is derived with regard of bulk spatial dispersion. The results are applied to free carrier excitations of superlattices. Dispersion relations are given for intra‐ and intersubband interface plasmon modes. It is shown that collective electronic excitations of superlattices considered in previous papers are limiting cases of interface plasmon modes. Experimental data on plasmon dispersion can be understood in terms of interface plasmon modes if carrier confinement is taken into account.
Thermodynamic Functions and Self‐Energy of Interacting Coulombic Systems. Numerical Results
Authors: Rösler M. , Stolzmann W.
Published: July 2007

The thermodynamics of multicomponent plasmas is investigated using a theoretical description where the free energy as a function of density and temperature is the basic quantity. The correlation contribution to the free energy and the chemical potential as well as the excess pressure (exchange plus correlation) are calculated at arbitrary degeneracy by summation over the complex Matsubara frequencies. With increasing degeneracy the numerical effort may be reduced with the help of the large wave‐number approximation or plasmon—pole expressions for the dielectric function. The temperature dependent self‐energy is calculated in the same way. Numerical results are given for the electron system, a hydrogen plasma, and for the electron—hole plasma in highly excited semiconductors.
Electronic Structure of Intrinsic Point Defects in Perovskite‐Type Crystals
Authors: Fisenko A. V. , Prosandeev S. A. , Sachenko V. P.
Published: July 2007

Using a tight‐binding model for the transition‐metal perovskite‐type oxides ABO3 a method of electronic Green function calculation is developed. In the framework of this technique the analytical equations for the local levels of atomic vacancies are obtained. The vacancy potential is calculated in the dielectric approximation including the ion displacements on the long ranges from the vacancy site and the electronic screening in the vicinity of the defect. It is shown that the appearance of the local levels in the forbidden gap is caused by the perturbation potential on the first neighbours of the vacancies. The energies of electronic excitations from the local levels are evaluated using the generalized transition state scheme. Donor levels close to the conduction band for the oxygen vacancy and acceptor levels for the A and B ion vacancy are obtained. The correlation between the results obtained and the physical properties of the perovskite oxides is discussed.
The Effect of Spin—Orbit Interaction on the Intensity of Transitions from 4A2 and 2E States in Strong Crystal Fields
Authors: Veremeichik T. F. , Kalinkina I. N.
Published: July 2007

The effect is studied of spin—orbit interaction and low‐symmetry field on the absorption ratios of the wide bands of the excited‐state absorption spectrum and similar bands of the ground‐state absorption spectrum of d3‐configuration. The study includes calculations of the energy of the pure electron transitions and the analysis of the composition of the eigenfunctions of the Cr3+ ions in ruby, magnesium oxide, and emerald in the cubic field approximation and taking mixing of all terms of the double trigonal group into account. The results are compared to the experimental values of the ratios of the oscillator strengths of the transitions compared. It is shown that the ratios depend strongly enough on the crystal field parameters and the parameter of the spin—orbit interaction. The influence of the interaction under consideration on the lasing parameters is discussed.
Influence of Macroscopic Inclusions on Excitonic Luminescence of InSe Crystals
Authors: Korbutyak D. V. , Kryuchenko Yu. V. , Demchina L. A. , Litovchenko V. G. , Sugakov V. I.
Published: July 2007

Low temperature (from 4.2 up to 30 K) photoluminescence spectra of InSe crystals containing macroscopic inclusions of neutral slightly soluble impurities (gadolinium, anthracene, fluorene etc.) are studied. The inclusions are of 0.1 to 0.5 μm in size. A substantial rise (by one or more orders in magnitude with respect to the initial spectrum of undoped crystals) of the exciton luminescence intensity is observed at relatively moderate doping doses (the impurity concentration less than 0.5 wt%) in the region of longitudinal exciton frequency ωL. To explain this effect a model is developed, which takes into account the scattering of lower branch polaritons at macroinclusions and their transformation into polaritons of the upper branch without changes in energy. The calculated temperature and frequency dependences of upper polariton branch luminescence are close to those measured experimentally despite of the practically non‐parametric way of the calculations.
Multipole Theory of the Polarization of Solids by Point Defects. III. Transition Energy Calculations, the Approximation of Non‐Point Charges
Authors: Kantorovich L. N.
Published: July 2007

The method developed in Parts I, II for calculating the polarization energy of crystals by point defects self‐consistently with their electronic structure is extended. (i) Along with the initial (e.g. ground) state the final (e.g. excited) electronic state of the crystal containing the defect and the corresponding vertical transition energy is considered. (ii) The uncompensated charge density simulating the defect and its neighbourhood is treated now by a set of non‐point charges. (iii) The polarization potential is calculated. This generalized method may be useful for self‐consistent calculations of a wide class of actual point defects in ionic solids.
Orbit—Lattice Interaction and Mössbauer Quadrupole Splitting of 57Fe2+ in GeFe2O4
Authors: Choudhary S. N. , Sinha T. P. , Srivastava K. K. P.
Published: July 2007

The temperature dependence of Mössbauer quadrupole splitting of 57Fe2+ in GeFe2O4 is theoretically analysed by taking into account the effect of orbit—lattice interactions from 77 to 1000 K. It shows a good agreement between theoretical and experimental results, brings consistency between Mössbauer and magnetic susceptibility measurements, and emphasizes the importance of the dynamic crystal field approach over a wide range of temperatures.
Damping of Polaritons in Bilayer Systems. Long‐Range Surface Phonon‐Polaritons
Authors: Wendler L. , Haupt R.
Published: July 2007

The damping properties of surface phonon‐polaritons, especially long‐range surface phonon‐polaritons in bilayer systems are analysed. An analytic expression is derived for the damping function by means of a perturbative method. The different behaviour of the damping properties of surface phonon‐polaritons for various physically different bilayer systems are discussed. It is shown that not all possible cases yield long‐range surface phonon‐polaritons. In some cases one or two modes show the behaviour of long‐range surface phonon‐polaritons. In special cases, there two longe‐range surface phonon‐polariton modes exist, one of these shows in a small frequency range a significantly increased propagation length. The propagation properties are analyzed numerically in dependence of frequency and of the thicknesses of the two layers of the bilayer system. Dispersion curves, the electromagnetic fields, the energy flow, the power flow, and the propagation length of the surface phonon‐polaritons are given in graphical form and in tables.
Photoluminescence Study of the Gallium Defect Spectrum at ≈ 0.928 ev (ga3) in Irradiated Silicon
Authors: Thonke K. , Schall U. , Sauer R.
Published: July 2007

A detailed investigation is made of the Ga3 luminescence spectrum observed in gallium doped silicon after electron irradiation and annealing at ≈ 250 °C. The spectrum exhibits exciton singlet—triplet pair no‐phonon transitions at 0.9291 or 0.9272 eV, respectively, and quasi‐localized modes of vibration energy 10.3, 24.4, and 56.9 meV along with combination modes and lattice phonon bands. An additional higher energy no‐phonon transition is due to an excited exciton state at excess energy of 3.8 meV thermalizing with the singlet—triplet states. The isotropic Zeeman splitting of the triplet (g = 2.00 ± 0.05) shows that the orbital momentum of the excitonic hole is entirely quenched. This and the low thermal dissociation energy of the exciton (16 meV) are used to deduce a strong hole binding energy (≈ 226 meV) whereas the 16 meV energy is attributed to the electron localization. The low electron binding energy is consistent with uniaxial stress measurements which are quantitatively explainable in terms of effective‐mass valley—orbit electron states. This defect represents the first example of a luminescent center in silicon which under uniaxial stress exhibits the whole multiplicity of valley—orbit‐states. The valley—orbit nature of the electron and the quenching of the hole orbital momentum render a symmetry classification of the defect impossible.
Anharmonic Properties of the Ternary Semiconductor CuGe2P3
Authors: Prabhakar N. V. K. , Sanyal S. P.
Published: July 2007

The third order elastic constants and the pressure derivatives of second order elastic constants of ternary zinc blende semiconductor, CuGe2P3 are analysed using two theoretical approaches. The first model consists of Born‐Mayer type short‐range potential in addition to the long range many‐body interaction and Coulomb potentials, while the second model is derived from Martin's bond‐bending and stretching force model. It is shown that although the bond bending force model gives a reasonably good fitting to these properties, the present model including three body forces explains adequately the features of the anharmonic behaviour reasonably well.
Effective Potentials and Crystalline State Properties of Some Fluorite‐Structure Crystals
Authors: Dutt N. , Kaur A. J. , Shanker J.
Published: July 2007

Seven different potential forms for the overlap repulsive energy proposed by various investigators as reported in the literature are examined for the fluorite‐structure crystals, viz. CaF2, SrF3, BaF2, PbF2, Sr(NO3)2, Ba(NO3), and Pb(NI3)2. A number of crystalline state properties such as cohesive energy, bulk modulus and its pressure derivatives, Grünesien und Anderson parameters, first and second order volume and pressure derivatives of dielectric constants are calculated using different potential functions. The results obtained are found to compare well with available experimental data, and also reveal the usefulness of the approach based on interionic potentials in predicting new results.
X K Edge Absorption of Titanium and L Edges of Tantalum in (Ti, Ta)O2 Rutile Solid Solutions
Authors: Poumellec B. , Marucco J. F. , Touzelin E.
Published: July 2007

An analysis of the Ti K edge and Ta L edges yields that the chemical shift amounts to 0.8 eV by a valence charge increase. An exciton line is resolved like in other rutiles. Exciton binding energy amounts to 1.1 eV and decreases to 0.6 eV for high tantalum content. The coordination polyhedron size of titanium increases to 11 × 10−3 nm with the tantalum content. This is essentially due to the increase of ionic radius of titanium by the valence charge increase. The crystal field splitting is found to be 3.1 eV for TiIV, 2.1 eV for TiIII, and (5.5 to 6.0) eV for TaV. It seems that the crystal field splitting is well correlated with the valence charge. Additionally a procedure is described to add several × absorption spectra recorded on several samples with the same composition but with different thicknesses.
Electronic Structure of the Ce Systems I. General Theory
Authors: López‐Aguilar F. , Costa‐Quintana J.
Published: July 2007

A model is deduced for the electronic structure of the Ce systems. It includes a pseudopotential which considers the strong correlation effects due to the fluctuation of the number of f‐electrons in the Ce atoms. The pseudopotential affects the f‐symmetries according to their own occupations and is different from zero when the f‐states are spd/f hybridized. The APW method is modified in order to introduce the pseudopotential within the band structure Hamiltonian. Finally, the so‐called „three‐peak”︁ 4f structure which appears in the Ce systems is analyzed with our model.
Effect of Non‐Spherical Potential on the Band Structure of Metallic Dysprosium
Authors: Mukhopadhyay P. K. , Das K. C.
Published: July 2007

The effect of non‐spherical part of the crystal potential on the band structure of dysprosium metal in the warped muffin‐tin approximation has been studied using the augmented plane wave method. The band structure is not significantly affected by this factor in the present case. The maximum change in energy eigenvalues is 0.002 Ryd.
Dynamical Corrections to the Physical Characteristics of Solitons in Polyacetylene
Authors: Fedyanin V. K. , Osipov V. A. , Stoyanova I. S.
Published: July 2007

The soltion solutions to the continuum equations of motion of the trans‐(CH)x model in the limit of slowly moving solitons are obtained. The effects due to the soliton motion are considered. The main physical characteristics of moving solitons (the energy of creation, mass, charge, width, electrical moments) are calculated. In the case of the kink excitation the effects of motion are reduced by the small parameter ϵ ≈ 0.07, whereas for the polaron excitation the correction terms proportional to the polaron velocity are considerable.
Interaction of Crystal Impurity Centres with a Coherent Electromagnetic Field
Authors: Apanasevich P. A. , Kruglov V. I. , Tralle I. E.
Published: July 2007

By the projection operator technique the non‐Markovian kinetic equation for the density matrix of a crystal impurity centre in an external electromagnetic field is derived, taking into account the electron–phonon interaction. The analytical solutions of this equation are obtained for the external field as an ultrashort pulse and in the case of a monochromatic wave.
Photon Recycling in Double Heterostructures II. The Case of Non‐Perfect Optical Confinement
Authors: Enders P.
Published: July 2007

The influence of photon recycling (PR) onto the minority carrier distribution in double heterostructures is investigated. For non‐perfect optical confinement of the luminescence radiation in the active zone, the corresponding integro‐differential equation (without drift term) of the diffusion model cannot longer be solved exactly. The enlargement of the radiative lifetime, as measured in photoluminescence experiments, e.g., is calculated approximatively by means of perturbational methods. Particular attention is devoted to the dependence of this enlargement due to PR upon the thickness of the active zone. In very thin active zones, PR should play a minor role due to the loss of luminescence radiation.
The Generalized Curie‐Weiss Law
Authors: Fähnle M. , Souletie J.
Published: July 2007

It is shown that the zero‐field suscetpibility χ of many ferromagnets may be described with high accuracy by a modified power law (generalized Curie‐Weiss law) χT ∼ [(T – TC)/T]−γc in the whole paramagnetic regime, where γc is the critical susceptibility exponent. In contrast, the conventional power law χTC ∼ [(T – TC)/TC]−γc holds only for the very small critical regime. Application of the scaling variable (T – TC)/T, which is non‐linear in T, to other physical situations (spin glasses, percolation) is discussed. Altogether the temperature range where scaling ideas are useful appears to be much wider than generally expected.
Quantum Theory of Free‐Carrier Absorption in Quasi‐Two‐Dimensional Semiconducting Structures. Degenerate Carrier Case
Authors: Comas F. , Giner C. Trallero , León H.
Published: July 2007

Free‐carrier absorption coefficient of light is calculated for the case of quasi‐two‐dimensional (Q‐2D) semiconducting systems (of the kind of “quantum well” structures) assuming the conditions for highly degenerate carrier statistics are fulfilled. A comparison with recent results for the case of non‐degenerate carrier statistics is made: higher absorption coefficient for lower values of the quantum‐well width is predicted (as in the non‐degenerate case), but the absorption coefficient is lower for the degenerate carrier case.
Electronic Structure of PbTe–Pb1−xSnxTe Superlattices
Authors: de Dios Leyva M. , López‐Gondar J.
Published: July 2007

In the framework of the envelope function approximation a simplified effective Hamiltonian for each valley in the PbTe–Pb1−xSnxTe superlattice is obtained. Analytical expressions are given for the dispersion relations connected with different valleys as a function of the kx, ky transverse wave vector components.
Time‐Resolved Degenerate Four‐Wave Mixing in CuCl under Nanosecond Pulsed Excitation
Authors: Frindi M. , Hönerlage B. , Levy R.
Published: July 2007

Using nanosecond laser pulses the time behaviour is investigated of degenerate four‐wave mixing close to the biexcitonic resonance in CuCl. The signal emission presents different time evolutions depending on excitation conditions. Its generation rate shows intensity changes and retardation effects. These effects are explained by an attenuation of the pump pulse through a time‐dependent nonlinear absorption and, at the resonance, by a loss of coherence related to collisions with bi‐excitons.
Modelling of Angle Resolved Photoemission Spectra at Low Energy
Authors: Beckmann A.
Published: July 2007

The calculation of low‐energy angular resolved photoemission spectra is based on the method of surface Green's function. This approach enables excitations from bulk and surface states to be treated uniformly, thus elucidating the connection between peak characteristics and lifetimes of electrons and holes better than before. Furthermore, the method is suitable to describe surface disorder. Experimental results obtained by Gartland and Slagsvold for Cu(111) are used for discussion.
Diffusion of Non‐Interacting Quantum Particles in a Disordered System. The Transport Length in the Classical Limit
Authors: Lenk R.
Published: July 2007

Within the model of uncorrelated scatterers and a long mean free path the recently proposed structure of the density matrix in a current‐carrying state is shown to be produced and reproduced by the superposition of all scattered waves. The corresponding transport length corresponds to its classical value, but the physical picture is a new one.
Reconstructed Structures of Symmetrical 〈011〉 Tilt Grain Boundaries in Silicon
Authors: Kohyama M. , Yamamoto R. , Doyama M.
Published: July 2007

The energies of reconstructed structures of symmetrical 〈011〉 tilt grain boundaries with θ > 70.53° in silicon are calculated using the tight‐binding type electronic theory (bond orbital model). The energies of the most stable reconstructed boundary structures are in the same range or a little larger than those with θ ≦ 70.53°. These reconstructed structures are more stable than those with dangling bonds. There exists the continuity of boundary structure in the range 70.53° ≦ θ ≦ 148.41° (Σ = 27). Shallow cusps in the energy against θ curve can be found at special boundaries of type Σ = 3 (θ = 109.47°), Σ = 11 (θ = 129.52°), and Σ = 27 (θ = 148.41°), all of which are composed of one kind of original patterns. For θ > 148.41°, boundary structures are composed of an array of a0[100] edge dislocations.
Damping of a Kinked Dislocation by Electrons in Metals at Low Temperatures
Authors: Das Amal K.
Published: July 2007

The mobility of a dislocation in metals at low temperatures is limited by the damping due to conduction electrons. The kink model of a dislocation is pursued in order to study this damping force and hence the mobility. Calculations are presented for one, two and three dimensions. The dependence of the mobility and electron density is exhibited for different dimensions. New results are derived.
The Effect of Higher‐Order Cumulants in the Random‐Field Ising Model
Authors: Janiš V.
Published: July 2007

It is shown that below a critical dimension dc = 4 higher‐order random‐field cumulants contribute to the divergent part of the free energy. They renormalize the standard diagrammatic expansion into the most divergent terms in weak random fields in such a way that the perturbative effective dimensionality in the scaling region of the d < 4 model is two. It is argued that any dimensional reduction based on a diagrammatic expansion is doubtful in lower dimensions.