bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical Treatment of the Diffusion Coupled with Reaction, Applied to the Example of a Binary Solid Compound MX
Authors: Maier J. , Schwitzgebel G.
Published: July 2007

It is shown, that the coupling of diffusion processes with reactions can be treated as a formal ambipolar diffusion of ensembles of particles, which are conserved in the reaction scheme. In this way an extension of the flow equations to a complex defect chemistry becomes possible, as exemplified by the ionization processes of the ionic point defects in a binary solid MX. In a first approximation Fick's laws with generalized diffusion coefficients are obtained. The boundary conditions are formulated for several important cases by considering the equations of balance of the conservative ensembles. It is further shown, how the new evaluation of the usual electrochemical measurements of transport quantities (e.m.f., polarization and permeation) has to be performed. Additional terms, caused by the ionization processes, appear in all evaluation formula.
Polariton Theory in Electron—Hole Configuration Space. Bulk Solution for a Wannier‐Type Model
Authors: Stahl A. , Balslev I.
Published: July 2007

A pair of coupled wave equations for electromagnetic radiation and electron‐hole pairs in the Wannier limit is solved for the case of an unbounded crystal. The solution is discussed with respect to the following phenomena: (i) excitonic resonances, (ii) Coulomb enhancement of continuum, (iii) local current structure, (iv) spatial dispersion.
Band Effects on Charge Ordering in Fe3O4
Authors: Lorenz B. , Ihle D.
Published: July 2007

The stability of the electronic long‐range order in Fe3O4 with respect to the band term is investigated using a spinless fermion model with Coulomb interactions between nearest (U1) and next nearest neighbours (U2), where U1 > t > U2 (t overlap integral) is assumed. This model is treated by perturbation theory up to third order in t and an effective spin Hamiltonian is constructed. From an approximate solution the long‐range order condition at T = 0 is found to be \documentclass{article}\pagestyle{empty}\begin{document}$ t< 0,85\sqrt[3]{{U_1^2 U_2 }} $\end{document} and thedependence of the Verwey transition temperature TV on t is calculated. The experimentally observed pressure dependence of TV is explained.
Formation of Lattice Defects by Two‐Photon Excitation in Alkali Halides
Authors: Provoost J. , Debergh P. , Hoebeeck G.
Published: July 2007

Measurements by two‐photon excitation of the second harmonic of a ruby laser are made of the formation of lattice defects in four alkali halides; RbBr, KBr, RbI, and KI. The spectra and measured curves reveal the crucial role of the experimental conditions in the defect formation. In the measured alkali halides the F and H bands at LHeT are thermally stable, but can be bleached by a very small amount of probe light. Time resolved measurements in RbBr show that the ratio of the STE and F centre absorption depends on the intensity of the laser beam. In KBr the possibility is shown of excitation at LHeT with the second harmonic of the ruby laser. In RbI the F band is formed even at LHeT, and in KI the F′ band at LHeT is larger than the F band after two‐photon excitation.
New Theoretical Method for Studying Elementary Excitations in Crystalline Disordered Alloys. Application to Phonons in NixPt1−x
Authors: Gómez‐santos G. , Yndurain F.
Published: July 2007

A new method for studying elementary excitations in disordered crystalline binary alloys is presented. The complex self‐energy due to positional disorder is calculated in a Bethe lattice and taking proper account of the short‐range order. The crystalline alloy is then described by a non‐hermitean Hamiltonian which includes the calculated self‐energy. Densities of states, as well as dispersion relations, can be calculated. The method is applied to the study of lattice vibrations in crystalline NixPt1–x random alloys. The results are in good agreement with neutron diffraction experiments. The experimentally observed phonon densities of states, lineshapes, and dispersion relations are well described in the model.
Influence of surface stress on the surface elastic waves in anisotropic cubic crystals
Authors: Velasco V. R.
Published: July 2007

The effects of surface stress on the surface dynamical properties of cubic crystals are discussed. Analytic expressions for the dispersion relations of surface waves propagating along [100] and [110] directions in the (001) surface are obtained. It is found that these surface waves are dispersive and that the dispersion relation exhibits a cut‐off. The same is true for an arbitrary direction of propagation. An application is made for W. It is found that the shifts in the velocities of Rayleigh waves due to the surface stress can not be detected by using the experimental techniques available now. The possible effects of surface stress on the thermodynamical properties of cubic crystals are also discussed briefly.
Electronic excited state and the phenomena of photo‐induced polarization and depolarization
Authors: Prakash J.
Published: July 2007

That the activation energy for the orientation of impurity—vacancy dipoles in the electronic excited state (ees) should be less than that required in the electronic ground state (egs) is not the sufficient condition for the occurrence of PIP and PID. It is observed that the activation energy is not the only parameter which controls the occurrence of PIP and PID. The fundamental relaxation time also participates in deciding the situations in which PIP and PID take place. The necessary condition found, is that the product of the activation energy and the fundamental relaxation time in ees should be less than the corresponding value in egs.
The dielectric response of a charge density wave (CDW) distorted electron—phonon system in the precritical region II. Macroscopic dielectric function and conductivity
Authors: Heiner E.
Published: July 2007

Starting with a selfconsistent approximation (I) for the calculation of the selfenergies of the electron and phonon system, the dielectric function is calculated in the small momentum limit for a quasi‐one‐dimensional system in the precritical region above the Peierls temperature.
Symmetrical grain boundaries with special angles and their coincidence site lattices
Authors: An Nguyen , Worm G. , Herrmann R.
Published: July 2007

A simple method for an explicite calculation of the coincidence site lattices is presented. This method based on clear geometrical conditions is convenient for the interpretation of experimental results. The indices of the grain boundary planes for symmetrical rotation axes are estimated. The type and the orientation of the Bravais lattice of the coincidence site lattices in cubic crystals are obtained.
Broadening and shift of diffraction lines due to partial coherence of X‐rays
Authors: Čerňanský M.
Published: July 2007

The arrangement with the high angle X‐ray monochromator produces pure diffraction profiles which are narrower and shifted as compared to the arrangement without monochromator. These effects can be explained by the theory of partial coherence of radiation.
Diamagnetic excitons and the indium antimonide energy band parameters
Authors: Efros Al. L. , Kanskaya L. M. , Kokhanovskii S. I. , Seisyan R. P.
Published: July 2007

The oscillatory absorption edge spectrum of InSb crystals exhibiting the discrete structure of the Wannier‐Mott exciton at H  0 is studied at 1.8 K in magnetic fields of up to 80 kOe. The experimental data obtained are analyzed with a computer taking into account the excitonic nature of the phenomenon and the effect on the Landau levels of the nonlocality of potential resulting from the electron—electron exchange interaction. A set of band parameters is presented which provides a minimum discrepancy between the theoretical and experimental spectra of not more than 0.65 meV per datum point: Ep  23.42 eV, F  − 1.18, N1  − 0.33, γ2  36.41, γ2   15.95, γ3  16.99, k  15.22, q  0.31, χ0  17.9. The possible contribution of the higher bands and the reasons for disagreement with other avialable parameters are discussed.
Electronic structure of intermetallic compounds by interpolation schemes
Authors: Rennert P. , Gläser U. H. , Hergert W. , Timoshevskii A. N.
Published: July 2007

The applicability is studied of a d‐band LCAO interpolation scheme and of the recursion method to intermetallic compounds with CsCl structure of two transition metals. Input data are APW energy eigenvalues, needed for this purpose at only a few points of high symmetry in the Brillouin zone. The interpolation scheme works with an accuracy that is sufficient for calculating densities of states, but works much faster than the APW calculation. From the LCAO hopping parameters local densities of states are obtained with the recursion method.
Coherent‐potential approximation for vibrations in Ga1—xInxSb and Ga1—xInxAs mixed crystals
Authors: Kleinert P.
Published: July 2007

The vibrational properties of the mixed crystals Ga1—xInxAs and Ga1—xInxSb are investigated using the coherent‐potential approximation (CPA). These III—V systems switch from one‐mode to two‐mode behaviour as the composition is changed. Results are obtained for a one‐dimensional model including self‐consistently mass defects and force constant changes. The calculated density of states and the dielectric susceptibility are compared with results of the mass CPA. In addition, a three‐dimensional CPA Bethe lattice treatment is carried out to calculate the optical properties of these disordered systems showing mixed‐mode behaviour. The force‐constant change is included by a virtual‐crystal interpolation. The theory explains the mode switching behaviour and agrees fairly well with experimental peak positions and line shapes of the response function.
Elastic‐wave surfaces in solids
Authors: Ledbetter H. M. , Kriz R. D.
Published: July 2007

Based on Christoffel‐equation solutions, some interesting wave‐surface topological features are described for anisotropic media. These features include crossovers of transverse—longitudinal surfaces and continuous transverse—longitudinal mode conversion over a single surface. For orthorhombic symmetry (mmm), crossovers of transverse—transverse surfaces occur for all crystals: the transverse surfaces interconnect and form a single surface. Beyond this, some orthorhombic crystals exhibit a longitudinal—transverse crossover that causes all three surfaces to interconnect into a single surface. Crossover of longitudinal and transverse surfaces means that a transverse wave velocity will exceed a longitudinal wave velocity. A longitudinal—transverse mode conversion means that both longitudinal and transverse modes exist on the same wave surface.
Cellular density functional theory of the heat of formation of disordered simple alloys
Authors: González D. J. , Alonso J. A. , Iñiguez M. P.
Published: July 2007

A cellular model is used to calculate the heat of formation in disordered solid MgxCa1—x alloys. The computations of the electron density and the energy are performed by the density functional formalism. The influence of the cell‐partitioning is analyzed. Neutral atomic spheres are the most convenient type of atomic cells for completely disordered alloys because an optimum number of physically satisfactory matching conditions can be imposed at the surface of the neutral atomic cells.
Integral representation of the incoherent field in disordered materials. V. Wave Diffusion in a Correlated Medium
Authors: Lenk R.
Published: July 2007

Based on a generalized representation obtained in IV, the diffusion current in a macroscopically homogeneous disordered medium with varying wave intensity or quantum particle density, respectively, is determined. The conductivity can be calculated from the averaged diffusion coefficient. In the limit of long mean free path the theory essentially confirms the result of a semi‐classical treatment where each scatterer together with its correlation cloud acts as an effective scattering element whose scattering properties are influenced by interference effects.
Conduction spin density and knight shift of copper
Authors: Bross H.
Published: July 2007

Using Chodorow's potential the energy eigenvalues and wave functions of metallic Cu are obtained by the MAPW formalism. In accordance with other investigations the Fermi surface is in close agreement with de Haas‐van Alphen experiments. The Fermi‐surface average of the probability 〈Ψnk(0)|2〉FS yields the spin density at the nuclear sites. Due to many‐body corrections of the Pauli spin susceptibility a reliable value of the Knight shift cannot be derived from first principles. By comparing with the experimental value of the Knight shift it is found that many‐body effects enhance the Pauli susceptibility by a factor of 1.25.
Dynamics of two‐spin cluster in spin glass III. Magnetization process at finite, but low temperature
Authors: Heber G. , Zymierska D. , Beyer H.
Published: July 2007

Based on the two‐spin cluster model of spin glass, the magnetization process at low T is investigated. The short‐range inter‐cluster interaction is taken into account by excitation waves, treated as free Bosons. They feel, however, additionally a Weiß‐type molecular field of long range. For certain parameter sets in the range T < TF magnetic instabilities are found, as well as metastable branches of m(H, T)‐curves, leading to magnetization jumps, induced by external magnetic field. The phase diagram shows a line of first‐order transitions and a point of a higher‐order transition. Hysteresis and domain patterns are the usual behaviour in this region.
Electron state associated with the unreconstructed (111) surface in diamond
Authors: Persson Astrid
Published: July 2007

A calculation of the electron states associated with the unreconstructed (111) diamond surface is performed using Slater‐Koster parameters evaluated from first principles. The calculation gives a surface band in the lower half of the band gap. Close agreement with a self‐consistent pseudopotential calculation is found when the matrix elements of the surface atom is modified to contain non‐spherical parts of the potential.
Repulsive energy and the grüneisen parameter of alkali halides calculated on the basis of a quantum‐statistical ab initio theory
Authors: Kucharczyk M. , Olszewski S.
Published: July 2007

The Grüneisen parameter of alkali halides is calculated by an ab initio quantum‐statistical method and then compared with the experimental data. The crystal model applied assumes the crystal ions to be compressible but impenetrable spheres. The ions are described with the aid of a modified Thomas‐Fermi theory with exchange. At the next step it is possible to calculate the energy needed to transform the system of the non‐interacting ions into the ionic system represented by the crystal lattice. This calculation allows for an ab initio estimate of the parameters entering the Born, or the Born‐Mayer, repulsive part of the crystal energy. The parameters are then used in the calculation of the Grüneisen parameter and its dependence on the crystal compression.
Debye‐ waller factors for alkaline earth oxides
Authors: Singh R. K. , Sanyal S. P.
Published: July 2007

A theoretical calculation of the Debye‐Waller factors and the mean square displacements is carried out for the alkaline earth oxides. The eigenfrequencies and eigenvectors required for this purpose are obtained from the recently proposed eight parameter generalised three body force shell model. A discussion in the light of available experimental data and earlier theoretical results revealed that the values are in excellent agreement in the cases of MgO and CaO and follow a systematic trend in the case of other members of the alkaline earth oxide family.
Dielectric and anharmonic behaviour of alkali halide crystals
Authors: Shanker J. , Sundaraj R.
Published: July 2007

The dielectric and anharmonic behaviour of alkali halide crystals is studied by determining the volume derivatives of low frequency dielectric constant and polarizability. An interionic force model which takes into account the contributions arising from the short range overlap repulsion and the van der Waals interactions is used in calculations. The implicit and explicit temperature derivatives of low frequency and high frequency dielectric constants are obtained separately for all the alkali halide crystals with NaCl and CsCl structures. The anharmonic contribution to the dielectric constant is calculated using theoretical models due to Szigeti and Havinga. The recent analysis of pressure dependence of effective charge parameter and dielectric constant of ionic crystals performed by Varotsos is shown to be invalid.
Dispersion curves of stimulated Raman effect in piezoelectric cubic crystals
Authors: Schulz V. , Merten L.
Published: July 2007

It is shown that Stokes and anti‐Stokes waves for general directions of propagation in piezoelectric crystal couple anharmonically both with transverse and longitudinal polaritons (stimulated Raman effect). Only for special directions in the crystal pure transverse polaritons are excited. The dispersion functions for such a special direction in GaP are calculated numerically. If the damping of the polariton waves is neglected the wave vectors of the coupled waves are either real quantities or pairs of conjugate complex quantities. The complex wave vectors are represented in the complex plane. The results show that exponentially amplified polariton waves exist. The properties of these waves are discussed in detail.
Triplet exciton trapping in p‐terphenyl crystals studied by delayed fluorescence and phosphorescence
Authors: Takase T. , Morikawa E. , Kotani M.
Published: July 2007

Triplet exciton trapping in p‐terphenyl crystals is investigated by delayed fluorescence and phosphorescence. Trap depth ((2800 ± 100) cm−1) and concentration (10−6 mol/mol) are deduced from the temporal behaviour of delayed fluorescence. The same trap is also found in the phosphorescence spectrum. The bimolecular annihilation rate constant between free and trapped excitons (1 × 10−13 cm3 s−1) is found to be orders of magnitude smaller than that for free—free annihilation.
Optically detected magnetic resonance in Zn1−xMnxTe
Authors: Malyavkin A. V.
Published: July 2007

Optically detected magnetic resonance (ODMR) due to ESR of Mn in Zn1−xMnxTe is observed. The resonance is displayed in reconstruction of recombination spectra of free exciton, exciton bound to a neutral acceptor, and donor‐acceptor pairs in magnetic field. Two cases are treated: when electron and hole spin‐splitting energies are less or comparable to their spin temperatures (at Mn concentration x = 0.0025), and when spin‐splitting energies many times exceed spin temperatures (x = 0.08).
Donor‐related edge luminescence in ion‐implanted ZnTe
Authors: Verity D. , Nahum J. , Bryant F. J.
Published: July 2007

Single crystal samples of ZnTe are implanted at 77 K with chlorine, bromine, copper, or silver ions of 100 keV energy. The low temperature cathodoluminescence spectra of the sample are recorded as a function of post‐implantation isochronal (30 min) annealing temperature in vacuum in the range 50 to 500 °C and compared with the spectra of similarly‐annealed unimplanted and argon‐implanted reference samples. Chlorine, bromine, and copper each induce similar changes in the emission spectrum and, in addition to enhancing strongly donor‐acceptor pair bands at 547.3 (CII) and 553.6 nm (BII), produced new edge emission bands at 530.2, 535.2, and 530.6 nm, respectively. These three new bands are each assigned to a transition from a deep donor level to the valence band. The deep donor defects are attributed to the complexes (VZn2ClTe), (VZn2BrTe), and (VZn2Cui) or (VZn 2CuTe), with electron binding energies of 65, 88, and 69 meV, respectively. In contrast, annealing of silver‐implanted ZnTe causes emission spectrum changes comparable to those induced by argon ion bombardment and produces no effects which can be attributed to the silver impurity.
Conduction electron self‐energy effects in ferromagnetic semiconductors
Authors: Eränen S. , Sinkkonen J.
Published: July 2007

The interaction between conduction electrons and localized lattice spins is studied. The problem is considered in broad conduction band ferromagnetic semiconductors at low temperatures using spin‐wave approximation to obtain the lattice moment correlation functions. Calculating the self‐ energies in generalized Born approximation for both the conduction electron spin directions it is shown that a spin gap does not exist i.e. even at zero temperature the spin down band has a tail extending down to the spin up band edge. Consequently the relative conduction electron spin polarization does not reach the ideal value 100%, but ‐ with parameter values appropriate for EuS ‐ remains always smaller than 93% and further keeps practically constant at low temperatures. At T = 10 K the value 93% exactly coincides with the known experimental result for EuS. In addition the mobility including self‐energy terms only is evaluated. For spin down electrons the mobility is found to be several orders of magnitude smaller than for spin up electrons. Moreover, due to the non‐existence of the spin gap the spin flip scattering is always present, which tends to reduce the mobility of spin up electrons from the ordinary Boltzmann result.
On the interaction between a vacancy and an interstitial impurity atom in B.C.C. transition metals
Authors: Masuda K.
Published: July 2007

The binding energies are calculated between a vacancy and an interstitial impurity (like carbon) atom in b.c.c. transition metals (α‐Fe, Mo, and W). The changes in the total energy due to the introduction of lattice defects are calculated using a tight‐binding type electronic theory and the Born‐Mayer repulsive potential. A comparison is made with previous theoretical calculations as well as with recent experimental observations (positron annihilation experiments). In addition the detailed electronic structure is investigated of the impurity atom, assuming both, interstitial and substitutional occupations and using the recursion method by Haydock et al. It is shown that there are marked differences in the local p‐orbital density of states between the octahedral and substitutional impurity (carbon) atoms. These calculations enable one to understand the stability of the vacancy‐interstitial impurity pair in b.c.c. transition metals.
A microscopic interpretation of the spin‐lattice coupling coefficients for S‐state 4f7 ions
Authors: Pastusiak W.
Published: July 2007

A previously reported model for spin–lattice coupling for 4f7 electrons, based on the assumption that intraionic interactions are perturbed by relativistic interaction with crystal lattice, is developed. Analytical formula for second‐ and fourth‐order spin‐lattice coefficients are derived. Using these formula and the concept of the relativistic crystal field in the point charge approach the values of the spin‐lattice coefficients for Gd3+ in CaF2 are calculated. These values are in much better agreement with the experimental values than those calculated so far. To the best of our knowledge the fourth‐order spin–lattice coefficients are calculated the first time and their values are about ten times smaller than the experimental ones.
Second‐order perturbation treatment of correlations in transition metal alloys
Authors: Drchal V. , Kudrnovský J.
Published: July 2007

Correlation effects in disordered transition metal alloys are studied within the degenerated Hubbard model using the perturbation theory limited to the second order in U/w (U Coulomb integral, w bandwidth). The influence of disorder is included within the coherent potential approximation. The theory is illustrated on a simple model of the copper–nickel alloy.
Parameters of bound multiexciton complexes in direct polar semiconductors
Authors: Haufe A. , Henneberger K. , Röseler J. , Wünsche H.‐J.
Published: July 2007

Using a generalized density‐functional method, which takes electron–phonon interaction into account, the ground state energies of donor BMEC's D0Xm(m = 1,…, 20) are calculated for the polar semiconductors ZnS, CdS, and CdSe. The highest stability is obtained for complexes possessing filled electron shells (m = 1, 7, 19), whereas gaps of instability are located between these complexes. The spectral locations of luminescence lines connected with the radiative decay of BMEC's are predicted and found to be concentrated between the luminescence lines of the bound exciton and the free exciton. Furthermore, analytic approximate expressions for the electron and hole densities are obtained and their shapes are extensively discussed.
Theory of coherent quasi‐steady states in NQR multipulse experiments
Authors: Ermakov V. L. , Ya. Osokin D.
Published: July 2007

On the basis of the average Hamiltonian theory the influence of the pulse sequence t — (τ — t — 2τ — t — τ)n on a system of two dipolar coupled equivalent spin‐1 nuclei each subject to local electric field gradients is studied. Using a fictitious spin‐1/2 formalism a general method is developed for solving the Liouville equation for such spin systems, leading to results which reproduce the experimentally observed quasi‐steady state of magnetization in nitrogen‐14 pulse experiments. Expressions for the density matrix of the system in terms of the effective time n4τ and the parameters characterizing the quadrupolar and dipolar interactions are obtained.
Forbidden luminescence and resonance Raman scattering of bound exciton states in CdS
Authors: Baumert R. , Broser I. , Gutowski J. , Hoffmann A.
Published: July 2007

A series of six energy levels is found at energy distances 2.5, 3.8, 4.7, 6.1, 7.0, and 8.3 meV on the high energy side of the neutral‐acceptor bound exciton I1 in CdS. They appear in the emission and absorption spectra of some freshly grown virgin crystals and of some crystals damaged by laser‐irradiation. Resonance Raman scattering (RRS) establishes the new levels as intermediate states in every high quality crystal. These levels are interpreted as excited states of the bound exciton I1. The direct radiative recombination is forbidden due to parity selection rules. These selection rules are removed in symmetry disturbed crystals, at high excitation intensities, or by recombination with phonon participation. In consequence they are detected as forbidden luminescence, as LO phonon replica, or as 1‐LO‐ and 2‐LO‐Raman lines. The 2‐LO scattering cross section of the energy region of bound excitons in CdS is determined.
Anomalous transient‐time dispersion in amorphous solids: Oxadiazole
Authors: Crisa N.
Published: July 2007

Hole transport in thin films of an organic disordered solid (oxadiazole) is studied by time‐of‐flight techniques, as a function of sample thickness and applied field. The experimental data are analyzed in terms of the theory of stochastic non‐Gaussian transport by Scher and Montroll, showing the validity of this model. The last section is devoted to an evaluation of the hole drift mobility: its resulting values are extremely low (<10−6 cm2/Vs).
A new semiempirical formula for superconducting transition temperatures of metals and alloys
Authors: Surma M.
Published: July 2007

A new formula describing the superconducting transition temperature is presented. The electron–phonon coupling constant λ and the phonon frequency spectrum parameters of a material are the quantities which define the transition temperature Tc of superconductors. A comparison between the experimental and calculated values of Tc for metals and alloys confirms the validity of the new formula presented in this paper.
Far‐infrared photoconductivity spectroscopy of shallow acceptors in InSb
Authors: Meisels R. , Kuchar F.
Published: July 2007

Acceptor excitation as well as ionization spectra are observed in InSb doped with Ge and Cd using Fourier transform spectroscopy and photoconductivity techniques. Best agreement with theoretical results is obtained with the following values for the Luttinger parameters: γ1 = 36.13, γ2 = 16.24, and γ3 = 17.34 which were recently deduced by Littler et al.
Spin‐lattice relaxation theory of excited triplet‐state impurities in molecular crystals
Authors: Sugakov V. I. , Shtepa Yu. D.
Published: July 2007

The processes of spin‐lattice relaxation of the excited local triplet states in molecular crystals at low temperatures are investigated. The coupling of the translational and rotational motion of the molecule (TRC relaxation mechanism) is chosen as the major phenomenon, leading to the coupling of the spin system with acoustic vibrations. The general expressions for the spin‐lattice relaxation time of the triplet centre in molecular crystal with impurities are derived taking into account the change of force constant due to the impurities. The angular, field, and temperature dependences of SLR time are calculated for an isotopic impurity in naphthalene crystal. The results of calculation are in accordance with available experimental data.
Study of various approximations for the continuum planar potentials at channeling I. Monoatomic crystals
Authors: Dedkov G. V. , Gorshokov A. M.
Published: July 2007

A detailed analysis of different approximations for the continuum planar potentials under the channeling conditions is given. The continuum potentials for positrons (electrons) channeled in (110) and (100) planes in crystals of diamond, silicon, and tungsten are calculated in different approximations to the screening function. The model of an atom with a piecewise exponentially decaying electron density is discussed. The influence of the thermal vibrations and the atomic shell structure on the potentials is investigated. Some estimates of the photon energy radiated at channeling of 56 MeV electrons in (110) planes of C are given.
Magnetooptical study of the fundamental absorption edge in Cd1−xMnxTe
Authors: Khoi Nguyen The , Ginter J. , Twardowski A.
Published: July 2007

The fundamental absorption edge in Cd1−xMnxTe for x = 0.01, 0.05, 0.10, 0.20 in the Faraday configuration, at magnetic fields up to 5 T and in the temperature range 1.8 to 293 K is studied. Moreover the exciton spectrum of extremely thin samples (of less than 1μm) is measured. An additional structure on the σ‐edge is found. The appearance of that structure is supposed to be the effect of the mixing of exciton states in the zincblende structure.
Quantum transport in quasi‐one‐dimensional systems
Authors: Arora V. K. , Prasad M.
Published: July 2007

The results of quantum transport investigations in quasi‐one‐dimensional systems assumed to be of cylindrical geometry are presented. Quantum size effect is important when the de Broglie wavelength (λD) of confined electrons becomes comparable to radius R0 of the cylindrical geometry. In the extreme quantum‐limit, λD ≫ R0, the resistivity is found to be proportional to λ2D/A, where A = πR20 is the area of cross‐section. Similar behavior is found for electrons confined to a strong magnetic field, λD ≫ λ = (hc/eB)1/2, when A = πλ2 is the area of the cyclotron orbit of radius λ in a magnetic field of strength B. The quantum transport behavior of electrons confined in thin wires is found analogous to those confined in a magnetic field.
Localization effect in CrRh alloys
Authors: D˘adålat D. , Petrişor T. , Giurgiu A. , Pop I.
Published: July 2007

Magnetic susceptibility measurements of CrRh alloys are reported. Below the Néel temperature a temperature independent magnetic susceptibility is observed for the samples containing 10.9 and 14.5 at% Rh. A decrease of the magnetic susceptibility as a function of temperature is observed for samples containing between 5.14 and 14.5 at% Rh in the paramagnetic region. A slight localization, induced by Rh impurities on the host Cr atoms, is proposed in order to explain these peculiar behaviours. The magnetic phase diagram is also given. The Néel point rapidly increases at the beginning, with increasing Rh concentration, and then slowly decreases, giving rise to a large maximum at about 6 at% Rh.
On the schrödinger spin‐exchange ferromagnet
Authors: Brown H. A.
Published: July 2007

The thermodynamic behavior of a ferromagnet whose interactions are described by the Schrödinger spin‐exchange operator is investigated. The transition temperatures, magnetization, susceptibility, exchange energy, heat capacity, and entropy are calculated for general spin and lattice coordination number using the constant‐coupling approximation. Both first‐ and second‐order phase transitions are found. Comparison with some results of the high‐temperature expansion method shows some qualitative agreement.
Quantumchemical studies of chemisorption at anion or cation sites of the GaP (110) surface. Coverage dependence of adsorption energy and equilibrium in cases of positive and negative partial charge injection
Authors: Lorenz W. , Engler C. , Nechwatal A. , Adler G.
Published: July 2007

Chemisorption of electronegative or electropositive atoms at anion or cation sites of a relaxed GaP (110) surface is investigated in the TB approximation, with parameters corresponding to fluor or lithium adsorption. Local densities of states and the dependences of the surface Fermi energy, partial charge injection, adsorption energy, and adsorption equilibrium upon the adsorption density are reported.
Electromagnetic wave propagation in the uniaxial antiferromagnetic crystals FeF2, CoF2, and MnF2
Authors: Brandmüller J. , Lehmeyer A. , Häussler K. M. , Merten L.
Published: July 2007

In the most general form the matter equations of electrodynamics contain the magneto‐electric tensor in addition to the electric and magnetic permeability tensor. By group‐theoretical methods, however, it is shown that for crystals belonging to the magnetic point group 4′/mmm′ the magneto‐electric tensor vanishes. Important examples for this point group are the antiferromagnets FeF2, CoF2, and MnF2. Regarding this essential simplification a generalized Fresnel's equation of the wave normals is derived for these antiferromagnetic crystals. The Fresnel's equation has two solutions describing the magnitudinal and directional dispersion of the refractive index for the two corresponding waves. The limiting cases of these two solutions agree with results already known from literature. Besides, the polarization of both waves is determined.
Low‐temperature fluctuational motion of dislocations in crystals I. Quantum and dissipative effects
Authors: Osetskii A. I.
Published: July 2007

A mechanism underlying fluctuational bending of dislocation segments is considered which is based on periodical ordering in the thermal motion of atoms near the dislocation core. Basic principles of calculating the probability of dislocation unpinning from local obstacles due to the fluctuations are formulated. As found, the interaction of dislocation segments experiencing fluctuational bending with phonons and/or conduction electrons, described in the framework of the usual dissipation model, can result in a reduction of the energy of atoms near the dislocation core during the fluctuation. Consequently, the effective activation energy of the dislocation unpinning from local obstacles is increased, i.e. the mean velocity of the activated dislocation motion drops. Activated dislocation motion at low temperatures is analysed with an account of the possible quantum mechanical fluctuations which can arise in condensed structures owing to the correlation of atomic zero point vibrations. An expression is derived for the probability of low temperature fluctuational dislocation with allowance for quantum mechanical effects.
Spin dependence and the interrelation between staggered and uniform field properties in the exact solution of one‐dimensional mixed‐spin ising ferrimagnets
Authors: Yousif B. Y. , Bowers R. G.
Published: July 2007

An exact statistical mechanical calculation of certain properties of a one‐dimensional mixed‐spin Ising model is presented. Particular attention is given to the behaviour at very low temperatures. Investigations are centred on questions of spin dependence and of the relation between uniform and staggered field properties.
Nonlinear Excitation of New Emission Bands in Kcl Crystals Containing Color Centers
Authors: Casalboni M. , Francini R. , Grassano U. M. , Pizzoferrato R.
Published: July 2007

New luminescence bands are observed in KCl containing F and F‐aggregate centers under strong excitation with an infrared laser. All these emission bands occur at energies higher than those of the pumping beam. These „up‐conversion”︁ emissions are assigned to transitions from higher excited states of color centers populated by means of multiphoton absorption. Specific relations between the centers and the emission bands are also discussed.
Kumakhov Radiation Energy Losses of Ultrarelativistic Electrons and Positrons
Authors: Kanloev A. M.
Published: July 2007

The process of spontaneous radiation from ultrarelativistic electrons and positrons in planar channels (the Kumakhov effect) is studied by means of computer modelling. The energy loss for radiation is investigated depending on the angle of particle in the crystal. A significant difference in the absolute energy losses of electrons and positrons is noted.
Investigation of the Electronic Structure of VFeH Alloys by Positron Annihilation
Authors: Szuszkiewicz M. , Chojcan J. , Ostrasz A.
Published: July 2007

The angular distributions of annihilation quanta (AD) coming from the two‐photon annihilation of a positron‐electron pair are measured for V0.96Fe0.04Hx alloys. The results are discussed in view of application to the description of the electronic structure of the system under consideration within the rigid band approximation. It is found that the explanation of results obtained requires the assumption of d‐like electron states in the vicinity of the Fermi energy capable to be occupied, beginning from a certain concentration of (4s + 3d)‐like electrons.
Investigation of Structural Defects in the Amorphous Ferromagnetic Alloy Fe40Ni40P14B6
Authors: Grimm H. , Kronmüller H.
Published: July 2007

Structural defects in the amorphous ferromagnetic alloy Fe40Ni40P14B6 are investigated by measuring the magnetic polarization in the region of approach to ferromagnetic saturation. From the field dependence of the high‐field susceptibility it is concluded that the main source of internal stresses may be characterized as quasi‐dislocation dipoles. These structural defects are shown to be rather stable under annealing treatments and may be produced by small plastic deformations.
Optical Phonons in Amorphous Silicon Oxides. I. Calculation of the Density of States and Interpretation of LoTo Splittings of Amorphous Sio2
Authors: Lehmann A. , Schumann L. , Hübner K.
Published: July 2007

Until now there exists no calculation of the phonon density of states of amorphous SiO2 and related materials, which could interpret the experimentally detected splittings between longitudinal and transverse modes in their optical phonon spectra. This problem is solved by taking into account the influence of the vibrationally induced polarization on the motion of atoms. A simple model for the description of this polarization in non‐crystalline SiO2 is developed and used for the calculation of the density of states of LO and TO phonons in the framework of a coherent potential approximation for structurally disordered systems. This new method leads also to the determination of the IR response function and to simple analytical expressions for the microscopic parameters of the model, i.e. the bond angle at the oxygen atom and transverse dynamic effective charges. The values of these parameters are determined from IR transmission spectra of thin SiO2 layers prepared with the help of different methods. The calculated density of states and IR response function are in good agreement with corresponding experimental results.
Manifestation of Spin–Rotation Coupling in Intersystem Crossing in a Rotating Impurity Molecule
Authors: Sildos I. , Sild O. , Dolindo I.
Published: July 2007

The dependence of the probability of the singlet‐triplet S1–T1 radiationless transition on the total angular momentum J is observed for an NO impurity molecule in the KCl matrix. This dependence disappears in the presence of a magnetic field (H ≧ 0.2 T), which breaks the spin‐rotation coupling.
The Quasimolecular Approach to the Electronic Structure Calculations for Silver and Copper Halides
Authors: Ermoshkin A. N. , Evarestov R. A. , Kuchinskii S. A. , Zakharov V. K.
Published: July 2007

The quasimolecular large unit cell (QLUC) approach in the framework of complete neglect of differential overlap (CNDO) approximation is used for the one‐electron energy level and wave function calculations in symmetry points of the Brillouin zone of silver and copper halides AgX (X = F, Cl, Br), and CuX (X = Cl, Br). The parametrization scheme of the CNDO approximation is discussed. The results obtained are in agreement with the results of other band structure calculations and with the experimental data. The calculations of the impurity ion Cu+ in the silver site are also carried out in the framework of QLUC‐CNDO approach for AgCl and AgBr crystals. The impurity levels have t2g and eg symmetry and are situated near the top of the valence band. The centres of impurity bands, caused by the periodical distribution of defects in this model, are shown to converge with the increasing quasimolecule size to the energy levels of a single defect.
On the Elastic Constants of a Nematic Liquid Crystal
Authors: Schiele K. , Trimper S.
Published: July 2007

Comparing an extended form of the Landau‐de Gennes model for the order parameter with the free elastic energy for the director field of a nematic liquid crystal the temperature dependence of the Frank elastic constants is found. The theoretical results are in agreement with both the recent measurements and with molecular statistical theories.
Deuteron Dynamics in Crystal of Sodium Trideuterium Selenite
Authors: Fedotov A. P. , Shabanov V. F.
Published: July 2007

A model of the deuteron dynamics in NaD3(SeO3)2 crystals without quantum mechanical tunneling is proposed. The possibility of deuteron jumps in hydrogen bond double‐minimum potentials is shown which is due to the deuteron‐deuteron interaction. The effect of the deuteron dynamics on the Raman spectrum is discussed. It is found that taking into account only dipole‐dipole interaction between the nearest neighbours is insufficient for the correct description of the deuteron dynamics and phase transition.
Critical Temperature of a Quasi‐One‐Dimensional Ising Model
Authors: Sokalski K. , Wójcik W.
Published: July 2007

A quasi‐one‐dimensional Ising system containing n chains (n = 1, 2, 3) in an effective field is investigated. The dependence of the critical temperature on the coupling parameter with the effective field and on the number of chains in the system is shown.
Electron–Phonon Interaction in Periodic Two‐Layer Structures
Authors: Pokatilov E. P. , Beril S. I.
Published: July 2007

The Hamiltonian of the interaction of electrons and spatially extended optical interface modes in two‐layer periodic structures is developed.
Distribution of the Atoms on Grain Boundaries and in the Bravais Cell of the Coincidence Site Lattice
Authors: An Nguyen , Herrmann R.
Published: July 2007

The distribution of the atoms on symmetrical and asymmetrical coincidence grain boundaries in s.c., f.c.c., b.c.c., diamond, and zinc blende lattices is calculated. A general method is developed which allows to determine the number of atoms and the structure of the two‐dimensional lattice of any grain boundary plane and of the planes next to grain boundary.
The Impurity Effect on the Phase Transitions in Quasi‐One‐Dimensional Conductors
Authors: Seidel Ch. , Prigodin V. N.
Published: July 2007

Using bosonization and replica methods the influence of impurities on phase transitions in quasi‐1D systems is studied within the framework of the 1D Fermi gas model. The random impurity potential is taken as a white‐noise type. It is shown that nonmagnetic impurities suppress the dielectric transition but do not influence the superconducting state. Magnetic impurities suppress both the transitions, but the dielectric one considerably stronger than the superconductor one. Some special features occuring in the commensurate case are studied. Implications for organic 1D conductors are discussed.
A Relationship Between the Universal Law of Susceptibility Response and the Correlation Weakening Principle
Authors: Nigmatullin R. R. , Hill R. M.
Published: July 2007

The observation of the universal response of electrial, mechanical, acoustic, and magnetic systems in which power law behaviour in the time and frequency domains is seen indicates the universal behaviour of correlation functions in these systems. It is shown that this behaviour can be obtained by making use of the correlation weakening principle which plays an essential role in quantumstatistical physics and which does not depend on the detailed structure of the many‐body system involved.
Photothermal Creation of Cu°v Centres and Mobility of Anion Vacancies in X‐Irradiated KCl:Cu Crystals
Authors: Zazubovich S. , Usarov A. , Egemberdiev Zh.
Published: July 2007

Associates of Cu° atoms with anion vacancies v (Cu°vcentres) are used as luminescent detectors for the study of anion vacancy mobility in KCl crystals. It is found that near 220 K, Cu°v centres may be thermally formed because of the migration of v only to its neighbouring Cu° atom (i.e. the process occurs in {Cu°–v} pairs). An analogous process in {Cu+–v} pairs results in the creation of Cu+v centres. After Cu+v have trapped electrons released at the thermal destruction of Cu°, Cu°v centres are formed. {Cu+–v} and {Cu+–v} pairs are produced at low temperatures by the transfer of an F electron onto an impurity ion in irradiation‐generated {Cu+–F} and {Cu2+–F} pairs. If the crystal contains Cu centres, Cu°v centres may be formed at T > 370 K by the interaction of mobile VF centres, produced at a thermal dissociation of Cl3 molecular ions, with Cu centres. It is proposed that at T < 300 K it is not the isolated but the “associated” anion vacancy that can migrate. This process occurs in the field of some defects and needs a comparatively lower activation energy than the motion of isolated v. The isolated or “free” v which are believed to be mobile at higher temperatures, are practically absent in X‐irradiated crystals at T > 250 K due to the interaction of v with mobile cation vacancies or interstitial halide ions as well as due to the thermal association of v with neighbouring lattice defects.
Statistical Thermodynamic Approach to Some Hyperstoichiometric Dihydrides of Lanthanides (La, Ce, Pr, Nd, Sm) and Actinides (Np, Pu, Am)
Authors: Shohoji N.
Published: July 2007

Statistical thermodynamics is applied to available equilibrium hydrogen pressure‐temperature‐composition relationships for cubic hyperstoichiometric dihydrides, MH2+x (O < x < 1, M = La, Ce, Pr, Nd, Sm; Np, Pu, Am). A statistical model is constructed based on a perfect face centred cubic lattice of metal atoms with fully occupied tetrahedral interstitial sites by hydrogen atoms and partly and randomly occupied octahedral interstitial sites by the rest of hydrogen atoms. Atomic partition function of hydrogen in MH2+x is found to be similar for all MH2+x. On the other hand, the atomic partition function of metals in MH2+x for different M is found to be different.