bluuebunny/crossref_metadata_2025 huggingface dataset:
Relations between Characteristics of Point Defects and Bulk Properties of Solids a Criticism of the Varotsos‐Alexopoulos Model
Authors: Bekf D. L. , Neumann G. , Szabó I. A.
Published: July 2007

Using the similarity of the effective potentials seen by ions in solids general relations between characteristics of point defects (i.e. the volume vi, the entropy si and the energy Qi of formation, migration, and activation) and bulk properties of solids are derived. It is shown that the basic assumption of the Varotsos‐Alexopoulos‐model (VA‐model) is not necessarily valid. Using dimensional analysis the validity conditions of this model are expressed in a form of partial differential equation from which relations “derived” by Varotsos and Alexopoulos are obtained. Their relations are different from the here obtained results because the ratios of vi, si, and Qi are proportional to expressions containing only bulk parameters of the solid, while in the relations obtained by us there are proportionality factors depending on i, i.e. these constants are different for the formation, migration, and activation (formation plus migration) processes. It is also shown that the basic assumption of the VA‐model is not valid for a harmonic solid. Experimental data for f.c.c. metals definitely confirm these results instead of those obtained by Varotsos and Alexopoulos.
Correlation Mechanism to Change Lattice Parameters and Phase Transitions in Compounds with Intermediate Valency
Authors: Manakova L. A. , Ionov S. P.
Published: July 2007

For compounds with intermediate valency (IV) a microscopic mechanism of changing the parameters of crystal lattice depending on the impurity concentration (or pressure) and temperature is suggested. The mechanism is based on the exact account of the Coulomb correlation and hybridization between f‐ and d‐electrons. For that the “non‐Fröhlich” terms in the electron—phonon interaction proportional to the first and second derivatives of the Coulomb interactions and overlapping integrals of f‐ and d‐electrons are considered. These terms in the electron—phonon interaction contain an explicit dependence on the interatomic distance and cause additional (as compared to the termal) shifts of the lattice sites on account of the Coulomb correlations. The corresponding additions for the lattice parameter depend on the concentration of the impurities x (or pressure) and temperature T. The dependences of the lattice parameter on x and T are obtained for both, the semiconductor and metallic phase. The equation for the transition temperature into the state with IV and the existence condition of this state with concentration are obtained. A comparison of the dependences with experimental ones obtained on the basis of the correlation mechanism is made on the example of the solid solutions Sm1−xRS (R3+ is a rare earth element for instance Gd, Y etc.).
Collective Excitations in Microcrystal Aggregates and Composites
Authors: Monecke J.
Published: July 2007

The optical properties of aggregates of microcrystals are dominated by surface excitations. The Raman and ir absorption spectra of polar microcrystals show, however, peaks at the bulk TO‐phonon frequencies, too, the occurrence of which depends on the sample preparation, i.e. on the aggregate microstructure. A recently developed theory for the dielectric function of composites of different topologies is applied to the calculation of the phonon spectra. It is shown that this theory explains the occurrence of bulk TO‐phonon peaks in appropriate microstructures.
Theoretical Investigation of XPS, UPS, and BIS in Rare‐Earth Compounds — Thulium Monochalcogenides
Authors: Nizhnikova G. P. , Farberovich O. V.
Published: July 2007

The investigation of electron spectra in thulium monosulphide and monoselenide is based on the calculations of relative intensities for multiplet transitions fN → fN±1 in the intermediate coupling using the results of self‐consistent band calculations as a zero approximation of perturbation theory. Correct interpretation of all experimental data in rare‐earth compounds is shown to be actual only in the framework of the common calculational model of excitations from highly localized 4f‐states to delocalized band states.
Polariton Theory of Light Propagation in Crystals II. Crystal Optics Approximation
Authors: Koinov Z. G. , Glinskii G. F.
Published: July 2007

Using the method of successive integrations first over rapidly varying photon fields and then over slowly varying photon fields a new tensor ϵαβ(Q, ωv) is defined that provides both, the equation for obtaining the polariton spectra ωv(Q) and the relation between the displacement D(Q) and the electric field E(Q) corresponding to the (v, Q) normal mode in crystals.
Optical Studies of Electronic Transitions of Mn2+ Ion in Na2Mn3Cl8 Single Crystals
Authors: Talapatra D. , Mukherjee R. K.
Published: July 2007

Low temperature polarised absorption, and emission as well as excitation spectra of Mn2+ in Na2Mn3Cl8 single crystals are reported. Eight d—d bands are observed in the optical spectra with some additional fine features. Analysis of the spectra indicates octahedral site symmetry around the hexacoordinated Mn2+ ion. The general features of the spectra are analysed quite satisfactorily within the framework of the strong field scheme of Tanabe and Sugano including Trees' correction. The finer spectral details are accounted for by invoking vibronic interactions involving different sites.
Electronic Properties of Vacancy‐Induced Structures in AlAs/GaAs Superlattices
Authors: Wang En‐Ge , Zhang Li‐Yuan , Wang Huai‐Yu
Published: July 2007

For the first time a detailed description is presented of electronic properties in (AlAs)n/(GaAs)n (n = 1, 3) superlattices with different vacancies. The local‐ and partial‐DOS of the vacancy neighbor atoms are calculated within the tight‐binding approximation including nearest and next‐nearest neighbor interactions. The shift of the band‐edge in the point‐defect structures is about 0.14 eV, which is very similar to that of bulk GaAs material and confirm that the top of the valence‐band of these kind of thin superlattices is derived from the As anion p states, and the bottom of the conduction‐band from the Ga or Al cation. Also a negative center is found around a group‐III atom vacancy in the bulk and a local field, which is similar to that of an acceptor impurity, across the interface. A comparison with AlAs/GaAs superlattices with and without impurities is given.
Hole Velocity in an Electric Field for Valence Bands Degenerate at k = 0
Authors: Dargys A.
Published: July 2007

In p‐type semiconductors having spherical constant energy surfaces and parabolic dispersion law the velocity of the hole immersed in an electric field F, as known, is described by the simple formula Ft/mh,1, where t is the time and mh,1 the effective mass of heavy or light holes. The paper shows that inclusion of intervalence tunneling between heavy and light mass subbands leads to a more complicated time dependence of the hole velocity, especially near the point of degeneracy, where the wave vector k = 0. The curves of the time dependence of hole velocity for parameters typical to p‐Ge and for some values of the electric field and initial hole wave vector are presented. The paper also demonstrates how the theory of angular momentum can be adapted to the problems of hole dynamics in valence bands degenerated at k = 0.
Theory of the Residual Resistivity Dipole Diffusion of Non‐Interacting Particles Around a Fixed Point Scatterer
Authors: Lenk R.
Published: July 2007

Employing a superposition representation of the microcanonical density matrix the perturbation of the stationary diffusion process by a fixed point scatterer is treated approximately. The primary dipole distributions of carrier and current density are calculated and interpreted without additional assumptions. The stationarity of the current is shown explicitly. The primary dipole is localized in a region limited by the mean free path. The classical diffusion process which follows removes this localization, the electrostatic screening by the ensemble of all electrons restores it. The ideas of Landauer are confirmed within a coherent theoretical scheme.
Spin‐Canting Effect and Magnetic Moment of LiZn Ferrite
Authors: Zhang Shouye , Fong Guangyao , Mao Ming
Published: July 2007

The magnetic moment at 0 K of zinc‐substituted lithium ferrite is calculated, based on the spin‐canting model, for an accurate relationship between the moment and the substituent level of zinc. A modified computational plan is adopted in the calculation in contrast with those of recent publications so that a hitherto optimal fitting between the theory and experiment is realized. A generalization is also made to the case of double‐substituted LiTiZn ferrite.
NMR of 167Er and Magnetic Structure of Er3Fe5O12 Garnet
Authors: Savosta M. M. , Doroshev V. D. , Novák P. , Veltruský I.
Published: July 2007

NMR of 167Er in Er3Fe5O12 garnet is studied at temperatures 2.1 to 4.2 K using the spin echo method. Two spectra centered at 448.5 and 691 MHz are observed and an unambiguous correspondence between them and two inequivalent positions of Er3+ ions is established. With raising temperature the relaxation time of the spectrum at 448.5 MHz decreases rapidly and at 4.2 K its detection is difficult. The magnitudes of the magnetic moments of the Er3+ ions are determined and the resonance frequencies are discussed within the framework of the crystal field theory and isotropic exchange approximation for the Er3+ ion.
Optical Studies on Europium Doped K2SO4
Authors: Kumar V. S. Kishan , Sastry S. B. S. , Acharyulu B. S. V. S. R.
Published: July 2007

Optical absorption (OA), fluorescence, and thermoluminescence are studied on γ‐irradiated K2SO4:Eu crystals. The OA spectrum shows a prominent peak around 5.33 eV which is identified as due to europium ions in their divalent state (Eu2+). On irradiation, the population of Eu2+ is found to increase. Fluorescence shows the characteristic Eu2+ emission around 3.05 eV. The glow curve consists of an intense peak around 365 K with two weaker ones around 415 and 435 K. The emission under all these glow peaks is the same and consists of a single band around 3.06 eV attributable again to the characteristic emission of Eu2+ ions.
The Quantum Theory of the Optical Activity of Ionic Dielectric Crystals
Authors: Kotsur S. S. , Stasyuk I. V.
Published: July 2007

In the frames of the field theoretical approach an analytical expression for the dielectric permeability tensor of ionic dielectric crystals, containing the contributions of higher multipole momenta, is obtained, taking into account the spatial dispersion. The expression for the tensor describing optical activity is separated; its analysis is performed. The Hubbard operator formalism and the two‐time Green's function method are used.
Effects of Correlated Rotation of Linear Molecules in the Low‐Temperature Dynamics and Thermodynamics of Weak Molecular Solutions
Authors: Kokshenev V. B. , Zholonko N. N.
Published: July 2007

The paper proposes a pseudospin approach to describe low‐temperature dynamics and thermodynamics of weak molecular solutions of linear molecules in atomic cryomatrices. On the basis of a microscopic Hamiltonian, a direct analogue of a weak ortho‐parahydrogen solution, the characteristics energies of various types of intermolecular interactions are estimated, i.e. the direct quadrupole‐quadrupole one and the indirect ones, dynamic and static, which involve phonons. The thermodynamic characteristics (specific heat, thermal expansion, and the Grüneisen parameter) of the rotational subsystem of the 14N2‐Ar solution are calculated with consideration of longrange correlations in the molecule rotation. A comparison with experimental data on the specific heat of 14N2‐Ar is made.
Gap Modes and Local Density of States of Impurities in RbCl and KBr
Authors: Rath B. K. , Ram P. N.
Published: July 2007

A nearest‐neighbour defect model is employed to calculate the local density of states of substitutional impurities K+ and Br− in RbCl and Rb+ and Cl− in KBr. A detailed investigation of resonance condition is made and possible resonance modes are identified. Contrary to earlier assumption of gap modes in all the systems, resonance modes are identified except in KBr:Rb+ in which a gap mode is found.
Quantum‐Chemical‐Calculations of Models for Surface Defects in Dehydroxylated Silica
Authors: Lygin V. I. , Serazetdinov A. D. , Chertikina O. I. , Ryabenko E. A.
Published: July 2007

A quantum‐chemical analysis is made of surface defects formed during dehydroxylation of silica. Calculations are made using the MNDO method with optimization of geometric parameters of silica clusters. The results obtained show that surface dehydroxylation leads mainly to the formation of siloxane bridges with different rates of strain. The formation of other types of defects can occur only at high rates of dehydroxylation.
Densities of Interstitial Electrons in Pd(Ni)Noble Metal Alloys
Authors: Dębowska E. , Rudzińaska‐Girulska J.
Published: July 2007

The ion core radii rc and interstitial electron densities nint for Pd, Ni, Ag, and Cu and for binary random alloys of these metals throughout the concentration ranges have been calculated on the basis of positron annihilation data. The differences in rc and nint for pure metals are suggested to be the main reason of different electronic structure of Pd–Cu and Pd(Ni)–Ag(Cu) alloys being the representatives of VCA and split‐band alloys, respectively.
Correlation Effects of the Antiferromagnetic Phase in the Hubbard Model
Authors: Wel G. Z. , Nie H. Q. , Zhang K. Y.
Published: July 2007

Using series expansion of local approach, the electron coupling between nearest‐neighbour sites is considered to investigate the electron correlation effect in the Hubbard model. The results show that when the off‐diagonal elements of the density matrix are considered in the calculation, it leads to an enhancement of double occupancy and are duce of the formation of the local moments.
Ab Intio Calculations on the Adsorption of H, O, and CO atop Cu2+ on MgO(001)
Authors: Bacalis N. C. , Kunz A. B.
Published: July 2007

The adsorption of hydrogen, oxygen, and carbon monoxide atop Cu2+ in a Mg‐vacancy on the MgO(001) surface is studied in a cluster Hartree‐Fock approximation. The H atom binds slightly atop Cu2+ with binding energy of 0.05 eV. The O atom binds with Cu2+ in a quartet spin state with binding energy of about 0.12 eV. In the doublet state it does not bind to Cu2+. The CO molecule in a Cu2+CO geometry perpendicular to the surface binds to Cu2+ with binding energy of about 0.2 eV. In the Cu2+OC geometry it binds directly with Cu2+ with about 0.33 eV. In both geomtrie, the atom of CO closer to the Cu2+ has some more electronic charge near the surface, than in free space. In all three substances, mutual exchanges of electrons occur between z‐directed orbitals. It seems that the adsorption of CO is rather electrostatic in nature (via the dipole moment) than covalent, where a donation and a back donation occurs. In both geometries the donation is mainly from the 5σ CO orbital. The back donation from Cu2+ occurs mainly with the 2σ* CO orbital in the Cu2+OC geometry. In the Cu2+CO geometry it occurs mainly with a σ orbital.
The Enrgy Transfer between Free Excitons, Bound Excitons, and the 3d‐Shell of Mn2+ in Zn1−xSe Mixed Crystals and the s–d Type Exchange Interaction
Authors: Stutnbäumer U. , Gumlich H. ‐E. , Zuber H.
Published: July 2007

Excitation spectra of the Mn2+ emission band 4T1 → 6A1 of Zn1−xMnxSe and their dependence on the crystal temperature and the manganese concentration (x ≦ 0.016) gives evidenve of an excitonic energy transfer from free and bound excitons to the 3d‐shell of Mn2+. Reflection spectra and the comparison of excitation maxima of free and bound excitons reveal some steps of the transfr of excitonic energy to the luminescence centre Mn2+ in Zn1−xMnxSe. The full spinsplitting of bound excitons which is not determined by former luminescence experiments is observed (up to 90 meV at B = 5 T) in excitation spectroscopy of the Mn2+ emission band and the s–d exchange constant relation in Zn1−xMnxSe is determind to β/α = −5.5 ± 0.6.
Impurity Scattering in Weakly Compensated Gapless Semiconductors at Low Temperatures
Authors: Mikheev V. M.
Published: July 2007

An anomalous decrease in mobility with rising temperature is attributed to the contribution by scattering on acceptor–donor pairs. The change in the concentration of acceptor–donor pairs is related to the merging of pairs into large entities — “clusters”. In the case of “clusters” the scattering on charge fluctuations is essential which is more effective than that on individual pairs. An increase in cluster volume with rising temperature results in lower mobility. Within the framework of the model suggested, the electrical conductivity is calculated numerically for a material that has the parameters of HgCdTe. The results of numerical calculations agree well with experiment.
New Approach to the Theory of High‐Temperature Superconductivity
Authors: Fukuhara M. , Smith G. W.
Published: July 2007

A new approach to the theory of high‐Tc superconductivity in cuprate oxides is proposed, based on the possibility of superelectrons derived from inner closed shells in shrunken atoms induced by a strong screened attractive interaction between an electron and an ion. An electrostatic fluctuation connected with an electronic polarizability halts the breaking‐up of the pre‐existing pairs of electrons with antiparallel spins in the shells. The transport of superelectrons can be explained by the virtual tunneling.
The Long‐Wavelength Limit of the Structure Factor of Liquid Alkali Metals Using a One‐Component Plasma System
Authors: Derouiche A. , Bretonnet J. L.
Published: July 2007

The one‐component plasma (OCP) is used as reference system to calculate the long‐wavelength limit S(0) of the structure factor of liquid alkali metals within the random phase approximation (RPA). It is found that the OCP system is suitable to describe S(0) when the plasma parameter corresponds to the minimum variational upper bound to the free energy.
Structure and Electrical Resistivity of Alkali–Alkali and Lithium‐Based Liquid Binary Alloys
Authors: Mishra A. K. , Sahay B. B. , Mukherjee K. K.
Published: July 2007

Harmonic model potential, developed and used for simple metals is applied here to evaluate hardsphere diameters, which ensure minimum interionic pair potential for alkali–alkali (Na–K, Na–Rb, Na–Cs, K–Rb, K–Cs) and lithium‐based (Li–Na, Li–Mg, Li–In, Li–Tl) liquid binary alloys as a function of composition for use in the determination of their partial structure factors. These structure factors are then used to calculate electrical resistivities of alloys considered. The computed values of electrical resistivity as a function of composition agree both, in magnitude and gradient reasonably well with experimental values in all cases except in Cs systems, where the disagreement is appreciable.
Computation of Vacancy Formation Energy of Liquid Metals
Authors: Das T. , Nammalvar T. , Bhuiyan L. B. , Joarder R. N.
Published: July 2007

A formulation of the vacancy formation energy involving pseudopotential and total pressure is utilized to calculate the vacancy formation energy for liquid alkali (monovalent) metals and liquid In, Al, and Pb (polyvalent). Both, the one component plasma and the charged hard sphere models are employed to simulate the systems studied. The calculated vacancy energies are in good agreement with experiment.
Orientational Effects Due to Traveling of Fast Neutrons and Hard γ‐Quanta through Crystals
Authors: Baryshevskii V. G. , Zaitseva A. M.
Published: July 2007

The paper presents orientational effects, which are observed during the passage of neutrons and γ‐quanta with an energy of the order of a few MeV through crystals. It is shown that for neutral particles channeling effects are also rather strong. Calculated results agree with the experiment.
A Contribution to DC Transport in Thin Bi Films
Authors: Gerlach E.
Published: July 2007

A generalization of the energy‐loss transport concept, which includes carrier–carrier‐scattering, is applied to a thin (two‐dimensional) Bi‐film deposited on an e.g. CaF2‐substrate. In particular, scattering by ions is considered and by charge density fluctuations. An additional scattering channel is taken into account by a scattering frequency ωτ. If the concentrations of electrons and holes are about equal, the resulting resistance of the Bi‐film at not too low temperatures is governed by scattering by fluctuations.
Effect of Lattice Vibrations on the Change in Work Function
Authors: Patra S. , Kumar A.
Published: July 2007

A change in work function is calculated by extending previous calculations beyond Hartree‐Fock approximation. The chemisorption energy and the change in the work function are also found to depend on the strength of adatom–electron interaction by including the effects of lattice vibrations on adatoms on a metal surface.
Auger Recombination in Disordered Semiconductors. Effect of the Band‐Gap Fluctuation
Authors: Quang Doan Nhat
Published: July 2007

The Auger recombination coefficient is calculated for disordered semiconductors with fluctuating band gap which can be described by the smooth random Gaussian field model. The effect of different random bending of the conduction and valence bands on Auger transitions is investigated for the case of non‐degenerate carrier statistics. An integral relationship for the Auger coefficient is derived which is analogous to the Dexter representation for the optical absorption. For various recombination processes general analytic expressions of the Auger coefficients are obtained, which exhibit simple linearly exponential functions of the random‐field parameters, and the Auger recombination might be strongly enhanced as compared to that in the appropriate fluctuationless system. In contrast to the case of case of uniform random band bending, the influence of the band gap fluctuation is shown to depend on the type of recombination process and the effective masses of the material as well. Moreover, this effect is especially sensitive to the kind of relative band bending. Numerical estimates indicate that the Auger transition might still remain as one of the most effective recombination mechanisms for non‐crystalline semiconductors.
Dependence of the Line Strength of f–f Transitions on the Manifold Energy. I. Projector on the Basis of Nonorthogonal Functions
Authors: Kornienko A. A. , Kaminskii A. A. , Dunina E. B.
Published: July 2007

The method of construction of effective operators in nonorthogonal basis states with the help of projection operators is suggested. According to this method the localization of the ground and excited states are preserved. The expressions for the effective operator are given in third order of perturbation theory.
The Electronic Structure of Copper Oxide Crystalline Compounds. I. LUC‐CNDO Approach to the Electronic Structure of Cu2O and CuO Crystals
Authors: Evarestov R. A. , Veryazov V. A.
Published: July 2007

LUC‐CNDO computation scheme and its valence theory applications are discussed. Electronic structure calculations of well‐known semiconductors Cu2O are reviewed. The parameters of CNDO scheme for copper–oxygen compounds are calibrated using the results of Cu2O band theory calculations and experimental data.
Effect of Fluctuations in Delta‐Doping Superlattices on Donor–Acceptor Pair Luminescence
Authors: Zehe A. , de la Cruz D.
Published: July 2007

Donor–acceptor pair luminescence represents a recombination channel depending on the crystal site separation of donors and acceptors in semiconductors. This fact turns out prolific when doping superlattice structures are concerned. The donor–acceptor‐pair distribution function is very sensitive in respect to the mutual positions of doped layers in superlattice structures. Effects of disorder or non‐ideal arrangement of doped layers in the one‐atomic length scale are thus accessible to luminescence experiments.
First‐Order Quantum Corrections to the Current Driven by a Force and Einstein Equivalence
Authors: Lenk R.
Published: July 2007

Employing a superposition representation of the density matrix for a fixed energy the first order quantum corrections to the current driven by a weak constant force are calculated for independent charge carriers scattered by uncorrelated point‐like scatterers. This complements a preceding treatment of the corresponding diffusion problem and demonstrates the Einstein equivalence as a property of the density matrix itself. The changes in kinetic energy caused by the force field lead to a reformulation of the correction terms which resemble now the known vertex structure of the Bethe‐Salpeter equation. The influence of the force is considered explicitly in the propagator construction.
The Magnetic Field Influence on a Superconducting Bisoliton Condensate
Authors: Brizhik L. S. , Davydov A. S. , Ermakov V. N.
Published: July 2007

It is shown that in terms of the bisoliton model the effect of a magnetic field on ceramic high‐temperature superconductors is qualitatively different from that on superconductors that satisfy BCS theory. The energy gap and the critical temperature change little with increasing field and sharply become zero in fields exceeding the critical value.
Mean‐Field Theory for Spin Glasses with Many Macroscopic States
Authors: Janiš V.
Published: July 2007

Assuming from the beginning that there may be many macroscopic states in the Sherrington‐Kirkpatrick model, a new many‐state free energy is derived for one configuration of spin‐spin couplings and a new technique for averaging it. It is shown that the free energy of Thouless et al. must be extended to define the averaging scheme or the thermodynamic limit with many macroscopic states.
Magnetic Properties of Single Crystals of Bismuth Doped with Lead and Tin
Authors: Banerjee D. , Bhattacharya R.
Published: July 2007

Measurements made of magnetic susceptibility of single crystals of bismuth doped with tin and lead (two each) are reported in the temperature range 100 to 300 K. Both, tin and lead have marked effect in changing the band structure of bismuth. The temperature variation of χ∥ shows that in one region there is an increase in observed diamagnetism followed by a region of decrease of diamagnetism. This is shown to be due to the decrease of both, the dia‐ and paramagnetic contribution of excited free carriers residing in different bands.
Field Influence on the Parameters of Ferromagnetic Semiconductor Superlattices Formed by Coherent Light Beams
Authors: Grechko L. G. , Ogenko V. M. , Semchuk A. Yu.
Published: July 2007

The behaviour is studied of the electron–magnon system of a ferromagnetic semiconductor in an external constant electric field and a high‐frequency field of coherent light beams. It is shown that in a field of coherent light beams the system of nonequilibrium electrons and magnons of a ferromagnetic semiconductor forms several superlattices differing in their parameters. The superlattice parameters are found to depend on the electron density, sample temperature, heating field strength, and on the amplitude and frequency of the coherent light beam field.
Moment Expansion of the Optical Second‐Harmonic Response Tensor of Condensed Media
Authors: Keller O.
Published: July 2007

With main emphasis on studies of optical second‐harmonic generation in centrosymmetric media new expressions for the local and near‐local parts of the nonlinear conductivity response tensor are derived. The expressions are obtained via a moment expansion of the fully nonlocal response. The new results allow a direct comparison between the multipole expansion formulation used in studies on dielectrics and the delocalized approach often adopted in works on the metals. Contact is made to recent investigations of the nonlinear response of a flat surface.
Investigation of Electron Mobility in a Two‐Dimensional Electron Gas. GaAs–AlxGa1−xAs Heterostructures, Application of the Hydrostatic Pressure Method
Authors: Kończewicz L. , Litwin‐Staszewska E. , Maslowska A. , Piotrzkowski R. , Robert J. L. , André J. P.
Published: July 2007

Hydrostatic pressure experiments are a very useful tool for 2D electron transport effect investigations as they allow the electronic properties of the heterojunction to be precisely modified without changing the intrinsic properties of the material. The hydrostatic pressure technique is applied in 2D gas mobility investigations in GaAlAs/GaAs heterojunctions in liquid helium temperature range. The experimental mobility versus 2D electron concentration and temperature dependences are compared with the theoretical predictions and it is shown that the mobility variations are generally described by the scattering by charged centers in the GaAlAs layer.
Optical Behaviour of Lead Selenide Films
Authors: El‐Ocker M. M. , Sharaf F. , Yossef S. M. , Metawe F. , El‐Desouki M.
Published: July 2007

Lead selenide thin films of thicknesses between 80 to 250 nm, are prepared by thermal evaporation under vacuum of ≈ 1.3 × 10−3 Pa. X‐ray diffraction studies reveal the formation of polycrystalline films. The optical gap is determined as a function of film thickness. It is observed that the width of the optical gap lies between 0.3 to 0.4 eV, additionally it shows the tendency to decrease with increasing film thickness. Ellipsometry is used to estimate the optical constants and the absorption coefficient. Two well defined peaks are observed at energies of about 1.8 and 2.9 eV. The data are analysed in the frame of the band structure of PbSe2.
Transfer Matrix Method to Compute Energy Levels of Superlattices
Authors: Pavesi L. , Reinhart F.‐K.
Published: July 2007

Using the transfer matrix method the Kronig‐Penney model is generalized to superlattices formed by whatever successions of layers. Imposing the boundary conditions, the miniband structure as well as the envelope wave functions are obtained. As examples, the “enlarged well in a superlattice” problem and the Fibonacci superlattices are discussed. The Lyapunov coefficients as a function of energy are calculated to determine the localization properties of the eigenstates in the entire superlattice spectrum. The single channel Landauer formula is used to get the conductance spectrum. Results are presented for random disordered superlattices.
Magnetisation and ESR Studies of Gd(Ni1−xRhx)5 Compounds
Authors: Tari A. , Hwang J. S.
Published: July 2007

Magnetisation and electron spin resonance measurements are performed on Gd(Ni1−xRhx)5 compounds with x = 0, 0.1, 0.2, 0.3, 0.5, and 1.0. In the ordered state the value of the spontaneous moments is 6.5 μB/f.u. in GdNi5, rising to 6.7 μB/f.u. at the GdRh5 end of the series. Both θp and Tc show rapid variation with Rh substitution, particularly at the GdNi5 end of the series where dθp/dc ≈ 1.1 K/at% Rh. The ESR measurements indicate that the system is bottlenecked. The substitution of Rh for Ni reduces the slope of the linewidth from 7 × 10−4T/K in GdNi5 to 4 × 10−4T/K in GdRh5 and raises the g‐value from 1.923 in GdNi5 to about 2.000 at the GdRh5 end of the series; both indicative of a change in the character of the conduction electrons from predominantly d‐like in GdNi5 to predominantly s‐like in GdRh5. From the g‐shift a value of J = −0.005 eV is obtained for the effective exchange integral. Both, ESR and magnetic data indicate that the density of d‐states goes through a minimum close to the GdNi5 end of the series.
Reflection Spectra of Some Garnet and Orthoferrite Single Crystals in Vacuum Ultraviolet
Authors: Kučera M. , Kolobanov V. N. , Mikhailin V. V. , Orekhanov P. A. , Makhov V. N.
Published: July 2007

The near normal reflectivity of single crystal garnets (Y3Fe5O12, Y3Fe4GaO12, Ca3Fe2Ge3O12, Y3Al5O12, Gd3Ga5O12), and yttrium orthoferrite (YFeO3) are studied over the energy range from 5 to 30 eV using synchrotron radiation. The spectra of complex permittivity ε1, ε2, the optical constants n, k, α, and the energy loss function Im(1/ε) are obtained by means of Kramers‐Kronig analysis. The optical spectra are very similar in this region and are only slightly affected by the different chemical composition of the samples. An interpretation of basic features of the spectra is proposed in terms of a simple energy band model.
Diffusion in Crystals with Competing Interactions of Migrating Ions
Authors: Zubkus V. , Kundrotas P. , Lapinskas S. , Kashcheev V.
Published: July 2007

Transport processes are investigated by means of the lattice gas model with nearest and nextnearest neighbour attractive and repulsive interactions. The temperature and concentration dependences of self‐diffusion coefficient and electroconductivity in the disordered homogeneous phase of these systems are calculated by the cluster approximation. The regions of deviation from the Arrhenius law are determined. The spatial‐time distributions of migrating particles are obtained. The influence of strong short‐range interactions on the relaxation time of the diffusion process is investigated. Several experimental results of diffusion in systems with strong shortrange correlations are discussed by the theory proposed.
Calculation of Energy Loss Characteristics of Accelerated Ions Passing through Thin Targets
Authors: Akkerman A. F. , Fain V. E.
Published: July 2007

The possibility is discussed of checking the formula for the impact parameter dependence of energy losses in an inelastic collision, comparing calculated and experimental mean energy losses and straggling of ions passing a thin foil. Some results of Monte‐Carlo calculations are presented.
Equations of Order–Disorder Kinetics in Alloys with B. C. C. and F. C. C. Lattices. Tetrahedron Approximation
Authors: Bakai A. S. , Fateev M. P.
Published: July 2007

The kinetic equations describing the order‐disorder in alloys with b. c. c. and f. c. c. lattices are derived taking into account atom correlation. The consideration is based on the master equation using a superposition approximation. It is supposed that atom movements are controlled by a vacancy mechanism. For the first time, the closed kinetic equations describing the kinetics of ordered alloys with b. c. c. and f. c. c. lattices in four‐particle tetrahedron approximation are obtained. As clusters the tetrahedron atom configuration is chosen. For the system in a stationary state the equations obtained give the same results as a variation cluster method.
Crystal Growth, Raman and Infrared Spectra of BaFI Single Crystals
Authors: Sieskind M. , Ayachour D. , Merle J.‐C. , Boulou J.‐C.
Published: July 2007

Single crystals of BaFI are synthetized and their Raman and infrared properties are investigated at room temperature. The reflectivity is measured between 50 and 4000 cm−1 using a Fourier transform infrared spectrometer. The experimental reflection spectra are analysed by means of the classical oscillator model and the Kramers‐Kronig transformation to obtain the TO and LO frequencies and the optical constants. The position of the Raman and infrared modes of the matlockite series (PbFCl) are qualitatively discussed. The most significant force constants of the dynamical matrix are estimated from the high frequency Raman modes.
Lattice Dynamics of Alkali Metals
Authors: Mishra S. K. , Singh T. N.
Published: July 2007

The lattice dynamics of alkali metals is investigated on the basis of a phenomenological model which is consistent with the translational symmetry of a lattice. The potential energy is considered due to (1) ion‐ion and (2) ion‐electron interactions. The contribution to ion‐ion interaction is evaluated applying the valence force field. The contribution to the potential energy from ion‐electron interaction is considered through the screened Coulomb potential. The theoretical model developed is employed to compute the phonon dispersion curves of Na, K, Li, Rb, and Cs. Computed phonon frequencies are used to interpret the Debye characteristic temperatures at different temperatures. The calculated results for phonon dispersion curves and Debye characteristic temperatures compare well with the experimental ones.
Charge Carrier Energy Spectrum in Multilayer Structures and Superlattices in the Field of the Self‐Action Potential
Authors: Pokatilov E. P. , Beril S. I. , Semenovskaya N. N. , Fahood M.
Published: July 2007

The manifestation of the self‐action potential in spatially heterogeneous structures is investigated theoretically. The expressions for the self‐action potential energy of a test charge placed in a central layer of the three‐layer structure or in either of the superlattice layers are obtained. The numerical results which correspond to the exact formulae are compared with the approximate ones which correspond to the analytical expressions, for which the criteria of validity are deduced from this comparison. The electron energy spectra are obtained from both, numerical and analytical calculations accounting for the self‐action potential energy. The applicability range of analytical results is established.
A Tight‐Binding Calculation of Band Discontinuities in the AlxGa1−xAs/GaAs(AlAs) Heterostructures
Authors: Gushchina N. A. , Nikulin V. K.
Published: July 2007

Valence (ΔEv) and conduction (ΔEc) band discontinuities at the AlxGa1−x As alloy‐GaAs(AlAs) crystal interface are calculated within the tight‐binding approximation basing on the bulk electron properties of the individual components and taking into account the presence of a dipole layer at the interface. The alloy is considered in the virtual crystal approximation using the self‐consistent parameters of the sp3s* Hamiltonian obtained for the GaAs and AlAs crstals. The calculated value of the ratio of Qv = ΔEv/ΔE to Qc = ΔEc/ΔE, where ΔE is the difference between the direct band‐gap widths for the crystal and the alloy, is found to be 0.33:0.67 for all values of the Al concentration which is in good agreement with recent optical measurements. These parameters permit to obtain within the tight binding approximation the values of concentration x at which a direct‐indirect crossover occurs in the alloy (xc = 0.38) and a transition occurs from the first to the second type of heterojunction in the AlxGa1−xAs/AlAs structure (xc2 = 0.25), values being close to the experimental data.
The One‐Dimensional Plasma Model
Authors: Yanetka I.
Published: July 2007

The Hamiltonian of the one‐dimensional electron‐ion system is proposed. The relevant forces between electrons and ions are of the Coulomb‐type. Besides, the repulsive pseudopotential in the form of the delta‐function is included in the electron‐ion interaction. The stability of the system against lattice distortion is examined. The effects of the periodicity appear to strengthen the transition to the state with lattice distortion. The region of the parameter values describing the stable system against lattice distortion is defined.
Dielectric Function of La2CuO4
Authors: Vielsack G. , von Baltz R.
Published: July 2007

For La2CuO4 the longitudinal, frequency and wavenumber dependent dielectric function is calculated on the basis of a two dimensional LCAO model. The results are in agreement with much more elaborated investigations, provided these are available.
Magnetic Field Splitting and Dispersion of the Lower 1S‐Exciton Polariton in KI and RbI
Authors: Fröhlich D. , Kirchhoff St. , Köhler P. , Nieswand W.
Published: July 2007

Resonances on the lower polariton branch of KI and RbI are measured by the use of three‐photon difference‐frequency generation (TP‐DFG). From these data the energy of the lowest transverse exciton state is derived. Due to the small linewidth of the resonances it is possible to measure the k‐dependent magnetic field splitting of the lower branch for the first time. The results allow to determine the exchange energy, the longitudinal‐transverse splitting and the g‐values of the uppermost valence band (Γ‐symmetry) and the lowest conduction band (Γ‐symmetry) with much higher accuracy than before.
The Crystalline Cluster Method for Ionic Solids. The F Center in NaCl
Authors: Guimarães P. S. , de Siqueira M. L.
Published: July 2007

The electronic structure of the F center in sodiun chloride is investigated by the multiple scattering Xα method with proper crystalline embedding conditions. Several different clusters are used in order to determine the influence of the choice of the muffin‐tin radii and Xα parameters on the cluster energy spectrum. The calculated value of the first absorption line is compared with the experimental results and other theoretical work. It is found that the results obtained agree very well with experiment, provided clusters with at least 27 atoms are used in the calculation.
Coulomb Gap in n‐ZnSe and Its Critical Behaviour in the Region of the Metal–Dielectric Transition
Authors: Kasiyan V. A. , Nedeoglo D. D. , Simashkevich A. V. , Timchenko I. N.
Published: July 2007

In the temperature interval from 1.65 to 300 K the conductivity of n‐ZnSe crystals on the dielectric side of the metal‐dielectric transition (MD) induced by a change of the compensation from 0.39 to 0.98 or of the concentration of the donor impurity from 2 × 1016 cm−3 to 2 × 1018 cm−3 is studied. It is established that at sufficently low temperatures on the dielectric side of the MD transition Q ∼ exp (ToT)1/2 which is indicative of the presence of a parabolic Coulomb gap at the Fermi level. The main parameters of the Coulomb gap: width, depth, density of states are determined and their concentration dependences also investigated. It is shown that the compensation blurs the MD transition and shifts it to the region of large critical concentrations. The critical behaviour of the Coulomb gap parameters within the scope of the scaling theory and the dynamics of its collapse at the transition are described.
Weak Pair Exchange Interactions between Cobalt Ions in Quasi‐One‐Dimensional Antiferromagnet CsCoCl3
Authors: Voronkova V. K. , Mosina L. V. , Usachev A. E. , Yablokov Yu. V.
Published: July 2007

The EPR spectra of single Co2+ −Co2+ pairs in CsMg1−xCoxCl3 (x = 0.0001 to 0.04) in X‐ and Q‐ bands are investigated. One type of single Co2+ spectra and four types of additional spectra attributed to Co2+ Co2+ pairs coupled by weak spin‐spin interactions are observed. Using computer simulation of EPR spectra the parameters of spin‐spin interaction are determined. The observed EPR spectra and the expected pair Co2+ Co2+ interactions in CsMg1−xCoxCl3 are analysed. The exchange parameters for the nearest neighbour Co2+ −Co2+ pairs in the ab‐plane are estimated. The nature of the exchange anisotropy is discussed.
Embedded‐Atom Method for Surface Lattice Dynamics in Copper
Authors: Luo Ningsheng , Xu Wenlan , Shen Shuechu
Published: July 2007

The embedded‐atom method (EAM) based on results from density functional theory is applied to surface lattice dynamics. From the model within the framework of the EAM, the formula of surface interplanar force constant and surface lattice dynamical matrix are derived. It is found that the EAM can distinguish well the difference between atomic interactions in the bulk and near the surface without any additional parameter by its description of many‐body interaction. It is also found that the changes of surface force fields in the EAM are caused not only by the changes of interatomic distances but also by the changes of electron density background, which is quite different from ordinary empirical potential models. As an example, in the quasiharmonic approximation, the surface interplanar force consants and the surface‐phonon dispersions on unrelaxed Cu(100), (110), and (111) surfaces are calculated and analyzed without any adjustable parameter. The calculated results are in good agreement with experimental data by electron‐energy‐loss‐spectroscopy (EELS) and inelastic helium scattering. Although only the vibrational properties of Cu unrelaxed surfaces are calculated, it can be concluded that the EAM gives two competitive physical reasons affecting surface force fields: interatomic distances and electron density backgrounds.
Excitons in Indium Halide Thin Films
Authors: Brothers A. D. , Feuerborn F. J. , Bauer S. E.
Published: July 2007

Optical absorption spectra for thin films of InBr, InCl, and Inl are studied as a function of temperature from 90 to 300 K and as a function of hydrostatic pressures up to 0.35 GPa. Temperature shifts of the exciton peaks in InBr, InCl, and InI are −1.87, −0.5, and −1.1 in units of 10−4 eV/K, respectively. Pressure shifts of these peaks are reported as −2.5, −0.9, and −3.0 in units of 10−10 eV/Pa, respectively. A discussion of the implications of these results on the band structure of these materials is presented.
A Contribution to Reversed Transport
Authors: Gerlach E.
Published: July 2007

A generalization of the energy‐loss transport concept including carrier‐carrier scattering is applied to a periodic arrangement of pairs of conducting films. It is shown that for certain parameters a metal‐p‐semiconductor system should exhibit reversed transport (carrier‐drag) in the semiconductor, i. e. the holes move in the opposite direction relative to the external electric field.
On the Density of States of a Disordered Peierls‐Fröhlich Chain
Authors: Fischbeck H. J. , Hayn R.
Published: July 2007

Using the supersymmetry method an exact analytic formula for the integrated density of states of a disordered half‐filled Peierls‐Fröhlich system is derived. All kinds of disorder which can be handled in the continuum approximation, i. e. forward and backward scattering as well as (alternating) bond disorder, are included. Special cases are discussed.
Temperature Dependence of EPR Linewidth of Impurities in Van‐Vleck Paramagnets
Authors: Aminov L. K. , Kostetskii Yu. Yu.
Published: July 2007

The specific spin‐lattice relaxation mechanisms (“fluctuation” and “Waller” mechanisms) of impurity paramagnetic ions in rare‐earth Van‐Vleck paramagnets are shown to be the effective mechanisms of EPR line broadening too. On the basis of these mechanisms one can interpret qualitatively and, in a great extent, quantitatively all peculiarities of the temperature dependence of Gd3+ impurity ion EPR linewidth in rare‐earth zircons, observed by Mehran, Stevens et al. It is concluded, that in interpretation of their experimental results one can avoid the consideration of the hardly perceptible quasi Jahn‐Teller effect on excited doublets of the host Van Vleck ions.
Energy Transfer Mechanisms in Zn1−xMnxSe (0.005 < x < 0.017) Detected by ODMR
Authors: Zink K. , Krost A. , Nelkowski H. , Sahm J. , Stutenbecker H.
Published: July 2007

ODMR investigations of Zn1−xMnxSe (0.005 < x < 0.017) are presented. Starting from the observation of the resonance of the Mn2+ ground state in the Mn emission and in the SA emission as well, the necessary energy transfer is investigated using excitation dependent ODMR. The spectra, taken for the first time for this system show that the occurrence of the Mn2+ ground‐state resonance in the Mn emission can be referred to a spin dependent transfer of optical excitation energy. In the SA emission, on the contrary, a cross relaxation between manganese and donor‐acceptor pairs is responsible for the occurrence of the Mn2+ resonance in the ODMR spectra.