bluuebunny/crossref_metadata_2025 huggingface dataset:
The Thermodynamic Properties of Liquid Transition and Rare‐Earth Metals Based on CHS Model
Authors: Joarder R. N. , Das T.
Published: July 2007

The thermodynamic properties of liquid transition and rare‐earth metals are discussed on the basis of CHS model. The validity of the Gibbs‐Bogolyubov inequality within the thermodynamic perturbation theory is assumed. The small perturbation term is approximated by s‐d hybridisation energy in the atomic sphere approximation. The results indicate that the method yields thermodynamic quantities quite in agreement with the OCP model and much better than corresponding hard sphere results. Unlike the OCP model the expressions are obtainable analytically and in this sense the model is advantageous. The role of charge and packing on entropy calculation is also discussed in some length.
Heat Capacity and Lattice Anharmonicity in Cu‐III‐VI2 Chalcopyrite Compounds
Authors: Neumann H. , Kühn G. , Möller W.
Published: July 2007

The molar heat capacity at constant pressure is measured for CuGaS2 in the temperature range from 300 to 600 K and for CuGaTe2, CuInS2, CuInSe2, and CuInTe2 from 300 to 500 K with an estimated accuracy of ±1 J/mol K. The anharmonic contributions to the heat capacity at constant pressure and at constant volume are estimated and analysed in terms of perturbation theory. The volume Grüneisen functions of the compounds are evaluated and compared with existing literature data.
Electronic and Vibrational Properties of One‐Dimensional Quasi‐Crystals
Authors: He H.
Published: July 2007

The electronic structure of a one‐dimensional quasi‐crystal in the form of a Fibonacci sequence in tight‐binding model is extended to include next nearest neighbor couplings in addition to the nearest neighbor couplings previously included. The vibrational properties are also studied using negative eigenvalue theorem. The results of density of electronic states and square phonon frequencies are presented.
Material Constants of Composites. III. Development of the General Theory and Application to Alloys with Off‐Diagonal Disorder
Authors: Monecke J.
Published: July 2007

It is shown that the fourth order differential equation for the determination of effective material constants of composites or alloys from those of their constituents, derived for special cases in two previous papers, is universal in the sense that it can be applied to a wide class of physical properties. It is proved that the solution of this equation avoids multiple occupancies. Especially it can be applied to alloys with off‐diagonal disorder, too.
The Metal‐Semiconductor Transition in Amorphous Si1−xCrx Films Detection of the Transition by Room‐Temperature Measurements
Authors: Möbius A.
Published: July 2007

The relation between σ and dσ/dT is studied at room‐temperature. All samples investigated, which show activated conduction at low temperatures, fulfil σ(293 K) < 360 Ω−1 cm−1, whereas all samples with metallic conduction at low temperatures fulfil σ(293 K) > 440 Ω−1 cm−1. Within the regions 100 to 360 Ω−1 cm−1 and 440 to 1400 Ω−1 cm−1 nearly linear relations between σ and dσ/dT are observed. It seems likely that dσ/dT as a function of σ exhibits a knee at about 420 Ω−1 cm−1 indicating the presence of a sharp metal‐semiconductor transition at room‐temperature. The experimental results confirm quantitatively a phenomenological model of σ(T, x) in the semiconducting region, which is developed mainly on the basis of low temperature measurements in two preceding papers. On the other hand, doubts arise on the physical meaning of the ansatz α + βT1/2 + γT often used to analyse the metallic region at low T.
Optical Band Edge Absorption in CuInSe2
Authors: Medvedkin G. A. , Rud Yu. V. , Tairov M. A.
Published: July 2007

Optical properties are studied of (100) oriented CuInSe2 (D) crystals near the absorption edge. While the edge absorption spectra in the range 80 to 370 K are found to follow Urbach's rule, there are two regions in α(ħω) differing in the temperature dependence of Urbach's parameter. The results obtained suggest that the optical band edge absorption in CuInSe2 is dominated by electron–phonon interaction. Linear optical dichroism of the edge absorption in chalcopyrite‐structure CuInSe2 is revealed and investigated. CuInSe2 is established to differ from the other chalcopyrite crystals in a negative linear dichroism near the absorption edge.
Study of the Environment of Copper in Some Ternary Chalcogenides by EXAFS Spectroscopy
Authors: Deshpande A. P. , Sapre V. B. , Mande C.
Published: July 2007

The EXAFS data of the K absorption discontinuity of copper in some ternary chalcogenides obtained earlier are reanalysed using the graphical method originally proposed by Lytle, Sayers, and Stern. It is shown that the graphical method gives useful information about bond lengths, and also the environment of the absorbing atom. While the phase parameters of the first coordination shell α1 and β1 provide information about the forward and backscattering atoms, respectively, the total phase shift δ1 gives an idea about the periodicity of the EXAFS function. It is also shown that the periodicity is affected by the presence of the other atoms in the second coordination shell in the ternary compounds studied. The potentiality of the graphical method to probe the environment of atoms is demonstrated.
Theory of Higher‐Order Corrections to the Electronic Stopping Power of Solids
Authors: Shindo S. , Minowa H.
Published: July 2007

Higher‐order perturbation corrections are considered to the stopping power formula, i.e., the Bloch correction and the Barkas effect, which causes a difference of the stopping power for positive and negative charged particles. For this purpose, a semiclassical harmonic oscillator model is presented for the theory of the stopping power. It is shown that the contributions from the close collision is more important than those from the distant collision for the Barkas effect.
Mean Square Displacements of Neighbours and Phonon Resonance Due to a Vacancy in HgTe and CdTe
Authors: Madhavan Y. , Ramachandran K. , Haridasan T. M.
Published: July 2007

The mean square displacements (MSD) of the neighbours around a vacancy in HgTe and CdTe are worked out using Green's function technique and scattering wave formalism. The results are compared with the values of the perfect systems. The phonon resonance due to the vacancy for the above systems are also simultaneously worked out.
The Optical Absorption Edge in Thin Amorphous Oxide Films Based on Cerium Dioxide
Authors: Al‐Dhhan Z. T. , Hogarth C. A. , Riddleston N.
Published: July 2007

Measurements of optical absorption of thin film samples of CeO2/SnO2 and CeO2/GeO2 prepared by a co‐evaporation process at a pressure in the range 1.06 to 1.33 mPa, and having a thickness of ≈300 nm are reported. It is found that the fundamental optical absorption edge is sharp, much more like that expected for a crystalline material rather than the usual form for amorphous material, and the optical gap is increased by the addition of CeO2 to SnO2 and GeO2 films.
Temperature Dependence of Exciton Linewidth in CsI and CsBr
Authors: Beerwerth F. , Fröhlich D. , Leinweber V.
Published: July 2007

The temperature dependence of the linewidth of the lowest two Γ excitons in CsI and in CsBr and of a very narrow Γ exciton in CsI is reported. All spectra are measured by three‐photon spectroscopy. The analysis of the experimental results allows conclusions about the decay mechanism of excitons in alkali halides. The temperature dependent broadening is quantitatively described by taking into account exciton—phonon interaction.
Optical Properties of InS under High Pressure
Authors: Takarabe K.
Published: July 2007

Pressure effects are studied on the fundamental indirect gap (Eg), direct gap (B0), and next higher critical point (B) in InS single crystals by using a diamond‐anvil cell for the first time. A synchronized decrease of their energies is observed with pressure increasing up to 2 GPa. This unusual behavior is inferred partly due to a softening of InIn bond under pressure.
Theoretical Study of the Optical Absorption, Mössbauer, and EPR Spectra of Fe2+ Ion in Orthopyroxene
Authors: Zhou Yi‐Yang
Published: July 2007

The optical absorption, Mössbauer, and EPR spectra of Fe2+ ion in orthopyroxene, (Mg, Fe)SiO3, are theoretically studied using the approximated SCF d‐orbit of Fe2+ proposed by Zhao and Du. Most of the optical absorption spectra, especially the bands which have not been considered in past works, are assigned uniformly, and the Mössbauer data are fitted satisfactorily using these calculated values of the spectra. In addition, a comparison of EPR zero‐field splitting between the theoretical results and the experimental value shows that the contribution of the triplet of the Fe2+ ion in crystal is important for the zero‐field splitting.
The Faraday Effect Approach to a Study of AIVBVI Narrow‐Gap Semiconductor Band Structure
Authors: Martynchuk E. K. , Sizov F. F.
Published: July 2007

The magnetooptical Faraday effect in the transparency range ħhω ≦ Eg of Pb1−xSnxTe (0 ≦ x ≦ 0.23) and PbSe single crystals with carrier concentrations from 2 × 1016 to 2 × 1018 cm−3 is investigated under applied magnetic fields up to 6 T over the temperature range T = 4.2 to 300 K. g‐factors and effective masses of carriers are determined within the range of T = 4.2 to 300 K. It is shown that the experimental results with or without quantization of the energy spectrum may be interpreted only under condition ∣gc∣ > ∣gv∣, where gc and gv are the g‐factors of electrons and holes. Some new values of Dimmock six‐band model parameters g and g are obtained taking into account ∣gc∣ > ∣gv∣ condition.
The Role of Diffuse Scattering on Microdefects in the Precise Lattice Parameter Measurement
Authors: Holý V. , Härtwig J.
Published: July 2007

Diffuse scattering of X‐rays on microdefects play a substantial role in the interpretation of the results of lattice parameter measurements according to the Bond method. A lattice parameter can be defined on the basis of the elastically quasiperfect reference crystal. To determine this lattice parameter a diffuse scattering correction must be used.
Double‐Valued Band Representations of Monoclinic and Orthorhombic Space Groups
Authors: Evarestov R. A. , Smirnov V. P.
Published: July 2007

The simple double‐valued band representations of the space groups C, C, C, C, D, D, D‐D are generated. These space groups describe the symmetry of the most widely spread low‐symmetry crystal structures. The dependence of the band representations on the choice of origin and setting in the space group description is discussed.
Single Electron Tunneling
Authors: Apell P. , Tagliacozzo A.
Published: July 2007

Some of the most basic physical ingredients are discussed characterizing the observation of single particle tunneling in ultra small tunnel junctions. This includes charging energy versus image interaction and temperature, normal resistance of tunneling compared to the quantum resistance, and tunneling time versus time scale of introducing charges into the junction. Bounds are obtained for the resistance and capacitance of the junction to observe single particle tunneling and particularly the use of the recently developed scanning tunneling microscope to monitor this fundamental tunneling process is investigated.
Rigorous Ground State Energy of a Linearized Model of a Polaron in a Magnetic Field
Authors: Brosens F. , Devreese J. T.
Published: July 2007

An explicit analytical expression is derived for the ground state energy of a linearized polaron model in a constant and uniform magnetic field. In this model, the validity of the Feynman inequality in a magnetic field is investigated. In a fixed magnetic field, it is shown that even for arbitrarily small electron—phonon coupling, the presence of a magnetic field is prohibitive for the application of the Feynman inequality.
Density of States of a Binary Alloy with Short‐Range Order in a Bethe Lattice
Authors: Nazareno H. N. , Moreno I. Fonseca , Ribeiro‐Teixeira R. M. , Iglesias J. R.
Published: July 2007

A study is made of the electronic structure of a binary alley in a Bethe lattice with coordination number 4. The basis set of orbitals sp3 was chosen thinking in the tetrahedral structure shown by most semiconductors. Short range order (SRO) correlation is included in the occupancy in the lattice. The results show a strong dependence of the density of states on the assumed values of the SRO parameter.
The Size‐Quantized States of the Wannier‐Mott Exciton in Structures with Superthin Films
Authors: Pokatilov E. P. , Beril S. I. , Fomin V. M. , Litovchenko V. G. , Korbutyak D. V. , Lashkevich E. G. , Mikhailovskaya E. V.
Published: July 2007

Results are given of the experimental investigation of exciton absorption spectra in superthin films of PbI2 deposited on various substrates (glass, mica, metal). A theory of exciton states in thin polar films is developed, on the basis of which excitonic and band size effects are analyzed. The theory is in qualitative and quantitative agreement with the experiment.
Impurity Binding Energies in Heterostructures
Authors: Costa W. B. , Albuquerque E. L.
Published: July 2007

The transfer‐matrix theory for a one‐dimensional heterostructure is discussed. The applicability of effective‐mass approximation to the calculation of impurity binding energy is also presented. It is shown that both treatments lead to almost the same results which indicates the suitability of this approximation. Numerical results of the density of states are performed and their features discussed.
Low‐Temperature Conductivity of a Weakly Disordered, Strongly Anisotropic Two‐Dimensional System with Electron—Electron Interaction. Application to Germanium Bicrystals and to Surface Superlattices
Authors: Firsov Yu. A. , Nakhmedov E. P. , Prigodin V. N. , Weller W.
Published: July 2007

In the framework of a simple model for the electron gas in the grain boundary (inversion layer) of a semiconductor bicrystal or in a surface superlattice the quantum corrections to the conductivity tensor due to the electron—electron interaction are calculated. The results have a typical two‐dimensional form for w ≫ kBTħ/τ and show a crossing‐over to typical one‐dimensional behaviour for w ≪ kBTħ/τ. Here w⟂ is the transverse overlap integral betwween neighbouring dislocation tubes, τ the relaxation time due to scattering on the disorder. Also the weak localization corrections are calculated.
Spin Coherent States, Holstein‐Primakoff Transformations for Heisenberg Spin Chain Models, and Status of the Landau‐Lifshits Equation
Authors: Makhankov A. V. , Makhankov V. G.
Published: July 2007

The XXZ Heisenberg spin chain is considered. Following results are obtained: for the Bosonization of the spin Hamiltonians widely used Holstein‐Primakoff transformations are very sensitive to the way of breaking an initial system symmetry by the ground state. In the case of easy‐axis models only such transformations may be applied without previous analysis (at any rate when an external magnetic field vanishes). Others are subjected to the corresponding symmetry analysis.
Theory of Two‐Step Spectral Hole Burning by Pulses
Authors: Rebane I. K.
Published: July 2007

A two‐step spectral hole burning (SHB) is considered, where the selective excitation of the first step is fixed by a photochemical transformation through the second excitation step. It is shown that in a three‐level system excited by two consecutive pulses, the kinetics of the forming spectral hole differs essentially from the one on stationary irradiation. In the case of a short second pulse the shape of the hole in the inhomogeneous distribution function (IDF) of the purely electronic line frequency Ω01 is dependent on the time interval T between the first and second excitation steps and under certain conditions a hole can be obtained which is narrower than in the one‐step SHB with monochromatic light. It is considered how the hole in IDF is manifested in the absorption and two‐step pulse excitation spectra. It is shown that the hole in the absorption spectrum may be narrower than 2γ1 and in the two‐step pulse excitation, −γ1 (γ1 is the energy relaxation constant of the first excited level).
Bistability of Molecular Excitons
Authors: Antonyuk B. P. , Bussemer P. , Leine L. , Mukhamedov A. A.
Published: July 2007

The behaviour of a molecular crystal at high pumping level is described by a coupled system of two nonlinear differential equations for the density of excitons and the temperature. Analysis shows that a bistability and a step‐lie change of the temperature distribution and the quantum efficiency of luminescence occur for continuously varying pump intensity.
Thermal Effects on Thermophysical Properties of Alkali Cyanide Crystals
Authors: Chauhan S. K. S. , Agrawal R. , Pathak P. C. , Gupta B. R. K. , Goyal R. P.
Published: July 2007

An interionic lattice potential is developed on the basis of phonon‐pressure theory. This interionic potential is used to investigate the thermodynamic and thermoelastic properties of NaCN, KCN, and RbCN crystals. Thermal contribution is taken into account within the framework of HZ type short range overlap interactions, effective upto second neighbour ions. Cohesive energy and six distinct values of third order elastic constants (TOEC) for these crystals are calculated at both temperatures 0 and 300 K. Values of pressure and temperature derivatives of SOEC are also calculated corresponding to both temperatures. The results are discussed and compared with the available experimental data.
Multiple‐Scattering‐Theory Calculation of Elastic and Dielectric Properties of Alkali‐Halide Solid Solutions
Authors: Middya T. R. , Roy D. , Basu A. N.
Published: July 2007

With increasing concentration of defects in a crystal, when the defect–defect interaction starts dominating, the absence of a smallness parameter makes all the low concentration theories inapplicable. This interaction is treated here approximately via a mean field generated self‐consistently as envisioned in the multiple‐scattering‐theory. For the different alkali‐halide solid solutions considered, this approach reproduces both elastic and dielectric data at all concentrations without any parameter. The results thus obtained compare satisfactorily with experiment in most cases. Finally some of the limitations of this approach are pointed out.
The Behavior of Active Centers in a Laser Host. A Crystal Field Investigation on Dy3+ in LaF3 Single Crystal
Authors: Neogy D. , Purohit T.
Published: July 2007

The analysis of the observed absorption spectra of Dy3+ ion in the widely used laser host LaF3 is improved upon for D3h site symmetry and the magnetic thermal and hyperfine properties of this system are investigated using the crystal field parameters obtained. Except for one excited state, the fit to all the observed Stark states of the ground multiplet is quite good. The results of the study are discussed in the light of available experimental information.
Versatility of the Two‐Ion Quartic Spin Model of Spontaneous Metamagnetism
Authors: Szymański J. , Cofta H.
Published: July 2007

The interesting spin model, proposed in 1980 by Onyszkiewicz, involving a special two‐ion coupling of fourth order, is known to describe well and easily the spontaneous metamagnetism. Here one examines its possible power to describe also other classes of metamagnetism. For this purpose, thermodynamic properties of the model are calculated for many different values of interaction parameters. Indeed, two further classes of magnetism are found in some regions of the interaction space. Such a result confirms the supposed versatility of the model.
Temperature Dependence of Two‐Dimensional Optical Energy Gap for Germanium Selenide Single Crystals
Authors: Elkorashy A. M.
Published: July 2007

The interband absorption coefficient of GeSe single crystals near the fundamental absorption edge is analyzed on the basis of a two‐dimensional model and is found to correspond to an indirect forbidden transition. The optical energy gap at room temperature is found to be (1.066 ± 0.011) and (1.078 ± 0.010) eV for the a‐ and c‐crystallographic axes, which lie in the plane of cleavage. The temperature dependence of the two‐dimensional optical energy gap is studied from room temperature to near liquid nitrogen temperature. This dependence is found to be linear in the range from 96 to 250 K with a negative temperature coefficient dEg/dT equal to −0.48 and −0.46 meV per K for the a‐ and c‐axes, respectively. The results are compared with those obtained on basis of three dimensional model and agreement is found to be within the limits of experimental error.
Resonatorless Dissipative Optical Bistability. Influence of External Fields and Sample Thickness
Authors: Kochelap V. A. , Sokolov V. N.
Published: July 2007

The influence of sample thickness and carrier surface recombination on the characteristics of resonatorless dissipative optical bistability (RDOB) is investigated. The RDOB under consideration is caused by the excitation density nonlinearities of band‐edge absorption of laser radiation in semiconductors. The diversity of multivalued spatial carrier distributions in the sample of thickness d with arbitrary velocity of surface recombination S are considered. It is established that strong recombination can considerably affect the structure and properties of possible carrier distributions and hysteresis of the transmission.
Optical Bistability in Composite Media
Authors: Jungk G.
Published: July 2007

Optically bistable behaviour is demonstrated for systems consisting of optically nonlinear microcrystallites embedded in an optically linear metal or dielectric. The most simple version of the local‐field correction serves as feedback mechanism thus avoiding external mirrors. The dielectric function for macroscopic samples is calculated with Garnett's effective medium theory. Numerical examples for the reflectivity and the transmissivity are given for an embedded dielectric with exciton‐like nonlinearity.
Picosecond Plasma Dynamics and All‐Optical Data Processing
Authors: Collet J. H. , Pugnet M.
Published: July 2007

Using time resolved pulse and probe spectroscopy, the transmission modulation of CdSe platelets is investigated under excitation and generation of a dense electron–hole plasma. Induced absorption is related to exciton screening, band‐gap renormalisation, and energy relaxation in a high density electron–hole plasma. These investigations exhibit also interest from the point of view of optical signal processing because an all‐optical picosecond inverter gate and more generally a many input NOR gate can be built based on the induced absorption which occurs in the presence of a dense electron–hole plasma. The operation of such device is described and discussed in relation with the possible development of optical coprocessors (compatible with a parallel processing of data) and especially in relation with the critical problem of associating and cascading optical NOR gates. A first step is reported toward cascadable picosecond NOR gates by demonstrating the possible operation of an all‐optical inverter gate which uses an intermediate frequency doubling of the output signal. This implies that a cascadable all‐optical NOR gate operating in an effective one‐wavelength mode is possible. Experiments are carried out at liquid nitrogen temperature. The operation wavelength is around 673 nm in CdSe. The switching energy of the NOR gate is 30 μJ/cm2 per pulse at 337 nm.
Phonon Dispersion Curves of the Superconducting A15 Compounds Nb3Ge and Nb3(Al–Ge)
Authors: Gupta V. B. , Gupta H. C.
Published: July 2007

The phonon dispersion relations of superconducting compound Nb3Ge are calculated using an angular force constant model, the analysis being limited to third neighbour interactions only. The force constants are evaluated by fitting them to the measured values of phonons at the zone centre. The calculated results are found to be in very good agreement with the experiment in the case of Nb3Ge. With the help of this analysis the complete set of phonon dispersion relations in Nb3(Al–Ge) along the principal symmetry directions is determined. The obtained results nearly reproduce the measured Raman active phonons for Nb3(Al–Ge).
Single‐Ion Anisotropy and ESR Linewidth in Quasi‐Low‐Dimensional Heisenberg Magnets
Authors: Martynov S. N.
Published: July 2007

On the basis of the Kubo and Tomita theory the influence of single‐ion anisotropy on the ESR linewidth in a low‐dimensional Heisenberg magnet is estimated in the high‐temperature limit. Two spin correlation functions in r‐representation are used. The variation of the linewidth Δ𝒦(K) depends on the sign of anisotropy K, the value of spin S, and the magnetic dimension of system d. The linewidth can decrease only to a limited value, equal at S 𝒦, Δ𝒦(K) = Δ𝒦(0) · (2/5)(d+1)/3.
The Coherent Field in Infinite Disordered Systems of Point‐Like Scatterers. Refractive Index and Amplitude
Authors: Solbrig H.
Published: July 2007

The coherent field which is generated by a planar source is calculated as a sum of contributions arising from definite trajectories. The sum is performed by means of a method recently described by Warner and Gubernatis. Generalizing to effective propagators being capable to include returns and loops, comments are made on the primary‐field extinction and the quasi‐optical behaviour of the coherent field is analysed.
Infrared Reflectivity of High‐Tc YBa2Cu3O7–x and Related Ceramics
Authors: Petzelt J. , Kamba S. , Pačesová S. , Pollert E. , Šrámek J. , Smrčková O. , Sýkorová D.
Published: July 2007

Infrared reflectivity of several Y–Ba–Cu–O‐type ceramic samples with Tc ≈ 90 K is measured from 10 to 4000 cm−1 at room temperature and from 10 to 650 cm−1 as a function of temperature. The room temperature spectrum of the best‐surface Y–Ba–Cu–O sample is fitted using the model for the dielectric function consisting of the sum of the Drude term and five classical phonon oscillators. Due to the strong anisotropy the Drude term belongs predominantly to the E ‖ c electric field orientation whereas the phonon modes solely to the E ⊥ c orientation (Blu modes). The low temperature spectra in all samples show a systematic reflectivity increase below ≈ 180 to 220 cm−1 in agreement with recent results of other laboratories. The interpretation of this feature is discussed in terms of the gap opening below Tc. The smaller gap values than those obtained from tunneling experiments give an evidence for the strong gap anisotropy.
Raman Scattering from Vibrational Modes in Layered CdxHg1–xI2 Solid Solution
Authors: Anedda A. , Bongiovanni G. , Fortin E.
Published: July 2007

Raman spectra at 15 and 300 K of CdxHg1–xI2 solid solutions for 0 < x < 0.44 are investigated. One mode behaviour is observed for the low frequency phonons and the A1g mode, while a two mode behaviour is exhibited by the higher frequency vibrations. The expected mass, force, and temperature induced splitting of the B1g2 and Eg1 degenerated modes is not observed. This fact proposes another different experimental evidence of the low Raman cross section of the B1g2 rigid layer mode. For the first time the degeneracy of the Eg2 mode with the A1g is lifted and experimentally found.
Model for Orientational Defects in Quasi‐One‐Dimensional Ice Crystals
Authors: Yanovitskii O. E. , Kryachko E. S.
Published: July 2007

A model of the orientational kink defect in quasi‐one‐dimensional crystals of ice is proposed which takes the relaxation of the basic sublattice of the ice crystal into account. Some physical properties of the kink‐defect are evaluated within the continuum approach and compared with the experimental data. An original interpretation of the orientational defect is presented and discussed in view of elucidating the nature of the H‐bonding in ice and water. In particular, it is argued that a central H‐bond of the kink defect can be interpreted as a monomeric OH‐group.
On the Theory of Linear Diatomic Polymers
Authors: Fedyanin V. K. , Osipov V. A.
Published: July 2007

The model of linear diatomic polymers is considered. Both, the ground state and the phonon spectrum of a discrete AB chain are investigated. Taking into account the electron–phonon coupling as well as the finite‐band corrections, an expression for the sound velocity is derived. The continuum equations of motion in the finite‐band scheme are constructed. The kink solution is analyzed when the effects of an order of O(l/L), where L is a chain length, are included. It is shown that the low‐lying electronic excitations of an AB chain with fixed end atoms are the kink pairs where each kink has an irrational charge and a spin σ = 0 (or σ = 1/2). Depending on the value of the parameter α the odd‐membered chain with a free boundary is obtained to contain either a soliton or an electron–hole pair in its ground state. To compensate the irrational charge the creation of one soliton should be accompanied by a chain relaxation. In this case exactly one electron is removed from the valence band. At α = 0 all results obtained are valid for the trans‐polyacetylene model.
Theory of H(I) Centers in Vitreous Silicon Dioxide
Authors: Dianov E. M. , Sokolov V. O. , Sulimov V. B.
Published: July 2007

Electronic structure, optical, and ESR properties of a hydrogen atom bound in an oxygen vacancy in vitreous silicon dioxide are investigated by means of a semiempirical LCAO MO method (MINDO/3) in a molecular cluster approach. Three charge states of the defect are considered. The properties of the neutral defect prove to be in good agreement with experimental data on H(I) centers in silica glass.
New Aspects on the Electronic Conduction in Semi‐Insulating CdF2: Gd3+ Single Crystals
Authors: Triberis G. P. , Guskos N. , Kiriakidis G.
Published: July 2007

Experimental results on the DC conductivity of semi insulating CdF2:Gd3+ single crystals are presented. The effect of the electric field and the temperature is analysed using In σ versus E1/2 and T−2/5 plots as suggested in the multi‐phonon‐assisted small polaron hopping model introduced by Triberis. The agreement of the experimental results with this model is very satisfactory giving a meaningful interpretation of the transport properties of this material.
Extinction Length Variation in a Distorted Crystal
Authors: Smirnova I. A. , Entin I. R.
Published: July 2007

It is shown theoretically and confirmed experimentally that the correction to the extinction length τ depends on the displacement wavelength λs in a typical dispersion manner, which is associated with the resonance interbranch scattering at λs = τ. For long waves, the correction is negative and tends to zero as λ in the limit λs/τ → ∞. For λs < τ, the correction is positive and, in the limit λs/τ → 0, it is described by the Debye‐Waller factor. For arbitrary distortions the sign and the magnitude of the correction are determined by the characteristic size of the deformation field. Short‐wave distortions make dynamical reflections narrower and long‐wave distortions broaden them. In the former case diffraction occurs from the average lattice and is characterized by a reduced structure amplitude, whereas in the latter case scattering from the average lattice cannot be taken into consideration.
Elastic, Dielectric, and Photoelastic Constants of Alkali Halide Crystals
Authors: Hans D. , Sangachin A. A. S. , Kumar M.
Published: July 2007

The interionic potential model recently developed by taking account of the composite effect of short range repulsive and dispersion forces along with three body interactions is extended in the present work to calculate the (i) third and fourth order elastic constants, (ii) first and second order pressure derivatives of second order elastic constants, (iii) photoelastic constants, volume derivatives of dielectric constants, and optic mode Grüneisen parameters. The calculated values of various physical quantities are found to present close agreement with experimental data.
Single‐Electron Deexcitation of Volume Plasmons Induced by Heavy Ions in Thin Solid Foils
Authors: Burkhard M. F. , Rothard H. , Groeneveld K.‐O. E.
Published: July 2007

In heavy ion (C+, N+) induced electron spectra (Ee < 30 eV) differential in angle and energy, of thin C (x = 370 nm), Cu (x = 100 nm), and Al(x = 160 nm) targets pronounced structures in forward direction superimposed on the flanks of the “true” secondary electron peak are observed. The energies of these structures are well correlated with the plasmon energies hωp of the materials in question. The present data offer good evidence for the existence of the single‐electron deexcitation process of volume plasmons in thin solid films.
The Hyperfine Fields and Their Pressure Derivatives in (Y0.9Hf0.1)Fe2 and (Zr0.9Hf0.1)Fe2
Authors: Budzyński M. , Subotowicz M. , Niezgoda H. , Spustek H. , Tańska‐Krupa W. , Wasiewicz R.
Published: July 2007

Measurements of the hyperfine fields Bhf on Ta nuclei in (Y0.9Hf0.1)Fe2 and (Zr0.9Hf0.1)Fe2 are performed by means of TDPAC (time‐differential perturbed angular correlation) method. The pressure derivatves of Bhf are also calculated. The results confirm that on R site a magnetic moment exists. It stems from a diffuse negative spin density induced from Fe.
Local and Quasi‐Local Modes of Fe and Zn in CdTe
Authors: Lu W. , Ye H. J. , Yu Z. Y. , Zhang S. Y. , Fu Y. , Xu W. L. , Shen S. C. , Giriat W.
Published: July 2007

The far infrared absorption and reflection spectra of the mixed crystals Cd1−xFexTe and Cd1−x. ZnxTe with small x in the temperature range of 4.2 to 300 K and the wavenumber range of 20 to 350 cm−1 are reported. Local modes induced by Fe and Zn (196 and 173 cm−1), quasi‐local modes induced by FeTe and ZnTe (56 and 54 cm−1) in CdTe and CdTe‐like 2TA two‐phonon absorption (72 cm−1) are observed and estimated by use of the Green's function method. The random‐ element‐isodisplacement (REI) model is also used to calculate the two‐mode‐behaviors of the mixed crystals and to best fit the reflection spectra. Besides, the temperature dependences of the phonon absorptions are calculated and well explained.
Impurity Effects in the Raman and Luminescence Spectra of Nitrogen‐Doped GaP
Authors: Dos Santos M. P. , Hirlimann C.
Published: July 2007

Gallium phosphide doped with up to 1019 cm−3 nitrogen atoms, is used to show impurity induced effects in both, Raman and luminescence spectra. A simple way to distinguish between Raman and luminescence is given, which incidently allows to observe an intermediate behavior for some peaks. The results can be split into two different classes: the ones which are dependent on the nature of the impurity, like trapped exciton lines in the luminescence spectrum or localized Raman modes, and the ones which are independent from the nature of the impurity, like disorder activated first‐order Raman scattering (DAFORS). The Raman results are obtained under resonant conditions.
Modified Thomas‐Fermi Approximation for Accumulation Layers in MIS Structures
Authors: Übensee H. , Paasch G. , Zöllner J.‐P. , Handschack S.
Published: July 2007

A modified Thomas‐Fermi method for the calculation of the electronic structure of accumulation layers in MIS‐systems is presented which in spite of the simplicity of a “classical” method yields results in good agreement with extensive self‐consistent quantum mechanical calculations. It is shown that the modified Thomas‐Fermi method is suitable to describe the characteristics of bound and mobile carriers in accumulation layers. The method is used to calculate accumulation layers in n‐ and p‐type GaAs and to discuss the influence of the quantum effects at the interface on the dependence of the total charge in the accumulation layer on the surface band bending. Comparison is made with the classical results.
Effects of Electron–Electron Interactions on the Nature of the Eigenstates in Amorphous Materials
Authors: Brezini A. , Bendaoud M. , Sebbani M. , Behilil F.
Published: July 2007

A Hubbard model with random site energies is used to investigate the electron localization effects associated with both disorder and electron‐electron interactions on a Cayley tree. Extensive results are reported on the ground states properties of the system. In particular, two regimes are found: if the electron–electron interaction parameter U is greater than the disorder parameter W, it appears to be significant in localizing electronic states otherwise it may be viewed as weakly perturbating the effects of the disorder.
Localization in a System of N Coupled Chains. Recursion Method. I. General Approach
Authors: Weller W. , Kasner M.
Published: July 2007

The localization length of an electron is studied, which moves under the influence of a random potential in a system of N coupled chains. First, a closed recursion relation for the reflection amplitude for one chain is derived. It is based on a simple diagrammatic interpretation of this amplitude in the framework of Berezinskii's real space diagram analysis. Then, the method is generalized to the case of N coupled chains. The calculation of the localization length is reduced to the determination of a stationary probability density of some parameters characterizing the reflection properties of such a system in the asymptotic regime for large length. The general method provides a numerical algorithm for the calculation of interesting quantities too.
Stability of a Superconducting Condensate of Bisolitons
Authors: Davydov A. S. , Ermakov V. N.
Published: July 2007

An equation for the condensate wave function of a bisoliton gas is derived when the current carrier density is finite. The stability of a bisoliton condensate generating high‐temperature superconductivity in ceramic oxides is studied. The values of the critical condensate velocity and the electric current density are found. The weak dependence of the critical temperature on the heavy ion mass is explained. The dependence of the critical temperature on the charge‐carrier concentration is elucidated.
Calculation of Matrix Elements for Optical Transitions
Authors: Fritzsche V.
Published: July 2007

A careful analysis of the formulas for optical interband matrix elements yields expressions in which additional surface integrals appear. These additional terms cannot be neglected in calculations of optical properties, because both the initial and the final states have extended wave functions. – A calculation of the imginary part of ϵ(0, ω) for copper shows that an application of the ∇V‐matrix element leads to considerable errors at low exciting energies.
Electron Raman Scattering in Semiconductor Quantum Wells
Authors: Riera R. , Comas F. , Giner C. Trallero , Pavlov S. T.
Published: July 2007

Differential‐cross‐section for a type of electronic Raman‐scattering process in a semiconductor quantum‐well heterostructure is calculated considering inter‐valley transitions between the system of sub‐bands provided by electron confinement. The intermediate states consist of virtual electron–hole pairs belonging to sub‐bands from the conduction and valence bands respectively. The T = 0 K case is assumed and transitions with the participation of phonons are not included in the analysis. The quantum‐well is modeled with infinite potential barriers at the interfaces and the effective‐mass‐approximation is utilized. The dependence of the cross‐section with respect to the incident and emitted photon frequencies is analyzed for arbitrary polarization of the secondary radiation field. Singularities in the emission spectra as well as the step‐like character of the emission and excitation spectra are predicted. Numerical results and estimations of the process quantum‐efficiency for the case of a GaAs/AlxGa1−xAs heterostructure are reported.
Properties of the Quantum Hall Effect of the Two‐Dimensional Electron Gas in the n‐Inversion Layer of InSb Grain Boundaries under High Hydrostatic Pressure
Authors: Kraak W. , Nachtwei G. , Herrmann R. , Gliński M.
Published: July 2007

The magnetotransport properties of the two‐dimensional electron gas (2DEG) confined at the interface of the grain boundary in p‐type InSb bicrystals are investigated. Under high hydrostatic pressures and in high magnetic fields (B > 5 T) the integral quantum Hall regime is reached, where the Hall resistance ϱxy is quantized to h/e2j (j is the number of filled Landau levels of the 2DEG). In this high field regime detailed measurements are given of the resistivity ϱxx and the Hall resistance ϱxy as function of temperature T and current density jx. An unexpected high accuracy of the Hall resistance ϱxy at magnetic field values close to a fully occupied Landau level is found, despite the high value of the diagonal resistivity ϱxx. At high current densities jx in the quantum Hall regime (j = 1) a sudden breakdown of the quantized resistance value associated with a jump‐like switching to the next lower quantized value h/2e2 is observed. A simple macroscopic picture is proposed to account for these novel transport properties associated with the quantum Hall effect.
Theory for n‐Surface Charge Layers in Hg1−xCdxTe MIS Structures
Authors: Zöllner J.‐P. , Paasch G. , Gobsch G. , Übensee H.
Published: July 2007

The energy gap of the Hg1−xCdxTe‐system can be controlled by the mole fraction x of Cd. This offers the possibility to use this material as a model system for systematic investigations especially in the narrow‐gap region (corresponding to 0.16 ≦ x ≦ 0.25). The electronic structure, subband occupations, subband energies, and cyclotron masses are calculated for the two‐dimensional electron gas in MIS structures with this material using an approximate self‐consistent method. Both, inversion and accumulation conditions are considered. Comparisons with existing limited theoretical results and with experimental ones a) show the accuracy of the method used and b) allow to interpret the detailed experimental results from Shubnikov‐de Haas and cyclotron measurements available now. Predictions are made on the composition dependence of the electronic structure especially for inversion conditions.
Non‐Equilibrium Spin Polarization of the Si‐S1 Centre in Silicon Induced by Spin–Orbit Coupling
Authors: Minaev B. F. , Serebrennikov Yu. A. , Rempel H. D.
Published: July 2007

Electronic structure calculations of the neutral charged oxygen‐substitutional impurity in silicon are carried out within the framework of the semi‐empirical MINDO/3 method. The structure of the substitutional impurity in the silicon lattice is simulated by finite molecular cluster OSi4H12 with an equilibrium SiH bond length 0.152 nm. Account of electronic interaction for single and double excitations allows to explain the nature of the impurity displacement in silicon. The cluster choosed serves as a good model for Si‐S1 centres. The results of spin–orbit coupling calculations give information about processes determining the appearance of non‐equilibrium spin polarization in these centres. The quantitative interpretation of Si‐S1 centre ESR‐spectra angular dependence in the helium region of temperature is carried out on the basis of the solution of the equation of motion for a triplet density operator. The dependence of the stationary concentration of triplet centres on magnetic field and crystal orientation is obtained.
IR Absorption Peaks and Oxygen Vacancies in YBa2Cu3Ox
Authors: Shi T. S. , Zhao Y. G. , Cai P. X. , Gu H. F. , Xie L. M.
Published: July 2007

Three absorption peaks (620 to 640, 590, 550 to 580 cm−1) are observed in the IR spectra of YBa2Cu3Ox. The variation of intensity and frequency of these peaks with annealing temperature and oxygen content is investigated. The nature of IR peaks is studied and assignment to various CuO bond‐stretching vibrations is made and supported by the correspondence between peak frequency and appropriate CuO bond‐length. Based upon the correlation between peak intensity and oxygen content, these peaks are further interpreted as “quasilocalized” vibration mode of the CuO bond induced by the oxygen‐vacancy in the one‐dimensional CuO chain and two‐dimensional CuO network. The corresponding vacancy formation energy is found to be 0.34 and 0.93 eV, respectively. These peaks may serve as an useful indicator of oxygen content, crystal structure, and superconducting properties of YBa2Cu3Ox.
Elastic Constants and Phonon Spectra of Alkali Metals. Dynamical Treatment with Temperature‐Dependent Two‐Body Interaction
Authors: Singh N. , Banger N. S. , Singh S. P.
Published: July 2007

An exponentially damped two‐body interaction obtained in second‐order perturbation theory using a rational dielectric function and Heine‐Abarenkov model potential is used to calculate the elastic constants of alkali metals. The interaction is also used to predict the cohesive energy of these solids and the phonon spectra of rubidium. The agreement between predicted and observed values is as good as obtained with the corresponding theory for simple metals. The temperature dependence is included through a damping factor as obtained by Takanaka and Yamamoto.
Difference Equations in the Theory of Biphonons
Authors: Jäger E. , Mossa M. , Hehl K.
Published: July 2007

Using the method of difference equations the bound states of excitations in a linear chain are investigated. It is possible to obtain solutions of the partial difference equations and the energy of “biphonons” by a separation ansatz. The method of power series expansion is not so successful. The spectrum of the states with two phonons in two coupled linear chains (as a model of the system film‐substrate) is investigated additionally.
Optical Properties of Quasi‐1D n‐Merized Molecular Semiconductors
Authors: Yartsev V. M.
Published: July 2007

Interacting dimers and trimers in quasi‐one‐dimensional organic semiconductors are modelled by a linear supercluster of six molecules. The general expression for the complex frequency‐dependent conductivity of the compounds composed from such superclusters is derived and the electron coupling to the totally symmetric intrameolcular modes of vibration is taken into account. Electron eigenstates and eigenvalues of the supercluster are calculated in terms of Hubbard model in the limiting case of infinite Coulomb repulsion (U → ∞). Explicit calculations of the real part of the complex conductivity predict the shift of electronic excitation band to the lower energy as the interaction between dimers or trimers increase.
Calculation of Dipole Transition Matrix Elements in Crystals by the Relativistic Green's Function Method
Authors: Shilkova N. A. , Shirokovskii V. P.
Published: July 2007

An analog of Green's theorem for the Dirac operator is obtained, which allows to reduce the integration over the outer part of the Wigner‐Seitz cell volume to that through the muffin‐tin sphere surface. Within the relativistic variant of the Green's function method, matrix elements of the velocity operator are computed in the veritable relativistic and c → ∞ cases. A comparison is made with the momentum matrix elements calculated by the nonrelativistic technique.
Electric Conductivity of a Metallic Ferromagnet and Magnetization Process
Authors: Zakharov Yu. V. , Mankov Yu. I. , Titov L. S.
Published: July 2007

The dependence of the conductivity and the magnetoresistance of a multi‐domain metallic ferromagnet on the magnetization when current flows along a domain wall is studied. The expression for the conductivity is obtained as function of x‐coordinate which is normal to the domain wall plane. It is shown that the region of current localization in a compensated ferromagnet has a size in the x‐coordinate of the order of the cyclotron radius near a single domain wall and becomes of the order of the decreasing domain width under magnetization. The magnetoresistance sharply increases in an external magnetic field near the saturation region. A comparison is made with the experimental data on the magnetoresistance of a ferromagnet with domain structure. It is shown that the appearance of nuclei of reverse‐magnetized domains leads to a resistance drop in the beginning of the magnetization process.