bluuebunny/crossref_metadata_2025 huggingface dataset:
Exoemission theory: The exciton model
Authors: Merkin M. M. , Tsertsvadze A. A.
Published: July 2007

A possibility of defect annealing of the alkali halide structure with free exciton formation is discussed and a two‐stage exciton model of exoelectron emission (EEE) is suggested. According to this model, within the scope of the mechanism previously proposed by the authors, the defect annihilation results at the first stage in formation of an exciton which can ionize an electron centre (the second stage of the process). The probability of an elementary act for the NaCl crystal is calculated. The dependence of the exoemission parameters and energy spectra of exoelectrons on the crystal orientation is studied.
Zero‐field splitting and the d–d transitions of Mn2+ on Ca(II) sites in Ca5(PO4)3F
Authors: Wan‐Lun Yu , Min‐Guang Zhao
Published: July 2007

On the basis of the crystal field theory the energy matrices of a d5 ion in Cs (or C2) symmetry are derived, which are applicable to all orthorhombic and tetragonal symmetries. Using the perturbation theory the analytical expressions connecting the zero‐field splitting parameters with the crystal field parameters are obtained for a 6S‐state ion in arbitrary symmetry. It is shown that the zero‐field splitting of Mn2+ on Ca(II) sites in Ca5(PO4)3F, possessing Cs point symmetry, can be reasonably understood by the spin–orbit coupling mechanism, and that the d–d transitions can be interpreted well by the crystal field model proposed by Zhao et al.
Photoluminescence in a‐Si1−xCx:H films
Authors: Siebert W. , Carius R. , Fuhs W. , Jahn K.
Published: July 2007

A study of steady‐state and time‐resolved photoluminescence (PL) and optical properties of a‐Si1–xCx: H‐films prepared by glow‐discharge decomposition of gas mixtures of SiH4 and CH4 is presented. With increasing carbon content x the PL‐spectra shift to higher energies and broaden considerably. Simultaneously the quantum efficiency decreases at low x but increases again above x ≈ 0.6. The PL‐decay after pulse excitation (10 ns) is enhanced with increasing x, the lifetime spectra shift to shorter times by more than two orders of magnitude (x < 0.45). The data indicate a strong decrease of the average radiative lifetime with x. This behavior is explained by an enhancement of the Coulomb attraction between the recombining carriers and/or faster thermalization in the band tails.
Localized vibrations of hydrogen and deuterium in GaAs. A comparison with Ge and Si
Authors: Tatarkiewicz J. , Król A. , Breitschwerdt A. , Cardona M.
Published: July 2007

A detailed study is made of the localized vibrational stretching modes of hydrogen in GaAs using high‐energy proton implantation and low‐temperature Fourier transform infrared spectroscopy. New mode frequencies are found and confirmed with the help of deuterium implantation. A comparison with proton‐ and deuteron‐implanted germanium shows a different shape of the vibrational bands in comparison to GaAs. The oscillator strengths and effective charges are determined for the vibrational bands observed.
Cohesion in d‐state metals at high pressures
Authors: Walzer U.
Published: July 2007

A description of the d‐band structure in which the pseudopotential theory is combined with the muffin‐tin orbital theory serves for the calculation of the cohesive properties of d‐state metals. The coupling of free‐electron states and local, non‐overlapping d‐states is also taken into account. On this basis, two new models are developed. In both the models, it is assumed that the ratio of nearest‐neighbour separation to the atomic radius is independent of the pressure. In the first model (MB), the additional condition is that a redistribution of electrons between the two outer shells, which are not completely filled, is permissible. In the second model (MC), the requirement is that for elements with a filled d‐shell the theoretical bulk modulus at vanishing pressure is exactly equal to the experimental bulk modulus at vanishing pressure and that the pressure exactly vanishes for the zero‐pressure volume. In a comparison with various kinds of measured values, MC proves to be particular successful for all d‐state metals except Sc, Ti, and Nb. In both the models, the initial pressure derivative of the bulk modulus is not an input parameter, but can be predicted.
Screening of a proton moving through the electron gas. II. Kinetic Properties of the screening cloud
Authors: Stachowiak H.
Published: July 2007

An RPA type approximation developed in an earlier paper is used to study the kinetic properties of the screening electronic cloud forming around a proton moving through the electron gas. It is shown that the motion of the screening cloud in this approximation is compatible with no kinetic energy associated with this motion. The finite total electronic charge of the screening cloud is due to infinitesimal excitations of electrons with momentum in the immediate vicinity of the Fermi surface. If the proton is moving, the distribution of these excitations becomes anisotropic. Its angular dependence is a function of the proton velocity. A formula for this function is derived.
Ruthenium LII and rhodium LII,III X‐ray absorption‐edge structures
Authors: Sugiura C. , Kitamura M. , Muramatsu S.
Published: July 2007

The Ru LII absorption‐edge spectra of Ru(NH3)6Cl3, K3RuCl6, and RuCl3 · nH2O and the Rh LII absorption‐edge spectra of (NH4)3RhCl6 and RhCl3 · 3 H2O are measured together with the Rh LIII absorption‐edge spectrum from RhCl3 · 3 H2O, using a 50 cm bent crystal vacuum spectrograph. The RuCl3 · nH2O spectrum exhibits a double‐peaked LII‐edge structure, while the other spectra are characterized by an intense „white line”︁ at the LII or LIII edge. The Ru LII and Rh LII,III absorption‐edge structures are discussed by referring to atomic transition energies in Ru3+, Ru4+, and Rh3+ ions calculated on the basis of a self‐consistent‐field (SCF) method. From these calculated results, it is shown that the double‐peaked LII‐edge structure can be attributed to the 2p1/2 → 4d transitions in the Ru3+ and Ru4+ ions.
X‐Ray Reflection Curves of Si Crystals with Microdefects in the Laue Case
Authors: Holý V. , Kuběna J.
Published: July 2007

X‐ray diffraction in crystals with randomly placed microdefects, which are not resolvable topographically, is studied theoretically and experimentally. On the basis of the general theory published in a previous paper the procedure is described for computing the coherent and incoherent parts of the diffracted and transmitted intensities. Using a simple model for the defect deformation field the influence is studied of the size and the defect concentration on the computed reflection and transmission curves. The computations are compared with experimental curves measured by a triple crystal diffractometer in the symmetrical Laue case and a qualitative agreement is found.
Electronic Properties of Amorphous Semiconductors. Lattice Topology Effects
Authors: Latgé A. , Makler S. S. , Anda E. V.
Published: July 2007

The influence is studied of the lattice topology in the electronic properties of a tetrahedrally bonded amorphous semiconductor, typically a‐Si. The system is described by a nearest neighbour tight‐binding Hamiltonian, defined on a pseudolattice. The topology corresponding to the perfect gasket defined using hexagons as its basic structure is similar to the crystalline diamond lattice. The continuum random network is simulated by generating vacancies in the gasket lattice where dangling bonds are eliminated by saturation. The electronic density of states and the ring statistics of the disorder lattice are studied using a real space renormalization technique within the context of the Green function formalism.
Material Constants of Composites. II. An Improvement over the Coherent Potential Approximation for Crystals with Site‐Diagonal Disorder
Authors: Monecke J.
Published: July 2007

It is shown that the coherent potential approximation violates a fundamental requirement, called additivity in a previous work. A new approximation for the calculation of the self‐energy of crystals with site‐diagonal disorder is proposed, which interpolates between the two dilute limits of the average t‐matrix approximation and fulfils the requirement of additivity.
Electron Momentum Distribution in Silver and the Renormalised Free Atom Model
Authors: Sharma B. K. , Singh Hanuman , Perkkiö S. , Paakkari T. , Mansikka K.
Published: July 2007

The results are presented of a Compton profile study on polycrystalline silver. The measurements are carried out using 59.54 keV γ‐rays on three samples of different thicknesses − 0.5, 0.25, and 0.125 mm. Theoretical Compton profiles are calculated using (i) renormalised free atom model for 4d95s2 and 4d10Fs1 electron configurations and (ii) orthonormalised LCAO model approach. Best agreement between the measured and calculated values is found for the 4d105s1 configuration within the RFA model.
AC Losses in a Type‐II Superconducting Slab Including Surface Barrier Effects
Authors: Saif A. G.
Published: July 2007

The structure of a surface barrier, in the frame of the general critical‐state theory, of a type II superconducting slab subjected to parallel magnetic fields (in which the direction of the de bias and ac field are left arbitrary) is studied. The internal magnetic field distribution is developed. In the case of the amplitude of ac field (b0) less than the magnitude of the dc bias field (B0), analytic expressions for ac losses are obtained. Maximum values of FL cutting and flux pinning losses are also determined. A good agreement with the previous results (in this case of α0 = 0 with absence of surface barrier effects) is obtained.
Transmission of Spin Waves through Domain Walls
Authors: Patterson J. D.
Published: July 2007

The transmission coefficient of spin waves incident perpendicularly on 180°‐Bloch walls are calculated. A standard semiclassical continuum approximation is used and the results show typical wave interference effects. In particular the transmission coefficient is unity when twice the wall thickness equals an integral number of wave lengths.
Concentration Effect on Delayed Emissions: 1,4 Dibromonaphthalene Host—Pyrene Guest Mixed Crystal System
Authors: Pal A. J. , Talapatra G. B. , Misra T. N.
Published: July 2007

In 1,4 dibromonaphthalene (DBN) crystal, phosphorescence emission is observed in conformity with quasi‐one‐dimensional exciton motion. In pyrene doped DBN crystal, fairly intense delayed fluorescence (DF) and delayed excimer fluorescence (DEF) are observed on host singlet excitation though prompt excimer fluorescence (EF) is absent. EF emission, however, is observed on pyrene singlet excitation. These results suggest that host triplet excitons are trapped more efficiently at monomeric pyrene sites than at nascent excimeric pyrene sites. From temperature and guest concentration dependence studies of delayed emission intensities and lifetimes it is concluded that homofusion of an adventitious defect triplet and heterofusion of this defect with guest triplets produce DF and DEF at low temperature but at high temperature homofusion of guest triplets is the generative event of these delayed emissions.
UHF MC INDO Calculations of the Interstitial Hydrogen in Alkali Halides
Authors: Matos Maria
Published: July 2007

The applicability of a general scheme of parametrization of semiempirical methods for ionic crystalline systems is investigated in the study of pure LiF, NaF, LiCl, and NaCl crystals using different molecular clusters, and in the investigation of optical and magnetic hyperfine properties of the interstitial hydrogen (U2‐center) in these crystals. MC‐INDO (for the pure crystal) and UHF MC‐INDO (for the defect) calculations are performed to investigate the band crystalline structure and to estimate the U2‐band absorption peak energy and the electronic spin density at the nucleus of the interstitial atom. It is shown that the procedure of scaling the defect Slater‐type orbitals (STO) is not appropriate for the U2‐center and that this fact is related to the covalent nature of the binding of the interstitial atom to the crystal.
Screening of a Proton Moving through the Electron Gas III. Computation of the Momentum Distribution of the Screening Cloud
Authors: Stachowiak H.
Published: July 2007

A formula for the momentum distribution of the screening cloud around a proton moving in an electron gas has been derived in an earlier paper of the author in RPA approximation. This formula is investigated in details and the actual momentum distribution computed in the whole range of proton velocities. For velocities κ >λ (λ = 0.83355656) singularities appear in the momentum distribution (κ is given in units of the Fermi velocity). Another drastic change of the momentum distribution occurs at κ = 1. Two properties of the momentum distribution remain, however, remarkably steady: the whole screening charge is equal to one electronic charge and its total momentum is equal to one third of the momentum of an electron moving with the velocity of the proton.
On SCCF Treatment of the Hubbard Model of 1‐D Alloys. Auger Densities
Authors: Šlechta J.
Published: July 2007

There is presented an application of the SCCF method to the Hubbard model of 1‐D binary alloys which makes possible to progress beyond the narrow band restriction. The Hubbard split is demonstrated upon realistic densities of states provided by SCCF. There are also studied the multicentre densities relevant, e.g., for theory of the Auger effect and shown that for the alloys studied here the shapes of those densites do not depend upon the concentration of the components of those alloys. Thus clusters of more than one atom may play a role of a kind of “letters” or “words” in the genetic code.
Nonequilibrium Dynamics of Carriers and Phonons in GaAs after High Excitation by Short Time Laser Pulses
Authors: Asche M. , Sarbei O. G.
Published: July 2007

Taking into account the electron–hole interaction and the nonequilibrium of optical phonons, the plasma cooling in GaAs after short time excitation by a laser pulse is investigated numerically. The energy transfer from electrons to holes is essential for times up to the order of picoseconds. Results are presented for a concentration of 1018 cm−3 carrier pairs.
Self‐Consistent Tight‐Binding Investigation of Chemical Trends for Native Defects in III–V Semiconductors
Authors: Kühn W. , Strehlow R. , Hanke M.
Published: July 2007

Using the recently developed charge self‐consistent version of the empirical tight‐binding method (ETBM) in conjunction with the Koster‐Slater scattering‐theoretic approach the electronic properties induced by the substitutional native point defects (anion and cation antisite defect, anion and cation vacancy) in the six III–V semiconductors GaP, GaAs, GaSb, InP, InAs, and InSb are investigated. The calculations include the neutral and possible charged states of the defects. Chemical trends in the energetic position of gap states, the orbital composition, and the localization of the defect wave function are discussed and compared with other theoretical and experimental findings.
Spin Hamiltonian of Cr3+ in a Strained Ruby Crystal
Authors: Lacroix R.
Published: July 2007

As an application of the theory of the ligand field in a strained corundum crystal previously published, the spin Hamiltonian of Cr3+ in strained ruby is written down. Numerical calculations are performed, which show that the effect of the internal strains is by no means negligible.
Impact Parameter Dependent Electronic Energy Losses
Authors: Azziz N. , Brannon Karen W. , Srinivasan G. R.
Published: July 2007

Electronic stopping power for an ion traveling through a cloud of electrons is calculated in the linear approximation. The formalism allows for an explicit dependence of the stopping power on the impact parameter between the ion and the electron in the medium. This will facilitate the calculation of stopping power for semiconductors where the electronic density varies appreciably. Comparison is made with Firsov, Oen‐Robinson, and Lindhard formulas by averaging over a Thomas‐Fermi electron density distribution. The consistency of the approach with those mentioned above is clearly demonstrated.
Surface Optical Polaron in Magnetic Field
Authors: Bhattacharya M. , Mukhopadhyay S. , Mitra T. K.
Published: July 2007

An extrinsic surface optical polaron is investigated in a magnetic field perpendicular to the surface by canonical transformation method. Ground state energies are found for different values of the coupling constant (as) and the reouced magnetic field strength (Λ). The motion in the z direction is completely isolated, and the energy due to localisation in the z direction is neglected, so that the ground state energy is due to 2D motion perpendicular to the magnetic field. The importance of the problem in relation to cyclotron resonance and magneto‐optics is discussed.
EPR Study of Cu2+ Ion in K2Cd(SO4)2 · 6 H2O
Authors: Yi‐Yang Zhou
Published: July 2007

The admixture of d‐orbitals in the ground state wavefunction of Cu2+ ion in K2Cd(SO4)2 · 6 H2O crystal and, further, the EPR parameters g and A are calculated with the aid of the approximate SCF d‐orbit of Cu2+ ion. The results (compared with that obtained by Satyanarayana recently) show that the admixture fitted by Satyanarayana is reasonable.
Time‐Resolved Spectroscopy of the Low‐Energy Emission Bands of Highly Doped Zn1–xMnxS
Authors: Benecke C. , Busse W. , Gumlich H.‐E. , Moros H.‐J.
Published: July 2007

The luminescence excitation and emission spectra of Zn1‐xMnxS with high Mn concentration (0.01 ≦ x ≦ 0.25) is studied by time‐resolved laser spectroscopy at temperatures 10 K ≦ T ≦ ≦ 300 K. Besides of the emission bands peaking at ṽ = 17000 cm−1, ṽ = 15750 cm−1, and ṽ = = 13500 cm−1 a fourth band peaking in the IR at ṽ = 10600 cm−1 is registered. This IR band is closely connected to two excitation bands in the UV peaking at ṽ = 26900 cm−1 and ṽ = 28750 cm−1 (T = 10 K, x = 0.034). Four different Mn dominated centers are distinguished, which are partly connected by energy transfer.
Thermal Properties of Tetrahedral Compounds
Authors: Kagaya H.‐Matsuo , Soma T.
Published: July 2007

The thermal properties of tetrahedral compounds are studied theoretically from first principle using the lattice dynamical method recently developed. Numerical calculations about the specific heat are performed for AlP, AlAs, AlSb and about the thermal expansion coefficient for AlP, AlAs, AlSb, ZnS, ZnSe, ZnTe, and CdTe.
Dispersion and Damping Properties of Plasmon Polaritons in Superlattice Structures II. Semi‐Infinite Superlattice
Authors: Haupt R. , Wendler L.
Published: July 2007

The dispersion and damping properties of surface plasmon‐polaritons in semi‐infinite superlattices are analyzed using a local theory with retardation. The position of the dispersion curves of the surface plasmon‐polaritons relatively to the bands of the plasmon‐polaritons in infinite super‐lattices is discussed. It is shown that in a definite range of the thickness ratio of the single‐layers in an infinite superlattice a degeneracy point of the two plasmon‐polariton bands exists which is crossed by one of two surface modes existing in semi‐infinite superlattices. An analytic expression for the damping function is derived by means of a perturbative method. The different behaviour of the damping of surface plasmon‐polaritons in various semi‐infinite superlattice structures is discussed. The dispersion curves, the electromagnetic fields, and the damping function are calculated and presented in graphical form.
Symmetry of Localized Crystalline Orbitals and Double‐Valued Band Representation
Authors: Evarestov R. A. , Smirnov V. P.
Published: July 2007

The double‐valued band representations of space groups are considered. The simple double‐valued band representations of the space groups Oh5, Oh7, and Td2 are generated for the first time. The tables of the induced double‐valued representations of the crystallographic point groups are given. These tables are necessary for some applications of the simple band representations of space groups in physical problems. The symmetry of localized orbitals in Ge crystals is analyzed basing on the results of the relativistic band structure calculation.
Monovalent‐Copper (I; 3d10,S= 0) Induced Oxygen‐2p Valence Band Holes with Concomitant Attractive Interaction inside Low‐Dimensional Oxygen Networks
Authors: van Bruggen C. F.
Published: July 2007

Results of electrical transport and magnetic measurements on coldpressed compacts of La2CuO4and La1.70Ba0.30CuO4provide strong evidence for their p‐type metallic and Pauli paramagnetic character at room temperature connected with the existence of low density‐of‐states oxygen‐2p valence band holes induced by ground state monovalent copper (3d10,S= 0). The host La2CuO4exhibits an electronic phase transition at ≈85 K from a p‐type broad‐band metal (p= 2.5 × 10−2holes/La2CuO4, μh= 10−4m2/Vs,m= 0.5m0,EF= 0.26 eV,N(EF) = 0.15 states/eV‐La2CuO4) to a p‐type polaron‐mediated ICDW state (p≈ 2.5 × 10−2holes/La2CuO4, μh≈ 10−4m2/Vs,m≈ 10m0,EF= 11 meV,N(EF) = 3.4 states/eV‐La2CuO4). The attractive interaction attending the O‐2p VB holes inside low‐dimensional oxygen networks is discussed in the framework of recent papers on structural distortions, electronic structures, and phase transitions in copper containing oxidic hosts and doped superconducting derivatives.
The Coexistence of Unstable, Metastable, and Separated Frenkel Pair Defects in Solids. II. Theory of the Impact Mechanisms for Defects Formation in Metals
Authors: Khabibullaev P. K. , Dzhumanov S.
Published: July 2007

The coexistence of unstable, metastable, and separated Frenkel pair defects in metals under nonequilibrium conditions generated via impact displacement of lattice atoms by incident particles is studied theoretically. The extent of a spontaneous annihilation (SA) zone of defects along their attraction directions is determined in the framework of elasticity theory. The value of defect formation threshold energy (DFTE) along the basic crystallographic directions is calculated and the degree of DFTE anisotropy is found. The displacement probability function and the damage function depending on the kinetic energy E of knock‐on atoms, the scattering law of colliding atoms, and the degree of DFTE anisotropy are obtained. The damage function found differs essentially from the well‐known Kichin‐Pease formula. It is shown that the cascade multiplication of defects is possible at E ≧ Ed, max ≈ 3 × Ēd (where Ed, max and Ēd are the maximum and average values of DFTE, respectively). The damage function found takes also into account the SA effect of metastable and separated pair components at overlapping of their existence zone with the SA zone of another partner. This effect is noticeable especially at low knock‐on energy. The results obtained are in good agreement with the experimental data.
Theoretical Calculations of EPR Parameters and Absorption Spectra in Chromium‐Doped α‐LiIO3 Crystals and Determination of Local Compressibilities
Authors: Zheng Wen‐Chen
Published: July 2007

The EPR parameters (g∥, Δg, D) and the absorption spectra of Cr3+ in α‐LiIO3 are calculated by crystal‐field theory. In the calculation the influence of Li+ vacancies in the vicinity of Cr3+ ions is taken into consideration and four parameters (N, μ, q2, α) are used. The results calculated agree well with experimental data. On this basis it is found that the host crystal compressibility can not be assumed to be the local compressibility in the vicinity of Cr3+ ions to explain the effect of pressure on EPR spectra. The possible local compressibilities are obtained from the EPR spectra under pressure. These values are supported by macroscopic thermodynamic analysis and microscopic crystal‐field theory.
Transport Coefficients and Einstein Relation in a High Density Plasma of Solids
Authors: Mnatsakanov T. T.
Published: July 2007

General expressions for charge carriers transport in the high density isothermal plasma of semiconductors and semimetals are derived. In addition to the usual drift and diffusion ones extra terms arise which describe a mutual drag of charge carriers due to electron‐hole scattering. A generalised matrix form of the Einstein relation is established. It is shown that nondiagonal elements in transport coefficients matrices arise due to electron‐hole scattering.
Optical Properties of the Alloy System SnTeGeTe from Reflectance Measurements
Authors: Lewis J. E.
Published: July 2007

The reflectance spectra of bulk samples of the alloy series Sn1−xGexTe are measured at room temperature in the wavelength range 0.2 to 50 μm. The principal optical constants are obtained using Kramers‐Kronig analysis. The variations of the optical dielectric constant, the carrier effective mass, the optical mobility, and the optical energy gap are given as functions of the alloy fraction x. For the Sn‐rich alloys (x < 0.3) the energy gap and the dielectric constant do not follow the trends established for the series at the Ge‐rich side, and are possibly related to the cubic‐rhombohedral phase transition that occurs in this system. The mobility and effective mass, though, behave reasonably for all alloy fractions.
X‐Ray Brillouin Scattering of Mössbauer Radiation by an Oscillating Quartz Crystal
Authors: Jex H. , Müller U.
Published: July 2007

The elastic and inelastic scattering from x‐cut quartz single crystal is investigated by means of Mössbauer diffractometry from the (1120) Bragg plane using 57Co‐source and a Mössbauer absorber for the energy analysis. When the crystal is excited to piezo‐electric vibrations in the MHz‐range, inelastically scattered quanta are resolved. The phonon energies are in the range 40 neV < < ħΩq < 150 neV.
Electron‐Phonon Interaction in Semiconductor Superlattices
Authors: Wendler L. , Haupt R.
Published: July 2007

The single‐particle Hamiltonian of the electron–(long‐wave optical) phonon interaction is discussed for semiconductor superlattices. In detail the dispersion relations are analyzed of long‐wave optical phonons, i.e. of the interface‐like bulk phonons (IBP's) and of longitudinal optical (LO) phonons, of the polar semiconductor superlattice. The IBP's are composed from ordinary interface phonons on adjacent interfaces coupling through their macroscopic electric fields to form a collective excitation of the whole superlattice. The coupling function of the electron–phonon interaction is calculated using a continuum theory. The coupling functions describing the modified Fröhlich interaction of a single electron with the continuous field of polarization of the IBP's and LO phonons are discussed and presented in graphical form.
Phenomenological Treatment of the Departure from the Picture of Ideal One‐Dimensional Conductor
Authors: Bâldea I.
Published: July 2007

A statistical distribution w(kF) of the Fermi wavevectoi values is postulated for the one‐dimensional (1D) electron–phonon system in order to account for some effects inherently present in real (quasi‐) 1D conductors. The relevant properties are obtained by taking the ensemble average with the distribution w(kF). It is found that the charge‐density‐wave (CDW) instability is suppressed if the spread ∧ in kF is large enough. For lower values of ∧, the Peierls‐CDW distortion still exists, but the electronic gap is reduced. In this case, the broadening of the optical conductivity peak as well as the increase in the value of the phason pinning frequency measured in neutron experiment with respect to that deduced from optical data are obtained. A gapless CDW state may also occur. Fair agreement with the data reported for KCP is found.
Thermal E.M.F. Anomalies Due to Axial Compression and the Band Structure of Bi1‐xSbx (x = 0.27) Alloys
Authors: Brandt N. B. , Lavrenyuk M. Yu. , Minina N. Ya. , Savin A. M. , Kraak W. , Herrmann R.
Published: July 2007

The electron Fermi surface (FS) in single‐crystal Bi0.73Sb0.27 alloys and its change under axial compression are investigated with the help of magnetoresistance quantum oscillations. An additional frequency in the oscillation picture due to the minimum FS cross sections is observed. This is consistent with the assumption that the FS in the unstrained alloy investigated consists of three dumbbell‐shaped pockets. When the specimen is compressed the maximum FS cross‐section abruptly decreases by a factor of two at some critical deformation ϵcr. This phenomenon indicates that the “neck” of the dumbbell‐shaped FS disrupts at the moment when under compression the Fermi level crosses the saddle point of the energy spectrum. In the vicinity of the singular point the strain dependences of thermal e.m.f. and resistance reveal an anomalous behaviour what is a manifestation of the electronic topological transition.
Some Exact Results on the Ising Antiferromagnet with General Spin S on the Triangular Lattice
Authors: Hajdukovć D. , Vujičić B. U.
Published: July 2007

A triangular lattice is considered which consists of five chains, i.e. a lattice with N = 5 × L sites. In the thermodynamic limit (L → ∞) and in the critical magnetic field, the exact ground‐state entropy for such a lattice is obtained as an explicit function of spin S. It is shown that this exact result for the lattice finite in one and infinite in the other dimension is an excellent approximation for the triangular lattice infinite in both dimensions.
Fine Structure in the Free Exciton Region of the Emission Spectrum of γ‐InSe
Authors: Paraskevopoulos K. M. , Julien C. , Balkanski M.
Published: July 2007

The fine structure of the emission spectrum of γ‐InSe in the free exciton region has been investigated. The complete behaviour is given and the quantitative analysis is described in detail. Also, the doublet structure is examined and the possible stacking faults contribution is considered. It is found that the origin of the second peak is bound excitonic. The phonon modes involved in the temperature variation of the exciton energy are obtained from fitting the experimental data to previous theoretical calculations. An explanation is given for the temperature variation of the half width anomaly.
Noncentrosymmetry Observed in CaC4H4O6 · 4H2O Crystals by Nonlinear Optical Measurements
Authors: P. C. Medrano , Günter P. , Arend H.
Published: July 2007

Calcium tartrate tetrahydrate (L‐CTT) is reported to have point group symmetry mmm at room temperature. Second harmonic generation experiments indicate that calcium tartrate tetrahydrate prepared from pure L‐tartaric acid has the noncentrosymmetric point group mm2. All the nonlinear optical d‐coefficients are determined. Linear optical properties of this material such as: refractive indices and absorption constant are measured in the range 200 nm < λ < 2000 nm and the phase matching conditions for optical frequency doubling are derived. Non critical type I phase matching can be achieved in L‐CTT at room temperature for a fundamental wavelength of λ = 816 nm.
Overtone Spectroscopy of the OH and OD Stretch Modes in LiNbO3
Authors: Förster A. , Kapphan S. , Wöhlecke M.
Published: July 2007

Infrared absorption bands due to the fundamental and the overtone of OH and OD stretch modes are investigated in undoped, Mg‐doped, and benzoic acid treated LiNbO3. The different bands are interpreted using an anharmonic oscillator model. Spectroscopic parameters determined from the isotope shift and the overtone frequencies are in good agreement. The anharmonicity found is comparable to that of OH in other ABO3 compounds and in alkali halides.
A New Luminescent Band Induced in NaF by Ion Implantation
Authors: Afonso C. N. , Gomez I. M. , Ortiz C.
Published: July 2007

High concentrations of defects are induced in a thin surface layer of pure and doped NaF crystals by ion implantation with 280 keV H+, H+2, B+, and Ne+ ions. Laser induced luminescence in these crystals shows a new band peaking at 750 nm. It presents a high stability under thermal annealing and optical treatments. The experimental results point out that those centers responsible for this emission band are F2 disturbed centers which are only created under ion implantation.
VUV Optical Constants of Liquid and Solid Xenon at the Triple Point
Authors: Subtil J. L. , Laporte P. , Reininger R. , Saile V.
Published: July 2007

Accurate optical constants for solid and liquid Xe are reported. The samples are prepared under ideal conditions in a closed cell. Reflectivity spectra between 7 ≦ hv ≦ 11 eV are recorded for normal and grazing incidence of the radiation and at temperatures T = 161.2 and 163.2 K. By a special evaluation technique optical constants are deduced. They are compared to previous data for solid Xe and the differences are found to be very large. The reasons for these discrepancies and the consequences are discussed.
Alternative Quasi‐Classical Transport Equations. III. Charge Transport in MIS Systems with Inclusion of Diffuse Surface Scattering and Quantum Size Effects
Authors: Scherf S. , Schnittler Ch. , Paasch G.
Published: July 2007

On the basis of the results of two preceding parts in this paper new expressions for the current density, the conductivity, and the local and effective mobility in the channel of a MIS field effect transistor are presented. For the first time in these expressions the following two different effects of the interface are taken into account simultaneously: (i) the vanishing of the wave functions at the interface as a quantum mechanical effect and (ii) incomplete specular reflection at the interface. Numerical results are given for the SiSiO2 system both, in the constant field approximation and for self‐consistent potentials in the range of surface fields between 2 × 104 and 3 × 105 V cm−1. The main new result is that the decrease of the mobility due to surface scattering is reduced by wave function effects giving so a much better agreement with experimental data of other authors. Problems of the influence of temperature and the consideration of an energy dependent relaxation time are discussed.
Displacement Cascades. Dynamics and Atomic Structure
Authors: Seidman D. N. , Averback R. S. , Benedek K.
Published: July 2007

The seminal ideas of Seeger, in 1958 and 1962, on the nature of a displacement cascade (DC) in irradiated crystalline materials have played an important role in the field of radiation damage. This paper first reviews the early history of displacement spikes, replacement collision sequences (RCSs), “diluted” or depleted zones and thermal spikes. Then the use is discussed of molecular dynamics (MD) simulations and field‐ion microscopy as techniques by which one can study the dynamics of the formation of DCs and the spatial array of point defects — on an atomic scale — associated with quiescent DCs, respectively. These techniques demonstrate quantitatively that a DC consists of a vacancy‐rich core surrounded by a distribution of self‐interstitial atoms (SIAs) — the latter are predominately created by RCSs. Recent results on the application of MD to the behavior of 5 keV cascades in copper are presented. Some of the results of the application of field‐ion microscopy to the study of DCs in tungsten are reviewed, and compared with the MD results.
Eigenstates of Kinked Dislocations in Crystals
Authors: Mann E.
Published: July 2007

In a first contribution, the unstable equilibrium configuration of a dislocation with a kink pair at high effective stresses is established and the small‐amplitude eigenvibrations on it are derived. The contribution of the eigenfrequencies to the entropy is calculated in both the high‐ and the low‐temperature approximation. The second part deals with the influence of small effective stresses on the breather oscillations. The homogeneous solution of the perturbed Enneper (sine‐Gordon) equation is found in general form by virtue of Bäcklund transformations. The inhomogeneous initial‐value problem is reduced to a system of two integral equations, a special solution of which is obtained by asymptotic expansion.
Electronic Properties of Glassy Metals
Authors: Krey U. , Ostermeier H. , Zweck J.
Published: July 2007

Electronic properties of glassy metals, including transport properties, are studied by numerical calculations for realistic structural models. For the Fe1‐xBx and Cu1−Zrx system results are presented for total and partial densities of states and the electrical resistivities. For Cu0.65Zr0.35 also the Hall constant is calculated. The results agree with recent experiments. The projections of the densities of states on the B‐p‐bands give the possibility to distinguish between different structural models with similar radial distribution function.
A Reexamination of the Continuum Approach to the Calculation of Lattice Sums
Authors: Mohazzabi P. , Behroozi F.
Published: July 2007

The continuum approach of Born and his collaborators to the calculation of simple lattice sums, originally suggested for large‐inverse‐power interatomic potentials in cubic crystals, is reexamined. Explicit calculations show that the model is an effective approximation not only for large powers of the inverse interatomic potential, but for all powers as low as n = 4. Furthermore, the method is shown to be applicable to any solid regardless of its crystal structure. The hexagonal close‐packed case is considered as an example. The model is expected to be useful for amorphous solids and liquids.
Electronic Spectra of Radiation‐Induced CrO in 3 CdSO4 · 8 H2O Crystals
Authors: Murthy T. S. N. , Salagram M.
Published: July 2007

The optical absorption spectra of 3 CdSO4 · 8 H2O crystals doped with different concentrations of CrO impurity are analysed at different stages of X‐irradiation. The products of radiation damage are identified to be SO and a new species formed out of CrO impurity. The ligand‐field (LF) nature and the charge transfer (CT) nature of the optical peaks suggest that the new species is a CrO ion. The assignment of the bands indicate that the approximate site symmetry of CrO ion in the lattice is D2d. The radiation damage mechanism for the formation of CrO ion in the lattice is discussed.
The Model Description of Neutron Scattering by Quasilocal Vibrations and Phonons in Glasses
Authors: Plakida N. M. , Stamenković S. , Vujičić G. M.
Published: July 2007

In the frame of an appropriate dynamical model of glasses the differential cross sections for inelastic neutron scattering by quasilocal vibrational excitations (QVE) and phonons are evaluated. It is shown that at low energies (E ≦ 10 meV) these cross sections are of the same order and could explain the corresponding experimental results.
Structural Phase Transition of Alkali Halides Using a Modified Interaction Model
Authors: Kumar M. , Hans D. , Sharma A. K. , Shanker J.
Published: July 2007

A new interionic potential model is developed by taking into account (i) the effect of three body interactions, (ii) the recent values of van der Waals (vdW) potentials estimated from Kirkwood‐Muller formula, (iii) the repulsive hardness parameters evaluated from the overlap integrals using the exchange charge model, and (iv) the effect of coupling between overlap repulsive and van der Waals forces in terms of the Drude oscillator model. The modified potential model is capable of predicting correctly the relative stability of two structures viz. NaCl and CsCl structures. The calculated values of transition pressure are found to present close agreement with experimental data.
The Influence of the Compression Effect on the Phase Diagrams of Si–Ge System
Authors: Kagaya H.‐Matsuo , Soma T.
Published: July 2007

The pressure effect on the phase diagrams of the Si–Ge system are studied using an electronic theory based on pseudopotentials and considering lattice vibrational contributions. A region of phase mixture is increased under pressure, and the top of phase mixture rises at a rate of about 6 K per 109 Pa. The melting curves under compression show a drop in temperature.
Self‐Consistent MAPW Calculation with a Warped Muffin‐Tin Potential. II. The Electronic Structure of Li and Its Pressure Dependence
Authors: Bross H. , Stryczek R.
Published: July 2007

The electronic structure of lithium is self‐consistently evaluated in the b.c.c. and f.c.c. phase on a wide range of the lattice parameter, using a formalism which allows the incorporation of the leading warped‐muffin‐tin correction. With increasing compression the band structure exhibits rather peculiar modifications. At high compression the conduction band has its lowest energy at the N‐point whereas the Γ‐point is shifted above the Fermi level. With this information the free‐energy per Wigner‐Seitz‐cell at zero‐temperature and the equation of state are straightforwardly derived. Using the Hedin‐Lundqvist expression for exchange and correlation the b.c.c. phase is found to be more stable for all pressures up to 2 × 1011 Pa, contrary to recent investigations. However, a final conclusion about the stability of the phases is not possible as long as no similar investigations for the h.c.p. phase have been performed.
Localized Electron Levels in Isotopically Ordered Crystals (Isotopic Quantum Wells)
Authors: Berezin A. A.
Published: July 2007

An infinite defect‐free crystal lattice with large‐scale variations of isotope distribution is considered. It is shown that a sufficiently large “isotopic island” could form a potential well with localized energy levels. An example is given for CaO lattice with isotope 40Ca constituting a “host” part into which a spherical region with 48Ca is incorporated without any lattice misfits. The critical radius of such region which is able to act as an electron trap with a localized electron level is estimated as about 5 nm.
Nonlinearities in Excitonic Luminescence of Direct‐Gap Semiconductors under Very Low Optical Excitation Levels
Authors: Brodin M. S. , Gushcha A. O. , Khotiaintsev V. N. , Shevel S. G. , Taranenko L. V. , Tishchenko V. V.
Published: July 2007

The dependences of peak luminescence intensity Ilum for free and bound excitons on the level Iexe of the above‐gap optical excitation are studied in direct‐gap semiconductors PbI2 and CdSe within a wide range 1011 ≦ Iexe ≦1019 quanta cm−2 s−1 which included the never explored region below ≈ 1016 quanta cm−2 s−1. The experimental dependences Ilum = f(Iexe) are well represented by nonlinear power law Ilum ∼ I, 1 ≦ γ ≦ 1.75. The degree of nonlinearity (i.e. the value of γ) increases gradually with the decrease of Iexe and also becomes higher with increasing of temperature. Linearity (γ = 1) is sometimes observed only at sufficiently high Iexe above 1016 quanta x x cm−2 s−1. A phenomenological theoretical model is proposed which accounts for non‐radiative surface recombination and non‐uniform spatial distribution of the excited carriers and provides satisfactory qualitative agreement with the experimental findings.
Structure Factors of Liquid Zn, Cd, and Hg
Authors: Zou Xian‐Wu , Jin Zhun‐Zhi , Tian De‐Cheng
Published: July 2007

The structure factors a(Q) of liquid metals, Zn, Cd, and Hg are calculated by using the hard sphere cluster model. The calculated concentration and relative sizes of the two kinds of hard spheres, which correspond to the two kinds of clusters, respectively, coincide with the characteristics of solid structures.
On the Mechanisms of Photoreflectance in Multiple Quantum Wells
Authors: Enderlein R. , Jiang Desheng , Tang Yinsheng
Published: July 2007

Mechanisms of photoreflectance of MQW structures are studied. The photomodulation of the built‐in surface electric field is analysed. It is argued that in MQW structures unlike to bulk crystals the electric field does not result in a Franz‐Keldysh‐effect but rather in a Stark effect of subbands and excitons and a corresponding change of wavefunctions and oscillator strengths. The photoreflectance line shape is not of third derivative but of first derivative type in the case of MQW structures. Experimental photoreflectance spectra are reported which confirm these conclusions and allow the derivation of sublevel energies, broadening parameters, and exciton binding energies.
Trapping of Minority Carriers in Thermal U−‐Donors in n‐Si
Authors: Makarenko L. F. , Murin L. I.
Published: July 2007

Theoretical studies are made on the trapping behavior of the semiconductor containing U−‐donors. The theoretical analysis results are compared with the experimental data for the two sets of bistable thermal donors in n‐Si.
Two‐Mode Approximation in the Low‐Temperature Region. Biquadratic Exchange and Single‐Ion Anisotropy
Authors: Kazakov A. A. , Kleshchev S. M.
Published: July 2007

For the case of strong biquadratic‐law exchange and large energy of single‐ion anisotropy taken into account in the Heisenberg model, it is shown that the low‐temperature thermodynamics is not always reduced to the spin‐wave approximation. The hybrid character of the magnon excitation energy along with the excitation branches stipulates a more intensive temperature dependence of magnetization.
Effect of Doping on Parametric Amplification in Piezoelectric Semiconductors
Authors: Aghamkar P. , Sen P. , Sen P. K.
Published: July 2007

Using the straightforward coupled‐mode theory, the parametric amplification is analytically investigated in a doped piezoelectric semiconductor. The origin of the nonlinear interaction is taken to be in the second‐order optical susceptibility χ(2) arising from the nonlinear induced current density. The threshold value of the pump electric field E0th and the corresponding excitation intensity are obtained for crystals for various concentrations. E0th is found to decrease with rise in doping concentration. The investigation also reveals that the phenomena of self‐focusing and self‐defocusing can be enhanced in these crystals. The parametric gain constant is obtained for both, lightly and highly doped semiconductors. Numerical estimations made for n‐InSb crystal, at 77 K duly irradiated by frequency doubled pulsed 10.6 μm CO2 lasers, yield ∣χ(2)∣ comparable with that observed experimentally.
Microscopic Mechanisms of Paramagnetic Probe—Lattice Interaction for the System RbCdF3:Cr3+ near the Structural Phase Transition
Authors: Chernitskii M. V. , Nikiforov A. E. , Krotkii A. I.
Published: July 2007

Microscopic mechanisms of the interaction of the Cr3+ ion spin system with fields due to the lattice distortions in RbCdF3 crystals are studied. These mechanisms determine changes in the EPR spectrum below 124 K after structural phase transition. The order parameter dependence of the spin‐Hamiltonian parameters both, for centres with local charge compensation and for non‐local charge compensated centres is obtained. The significance of crystal distortions caused by the second order uniform deformation is revealed. Such distortions change considerably the constants determining the interaction of the spin system with the order parameter. To obtain reliable information about the phase transition using an EPR method it is necessary to apply microscopic analysis. The obtained results are compared with experimental data.
Fourth‐Order Elastic Constants of Fluorite Structure Solids
Authors: Goyal S. C. , Sharma A. K.
Published: July 2007

The homogeneous deformation method is extended to study the fourth‐order elastic constants for nonprimitive lattice structure solids. The extended theory is used to study the fourth‐order elastic (FOE) constants of CaF2, SrF2, and BaF2 solids. The second‐order pressure derivatives of the second‐order elastic (SOE) constants for these solids are also calculated.
An Application of the Extended CPA Method to Electron Work Function and Surface Charge Calculation for Cu Single Crystals
Authors: Romanowski S.
Published: July 2007

Electron work function calculations from Cu(100), Cu(110), and Cu(111) monocrystals as a function of boundary conditions and surface charge of metal are carried out by means of the extended coherent potential approximation (ECPA) method. A special parameter describing the polarization of a metal sample by an external source of voltage is also introduced. The work function dependence on crystallographic faces of monocrystalline sample has been explained on the basis of local Fermi level idea.