bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature Dependence of the Baric Magnetization Coefficient of a Band Ferromagnet
Authors: Ponomarev B. K. , Thiessen V. G.
Published: July 2007

An expression for the temperature dependence of the baric magnetization coefficient (1/σs) × (∂σs/∂p)T is obtained in the framework of Wohlfarth's model for a very weak band ferromagnetism. Good agreement is obtained between calculated and experimental values for nickel and iron‐nickel alloys of invar composition. The derivative of the effective exchange interaction parameter with respect to pressure, dI/dp, is estimated for nickel and iron‐nickel invar Fe0.66Ni0.34. It is shown that anomalously high values of the quantity dI/dp in a band ferromagnet are not the necessary condition to obtain great values of the quantity (1/Tc) (dTc/dp) specific for invars.
Band Structure Calculation of Electronic Properties in CuIn1–xGaxSe2
Authors: Chizhikov V. I. , Panyutin V. L.
Published: July 2007

The energy band structure of CuIn1–xGaxSe2 solid solution is calculated in the center of the Brillouin zone by empirical pseudopotential method, using the model of average statistical substitution. The results of calculations are in good agreement with experimental data. An estimation of the acoustic deformation potential and temperature deviation coefficients of lattice parameters is also made.
The Phenomenon of Self‐Induced Transparency in the Range of the Proper Lattice Absorption
Authors: Kiselyova E. S.
Published: July 2007

Soliton wave pulses are investigated in the phonon range of the spectrum. The phenomenon of self‐induced transparency (SIT) is considered taking into account both, the amplitude and the phase modulations of the soliton wave packets. In essence new dispersion laws and the wave vector dependence of the soliton velocity are obtained. Criteria of SIT theory application are given, and also new possibilities of observing non‐linear optical phenomena in the phonon range of the spectrum are pointed out.
Effect of Electron–Phonon Interaction on the Optical Conductivity of Alloys
Authors: Taranko R. , Taranko E. , Wysokińaski K. I.
Published: July 2007

The effect of electron‐phonon interaction on the optical conductivity γ(ω) of disordered alloys is investigated in the framework of coherent potential approximation. The formalism is used for the case in which the one‐particle Hamiltonian includes thermal diagonal and compositional diagonal and off‐diagonal disorder. The results of numerical calculations of γ(ω) are presented in the case of the model binary alloy AxB1–x for various temperatures.
Impurity Vibrational Properties of Ga in InP Crystals
Authors: Jahne E. , Kleinert P. , Kh. Bairamov B. , Topoprov V. V.
Published: July 2007

A theoretical and experimental study is undertaken on the vibrational mode frequencies, infrared and Raman response functions of isolated substitutional Ga impurities in InP. The calculations are based on the existing rigid‐ion and defect models for isolated impurities in zincblende lattices. They yield an impurity‐induced infrared absorption and Raman scattering in the regions a few cm−1 above the maximum TA, LA, and optical phonon frequencies. Experimentally the infrared reflection and Raman scattering are measured of InP containing 1% Ga impurities. A local mode is observed 4.5 cm−1 above the LO(Γ) frequency of InP. From these experimental data the relative force constant change around the Ga impurity is estimated to Δf/f ≈ 0.14.
Model Pair Distribution Functions for Amorphous Binary Metal–Metalloid Alloys Based on Crystal‐Like Short‐Range Order
Authors: Mattern N. , Hermann H.
Published: July 2007

A model is introduced for the description of the atomic structure of amorphous metal–metalloid alloys. The model is based on the assumption that the local arrangement of the atoms in the amorphous state is similar to that in the corresponding crystalline phase. Short‐range order of the Fe3C and Ni3P structure types is used for modelling amorphous metal–metalloid alloys. The partial and the total pair distribution functions are calculated and compared with experimental values. The Ni3P unit cell is suitable as a structural unit for modelling the amorphous atomic structure. The Fe3C unit cell is not realized as a structural unit in amorphous metal–metalloid alloys in general.
Local Quasiparticles in the Anderson Model
Authors: Nolting W. , Oleś A. M.
Published: July 2007

With a new spectral density approach it is shown firstly that the ground state of the Anderson model is nonmagnetic. Local quasiparticles corresponding to transitions between different occupancies of the impurity orbital are shown to be well defined and well separated for realistic values of the parameters of the Anderson model. Lifetimes of the quasiparticles strongly depend on the hybridization coupling constant Vdk and are almost independent of temperature. The maxima of the spectral density Adσ(ω) are in qualitative agreement with the results of the optical absorption experiments performed on noble metal alloys containing 3d magnetic impurities.
Acoustic Wave Scattering by Mixing Modes in Piezoelectric Media
Authors: Albuquerque E. L.
Published: July 2007

The scattering process for waves propagating in piezoelectric media is presented and its features are discussed. The coupling between the acoustic and electromagnetic field, for the geometry presented in this paper, allows these waves to be polarized in the xz plane showing a new aspect of the scattering process. Some limiting cases are presented and they are in agreement with the general treatment.
Negative Volume Plasmon Dispersion in Be for k > kF
Authors: Priftis G. D. , Boviatsis J.
Published: July 2007

Measurements of volume plasmon dispersion in beryllium excited by X‐rays are reported for k up to 1.5kF. The dispersion curve shows a flattening for k near kF and for k ≈ 1.2kF its slope becomes negative. These results are consistent with previous X‐ray scattering experiments and with some electron scattering experiments. However they cannot be explained in the context of the existing RPA theories.
Effects of Microstructure Scale on the Temperature Coefficient of Electrical Resistivity
Authors: Aubauer H. P. , Rossiter P. L.
Published: July 2007

The temperature coefficient of resistivity dϱ/dT is shown to contain a component which depends upon the microstructure scale R0 and the conduction electron mean free path Λ. The size of this contribution depends upon the rate of change of Λ with temperature but in general dϱ/dT will be larger when R0 < Λ. The general predictions of the theory are shown to be in agreement with the limited experimental data available.
Stability Region of the Two‐Dimensional Electron Crystal in Strong Magnetic Fields
Authors: Lozovik Yu. E. , Musin D. R.
Published: July 2007

In the self‐consistent harmonic approximation equations are obtained for the frequency spectrum of a two‐dimensional electron crystal in a magnetic field H. From the condition of the existence of “soft” mode peculiar for the crystal at H ≠ 0 the absolute stability curve of the crystal is analyzed. The stability region increases with H. In the H‐n phase diagram at temperature T = 0 (n is the electron density) along the stability curve obtained the phenomenological Lindemann‐type rule 〈u2〉/b2  γ = const. is fulfilled for strong H with γ ≈ 0.06 (〈u2〉 is the mean square amplitude in the harmonic approximation, b is the lattice period). At H → 0 the Lindemann‐type constant γ decreases. The phase diagram of the two‐dimensional electron system and effects of H on the electronic crystal‐isotropic liquid and liquid‐electronic crystal‐isotropic liquid transitions are discussed.
Effective One‐Particle Crystal Field Potentials in the Point Charge Approximation
Authors: Adam S. , Adam Gh. , Corciovei A.
Published: July 2007

A generał method is developed for the derivation of the one‐particle crystal field potential characteristic of a given crystallographic symmetry, for arbitrary effective interatomic forces. Calculations performed for cubic and hexagonal structures lead to the standard representations in spherical harmonics with the coefficients given, however, by power series of rn rather than by single rn terms as obtained within the usual hypothesis of Coulombian interatomic forces. This result is found to have implications on the interpretation of some recently published theoretical and experimental data.
A Study in the Homology of Lattice Mechanical Properties of Alkali Metals
Authors: Sarkar S. K. , Sen D. , Sengupta S.
Published: July 2007

The nature of the dependence of the Heine‐Abarenkov model pseudopotential parameters rc and u for different alkali metals, obtained through a comprehensive unified study of the metals, on the atomic number, Z, is investigated, leading to simple empirical relations between Z and the parameters. The product of Z1/2 and the dynamical matrix, D, calculated using the parameters yielded by these relations is found to satisfy a relation of the form DZ1/2 = C1(q, e) for various Z‐values, C1(q, e) being a constant depending on the phonon wave vector q and the polarization e. This at once leads to the relation m1/2Z1/4v = C2(q, e) which is verified empirically and thus implies that the lattice vibrations of alkali metals are homologous. The relations obtained are used for a comprehensive unified study of Cs and for predicting its dispersion relation. The importance of the approach lies in the fact that knowing only Z, a unified study of a large number of lattice static, dynamic, and electronic properties of any alkali metal can be carried out within the framework of the pseudopotential theory.
Magnetic Configurations in Anhydrous Sulphates and Selenates of the 3d Transition Metals
Authors: Hautecler S. , Wegener W.
Published: July 2007

The various magnetic structures compatible with the crystallographic symmetry in the anhydrous sulphates and selenates of the 3d transition metals are determined using the matrix method of Bertaut. The stability conditions and the minimum range of the magnetic interactions needed to stabilize the different configurations are derived.
Luminescence et interaction phonon—électron dans le luminophore MgS:Bi3+. I. Expérience générale
Authors: Asano S. , Yamashita N.
Published: July 2007

Emission and excitation spectra of MgS:Bi3+ phosphors are measured at 300, 80, and 6 K. Two emission bands 3T1u, 3A1u → 1A1g and three excitation bands A, B, C peculiar to Tl+‐type ions are observed. Furthermore, the D′ excitation bands corresponding to charge transfer transitions are also observed. The phonon structure on the emission and excitation bands for MgS:Bi3+ phosphors is compared with that for CaO:Bi3+ and CaS:Bi3+ phosphors. The values of the energy parameters of Bi3+ center are determined from the position of the three excitation bands.
Electron Paramagnetic Resonance Studies of Cu2+ in NH4Cl1−xBrx Single Crystals
Authors: Aruna B. , Ramakrishna Y. V. S. , Narayana P. A.
Published: July 2007

Electron paramagnetic resonance studies of Cu2+ in mixed single crystals of NH4Cl1−xBrx (0 ≦ x ≦ 0.2) are carried out in the temperature range of 100 K ≦ T ≦ 290 K. The variation of the spin Hamiltonian parameters with temperature and the dependence of the linewidth on the nuclear magnetic quantum number MI indicate that Cu2+ is coordinated only by Cl− for x ≦ 0.08. In the range of 0.08 ≦ x ≦ 0.2 Cu2+ is coordinated by both, Cl− and Br−.
Peculiarities of an Electromotive Force Arising in a Scemionductor in a Non‐Homogeneous Magnetic Field (II)
Authors: Gulamov Sh. A. , Katulevskii Yu. A. , Muminov R. A.
Published: July 2007

The spatial distributions of current carriers and the electric field as well as the expression for the electromotive force are found for the homogeneous semiconductor placed only into a non‐homogeneous magnetic field for the case when the semiconductor length in the direction of the magnetic fields gradient far exceeds the bipolar diffusion length of current carriers. The magnetic field is supposed to be non‐quantized. It is shown that these characteristics strongly depend on the spatial structure of the magnetic field. The special case of linear coordinate dependence of the magnetic field is considered.
A New‐Type V Centre in SrO
Authors: Seeman V. , Reifman S. , Lehto T.
Published: July 2007

A new type of V centre with orthorhombic symmetry is observed in SrO crystals. An investigation of the superhyperfine structure of the ESR spectra shows that this centre consists of an O− ion near a cation vacancy surrounded by two OH− groups. The values of the superhyperfine structure constants are used for the calculation of the distance between the O− ion and the OH− groups.
Nonlocality of Carrier Multiplication in Semiconductor Depletion Layers
Authors: Gribnikov Z. S. , Ivastchenko V. M. , Mitin V. V.
Published: July 2007

Analytical and numerical calculations of multiplication coefficients in the Schottky junctions are performed with inhomogeneity of the electric field taken into account. It is shown that in the thin depletion layers distribution functions of electrons and holes and their impact ionization (II) coefficients is nonlocal, i.e. they depend not only upon the electric field strength at a certain point but also on the fields in the vicinity of the point. In such nonlocal cases the introduction of the II coefficients is justified only for small multiplication coefficients, for the large ones they make no sence. If one assumes that the carriers — holes and electrons — differ from each other by the sign of the charge only, the nonlocality effect does not change just the values of the II coefficients obtained in the local approximation, but provides as well the difference between the electron and hole multiplication coefficients. This is due to the electric field asymmetry in the depletion layer.
Theory of Second‐Harmonic Generation by Soft Ultrasonic Shear Waves in Transforming A‐15 Compounds
Authors: Agyei A. K.
Published: July 2007

A theory of second‐harmonic generation by pulsed “soft” ultrasonic shear waves in a transforming A‐15 compound, A3B, is presented which accounts for the generation observed in V3Si by Testardi. It is shown that this phenomenon, which is forbidden on the basis of static symmetry considerations, is possible under conditions of quasi‐static distortion of the crystal by the free A‐atom oscillations excited by the front of the driving‐wave pulse. Under these conditions not only should the driving‐wave‐amplitude dependence of the second‐harmonic waves be cubic at low drive levels and linear at high drive levels, as observed by Testardi, but the generated waves should also be amplitude modulated by a sine curve (in the absence of damping) of a period equal to the free‐oscillation period of the A atoms. This latter effect can thus be used to measure the soft‐phonon frequency as a function of temperature and the phonon dispersion at very low wave numbers.
On the Electrical Conductivity of the Periodic Anderson Model
Authors: Elk K.
Published: July 2007

The periodic Anderson model is used as a simple model describing intermediate valence compounds. In that model a strong electron—electron correlation is taken into account. The correlation term is decoupled using the alloy analogy approximation, which yields a two‐peak structure of the density of states of the f‐electrons. Then the electrical conductivity can be obtained using the Kubo‐Greenwood formula. By numerical methods the density of states and the temperature dependence of the conductivity are calculated. The density of states shows an interesting structure in the centre of the hybridized f‐band. For the conductivity σ an increase with increasing temperature is found. A remarkable pressure dependence of σ is not observed in the model used.
The Exponential Shape of the Optical Absorption Edge Tail
Authors: Szczyrbowski J.
Published: July 2007

A simple model of light absorption for photon energy smaller than the width of the forbidden band‐gap Eg is proposed. Assuming that the shape of the fundamental absorption edge is influenced by elastic fluctuations in Eg caused by both, permanent and temperature lattice disorder, a general expression for the exponential shape of the optical tail is obtained, which is valid for both, amorphous and crystalline solids. The so‐called Urbach rule (exponential dependence on energy and temperature) represents an asymptotic relation of this formula when the permanent disorder is neglected. The proposed model also predicts quantitatively the values of the steepness parameter for several materials which show the Urbach shape of the absorption edge tail.
Spectral Density Method for the Spinless Fermion Model
Authors: Ihle D. , Lorenz B.
Published: July 2007

A self‐consistent approximation scheme for the spectral densities of the one‐dimensional spinless fermion model is developed, where the one‐particle spectral density fulfils the first four moments. At T = 0 the one‐particle excitation spectrum is calculated in dependence on the Coulomb and band energy parameters. The exact results for the atomic and band limits and the gas—liquid like phase transition are reproduced.
Correlated Hartree‐Fock Theory of the Electron—Hole Plasma Containing Exciton Bound States
Authors: Stolz H. , Zimmermann R. , Röpke G.
Published: July 2007

For the electron—hole plasma the Hartree‐Fock scheme is extended to include those pair correlations leading to excitonic bound states. Thus an effective single‐exciton Schrödinger equation is obtained systematically accounting for the interaction of an exciton with others in a mean‐field approximation valid to linear order in the density. This single‐exciton equation may also be derived from a many‐exciton Hamiltonian subject to Hartree‐Fock decoupling. The excitons are represented by Boson operators, the Pauli principle for the electrons and holes being maintained by a proper antisymmetrization of the interaction matrix element.
Size Effect and Temperature Dependence of the Electrical Resistance in Bismuth Single Crystals of High Perfection
Authors: Kraak W. , Kuhl R. , Thom F. , Haefner H. , Schmidt P.
Published: July 2007

Bi single crystals of high perfection (having extremely high resistance ratios r4.2 = ϱ290 K/ϱ4.2 K) are grown by the mould growing method. In these crystals, the thickness dependence of the resistance ratio r4.2 and the temperature dependence of the electrical resistance are investigated in a temperature range between 2 and 77 K. In the range of the diffusion size effect, a markedly different behaviour is found to occur in the thickness dependence of the two crystal orientations investigated (j ‖ C3; j ‖ C2). An anisotropy of the slope of the T2‐terms is detected in the temperature dependences of the electrical resistance in the temperature interval from 2 to 16 K.
Role of Free Electrons and Holes in the Exciton Condensation Process in Semiconductors
Authors: Ashkinadze B. M.
Published: July 2007

An experimental study of hysteresis in the luminescence of electron—hole drops in Ge upon the action of a magnetic field is presented. In the magnetic field the free electron and hole flows onto a drop decrease. This results in an increase of the nucleation rate of the drops and also the decrease of the drop radius. The principal experimental results are: 1) The threshold phase diagram obtained in the presence of magnetic field is different from that for H = 0; 2) the new hysteresis loop is found on varying the magnetic field; 3) the contribution from free carrier current in the stationary drop radius is determined and it follows that the value of the cross‐section for the exciton formation process is of the order of 2 × 10−11 cm2.
Non‐Linear Optical Properties of the Degenerate Band Charge Carrier Gas
Authors: Klymin S. N. , Pokatilov E. P.
Published: July 2007

A theory of non‐linear optical properties of a band charge carrier gas of arbitrary degeneracy due to the influence of a strong electromagnetic radiation on the probability of the carrier scattering by phonons or ionized impurities leading to intraband transitions is developed. A general expression for the polarization created by the carrier response in a laser field in the presence of other electromagnetic fields is obtained at arbitrary temperature and concentration of carriers. The explicit expressions for a generalized dielectric susceptibility tensor and a generalized generation tensor are obtained for all principal scattering mechanisms and arbitrary laser field intensity. The non‐linearity of the absorption coefficient of laser radiation and the dependence of the absorption coefficient of the weak probe wave on the intensity of the laser field are studied.
The Fundamental Absorption Edge of MgSiP2
Authors: Ambrazevičius G. , Babonas G. , Rud Yu. V. , Šileika A.
Published: July 2007

The absorption and wavelength‐modulated absorption spectra of MgSiP2 single crystals are investigated in the region of the fundamental absorption edge at 4.2, 77, and 300 K and under hydrostatic pressures up to 770 MPa. MgSiP2 is determined to be a pseudodirect bandgap compound with a forbidden energy gap of 2.409, 2.404, and 2.33 eV at 4.2, 77, and 300 K, respectively. The pressure coefficient of the absorption edge is −2 × 10−5 eV MPa−1. The spin‐orbit splitting at the top of the valence band of MgSiP2 is close to that for phosphides of the AIIBIVCV2 type. The crystal‐field splitting of the higher valence band is proposed to be negative.
Acceptor Bound Exciton States in High‐Purity CuCl
Authors: Sauder T. , Stébé B. , Certier M.
Published: July 2007

The magneto‐optical properties of high‐purity CuCl at 4.2 K show that the two lines ν½ (25665.2 cm−1) and ν (25697.6 cm−1) are due to transitions to (or from) excitons bound to the same neutral acceptor, the two holes arising from the lowest Γ8 valence band. They are attributed, respectively, to the J = 5/5 and J = 1/2 states of the (A0, X) complex. These results are explained by the importance of the electron—hole exchange interaction and by a dominant occurrence of the Γ8 rather than the Γ7 neutral acceptors. As a consequence, free holes should chiefly arise from the Γ7 valence band. This is due to the particularly low value of the valence band spin—orbit splitting (ΔSO ≈ 600 cm−1), compared to the free‐exciton binding energy (EX ≈ 1523 cm−1) or to the estimated Γ7 acceptor ionization energy (EX ≈ 17000 cm−1). These conclusions support the previous assignment of line ν2 (25654 cm−1) to a transition between a free Γ7 hole and a Γ6 charged exciton X.
Diffusion of Interstitial Solute in a Potential Well Characteristic of a Moving Dislocation
Authors: Fuentes‐Samaniego R. , Hirth J. P.
Published: July 2007

The equation governing the flux of interstitial solute near a moving dislocation is found. Such an equation is non‐linear, a feature which allows for the realistic situation that the solute concentration is bounded by the concentration of interstitial sites. However, in order to find analytical solutions, the alternative of a two‐dimensional, symmetric potential well characteristic of the dislocation is considered. Analytical determination of the properties of such a potential well as either a trap or a carrier of interstitial solute is made. The solute drag force‐velocity curves for the steady‐state motion of a potential well are obtained.
Thermal Conductivity of a Crystal with Impurities of Internal Degrees of Freedom
Authors: Segal A.
Published: July 2007

The thermal conductivity of a crystal with impurities with internal degrees of freedom is theoretically investigated using the double‐time Green's function methods and the Kubo formalism, considering non‐diagonal terms in the heat‐current operator as proposed by Hardy. Mass changes as well as impurity‐phonon interaction are taken into account explicitly. It is shown that the total conductivity can be written as a sum of two contributions: one diagonal and the other nondiagonal. An expression for the inverse relaxation time is obtained and compared with that deduced from the Callaway theory.
Non‐Linear Fluorescence Quenching in Molecular Crystals. III. Recombination of Static Excitations
Authors: Benderskii V. A. , Brikenshtein V. Kh. , Filippov P. G. , Shamovskii I. A.
Published: July 2007

Non‐linear quenching of the excimer fluorescence of pyrene crystals is investigated. The transition between diffusing (T > 150 K) and static excitation (T < 70 K) affects the relationship between the quantum yield of fluorescence Φ and pumping I (from Φ ∞ I−1/2 to Φ ∞ I−2/3 at high I, respectively), and the shape of its initial decreasing portion L (from L ∞ t−1 to L ∞ t−1/2 for a δ‐pulse of pumping). The rate constant of bimolecular excimer recombination γ is (5 × × 1.5) × 10−11 cm3 s−1 at 300 K and decreases exponentially with temperature in correspondence with the decreasing diffusion. The Förster radius R0 corresponding to the measured value of γ is 30 Å and agrees well with the value calculated from the spectral overlap. It is shown that the low‐temperature non‐linear quenching is described quantitatively by the static dipole—dipole excitation recombination model with approximate allowance for the three‐particle correlations. Allowance for higher order correlations is found to be insignificant up to the maximum experimentally attainable excitation density n corresponding to (4π/3) Rn ≈ 8.
Influence of Ni Impurity on the Absorption Spectrum of Layered GaSe Crystals
Authors: Gnatenko Yu. P. , Kovalyuk Z. D. , Skubenko P. A.
Published: July 2007

Absorption spectra of the layer crystal GaSe doped with Ni of different concentrations (0.5, 0.1, 0.05, and 0.01%) are investigated in the temperature range 4.2 to 300 K. In the region 1.5 to 2.0 eV an impurity absorption due to intra‐centre transitions between the 3F and 3P states of the ion Ni is revealed. The energies of phonons participating in the absorption and an intermediate type of electron‐phonon interaction in GaSe are determined. The observed doublet structure can be attributed to the Jahn‐Teller effect for the excited state.
A New Method to Determine the Coupled Mode Parameters of a Plasmon‐Multiphonon System
Authors: Gelten M. J. , Bosch L. A.
Published: July 2007

A new (numerical) method is described to determine all coupled mode parameters of a complex plasmon‐phonon system in a semiconductor. First a general theoretical description is given of the complex dielectric function describing such a system. Some paractically important relations are deduced. Starting from an experimentally determined reflectivity spectrum, the dielectric function is determined at a number of discreate frequencies by means of Kramers‐Kronig analysis. A complete background is given for a numerical method to determine all relevant mode parameters, especially the coordinates of poles and zeros in the complex frequency plane, from this spectrum. Finally the new method is applied to a test spectrum. The results are discussed and compared with those of other methods.
Thermodynamic Properties of Liquid Metals and Alloys
Authors: Khanna K. N. , Khandelwal D. P.
Published: July 2007

An analysis of thermodynamical properties, such as excess entropy, softness of the constituent pseudoatoms, and specific heat, is presented for liquid metals and alloys in the hard sphere reference system. The formalism, based on the concentration and excess volume dependent hard‐sphere behaviour, provides a basis for microscopic understanding of the thermodynamics of alloys.
Resonant photon–photon scattering and approximate spherical symmetry in CuCl
Authors: Van Hieu Nguyen
Published: July 2007

It is shown that if the contribution of the holes in the lower valence bands Γ8 in CuCl is neglected, the state vectors of the stationary states of the excitons and hiexcitons in this semiconductor belong to irreducible representations of the rotation group, and their interaction is spherically symmetric. Due to this approximate symmetry, the partial wave expansion may be applied to the photon‐photon scattering amplitude and the angular dependence of the cross‐section of the resonant scattering via a definite biexciton state may be predicted.
Helium desorption from a tungsten single crystal
Authors: Mhaskar S. P. , Naik P. K.
Published: July 2007

Entrapment of helium in molybdenum and tungsten has been investigated both theoretically and experimentally in detail in the past. In general, peaks observed in the thermal desorption spectra of helium are related to the detrapping reactions of helium from HenV (n = 1 to 4) complexes. A computer programme is developed to simulate the activation energies of these detrapping reactions in the bulk of the solid and in the vicinity of the crystal surface. Experimental values of the activation energies of these reactions reported earlier by Kornelsen show good agreement with the values calculated for the same reactions occurring in the bulk. Values calculated for these reactions near the surface are much lower than the experimental values. It is further shown that an agreement between the experimental and the calculated values of activation energies for the surface reactions is possible only if the entrapment is made to occur near the surface.
Damping of acoustic modes in one‐dimensional systems
Authors: Rowlands G.
Published: July 2007

The propagation of long‐wavelength disturbances in a 1D lattice with N atoms per unit cell is considered. The appropriate value of the sound velocity is obtained. It is shown that for N sufficiently large such disturbances will be damped, and an expression for the damping rate is given. The analysis is made applicable to disordered systems by considering the limit N = ∞).
Local susceptibility calculation for intermediate‐valence solids
Authors: Schlottmann P. , Falicov L. M.
Published: July 2007

A calculation of the local susceptibility of an intermediate‐valence solid is presented. The mode which resembles some thulium salts, consists of an ionic doublet (with n electrons) or an ionilc triplet (with (n + 1) electrons) and a variable number of electrons in a “conduction” band. Hybridization is fully taken into account locally. The calculation is performed in the micro‐grand‐canonical ensemble. Both a Curie‐ and a Van‐Vleck‐type susceptibilities are found. The contributions from doublet, triplet, and itinerant electrons and their interferences are analyzed. Under some conditions it is found that the hybridized intermediate‐valence compound has a lower effective magnetic moment than both the doublet and the triplet.
Two‐photon absorption of F‐centres in KCl
Authors: Vogler K.
Published: July 2007

Picosecond light pulses are used to give evidence of the occurrence of two‐photon absorption of F‐centres in KCl. The results, especially the unchanged fluorescence decay, are discussed on the basis of the mixed‐state model of F‐centres.
Die Bestimmung der Valenzladungsdichte von Elementhalbleitern und III–V‐Verbindungen mit Röntgenbeugungsmethoden. I. Grundlagen
Authors: Brühl H. G.
Published: July 2007

Einige Methoden zur Berechnung der Valenzelektronendichte an Elementhalbleitern und III‐V‐ Verbindungen werden vorgestellt. Eine kurze Diskussion der Voraussetzungen für die Anwendbarkeit der Modelle und der erreichbaren Aussagen erfolgt. Die angeführten Methoden sind nicht gleichermaßen für die Diamant‐ und Zinkblendestruktur geeignet. Eine ausführliche Beschreibung des Tetraeder‐Deformations‐Modells (TDM) schließt sich an. Der Strukturfaktor |F| wird in erster Näherung für die Zinkblendestruktur abgeleitet. Die Valenzelektronendichte kann für einen beliebigen Punkt der Elementarzelle berechnet werden. Die Vor‐und Nachteile des TDM‐ Modells werden diskutiert.
On the equivalence of different approximations in linear electron transport theory
Authors: Van Royen J. , Beleznay F. , Devreese J. T.
Published: July 2007

A critical comparison is made of different methods to obtain the mobility in linear electron transport theory. An approximation to the Kubo formula is studied, a calculation based on the displaced Maxwellian distribution function, and a method introduced in path integral calculations. It is shown that the three methods are equivalent: they lead to the same resistance in the weak scattering limit, for small electric fields. Special attention is given to the weak coupling Frohlich polaron mobility.
On self‐deconvolutions in Auger spectroscopy
Authors: Bartoš I. , Máca F.
Published: July 2007

Two self‐deconvolution procedures are discussed in the context of CVV Auger processes and their stabilities with respect to signal distortions from self‐convoluted forms are tested. In the sequential method the results are highly sensitive to random fluctuations in general, and especially at the edge of spectra; the reverse run starting at the low‐energy end is often useful in handling the data. The Fourier transformation method, being a global one, is far less sensitive to the above‐mentioned distortions; the resulting increased bandwidths can be suppressed by proper corrections.
Electron transfer within F center pairs in KCl from optical emission and absorption
Authors: Mezger A. C. , Jaccard C.
Published: July 2007

Transient F luminescence and F′ population after F band excitation and the effect of a static magnetic field (0.5 T) are measured in KCl at 10 K for F center concentrations between 5 × 1016 and 1 × 1018 cm−3. The results are explained by an F center pair model involving a radiative decay (time τr), direct and inverse non‐radiative electron transfers within the pair depending exponentially on the separation: τ = τ0 exp (r/λ), and a Poisson spatial distribution of the individual centers, with the following parameters: τr = (890 ± 5) ns; τ0 = (0.4 ± 0.2) ns, λ = (0.8 ± 0.1) nm (direct); τ0i = (2 ± 1) ms, λ′ = (0.8 ± 0.1) nm (inverse).
Force constant change of Mössbauer 119Sn impurity in copper host
Authors: Puri R. K. , Gupta L. R.
Published: July 2007

The temperature dependence of Mössbauer recoil‐free fraction of 119Sn atom, present as a dilute impurity in copper host, has been measured. The experimental data are analysed in terms of the moments, ω(−2), of the phonon frequency distribution of the 119Sn impurity atoms in the copper lattice to obtain the weighted mean frequency and characteristic temperature. The ratio of impurity‐host to host‐host force constant is found reduced to (0.61 ± 0.01) as calculated on the basis of the nearest‐neighbour central force model using Debye phonon frequency spectra.
A comparative study of the pseudopotentials for Cu, Ag, and Au
Authors: Idrees M. , Khwaja Farid A. , Razmi M. S. K.
Published: July 2007

An analysis of the pseudopotentials proposed for copper, silver, and gold is presented. Three tests based on the calculation of band gaps, liquid (molten) metal resistivities, and thermoelectric power are carried out. Numerical results obtained are compared with the corresponding experimental values. It is concluded that the pseudopotential of Borchi and De Gennaro is the only one giving some overall agreement with experiment. It is pointed out that the s‐d hybridization contribution in the Borchi‐De Gennaro pseudopotential needs to be suitably modified if closer agreement with experiment is to be achieved.
A Renormalization Group Approach for Randomly Distributed Conductance
Authors: Kazakov V. A. , Satanin A. M.
Published: July 2007

The recursion equation for the probability scaling behaviour of conductance for a randomly inhomogeneous resistive medium is obtained. The equation enables one to calculate percolation thresholds and a complete set of critical indices in the continuous percolation problem for which in a d‐dimensional space the following formulae are derived: t=v(d − 1), q = v, s = (d − 1)/d, m = (dv)−1. For the cubic lattice t = 1.23 in a two‐dimensional and t = 1.74 in a three‐dimensional space are obtained. The scaling behaviour of dispersion and of the mean value of the normal distribution near a homogeneous fixed point is found.
A New Interpretation of EXAFS Spectra in Real Space: II. A Comparison of the Regularization Technique with the Fourier Transformation Method
Authors: Ershov N. V. , Ageev A. L. , Vasin V. V. , Babanov Yu. A.
Published: July 2007

The errors in the initial EXAFS data are simulated numerically. The comparative sensitivity of the regular methods and of the Fourier transformation method to these errors is analyzed. The theoretically calculated normalized oscillating part of the absorption coefficient χ(k) for polycrystalline copper at T = 77.4 K is compared with the experimental spectrum obtained with the use of a synchrotron facility. The spectrum involved is processed using regular methods. A substantial improvement of the result in comparison with the Fourier method is shown.
Near‐Infrared d–d Transitions of NiI2, CdI2:Ni2+, and CoI2
Authors: Kuindersma S. R. , Boudewijn P. R. , Haas C.
Published: July 2007

The absorption spectra of NiI2, CdI2:Ni2+, and CoI2 in the wave‐number region 4000 to 16000 cm−1 are reported. The spectra are interpreted in terms of d‐d transitions of Ni2+ and Co2+ ions. Parameters of the electron repulsion, cubic crystal field, and spin‐orbit coupling are estimated. From the latter the transferred spin densities are deduced and compared with Mössbauer data. Birefringence experiments as a function of the temperature show the presence of two phase transitions in niI2.
Relativistic Effect on a Deformed Semi‐Infinite Crystalline Surface
Authors: Yuyi Shen
Published: July 2007

The deformed surface described by some parameters has been studied by Volkenstein [1] in 1947 and by Steslicka [2] in 1974 in the non‐relativistic case. In this paper, the study is extended to the relativistic case. When the parameters take certain values, the relativistic equations of the deformed surface correspond to the relaxation, segregation, or relaxation‐segregation case, respectively. When c → ∞, i.e. for the non‐relativistic limit, the equations will approach the results obtained by Steslicka.
Electron–Hole Plasma Expansion in Direct‐Gap GaAs1−xPx
Authors: Modesti S. , Frova A. , Staehli J. L. , Guzzi M. , Capizzi M.
Published: July 2007

The e‐h plasma in the ternary alloy GaAs1−xP2 below direct‐indirect crossover is studied at 2 K by means of very high excitation photoluminescence measurements. The most interesting feature observed is an extremely efficient expansion of the excited region when the particle density reaches the value n* needed to minimize the free energy. This behaviour and the corresponding spectral evolution with power density are accounted for by a model where pressure‐induced flow of particles away from the pumped region becomes competitive with recombination mechanisms.
Electronic State Calculations of Dislocations in Cadmium Telluride
Authors: Öberg S.
Published: July 2007

Using a tight binding method the localized electronic states in CdTe connected with the core of 30° and 90° glide partials are calculated. Periodic arrays of large unit cells each containing two 30° or 90° partials with Burgers vectors of opposite sign are considered. Several models of the core structure of each partial are analyzed. The electronic states of dissociated screw and 60° glide dislocations are discussed.
X‐Ray L1 Spectra and XPS Investigation of the Electronic Structure of Indium Compounds
Authors: Gusatinskii A. N. , Blokhin M. A. , Shumichen P. , Bunin M. A. , Mikhailova S. S.
Published: July 2007

Metallic indium and some of its compounds with various types of chemical bonds are investigated by means of X‐ray and X‐ray photoelectron spectroscopy (XPS). In the first part of this work emission InLγ2,3‐lines (transition L1‐N2,3) and InN2,3 XPS are obtained. The splitting of the Lγ2,3‐ lines is detected for In and the spin‐doublet N2,3‐level parameters are measured. Furthermore the L1‐emission bands and L1‐absorption spectra are (apparently for the first time) used to obtain information on the band structure of some semiconductor compounds.
Changes of EPR Spectra by Inhomogeneous Plane Stress Distributions in a Two‐Layer System
Authors: Gehlhoff W. , Segsa K. H.
Published: July 2007

The influence of plane stress distributions on lineshapes and line positions of EPR transitions in a two‐layer system (film‐substrat) is investigated. Basing on previous calculations explicit expressions for the lineshape of EPR transitions are derived also for the case of an inhomogeneous stress distribution. The effects of homogeneous and inhomogeneous stress distributions on the EPR spectra of paramagnetic systems with S = 1 are represented. The possibility to confuse stressinduced changes of the lineshape with hyperfine and superhyperfine structures is demonstrated.
Small Wave Vector q and Frequency ω Dependent Longitudinal Dielectric Funtion for a Model Semiconductor
Authors: Sharma A. C. , Auluck S.
Published: July 2007

A modified calculation of the small wave vector q and frequency ω dependent longitudinal di‐electric function for Milchev's model semiconductor is presented. To evaluate the matrix elements, two‐band wave functions are used instead of plane waves (as used by Milchev). The energy difference between the valence and conduction bands is taken to be the same as Milchev's. It is found that the general behaviour of the real and imaginary parts of the dielectric function ϵ1(q, ω) and ϵ2(q, ω) are similar to that found by Milchev and by Walter and Cohen using a realistic band structure. The static dielectric function ϵ1(q, 0) agrees well with the exact calculation of Walter and Cohen. ϵ(0,ω) accounts properly for the Kramers‐Kronig relation and up to a reasonable extent for the f‐sum rule. It is also in good agreement with optical data. For small q the plasmon dispersion is given by Ep(q) = Ep(0) + Aq2 where A is an analytical function of Δ = Eg/4EF where Eg is the energy gap parameter and EF the Fermi energy.
Mixed Valence Phase Transition in Ferromagnets
Authors: Valkov V. V. , Ovchinnikov S. G.
Published: July 2007

The electronic phase transitions in ferromagnetic semiconductors connected with redistribution of electrons between localized and band states due to s‐d Coulomb and exchange interactions and hybridization is investigated. There are several regions on the phase diagram of the system at T = 0 with integral or mixed valence. The temperature behaviour is determined by the ground state and the transition between different valence state may be smooth as well as sharp.
Effective Pair Potential and Thermodynamic Properties of Liquid Noble Metals
Authors: Joarder R. N. , Rao R. V. Gopala
Published: July 2007

The variational method associated with the Gibbs‐Bogolyubov inequality in the thermodynamic perturbation method allows the determination of the thermodynamic properties of liquid noble metals. The interionic potentials are generated from the energy‐wave‐number characteristics obtained from a model pseudopotential in conjunction with the dielectric response function of the interacting electron gas. The variation of the packing density with volume and temperature shows a relatively hard core pseudoatom and it is shown that under pressure, along the melting line, the packing density actually increases for noble metals unlike the simple metals. In addition, the results compare favourably with experiments as well as the calculations of Moriarty based on Harrison's generalised transition metal pseudopotential.
Equation for the Melting Curve of Solids under High Pressure
Authors: Boguslavskii Yu. YaL
Published: July 2007

Simon's equation of the melting curve is obtained using the Clausius‐Clapeyron equation in the linear approximation of the pressure dependence of the melting entropy and the volume change at the melting point. The constants in Simon's equation are calculated in this approximation for the alkali metals Li, Na, K, Rb, Cs and also for hydrogen, H2, and argon. It is shown that one can obtain the constants of Simon's equation in a pressure range which is wider than the region of the thermodynamical validity of Simon's equation by averaging the values of the constants determined in different points of the melting curves. The constants obtained by this manner agree well with the experimental data.
Spin–Orbit Interaction in Superexchange with Orbital Degeneracy. I. Coupling Coefficients and Operator Algebra for the Chain 0* ⊂ SU2
Authors: Blaszak M.
Published: July 2007

Because the spin–orbit splitting in the strong crystal field approximation leaves more degeneracy than that expected from the group theory, all considerations should be extended from one symmetry group to the chain of groups. The nΓγ and nΓ symbols for octahedral spinor group O* and the njΓ and njΓ symbols for the chain O* ⊂ SU2 are introduced. Irreducible operator sets for both groups are defined and the connections between them are found. The connection between operators from associated spaces for the O* group is found also.
Electron‐Energy‐Loss Spectroscopy of Different Al2O3 Modifications. I. Energy Loss Function, Dielectric Function, Oscillator Strength Sum Rule and the Quantity ϵ2E
Authors: Tews W. , Gründler R.
Published: July 2007

Using 40 keV electrons electron‐energy‐loss measurements are performed on amorphous, polycrystalline γ‐ and α‐Al2O3 thin films. From the electron‐energy‐loss spectra measured in the energy range up to 40 eV the energy‐dependent energy loss function Im(–ϵ−1) is derived. The dielectric function, optical constants, and neff(E) are computed using the Kramers‐Kronig analysis and wellknown formulae. The quantity ϵ2E is compared with a simple joint density of states model.
Coherent Motion of Triplet Excitons in Molecular Crystals with Two Differently Oriented Molecules per Unit Cell. Application to the Charge Transfer Crystal Anthracene/Tetracyanobenzene
Authors: Schmid U. , Reineker P.
Published: July 2007

The dynamics of triplet Frenkel excitons moving coherently in a molecular crystal is investigated by a model calculation. The model assumes that the crystal has two differently oriented molecules per unit cell and that the interaction between the molecules in different crystal directions is strongly anisotropic. The Hamiltonian of the model consists of an electronic and a spin part. The first part contains the excitation energy of the electron‐hole pair and the interaction matrix element for the coherent exciton transport. The second part comprises the Zeeman energy of the triplet spin and the fine‐structure terms of the differently oriented molecules. The eigenvalue problem is solved for parameters of anthracene/tetracyanobenzene in the case of zero and high magnetic fields. In zero field the experimentally observed doublet structure of the ODMR lines can be explained, in high field especially the ESR line positions for several orientations of the crystal in the field are discussed.
Thermoluminescence of Coloured Alkali Halides
Authors: Murti Y. V. G. S. , Sucheta N.
Published: July 2007

A rigorous analysis of the thermostimulated luminescence of several irradiated alkali halide crystals is presented. The accuracy of the fitting of glow curves to theoretical models is examined with the help of a statistical chi‐square test and the applicability of the different physical models is discussed.
Electron–Hole Correlation Effects on Donor–Acceptor Pairs
Authors: Kartheuser E. , Babcenco A. , Evrard R.
Published: July 2007

The effect of the electron–hole correlations on the effective mass electronic states of donor–acceptor (DA) pairs in polar semiconductors is investigated. Variational calculations are carried out and compared with previous theoretical work in the case of GaP and ZnSe. Analytic expressions for the energy, charge density, and pair distribution function of DA pairs are obtained as a function of the pair separation R. It is shown that whereas the energy and the charge distribution of a DA pair are only slightly affected by the electron–hole correlations, the pair distribution function shows a strong enhancement at short electron–hole distance.
Modified Self‐Scattering Procedure for Monte‐Carlo Simulation of Hot Electrons
Authors: Kozlov V. A. , Nefedov I. M.
Published: July 2007

The generalization of the self‐scattering procedure of the electron trajectory simulation in semiconductors for the arbitrary scattering rates w(p, p′) from the state with momentum p to p′ is suggested. In the method proposed instead of the total scattering rate per unit time v (p) = f w(p, p′) d3p′ the random function ξ(p) with the average value 〈ξ(p)〉 = v(p) is introduced. Such simulation procedure corresponds to the calculation of v(p) along the simulated trajectory by means of the Monte‐Carlo method. The comparison of the commonly employed self‐scattering procedure with the suggested one is performed.