bluuebunny/crossref_metadata_2025 huggingface dataset:
Coexisting phases in an electron—hole plasma
Authors: Ebeling W. , Kraeft W. D. , Kremp D. , Kilimann K.
Published: July 2007

The thermodynamic stability of the optically excited electrons and holes in a semiconductor is investigated in a wide temperature—density range. In dependence on the position of the instability region with respect to the Mott curve the two coexisting phases (i), (ii) may have very different properties: e.g. (i) drops of degenerate electrons and holes and (ii) a partially ionized plasma of excitons, electrons, and holes. For Ge the region of instability is determined. For this purpose the chemical potential is calculated exactly at all temperatures in the ideal and Hartree‐Fock contributions and approximately in the correlation part, using exact formulae for the highly degenerate and the non‐degenerate cases. In the low density region the mass action law is taken into account.
High‐temperature conductivity of organic bilinear addition compounds
Authors: Phillips J. C.
Published: July 2007

A specific model is proposed that correlates the power index n = − (d log σ/d log T), in fits to the longitudinal conductivity σ(T) at temperatures above the maximum in dσ/d(T−1), with structural characteristics of addition compounds of TCNQ and similar molecular crystals.
Shubnikov‐de Haas oscillations in HgSe at 77 K in pulsed magnetic fields
Authors: Byszewski P. , Kowalski J. , Gałązka R. R.
Published: July 2007

Results are given of measurements of the Shubnikov‐de Haas effect at 77 K in pulsed magnetic fields up to 32 T. HgSe single crystal samples are measured in the electron concentration range from 2.2 × 1017 to 2 × 1018 cm−3. To explain the experimental data the curvature of the lowest Luttinger level versus kz has to be taken into account. The change of the Fermi energy with magnetic field is also computed. The results show good agreement between experimental data and theory.
Zero‐field splitting of 6S(d5) ions in tetragonal and rhombic symmetry. II. Application of complete d‐electron calculations to the zero‐field splitting in Mn:MnF2 and Mn:ZnF2
Authors: König E. , Schnakig R.
Published: July 2007

The results of a complete d‐electron calculation for configuration 3d5 of orthorhombic symmetry are applied to a study of zero‐field splitting in Mn2+:MnF2 and Mn2+:ZnF2. The parameters are determined from the optical spectra of the crystals by a least squares fit and adjusted by conditions governing the zero‐field splitting. The results for Mn2+:ZnF2, D = −180.0 × 10−4 cm−1, E = −42.2 × 10−4 cm−1 are in excellent agreement with the experimental values D = (−186 ± 7.5) × 10−4 cm−1, E = (−41 ± 12.5) × 10−4 cm−1. The data for Mn2+:MnF2 are used to estimate the unknown covalency contribution. All contributions fit into a regular and consistent pattern.
Theory of the hopping conductivity in non‐crystalline systems
Authors: Moseley L. , Tripathi R. S. , Lukes T.
Published: July 2007

The Kubo equation for the ac conductivity of a disordered system at T = 0 is evaluated for non‐interacting electrons. An expression for this is given in closed form and agrees in limiting cases with that discussed by Mott.
On the theory of the structural phase transition in squaric acid
Authors: Zinenko V. I.
Published: July 2007

Crystals of the squaric acid C4H2O4(H2SQ) and C4D2O4(D2SQ) exhibit a structural phase transition at T0H = 370 K and T0D = 520 K. The phase transition is associated with the ordering of protons, moving above T0 in the symmetric double well potential of OHO bonds. The statistic of the phase transition in H2SQ and D2SQ is investigated in the cluster approximation with account of the tunneling of the protons. It is shown that the phase transition in the model is always of second order and that the increase of the ordering parameter with the decrease of temperature takes place in a large temperature range. The predictions of the theory are compared with experimental data at hand.
Electronic structure of BiI3
Authors: Schlüter M. , Cohen M. L. , Kohn S. E. , Fong C. Y.
Published: July 2007

The band structure of BiI3 is calculated using the empirical pseudopotential method (EPM). Measurements of the reflectivity and wavelength modulated reflectivity spectra at liquid helium temperatures are presented. For energies below 4.5 eV, fair agreement is obtained between the experimental and theoretical reflectivity spectra. The valence band density is compared to recent X‐ray photoemission (XPS) data. Charge density distributions of distinct groups of valence bands are calculated and combined with the density of states spectrum. These are used to explain the nature of the chemical bonding in BiI3.
A Green‐function theory of an anisotropic ferromagnet
Authors: Pandey R. B. , Bhandari C. M.
Published: July 2007

Green's function technique is used to investigate the magnetic properties of a wide range of anisotropic ferromagnets. A method of terminating the series of equations through the standard spin identity for spin‐1 systems is used to treat the Green functions without decoupling and to obtain a set of coupled equations. Magnetization 〈Sz〉 and 〈(Sz)2〉 are studied as a function of different anisotropic parameters, i.e. single‐ion crystal‐field anisotropy parameters D1 and D2, exchange‐anisotropy parameters η1 and η2, and temperature. Analytic expressions are derived in some limiting cases. In the absence of crystal‐field anisotropy expressions for magnetization and critical temperature as functions of exchange anisotropies are obtained by series expansion methods. All these physical properties are discussed in different models and are in limiting cases in agreement with some earlier results.
Volume dependence of the electronic structure of lanthanum and cerium
Authors: Glotzel D. , Fritsche L.
Published: July 2007

Results of a band structure calculation for the f.c.c. lattices of lanthanum, γ‐and α‐cerium are presented. The computation is based on Altmann's rigorous cellular method. A local potential is used generated according to the standard method. Charge densities of the free atoms are obtained from a SCFXα‐integration of the Dirac equation. Earlier band calculations using the full Slater exchange led to f‐levels far too low in energy. Instead the more adequate Gaspar‐Kohn‐Sham value of 2/3 is used for the exchange scaling factor, which yields reasonable f‐band energies. The Permi level proves to be well below the f‐bands for both metals. As follows from an analysis of the occupied band states in terms of angular momenta, there are f‐admixtures of 0.3 and 0.6 electrons/atom for La and Ce, respectively, which increase slightly under pressure. As the pressure rises the Fermi level moves across three van Hove singularities in the electronic density of states at –15, 10, and at 60 kbar. This is also reflected in topological changes of the Fermi surface which can give rise to isostructural phase transitions. Recent Bremsstrahlung‐ and XPS‐spectra of La and Ce are in fair agreement with the results on the density of states.
The absorption spectra of Y3Fe5O12 and Y3Ga5O12:Fe3+ to 5.5 eV
Authors: Scott G. B. , Page J. L.
Published: July 2007

The absorption spectrum of Y3Fe6012 (YIG) is obtained to 40000 cm−1 and analysed by Gaussian fitting to the spectrum of ε0 obtained from the absorption data and the known refractive index behaviour. The energies of the major components agree well with those obtained in Y3Ga5O12:Fe3+. Comparison of the spectra of Y3Fe0.09Ga5.O12 and YIG indicates that all transitions below 35000 cm−1 involve Fe3+ pairs and that the first charge transfer transition between the oxygen 2p band and the iron d‐like bands occurs at 35600 and 37400 cm−1 in YIG and Y3Fe0.09Ga4.91O12 respectively with an oscillator strength of approximately 0.1. An assignment is forwarded for the pair bands in terms of intersublattice Fe3+ charge transfer transitions with reasonable agreement between the expected and observed energies and intensities. An example of the use of the density of states plot, obtained from the analysis, to explain the features of impurity absorption spectra in YIG is given for YIG:Si.
Effect of stress on the raman spectra of Mg2Si and Mg2Sn
Authors: Onaki S. , Cardona M. , Schnherr E. , Stetter W.
Published: July 2007

The effect of uniaxial stress along [100] and [111] on the Raman‐active phonon of Mg2Sn is reported. The effect of a [111] stress on the corresponding phonon of Mg2Si and on the Fröhlichinteraction‐induced LO and 2LO lines of this material is also presented together with complementary measurements of the 2LO line under pure hydrostatic stress. The results are discussed in terms of anharmonic force constants and compared with similar results reported earlier for the F2g phonons of germanium and silicon.
Crystallographic aspects of elastic dispersion in body‐centered cubic transition metals
Authors: Parikh P. , Hay D. R.
Published: July 2007

Elastic dispersion in the transverse mode in body centered cubic transition metals is described in terms of local lattice dilation and non‐homogenous deformation. The model considers three types of bonding forces. One derives from local lattice dilation with respect to nearest neighbor and next nearest neighbor reference volumes and two derive from angular distortions of the nearest and next nearest neighbors. Elastic dispersion equations are developed and dispersion curves obtained for V, Nb, Ta, Cr, Mo, W, and Fe. Good agreement with experimental data is obtained for these metals except in the region of Kohn and other anomalies and negative dispersion in Nb is interpreted as a result of increasing contribution from local lattice dilatation at higher frequencies.
On the theory of phonon‐modulated electronic transport in off‐diagonal random alloys
Authors: Kolley E. , Kolley W.
Published: July 2007

The theory of the electric conductivity including electron‐phonon interaction is extended to off‐diagonal random alloys. The functional‐derivative technique is used to show the equivalence of the statically approximated tight‐binding model to a generalized stochastic model. Vertex corrections are evaluated by solving Bethe‐Salpeter‐type equations within a modified CPA for the additive limit.
Investigation on the far‐infrared response of DKDP by asymmetric fourier spectroscopy
Authors: Happ H. , Rother G.
Published: July 2007

The complex amplitude reflection coefficient of K(H1−xDx)2PO4, x = 0.86, is measured with E ∥ c and E ⟂ c between 15 and 360 cm1 at room temperature and between 15 and 130 cm−1 at three lower temperatures using an asymmetric Fourier spectrometer. The spectra of both ε′(ω) and ε″(ω) derived thereof are presented. For E ∥ c the ferroelectric mode at room temperature is seen below ω ≈ 30 cm1 and can be described by a Debye formula with ωr = 1/τ  3.12 cm1. For E ⟂ c there is also a low frequency mode due to deuteron motion which can be described by an overdamped oscillator at ω0 = 22 cm1.
Photo and dark conductivity of doped amorphous silicon
Authors: Rehm W. , Fischer R. , Stuke J. , Wagner H.
Published: July 2007

The photo and dark conductivity of boron‐ and phosphorus‐doped amorphous silicon is measured in the temperature range from 100 to 400 K, for various doping levels. Increasing doping generally decreases the activation energy of dark conductivity, down to 0.2 eV, and also decreases the σ0‐value (extrapolation of dark conductivity forT→ ∞). This lowering of σ0is explained by a temperature shift of the Fermi level plus a change in the conduction mechanism. The activation energy of photoconductivity is also lowered, at least by phosphorus doping, which is understood by an influence of doping on the tailing of the bands.
Temperature dependence of the A1 and E optical phonons in BaTiO3
Authors: Scalabrin A. , Chaves A. S. , Shim D. S. , Porto S. P. S.
Published: July 2007

Measurements of all A1 and E transverse and longitudinal phonons in BaTiO3 are reported in the temperature range 10 to 132 °C where the crystal is in the tetragonal phase. A least squares fitting of the spectral shapes is done and the resulting temperature dependence of the frequencies, linewidths and coupling parameters is presented. The temperature dependence of the phonon and electron contribution to the dielectric constant, ε, in the c‐ and α‐axes is determined and compared to the capacitance measurements. It is found that the dielectric constant discrepancy is highly anisotropic.
Residual resistivity of noble metals
Authors: Singh N. , Singh J. , Prakash S.
Published: July 2007

The residual resistivities of noble metals are investigated by alloying 1 at% of monovalent and divalent elements. The form factors due to Singh and Prakash and Moriarty are used for noble metals and the generalized pseudopotential form factors of Veljkovic and Slavic are used for the other elements. The second‐order perturbation theory is applied to include the lattice distortion effects and the results are compared with available experimental values. The agreement is reasonable for some alloys while it is not so for others.
Thermally modulated ultraviolet absorption spectra of CaF:Ce3+ crystals
Authors: Loh E. , Samoggia G. , Reguzzoni E. , Nosenzo L.
Published: July 2007

The thermally modulated absorption spectra (TMA) of Ce3+ in CaF2, crystals at 90 K show the zero‐phonon and vibronics of the 4f → 5d ∣ε〉 transitions. They are more distinctive than that in the conventional absorption spectra. In TMA, the absence of the overlapping background in the composite band between ≈32000 and ≈34000 cm−l enables one to compare the band widths of various orders of vibronics readily and hence to reveal the possible presence of the weak and/or broad 4f → 5d ∣θ〉 absorption at ≈33100 cm−l.
Corrections of the long beating length in quantum modulated electron beams
Authors: Hora H.
Published: July 2007

The quantum mechanical modulation of electron beams crossing a laser beam within a solid, results in an experimental long beating wavelength. The values calculated from a nonrelativistic or from a relativistic theory differ remarkably. A full agreement is reached, if the relativistic dielectric correction of Varshalovich and Dyakonov is included in the relativistic expression for the long beating lengths. The agreement with the Dirac theory is checked and the velocity of the electrons has to be used as given from the relativistic kinetic energy.
Electrical resistivity recovery of cobalt after low‐temperature electron irradiation
Authors: Dander W. , Schaefer H. E.
Published: July 2007

After electron irradiation at 4.2 K with different doses the annealing behaviour of the electrical resistivity of Co is studied between 15 and 550 K. Two stages at 65 and 300 K show a dose‐dependent temperature shift which may be attributed to the free migration of defects. As the annealing spectrum is found to be rather complex the influence of impurity atoms on the annealing of the radiation‐induced defects cannot be excluded.
Light scattering and spin diffusion in doped semiconductors
Authors: Vasconcellos A. R. , Luzzi R.
Published: July 2007

Inelastic scattering of light by single‐particle spin‐flip electron excitations and paramagnetic spin‐waves in semiconductor magneto‐plasmas is considered. The generalized Landau quasiparticle picture is used to calculate the Raman cross section. Particular attention is given to n‐doped GaAs for carrier concentrations in the metallic region (rn = 1.6). Scattering of the carriers with lattice vibrations is considered to discuss the spin‐diffusion coefficient, in uniform quasi‐equilibrium conditions, as a function of the magnetic field and the electron effective temperature. Spin‐flip magneto‐Raman investigations in doped semiconductors can produce valuable information on the “hot metallic electron liquid” and its interactions.
Atomic hydrogen levels near a metallic surface
Authors: Iadonisi G. , Ramaglia V. Marigliano , Zucchelli G. P.
Published: July 2007

It is found that the ground state of an isolated hydrogen atom becomes a resonance when it moves towards a metallic surface. The center and the width of the resonance are calculated as functions of the distance. A comparison with numerical calculations is made.
Optical properties and plasmon dispersion in epitaxial (SN)x by electron energy loss spectroscopy
Authors: Stolz H. J. , Petri E. , Otto A.
Published: July 2007

Wavevector dependent electron energy loss spectra of epitaxial films of (SN)x between 0 to 30 eV up to wave vector transfers of 1 Å+1 are measured for k parallel and perpendicular to the chain axis. The optical constants ϵ1, ϵ2 and the reflectivity R are derived by a Kramers‐Kronig analysis for zero wavevector. Peak positions in R agree with optical measurements. A change in the dispersion of the bulk plasmon of the conduction electrons is attributed to the interference with free‐electron‐like single particle excitations.
Formation and properties of the impurity band in n‐ZnSe
Authors: Nedeoglo D. D.
Published: July 2007

The Hall effect, conductivity, mobility, and magnetoresistance of low‐resistivity n‐ZnSe crystals in the temperature range from 4.2 to 300 K are investigated. At room temperature, the electron concentration of the samples investigated is 2 × 1016 to 5 × 1017 cm―3. The maximum electron mobility observed at 60 K is equal to U = 2200 cm2/Vs. Following experimental results are obtained: a maximum on the temperature dependence of Hall cofficient, the negative magneto‐resistance in the helium temperatures range described by Toyozawa's theory and discussed assuming the existence of an impurity band in the samples investigated. A model of the impurity band formation in n‐ZnSe is proposed and its basic parameters are determined.
Lattice dynamics and thermal properties of simple metals based on model pseudopotential calculations
Authors: Prasad B. , Srivastava R. S.
Published: July 2007

The phonon frequencies and Grüneisen parameters are calculated for different values of the phonon wave vector in the three principal symmetry directions for b.c.c. rubidium and cesium using a new form of the local model potential proposed by Krasko and Gurskii. The average values of the Grüneisen parameter γM are also calculated for different temperatures and the results are compared with available experimental and theoretical results.
A simple explanation for the “4kF anomaly” in TTF‐TCNQ
Authors: Phillips J. C.
Published: July 2007

In addition to a Peierls charge density wave with q = 2kF an additional scattering anomaly with q = 4kF is observed by X‐ray scattering. The anomalous temperature dependence of the 4kF anomaly can be explained by impurity ordering stabilized by condensation of Peierls CDW.
Simultaneous Kinetics of Drops and Excitons in Silicon during the Drop Formation
Authors: Collet J. , Barrau J. , Brousseau M. , Maaref H.
Published: July 2007

Using an homogeneous excitation, the simultaneous kinetics of drop and exciton luminescence are measured during the initial regime of drop formation in silicon. Theoretical simulations of the exciton density developped, do not fit well the experimental lineshapes. From an analysis of the experimental curves, it is suggested that a coalescing effect of drops must be taken into account to improve the fit of the experimental kinetics by the theoretical model.
The Critical Voltage Effect in Transmission Electron Microscopy. VIII. The Qualitative Influence of Non‐Systematic Reflections on the Critical Voltage
Authors: David M. , Gevers R.
Published: July 2007

The linear second order critical voltage effect often still occurs when also non‐systematic reflections are excited. However, the critical voltage Vc will in general be different from the purely systematic value V. A simple formula is given to derive a quick estimate for the critical voltage shift, from the structure of the dispersion surface (calculated at V) for the zone considered. This estimate can be useful to guide experiments and exact numerical calculations.
Effective Pair Potential and Thermodynamical Properties of Liquid Gallium
Authors: Badiali J. P. , Defrain A. , Dupont M. , Regnaut C.
Published: July 2007

The thermodynamic properties of liquid gallium are investigated from an effective pair interaction between ions, which explains the properties of the crystalline phases at 0 K for this metal. The results of pseudopotential theory are used and the thermodynamic perturbation method associated with the Gibbs‐Bogoliubov inequality. Computed values agree with the experimental data for volume, compressibility, latent heat, and auto‐diffusion coefficient. Some particularities are predicted of gallium such as low melting point and the fact that the liquid is more compact than the stable crystalline phase.
Phonon Contributions to the Anomalous Specific Heat of Triglycine Sulfate near Tc
Authors: Winterfeldt V. , Schaack G.
Published: July 2007

The ferroelectrics of the triglycine sulfate (TGS) family demonstrate an anomalous contribution to the specific heat near Tc whose peak height can not be accounted for in the usual mean field approximation. It is shown for TGS that this anomaly is due to temperature dependent frequencies of optical phonons near the phase transition temperature.
Influence of Random Electric Fields on the Polarization of Deep Centre Electroabsorption
Authors: Georgobiani A. N. , Ozerov Yu. V. , Strumban E. E. , Vinogradov V. S.
Published: July 2007

Deep centres of zinc sulfide single crystals doped with indium to different concentrations are investigated by impurity electroabsorption (IEA) and luminescence. The phenomenon of IEA sign reversal with change of light polarization direction relative to the modulating electric field from the parallel to the perpendicular direction is experimentally observed. An explanation of this effect is proposed assuming a strong EA dependence on the angle between the acting electric field and the light polarization in a crystal with random electric fields.
The Electron‐Two Short‐Wavelength Phonon Scattering in Non‐Polar Semiconductors
Authors: Kubakaddi S. S. , Krishnamurthy B. S.
Published: July 2007

The transport properties are calculated in n‐silicon with the electric field along the 〈111〉 direction, taking into account the electron‐two (short wavelength)‐phonon mechanism besides the usual one‐phonon mechanisms. The variation of warm electron coefficient and zero field mobility with lattice temperature are in good agreement with experimental observations in a wide range of temperature. However, the disagreement noticed in the variation of drift velocity with the electric field in the high field region requires the inclusion of non‐parabolicity of the conduction electrons in the calculation.
Resonant Acceptor Level in Zero‐Gap Semiconductors. Single and Multiple Scattering Effects
Authors: Bastard G.
Published: July 2007

A theory of a resonant acceptor level in zero gap semiconductors is developed taking into account the multiple scattering processes. It is shown that the results accounting only for contribution proportional to the impurity concentration describe properly the evolution of the activation energy in Hg1—xCdxTe alloys. However, it is necessary to include the multiple scattering effects to obtain reliable expressions of the density of states and the conductivity. Analytical expressions are obtained and discussed. Comparison with experimental data is made when possible.
Hypercircle Approach and Complementary Variational Principles for Lattice Thermal Conductivity
Authors: Srivastava G. P.
Published: July 2007

A connection is established between the hypercircle approach and two previously developed methods of deriving complementary variational principles for lattice thermal conductivity.
Optical and Modulated Absorption Spectra in the Band Gap Region of Amorphous Vanadium Zinc Borate Semiconductors
Authors: Koffyberg F. P.
Published: July 2007

The optical absorption spectra of V2O5—B2O3—ZnO semiconducting glasses, prepared by rf sputtering, indicate that the interband transitions are direct forbidden; for glasses with 50% to 90% V2O5 the optical band gap ranges from 2.51 to 2.31 eV at 100 K. Below the absorption edge the spectra exhibit an Urbach tail. The shapes of the electro‐ and thermomodulation spectra agree qualitatively with theoretical predictions based on the Franz‐Keldysh effect. From a quantitative analysis of the electromodulation spectra in the Urbach tail region a reduced effective mass of 10 mo is derived and band gap values which are 0.2 eV larger than the values obtained from the absorption spectra.
Electron–Phonon Interaction in a Superlattice
Authors: Shmelev G. M. , Chaikovskii I. A. , Pavlovich V. V. , Epshtein E. M.
Published: July 2007

The peculiarities of the phonon spectrum in semiconductors with superlattice are investigated. The dispersion equation is solved for the case of high temperatures: 2Δ ≪ kT (2Δ is the miniband width, T the absolute temperature). The influence of electron‐phonon interaction on the effects under consideration is taken into account.
Moments of Electroabsorption of Jahn‐Teller Impurity Centres
Authors: Perlin Yu. E. , Tsukerblat B. S. , Dod T. Singh
Published: July 2007

The linear and quadratic electroabsorption in the wide electron‐vibrational bands of the cubic centres are studied theoretically on the basis of the matrix formalism of the method of moments. The formulae for the field‐induced changes of the first three moments are derived, and it is shown that the dichroism effect of the electroabsorption allows to determine both the symmetry of the impurity centres and the parameters of Jahn‐Teller interaction.
Dynamic Theory of Conversion Electron Mössbauer Spectroscopy
Authors: Chugunova G. P. , Mitin A. V.
Published: July 2007

The dynamic theory of the conversion electron Mössbauer spectroscopy (CEMS) is considered with the help of the electron–nuclear density matrix. The cross section of the Mössbauer conversion process, electron and conversion linewidths are calculated. As an application of the developed theory, the ultrasonic modulation by Rayleigh waves and the γ‐ray magnetic resonance effect are investigated. The possibility of the CEMS application for surface phenomena is discussed.
A Theory of the Hall Effect in the Hopping Region in Disordered Systems. II. Random Resistor Network and Hall Effect
Authors: Böttger H. , Bryksin V. V.
Published: July 2007

Linearizing the rate equation for the diagonal elements of the density matrix with respect to the electrical and magnetic field the basic equations for the Hall effect in the hopping region are derived. The equations hold for ordered as well as for disordered systems. The problem of solving these equations is shown to be equivalent to an investigation of a random resistor network with capacitors and extrinsic currents. For some special cases the Hall mobility is calculated by these equations.
Finite Dislocation Elements in Finite Media. II. Trapezoidal Splitting of Partial Dislocations in Thin Foils
Authors: Korner A. , Karnthaler H. P. , Kirchner H. O. K.
Published: July 2007

Partial dislocations in thin foils split trapezoidally. In numerical calculations the presence of the surface and the elastic anisotropy of the medium is taken into account. In Cu–10 at% Al this tendency for nonparallel orientation of the partials is stronger than in silver. The computation is done on the force level by adjusting the geometry of the arrangement till forces and moments are zero. Deviations from isotropic fitting are considerable. The tendency of screw dislocations to cross slip at surfaces is confirmed.
Magneto‐Optical Investigation of the Ground State of the Indirect Free Exciton in Si and Gap
Authors: Donecker J. , Kluge J.
Published: July 2007

Measurements are made of the magnetic circular dichroism (MCD) at the indirect absorption edge of intrinsic Si and GaP at low temperatures and at magnetic fields up to 5 T. By comparison with wavelength modulated spectra it is concluded that the thresholds of the indirect absorption edge for σ+‐ and σ−‐ light are rigidly shifted against each other by the magnetic field. The direction of this shift is connected with the type of assisting phonons. This effect can be explained by the splitting of the 1s ground state of the indirect exciton using group theoretical arguments and can be used to estimate hole g‐values.
A Model for Vacancies in Simple Metals Including an Ionic Potential in the Self‐Consistent Density Calculation
Authors: Perrot F.
Published: July 2007

A calculation is made of the vacancy formation energies for Li, Na, K, and Al in the density functional formalism. The difference between the formation energy and the binding energy results from two self‐consistent calculations, one for the perfect lattice with N atoms and one for the defect lattice with N − 1 atoms in the same volume. In both the calculations, the effect of ions is self‐consistently included as a spherically averaged superposition of pseudopotentials. The results improve the “semilinear screening” values due to Evans and Finnis, but remain too large compared with experiment.
Pseudopotential Theory and the Problem of Covalent Bonding. II. Crystals of Si, Ge, and α‐Sn as‐Ordered Systems
Authors: Makhnovetskii A. B. , Krasko G. L.
Published: July 2007

A recently developed theory of the covalent bonding using the Brillouin‐ Wigner perturbation series is applied to the analyses of the static stability of Si, Ge, and α‐Sn crystals with diamond lattice. The model proposed considers the latter as an ordered superstructure arising from the b.c.c. disordered solid solution of the element atoms with vacancies. The calculations of lattice stability are performed firstly for pairwise interaction (second order pseudopotential perturbation theory) and secondly for covalency effects. It is the latter that stabilizes the diamond lattice for the three elements. The spatial covalent charge distribution ϱcov(r) is calculated along the [100], [110] and [111] directions and in (110) plane of the b.c.c. lattice. ϱcov(r) is shown to have pronounced maxima just in the points where Phillips' “bonding charges” are localized, but being of a more complicated structure with deep holes in the “empty” sites.
Theory of Two‐Photon Absorption by Surface Polaritons
Authors: Nkoma J. S.
Published: July 2007

It is shown theoretically that observation of surface polaritons by two photon absorption (TPA) is possible. The expression for the TPA coefficient is calculated by using surface polariton response functions, and is found to have a Lorentzian shape. As a result of the TPA, a surface polariton beam grows and its intensity is related to the TPA coefficient. The photon component of the generated surface polariton beam can be detected by a prism‐coupling method. Results applicable to GaP are shown.
On the Optical Absorption Spectrum of Co2+ in CaF2
Authors: Alonso P. J. , Alcalá R.
Published: July 2007

The temperature evolution is measured of the absorption band centered at about 540 nm in the absorption spectrum of Co2+in CaF2 at temperatures in the range from 80 to 320 K in order to check previous designations of the transitions that produce the band. Four peaks at 19100, 18560, 18450, and 17800 cm−1 are observed. The peak at 18565 cm−1 is assigned to the 4A2g(4F) → 2T1g(2G) transition of the Co2+ in a cubic environment with values of Dq = 360 cm−1, B = 930 cm−1, and C/B = 4.47 for crystal‐field and Racah parameters, respectively. The other three peaks are assigned to transitions from the 4A2g(4F) ground state to the Jahn‐Teller split 4T1g(4P) triplet with the same Racah and crystal‐field parameters and with Jahn‐Teller parameters c2 = 0.42 eV, ħω = 247 cm−1, γ = 0.57. With these designations the temperature evolution of the peak positions and of the integrated intensity of the band are in good agreement with those predicted by the theory.
Pseudo‐Spin Motion in Solid Solutions of the Type of the Type K(H1−xDx)2PO4
Authors: Schreiber J.
Published: July 2007

Within the pseudo‐spin model which includes occupation dependent tunnelling frequencies Γi, dipole moments μi, and interactions Iij ∼ μitijμj, the dielectric susceptibility is obtained by a linearized Bloch equation for ferroelectric solid solutions of the type K(H1−xDx)2PO4. The single site and the Bethe‐Peierls version of CPA are used to solve the disorder problem. Using a suitable set of ingoing parameters the Curie temperature, the spontaneous polarization, the intensity of Raman scattering, and the static susceptibility are calculated for various deuterium concentrations, x, and discussed in relation to experiments.
Magnetic and Thermodynamic Properties of Heisenberg Open Finite Chains. I. Susceptibility in Zero Field
Authors: Rakitin Yu. V. , Kalinnikov V. T. , Hatfield W. E.
Published: July 2007

Calculations of spin levels and effective magnetic moments as functions of reduced temperature, μeff(kT/J), are carried out for Heisenberg open finite chains with ion spins of 1/2 to 5/2 containing up to 11 subunits (N ≦ 11). Some relationships useful in interpretation of experimental results are considered. A detailed comparison of the results with literature data is given.
Lattice Dynamics of Rubidium Halide Crystals
Authors: Goel C. M. , Dayal B.
Published: July 2007

Phonon dispersion curves of rubidium halide crystals are calculated in three principal symmetry directions on the basis of the three‐body force shell model by incorporating charge transfer effects in an appropriate manner. A comparison of the theoretical results with the neutron scattering measurements of Raunio and Rolandson exhibits very good agreement.
Two‐Photon Absorption by Envelope–Hole Coupled Exciton States in Cubic ZnSe. II. Polarization Dependence in the Case of Degenerate Valence Bands
Authors: Sondergeld M.
Published: July 2007

The general angular dependence of two‐photon absorption for elliptical polarization is given in terms of complex polarization vectors in the case of cubic and spherical symmetry. The two‐photon transition probability for p‐like envelope–hole coupled exciton states is treated within an extended two‐band model which, for the case of degenerate valence bands, includes Hd‐term contributions of the exciton Hamiltonian. By use of the effective Bohr radii, resulting from the envelope–hole coupled exciton wave functions, the experimental polarization ratios yield good quantitative agreement with the values predicted theoretically.
Polarons in the Degenerate‐Band Case. II. Weak to Intermediate Hole–Phonon Coupling
Authors: Trebin H.‐R.
Published: July 2007

Due to the valence band degeneracy it is a special feature of the hole‐phonon interaction in cubic polar semiconductors that the holes cannot only be intraband but also interband scattered by virtual LO phonons. Within the k · p‐formalism this amounts to a change of the \documentclass{article}\pagestyle{empty}\begin{document}$ J = \frac{3}{2} $\end{document} spin component along the hole wave vector. It is shown that because of this spin scattering new correlations between the emission of successive phonons are introduced in addition to those caused by the hole recoil. A generalized Glauber‐state ansatz is made for the wave function of slowly moving hole polarons containing spin‐dependent displacement amplitudes. With the use of the Fröhlich interaction term renormalization relations for the band parameters γ1 and μ are established valid for weak to intermediate hole–phonon coupling.
Fluctuations of the Superconducting Order Parameter below Tc by Tunnel Effect
Authors: Burrafato G. , Giaquinta G. , Mancini N. A. , Pennisi A. , Simone F. , Troia S. O.
Published: July 2007

Tunnelling anomalies in Al–formvar–Sn junctions operated at temperatures slightly below the transition temperature of the Al arm, are detected as function of externally applied magnetic fields. Evidence is given to interpret these anomalies as due to the thermodynamic fluctuations of the superconducting order parameter in the Al side. A transverse fluctuation mode appears to be present, in good agreement with the theoretical model proposed by Simanek and Hayward.
Enhancement Effect on Positron Annihilation with Tightly Bound Electrons
Authors: Chiba T. , Dürr G. B. , Brandt W.
Published: July 2007

Coulomb correlation is taken into account in the annihilation process of positrons in solids with tightly bound electrons by first‐order perturbation theory. A momentum‐dependent enhancement formula is derived which is applicable to insulators with high electron density. With the use of the Thomas‐Fermi approximation the formula is reduced to give an annihilation rate depending solely on the overlap density of electrons and a positron. It is shown that the annihilation rates in insulators follow an enhancement curve different from that for metals.
Optical Constants of GaSe at the Fundamental Absorption Edge
Authors: Antonioli G. , Bianchi D. , Canevari V. , Emiliani U. , Podini P.
Published: July 2007

The three‐dimensional mechanical character of GaSe single crystals makes it very difficult to study the optical properties along the c‐axis. A technique is developed based on the analysis of the interference pattern as a function of the wavelength. Using the envelope of maxima or minima it is possible to normalize the data in respect to the intensity of the incident light so to have an absolute measurement. The fit of the experimental normalized data gives the index of refraction n and the extinction coefficient k with an error of the order of 1\documentclass{article}\pagestyle{empty}\begin{document}$ {\raise0.7ex\hbox{$0$} \!\mathord{\left/ {\vphantom {0 {00}}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${00}$}} $\end{document} for n ⟂ c and 3\documentclass{article}\pagestyle{empty}\begin{document}$ {\raise0.7ex\hbox{$0$} \!\mathord{\left/ {\vphantom {0 {00}}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${00}$}} $\end{document}, for n ∥ c. An error ten times greater is obtained for k⟂ and k∥. It must be emphasized that in the presence of a variation of the refractive index as small as 1\documentclass{article}\pagestyle{empty}\begin{document}$ {\raise0.7ex\hbox{$0$} \!\mathord{\left/ {\vphantom {0 {00}}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${00}$}} $\end{document} between two neighbouring maxima or minima the more traditional methods based only on the determination of the position of extrema are quite inconsistent. On the contrary the envelope method proposed can be used on a wider range. Moreover this method allows to determine k also in a region of low absorption, where 4πk/λ ≪ 1/z0. The transmission between 8000 and 5900 Å is measured and n⟂ land k⟂ are obtained on the entire region while for n∥ and k∥ the determination is possible only in the exciton tail. In this region a ratio k∥/k⟂ ≈ 3 is found.
Statical Charge Waves in Crystals
Authors: Ionova G. V. , Makarov E. F. , Ionov S. P.
Published: July 2007

A theory is developed for the disproportion of the electron density on crystal sites. A criterion of the existence of charge ordering together with a lattice distortion is obtained.
Structure of the Thermomagnetic Tensor in Semiconductors of n‐Ge Type
Authors: Baranskii P. I. , Buda I. S. , Shcherbina L. A.
Published: July 2007

Using the anisotropic scattering theory the general expression of the components of the thermomagnetic tensor α(H) is obtained for semiconductors of n‐Ge type for arbitrary Orientation of the magnetic field vector H with respect to crystallographic axes. The formulas obtained can be generalized to the case of crystals with non‐cubic symmetry. The structure of thermomagnetic tensor α(H)in n‐Ge is studied in detail for the three most typical cases of orientation of the “laboratory” coordinate system with respect to crystallographic axes. The expressions found allow to compare the theoretical conclusions with the experimental data.
Calculation of the Energy Flows between Subsystems by Means of the Non‐Equilibrium Statistical Operator
Authors: Skrebnev V. A.
Published: July 2007

The exchange of energy between subsystems in the case when there is a great difference in the frequencies at which this exchange is possible, is treated by non‐equilibrium statistical operator method. It is shown that agreement with the experiment can be achieved taking into consideration only pair correlations when calculating the streams of energy. This fact is discussed from the viewpoint of the ergodic theory. A thermodynamic parameter is juxtaposed to the interaction between the subsystems which results in regarding interaction as energy capacity and in the Gibbs' distribution with establishment of equilibrium in the system.
Scattering of Electrons by the Deformation Potential in Doped InSb
Authors: Demchuk K. M. , Tsidilkovskii I. M.
Published: July 2007

An analysis of the experimental data for InSb and of the theoretical considerations used for the determination of the deformation potential constant of the conduction band ϵl is made. It is shown that the correctly calculated value of |ϵ1| is about 30 eV. The small values of |ϵ1| obtained in some works by observing transport phenomena in the hot electron region at lattice temperatures of 4.2 K in samples with electron densities n from 1015 to lo17 cm−3 for which the Boltzmann transport equation is valid, are caused by a distortion of the bottom of the conduction band due to fluctuations of the impurity potential.
Luminescence of Ph2+ and Bi3+ Centres in Alkali‐Earth Sulphides and Oxides
Authors: Ellervee A. F.
Published: July 2007

Luminescence centres connected with Bi3+ and Pbz+ ions are investigated in CaO, SrO, CaS, and SrS crystals at 5 to 300 K. It is found that the ground state and the two lowest excited states of the Bi3+ as well as the Pb2+ centres correspond to the energy levels of the free impurity ions and have A1g, 3A1U, and 3T1u, symmetries, respectively. Emission is observed which is caused by electronic transitions from both the excited states. The 3A1u, → Alg transition becomes slightly allowed due to the interaction of an electron with the vibrations of T1u, symmetry. The anti‐Stokes and zero‐phonon lines are observed in the vibronic structure of spectra. The main series of lines are caused by the interaction of electrons with the totally symmetric Alg, vibrations. In the emission spectra of CaS: Bi and SrS: Bi electronic transitions in the host lattice perturbed by impurities are also evident besides intra‐centre electronic transitions.
Carrier Concentration and Magnetic Susceptibility of Doped Ferromagnetic Semiconductors with Application to EuO: Eu
Authors: Spalek J. , Lubecka M. , Węrzyn A.
Published: July 2007

Slightly oxygen deficient crystals of EuO exhibit a metal‐semiconductor transition below the Curie temperature accompanied by a strong temperature dependence of the carrier concentration in the conduction band. For the description of this case a model is proposed which takes into account two donor levels: the first one is localized near an oxygen vacancy, and the other one is connected with other impurities or with conduction band tails formed by disorder in heavily doped samples. For such a general case the temperature dependence of carrier concentration is discussed. To get some information about the magnetic state of localized pairs of electrons near an oxygen vacancy the static magnetic susceptibility in the paramagnetic regime for the triplet state is calculated. It depends on carrier concentration and thus a correlation between transport and magnetic properties is obtained. The correspondence with the concept of the molecular magnetic polaron is briefly discussed.
Quadratic Jahn‐Teller Effect and Tunneling Splitting in Octahedral Clusters
Authors: Bacci M.
Published: July 2007

The tunneling splitting due to transitions between coexisting minima of different symmetry is considered and approximate expressions for this splitting are given. The contemporary presence of quadratic Jahn‐Teller terms and anharmonic restoring forces is also discussed. The results are a.pplied to the rase of MgO:V2+ and an hypothesis for the origin of the strirctrrre of the no‐phonon line (4A2g → 4T2g) in the absorption spectrum is proposed.
Channel Model in Isotropic Exchange Theory (II)
Authors: Eremin M. V. , Kakitin Yu. V.
Published: July 2007

An extension is made of the exchange channel model suggested in previous communications. The relationships between exchange parameters are derived that take into account configuration interactions and are applicable to various pairs of ions in excited as well as ground states and with nonquenched as well as quenched orbital moments. Good agreement of the semiempirical theory developed with experiment is demonstrated for a number of solids with perovskite and rock‐salt‐type structures containing bivalent ion pairs. The theory is used to predict spin exchange parameters for various homo‐ and heteronuclear ion pairs linked by 180° bridges in perovskite and rock‐salt structures.
Recombination in Tellurium at High Optical Excitation
Authors: Galkin G. N. , Bobrova E. A. , Herrmann K. H.
Published: July 2007

Photoconductivity (p.c.) and photomagnetoelectric effect (p.m.e.) are observed in tellurium at high levels of optical excitation by means of a Q‐switched ruby laser. The characteristics observed for the stationary case are analysed in terms of a spatial distribution of carriers governed by diffusion and interband Auger recombination. The coefficient of Auger recombination in tellurium C = 1.5 × 10−28 cm6 s‐l is obtained.
On the Ground State Energy of a Gas of Interacting Polarons
Authors: Lemmens L. F. , Brosens F. , Devreese J. T.
Published: July 2007

Using a canonical transformation the Hamiltonian of a gas of interacting polarons is studied. After elimination of the phonons the reduced Hamiltonian describes a system of charged particles interacting through an effective interparticle potential. The Hartree‐Fock ground state of the reduced system is analysed.
X‐Ray Spectroscopical Investigation of the Electronic Structure of Fe—Cr
Authors: Niederlag W. , Blau W. , Kleinstück K.
Published: July 2007

Fe‐ and Cr‐L2,3 emission and self‐absorption spectra of the disordered alloy Fe70Cr30 are measured using a Bragg‐Soller spectrometer in primary excitation. The intensities are corrected and both, the emission and the absorption spectra are converted into absolute emission probabilities. The results are inconsistent with the rigid band model and are interpreted on the basis of a dilute alloy model. The existence of unoccupied virtual bound states at chromium sites as demanded from theory is experimentally proved.
Soft X‐Ray Spectra of A1 and the Laves Phase CaAl2
Authors: Taut M. , Radwan A. M.
Published: July 2007

Based on previous band structure calculations, the soft X‐ray spectra of pure Al and CaAl2, are calculated for transitions to Al core states. Pseudo‐wave functions (without orthogonalization correction to the core states) are used for the calculation of the transition probability. The orthogonalization correction is unimportant for the K‐spectra and doesn't produce any additional peak structure in L‐spectra.
Influence of Surface Carriers upon the Gralvanomagnetic Effects in Slightly Doped Tellurium Samples at Low Temperatures
Authors: Czaya C.‐P. , Keck K. , Rüthlein H.
Published: July 2007

The measured temperature dependence of the Hall coefficient of slightly Sb‐doped tellurium samples at low temperatures (T < 5K) is interpreted in light of surface carriers using a kind of two‐carrier‐model. By application of Fermi statistics the position of acceptor states in slightly Sb‐doped tellurium crystals can be determined from the temperature dependence of the bulk Hall coefficient.
On the Heating of Ultracold Neutrons
Authors: Blokhintsev D. I. , Plakida N. M.
Published: July 2007

The probability of ultracold neutron heating by inelastic scattering in neutron “bottles” is calculated by van Hove's method. The probability is low (⪅ 10−5) for pure substances but is greatly enhanced if light impurities with large incoherent cross‐section (like hydrogen) are present in the surface layers of the “bottle”.
Effective Electron‐Hole Interaction for Intermediate and Strong Electron‐Phonon Coupling A Comment
Authors: Bajaj K. K. , Aldrich C.
Published: July 2007

The values are calculated of the binding energy of a Wannier exciton in several polar crystals including thallous halides and cuprous chloride using both the potential derived by Barentzen and Haken's potential. It is found, contrary to what is claimed by Barentzen, that the values of the binding energy obtained from these two different effective interaction potentials hardly differ from each other. The reason why this is so is discussed.