bluuebunny/crossref_metadata_2025_split huggingface dataset:
Study of NH4 motion in some antiferroelectric crystals by inelastic scattering of cold neutrons
Authors: Dimic V. , Osredkar M. , Slak J. , Kandušsar A. 
Published: July 2007

AbstractThe spectra of the inelastic scattering of cold neutrons by antiferroelectric NH4H2PO4, NH4H2AsO4, and (NH4)2H3IO6 have been measured above and below their transition point, using a rotating crystal spectrometer together with a cold source. The low temperature neutron measurements show very little change as compared with the room temperature results. The torsional bands are narrow in both phases. The broadening of the bands is inversely related to the hindrance, and therefore the measurements suggest that the rotation of the NH+4 ion is strongly hindered, which is supported by the angular distribution of elastic scattering. The deuteron magnetic resonance of ND4D2AsO4 indicates that in addition to rotational motion of the ND4 group this group might oscillate in the paraelectric phase. The oscillational mode condenses at Tc. In order to check the assignments obtained from the inelastic neutron scattering studies the ir absorption spectra of NH4H2PO4, NH4H2AsO4, and (NH4)2H3IO6 and their deuterated analogues were measured at room temperature.
Binding of an exciton to a donor‐acceptor pair in a semiconductor. I. Critical binding in the case of a Coulomb interaction
Authors: Munschy G. , Stébé B. 
Published: July 2007

AbstractIt is shown that a stable binding of an electron and a hole in the finite dipole field of both an ionized donor and an ionized acceptor cannot occur, if the donor‐acceptor pair distance is smaller than a certain critical value depending on the dielectric constant ϵ and on the electron and hole effective masses m and m. Accurate upper and lower bounds of the critical distance are derived in the model defined by these physical constants, and compared to the experimental data available from the observed luminescence spectra in doped ZnSe and AlSb.
Contribution of configuration interaction to the intensity of crystal field transitions in tetrahedral Fe2+ion
Authors: Basu C. , Ghosh U. S. 
Published: July 2007

AbstractThe effect of configuration interaction via crystal field on the intensity of crystal field transitions in tetrahedral Fe2+ions has been discussed. The mixing of the excited configurations 3d54p and 3d54f to the ground 3d6configuration of the tetrahedral Fe2+ion via crystal field results in a parity allowed electric dipole transition between crystal field levels of E and T2symmetry. The contribution of such configuration interaction to the oscillator strength of crystal field transition of the tetrahedral Fe2+ion in ZnS has been estimated within the framework of crystal field theory using Racah formalism and Richardson functions for the radial integrals. This contribution to the oscillator strength is found to be quite significant and approximately equal to 0.28x10−4.
Orientation‐dependent resonant raman scattering in InSb and GaSb at the E1–E1+Δ1 region
Authors: Dreybrodt W. , Richter W. , Cerdeira F. , Cardona M. 
Published: July 2007

AbstractThe resonance of the first order Raman scattering by LO and TO phonons near the E1 and E1 + Δ1 electronic gaps of InSb and GaSb is presented. Whereas for forbidden LO scattering the spectra are independent of the orientation of the crystal surface, for allowed TO and LO scattering the resonance spectra depend strongly on the surface orientation. This orientation dependence is discussed in terms of both allowed and first order vector induced (q‐dependence, electric field, etc.) scattering processes.
Crystal potentials and one‐electron states near the fermi surface of calcium
Authors: Perrot F. 
Published: July 2007

AbstractUsing the APW method an investigation is made of the band structure of f.c.c. calcium in two directions which are not explored in recently published APW calculations on solid Ca. The first of these is related to a detailed description of the crystal potential including the correct Hartree‐Fock core‐valence exchange (exact under the restriction of the frozen core approximation), a local valence‐valence exchange and correlation potential and a crude core polarization contribution. The corresponding self‐consistent band structure is very similar to the classical one obtained with a free‐electron Kohn‐Sham exchange. The modification of the band structure appearing when the core polarization is deleted is in qualitative concordance with the pseudo‐potential results of Vasvari et al., but a quantitative agreement is excluded. The second part of the work is interested in the effect of the non spherical potential on the band structure. It is not strong enough to modify the ordering of the levels near the Fermi surface found in the Kohn‐Sham approximation and does not confirm the results of the OPW calculation of Chatterjee and Chakraborti.
Underdamped soft tunnelling modes in hydrogen‐bonded ferroelectrics below T
Authors: Blinc R. , Lavkenčič B. , Levstek I. , Smolej V. , Žekš B. 
Published: July 2007

AbstractUnderdamped soft tunnelling modes, which disappear on deuteration, are observed in KHZP04 type crystals below Tc in addition to a central mode which exists both above and below Tc. These results can be understood in terms of a Bloch type description of pseudo‐spin motion.
A further synthesis of sextic and integral theories for dislocations and line forces in anisotropic media
Authors: Asaro R. J. , Hirth J. P. , Babnett D. M. , Lothe J. 
Published: July 2007

AbstractA further synthesis of the sextic and the integral theories for the elastic fields of line forces and dislocations in anisotropio media is made. The expressions derived for the distortions and displacements are believed to be the most convenient ones for purposes of numerical computation. In addition expressions are given for the derivatives of the field quantities for use in Lothe‐Brown‐Indenbom‐Orlov theorems which allow for the construction of three dimensional fields from two dimensional data. Some numerical examples for cubic media are included.
A decoupling procedure for green functions: Test at the ising model and application to an extended hubbard model
Authors: Ihle D. , Lorenz B. 
Published: July 2007

AbstractA decoupling procedure for the double‐time three‐particle Green functions is presented. The formalism is tested at the spin ½ nearest‐neighbour interaction Ising model yielding more accurate results than Bethe's first approximation. The decoupling procedure is applied to the problem of electron‐charge ordering on the basis of an extended Hubbard model investigated by Bari, the corresponding phase diagram being calculated.
Photoemission and density of valence states of the H‐YI compounds II. ZnSe, CdS, and HgS
Authors: Shevchik N. J. , Tejeda J. , Langer D. W. , Cardona M. 
Published: July 2007

AbstractThe density of valence states of the II‐VI compounds ZnSe, CdS, and HgS has been determined with far uv (h v = 21.1 and 40.8 eV) photoemission. Discrepancies in the widths of the top portions of the valence bands between theory and experiment, similar to those reported for other II‐VI compounds in part I, are also found here. The results for ZnSe are compared to those obtained by others using synchrotron radiation and monochromatized X‐rays.
Screening of a static charge in the electron liquid
Authors: Jain Madhu , Jain S. C. 
Published: July 2007

AbstractNumerical results are presented for the screening charge density around a static foreign charge in an electron liquid in the metallic density regime using a recently proposed expression for the local field correction term G(q) on the basis of the theories of Singwi et al. and using additional information of g(0) from the partial‐wave phase‐shift analysis. Present values are found to lie in‐between the values of Singwi et al. (Theory I) and of Vashishta and Singwi (Theory V).
Isoelectronic Impurity States in Tellurium
Authors: Takita K. , Suzuki K. , Tanaka S. 
Published: July 2007

AbstractThe galvanomagnetic properties of Se‐doped tellurium measured in the temperature range from 1.2 to 77 K were compared with those of undoped tellurium. The hole concentrations were not changed by the Se‐doping (from several p.p.m. t o 1500 p.p.m. (0.15%)). The temperature dependence of the weak field Hall coefficient was greatly changed by doping and showed two peaks at 4 to 6 and 20 to 25 K in sufficiently Se‐doped samples. The temperature dependence of the resistivity was also changed and showed a broad peak around 15 K. These results were interpreted on the base of a two‐carrier model in which the impurity band‐like states are degenerate with the valence band as was proposed previously and the number of the states increases with the Se‐concentration. This means that the impurity band‐like states of undoped tellurium may also consist of isoelectronic impurity states due to the residual Se‐impurity. The nature of the short range potential of the Se‐impurity in tellurium is discussed.
On Interaction Potential, Correlation Factor, Vacancy Mobility, and Activation Energy of Impurity Diffusion in Diamond Lattice
Authors: Hu S. M. 
Published: July 2007

AbstractAn intuitive concept was proposed earlier by the author that for the activation energy of impurity diffusion in the diamond lattice to be smaller than that of self diffusion, there must exist a long‐range vacancy‐impurity interaction potential. The present work explores more quantitatively certain required characteristics of the potential. The relationship between the activation energies of impurity diffusion and of self diffusion, QA and QB, respectively, is approximately given by QA ≈ QB − u(r3), where u(r3) is the potential when the vacancy is at the third coordination site of the impurity atom. Most of known physical models of interaction do not satisfy the required characteristics of u(r), leaving that of the extended vacancy as a probable one. Irrespective of the interaction potential, the relationship between the migration energies of the simple vacancy and the vacancy‐impurity complex, H and H, respectively, is simply given by H = H + QB − QA − Eb, where Eb is the binding energy. Utilizing this relationship and published experimental data of impurity diffusion, the migration energy of simple vacancy in silicon is found to be N (0.5 ± 0.1) eV. It is also shown that the impurity‐vacancy saddle point potential will not affect QA, contrary to common concept.
Exciton Spectra with Spatial Dispersion in Case of Non‐Normal Incidence
Authors: Skettrup T. 
Published: July 2007

AbstractThe general case of non‐normal incidence of light on a crystal exhibiting spatial dispersion is treated. The light wave will in general excite two transverse and one longitudinal polariton in the crystal. Additional boundary conditions are obtained for the three‐dimensional case by means of the Lagrange formalism. The Fresnel formulae for reflection and refraction are derived, and exciton spectra for CdS are computed. Additional structure in the reflection spectrum near the longitudinal resonance energy is obtained for non‐normal incidence when the electric field of the light wave is parallel to the plane of incidence.
Valence and Conduction Band Structure and Infrared Optical Properties of Tellurium in the Presence of Pressure
Authors: Enderlein R. , Hache A. 
Published: July 2007

AbstractThe influence of pressure on the valence and conduction band structure and on the infrared optical properties of tellurium is investigated by means of group and perturbation theory. The first peak of the 11 μm‐band which corresponds to a direct intervalence band transition at the BZ corner H is shifted linearly by stress of any symmetry. At a certain critical value of hydrostatic pressure or uniaxial pressure parallel to the c‐axis it changes over into a second peak of the 11 μm‐band which is due to a transition on the symmetry line P. For hydrostatic pressure and uniaxial pressure parallel to the c‐axis the alteration of the fundamental edge results from a strain dependence of the energy gap being approximately linear. The deformation potential constants of the gap and of the first peak of the 11 μm‐band are determined by means of experimental data.
GaAs Luminescence Transitions to Acceptors in Magnetic Fields
Authors: Willmann F. , Dreybrodt W. , Bettini M. , Bauser E. , Bimberg D. 
Published: July 2007

AbstractLow temperature luminescence spectra of pure and Sn‐doped epitaxial GaAs are reported. Distant donor‐acceptor pair transitions and conduction band to acceptor transitions are investigated in magnetic fields up to 12 T. The shifts of donor‐acceptor emission lines in unintentionally doped samples are in quantitative agreement with the calculated variation of the donor binding energy and of the Coulomb term. A considerable change in the magnetic behaviour at donor concentrations n > 5 × 1015 cm−3 is qualitatively discussed in terms of screening effects by free carriers on the impurity potential and impurity band formation. The shift of conduction band to acceptor transitions is linear up to the highest field.
The Anderson Model for Random Lattices with Statistically Distributed Overlap Potentials
Authors: Herscovici C. 
Published: July 2007

AbstractAnderson's model for random lattices, considering not only the lattice‐site energies, but also the overlap potentials statistically distributed, is investigated. Assuming for both of them Lorentzian distributions, a calculation is performed for the mobility edges in second order of perturbation theory, and it is found that the statistical character of the overlap potentials has an effect of delocalization. This conclusion is enforced by the results obtained for the upper bounds of the mobility edges.
Réflectivité infrarouge du cinabre synthétique (α‐HgS)
Authors: Marqueton Y. , Ayrault B. , Decamps E. A. , Toudic Y. 
Published: July 2007

AbstractNous avons mesuré à T = 300 et 80 K les spectres de réflectivité infrarouge du cinabre (α‐HgS) en lumiére polarisée. Pour la première fois, tous les modes de vibrations prévus par la théorie des groupes ont été observés et analysés en termes d'oscillateurs. Ces expériences ont été effectuées sur un cristal synthétique de bonne qualité. Elles complètent les résultats antérieurs et conduisent à un modèle satisfaisant pour la dispersion de la constante diélectrique.
Spinabhängige und optimal lokalisierte Funktionen geringer Energieunschärfe in Metallen
Authors: Krüger E. 
Published: July 2007

AbstractDie Theorie der verallgemeinerten Wannierfunktionen wird durch die Berücksichtigung des Elektronenspins erweitert. Auf diese Weise ist es möglich, “spinabhängige lokalisierte Funktionen” zu definieren, die eine weitere Verallgemeinerung der verallgemeinerten Wannierfunktionen darstellen. Sie werden zur Beschreibung der Elektroneneigenschaften insbesondere der Metalle geeigneter als die letzteren sein, da ihre Energieunschärfe kleiner ist. Darüberhinaus wird dargelegt, wie man lokalisierte Funktionen bilden kann, die eine vorgegebene Spinstruktur im Festkörper beschreiben. Dabei zeigt es sich, daß nicht jede Spinstruktur auftreten kann, die auf Grund der Kristallsymmetrie möglich wäre, sondern nur solche, die auch mit den Symmetrieeigenschaften der Blochfunktionen verträglich sind.
Dynamics of Single Flux Lines in Superconducting Films by Vortex Microscopy Experiments
Authors: Boersch H. , Lischke B. , Söllig H. 
Published: July 2007

AbstractTo study single magnetic flux lines in superconductors a Vortex‐Microscope is developed and the motion is investigated of a single vortex through a superconducting thin film. Extremely small jumps between pinning centers are observed, which are compatible with the Anderson model of flux creep. The motion of a single vortex is thermally activated in agreement with the flux‐creep model. Henceforth the interactions of single flux lines and well defined pinning centers can be studied by vortex‐microscopical observations.
Further Aspects of the Electrical Conduction in the Highly Anisotropic Organic Semiconductor TEA (TCNQ)2
Authors: Brau A. , Farges J.‐P. 
Published: July 2007

AbstractA complete report of experimental results on the dc electrical conductivity of TEA (TCNQ)2 is presented. In order to reconcile these results with the known structural and magnetic properties, it is suggested that the electron interactions play a fundamental role in the low lying magnetic excitations but have only a small influence on the much more energetic excitations to the conducting states. Conduction thus appears to resemble that of a band gap semiconductor modified by band tailing as a result of the cation disorder.
Luminescence Spectra Due to High‐Density Excitons in CuBrxCl1−x Mixed Crystals
Authors: Suga S. , Koda T. 
Published: July 2007

AbstractLuminescence spectra due to high‐density excitons are investigated on CuBrxCl1−x mixed crystals under the N2 laser lightexcitation. Fine structures observed in the excitonic molecule luminescence in CuCl and CuBr are interpreted in terms of a predicted model for the excitonic molecule transition taking acount of the spin exchange and polariton effects. Binding energies of the excitonic molecule are estimated to be about 32 and 21 meV for CuCl and CuBr, respectively. A possible effect of the random potential in the mixed crystals on the high‐density excitons is also discussed in order to interpret an unusual behaviour of the luminescence spectra in the intermediate concentration region.
X‐Ray K‐Spectra of Chromium in Cr3X‐Compounds
Authors: Kurmaev E. Z. , Belash V. P. , Nemnonov S. A. , Shulakov A. S. 
Published: July 2007

AbstractThe results of investigation of CrKβ5‐emission bands from pure metal and from Cr3X‐compounds are considered. A conclusion about the structure of the energy bands in Cr3X‐compounds is made after discussion of these data together with X‐ray spectra of the second component, theoretical calculation of the electron structure, and experimental results obtained by photoemission. A detailed comparison of the X‐ray spectra of V3X‐ and Cr3X‐compounds is made.
A Model for the Deposition of Amorphous Ge
Authors: Shevchik N. J. 
Published: July 2007

AbstractIt is proposed that the continuous random network structure of amorphous Ge depends upon the types of configurations of atoms which are stable at the surface of the film during the time it takes to form a monolayer. As the temperature is raised and/or the deposition rate decreased, the average bond angle distortion decreases and the number of higher membered rings increases at the expense of lowered membered rings. It is argued that these changes may, in part, account for the observed variations in the electronic properties of amorphous Ge.
Approximate Analytical Description of the E(k)‐Dependence of the Bands of Heavy and Light Holes in Si and Ge
Authors: Von Bozeszkowski J. 
Published: July 2007

AbstractExplicit analytical expressions of the E(k)‐dependence of the valence band of Ge and Si are derived. These expressions describe approximately the nonparabolicity of the bands of heavy and light holes which is caused by the interaction of these bands with the split‐off band. The approximate band structure is compared with the E(k)‐dependence of Kane's three‐band‐model of the valence band following from the (K. p.)‐perturbation theory.
Low‐Energy Phonons in Magnetite
Authors: Samuelsen E. J. , Steinsvoll O. 
Published: July 2007

AbstractAcoustic and some optic phonon branches have been observed by means of inelastic neutron scattering in magnetite at room temperature and near the Verwey transition at Tv = 119 K. No appreciable effect on the phonon spectrum was found in passing Tv. Sound velocities determined at room temperature were found to agree only moderately with ultrasonic data, and they show marked deviation from cubic symmetry, which possibly could be interpreted in terms of anharmonic effects.
Grüneisen Constant of Alkali Metal: Calculation of Its Wave Number Vector Dependence and Its Volume Dependence
Authors: Shimada K. 
Published: July 2007

AbstractIn the case of alkali metal, the wave number vector dependence of the Grüneisen constant and its derivative with respect to the volume are calculated on the basis of the lattice dynamical theory. From the results calculated using second order perturbation theory, the following statements are obtained: 1) the T1 mode, in which wave number vector and polarization vector are parallel to (110) and (110), respectively, gives largest change to the Grüneisen constant. 2) As temperature raises, the volume dependence of the Grüneisen constant corresponding to this T1 mode reaches to the formula obtained assuming that the adiabatic compressibility depends only on the volume.
A Model Pseudopotential in Metallic Properties
Authors: Srivastava S. K. 
Published: July 2007

AbstractA new pseudopotential model (distinct from the Ashcroft model) having only one parameter rc (core radii) of the metal has been proposed. The present model as well as Ashcroft's model have been used in evaluation of several properties like electrical resistivity and thermoelectric power of liquid metals, electrical resistivity of solid metals, temperature dependence of Knight shift at constant volume and constant pressure, and Fermi energy and density of states of simple metals. A new correction term for thermoelectric power has been given, which gives a better result for lithium and cesium. The results obtained have been compared with possible experimental and theoretical data. Good agreement has been achieved.
The Model Potential Method for an Extra Electron in Ionic Crystal
Authors: Abarenkov I. V. 
Published: July 2007

AbstractThe pseudo‐potential and mode. potential equations for an extra electron in ionic crystals are derived. The overlapping of the core orbitals is taken into account in the approximation linear in the overlap integrals. By means of this method the position of the bottom of the conduction band in KCl and NaCl is calculated.
Model for Long ‐ Wavelength Optical‐Phonon Modes of Mixed Crystals
Authors: Genzel L. , Martin T. P. , Perry C. H. 
Published: July 2007

AbstractA model is presented which describes the optical modes of a mixed crystal. The model emphasizes a realistic use of the local electric field and is completely defined by the macroscopic parameters of the pure end members. The concentration dependence of the optical frequencies can be categorized into four distinct classes in contrast to the usual two classes, “one‐mode” and “two‐mode”. We suggest that experimental evidence already exists which supports this new classification scheme.
Investigation of Hyperfine Interaction and Structure in FeSiF6·6HO2 by Mössbauer Measurements
Authors: Spiering H. , Zimmermann R. , Ritter G. 
Published: July 2007

AbstractMössbauer, proton resonance, and susceptibility measurements (T ≦ 4.2 K) on FeSiF6. · 6H2O can be understood with the hypothesis that there are three equivalent Fe(II) sites related by C3 operation and the effective symmetry of the ligand field at each Fe(II) ion site is c2h (or lower). The nonaxial susceptibility tensor χ is related to the measured axial tensor χm by the equation χm = ⅓(χ + C3χC3−1 + CχC). A new interpretation of proton resonance measurements on single crystals yields χxx/χyy = 1,54 being in good agreement with the value deduced from Mössbauer powder spectra. The hyperfine constants are reconsidered. The quadrupole moment Q of the 14.4 keV 57Fe state is determined to be Q =0.20 b.