bluuebunny/crossref_metadata_2025 huggingface dataset:
Dispersion of Generalized Surface Phonon Polaritons on α‐Quartz Observed by Attenuated Total Reflection
Authors: Falge H. J. , Borstel G. , Otto A.
Published: July 2007

Real and virtual excitation generalized surface polaritons are investigated on a 45° cut of an α‐quartz crystal. For the wave vector of the surface polariton coplanar with the normal to the surface and the optical axis they are transverse magnetic polarized extra‐ordinary waves. The experimental results agree with calculated dispersion curves. By attenuated total reflection for transverse magnetic polarized radiation it is shown that virtual excitation generalized surface polaritons exist only for certain ranges of the angle between the surface and the optical axis.
Application of the Random‐Element Isodisplacement Model to Long‐Wavelength Optical Phonons in CdSexTe1−x Mixed Crystals
Authors: Górska M. , Nazarewicz W.
Published: July 2007

Infrared reflection spectra from mixed crystals CdSexTe1–x (0 ≦ x ≦ 1) have been investigated in the spectral region from 90 to 300 cm−1 at room temperature. The spectra contain two distinct reststrahlen bands whose strengths and frequencies depend on the composition, x. Using Kramers‐Kronig analysis the frequencies of long wavelength optical phonons were obtained. It is established, that the dependence of the phonon frequencies on the composition of crystals does not show any discontinuity when changing the structure of the mixture from zincblende to wurtzite. The calculations, based on the random‐element isodisplacement (REI) model, of the dependence of long‐wavelength optical phonon frequencies on the composition x are in good agreement with the experimental results.
Electron Energy Loss Measurements on Fluorene (C13H10) Vapour and Single Crystals
Authors: Venghaus H. , Hinz H.‐J.
Published: July 2007

Electron energy loss spectra from 0 to 40 eV were measured with 55 keV electrons on fluorene single crystals. The momentum transferred to the crystal by the electrons scattered inelastically was taken to be parallel to the main axes of the dielectric tensor ε(E). The energy loss functions Im (−1/εii) (i = x, y, z) were derived from the spectra and the tensor elements εii(E) calculated from Im (−1/εii) by Kramers‐Kronig analysis. The results obtained are discussed in terms of excited molecular states determined by electron energy loss measurements on fluorene vapour in the present investigation. The results are also related to optical absorption measurements performed by other authors on fluorene in solution.
The Local and Quasi‐Local Impurity States in the Finite One‐Dimensional Hubbard Model
Authors: Mikhailov I. D. , Shelestina I. Yu.
Published: July 2007

An analytic expression for the Green's function of the ground state of the finite one‐dimensional Hubbard model and the density of the electronic states on the sites are obtained. It is shown that the introduction of a local perturbation can cause the formation of quasilocalized levels which may transform into localized states when the pertubation reaches some critical value.
Quantum‐Efficiency of Multiphonon Transitions According to the Static Coupling Scheme
Authors: Pässler R.
Published: July 2007

On the basis of the static coupling scheme a standard expression for the quantum‐efficiency of multiphonon transitions of an electronic system interacting with the quantized normal oscillation modes of the crystal lattice is given. For the case of band–imperfection transitions both in homopolar and heteropolar semiconductors, the quantum‐efficiency reduces to a simple function of three imperfection parameters to be determined from luminescence spectra data and the temperature. In order that imperfections with a level near the middle of the band gap in typical crystal phosphors may act as killer centres at room temperature, the associated deformation energies of the lattice must exceed an amount of about 11 to 17% of the width of the gap.
Elastic Electron Scattering in Symmetry‐Induced Zero‐Gap Semiconductors
Authors: Szymańska W. , Bogusławski P. , Zawadzki W.
Published: July 2007

Relaxation times for elastic electron scattering in the symmetry‐induced zero‐gap semi‐conductors of zincblende structure are calculated, taking into account the nonparabolic character of the conduction band both in the density of states and the electron wave functions. The calculation is based on the band structure proposed by Groves and Paul and uses exact k · p solutions for the three‐level model of Γ8, Γ6, and Γ7 levels. Electron scattering by charged impurities and heavy holes, optical phonons (screened polar interaction) and acoustic phonons is considered. Spin‐flip transitions play an important role for all scattering modes. An anomalous q‐dependence of the static dielectric function due to low energy excitations between the Γ8 heavy hole and conduction bands is shown to have small effect on the ionized impurity scattering at low temperatures. It is demonstrated that transverse modes give a large contribution to acoustic scattering. In all cases the Γ8 symmetry of the band‐edge and the nonparabolicity of the band at higher energies have essential effect on the scattering processes.
Spin–Lattice Coefficients of Gd3+ in CdF2
Authors: Falkowski K. , Szumowski J. , Krukowska‐Fulde B. , Jabloński R.
Published: July 2007

EPB spectra of Gd3+ in CdF2 under uniaxial stress were observed and the spin–lattice coefficients of second and fourth order were measured. The measurements were performed in X band in the temperature range 77 to 300 K. Experimental data show that the fourth order term gives an important contribution to the shift of the fine structure resonance lines. Comparison with the results of other authors at different frequencies does not reveal a frequency dependence. Also a temperature dependence of the spin–lattice coefficients is not found. From the value of the coefficient Clg responsible for the symmetric deformation it is deduced that the temperature changes of B are due to thermal expansion.
The Quasi‐Molecular Large Unit Cell Model in the Theory of Deep Levels in Imperfect Crystals: Point Defects in Graphitic Boron Nitride
Authors: Dobrotvorskii A. M. , Evaeestov E. A.
Published: July 2007

The possibilities of the quasimolecular “large unit cell” approach in the theory of deep energy levels in imperfect solids are discussed and some general principles of its realization are formulated. The success of this approach is illustrated by the application to the defect level problem in graphitic boron nitride (vacancies and impurity carbon atoms at boron and nitrogen lattice sites are considered). The results obtained are compared with available experimental data, and possible defect models are discussed.
On the Theory of Conductivity in Disordered Semiconductors
Authors: Keiper E. , Kleinert P.
Published: July 2007

Kubo's general formula for calculating the conductivity is studied for the case, where the electron–electron interaction is taken into account in disordered semiconductors. The configuration averaged Green's functions occurring in this connection satisfy equations of motion which are strongly modified in comparison with the well‐known expressions for crystalline materials.
The X‐Ray K Emission Band of Magnesium from Mg(OH)2 Single Crystals
Authors: Freund F.
Published: July 2007

The MgKß emission band has been measured of Mg(OH)2 single crystals. The MgKP band shows two maxima at 1297 and 1292 eV having an intensity ratio of about 2: 1. They are assigned to 3Tlu → 1s and 2Tlu → 1s transitions involving the lπ and 3σ molecular orbitals of the OH− ligands, respectively. The assignment is based on octahedral symmetry oh, even though the [Mg(OH)2]‐polyhedra in Mg(OH)2 have trigonal symmetry, D3d. The results are compared with known data from MgO and with recent MO‐LCAO calculations.
Lattice Dynamics and Anharmonic Effects in AgCl
Authors: Fischer K.
Published: July 2007

The lattice vibrations of AgCl are calculated from an extended shell model where the shell of the Ag‐ion has a quadrupolar and a rotational degree of freedom. The model parameters are discussed in connection with the electronic 4d‐shell of the Ag‐ion. Thermal expansion, infrared reflection, and anharmonic frequency shift are calculated from a simple extension of the harmonic model, and are found to be in good agreement with experimental data. The anharmonic effects are comparable to those of the alkali halides.
A Shell Model for Molecular Crystals
Authors: Luty T. , Pawley G. S.
Published: July 2007

The rigid molecule approximation for the lattice dynamics of molecular crystals is not good when there is considerable molecular polarisability. A shell model is presented which takes account of the polarisability, and this may be called the rigid shell model. The relative arrangements of the cores throughout a molecule is rigid, and the relative arrangement of the shells is also rigid, but the shell array is free to move with respect to the core array. Each shell is related to its core by an identical isotropic force constant. In the course of lattice vibrations small dipole moments are generated at the atomic sites, and the interaction between these dipoles needs to be calculated. This is done explicitly between the dipoles within neighbouring molecules, but for the more distant neighbours it is assumed that only the dipole moment summed over the whole molecule is important, and the interaction between these is done using Ewald's transformation. The short range repulsive forces and the medium range Van der Waals attractive forces are included by using a 6‐exp atom‐atom potential function.
Elastic Interaction between Atomic Defects
Authors: Kronmüller H. , Schaefer H.‐E.
Published: July 2007

The elastic interaction between defects of orthorhombic symmetry is considered. Explicit results for the elastic stress tensor are derived for defects with finite extensions assuming a Gaussian distribution of the quasiplastic deformations. Within the framework of the dipole approximation it is shown that 〈110〉‐crowdions form bound pairs at small separation whereas their interaction may be reptilsive or attractive at large separation.
Exciton–Phonon Interaction in Crystals of Linear Polyacenes
Authors: Kurik M. V. , Tsikora L. I.
Published: July 2007

Investigation results of the exciton absorption band shape, its long‐wavelength edge, and maximum position and half width as a function of temperature are systematized for linear polyacene crystals from naphthalene to pentacene. It is found that excitons interact mainly with optical phonons resulting in a broadening and shift of the exciton absorption band. The shape of the absorption bands is found to be complicated and may be described by the Voigt function. Some assumptions on the character of the exciton–phonon interaction are made.
Mean Spherical Model and Structure Factor of Liquid Mercury and Aluminium
Authors: Rao R. V. Gopala , Murthy A. K. K.
Published: July 2007

A closed form expression for S(k), the structure factor, has been derived, taking the square well potential as a perturbation in the mean spherical model approximation. This expression has been used to calculate the structure factor and radial distribution function, g(r), for liquid mercury and aluminium at various temperatures and densities. The very good agreement between theory and experiments suggests that the representation of the attractive tail by a square well potential is a satisfactory one and the concept of pairwise interactions is a useful tool in understanding some of the structural problems of liquid metals.
Linear Magnetic Birefringence and Near Infrared Absorption of K2NiF4
Authors: Kleemann W. , Pommier J.
Published: July 2007

Evidence of 2D magnetic short‐range order has been observed in the linear magnetic birefringence (LMB) of K2NiF4 up to 3TN (≈︁300 K) in agreement with specific heat and Raman data. Below TN/2 the LMB is proportional to the long‐range order parameter M2 Both results are explained by magnetostriction due to 2D and 3D Spin–order, respectively. The fine structure of the 3A2g → 3T2g absorption band at LHeT is discussed. The MCD of the lowest Spin–orbit component τ3 yields g ‖ = 2.27 ± 0.04 for the ground state. The Debye temperature θ ≈︁ 335 K is obtained from the temperature dependence of the τ3 line. Only about 5% of the thermal line shift below 0.7 TN is estimated to be due to the decrease of the exchange stabilization of the ground state.
Dislocations and line charges in anisotropic piezoelectric insulators
Authors: Barnett D. M. , Lothe J.
Published: July 2007

The six‐dimensional framework developed by Stroll to treat dislocations and line forces in classical anisotropic elastic media is extended to an eight‐dimensional formalism to treat dislocated piezoelectric insulators. The solutions are recast into a four‐dimensional form using the so‐called integral formalism developed previously by the authors. This technique allows one to compute the gradients of the mechanical displacement and the electrostatic potential as well as the total energy of the dislocated piezoelectric by merely computing three constant real (4 × 4) matrices using numerical integration, numerically a considerably simpler method than the prescription attached to the original Stroh formalism.
Polarons and effective electron‐hole interaction in anisotropic polar crystals
Authors: Fock H. , Kramer B. , Büttner H.
Published: July 2007

Starting from the Fröhlich electron‐phonon interaction for crystals with anisotropic band masses and dielectric constants, the polaron self‐energy and effective mass are calculated. An expression for the effective electron‐hole interaction in anisotropic crystals is derived. The effect of the electron–phonon interaction on the anisotropy of the electron–hole potential is discussed.
Structure of the infrared absorption bands and the phonon sidebands of SH‐impurity molecules in potassium halide crystals
Authors: Mauring T. , Rebane K.
Published: July 2007

The structure of the SH− stretching hand and its phonon sidebands in KCl, KBr, and KI crystals from 2 to 300 K has been studied. A splitting of the stretching bands was observed at low temperatures into three or more components that may be attributed to the formation of the SH− clusters. The intensity distribution of the sidebands corresponds, in general, to the phonon density of states of the pure host lattice. A local gap mode was observed in the phonon spectrum gap region at the frequencies 95 and 78 cm−1 in KBr and KI, respectively. The sharp maxima at 136 cm−1 in KI and at 158 cm−1 in KBr, which are situated in the lattice vibration region, were related with the pseudolocal modes.
The lattice distortion around a vacancy in F.C.C. metals
Authors: Miller K. M. , Heald P. T.
Published: July 2007

The lattice distortion produced by an isolated vacancy in aluminium and the noble metals has been calculated using a discrete lattice model which assumes non‐equilibrium pairwise interactions extending out to second nearest neighbours. It is shown that, to first order, the relaxation volume associated with a vacancy can be expressed in terms of the elastic constants. For aluminium the asymptotic displacement field calculated from the lattice model is compared with the corresponding elasticity results.
Theory of inter‐valence‐band electronic raman scattering in cubic semiconductors without and with an external electric field
Authors: Bechstedt F. , Enderlein R. , Peukere K.
Published: July 2007

Interband electronic resonance Raman scattering for excitation from the split‐off valence band to the light‐hole and heavy‐hole band and from the light‐hole band to the heavy‐hole band in p‐type semiconductors with diamond or zincblende structure has been studied. The polarization dependence of these electronic Raman transitions is stated. The influence of a strong electric field on the form and magnitude of the scattering cross section is calculated. The Raman spectrum will be depressed in the presence of the strong electric field. All explicit results are given for band parameters of Ge.
The translational symmetry in the molecular models of solids
Authors: Evarestov R. A. , Petrashen M. I. , Ledovskapa E. M.
Published: July 2007

It is shown that the translational symmetry of a solid when the molecular models are applied, can be taken into account in the framework of the “large unit cell (LUC)” approach. The method of choosing of LUC, suitable for a calculation of the energy levels and one‐electron wave functions corresponding to some given values of the wave vector is given. The way of connecting the molecular orbitals obtained in the LUC calculation with the one‐electron states of the infinite crystal is discussed.
Schottky‐type anomaly in the linear optical birefringence of DyPO4
Authors: Jahn I. R. , Smith S. H.
Published: July 2007

The linear optical birefringence of DyPO4 shows anomalous temperature behaviour between 10 and 150 K. The effect is considered to be due to the excitation of a higher crystal field component of the 6H15/2 ground state of Dy3+. From this the separation between the lowest and the first excited state is determined as Δ/k = (97 ± 3.5) K.
Studies on substitutional defects in alkali halide crystals
Authors: Paul S. , Sengueta S.
Published: July 2007

A simple model is developed for the study of substitutional defects. Region I consists of nearest neighbours to the defect site. In region II, the relaxation is taken ∼1/r2 and the constant is determined from the equilibrium condition. Using harmonic approximation an extremely simple expression for the heat of formation is obtained. Values of heat of formation obtained from this expression show good agreement with experiment and with those obtained from more sophisticated models.
Thermomagnetic effects in tin‐doped bismuth
Authors: Uher C. , Goldsmid H. J. , Drabble J. R.
Published: July 2007

Several of the thermo‐galvanomagnetic phenomena have been studied for bismuth doped with about 0.24 at % Sn in the temperature range from about 5 to 300 K. The Hall coefficient, magneto‐resistance, Seebeck coefficient, Nernst coefficient, and thermal conductivity have been measured in magnetic fields of up to about 6 T with current flow in the trigonal and bisectrix directions. The most remarkable observation has been that of a negative Seebeck coefficient for a range of temperatures well below that at which electrons in the main conduction band should be excited. A group of electrons, possibly closely associated with the light hole band, is invoked t o explain the results.
Crystal equilibrium and phonon dispersion in rare‐earth H.C.P. metals
Authors: Sinha H. P. , Upadhyaya J. C.
Published: July 2007

The phonon dispersionin two h.c.p. rare‐earth metals Tb and Ho are investigated on the basis of a model which uses a 5‐neighbour−central pair potential for close‐range forces and a screened Coulomb interaction for long‐range forces. The model considers completely the equilibrium of the ionic lattice in a medium of an uniform electron gas. Consequently two equilibrium conditions, necessary for the h.c.p. structure, are deduced and used in the study.
Optical lineshape for the A1−E transition − comparison of IOA with numerical method
Authors: Muramatsu S. , Nasu K.
Published: July 2007

Optical lineshape functions for A1−E transitions coupled with phonon modes of a1 and e symmetries are calculated using the independent ordering approximation (IOA) and the numerical method of Longuet‐Higgins et al. (EXM). The two kinds of results are compared and it is found that the IOA well reproduces qualitatively the peculiar feature of the lineshape of EXM, as long as considering only the global profile of the lineshape and disregarding its possible structure within the range of the phonon frequency. The condition of the validity of IOA is discussed.
Ultraviolet absorption and emission of CN− centres in alkali halides
Authors: von der Heyden E. , Fischer F.
Published: July 2007

The ultraviolet absorption of CN− centres is studied in NaCl, KCl, RbCl, and NaBr single crystals, as well as the ultraviolet luminescence of NaBr:CN−. At low temperatures several CN− absorption bands are resolved from the low energy tail of the ultraviolet host absorption. These bands refer to the generation of valence vibrations of the excited CN− molecule. The ultraviolet luminescence of NaBr:CN− consists of well separated nearly equidistant bands (ΔE ≈︁ 0.26 eV) in the range between 213 and 374 nm. This structure is refered to valence vibrations of the ground state CN−. The valence vibrations are described by Morse potentials. The experimental data allow the determination of the potential parameters, especially of the bond dissociation energy and of the change of the internuclear distance due to the electronic transition.
Nichtparabolizität des valenzbandes von Bi2Te3 gefolgert aus transporteigenschaften
Authors: Stordeur M. , Kühnberger W.
Published: July 2007

Messungen der galvanomagnetischen und thermoelektrischen Effekte an p‐Bi2Te3 werden im Rahmen eines nichtparabolischen Einvalenzbandomodells mit Vieltalstruktur interpretiert. Für dieses Modell werden die Ausdrücke für die galvanomagnetischen Koeffizienten und die Thermokraft angegeben. Für akustische Gitterstreuung werden die Streufaktoren der Hall‐ und Magnetowiderstandskoeffizienten theoretisch berechnet. Zur Ermittlung des reduzierten Ferminiveaus und der Breite der verbotenen Zone wird ein grafisches Lösungsverfahren angegeben. Der anomale Anstieg des Hallkoeffizienten bei steigender Ladungsträgerkonzentration wird durch den Einfluß des „nichtparabolischen”︁ Streufaktors erklärt. Entsprechend dem Erwartungswert für akustische Gitterstreuung ergibt sich der Temperaturgang der „isotropen”︁ Beweglichkeit proportional T−3/2 im Gegensatz zu den bisherigen Interpretationen mit parabolischem Band.
Antiresonance in vibronic absorption spectra of a two‐impurity system
Authors: Iida T. , Aegerter M.
Published: July 2007

Antiresonance effect is studied in a system composed of two‐impurity centres in a crystal; one has a broad absorption band while the other has sharp phonon‐line structure in its absorption spectrum when no interaction occurs between the two defects. A general expression of the change in the absorption spectrum due to interaction between them is given. The antiresonance effect on a sharp zero‐phonon line is investigated in detail and the favourable conditions for the observation of such an effect are also discussed.
Study of the electric quadrupole interaction (EQI) in (Pb0.925Ba0.075)ZrO3 by perturbed angular correlations
Authors: Saitovitch H. , Fechner J. B.
Published: July 2007

The EQI in polycristalline (Pb0.925Ba0.075)ZrO3 has been investigated in the temperature range 22 to 240 to 22 °C by time differential perturbed angular correlation measurements. For the application of this technique (Pb0.925Ba0.075)ZrO3 was doped with small amounts of radioactive Hf181. According to the crystal structure studies this compound exhibits phase transitions at 175 °C (antiferroelectric → ferroelectric) and at ≈︁205 °C (ferroelectric → paraelectric); it exhibits also a large temperature hysteresis at the lower transition. It is shown that the electric field gradient (EFG) exhibits a sharp discontinuity at approximately 170 °C, corresponding to the first phase transition, and a slightly sharp discontinuity at approximately 200 °C, corresponding to the second phase transition. In the cubic phase a remaining EFG is observed, which is probably caused by lattice imperfections in the source. No any variation of the EFG following the hysteresis loop is found.
The path‐integral method and the motion of a phonon–impurity system in a magnetic field
Authors: Gifeisman Sh. N. , Malkoch Yu. G.
Published: July 2007

It is shown that the variational principle connected with a quadratic trial Lagrangian in Feynman's path‐integral representation can be applied for calculating the energy eigenvalues and thermodynamic functions of systems in a uniform arbitrary external magnetic field. As an example the ground‐state energy of a hydrogen‐like atom in a magnetic field is calculated. A theory of shallow F‐centres in the presence of an arbitrary external magnetic field is developed. The results are valid for arbitrary magnitudes of the polaron coupling constant.
Structure of the Impurity Band and Magnetic‐Field‐Induced Metal‐Non‐Metal Transition in n‐Type InSb
Authors: Ferre D. , Dubois H. , Biskubski G.
Published: July 2007

Several samples of n‐type indium antimonide in the doping range 5.7 × 1013 to 3.9 × × 1014 cm−3 donors have been investigated. The electrical resistivity has been measured from 1.6 to 4.2 °K in magnetic fields up to 5 T. Activation energies ϵ1, ϵ2, ϵ3 which are characteristic of the resistivity versus temperature curves are measured and their variation with magnetic field are investigated. A transition from metallic conduction to an activated conduction process is observed for a critical magnetic field the value of which increases with the concentration of impurities. These features are interpreted with an impurity band structure which varies when the magnetic field is increased. A model of the impurity band and its changes with magnetic field are suggested. This model is consistent with the ideas of Mott and Davis on impurity conduction and the D− band model of Fritzsche suggested by Hubbard. These results are comparable with those which have already been obtained in germanium and silicon when the magnetic field or impurity concentrations are varied.
EPR of Non‐Kramers Rare‐Earth Ions in Tetragonal Crystals
Authors: Schwab G. , Hillmer W.
Published: July 2007

Electric ΔM = 0 and magnetic ΔM = 0 and ΔM = 1 transitions were measured with X‐, K‐, and Q‐band frequencies within the ground state doublets of the rare‐earth ions in (Y, Tb)PO4, (Y, Tm)VO4, (Y, Tm)AsO4, and (Y, Ho)AsO4. The g‐factors and hyperfine constants were determined. The doublets show a zero field splitting Δ which can be attributed to random strain. The half width of the distribution of the splitting Δ, the most probable splitting, and the natural line width were evaluated from the line shape and line position. The intensities of the electric transitions were compared with those of a crystal field calculation.
Theory of Interband Resonance Raman Scattering in Small‐Gap Semiconductors
Authors: Enderlein R.
Published: July 2007

A theoretical analysis is given for the interband resonance Raman scattering in small gap semiconductors. For a certain interval of incident photon energies the scattering spectrum shows a strong resonance peak. By measurements in the visible frequency region, especially the energy gap can be determined from the scattering spectrum. The possibilities of an experimental observation of the predicted effects are discussed for some representative small‐gap semiconductors.
Raman Scattering from Five Phases of Silver Iodide
Authors: Hanson R. C. , Fjeldly T. A. , Hochheimer H. D.
Published: July 2007

The Raman scattering is investigated from five phases of AgI that can be reached with moderate pressure and temperature. Particularly noteworthy is the strong negative pressure coefficient of the 17 cm−1 E2 mode in the wurtzite phase. Its mode gamma is −7.5. The intensity of this mode decreases above 1 kbar and shows hysteresis during pressure cycles. A possible interpretation of this hysteresis in terms of a sluggish phase transition from wurtzite to zinc blende is discussed. In the narrow phase between 3 and 4 kbar (IV) two strong modes are observed. At low temperature the sluggish phase transition does not occur and wurtzite transforms directly to rock salt at 3 kbar. The rock salt phase shows a second order spectrum similar to AgBr. The Raman spectrum of the disordered phase shows strong broad Rayleigh like scattering extending out to 50 cm−1 and a broad lattice like mode at 100 cm−1.
Born Model Calculations of the Properties of the Alkali Azides I. Cohesive Energies, Lattice Parameters, and Specific Volumes
Authors: Danemar Å. , Royce B. S. H. , Welch D. O.
Published: July 2007

Calculations have been performed in which potentials developed for Born model calculations of the defect properties of KN3 have been used to evaluate the cohesive energies of the alkali azides, KN3, NaN3, RbN3, and CsN3. The stability of the lattices under small deformations about the experimental values of the lattice parameters has also been studied. Three forms for the azide repulsive potential are employed all of which give reasonable agreement with experiment for the body centered tetragonal structures but only one of which provides agreement across the series of alkali azides. Comparisons are made to previous calculations.
Monovalent Mercury‐Like Ion Dimer Centres in Alkali Halide Crystals
Authors: Ermoshkin A. , Evarestov R. , Gindina R. , Maaroos A. , Osminin V. , Zazubovich S.
Published: July 2007

Polarized luminescence of (Ga+)2, (In+)2 and (Tl+)2 centres in KCl crystals is investigated experimentally. In the series of KCl, KBr, and KI crystals the (Tl+)2 centres are studied in order to clear up the influence of the anions on the (A+)2 centre properties. The quasimolecules [Tl2X10]8‐ and [T12X11]9‐, where X are Cl− or I− ions, with the symmetry D2h and D4h, respectively, were theoretically considered by a MO method. An attempt is made to prove the model of the (A+)2 centres basing on a comparison of the results of the experimental and theoretical investigations.
Magnetic‐Field‐Induced Energy Gap in HgTe Observed in Transport Measurements
Authors: Byszewski P. , Dziuba E. Z. , Gała̧zka R. R. , Szlenk K. , Szymborski K. , Walukiewicz W.
Published: July 2007

Experimental results are given of galvanomagnetic measurements of p‐HgTe in strong pulsed magnetic fields performed up to 35 T in the temperature range 77 to 340 K. Co cen‐tration and mobility of holes in the intrinsic conductivity region were determined. To explain the experimental data it is necessary to introduce a valence and conduction band separation of 0.6 meV/1 T induced by magnetic field. The temperature dependence of the mobility shows that the hole mobility is limited by phonon scattering.
Charge‐Ordered State in Semiconducting Crystals of the Cs2SbCl6 Type and the Electronic Structure of Sb Ions
Authors: Makarov E. F. , Amelin I. I. , Ionov S. P.
Published: July 2007

Results are presented of a calculation of the ground state energy for the various configurations of Sb ions. It is found, that the 5s25p2, 5s15p2, and 5s15p1 atomic configurations correspond to the SbCl, SbCl, and SbCl complexes, respectively. These configurations are near to occupations obtained in the LCAO‐MO terms. The nature of an effective electron attraction on the aig‐orbital in SbCl is discussed. In an atomic approach the energy gap value ΔE is determined. This quantity (ΔE = 0.043 a.u.) is found to be in good agreement with observations.
Symétrie du paramètre d'ordre de la transition de phase ferroélastique du phosphate de plomb
Authors: Torres J.
Published: July 2007

On examine d'un point de vue théorique la transition de phase ferroélastique du phosphate de plomb Pb3(P04)2 située à 180 °C. On remarque que le changement de symétrie spatiale R3m → B2/b qui se produit à la transition, s'accompagne d'un doublement du nombre d'atomes dans la maille élémentaire. Le phosphate de plomb est ainsi un matériau ferroélastique de type impropre. La symétrie du paramètre d'ordre de la transition est etudiee par des methodes classiques de theorie des groupes. On montre que le parametre d'ordre se transforme suivant une representation irreductible tridimensionnelle de R3m correspondant au point L de la frontiere de la premiere zone de Brillouin du reseau rhombo‐edrique. Toutefois deux representations irreductibles distinctes sont susceptibles d'induire la transition observee, laissant done subsister une ambiguite sur la symetrie reelle du parametre d'ordre. L'energie libre qui decrit revolution des proprietes physiques macro‐scopiques au voisinage de la transition prend la meme forme dans les deux cas.
Inelastic Neutron Scattering Study of the 2kF Instability in K2Pt(CN)4Br0.30 x D20 (KCP)
Authors: Comes E. , Renker B. , Pintschovius L. , Currat R. , Gläser W. , Scheiber G.
Published: July 2007

Besides the modifications of the longitudinal acoustic dispersion in the c‐direction, ill‐elastic neutron scattering measurements of the 2kF instability in KCP between room temperature and 60 K reveal an additional excitation at a frequency of 0.6 THz for the wave vector 2kF. This excitation obviously corresponds to the additional structure earlier observed at low temperatures by infrared measurements but the extension in wave vector 2kF – 0.2c* < qc < 2kF + 0.1c*, the very small dispersion, and the fact that, though much weaker than at low temperature, it is still observable at room temperature, casts serious doubts on its interpretation in terms of the pinned charge density wave (pinned Fröhlich mode). The existence of a low frequency longitudinal optical phonon branch is more likely, despite the unexplained weak intensity of the corresponding phonons.
Low‐Temperature Galvanomagnetic Effects in n‐Type PbS
Authors: Finlayson D. M. , Johnson I. A.
Published: July 2007

The resistivity and Hall effect of lead sulphide single crystals containing 1016 to 1017 electrons cm−3 have been measured at temperatures down to 0.2 °K in magnetic fields up to 40 kG. The number of conduction electrons is independent of both magnetic field and temperature. A negative longitudinal magnetoresistance at low fields gives place to a positive effect at high fields and both are strongly dependent on electron concentration.
NMR Evidence for the Motion of Br−Ions in the One‐Dimensional Conductor K2Pt(CN)4Br0.3 nH20
Authors: Nagasawa H. , Miyauchi N.
Published: July 2007

The motion of Br ions in the one‐dimensional conductor K2Pt(CN)4Br0.3 2.6 H2O at room temperature is observed in the measurements of the NMR line width of Br81 nuclei. The temperature dependence of the NMR line width of H1 nuclei in K2Pt(CN)4Br0.3 × × 2.6 H20 shows that the rotational motion of water molecules is strongly correlated with the gradual metal‐to‐insulator transition in KCP.
On the Hc2 anisotropy in uniaxial superconductors
Authors: Teichler H.
Published: July 2007

For impurity‐free uniaxial superconductors the temperature dependence of the anisotropy of Hc2 is evaluated for the whole temperature range below Tc. In particular it is shown that the residual value of the relative anisotropy of Hc2 at Tc and the temperature dependences of the various angular momentum components of ΔHc2/H̄c2 reflect effects of Fermi surface and energy gap anisotropies.
Interactions hyperfines du centre F+ dans ZnO
Authors: Gonzalez C. , Galland D. , Herve A.
Published: July 2007

Des mesures de RPE sur la lacune d'oxygène ayant piégé un électron (centre F+) dans ZnO, and permis de déterminer les paramètres des interactions hyperfines magnétique et quadrupolaire électrique, avec un noyau Zn67 adjacent à la lacune. Ceci dans les deux cas possibles de configuration: celui qui correspond à un Zn67 dans un site axial par rapport à la lacune, et l'autre, correspondant au noyau dans un des trois sites non‐axiaux. A partir des valeurs expérimentales du paramètre quadrupolaire P‖ = (3,13 ± 0,01) MHz (axial) et (3,1 ± 0,2) MHz (non‐axial) nous montrons que les quatre ions Zn++ premiers voisins, relaxent vers l'extérieur de la lacune.
Effects of excitation‐induced optical absorption in highly excited semiconductors
Authors: Kushida T. , Moriya T.
Published: July 2007

Optical gain spectra are calculated for radiative exciton–exciton collision and biexciton recombination processes. The result well explains the red shift of stimulated emission bands of various semiconductors with increasing the pumping intensity or the excitation length. Several effects of excitation‐induced negative gain are also discussed.
Quasi‐cubic state and piezobirefringence of CdS and CdSe
Authors: Réza A. , Babonas G. , Šileika A.
Published: July 2007

The dependence of intrinsic birefringence Δ of CdS and CdSe on uniaxial stress X has been investigated at 77 K in the spectral range 0.6 eV to the absorption edge. The measurements are carried out up to stresses of 9400 and 6700 kp/cm2 at X ‖ c (2000 and 1900 kp/cm2 at X ⊥ c) for CdS and CdSe respectivitely. The dispersion of the linear stress coefficients of birefringence dΔn(E)/dX is determined. In the case X ‖ c the CdS and CdSe crystals are compressed up to the quasi‐cubic state of the upper valence bands at point Γ in the Brillouin zone. The experimental results of piezobirefringence are interpreted by the stress‐induced changes of the parameters of the optical transitions at the absorption edge and in the higher energy region.
Spectrum of collective electronic excitations in semiconducting crystals of the Cs2SbCl6 type
Authors: Tonov S. P. , Lubimov V. S. , Makarov E. F. , Ionova G. V. , Uimin G. V.
Published: July 2007

Crystals of A2SbCl6 type with a very narrow half‐filled band are investigated. As the temperature is lowered it can undergo a transition to an electron‐charge ordered state. In this state alternate atomic sites along the (100) crystal direction are doubly occupied (SbCl63‐–SbCl6−)N. The compounds are diamagnetic in both the states (ordered and disordered). On the basis of an Hamiltonian that incorporates the intra‐site (I) and inter‐site (V) electron–electron interaction as well as the hopping energy the analysis of the two‐electron collective excitation spectrum is made. The conditions are found in which the two‐particle spectrum lies within the energy gap of the one‐particle spectrum. In a narrow band the transition from the charge‐ordered state to the delocalized one when there are low‐energy collective excitations will be either of second order or without a phase transition as a continuous one. The model of the transition to a delocalized state is discussed too.
A new current transport mechanism due to interband tunneling without and with photon assistance
Authors: Henneberger K. , Enderlein R.
Published: July 2007

Zener tunneling is shown to contribute in two different ways to the total current: firstly, by the intraband contribution of free carriers generated by tunneling and secondly, by an interband current resulting from charge transfer through the gap due to tunneling. The latter contribution is studied in more detail. The considerations are generalized to photonassisted tunneling, i.e. Zener tunneling which is followed by photon‐induced intraband transitions. The interband current is independent of concentration and mobility of free carriers and, consequently, dominates if those are extremely small.
Mössbauer study of FM sidebands produced by the RF magnetic field applied to an iron foil
Authors: Kopcewicz M. , Kotlicki A. , Szefer M.
Published: July 2007

The formation of rf sidebands in the Mössbauer spectra of metallic iron as a result of the application of an external rf magnetic field is studied. The dependence of the sideband intensities on the rf field intensity and the thickness of the sample is discussed. Experiments show that the formation of sidebands is connected with atomic vibrations induced within the foil by the rf field applied. Basing on the experimental spectra obtained, modulation indices are calculated in accordance with the FM theory. The results obtained are interpreted in terms of the modified magnetostriction model. The dependence of sideband formation on the coating of the foil with a damping medium is studied. It is found that covering of the sample surface with a silicon grease, adhesive tape, or metallic zinc causes a considerable damping of the sidebands and in some cases causes their complete quenching. This effect has been attributed to the dissipation of energy generated within the sample by the rf field. If the acoustic resistivity of the medium covering the sample is higher than that of air the energy transferred from the foil to the environment increases and the sidebands are damped.
On the measurement of the conductivity density of states of evaporated amorphous silicon films
Authors: Brodsky M. H. , Döhler G. H. , Steinhardt P. J.
Published: July 2007

Data are analyzed on the conductivity across the barrier between amorphous and crystal Si. The analysis assumes that variable range hopping near the Fermi level is the dominant conductivity mechanism in the evaporated amorphous Si film and band conductivity is dominant in the crystal. The density of states is found to be 1.2 × 1020eV−1cm−3at the Fermi level in amorphous Si. The barrier heights are found to be larger when the crystal is p‐type. This is in contrast to the somewhat analogous case of metal–semiconductor Schottky barrier diodes in which n‐type crystals give higher barriers. The possibility of applications in circuits is pointed out.
Monte Carlo Simulation of NMR Line Shapes in Distorted Non‐Metallic Solids
Authors: Baumhoer W. , Thiemann J. , Kanert O.
Published: July 2007

The Monte Carlo method is applied to calculate the quadrupole distortion of the shape of the NMR spin echo signal caused by randomly placed defects (point defects or dislocations) in a cubic lattice. Specific evaluations are carried out in the case of Na23Cl and Na23Br disturbed by Ca2+ (valence effect), by Ag+ (strain effect), and by a defined number of dislocations, respectively. The theoretical results are compared with XMR measurements on Na23Cl and Na23Br single crystals distorted by 0.01 at% Ca2+, 0.5 at% Ag+, 6 × 107 dislocations/cm2, and 1.8 × 108 dislocations/cm2, respectively. It is found that the calculations agree well with the experimental data in the case of doped crystals, whereas some deviations between measurements and theory exist if the crystal is strained by dislocations. These discrepancies may be explained by a non‐uniform distribution of the dislocations in the crystal.
Theory of Biexcitons Using the Boson Formalism The Ground State Binding Energy
Authors: Sheboul M. I. , Ekardt W.
Published: July 2007

Using the boson formalism of Usui for the Hamiltonian of the many exciton system, a coupled system of integral equations for the biexciton amplitudes is derived by means of the method of Gell‐Mann and Low. In lowest order, this system is then reduced to only one Schrödinger equation for the bound states of two ls‐excitons. With that the ground state binding energies of biexcitons in CuCl and CuBr is calculated in good agreement with experiment.
Magnetic Interactions in Co3O4
Authors: Scheerlinck D. , Hautecler S.
Published: July 2007

Co3O4 is a normal spinel with only A‐A interactions which becomes antiferromagnetic at TN ≈︁ 40 K. The exchange integrals between one ion and its first and second neighbours are derived from the second moment of energy distributions of neutrons scattered by a polycrystalline sample in its paramagnetic state (155 K) and from a Green's function analysis of TN. The result, J1/kB = −(7.27 ± 0.25) K and |J2|/kB < 0.3 K, indicates that the interaction with the nearest neighbours is by far dominant.
Nonlinear Electromagnetic Properties of Semiconductors with a Superlattice
Authors: Ignatov A. A. , Romanov Yu. A.
Published: July 2007

The nonlinear high‐frequency conductance of superlattices due to the nonparabolicity of minizones has been investigated. Exact expressions for the current excited with a harmonic electric field are obtained. A selftransparence effect is predicted. It is shown that in the presence of a strong high‐frequency field the increase of the low‐frequency oscillations is possible. The frequency doubling and the high‐frequency signal detection in the presence of a strong constant electric field is also investigated.
Temperature Dependence of Raman Spectra of Cuprous Halides
Authors: Fukumoto T. , Nakashima S. , Tabuchi K. , Mitsuishi A.
Published: July 2007

The temperature dependence of the half linewidth and frequency shift of Raman lines in cuprous halides are investigated in the temperature range from 4.2 to 300 K. As for the LO and TO mode of CuI and CuBr, and the LO mode of CuCI, the experimental linewidth and frequency shift are in good agreement with the theoretical ones which have been calculated in terms of three‐phonon interaction. In order to explain the temperature dependence of a strong combination mode in conjunction with the TO phonons of CuCI, the hybridization theory proposed by Ruvalds and Zawadowski is employed in which hybridization between a two‐phonon and a single phonon state is postulated. A numerical analysis is made assuming that the acoustic combination mode (TA + LA) hybridizes with the TO mode. The anomalous frequency shift and line broadening of the strong combination line are explained by the temperature dependence of the coupling constant of two acoustic phonons (between TA and LA mode). The linewidth and frequency shift of the TO mode can be explained in some degree by this hybridization theory.
Isotropic Compton Profile of LiH in the Tight‐Binding Approximation
Authors: Grosso G. , Parravicini G. Pastori , Resta R.
Published: July 2007

The isotropic Compton profile of LiH is considered in the tight‐binding approximation. Firstly, it is shown how to simplify the inversion of the overlap matrix in LCAO calculations of crystals by fully exploiting the lattice symmetry. Then the wave function for the hydride ion in LiH is calculated variationally by performing restricted Hartree‐Fock calculation for a cluster with 27 centers. The corresponding calculated Compton profile is closer to the measured curves.
Approximate Methods for Taking Absorption into Account in the Dynamical Theory of Electron Diffraction
Authors: Andrew J. W. , Sheinin S. S.
Published: July 2007

Approximate methods for taking absorption into account in the dynamical theory of electron diffraction can be obtained from the exact theory by assuming that mixing between certain pairs of Bloch waves is unimportant and can therefore be neglected. The assumptions required to reduce the exact theory to the lower order perturbation theories previously published in the literature are examined. This is of interest since an understanding of these assumptions provides a physical basis for comparing these theories as well as giving the insight required to develop new approximations. The details of a new approximate theory, referred to as the sub‐matrix method, are also given. This approach has a number of advantages over other approximate methods. Firstly, it is applicable under conditions which lead to either non‐degenerate, quasi‐degenerate or degenerate Bloch waves. As well the method can give very close agreement with the exact theory while at the same time providing advantages from a computational point of view.
Renormalized Density of States by Conduction Electron–Localized Spin Scattering
Authors: Ausloos M.
Published: July 2007

An analytic expression for the change in density of electronic states in the presence of conduction electron scattering by localized spins is derived. The expression seems to be general enough to be applicable to any scattering by a localized potential. The change in the density of states at the Fermi level with respect to the free electron density of states is also explicitly given for the low temperature region.
Optical Absorption, Fluorescence, and Electron Spin Resonance of Ir4+ on Octahedral Sites in Y3Ga5O12
Authors: Andlauer B. , Schneider J. , Tolksdorf W.
Published: July 2007

Ir4+, incorporated on the octahedral a‐sites of Y3Ga5O12:Ir, has been investigated by electron spin resonance at 9 and 35 GHz, and by optical absorption spectroscopy in the range 5000 to 6000 cm−l. For an unequivocal optical identification Zeeman measurements were performed. In the same energy range fluorescence from Ir4+ could be observed. Analysis of the data obtained by electron spin resonance and optical techniques reveals that the properties of the 2T2g ground state of Ir4+ are partly determined by interaction with higher excited states.