bluuebunny/crossref_metadata_2025 huggingface dataset:
Galvanomagnetic properties of MnkHg1–kTe under hydrostatic pressure
Authors: Stankiewicz J. , Giriat W. , Bien M. V.
Published: July 2007

The variation of the Hall coefficient and the electrical conductivity under hydrostatic pressure up to 10 katm and in the temperature range from 77 to 300 K was measured in mixed crystals of MnkHg1–kTe (k< 0.10). From the analysis of the pressure and temperature dependences of the electron concentration the energy gap E0= E(Γ6) − E(Γ8) was determined for k = 0.012; 0.05; 0.07; and 0.09 in the temperature interval from 77 t o 300 K. If is found, that the energy gap dependence is linear with composition within the investigated range of composition. It is established, that α = dE0/dT and γ = dE0/dp are positive and equal 5 × 10−4 eV/deg, and 9 × 10−6 eV/atm, respectively. This is the first attempt to determine the dependence of the energy gap on the composition in MnkHg1−kTe mixed crystals for k< 0.10. The obtained results are quite similar to the energy dependence on composition in CdxHg1−xTe, but significant differences occur in quantitative values.
Additional long‐ and short‐range potentials for direct excitons in zincblende structure solids: II. Numerical results and comparison with experiment
Authors: Ekardt W.
Published: July 2007

The correction formulas derived in Part I are applied to the calculation of the III‐V compound exciton spectra. Obviously only four of the many possible correction terms give appreciable contribution for these materials: dynamical corrections due to electron−electron interaction and due to electron−phonon interaction, exchange interaction, and provision of the realistic one‐particle band structure. The comparison with experiment is good.
Interlayer interaction and optical properties of layer semiconductors: 2H and 4H polytypes of PbI2
Authors: Doni E. , Grosso G. , Harbeke G. , Meier E. , Tosatti E.
Published: July 2007

The consequences of layer compound polytypism on optical properties and electronic band structure are investigated for the case of 2H and 4H PbI2.
On the question of exciton−phonon complexes in II−VI compounds
Authors: Georgobiani A. N. , Ozerov Yu. V. , Friedrich H.
Published: July 2007

The peculiarities of the λ‐modulated reflection spectra of ZnSe and CdS lying above the exciton ionization energy and located near the energy Eex1 + hω0 (Eex1 energy of the exciton ground state, hω0, energy of LO‐phonon) are investigated. The observed structures are interpreted from the point of view of the exciton‐phonon complex formation.
Multiple scattering and central limit theorem: A monte carlo approach
Authors: Desalvo A. , Rosa R.
Published: July 2007

The angular distribution of 1 MeV protons across layers of amorphous SiO2 of various reduced thickness τ (from 0.2 to 20) is evaluated by Monte Carlo simulation for different interaction potentials. A simple γ−2 potential and a Coulomb potential truncated at the Thomas‐Fermi screening radius turn out to be good approximations at low and high τ values, respectively, while Meyer's detailed calculations are satisfactory on the whole thickness range. The final distribution depends critically on the chosen potential: an γ−1 dependence at small impact parameters is necessary in order to satisfy the central limit theorem at large thicknesses.
Reduction symmetry method in crystal field theory (I)
Authors: Kustov E. F.
Published: July 2007

An account is given of a new method used to determine the matrix elements of the operators with symmetry of the point groups. Emphasis is laid to the importance of the subgroup properties of the point groups and the method of connecting matrix elements of free atoms and of atoms in the crystal field is developed. Some examples are given of the application of the reduction symmetry method (SC method) for the calculation of the matrix elements of the crystal field operators, the electrostatic, spin–orbit, and Zeeman interactions and formulae for calculating the intensities of various transitions in crystals with different symmetries are derived.
Measurements of magnetic susceptibility on amorphous boron
Authors: Kubler L. , Gewinner G. , Koulmann J. J. , Jaéglé A.
Published: July 2007

Magnetic susceptibility measurements on amorphous boron samples have been performed between 77 and 1300 °K. Surface impurities present in the amorphous powder (according with a small particle size) have made measurements difficult. These impurity contributions being eliminated, a paramagnetic susceptibility signal, rising exponentially with the temperature above room temperature was found, just as in β‐rhombohedral boron. The presence of the same sort of electronic states in the forbidden band gap of the two compounds is concluded.
Calculation of the electrical conductivity above and below the Verwey transition in magnetite
Authors: Lorenz B. , Ihle D.
Published: July 2007

Using the Cullen‐Callen model of magnetite in the region U1 » |h| (U1 nearest‐neighbour Coulomb energy, h overlap integral) the electrical conductivity is calculated above and below the Verwey transition temperature Tv. From Tv = 119 up to 250 K an increase of the dc conductivity by a factor of five is obtained. At 383 K a conductivity maximum is found. The dc conductivities normal and parallel to the c‐axis are found to change abruptly at Tv by a factor of 110 and 70, respectively. From Tv down to 75 K a decrease in dc conductivity is obtained corresponding to an activation energy of 0.11 eV. The theoretical results agree very well with experiments.
Positronium‐like Á+‐center in potassium halides
Authors: Bisi A. , Bosi L. , Dupasquier A. , Zappa L.
Published: July 2007

The annihilation of positrons bound in Á+‐centers is studied. From a magnetic quenching experiment the value of |Ψ(0)|2 for the bound electron‐positron pair is obtained. Reasonable agreement is found with theoretical expectations.
Scattering of cold neutrons from an imperfect flux line lattice
Authors: Labusch R.
Published: July 2007

The scattering of cold neutrons from an imperfect flux line lattice (FLL) is analysed. It turns out to be possible by suitable experiments to distinguish between distortions due to pinning centres and those of grown‐in intrinsic defects of the FLL, and thus to obtain valuable information on the interaction forces between pinning centres and flux lines from the shape of the scattering intensity profiles.
A shell model for molecular crystals: Orthorhombic sulphur
Authors: Luty T. , Pawley G. S.
Published: July 2007

The recently proposed shell model for molecular crystals has been used in the case of orthorhombic sulphur. The value of the model is assessed by calculating a goodness of fit value using the measured zone‐centre mode frequencies. These measurements include Raman, IR, and inelastic neutron scattering results. The goodness of fit value is considerably better in the case of the shell model calculations than in the unpolarisable' molecule calculations. The model results are compared with neutron measurements throughout the Brillouin zone, showing scope for further improvement. Some suggestions for improvement are made.
Crystal field splitting and superhyperfine structure parameters of Co2+ in CdTe
Authors: Biernacki S. W.
Published: July 2007

A semiempirical LCAO method is used to study the paramagnetic impurity Co2+ in CdTe. The cubic field splitting parameter Δ, the position of the impurity level with respect to the valence band, and the parameters of SHFS tensor are calculated and compared with experiments. The theoretical value of Δ is about 50% underestimated and the parameters of SHFS tensor are in good agreement with those from experiment.
Deformation of a Ni 〈111〉 film on Cu substrate
Authors: Dragila R.
Published: July 2007

The discrete lattice theory has been used to determine the deformation near the free surface of Ni 〈111〉 thin film. The force constants have been chosen to fit the elastic constants and phonon dispersion data. Also the deformation of Ni 〈111〉 film near the contact with bulk Cu base is calculated in the frames of both discrete and continuum theories.
Electronic structure and EPR spectra of V4+ in TiO2, SnO2, and GeO2
Authors: Michel‐Calendini F. M. , Fichelle G.
Published: July 2007

The energy levels of V4+ in a VO68− complex of D2h symmetry are computed according to an extended LCAO method with non‐orthogonal orbitals. The substitutional impurity is trapped in TiO2, GeO2, and SnO2 lattices. The results are compared with previous calculations on these structures. The interband transitions at the centre of the Brillouin zone are related to optical data with a good approximation. A first tentative is made to explain the X‐ray spectra of TiO2 according to the molecular diagram. The g‐factors and the hyperfine structure constants are computed and the covalency of molecular orbitals is taken into account. The following results are obtained for gx, gy, and gz:1.917, 1.893, 1.966 for V4+(TiO2), 1.937, 1.869, 1.953 for V4+ (SnO2) and 1.928, 1.911, 1.970 for V4+ (GeO2). The corresponding values of Ax, Ay, and Az in 10−4 cm−1 units are: 35, 40, and 141 for V4+(TiO2), 27, 40, and 148 for V4+(SnO2), and 41, 43, and 128 for V4+ (GeO2). A comparison is made with previous works using the point charge model. The LCAO calculations give results in agreement with the values deduced from EPR experiments.
Influence of magnetic anisotropy on light absorption and scattering in antiferromagnets
Authors: Gurylev V. K. , Vereshchagin V. P. , Dmitriev I. N. , Kurbatov L. V.
Published: July 2007

The temperature dependence of the integrated intensity of optical magnon sidebands and the differential extinction coefficient of inelastic light scattering in two‐sublattice antiferromagnets with spin S = 1 are calculated. The zeroth approximation of the self‐consistent‐field method (SCFM) with exact allowance of the single‐ion anisotropy is considered. The theory is applied to some antiferromagnets with Ni ions. The agreement of theoretical and experimental results is satisfactory.
Microtheoretical estimation of laser influence on superconductivity
Authors: Vujičič G. M. , Tošić B. S.
Published: July 2007

The microtheoretical estimation of the superconductive gap behaviour in the electromagnetic field is given. The approach to the problem is based on the mechanism of the retarded interaction between electrons. Ii is shown that the light quanta mainly decrease the magnitude of the superconductive gap. Also, the possibility to increase the gap is discussed, but the result is obtained under conditions near the limit of the validity of the method used.
Low‐Field Thermomagnetic Tensor for the Bi‐Like Band Structure
Authors: Uher C.
Published: July 2007

The low field thermomagnetic tensor components for the Bi‐like band structure have been calculated. The expressions are applicable for both the diffusion and the phonon‐drag transport mechanisms. The umkehreffect predicted for the α11(B1) coefficient is purely of phonon‐drag origin and can be used to determine the temperature range for which the phonon‐drag effect operates.
Electron Structure of Atoms of the 3d Transition Metals
Authors: Galperin F. M.
Published: July 2007

Electron structures (configurations) of the atoms of all the 3d transition metals are calculated. It is shown that the regularities to which the electron configurations of the atom of the crystalline elements (3d metals) follow are much similar to those ones for free atoms of the Periodic Table of Elements.
Energy Band Structure of Some Wurtzite Crystals
Authors: Chatterjee S. , Sinha P.
Published: July 2007

A parametrized band structure for the wurtzite crystal is formulated following the tight‐binding interpolation scheme of Slater and Koster. The parametrized secular determinant is factorized using group theory at the symmetry points and axes inside the Brillouin zone. Besides the group theoretic factorization restrictive assumptions regarding the nature of the overlap integrals were needed for further factorization to facilitate the numerical evaluation of the parameters. Using the OPW data of Herman at the high symmetry points γ, A, H, and K, the parameters for wurtzite SiC, ZnS, ZnSe, and ZnTe are found and a thorough study of the energy bands throughout the Brillouin zone for these solids is performed with the help of these parameters.
Change of Volume Due to Cahn's Inhomogeneity
Authors: Tichy G.
Published: July 2007

Spinodal decomposition takes place in such a way that the concentration changes continuously in the matrix, concomitant with the change of volume. Using the formalism of the nonlinear theory of elasticity, a relation is found between the dilatation and the mean square of the concentration deviation.
Impurity‐Induced Infrared Absorption in Silver Halides
Authors: Hattori T. , Ehara K. , Hamasaki M. , Mitsuishi A.
Published: July 2007

The impurity‐induced infrared absorption bands are measured at low temperatures for AgBr: Li, AgBr: Na, and AgCl: Li. The absorption bands consist of those due to the localized mode, including the fundamental band, the higher frequency sidebands and the third harmonic bands, and those due to the band modes. The force constant change at the impurity site is discussed using a diatomic three dimensional model. The sideband spectra of the lithium and sodium localized mode in silver bromide are described by the theory based on two‐phonon absorption processes. There are apparent differences in the phonon coupling mechanisms to the host lattice between the lithium localized mode and the sodium one in silver bromide. It is found that for AgBr: Li, the phonon coupling mechanism due to the cubic anharmonicity is dominant and the impurity is cupled to its surrounding ions more softly. On the contrary, for AgBr: Na, the contribution due to the second‐order electric‐dipole moment mechanism is nearly equal to that due to the anharmonicity in high frequency limit of the sideband. Some absorption bands due to the impurity‐induced band mode are observed at 30, 48, and 61 cm−1 in AgBr:7Li and AgBr: Na at 2 K.
Lattice Dynamics of Covalent Ionic Compounds (II)
Authors: Altshuler A. M. , Vekilov Yu. Kh. , Izotov A. D.
Published: July 2007

The lattice dynamics of covalent ionic compounds with ZnS structure is discussed in the light of the pseudopotential method. The calculated results of the phonon dispersion in high‐symmetry directions are given for a number of compounds. Adequacy of experiments and the “dielectric model” as well as the possibility of its further generalization are also discussed.
Magnetic Susceptibility and Electronic Structure of CoSi and CoGe Alloys at High Temperatures
Authors: Dovgopol S. P. , Antropov V. A. , Radovskii I. Z. , Geld P. V.
Published: July 2007

The magnetic susceptibility of CoSi and CoGe alloys is investigated by the Faraday method with the help of the pendulum scale in the temperature range 900 to 1800 °C. It is shown that the Curie‐Weiss law is fulfilled both for solid and liquid alloys with Si (Ge) concentrations up to 30 at%. For these alloys the number of noncoupled spins per Co atom and the paramagnetic Curie temperature are calculated after determination of the temperature‐independent contribution to the magnetic susceptibility. Considering the possible localization of 3d‐electrons, a band model of Co alloys is proposed. Basing on this model and considering the connection of the nearest order in the alloys with the crystalline structure of the solid state, the magnetic properties of both solid and liquid alloys are explained.
Contribution of Lattice Dilation to the Temperature Shift of the Energy Gaps in Trigonal Selenium
Authors: Lingelbagh W. , Weiser G.
Published: July 2007

The influence of thermal lattice dilation on the temperature shift of some optical transitions has been studied. Below 100 K the shift of both the direct gap at H and the indirect gap is determined by the lattice dilation perpendicular to the hexagonal axis c, caused by acoustical phonons which leads to an increase of the gap energy with rising temperature. At higher temperatures the electron‐phonon coupling (mainly by optical phonons) gains importance and decreases the temperature coefficient of the gap energies. The energy of the direct gap at Z is less sensitive to the thermal dilation of the lattice.
Optical Detection of Nuclear and Electronic Magnetic Resonances of Close F‐Center Pairs in KCl
Authors: Jaccard C. , Schnegg P. A. , Aegerter M.
Published: July 2007

The luminescence of close F center pairs (produced by short F band irradiation near RT) is measured at LHeT in a static magnetic field (kG) under the influence of a microwave and/or a rf field of suitable frequency. EPR, revealed by an increase of the luminescence, is modulated by the rf, giving ENDOR spectra similar to those obtained for single centers by conventional methods. The rf alone induces also changes of the luminescence (up to 1%) giving NMR spectra with an intensity depending on the optical excitation and the rf power. The relaxation rate, extrapolated to zero light intensity, is equal to the inverse electronic spin‐lattice relaxation time. These observations can be explained by a pair model with isotropic spin exchange, calculated in the high field limit (Zeeman energy ≫ hyperfine energy ⪆ exchange energy). The states, characterized by electronic and nuclear quantum numbers, have different luminescent probabilities and the optical cycle transfers the pairs into the non‐radiative states. Any magnetically induced transition tends to restore the population equilibrium and increases the luminescence. The calculated intensity ratio EPR:NMR:ENDOR agrees with the observations.
A Self‐Consistent Calculation of Magnetic Properties of Single‐Element Amorphous Materials beyond the Homogeneous Molecular Field Approximation
Authors: Šlechta J.
Published: July 2007

A self‐consistent calculation of magnetic properties beyond the homogeneous molecular field approximation is given. It was found that the Curie temperature, due to fluctuations of the exchange interaction only, increases. It is also shown that the magnetic properties inherently depend on higher spatial correlations of spins then pair‐pair ones.
The Bond Orbital Model Study of the Valence Bands of Cubic‐Tetrahedrally Coordinated Semiconductors
Authors: Ciraci S.
Published: July 2007

The valence energy bands of cubic‐tetrahedrally coordinated semiconductor sis investigated by employing the bond orbital model. In terms of the basic parameters of the model simple expressions are obtained for the gaps and widths at X and L, which allows to learn origins of a variety of the trends. The main features of the bands are found to depend exclusively on the polarity and the s‐p splittings of the atomic term values of the cation and the anion. As a further sophistication, the second‐neighbor interactions are included. Additional energy parameters representing second‐neighbor interactions are seen to have interesting relations to the polar and the metallic character of these solids. Finally, the pressure dependence of the valence bands is studied.
Invariance Properties of a Proposed Hamiltonian for Bloch Electrons in a Magnetic Field
Authors: Wannier G. H.
Published: July 2007

A matrix equation has previously been proposed for the equation governing the motion of a Bloch electron in a magnetic field. The matrix was obtained by concentrating on irrational fields. The matrix elements vary continuously with the parameters of the problem. It is shown that this Hamiltonian matrix has the normal invariance properties expected from quantum theory. When derived in a Landau base it has an energy spectrum which is invariant with respect to a rotation of the Landau axes about the field. The same matrix equation results for rational and irrational fields if one works in a cylindrical magnetic base.
Exciton Superradiance
Authors: Kopvillem U. Kh. , Samartsev V. V. , Sheibut Yu. E.
Published: July 2007

The statistical aspects of exciton systems are considered. Calculations show that the non‐Bose character of the statistics plays a fundamental role in forming the exciton echo signal, self‐induced transparency, and other superradiant effects. As application of the general theory new phenomena as exciton nutation and forced exciton induction are considered. Comparison with experiment is made for the self‐induced transparency in CdS0.75Se0.25.
Linear Electroabsorption Due to Substitutional Deep Impurities in Wurtzite‐Type Crystals. III. Model Analysis of Co2+ in CdS
Authors: Gardavský J.
Published: July 2007

Axial crystal field and Stark effect splittings are calculated and compared with experimental values. Reversed splitting pattern of 4T1(4F) is accounted for by the equivalent even‐parity field. The field‐induced changes of the absorption band intensities are explained as being due to the field‐induced change of the equivalent even‐parity dipole moment operator. The perturbed absorption spectra obey sum rules suggesting a conservation of the f‐sum rule already within the 3d7 configuration. A pronounced enhancement of the Stark effect through the axial crystal field is discussed.
Stimulated Extraordinary Polaritons in α‐Quartz
Authors: Chapet M. , Laval S.
Published: July 2007

From an experimental study, dispersion curves are plotted of three extraordinary and one ordinary polariton modes in α‐quartz in the range 350 to 500 cm−1. A theoretical formulation of the two‐beam method, experimentally used, makes the possible coupling between E and A2 vibration modes obvious and the simultaneous existence of ordinary and extraordinary polaritons. The experimentally obtained dispersion curves are compared to the corresponding calculated branches. The agreement is quite good.
Effective Electron–Hole Interaction for Intermediate and Strong Electron–Phonon Coupling
Authors: Barentzen H.
Published: July 2007

The interaction of an electron‐hole pair is studied in a polar semiconductor, each particle being coupled to the lattice by the Fröhlich interaction. The theory of Gross and Tulub is used for the description of a single polaron. This theory which gives satisfactory results for both intermediate and strong coupling is then generalized to the problem of two interacting polarons. For not too strong coupling the effective electron‐hole potential will be presented in form of a series with increasing powers of the coupling constant. The first term of this series gives the well known Haken potential, whereas the higher terms are corrections to the potential which will become important for stronger coupling. The results are discussed in connection with exciton screening in the thallous and cuprous halides.
Linear Magnetoresistance in Parabolic Semiconductors
Authors: Arora V. K.
Published: July 2007

A density matrix approach is described to calculate the complete ohmic magnetoresist‐ivity tensor for a non‐degenerate and non‐polar semiconductor with isotropic effective mass. The conventional methods using the Boltzmann transport equation cannot be used in a satisfactory way with magnetic fields of arbitrary strength. The presence of a magnetic field in a crystal introduces some non‐diagonal elements of the velocity operator, which could be averaged properly by using the density matrix. The results obtained agree with those obtained from the Boltzmann transport equation in the limit of low magnetic field including the value of zero.
The H− Ion as a Diffusion Trap for Divalent Ions in KCl
Authors: Castro J. C. , Robert R. , de Souza M.
Published: July 2007

The interaction between divalent ions such as Ca++, Sr++, and Ba+ + and its associate positive ion vacancy with substitutional H− and D− ions is studied. A model is proposed for the trapping of the divalent ion by the H− anions. The strong perturbation on the H−, and D− localized vibration introduced by the divalent ion and its vacancy is reported. The decrease on the barrier height for I.V. dipole relaxation was studied by dielectric loss technique.
Exciton Model of Spin Excitations in Magnetic Dielectrics. Biaxial Antiferrodielectrics with One‐Ion Anisotropy
Authors: Ostrovskii V. S. , Petrov E. G.
Published: July 2007

A generalized exciton model is used to investigate the acoustic and optical spin excitations of magnetic dielectrics in a magnetic field. Equations which determine the equilibrium spin configuration and which are valid for an arbitrary spin magnitude are derived. The frequencies and the polarization properties of the antiferromagnetic resonance (AFMR) for biaxial antiferrodielectrics with one‐ion anisotropy are considered in detail.
X‐Ray Study of PbxSn1−xTe
Authors: Bocchi C. , Ghezzi C.
Published: July 2007

The temperature and the (sin θ)/λ dependences of Bragg and diffuse scattering intensities have been measured in Pb0.63Sn0.37 Te powder samples. The X‐ray Debye temperature has been determined. Low temperature anomalies in the Debye‐Waller factor have been observed and are briefly discussed. An upper limit to the mean‐square static displacement of the atoms is established and possible causes for it are discussed.
The Influence of Low‐Frequency Oscillations on the Storage Time of Ultracold Neutrons The Relation to the Low‐Tebmperature Specific Heat Anomaly Of Disordered Solids
Authors: Ignatovich V. K.
Published: July 2007

The influence of the low‐frequency spectrum of solids on the heating of ultracold neutrons (UCN) per single interaction with the wall is reported. It is shown that the anomalously short storage time in traps can beexplained by UCN heating if there are clusters near the surface of the solid matter, vibrating at a frequency of 1010 Hz. The comparison withthe experimental data of the measurements of the heat capacities of disordered solids carried out at low temperature permits to make the conclusion that the clusters should be of about 70 to 60 Å. The influence on the UCN heating of other possible low‐frequency vibrations inside the solid matter is considered as well.
Vacancy Relaxation and Phonon Band Calculations for Solid Argon
Authors: Guse M. P. , Kunz A. B.
Published: July 2007

The relaxation about a vacancy in solid argon is calculated by approximating the crystal by a cluster of atoms and using the extended Hückel method to calculate the repulsive energy of the cluster while the Van der Waals attractive energy is added explicitly. The nearest neighbor atoms arc found to relax inward by about 0.06% of the original nearest neighbor distance. In addition the effect of a superlattice of vacancies on the phonon band structure of solid argon is calculated using the nearest neighbor Born‐von Kármán model.
Semi‐Empirical Calculations for Stacking Faults in Diamond‐Type Lattices
Authors: Weigel C. , Alexander H. , Corbett J. W.
Published: July 2007

The stacking fault energies, γ, for intrinsic, extrinsic, and twin faults in diamond and silicon are calculated by the extended Hückel theory. For both materials γint > γext > >γtwin. and γtwin ≈︁ γext/2. The number and distance of plane sequences containing boat‐shaped six‐membered atomic rings appear to determine the ordering of the γs. Charge distribution analyses allow intrinsic and extrinsic faults to be interpreted as superpositions of twin faults and suggest that lattice distortions occur in the vicinity of the faults.
The Luminescence Spectra of Bi3+ Ions in MgO and CaO
Authors: Hughes A. E. , Pells G. P.
Published: July 2007

The details of the luminescence spectra of Bi3+ in MgO and CaO have been examined over a wide range of temperature. It is found thatemission from both 3P0 and 3P1 excited states to the 1S0 ground state occurs, the former forbidden transition dominating at low temperatures and the latter at high temperatures. The bands can be resolved from one another, thus permitting the intensities and lifetimes of each transition to be followed. These measurements show that over a substantial temperature range the two excited states are in thermal equilibrium, and the decay characteristics are examined with theaid of a model developed in the Appendix. The radiative lifetimes of the 3PI and 3P0 states are found to be 4 × 10−8 and 2.9 × 10−3 s, respectivelyin MgO, and 2 × 10−9 and 3.8 × 10−3 s in CaO. With the aid of the discovery of the true zero‐phonon line for the 3P0 → 1S0 transition in CaO: Bi at 383.98 nm, the form of the vibronic series for this transitioncan be interpreted. The forbidden transition is made slightly allowed by a combination of a static perturbation and coupling to a vibrational mode which mixes the 3P0 and 3P1 states. This interpretation is consistent with the results of magnetic‐field enhancement of the zero‐phonon line intensity reported by Runciman et al.
Relaxation Time of Phonons Interacting with Charge Carriers in GaAs
Authors: Kravchenko A. F. , Magarill L. I. , Romanov A. A. , Semchukov N. F. , Skok E. M.
Published: July 2007

A theory of transmitted phonon drag effect (TPDE) in a sandwich n‐i‐n‐structure (n is an electron layer and i a semiinsulating one of the semiconductor) is considered. The solution is performed on the basis of a kinetic equation system for electrons and phonons. The calculated results enable a good qualitative description of the main dependences of TPDE in GaAs. An analysis of experimental results shows that the transmitted phonons are dragged by electrons mainly through the deformation potential. The transmitted phonons are represented by long‐wavelength acoustic ones the relaxation of which is described by Herring's theory, the constant of the phonon‐phonon interaction B is equal to 2 × × 10−17 cm deg−3.
The critical voltage effect in transmission electron microscopy. V. The case of non‐centrosymmetric crystals
Authors: Serneels R. , David M. , Gevers R.
Published: July 2007

The critical voltage effect is discussed for the case of noncentrosymmetric crystals (systematic reflections only). An appropriate measure of the noncentrosymmetry is found in terms of the phase angles θg and θ2g of the lowest order extinction distances i.e. Δ = θ2g–2θg. When |Δ| ≦ 3° a critical voltage effect is still expected. For larger |Δ|‐values usually no critical voltage effect will occur. As an application the case is considered of interstitial alloys such as V2H, Nb2H, and Nb2O. For these alloys the critical voltage effect may be used to determine whether the H or O are in octahedral or tetrahedral interstitial positions.
Additional waves and Fabry‐Perot interference of photoexcitons (polaritons) in thin II–VI crystals
Authors: Kiselev V. A. , Razbirin B. S. , Uraltsev I. N.
Published: July 2007

Within the frame work of Pekar's model for a spatially dispersive, bounded medium, optical spectra of thin crystals, reported by us recently, are analyzed, showing a structure due to the interference of additional waves and the Fabry‐Perot structure deviating markedly from the one expected in the classical case in the neighbourhood of exciton resonances. A comparison of the experimental reflectivity with that obtained numerically provides values for all physical parameters defining dispersion and damping of the photoexcitons. Some additional experimental results for excitons in thin crystals are reported: evidence for the induced surface wave, an effect of the forbidden exciton, etc.
Strain‐modulated electron spin resonance of MgO:Fe2+
Authors: Henning J. C. M. , den Boef J. H.
Published: July 2007

Flexural‐strain‐modulated ESR spectra of Fe2+ in MgO are reported. No first‐harmonic SMESR signal is detected. The angular dependence of the second‐harmonic signal is consistent with modulation of the tensor D by the periodic strain field. The magnitude of the effect is calibrated against the known width of the spin packets. The value of G11 = 680 cm−1 (±25%), deduced from this experiment, agrees well with literature data.
On the influence of a smooth gaussian random field on optical thresholds in disordered semiconductors and external electric field induced change
Authors: Esser B. , Kleinert P.
Published: July 2007

The interband dielectric function ϵ(ω) and its external field induced change Δϵ(ω, E) are calculated in the one‐electron approximation for optical transitions between initial and final states shifted randomly in space by a smooth Gaussian random field. All types of critical points are considered. Using an exact semi‐classical approach up to the order of h2 the influence of the smooth Gaussian random field in ϵ(ω) and Δϵ(ω, E) is shown to be representable as an averaging of the corresponding Franz‐Keldysh expressions. The M3 and M2 problems are shown to be reducible to the M0 and M1 ones. The plots for ϵ(ω) and Δϵ(ω. E) are presented for the isotropic M0 point and the M1 point with equal transversal masses.
Soft mode in thiourea by Raman measurements
Authors: Delahaigue A. , Khelifa B. , Jouve P.
Published: July 2007

Raman measurements have been carried out on the soft mode of thiourea and the wave vector and temperature dependence of the soft mode frequency (ωS) are observed. The longitudinal (ωSL) and transverse (ωST) frequencies are used to derive a comparison between the dielectric constant obtained from the LST relation and that obtained from standard measurements of the capacitance.
Band structure and density of states in SiP2
Authors: Farberovich O. V. , Domashevskaya E. P.
Published: July 2007

The band structure of SiP2 is determined by the orthogonalized plane wave method. This compound is found to be a semimetal. The density of states of the valence electrons n(ϵ) is calculated by the linear interpolation method. The function n(ϵ) is compared with the experimental X‐ray emission L‐ and K‐bands of silicon and phosphorus in SiP2.
UV optical properties of lithium formate
Authors: Baldini G. , Cottini M. , Doglia S. , Grilli E.
Published: July 2007

The optical properties of single crystals of lithium formate monohydrate have been studied between 5 and 10 eV by means of absorption and reflection techniques. The data, which show two peaks at 7.2 and 8.4 eV, are interpreted in terms of n → σ and π → π transitions whose energy, strength, and polarization are consistent with the electronic properties of HCOOLi and H2O molecules.
Anomalies of sound propagation near orientational phase transitions in antiferromagnets
Authors: Mitsek A. I. , Kolmakova N. P. , Sirota D. I.
Published: July 2007

The sound velocity and attenuation anomalies, connected with the orientational phase transitions (OPT) of first and second order, are investigated for antiferromagnets and ferromagnets. For the first order OPT the anomalies are determined by the motion of the domain walls of the transitional domain structure, while for the second order one they are determined by the interaction of the sound wave and nonequilibrium deviations of the magnetization. The consideration of the influence of the magnetic subsystem on the sound propagation is reduced to the renormalization of the elastic constants. The wave equation and general expressions for the renormalization of the elastic constants, sound velocity, and attenuation are received, taking the magnetostriction and piezomagnetism into account. The comparison of the results received and available experimental data reveals the agreement of the theory and experiments.
Molecular Cluster Approach to Magnesium and Calcium Oxide Crystals. II. F+ and F Centres
Authors: Ermoshkin A. N. , Evarestov R. A. , Kotomin E. A.
Published: July 2007

The model of the molecular duster (MC) which has been applied for perfect MgO and CaO crystals is applied now to F+ and F centres in these crystals. A method is proposed to estimate the optical transition energies within the SCCC technique. The calculated values of the absorption maxima of the F+ and F centres are in reasonable agreement with experimental data. The procedure is also suggested to estimate the location of the defect levels within the forbidden gap of the perfect crystal. For comparison the analogous calculations of the ionic KCI crystal (perfect and with F centre) are given.
Modulation Spectroscopy on MoS2, and MoSe2
Authors: Meinhold H. , Weiser G.
Published: July 2007

On MoS2 and MoSe2 crystals electroreflectance, piezoreflectance, thermoreflectance, and wavelength modulated reflectance is measured. From the evaluation of the spectra and from the comparison with bandstructure calculations it is most likely that the first direct transition is at point P of the Brillouin‐zone between bands formed by the non‐bonding states of the Mo atom. The four excitons near the fundamental edge correspond to this transition split by spin‐orbit coupling and by interlayer interaction. From electroreflectance above the absorption edge it is concluded that the valence bands of MoS2 correlated to the covalent bonds are less than 700 meV below the first occupied bands.
The Relationship between Correlation Factor and Isotope Effect of Diffusion in Crystals
Authors: Mehrer H. , Seeger A. , Steiner E.
Published: July 2007

For crystals of orthorhombic or higher symmetry the sufficient conditions for the equality of correlation factor and isotope effect of tracer self‐ or impurity diffusion are critically discussed. For the most important one of these conditions, which concerns the dependence of the correlation factor on the tracer jump‐frequencies, necessary and sufficient formulations are derived covering the general case of more than one tracer jump frequency. This leads to a natural classification of diffusion mechanisms according to whether the correlation factor contains one or more tracer jump frequencies and whether the necessary and sufficient condition is fulfilled or not. The available calculations of correlation factors are assigned to the four cases resulting from this classification.
A Self‐Consistent Cluster Approximation for the Disordered Hubbard Model
Authors: Löser W.
Published: July 2007

The influence of disorder on electronic systems with intra‐atomic electron correlation is considered within a self‐consistent cluster approach. The disorder is represented by random site energies and hopping integrals in the Hubbard Hamiltonian. Configurational averaged properties of these disordered systems like reservoir coupling, local magnetic moments, and free energy per site are calculated for zero temperature and related to those in the ordered case.
Impurity‐Induced Raman Spectra of Potassium Halides
Authors: Siederdissen J. höner Zu
Published: July 2007

Impurity‐induced Raman scattering spectra of monovalent (Cs+, Rb+, Tl+) and divalent (Pb++, Sr++, Ca++) centres in potassium halides are presented. In the caseof the divalent impurities different spectra are found for associations of one divalent impurity plus one vacancy and aggregates of such associations. Contrary to the monovalent defects these impurities cause a strong A1g spectrum. The Raman scattering cross sections of the Tl+ and Pb++ impurities are much stronger than those of the other defects (Cs+, Rb+, Sr++, Ca++).
Lattice Dynamics of ZnTe and CdTe
Authors: Plumelle P. , Vandevyver M.
Published: July 2007

The model used is a rigid‐ion model with an effective ionic charge including general interactions for nearest and next nearest neighbours and long range Coulomb interactions. It provides a good fit with recent neutron data for ZnTe and CdTe on condition that no hypothesis is made a priori on the interatomic forces and that the eleven parameters given by the model are used.
An Investigation of the Electronic Structure of GaSe and GaTe by Photoelectron Spectroscopy, Using a Synchrotron Source, and Electron Energy Loss Spectroscopy
Authors: Williams R. H. , McGovern I. T. , Murray R. B. , Howells M.
Published: July 2007

The valence band structure as well as the location of the Ga 3d levels have been investigated for vacuum cleaved GaSe and GaTe single crystals by photoelectron spectroscopy using a synchrotron source. In addition the conduction band structure has been probed by photoemission partial yield spectroscopy and by electron energy, loss spectroscopy. The experimental results are in reasonable agreement with theoretical predictions although calculated band widths are generally too narrow. A feature in the partial yield and electron loss spectra is tentatively interpreted in terms of surface states.
Anharmonic Effects in Heat Capacity and Saturation Vapour Pressure of Dilute Substitutional Alloys
Authors: Masharov S. I.
Published: July 2007

The contribution to the heat capacity and equilibrium pressure of the vapour over crystals due to the anharmonicity of atomic vibrations is calculated. It is shown that the anharmonicity changes the temperature dependence of the saturation pressure. This change is simply connected with the anharmonic addition to the heat capacity. In the high temperature region the vapour density over the crystal is increased several times due to the anharmonism. The corresponding temperature addition may be about 10%. The temperature dependences of the anharmonic contribution to the heat capacity at high and low temperatures are found. In the high‐temperature region the corresponding addition is linear in temperature. At low temperatures its behaviour is more complicated, especially in crystals containing heavy substitutional impurities an anomalous growth may be observed.
On the Theory of Tunnelling in Disordered Semiconductors
Authors: Belova I. M.
Published: July 2007

Tunnelling is considered in the metal‐insulator‐disordered semiconductor structure. Current carriers are supposed to tunnel from the metal to discrete deep levels in the gap of the disordered semiconductor and vice versa. The expression for the tunnel‐current density is obtained taking into account electron‐phonon interaction in the semiconductor. The conditions are investigated for the determination the density of states in disordered semiconductors from the tunnel‐current density.
In+vv Centres in a KCl‐In Crystal
Authors: Nagli L.
Published: July 2007

A new emission centre (El = 2.43 eV) has been found in a KCl‐In crystal. It is produced by ionizing radiation at 350 K or after thermal destruction of In2+vc− centres. It is assumed that the centre in question is an In+ centre asociated with a bivacancy vv; (In+vv. The spectral, polarization, and kinetic characteristics of this centre are studied.
Molecular Cluster Approach to Magnesium and Calcium Oxide Crystals III. Charge Distribution Analysis of Some Hole Centres
Authors: Evarestov R. A. , Ermoshkm A. N. , Kotomin E. A.
Published: July 2007

The molecular cluster model is applied to the analysis of charge distribution and of symmetry of some hole centres (V−, V0, Li0, Na0), and also to impurity atoms of aluminium and to nitrogen ions in MgO. The calculations yield theoretical evidence of the models of hole centres derived from experimental data. Besides the essential effect of charge redistribution in the vicinity of impurity it is shown, e.g., the aluminium ion may be considered as triply charged only accounting for the charge of its nearest environment.
The Band Edge Excitons in 2HMoS2
Authors: Neville R. A. , Evans B. L.
Published: July 2007

The optical constantsof synthetic 2HMoS2 have been determined over the range 0.05 to 3.75 eV. The absorption edge consists of two series of exciton bands due to direct allowed transitions. The Zeeman splitting and diamagnetic shift of these exciton lines are measured together with the effects of crystal confinement and temperature; the derived exciton parameters are different from those of molybdenite. Oscillatory magneto‐absorption occurs at the high energy side of each exciton series which gives a measure of the carrier masses.
On the Electric Field Dependence of the Conductivity of Holes in Si at Low Temperatures 1. The Drift Velocity under Consideration of the Non‐Equipartition of Acoustic Phonon Energy
Authors: Von Borzeszkowski J.
Published: July 2007

The field dependence of the drift pelocity of holes in Si is discussed within a non‐parabolic valence band model. Taking into consideration the non‐equipartition of acoustic phonon energy both in a field‐dependent distribution function and in the relaxation time of holes, a satisfactory description of the experimental drift velocity can be given at low temperatures in a wide field range.
Fermi Surface and Helicons in Semiconducting Bi1−xSbx Alloys
Authors: Oelgart G. , Herrmann R. , Krüger H. , Stegmann R.
Published: July 2007

In semiconducting n‐and p‐type Bi1−xSbx alloys, the propagation of helicons is observed at 35.5 GHz and helium temperatures. Besides the propagation of helicons in the asymptotic limit, helicons are detected also under the influence of time dispersion. Under the influence of time dispersion, the anisotropy of the isoenergy surface of the charge carriers substantially influences the dispersion of helicons. On the basis of this effect we are able to gain information on the Fermi surface and to confirm results obtained from previous cyclotron resonance measurements. The influence of the anisotropy of the isoenergy surface is shown for magnetic fields parallel to special directions of the crystal lattice. This influence can, however, also be found in the anisotropy of the field values of the spatial resonances of the helicon waves.
Scattering of Light by Magnetoelastic Excitations in Ferromagnets
Authors: De Almeida J. R. L. , Rezende S. M.
Published: July 2007

Inelastic scattering of light by acoustic magnetoelastic excitations in ferromagnets is studied theoretically. The scattering cross‐section is obtained using the Green's function formalism. The radiationis considered to interact with the system both through the elastooptical and the magneto‐optical couplings. The contributions of the various mechanisms are analyzed in detail and estimates for cases of experimental interest are presented.
Annihilation of Incoherent Excitons in Molecular Crystals with Allowance for Their Interaction
Authors: Onipko A. I.
Published: July 2007

The dependence of the exciton annihilation rate on the dynamic interaction between excitons in molecular crystals is studied. The equation of motion for the two‐particle density matrix of incoherent excitons is used to obtain an expression for the annihilation rate constant in one‐, two‐, and three‐dimensional crystals. It is shown that the interaction between excitons may considerably affect the annihilation rate and its temperature dependence, and that the contribution of exciton‐exciton interaction to the annihilation process depends on the dimensionality of the system in which the excitons move.
Optical Properties of a Polydiacetylene Single Crystal Evidence for van Hove Singularities
Authors: Reimer B. , Baessler H. , Hesse J. , Weiser G.
Published: July 2007

Reflection spectra of polydiacetylene‐bis(toluene‐sulphonate) single crystals have been measured in the temperature range between 9 K and 300 K. By means of Kramers‐Kronig analysis the dielectric constants ε1 and ε2 and the absorption constant a have been derived. As they display van Hove singularities it is concluded that the optical absorption results from transitions between delocalized one‐dimensional states of the polymer chain forming broad bands. All absorption bands are split into doublets, the splitting energy decreases from 32 meV at 9 K to less than 10 meV at 300 K. Splitting is interpreted in terms of a threedimensional band model and finite intermolecular interaction.
Anisotropic Size Effect in Thin Aluminium Films
Authors: Sato H. , Yonemitsu K.
Published: July 2007

A practical calculation of the anisotropic size effect of aluminum is made on the basis of the realistic 4‐OPW Fermi surface and the anisotropic relaxation time model. Measure‐ ments are done for three crystallographic orientations. The theoretical prediction is in good agreement with the experimental data.
The Properties of a Threefold Degenerate Orbital Magnetic Impurity in a Feromagnetic Crystal
Authors: Ivanov M. A. , Mitrobanov V. Ya. , Falkovskaya L. D. , Fishman A. Ya.
Published: July 2007

The spectrum of localized excitations of a magnetic matrix with impurity Jahn‐Teller magnetic ions with a triplet orbital ground state in a cubic crystal field is investigated. The two cases are examined of the spin‐orbit interaction of the impurity to be much less or much greater than the exchange interaction between the impurity and the matrix. The orbital exchange interaction is shown to lead to displacement and splitting of the pure spin excitation frequencies and under certain correlations between the exchange parameters it can change the ground state of the system. The specific features of the orbital degeneracy of the impurities can also occur in the electromagnetic wave absorption spectrum and in the temperature dependence of the thermodynamic characteristics of the impure crystal.