bluuebunny/crossref_metadata_2025 huggingface dataset:
Domain structure and transverse conductivity of a many‐valley semiconductor in the multivalued sasaki effect
Authors: Gribnikov Z. S. , Mitin V. V.
Published: July 2007

The transverse conductivity (TC) of a semiconductor is studied in the range of the multivalued Sasaki effect in a heating electrical field. The negative TC due to the Erlbach effect does not arise in this field. The TC depends on the surface conditions of the sample and on the domain structure arising in these fields. The case of a small TC caused by a displacement of a domain wall under the influence of a transverse current is possible together with the case of a large positive differential TC. Such conductivity can be both positive and negative depending on the direction of the wall displacement. The case when the driving field and the transverse current are homogeneoils has been studied.
Properties of a 5T2 in iron(II) under the action of arbitrary‐symmetry ligand fields and spin‐orbit coupling: II. On the electric field gradient and susceptibility tensor
Authors: Zimmermann R.
Published: July 2007

The susceptibility tensor χ and the EFG have been treated for a 5T2, ground state of highspin iron(II) under the action of an arbitrary symmetry ligand field and spin‐orbit coupling λL·S. It is shown that the combined evaluation of χ‐tensor and EFG can be split into two nearly independent parts, in one for the determination of the orbital eigenstates from the relative orientation of the χ‐tensor and EFG and in another for the determination of the energy splittings of the orbital states, spin‐orbit coupling constant, and covalency. The determination of the orbital states does not involve the diagonalization of the 15 × 15 matrix of the Hamiltonian of the ligand field and λL·S. The second part is simplified by the fact that all calculations can be done as if the orbital states were those of D2 symmetry, i.e. |yz〉, |xz〉, |xy〉. Furthermore it is discussed that the rhombic symmetries D2 and D′2 and the twofold symmetries C2, and C′2 (e.g. trans and cis configuration) can be differentiated from χ‐tensor and EFG data.
Phonon‐induced hyperfine interactions for 6S‐state ions in tetrahedral symmetry: Mn2+ in ZnS and ZnSe
Authors: Koch B.‐P
Published: July 2007

On the basis of a point‐charge model according to the theory of Šimánek and Orbach equations are derived for the temperature dependence of the hyperfine coupling constant of 6S‐state ions in tetrahedral symmetry without the approximation of small wave vectors for lattice phonons. Acoustical and optical phonons of the entire phonon spectrum are taken into consideration. The temperature functions calculated are compared with the measured values for ZnS:Mn2+ and ZnSe:Mn2+. The curves obtained very well correspond to the temperature dependence measured if always one parameter is taken from the experiment. The calculations, however, supply parameters which deviate by more than one order of magnitude from the experimental ones. The reasons for these deviations are particularly involved in the applied point‐charge model.
Complementary variational principles for lattice thermal conductivity
Authors: Srivastava G. P.
Published: July 2007

A method of complementary variational principles for the lattice thermal conductivity is presented. This two sided variational method gives sequences of upper and lower bounds on the exact thermal conductivity coefficient. The sequence of lower bounds can be recognised as the one presented by Benin. The sequence of upper bounds is a subsequence of an infinite sequence, with only odd number of terms present. One finds that the method of complementary variational principles is a very successful method of estimating the exact coefficient: it gives a controlled means of putting the estimated results into a band of desired accuracy around the exact transport coefficient.
Antiferromagnetism in not exactly half‐filled narrow energy bands (hubbard model)
Authors: Nolting W.
Published: July 2007

It is investigated if the ground state of a system of interacting electrons in a narrow energy band can be of antiferromagnetic order within the limits of the Hubbard model. By means of the “method of spectral moments” an attempt is made to find antiferromagnetic solutions for all values of the band occupation, which then are checked for stability toward para‐ and ferromagnetism. The numerical evaluation for the b.c.c. lattice yields that antiferromagnetism is stable toward paramagnetism in a small region around the half‐filled band, but is unstable against ferromagnetism. Special attention is paid to the determination of a spin dependent band shift, which turns out to be a decisive quantity for the possibility of antiferromagnetic ordering.
Many‐phonon vibronic spectra of molecular crystals: I. The role of anharrnonicity of intramolecular vibrations
Authors: Lalov I. J.
Published: July 2007

Many‐phonon vibronic spectra are investigated in molecular crystals in the weak coupling approximation. A theory is developed (using adiabatic approximation) in which the anharmonicity of the intramolecular vibrations is treated as an exciton‐phonon coupling mechanism. This theory is applied to vibronic spectra with slowly migrating phonons of nontotally symmetrical vibrations. Anharmonicity and the change of frequency have approximately the same impact on the intramolecular interaction and on the intensity of vibronic spectra, but due to its smaller constant, the anharmonirity is less essential to the appearance of one‐particle states. Anharmonicity widens the bands of the many particle states by several ten per cent.
Luminescence by exciton‐exciton collision in GaSe
Authors: Catalano I. M. , Cingolani A. , Ferrara M. , Minafra A.
Published: July 2007

The photoluminescence of GaSe at 80 °K at high excitation with one‐ and two‐photon pumping is reported. The investigated energy range includes the emission localized between the indirect and the direct edges. Only the free exciton emission is observed at low pumping intensities. As the pumping increases, an extra band (L band) is observed, 20 meV below the free exciton. This band shows stimulation effects and may be ascribed to an exciton‐exciton collision process. Photocurrent measurements, resonantly excited at the excitonic wavelength, are in agreement with this conclusion.
Dielectric response theory of phonon‐ and plasmon‐assisted cyclotron resonance
Authors: Chiu K. W. , Wallace P. R.
Published: July 2007

The dynamic dielectric response of a polar semiconductor is calculated, taking account of the interaction of the electrons with the longitudinal optical phonons and plasmons, which themselves may in turn be coupled. This permits the calculation of the optical absorption at the “mixed” cyclotron resonances in which the absorption or emission of a boson accompanies a Landau transition. Because of relatively weak coupling, only one‐boson processes need be considered; a canonical transformation eliminating the electron‐boson interaction to first order then gives the required response. The conditions under which two mixed resonances of comparable intensity may be observed are discussed.
Magnetic resonance and related studies in cerium‐tin alloy
Authors: Malik S. K. , Vijayaraghavan R. , Garg S. K. , Ripmeester R. J.
Published: July 2007

The results of nuclear magnetic resonance (NMR) and magnetic susceptibility measurements in CeSn3 are presented together with other available data. The magnetic susceptibility of CeSn3 shows a broad maximum around 135 K and a rapid rise below 30 K. The 119Sn Knight shift in CeSn3 in the temperature region 1.2 t o 300 K is not proportional to the bulk susceptibility. The electrical resistivity of CeSn3 shows a T2 term at low temperatures as expected in localised spin fluctuation systems, and the coefficient of the electronic specific heat is also very large. These results suggest sizeable 4f occupation near the Fermi level in CeSn3.
Tridimensional character of GaSe excitons in the Faraday effect
Authors: Bianchi D. , Emiliani U. , Podini Y.
Published: July 2007

Faraday measurements on GaSe were performed :with magnetic field of intermediate intensity (≈︁ 50 kG). Four exciton lines were clearly resolved. A simple analytical model was used to fit the experimental data successfully, obtaining the values of some relevant parameters such as the exciton resonance energies, the damping factor, and the relative oscillator strength. An analysis of the data with the Wheeler and Dimmock model for anisotropic exciton gives α ≈︁ 1, for the anisotropic parameter, and μ‖/μ⊥ = 1.3 for the reduced exciton masses ratio. These data show that in GaSe crystals excitons are isotropic. This agrees with recent experimental results supporting the tridimensional character of the bands responsible for excitons. A new and extremely interesting problem now arises: the different behaviour of GaSe, tridimensional in respect to optical and transport phenomena, bidimensional in respect to mechanical and crystallographic properties.
The free carrier magneto‐microwave effects in p‐type germanium
Authors: Mehra R. M. , Srivastava G. P.
Published: July 2007

The free carrier magneto‐microwave effects, namely, the Kerr effect, the Faraday rotation, and the magneto‐absorption, have been studied in detail in p‐type Ge in Faraday configuration. The experimental analysis applies to guided waves and treats multiple reflections exactly. The theoretical analysis takes account of the complex valence band structure and the contributions from the light and heavy holes, separately. The mixed scattering due to ionized impurities and lattice vibrations has been taken into consideration. The values of the ionized impurity scattering parameter and relaxation time obtained from Kerr effect measurements compare well with the values obtained by Faraday rotation measurements. Magneto‐absorption measurements have been done on the samples used for Faraday rotation measurements and the experimental results are compared with theoretical results using the transport parameters obtained from Faraday rotation measurements. Good agreement has been found between the two. The results of the above three types of measurements indicate the presence of larger impurity scattering and smaller relaxation time for light holes as compared with the heavy holes. The values of the relaxation times which has been estimated at 77 K (by Kerr effect measurements) compare well with the values obtained by earlier workers from cyclotron resonance and magneto‐plasma studies.
Propagation of stimulated polaritons and infrared emission in uniaxial damped crystals − observation in LiNbO3
Authors: Paraire N. , Koster A. , Biraud‐Laval S. , Reinisch R.
Published: July 2007

Propagation of stimulated polaritons generated in a damped uniaxial crystal is studied using Maxwell's formalism without any approximation relative to the wave transversality. The polarization of these polaritons is briefly discussed. The transmitted amplitude of the driven beam is calculated and compared to experiments performed in LiNbO2 in the range 10 to 20 cm−1. The observed position of the maximum intensity of the infrared radiation is in good agreement with previous calculations.
The influence of mechanical stress on the structural phase transitions in V3Si and Nb3Sn
Authors: Pietrass B.
Published: July 2007

The influence of external mechanical stress on the lattice transformations in V3Si and Nb3Sn is investigated on the basis of Landau's theory of phase transitions. The calculated phase diagrams for uniaxial [001] and [110] stress are found to have a critical and a tricritical point. Discrepancies are indicated between theoretical results and experimental observations of the effect of an uniaxial [001] pressure in V3Si.
Hopping photoconductivity in amorphous germanium
Authors: Fischer R. , Vornholz D.
Published: July 2007

The photoconductivity of amorphous germanium is measured as a function of temperature, annealing temperature, and intensity of the incident light. The photocurrent depends only weakly on sample temperature and annealing state, in contrast to the behaviour of the dark current. The dependence of the photocurrent on light. intensity is linear to sublinear. The results are explained by hopping photoconduction of excess carriers around the Fermi level. Results of model calculations are in good qualitative agreement with the experiments.
Linear electroabsorption due to substitutional deep impurities in wurtzite‐type crystals: II. Experimental study of Co2+ in CdS and CdSe
Authors: Gardavský J. , Boyn R.
Published: July 2007

Measurements are made of linear electroabsorption spectra in the range of the 4A2 → 4T1(4F) and 4A2 → 4T1(4P) absorption bands of substitutional Co2+ in CdS and CdSe crystals for electric fields parallel and perpendicular to the hexagonal c‐axis, at 77 and 300 K. The observed dependences of these spectra on field and polarization direction can be completely explained on the basis of the theoretical treatment given in the first part of this work. A moment analysis is performed for CdS: Co2+, and this leads to values for the linear Stark splittings and field‐induced changes in transition probability. The axial crystalfield splittings of the T, excited levels of CdS: Co2+ are also determined, and their similarity to the external field splittings for F ‖ c is emphasized.
Functional integral approach for an extended hubbard model. A graph scheme treatment
Authors: Röpke G. , Albani B. , Schiller W.
Published: July 2007

For the extended Hubbard model, generalized susceptibilities are expressed by functional integrals over fictitious stochastic fields. It is shown, that in the static approximation most of these fields can be eliminated. A diagram technique is developed to evaluate the susceptibilities. Corrections to the molecular field approximation are obtained. Phase transitions from the paramagnetic into long‐range‐ordered phases (magnetically or charge‐ordered) are considered for the atomic limit and special parameter values of the Coulomb interaction terms.
EPR investigations on the hole centres in CaWO4 at T = 4.2 K
Authors: Biederbick R. , Born G. , Hofstaetter A. , Scharmann A.
Published: July 2007

EPR investigations at T = 4.2 K show the principal g‐values of the radiation induced hole centres in CaWO4 being different from those obtained at temperatures ≧ 78 K, whereas the principal directions of the g‐tensor remain nearly unchanged. The g‐tensor at T = 4.2 K is quite well explained by a model in which the hole is assumed in an oxygen Pπ‐orbital, including some admixtures from other orbitals of the WO4 tetrahedron. In this model, combined with a weak bonding between two neighboured WO4 tetrahedra, the observed hyperfine interaction of the hole with two inequivalent W183 nuclei can be understood too. The transition to the results at higher temperatures is possible by a faster exchange of the hole between the two WO4 complexes.
Theory of disordered systems. IV. Coulomb interaction and screening
Authors: Fischbeck H. J.
Published: July 2007

The formal theory of interacting disordered systems is developed. Its main characteristic is that an external potential to which the system responds is coupled not only to the electronic but also to the disordered ionic charge density. This allows to describe the screening of individual potentials and potential fluctuations by the response properties of the averaged system. Electron–electron and electron–ion correlations constitute an effective screened interaction function from which the „screened”︁ single‐site approximation is derived.
Valence energy bands of solid hydrogen in H.C.P. structure. Ab initio LCMO calculation
Authors: Iacovo V. Dello , Resca L. , Resta R.
Published: July 2007

An ab initio LCMO calculation of the valence bands of solid ortho‐hydrogen in h.c.p. structure is reported. Firstly the molecular orientations are considered assuming the space group Pca21. Then in a spherical approximation the whole space group P63/mmc of the h.c.p. structure is considered. These two cases are related to each other on the basis of symmetry properties.
S− and Se− centers in the cuprous halides
Authors: Goltzene A. , Meyer B. , Schwab C. , Cho K.
Published: July 2007

Infrared absorption and electron spin resonance have been performed on chalcogen‐doped
copper halides. They suggest a substitutional S− or Se− ion at an anionic site of the lattice. A theoretical model, in terms of a dynamic Jahn‐Teller effect on a Γ5 triplet of the (chalco‐gen)‐ ion is given, which is in fair agreement with the experimental results.
Selfconsistent single‐site approximation for structurally disordered solids
Authors: Kramer B.
Published: July 2007

The self‐energy of an electron in a structurally disordered solid is calculated selfconsistently in the single site approximation. Short range order is taken into account by an atom distribution function with sharp peaks. For smooth atom potential form factors a nonlinear system of equations for the self‐energy is derived, which can be solved numerically.
The influence of band degeneracy and hybridization on correlations in narrow bands
Authors: Capellmann H.
Published: July 2007

Starting from a Hubbard Hamiltonian for two bands, including exchange and hybridization, a formal theory of correlations is developed. The possibility of different order parameters is pointed out: magnetic, excitonic, and mixed order parameters. A specific model is discussed and the renormalized band structures of three representative states are calculated: The magnetic insulator, the excitonic insulator, and the correlated metal. A comparison of the energies of these states in given.
Surface phonon‐polaritons on general crystal cuts of α‐quartz observed by attenuated total reflection
Authors: Schuller E. , Borstel G. , Falge H. J.
Published: July 2007

The theory of surface modes on general cuts of anisotropic crystals yields excitations of mixed polarization. This fact results in the existence of additional dispersion branches and pseudosurface modes. The dispersion of the pseudosurface phonon‐polaritons may be determined by means of the minima of the calculated reflectivity response. The theoretical results have been examined by attenuated total reflection and show a very good agreement with the experiment.
Optical absorption of defects in V2O5 single crystals: V2O5 doped with Ti, Mo, and Cu
Authors: Clauws P. , Vennik J.
Published: July 2007

The absorption spectra in the range from 0.3 to 2.1 eV of V2O5 single crystals doped with Ti, Mo, and Cu are presented. Besides of bands due to impurities, an enhanced absorption from oxygen vacancies is observed in Ti‐doped samples. Models of the various defect centres involved are discussed and it is shown that the absorption bands can be attributed to centres involving V4+ and an impurity. The model of Ballhausen and Gray for vanadyl complexes is used as a working model for the defects. Two weak bands are assigned to ligand field transitions of V4+. The more intense bands are assigned to charge transfer transitions into the orbitals of the neighbouring vanadium ions or the impurities.
Calculation of the electrical conductivity of disordered binary alloys with off‐diagonal randomness
Authors: Eifrig Th. , Kolley E. , Kolley W.
Published: July 2007

The dc conductivity of a disordered binary alloy with off‐diagonal disorder is calculated. On condition that the transfer integrals depend additively on the configuration the application of a modified CPA is possible. In this approximation the vertex corrections vanish. The numerical calculation shows a strong dependence of the conductivity on the occupied fraction of the band.
Phonon dispersion in sodium and potassium
Authors: Goel C. M. , Pandey B. P. , Dayal B.
Published: July 2007

The modified Sharma‐Joshi model has been employed to calculate the phonon dispersion relations in the b.c.c. metals sodium and potassium. The theoretical results thus obtained show an excellent agreement with available experimental data.
The influence of polariton motion on the polariton Raman scattering spectra
Authors: Strizhevskii V. L. , Yashkir Yu. N. , Ponath H.‐E.
Published: July 2007

The influence of the relative group velocity finiteness of the scattering polariton on the polariton Raman scattering (RS) intensities is discussed. It is proved that this effect is not very essential only in the region of polariton frequencies ωp near the frequency ωf of the corresponding polar mechanical phonon because here the relative group velocity ξp of the polariton is small. But the influence of the group velocity ξp finiteness appears to be essential in the case of sufficiently distant frequencies ωp from ωf. This influence results not only in essential quantitative changes but also in qualitative changes, in particular, of the angular dependence of the polariton RS intensity. It is manifested particularily in the vicinity of the „critical”︁ scattering angles where the neglection of this effect distorts absolutely the character of the above‐mentioned dependence. The ξp‐finiteness influence is illustrated by numerical calculations for a number of real crystals (ZnO, ZnSe, GaP, CdS, LiIO3). Experimental investigations are made of the RS‐intensity angular dependence in the region of the upper polariton branch in LiIO3 crystals in the vicinity of the critical scattering angle. The results confirm the conclusion of the theory about the essential influence of the effect mentioned above on the polariton RS intensity.
Spin–lattice relaxation of P donors in Si for different concentrations of majority and compensating impurities
Authors: Gershenzon E. M. , Semenov I. T. , Fogelson M. S. , Chambootkin D. P.
Published: July 2007

The influence of compensating impurities on the spin‐lattice relaxation of P donors in Si was studied for the concentration range ND = 8.5 × 1016 to 1.3 × 1018 cm−3 at temperatures 1.7 to 15 °K in the X‐band. For the concentrations ND = 8.5 × 1016 to 1 × 1017 cm−3 the compensation accelerates the relaxation rate Ts−1. Experimental results are in good agreement with Sugihara's theory. For ND ≧ 3 × 1017 cm−3 the increase of the compensation ratio causes a decrease of Ts−1 and increases its dependence on temperature. The latter is explained by weakening of the exchange interaction and retardation of the relaxation by exchange system.
The electron–electron interaction in gapless semiconductors
Authors: Gelmont B. L. , Ivanov‐Omskii V. I. , Ogorodnikov V. K.
Published: July 2007

The effect of the Coulomb interaction on the dielectric function, the mobility, and the energy spectrum of gapless semiconductors is analyzed. It is shown that the hole energy is strongly renormalized due to the nonlocality of the potential. The resulting shape of the valence band and its dependence on the electron concentration is considered. The nonlocality of the lattice potential diminishes the ground state energy of the acceptor.
The Magnetic Properties and the Metal‐Insulator Transitions in V2O3
Authors: Capellmann H.
Published: July 2007

Based on a formal theory of correlations in degenerate bands a qualitative description of the physical nature of the transitions in V2O3 is given. It is argued that the low temperature insulating phase is a magnetic insulator, the intermediate metallic phase is described by the correlated metal, and the high temperature insulating phase is an excitonic insulator. The driving force for the transitions is the interband mixing term contained explicitly in the Hamiltonian.
The Effect of Long‐Range Disorder on the Optical Spectra of Tetrahedrally Bonded Semiconductors
Authors: Maschke K. , Overhof H. , Thomas P.
Published: July 2007

A comparison is made of the configurational averaged spectral function for amorphous ZnS obtained from a pseudopotential (CPSP) and a muffin‐tin (CKKR) approach using the same scattering correlation function. This function leads to a relaxation only of the long‐ range order in the CKKR scheme whereas for the CPSP scheme short‐range potential fluctuations are also introduced. As a result, the CKKR conduction band states are affected equally by disorder. Only the pseudopotential states exhibit selective changes which are essential in order to obtain agreement with experimental spectra. It is, therefore, concluded that short‐range potential fluctuations are responsible for the difference in ϵ2 of amorphous and crystalline semiconductors.
Ground State Energy of the Truncated Anderson Hamiltonian
Authors: Rudra P.
Published: July 2007

The ground state energy of the Anderson Hamiltonian for infinite dd correlation energy is calculated using the method of thermodynamic Green's functions. The quantum mechanical ground state energy has a term proportional to exp (−α · ed/rv2) for rv2 ⪅ 1.5 where ed, r, v denote the energy of the d‐electron, the density of state and strength of interaction between the d‐electron and the conduction band electrons, all expressed in dimensionless units. The value of α remains almost constant over a wide range of the physical parameter rv2.
Recoilless Fraction of Iodine Ion in RbI, NaI, and KI by the Shell Model
Authors: Sneh  , Dayal B.
Published: July 2007

Recoilless fraction f(T) of iodine ion in RbI, NaI, and KI has been calculated at different temperatures using the simple shell model. The temperature dependence of the shell model parameters is taken into account by a method discussed by Pandey and Dayal in their study of thermal expansion of ionic crystals. The calculated values of f(T) are compared with available experimental results and good agreement is obtained.
Exciton Polaritons in HgI2 Crystals
Authors: Akopyan I. , Novikov B. , Permogorov S. , Selkin A. , Travnikov V.
Published: July 2007

At low temperatures a detailed study of the emission and reflection spectra of HgI2 crystals in the region of the ground state n = 1(Γ−Γ) exciton resonance has been carried out. It is found that the shape of the resonant exciton emission spectra in allowed and forbidden polarizations E ⊥ cand E ‖, and the relative positions of the emission and reflection spectra are similar to that in many direct gap semiconductors with the high oscillator strength of n = 1 exciton transition. The emission and reflection spectra of the HgI2 crystals are correlated with the energy structure of the exciton resonance using the polariton model. The interrelation between the resonant exciton emission and the exciton absorption is discussed. The polariton dispersion curves in the region of the n =1 exciton resonance in HgI2 crystals are calculated on the basis of experimental data.
Biexcitons in Indirect‐Gap Semiconductors: Applications to GaSe and AgBr
Authors: Quattropani A. , Forney J. J. , Bassani F.
Published: July 2007

The effective mass approximation for biexciton states is extended to semiconductors with indirect gap. An equation similar to that valid for direct‐gap semiconductors is obtained for the envelope function. The electron‐hole exchange contribution is formally the same as that for the direct‐gap case. Intervalley interaction lifts the degeneracy of the biexciton ground state. The procedure is applied to the case of GaSe and AgBr, where the ground state multiplet of the biexcitons is related to the band structures.
On the Connection between Electric and Magnetic Properties of the Hubbard Model I. Electrical Conductivity
Authors: Nolting W.
Published: July 2007

An approximate expression for the electrical conductivity σ is derived by means of the method of spectral moments. Therefore, σ is written in terms of a spectral density, for which a suitable ansatz is chosen. Free parameters of this ansatz are fixed by equating moments. The so obtained approximate solution of the conductivity tensor fulfils the well‐known sum rules and contains nearly all exactly calculable limiting cases. Furthermore, Moneckes statement that the conductivity exhibits free acceleration behaviour even in the case of strong electronic correlations, is shown to be incorrect.
Theory of Electro‐Absorption in Heavily Doped Semiconductors
Authors: Yang E.
Published: July 2007

The imaginary part of the interband dielectric function of a heavily doped semiconductor in the presence of a uniform electric field is calculated. The expression is calculated directly from the Kubo equation in the one‐electron approximation. The average over the impurity coordinates is carried out by means of the cluster integral technique and small time approximation. Under the weak field and low temperature approximation, the dielectric function of a heavily doped semiconductor in the presence of a uniform electric field may be calculated explicitly. It can be expressed in two terms; one is the Franz‐Keldysh effect and the other is due to the impurities in the semiconductors. The results are given for both n‐ and p‐type semiconductors.
The Critical Voltage Effect in Transmission Electron Microscopy IV. Influence of High‐Order Systematic Reflections
Authors: David M. , Gevers R. , Serneels R.
Published: July 2007

The second order critical voltage effect in centrosymmetrical crystals without absorption is discussed in the general linear (2n + 1)‐beam system. It is shown that the qualitative description in the three‐beam approximation remains valid, provided one corrects the value of the critical voltage. A study of the effect in noncentrosymmetrical and absorbing crystals is prepared; it is shown that a three‐beam approximation will be sufficient for these discussions as well.
Jahn‐Teller Effect on the Optical Absorption Spectra of CdS:Ti2+ and CdSe:Ti2+
Authors: Rosenfeld A. , Boyn R. , Ruszczynski G.
Published: July 2007

A study is made of electron‐phonon interaction at Ti2+ centres in CdS and CdSe by analyzing the temperature dependence of the 3A2 → 3T1(3F) and 3A2 → 3T1(3P) absorption bands of these systems. Triplet structure in the 3T1(3F) band due to coupling with τ2‐type vibrations is found for CdSe, just as earlier reported for CdS, while the 3T1(3P) bands do not exhibit pronounced structure for both host substances. This behaviour is discussed on the basis of a cluster model involving coupling only to the displacements of the impurity and ligand ions. A point‐charge estimate shows that for both the 3T1(3F) and 3T1(3P) excited states the overall coupling to τ2‐type vibrations should be rather strong. The different shapes of the two bands can be interpreted in such a way that the 3T1(3F) states are especially sensitive to a group of τ2 vibrations having very low frequency, whereas the 3T1(3P) states are mainly coupled to higher frequency τ2 vibrations.
Interface States in a Linear Model of Heterojunctions
Authors: Kandilarov B. D. , Detcheva V. , Petrova P. C.
Published: July 2007

The necessary and sufficient conditions for the existence of interface states in a heterojunction between a one‐atomic and a two‐atomic crystal are derived on the basis of a linear model using the scattering theoretical method of Saxon and Hutner. A discussion of the general features of the above conditions and their interpretation is given and particular cases are visualized applying numerical analysis.
Influence of the Form of the Random‐Field Probability Functional upon the Interband Dielectric Function and Its Modulation by an External Field
Authors: Esser B. , Herzog F.‐N.
Published: July 2007

A method is developed enabling one to take account of the influence of an arbitrary random‐field probability functional of a disordered semiconductor on both the interband dielectric function ε(ω) and its external field induced change ε(ω). The influence of a deformation of the Gaussian distribution upon ε(ω) and Δε(ω) is considered. The corresponding plots are presented. The change of the exponential linear tail in Δ2(ω) due to the random Franz‐Keldysh effect is found to be small. The functions Δε1(ω) and Δε2(ω), however, are more sensitive to the particular form of the random‐field probability functional.
A Criterion for Energy Gaps in Three‐Dimensional Disordered Alloys
Authors: Lehmann G.
Published: July 2007

A new general criterion for the existence of band gaps in disordered alloys is given. It is represented as a condition for the generalized logarithmic derivatives in the potentials of the alloying constituents at spheres circumscribing the Wigner‐Seitz cells. Other three‐dimensional gap criteria published up to now are contained as special cases. The application of the new criterion is not restricted to approximations for the potentials and to the condition of corresponding states in the systems of pure constituents. A comparison with criteria for one‐dimensional systems is given.
Pseudopotential Method and Thermodynamics of Alloys. II. The Calculation of Intermediate Phases in Alloys with limited Solubility
Authors: Fuks D. L. , Zhorovkov M. F. , Panin V. E.
Published: July 2007

To construct the thermodynamical potential of an ordering alloy the pseudopotential method is applied. The conditions of the intermediate phase formation are studied in systems with limited solubility. Possible reasons for a narrow homogeneity region of these phases are established. An analysis is made of the dependence of phase equilibrium on composition and temperature. Numerical calculations are given for the BaCd system, having a b.c.c. lattice.
The Optical Properties of In2Te3
Authors: Hughes O. H. , Nikolió P. M. , Doran C. J. , Vujatović S. S.
Published: July 2007

The fundamental band gap of In2Te3 as a function of temperature has been deduced from transmission measurements. The absorption edge is due to direct allowed transitions and the room temperature band gap is 1.13 eV. The Debye temperature is found to be 200 °K. Reflectivity measurements in the infrared range indicate the existence of a complex rest‐strahlen band with four peaks in the value of ε2 at 143, 155, 180, and 190 cm−1. The first peak is attributed to a TO lattice phonon mode and the other three to localised modes due to the statistically vacant In sites.
The β Parameter and Velický's Approximation in the Calculation of the Optical Properties of GaAs
Authors: González O. D. Castaño , de Dios Leyva M. , Alvarez R. Pérez
Published: July 2007

The optical properties of GaAs are calculated by the Kramers‐Kronig dispersion relation s A study is made of the method proposed by Velický, establishing a rigorous bound which permits to neglect an infinite number of terms in the series which gives the contribution of. the unknown part of the spectrum for the phase angle. A possible interpretation of the extrapolation parameter β is proposed. Also, the causes of the inadmissible values obtained for the extinction coefficient are made a bit clearer. Values for the interband transition energies in the range 0 to 25.3 eV, for the plasma frequency, and plasmon lifetime are reported.
Structural Phase Transitions in BaMF4 Antiferromagnets
Authors: Dvořák V.
Published: July 2007

Two‐dimensional antiferromagnets BaMF4 where M = Mn, Fe, Co, or Ni exhibit a linear magnetoelectric effect. It is known that this effect is forbidden by symmetry unless a structural phase transition takes place in these compounds. Recently such phase transition was discovered in BaMnF4. Using the group theory and the knowledge of magnetic structure and magnetoelectric effect, essential characteristics of this structural phase transition, i.e., symmetry of the phase transition parameter, possible symmetry changes, anomalous dielectric and elastic constants, are determined.
s–d Interaction in the Transition Range between Metals and Insulators: CdS: Mn
Authors: Spehlich G. , Múller H.
Published: July 2007

In metallic compounds with low charge‐carrier concentrations the Korringa relaxation rate can be so small that it reaches the order of magnitude of the direct spin–lattice relaxation rate. We report on an ESR investigation of CdS doped with the S‐state ions Mn and Eu, where between both contributions to the homogeneous ESR line‐width could be distinguished. Mn‐doped CdS turns out to have an extremely small Korringa slope of the ESR linewidth. We were able to vary this slope from zero to about 0.02 G/K by variation of the electric conductivity of CdS from insulator to a compound with metallic behavior.
Electric Field Gradients in Hexagonal Crystals of Cd, Zn, Gd, and Te Acting on Sm, W, and Os Impurities
Authors: Roesch L. Ph. , Kulessa R. , Horber F.
Published: July 2007

The quadrupole interactions of the 2 + states of Sm, W, and Os isotopes have been studied by using time‐integrated perturbed angular correlations. It has been possible to derive the electric field gradients by applying known data of the quadrupole moments of the implanted probe nuclei. A comparison with the calculated ionic contribution of the lattice shows that great importance has to be attributed to the conduction electrons.
Jahn‐Teller Effect in the Ligand Field Theory of Trigonal Bipyramidal V4+ Complexes
Authors: Bhattacharyya B. D.
Published: July 2007

A ligand field model for 3d1 ions with approximate D3h site symmetry has been developed which satisfactorily explains the fine structure parameters, the EPR spectra, and the magnetic susceptibility. The treatment includes the essential features of Jahn‐Teller distortion and covalency effects which are found to have important bearing on the nature of the Jahn‐Teller distortion. Finally by the spin Hamiltonian formalism of Pryce the g‐values and magnetic susceptibility arecalculated. Attempts are made to fit the experimental data of optical absorption, gr‐values, and magnetic susceptibility with a set of parameter values in the theoretical expressions at different temperatures. From this an estimate is made of covalency and Jahn‐Teller parameters.
On the Calculation of Mode Grüneisen Parameters in Rubidium Halides
Authors: Kress W. , Jex H.
Published: July 2007

Recently both authors have calculated the mode Grüneisen parameters of RbI using different methods. A critical comparison is made and the consequences of different approximations for the third order coupling constants are discussed. It is shown that the pressure dependence of at least five shell model parameters has to be taken into account in order to reproduce the measured pressure derivatives of the elastic and dielectric constants. In the most stringent approximation, however, where only the third order terms of the rigid part of the shell model are taken into account in the central potential approximation, qualitative agreement with the measured mode Grüneisen dispersion curves is still maintained.
The Parasusceptibility of Gadolinium
Authors: Hof H.‐j. , Kronmüller H. , Rothfuss B.
Published: July 2007

The parasusceptibility, 𝔵p, of Gd single crystals was measured in the temperature range between 4.2 and 290 K, and in the field range from 0.07to 3.0 T. Below 0.8 Tc the parasusceptibility depends linearly on temperature and the field dependence follows a H‐0.44. law. The experimental results are interpreted by means of the spin wave theory taking into account the indirect exchange coupling, the dipolar interactions, and the anisotropy of the energy spectrum of the conduction electrons.
Experimental Evidence of an Electronic Localization in n‐Type InSb Using a Microwave Technique
Authors: Ferre D. , Dubois H. , Biskupski G.
Published: July 2007

It is well known that, when irradiated by two microwaves of frequenciesv1 and v2, a sample of n‐type indium antimonide generates a beat at the intermediate frequency (i.f.) v [v1 _ v2]; that superheterodyne‐detecting property originates from the non‐linearity ofthe I‐U characteristic in weak electric field. It is shown here that, if the classical results about InSb bolometer mixer are valid whenelectrons are free, a new feature appears when localized and delocalized electronic states are both present in the crystal; in this case, the signal detected at i.f. frequency must be separated into two parts: the beat generated by free electrons at frequency v on the one hand and the “response” of localized electrons subjected to this beat on the other hand. It is also shown that information obtained about electronic localization is in good agreement with that one previously deduced from resistivity measurements. In conclusion, such a method appears to be a very convenient way of sonding electronic distribution in a given material.
Structure and Effective Interionic Potential of Liquid Titanium
Authors: Waseua Y. , Tamake S.
Published: July 2007

X‐ray diffraction patterns from liquid titanium have been obtained at 1700 and 1850 °C. After calculating the interference function in Fourier analysis, the atomic radial distribution function is obtained from which interatomic distance and coordination number are estimated. Comparingthe results with the structure of liquid calcium, a broadening of the firstpeak maximum is found in the pair distribution function of liquid titanium. The Born‐Green equation is analyzed numerically to derive the effective interionic potentials from the observed structural data. The temperature insensitive long‐range oscillatory potentials are found.
On the Connection between Electric and Magnetic Properties of the Hubbard Model II. Optical Absorption
Authors: Nolting W.
Published: July 2007

The optical absorption, caused by interband transitions between quasiparticle bands, is investigated within the limits of the Hubbard model for a system of correlated electrons in a narrow energy band. Thereby first of all the influence of magnetic orderings over absorption spectra is discussed. It is shown that a phase transition paramagnetism ↔ ferro‐magnetism is signalled by a significant modification of the absorption curves. The numerical evaluation is done for a simple cubic lattice for arbitrary band occupation n and arbitrary interaction constant I.
Image of a Stacking Fault
Authors: Indenbom V. L. , Sh. Slobodetskii I.
Published: July 2007

The wave field of X‐rays is investigated in a crystal with a stacking fault whose line of intersection with the scattering plane is parallel to the surface. The conditions are regarded in which the stacking fault acts as a diffraction lense focusing the X‐rays. The solutions found are illustrated by calculated semi‐shadow maps of the images of stacking fault and intensity distribution of the wave field in the scattering plane of the crystal.
Anderson's Hamiltonian for an Electron in a Distorted Crystal Lattice
Authors: Mezincescu G. A.
Published: July 2007

The matrix elements of the Hamiltonian of an electron in a randomly distorted crystal lattice are estimated for a complete and orthonormal system of localized functions, related to the Wannier functions. The estimates agree with the usual neglection of the stochastic behaviour of the off‐diagonal matrix elements. A numerical calculation for the conduction band of Na is in good agreement with the estimated values of the matrix elements.
Luminescence from the Excited States of Excitons in CdS and CdSe Single Crystals
Authors: Vlasov G. K. , Brodin M. S. , Kritskii A. V.
Published: July 2007

The photoluminescence in “pure” crystals of CdS and CdSe atT  1.4, 4.2, and 77 °K and high exciton concentration has been investigated. The correlation between arising (having mainly threshold character) of luminescence from the excited states of excitons A(n  2, 3) and B(n  1) and the existence of a fine structure, related to radiative recombination due to inelastic collisions between excitons (P, P2, P3, P4, and PB−lines) is observed. It is shown that with increasing excitation the “blue” shift of P‐line is twice than that for the A(n  1)‐line. The analysis of the problem using kinetic equations leads to conclusions about the possibility of stimulated enlargement (similarly to laser generation) of exciton concentration in their excited states due to Auger‐throwing of the excitons into these states at luminescence of P‐lines. The numerical estimations correspond to the experimental condition and the agreeement to the theory with experiment permitsto conclude that the observable luminescence from the excited states is dueto the stimulated generation of excitons in these states.
Galvanomagnetic properties of anisotropic semiconductors of the p‐Te type
Authors: Tomchuk P. M. , Shenderovskii V. A. , Gorley P. N. , Gavaleshko N. P. , Solonchuk L. S.
Published: July 2007

Expressions for the independent galvanomagnetic resistance tensor components in weak magnetic fields are obtained for p‐Te type semiconductors, using a variational method. Carrier scattering by optical lattice vibrations is considered without using the relaxation time approximation. With p‐Te as an example, the temperature dependence of the tensor components is studied both theoretically and experimentally for temperatures between 77 and 300°K. From comparison between the calculated and measured values of components, the constants for carrier interaction with polar optical phonons are estimated.
Influence of the electron–electron scattering on the kinetic effects in many‐valley semiconductors
Authors: Mitin V. V. , Tolpygo E. I.
Published: July 2007

The conductivity and magnetoresistance (MR) of a many‐valley semiconductor are calculated for the case when electron scattering on impurities and on the electrons of the same and of the other valleys is characterized by different coupling constants. It is shown that, with increasing anisotropy of the ellipsoidal energy surfaces, the contribution of the electron–electron scattering grows rapidly and MR decreases. The ratio of the coupling constants may be determined unambiguously through analysing the stress dependence of the conductivity and of MR in these semiconductors. The piezomagnetoresistance is shown to increase along with growing electron–electron scattering.
Theory of impurity excitation transfer via a phonon field
Authors: Godenko L. P. , Mashkevich V. S. , Shadchin E. A.
Published: July 2007

Processes of impurity excitation energy transfer are studied by the method of quantum correlation functions. The probabilities of the process are calculated. The probability dependence on the spacing of the interacting centres is determined. The contribution of the process to the guest centre spectral line width is investigated.
Near‐edge spontaneous photoluminescence in GaSe1–xSx
Authors: Kuroda N. , Nishina Y.
Published: July 2007

Photoluminescence spectra of GaSe1–xSx (0 ≦ x ≦ 0.2) are investigated at 4.2 K by the time resolving method. The shift in the photon energies of the luminescence lines with respect to the chemical composition x may be understood without any quantitative contradiction if one employs the empirical model that the direct conduction band edge lies 5 meV below the indirect one, whereas the ground state of the direct exciton lies 13 meV above that of the indirect one in ϵ, γ‐GaSe at 4.2 K. The direct–indirect gap reversal occurs near x = 1 × 10−2.
Ab‐initio pseudopotential calculation of static and dynamic crystal properties of H.C.P. metals
Authors: Hafner J. , Eschrig H.
Published: July 2007

Harrison's ab‐initio pseudo‐potential scheme is applied to Be and Mg. The influence of the core–valence and the valence‐valence electron exchange and correlation on the atomic radius, the binding energy, the bulk modulus and its pressure derivative, the equation of state, and the phonon spectra is investigated. Satisfactory agreement with experiment is reached.
Lattice dynamics of several A NB8–N compounds having the zincblende structure. II. Numerical calculations
Authors: Kunc K. , Balkanski M. , Nusimovici M. A.
Published: July 2007

The formalism of the deformable‐bond approximation, described in the preceding article of this series, is now applied to 15 compounds with zincblende structure. The phonon dispersion curves and densities of phonon spectra are calculated for ZnS, ZnSe, GaP, GaAs, InSb, SiC, GuCl, and CuI, the method developed for determination of 15 parameters characterizing the model is discussed. Analogous results obtained with an 11‐parameter version of the rigid ion model are presented for comparison.
Lattice dynamics of several ANB8–N compounds having the zincblende structure. III. Trends in model parameters
Authors: Kunc K. , Balkanski M. , Nusimovici M. A.
Published: July 2007

The parameters of the deformation‐dipole model which were determined in the preceding article of this series for 15 compounds with zincblende structure are now systematized into a regular scheme. It is found that short‐range forces between first neighbours vary as monotonic functions of interatomic distance and that static ionic charge varies linearly with Pauling's ionicity. The physical meaning of these regularities is discussed.