bluuebunny/crossref_metadata_2025_split huggingface dataset:
Polariton Effects in the Optical Spectra of Excitonic Molecules
Authors: Henneberger F. , Voigt J. 
Published: July 2007

AbstractStarting from a model Hamiltonian which describes an excitonic molecule‐excitonphoton system at not too high electron‐hole pair densities, an excitonic molecule‐polariton interaction is derived. By means of this interaction the influence of polariton effects on the optical emission, absorption, and scattering processes involving excitonic molecules is studied theoretically. It is shown, that the transition probabilities of these processes are diminished due to polariton effects, especially in substances with small molecule binding energies. Furthermore, some characteristic modifications of the optical spectra (additional lines, changes in the line positions and shapes) are reported.
Electron Paramagnetic Resonance of Two Stoichiometry‐Induced Intrinsic Defect Centres in CuGaS2
Authors: Von Bardeleben H. J. , Goltzené A. , Schwab C. 
Published: July 2007

AbstractRelated to the sulphur annealing conditions, two different EPR signals are observed in CuGaSg2 With liquid sulphur, an isotropic photosensitive line is observed at g = 2.024 ± 0.002. With gaseous sulphur, an anisotropic spectrum, independent of light excitation, is recorded with g∥[001] = 2.114 ± 0.005 and g ⊥ [001] = 2.005 ± 0.005. The suggested models for the intrinsic centres correspond respectively to a hole trapped on a copper vacancy and to a S molecule ion. The formation of both centres, important for the electrical compensation processes in CuGaS, is justified by the molecular composition of sulphur as a function of its physical state.
Calculation of Nonradiative Multiphonon Capture Coefficients and Ionization Rates for Neutral Centres According to the Static Coupling Scheme II. Alternative Trap Models
Authors: Pässler R. 
Published: July 2007

AbstractOn the basis of the static coupling scheme, explicit calculations are performed of nonradiative multiphonon capture coefficients for one‐band‐model traps in a direct semiconductor. The present investigations are more comprehensive than earlier ones in the following respects: 1) the interaction of the trapped carrier with both the longitudinal acoustic and the longitudinal optical crystal modes is taken into consideration, 2) comparable analytical results are derived for the alternative models of a delta‐function potential (isoelectronic traps) and a Coulombic potential (hydrogenic traps), 3) a comparison is made between the one‐ and two‐parameter descriptions of such traps, and 4) explicit results are given both for pseudo‐intraband and ‐interband processes.
Reflectance Measurements on Single Crystals of PbFCl, PbFBr, and PbBr2
Authors: Eijkelenkamp A. J. H. , Vos K. 
Published: July 2007

AbstractThe reflectance spectra of single crystals of PbFCl and PbFBr are measured between 2.8 and 6.5 eV at 84 and 295 K. The absorption spectra are calculated by Kramers‐Kronig transformation. Peaks at 4.55 and 5.13 eV in the absorption spectra of PbFBr and PbFCI, respectively, are attributed to cation exciton transitions. In the reflectance spectra of vacuum‐cleaved single crystals of PbBr2, which are measured at 84 K with different polarization directions, a new fine structure is observed. An interpretation is given based on an ionic model.
Photoconductivity of sublimated rubrene films during oxidation. The magnetic field effect
Authors: Frankevich E. L. , Tribel M. M. , Soroltk I. A. 
Published: July 2007

AbstractThe mechanism of carrier generation in sublimated rubrene films in the course of their oxidation is investigated. The technique of the simultaneous measurement of photoconductivity and fluorescence of the samples and their variation in magnetic fields up to 3 kOe is employed. The excited states whose deactivation leads to both, fluorescence and carrier generation in unoxidized films are found to be rubrene singlet excited molecules 1M1 and in films exposed to air, excited complexes of 1M1 with O2 molecules and with transannular rubrene peroxide molecules MO2. The probability of carrier generation in the interaction of 1N1 with O2 is higher than with MO2. The existence is shown of an efficient channel of generation of triplet rubrene excitons, associated with the deactivation of excited triplet complexes of rubrene with MO2. The influence is investigated of variations in the crystalline structure of films on the behaviour of photoconductivity.
The quadratic response of a Fermi gas: Electronic charge density of valence electrons in zincblende semiconductors
Authors: Pickenhain R. , Milchev A. 
Published: July 2007

AbstractThe electronic charge density of a free electron gas is exactly calculated up to second order in the perturbing potential, using a direct density matrix approach. The effect of the second order correction is analysed for both f.c.c.‐lattice metals and zincblende semiconductors and it is found, that the singularity of the quadratic filter function, introduced in this work, leads to the appearance of a covalent bonding charge in semiconductors. For illustration of the method, the charge distribution of Ge is calculated and compared with results of explicit summation of the wave functions.
Scattering of free carriers due to the strain field of screw dislocations in semiconductors
Authors: Farvacque J. L. , Gerlach E. 
Published: July 2007

AbstractUsing a method, based on the equivalence of energy loss and Joule heat the scatterin due to screw dislocations in semiconductors is calculated. In particular the effect of the deformation potential and the piezoelectric effect are considered.
New aspects of the magnetoluminescence of a band to acceptor transition in GaAs
Authors: Rühle W. , Göbel E. 
Published: July 2007

AbstractThe magnetic field dependence of the conduction band to acceptor (e, A°) transition involving the acceptor carbon is analyzed in high‐purity p‐type GaAs at very low and very high excitation intensities. The change in the density of states of the free electrons with magnetic field determines the lineshape and the non‐linear peak shift of these transitions. From the observed splitting into different electron Landau levels an effective mass of m*/m0 = 0.0663 ± 0.006 is determined. The variation of the low‐energy wing of the luminescence line with magnetic field gives an effective mass of m*/m0 = 0.0669 ± 0.0006 if the effect of a linear Zeeman splitting is included.
Systematic investigation of the sign change of the Hall effect in n‐InSb in the quantum limit
Authors: Fantner E. J. , Kuchar F. , Bauer G. 
Published: July 2007

AbstractAt liquid helium temperatures a change of the sign of the Hall effect of n‐InSb is observed in the magnetic freeze‐out regime. The experimental examination of possible explanations and their discussion show that only the contribution of the asymmetric impurity scattering (“skew scattering”) to the Hall effect explains the observed systematic behaviour of the sign change.
Hole mechanisms of H and F centre production in KCl crystals
Authors: Zazubovich S. , Osminin V. 
Published: July 2007

AbstractThe formation of interstitial halogen atoms (H centres) under the photodestruction of hole impurity Tl2+ centres in KCl crystals is observed by the method of thermally stimulated luminescence from 4.2 to 250 K. The existence of two new hole mechanisms of the H centre generation is established. It is shown that the H centres can be produced under recombination of holes (optically released from A2+ centres) with localized electrons (F or A° centres), as well as under a non‐recombination process caused apparently by the decay of holes into α and H centres.
Structural investigation of the laves phase MgZn2 with model potential calculations
Authors: Rennert P. , Radwan A. M. 
Published: July 2007

AbstractThe structure of the intermetallic compound MgZn2 is investigated using Shaw's model potential. The calculations are carried out using different model potential parameters of Zn. Variations of the axial ratio c/a and the parameter x in the Kagomé nets are studied. Charge transfer, which is taken into account in an approximate and simple way, plays an important role in the determination of the stable structure of MgZn2. Staking fault energy is also calculated.
Photoluminescence in amorphous silicon
Authors: Engemann D. , Fischer R. 
Published: July 2007

AbstractPhotoluminescence of amorphous silicon is measured in the range from 77 K to room temperature. The temperature dependence of the three bands, of which the total luminescence spectrum consists, could be determined seperately. It is found that all three of them decrease at high temperature with approximately the same activation energy of 0.13 eV. This decrease sets in at different temperatures for the different bands. The three bands are interpreted as transitions within electron‐hole pairs consisting of more or less deeply trapped carriers. The activated decrease at high temperature is explained by thermally activated dissociation of these pairs. The electron‐hole pair concept also explains why photoconductivity decreases about twice as fast as luminescence when the substrate temperature is lowered.
Cyclotron resonance and quantum oscillations in semimetallic bismuth‐antimony alloys with Eg  0
Authors: Braune W. , Kuka G. , Hess S. , Müller H.‐U. , Jung T. 
Published: July 2007

AbstractMeasurements are made of quantum oscillations of the surface impedance in the MHz range and cyclotron resonances of the electrons in the GHz range in two crystallographic planes in a semi‐metallic bismuth‐antimony alloy. The experimental results arc analyzed on the basis of Abriko‐sov's model of the gap‐less state of the bands at the point L of the Brillouin zone.
The quadratic response of a fermi gas. Building‐up of the covalent bonding in zineblende semiconductors. A nondiagonal density matrix treatment
Authors: Milchev A. , Pickenhain R. 
Published: July 2007

AbstractThe nondiagonal density matrix of a free electron gas, perturbed by a crystalline potential, is considered in the momentum‐space representation. One obtains for the second‐order correction to the unperturbed electron density on the diagonal an exact analytic expression for the quadratic filter function. It is used to evaluate the valence electron densities of Ge, Si, Sn, and GaAs. The influence of the separate pseudopotential formfactors, taken into account, on the bonding charge heaping up is thoroughly investigated.
Bound electron states close to the conduction band in germanium due to 60° dislocations
Authors: Winter S. 
Published: July 2007

AbstractElectron states with energies close to the conduction band are calculated in germanium with 60° dislocations within the framework of effective mass and deformation potential theories. These states are found to have energies down to 0.10 eV below the conduction band and have wave functions of a considerable spatial extent.
The superposition model in crystal field theory
Authors: Eremin M. V. , Kornienko A. A. 
Published: July 2007

AbstractA crystal field Hamiltonian is obtained with allowance for metal‐ligand interaction. Apart from the standard terms it contains the operators {U(k′)U(k″)}(p) whose role is discussed. Convenient expressions for the Hamiltonian parameters are obtained which make it possible to take into account the spatial distribution of all cluster electrons. Detailed calculations and comparison with experiments are made on KMgF3,:Mn2+ and KNiF3, crystals.
Factor group analysis of the Raman spectrum of Pb5Ge3O11
Authors: Müller‐Lierheim W. , Suski T. , Otto H. H. 
Published: July 2007

AbstractThe complete Raman spectrum of Pb5[GeO4/Ge2O7] is investigated. The results are analyzed by factor group analysis for external and librational modes taking polariton effects into account. The temperature dependence of the intensity of different modes in the depolarized spectra allows to distinguish between modes of pure E symmetry (at the Γ‐point for P3 space group) and those with mixed symmetry. The modes in the frequency region above 200 cm+1 are assigned to internal modes of the GeO4 tetrahedron and the Ge2O7 double tetrahedron. As Pb5Ge3O11 exhibits above its ferroelectric‐paraelectric phase transformation at Tc = 450 K additional anomalies of the birefringence and of the lattice constants the Raman spectrum up to 750 K is investigated.
Triplet exciton diffusion and double photon absorption in tetracene
Authors: Aladekomo J. B. , Arnold S. , Pope M. 
Published: July 2007

AbstractDelayed fluorescence from tetracene crystal excited at 8600 Å is studied. The luminescence decay is interpreted in terms of a model in which singlets are generated by triplet exciton fusion and by double‐photon absorption. The double‐photon absorption coefficient is estimated to be 5.8 × 10―27 cm s. The triplet exciton diffusion coefficient along the crystal b axis is determined by the spatial intermittency method to be (4 ± 1) × 10―3 cm2/s. The influence of this diffusion coefficient on other processes is discussed.
Ultraviolet Photoelectron Valence Band Studies on Phthalocyanine Compounds
Authors: Battye F. L. , Goldmann A. , Kasper L. 
Published: July 2007

AbstractUltraviolet photoelectron spectra excited by HeI and HeII radiation from the valence bands of H2Pc (H2C32H16N8), FePc, CuPc, and PtPc are measured for binding energies down to 18 eV. No satisfying agreement of the photoelectron spectra with available molecular orbital calculations is found. Although weak contributions of the central metal d‐electrons can be identified, the spectra are surprisingly similar for all Pc's and represent essentially the π‐electron ring system. They appear to be a superposition of the spectra of benzene and an inner porphin skeleton. A spectral change in H2Pc around 10 eV as compared to the metal Pc's is interpreted in terms of different σ‐bonds to the central atoms.
Finite Dislocation Elements in Finite Media. I. Basic Theory
Authors: Korner A. , Svoboda P. , Kirchner H. O. K. 
Published: July 2007

AbstractThe interaction of finite co‐planar dislocation pieces in a finite medium is considered with special reference to the case where two partial dislocations split trapezoidally in a thin foil. The calculation is carried out on the force and moment level. The presence of the surface is taken into account by Lothe's theorem. The remaining problem of the interaction of finite pieces in an infinite medium is considered in detail. It is shown that such problems require the execution of integrals involving only the energy E of dislocations (or interaction energy of parallel dislocations) within that plane — no derivatives of E are required. Formulae suitable for numerical evaluation are derived.
A Theory of the Hall Effect in the Hopping Region in Disordered Systems. I. Rate Equation in Presence of Electrical and Magnetic Fields
Authors: Böttger H. , Bryksin V. V. 
Published: July 2007

AbstractThe equation of motion of the density matrix in presence of an electrical and a magnetic field is derived. This equation holds for arbitrary electrical fields. It describes the charge transport in systems with localized charge carriers: in ordered systems in case of small polarons (strong electron–phonon interaction) as well as in disordered systems with an arbitrary strength of electron–phonon interaction. After linearizing this equation with respect to the electrical field it describes in particular the Hall effect in such systems.
Pseudopotential Theory and the Problem of Covalent Bonding. I. General Theory of Covalent Charge
Authors: Krasko G. L. , Makhnovetskii A. B. 
Published: July 2007

AbstractA theory of covalent bonding is developed, based on the Brillouin‐Wigner perturbation theory. The covalency (multi‐ion interaction) effects are shown to be associated with the existence of a so‐called “covalent charge distribution” ϱcov(g). The latter arises as a result of “coherent” electronic scattering on some Bragg planes. A procedure is suggested of sorting out “the main” terms of the Brillouin‐Wigner series and selfconsistent determination of the covalent charge Fourier‐transforms ϱcov(g), playing the fundamental role in the theory. As an example ϱcov(g) is found for the diamond lattice. Concrete calculations of ϱcov(g) and crystal lattice stability for Si, Ge, and α‐Sn will be presented in part II of the paper.
Two‐Photon Resonance Raman Scattering by Excitons via an Intermediate Excitonic Molecule. I. Bare‐Exciton Approach
Authors: Bechstedt F. , Henneberger F. 
Published: July 2007

AbstractTwo‐photon resonance Raman scattering from excitons via an intermediate excitonic molecule is studied theoretically. The line shape and dispersion of the scattering spectrum is calculated in terms of damping theory for excitation near the resonance energy of the two‐photon absorption for the molecule. By taking into account a finite band width of the exciting laser light the secondary radiation spectrum is decomposed into a Raman‐ and a luminescence‐like part. The relation between these parts is discussed in dependence on the dampings acting on the electronic intermediate and final states of the scattering process as well as the laser band width. Qualitative agreement is found with the experimental results for CuCl and CdS.
Wave‐Vector Dependence of the Electron Energy Losses of Boron Nitride and Graphite
Authors: Büchner U. 
Published: July 2007

AbstractWave‐vector (q)‐dependent energy loss functions, Im [−ϵ⟂(ω, q⟂)]−1, and dielectric functions, ϵ2⟂(ω, q⟂), of hexagonal boron nitride and graphite up to q⟂‐values of almost one third of the Brillouin zone and up to energies of 40 eV were derived from electron energy loss measurements. The results are discussed in terms of non‐vertical interband transitions of two‐dimensional band structures.
Deformation Effects on Biexcitons in Wurtzite‐Type Crystals
Authors: Goede O. 
Published: July 2007

AbstractThe effects of uniaxial stress on optical biexciton transitions in wurtzite‐type crystals are discussed in the framework of an effective‐mass approximation using the Bir‐Pikus Hamiltonian and taking into account electron–hole exchange interaction and JJ‐coupling of the two holes. The calculated stress‐induced energy shift, splitting, and polarization of the ground‐state biexciton line for CdS are compared with the available results of piezoemission measurements. Furthermore, the stress dependence of the optical transitions between the AA(Γ1) biexciton ground state and A(Γ6) and B(Γ1, Γ5,) exciton final states are discussed. In CdS the AA(Γ1) → A(Γ6) emission line may hardly be detectable for σ ⟂ c. The AA(Γ1) → B(Γ1, Γ5) lines, however, should be observable for σ ∥ c and also the expected values of the electron‐hole exchange interaction in the exciton states and the JJ‐coupling of the holes in the biexciton states.
Electrical Properties of (DEPE) (TCNQ)4
Authors: Somoano R. B. , Hadek V. , Yen S. P. S. , Rembaum A. , Hsu C. H. , Deck R. J. , Datta T. , Hermann A. M. 
Published: July 2007

AbstractConductivity measurements are performed on single crystals of two phases of (1,2‐di(N‐ethyl‐4‐pyridinium)ethylene2+) (7,7,8,8‐tetracyanoquinodimethane), (DEPE) (TCNQ)4. Conductivities are determined by four probe and Montgomery methods, and the highest values measured are those along theb0‐axis for the monoclinic phase (I) crystals, ≈ 50 (Ω cm)−1at room temperature. The resistivities are in general exponentially activated (semiconducting behavior). Thermoelectric power measurements are performed along the b0‐axis of phase I crystals and the thermopower tends to saturate at high temperatures to −60 μV/K, characteristic of a quarter‐filled correlated band. No evidence for the existence of a stable metallic state reported by Ashwell et al. was found. The preliminary crystal structures of both phases are discussed.
Zero‐Field Splitting in Trigonal Symmetry. Cr3+ and Fe3+ in α‐LiIO3
Authors: Karthe W. 
Published: July 2007

AbstractThe constant of zero‐field splitting B: of Cr3+ in α‐LiIO3 is calculated employing the methods of Rimmer and Johnston and Macfarlane. The lattice sums are determined from the structure of the chromium centre and by the use of the quadrupolar constant of lithium in chromium‐doped LiIO3. A good agreement between the theoretically and experimentally determined values of B is achieved. The contributions of most mechanisms to the zero‐field splitting of S‐states investigated so far are calculated for Fe3+ in α‐LiIO3 by using the above lattice sums because the structure of the centre is quite similar to that of the Cr3+ centre. The theoretically determined value proves to be too small by one order of magnitude. The relativistic mechanism in an overlap model is supposed to contribute to a better agreement.
A Theory of the Hall Effect in the Hopping Region in Disordered Systems. III. DC Hall Effect
Authors: Böttger H. , Bryksin V. V. 
Published: July 2007

AbstractThe random resistor network with extrinsic currents, which describes the Hall effect in the hopping region, is averaged by using ideas of percolation theory. Concrete results are obtained for the dependence of the dc Hall mobility on temperature and concentration of sites for weak and strong interaction with phonons. Both the limits of electron and hole conduction are investigated. The sign of the Hall mobility is shown not to change by passing from electrons to holes (p–n anomaly).
The Pair Correlation Function and the Dielectric Response in the Homogeneous Electron Gas
Authors: Brosens F. , Lemmens L. F. , Devreese J. T. 
Published: July 2007

AbstractRecent developments in the theory of the homogeneous electron gas relate the pair correlation function g(0) at the origin to a) the static limit of the exchange and correlation effects as well as to b) an integral over frequencies of the inverse dielectric function. It is shown that any static local field correction violates at least one of both the conditions, and that from the frequency‐dependent treatment of exchange, only the variational approach satisfies both the requirements.
Calculation of the Energy Band Structures and Carrier Mobilities in γ‐Platinum Phthalocyanine Single Crystals
Authors: Singh Jai 
Published: July 2007

AbstractTight‐binding calculations of the energy band structures for excess holes and electrons in γ‐PtPc are presented using single Slater‐type atomic orbitals. Carrier mobilities are also calculated using the constant relaxation time approximation. Hole energy bands containing the contributions from the platinum metal atom are shown to have as large band‐width as 3.6 eV along the crystal axis b. On the basis of the mobility calculations, it is predicted that the holes probably play a dominant role in the carrier transport in γ‐PtPc.
Interaction of Atomic Order with Localized Ferromagnetism
Authors: Billard L. , Chamberod A. , Villemain P. 
Published: July 2007

AbstractA pair model is used to describe atomic order and an inhomogeneous Ising model to describe magnetic properties of a binary alloy. Both ones are treated simultaneously in the mean field approximation. A comparison is made with experimental results on FeCo and FeNi3 alloys.
Theory of Temperature Dependence of the Magnetization in Rare‐Earth‐Transition‐Meta1 Alloys
Authors: Szpunar B. , Lindgård P‐A. 
Published: July 2007

AbstractIt is shown that the temperature dependence of the magnetic moments and Curie and ferrimagnetic compensation temperatures for Gdl‐xTx (T = Co, Ni, and Fe) and Y1‐xCox can be accounted for by a simple model assuming a RKKY interaction between the rare‐earth moments and the transition‐metal pseudo‐spin. The interaction is mediated by an effective alloy medium calculated using the CPA theory and elliptic densities of states.