bluuebunny/crossref_metadata_2025_split huggingface dataset:
Photoabsorption Coefficient of Alloys of AI with Transition Metals V, Fe, Ni and with Cu and Pr from 30 to 150 eV Photon Energy
Authors: Hagemann H.‐J. , Gudat W. , Kunz C. 
Published: July 2007

AbstractThe absorption coefficient of VAI3, FeAI, NiAI, NiAl2, CuAI2, PrAl2, and of disordered V–AI(16 at% AI, 28%, 41%) and FeAI (11%) alloys has been measured in the region of the M2, 3 absorption of the transition metals and the Labsorption of AI. The strong changes of the AI spectrum in the region of the 100 eV maximum upon alloying are explained as another evidence of the EXAFS (extended X‐ray absorption fine structure) nature of these structures. The broad, prominent absorption peaks from the 3p excitations in V and Fe and from the 4d excitations in Pr are influenced only little on allyoing and thus appear to be of atomic origin. The fine structure at the onset of the Pr 4d transitions is identical in the metal and the alloy but differs from that of Pr oxide. The only M2, 3 edge which is detectably shifted is that of Ni (up to 2.1 eV), whereas the onset of the AI L2, 3 edge is shifted in all the alloys (up to 1.1 eV). The shifts are interpreted in accordance with X‐ray fluorescence and nuclear resonance measurements as changes of the density of states in the valence band of the alloys.
On the Theory of Photon‐Assisted Zener Tunneling
Authors: Henneberger K. 
Published: July 2007

AbstractZener tunneling assisted by one‐photon absorption is studied theoretically. A general expression for the transition rate is obtained and further evaluated for both a two‐band and a three‐band process, where analytical results are presented in terms of Airy functions. Furthermore, an interband current is produced by photon assisted tunneling which is shown to be proportional to the corresponding transition rate. It can be concluded from these results that photon assisted tunneling noticeably contributes to macroscopically observable quantities like electroabsorption and electric current, especially, if two nearly degenerate valence or conduction bands exist. In the latter case an interband current is produced flowing opposite to the field direction.
Photoluminescence Excitation Spectra of Solid Krypton
Authors: Ackermann Ch. , Brodmann R. , Hann U. , Suzuki A. , Zimmerer G. 
Published: July 2007

AbstractThe excitation spectrum of the intrinsic photoluminescence of solid krypton has been investigated between 9.8 and 14eV. Samples with a clean surface yield a quantum efficiency which is within an uncertainty of ≈10% independent of the kind of initially excited states. Surface contamination reduces the steady‐state concentration of excited states leading to quenching of the intrinsic luminescence. In the region of excitonic excitation the energy transfer between excitons and the surface is investigated. Assuming exciton diffusion, a diffusion length L0 = 200 Å is obtained for the n= 1 and n′ = 1 excitons. Under the assumption of long range dipole–dipole interaction a critical radius R0 = 22 Å is deduced.
Generalized Locator‐Coherent‐Potential Approach to Ferromag‐netic Systems with Random Local Magnetic Fields and Moments
Authors: Pfeiffer H. , Haubenreisser W. 
Published: July 2007

AbstractThe locator formulation of the coherent‐potential approximation (CPA) is generalized and applied to a Hamiltonian with diagonal and off‐diagonal disorder where the latter comprises additional terms of the type a and ajak. The random variables can take an arbitrary number of values. The off‐diagonal elements are assumed to be separable. The resulting CPA‐equations are represented by relations between 2 × 2‐matrices. The formalism is applied in particular to the problem of inhomogeneous broadening of ferromagnetic resonance lines in polycrystalline materials. Taking into account the usual simplifications, a transcendental equation for the reduced, complex frequency shift is derived. The used approach comprises the results previously obtained by other methods.
Properties of Randomly Diluted Heisenberg Antiferrornagnets with Spin 1/2
Authors: Hattori K. 
Published: July 2007

AbstractThe properties of Heisenberg antiferromagnets with spin ½ randomly diluted with non‐magnetic atoms are theoretically investigated. It is shown that the critical concentration of magnetic atoms pc for which the Néel point becomes 0 K is determined from the relation pc[1 − (1 − pc)z] = l/(z − 1), where z is the number of nearest neighbours. The variations of the Néel point, spin expectation value, and specific heat above the Néel point with concentration of magnetic atoms are also obtained. The Schottky‐type maximum of the specific heat due to the residual short range order above the Néel point is found near the critical concentration of magnetic atoms.
Non‐Central Forces and Stability of the ANB8–N‐Tetrahedral Compounds. I. Bond Charges and the Valence‐Force‐Field Model
Authors: Rusakov A. P. , Anfimov M. V. 
Published: July 2007

AbstractStability aspects of the ionic‐covalent crystals are investigated in terms of the bond charge model. A definition of the covalent bond charge is proposed for the A‐NB8–N semiconductors with zinc‐blende structure. The possibility is discussed to use the bond charge as an order parameter to describe the transition from zinblende to NaCl type structures. The trends for the parameter change of the Keating‐Martin valence‐force‐field model with bond charge variations are investigated.
Dispersion of Phonons in Transition Metal Oxides (MnO)
Authors: Goyal R. P. , Sarkar K. K. , Goyal S. C. 
Published: July 2007

AbstractThe theory of the extended three body force shell model (ETSM) is applied to study the dispersion of phonons in transition metal oxides (MnO). The calculated values show a good agreement with the experimental values. The distinct behaviour of alkaline earth oxides and transition metal oxides is also discussed.
Anisotropic g‐Factor of the Conduction Electrons in Bi2Te3
Authors: Köhler H. 
Published: July 2007

AbstractShubnikov‐de Haas investigations were performed on n‐type Bi2Te3 in the concentration range from 2 × 1017 up to 2 × 1019 cm−3, at temperatures between 1.6 and 4.2 K. Magnetic fields up to 15 T induction were supplied by a superconducting coil (10 T) and a Bitter‐type magnet (15 T). In contrast to a commonly almost isotropic ratio of spin‐splitting to cyclotron‐splitting in semiconductors, this property shows an anisotropy ratio of more than 20 in n‐Bi2Te3. With knowledge of the electronic mass parameters of the lowest conduction band the angular variation and energy dependence of the corresponding g‐factors were obtained, where the magnitude and sign (g > 0) can he determined. The orbital part of the g‐factor is considerably large along two of the principal axes of the Fermi surface ellipsoids, but completely vanishes along the third principal axis. Hence the g‐factor essen‐rially describes an elliptic disc, the longest principal axis (g′2) tilted by an angle ϑ0 = 33.5° relative to the bisectrix axis within the mirror plane, while the second principal axis (g′1) is parallel to the binary axis. Perpendicular to this plane g′3 ≈ 2 is about an order of magnitude smaller. Values of gmc/(2mo) ≷ 1 may only be explained in a model of more than two interacting energy bands, where additional valence and conduction bands have to be tegarded.
Indirect Interactions and Nuclear Relaxation in TINO3
Authors: Villa M. , Avogadro A. 
Published: July 2007

AbstractMeasurements of the nuclear relaxations times T2T1and Tiq are performed on Tl203 and Tl205 in TINO3 from room temperature up to the melting point (210 °C). The indirect nuclear interactions and their effects on T2, T1 and T1q are discussed. From the experimental data, the values of the T1‐T1 indirect coupling constants and the values of the correlation times for Tl+ diffusion are deduced. Evidence for spin‐lattice relaxation processes driven by lattice vibrations is also shown.
Cyclotron Masses in Semiconducting Bi1−xSbx Alloys
Authors: Oelgart G. , Herrmann R. 
Published: July 2007

AbstractCyclotron resonance measurements are carried out on a great number of semiconductive Bi1–x‐Sbx; alloys in the range of 0.07 < x ≦ 0.17. The measuring frequencies are 35 and 70 GHz, and the measurements are carried out at helium temperatures. For the smallest cyclotron mass, a linear relationship to the antimony concentration is found. The more rapid increase of the great cyclotron masses and the associated increase of the elongation of the iso‐energy surfaces with increasing antimony concentration presents the greatest difficulty as compared to the known dispersion relations of the electron bands.
Long‐Wavelength Optical Phonons in Mixed Crystals. II. The persistence of common gaps
Authors: Jahne E. 
Published: July 2007

AbstractThe formalism developed in Part I in order to describe the long‐wavelength optical phonons in mixed crystals as a system of two coupled modes is now applied to the two mixed systems In1–xGaxP and GaAs1–xPx as representative ones for one‐ and two‐mode behaviour, respectively. Calculated mode frequencies, eigenvectors, and infrared oscillator strengths are presented and their behaviour is discussed in dependence on composition in both mixed systems. Contrary to earlier suggestions we state that in a traditionally named one‐mode system apart from the lattice‐like mode in addition an impurity‐like mode occurs which is always an in‐band mode. A two‐mode system is characterized by a gap between the two sets of optical phonons arising from a gap between the optical phonon bands of the end‐member crystals. A new criterion is given for one‐ or two‐mode behaviour in terms of an overlapping or the existence of a gap between the optical phonon densities of states of the end‐member crystals. It is suggested from several items that the persistence of common gaps of the end‐member crystals is a determining factor in the lattice dynamics of mixed systems.
Wavelength Modulation Spectra and Electronic Band Structure of SnS2 and SnSe2
Authors: Camassel J. , Schlüter M. , Kohn S. , Voitchovsky J. P. , Shen Y. R. , Cohen M. L. 
Published: July 2007

AbstractThe first detailed reflectivity and differential reflectivity measurements performed on the layer compounds SnS2 and SnSe2 for energies ranging from 2 to 6 eV are presented. The results are discussed on the basis of existing band structure calculations. The main structures in the optical response of SnS2 and SnSe2 are found to result from excitations from the top most anion p‐type valence states (crystal field split pz and pxy type levels) into the lowest conduction band (anion and cation s‐ and p‐type levels). The location of these structures in k‐space is found to be very close to symmetry determined “special k‐points” for SnS2.
Electron–Hole Droplet Formation Kinetics in Pure Germanium
Authors: Staehli J. L. 
Published: July 2007

AbstractThe Pokrovskii rate equations for electron–hole droplets (EHD) are generalized by adding a term accounting for statistical droplet nucleation in a supersaturated homogeneous gas of free excitons (FE). The surface tension, which is dominant in this new term, is also used to correct the evaporation coefficient. Examples of numerical solutions show that the size and density of the EHD created are strongly affected by how fast the FE density reaches the threshold at which embryo nucleation starts. Luminescence experiments using pulsed volume excitation are reported, and are in qualitative agreement with the theoretical predictions; e.g. a maximum in FE density occurring during EHD nucleation and a delay in the build‐up of the EHD phase are observed.
Luminescence of Highly Excited ZnTe Crystals
Authors: Opanowicz A. , Marinova K. , Liebing H. , Ruppel W. 
Published: July 2007

AbstractThe luminescent emission of undoped and P‐doped ZnTe single crystals is measured at 1.8 and 77 K under one‐photon excitation by a Cd‐He laser and a ruby laser. At 1.8 K, at low excitation intensities, radiative recombination through bound excitons and impurities predominates. At increasing intensity, inelastic exciton‐exciton collisions become the most important recombination process. At the highest excitation, ionisation of excitons occurs accompanied by band‐to‐band recombination. At 77 K, all bound excitons except those bound to donor–acceptor centres are destroyed. At low excitation intensities, radiation from the decay of free excitons, band‐to‐band transitions, and impurity centres are observed. At high intensities, radiation from inelastic exciton‐electron collisions is predominant.
Landau Emission in n‐InSb
Authors: Bauer G. , Overhamm M. , Grosse P. , Mütller W. , Gornik E. , Pötzl H. W. 
Published: July 2007

AbstractIn n‐InSb (n < 2 × 1013 cm−3) hot electron cyclotron‐resonance emission (Landau emission), absorption, and reflection are investigated at T = 4.2 K. Electric fields up to 20 V/cm and magnetic fields up to 5 T are applied to the samples. From a determination of the optical constants under hot electron conditions, the emission intensity profiles are deduced and the following transitions are identified: 1+ ↔ 0+, 1− ↔ 0− Landau level and 110 ↔ 000 impurity transitions. The population of the various states is not only influenced by the electric and magnetic field but also by the background radiation conditions as evidenced in the variation of the resistivity, the absorption and emission.
Positron Annihilation in Liquid Copper–Bismuth and Copper–Tin Alloys
Authors: Itoh F. , Kai K. , Kuroha M. , Takeuchi S. 
Published: July 2007

AbstractAngular correlation curves by the positron annihilation are measured on copper–bismuth and copper–tin alloys in the liquid state. A large positron affinity for bismuth and tin in these alloys is found in the concentration dependence of the angular correlation, which is ascribed to the atomic arrangement in the liquid alloys.
Raman and Vibronic Spectra of a New Oxygen Molecule Ion in CaF2
Authors: Bill H. , von der Osten W. 
Published: July 2007

AbstractSpectra are reported of an unidentified oxygen molecule center of monoclinic symmetry found in natural crystals of CaF2. Two alternative models consisting of a perturbed O and a O molecule ion substituting for two fluorine neighbors are discussed. In contrast to the well‐known unperturbed O center in CaF2, this new center has a visible absorption band. This band exhibits vibronic structure corresponding to an excited‐state vibrational frequency of (900 ± 10) cm−1 and coupling constants S ≈ 6 for this vibration and S ≧ 6 for the lattice phonons. This structure is enhanced in stress‐induced dichroism spectra. Resonance Raman scattering excited in this band shows the ground‐state vibrational fundamental at (1038 ± 1) cm−1 and the overtone at (2076 ± 2) cm−1. The polarization of these features is consistent with the 〈100〉 orientation of the center. The Lorentzian line‐shape, the width, and the relative intensity of the two‐ and one‐phonon scattering peaks are consistent with recent theories of the resonant Raman effect of impurity centers.
Fading Memory in the Theory of Electronic Excitation Transfer
Authors: Sokolov F. F. , Hizhnyakov V. V. 
Published: July 2007

AbstractWithin the framework of a model with the Hamiltonian of the electron–phonon interaction linear in the phonon operators, a memory function (MF) has been found which represents a kernel of the integral operator in a generalized master equation describing the migration of the electronic excitations in the crystal. At finite temperatures the microscopic elements of MF obtained fade in time. This is due to the presence of phonon‐assisted transitions which are competing with the zero‐phonon one. Therefore, the coarse‐grained MF has two characteristic fading times.
Influence of a Magnetic Field on the Susceptibility of a Heisenberg Ferromagnet near the Critical Point
Authors: Fechner B. J. 
Published: July 2007

AbstractStarting from the high temperature series expansion of the free energy, a magnetic equation of state is derived for the paramagnetic region valid for any crystal lattice and any spin value and solvable in various approximations. The equation is used in the Gaussian approximation to study the temperature and field dependence of the magnetic susceptibility near and above the Curie point. The results are compared with the experimental data for nickel. It is deduced that at the Curie temperature and in zero external field, the exchange coupling constant J = 132.6k‐ and the average atomic moment n0 = 0.65′B.
Nucleation Phenomena in Electron‐Hole Drop Formation in Ge and Si. II. Application to Observable Phenomena
Authors: Westervelt R. M. 
Published: July 2007

AbstractThe expressions derived in Part I for electron‐hole drop (EHD) nonequilibrium nucleation rates and dynamic equilibrium concentration are used to calculate the following results for Ge samples under continuous excitation: 1. very slow metastable time dependence of the EHD concentration, 2. EHD signal versus generation rate G for different histories of excitation, 3. mathematically well defined equilibrium and non‐equilibrium EHD thresholds, 4. procedure for accurately determining the EHD surface tension from threshold data at a given temperature, 5. modified EHD‐free exciton phase diagram including the effeets of metastability and finite lifetime, 6. EHD radius versus temperature for different historics of excitation. Results for the modified phase diagram of Si are also presented.
Photon Echo in CaWO4:Nd3+ and LiAl5O8: Cr3+
Authors: Samartsev V. V. , Usmanov R. G. , Khadiev I. Kh. , Kustov E. F. , Baranov M. N. 
Published: July 2007

AbstractThe conditions and peculiarities of the photon echo forming in monocrystals of LiAl5O8:Cr3+ and CaWO4: Nd3+ are experimentally investigated. The photon echo phenomenon for LiAl5O8:Cr3+ crystals is firstly discovered. A value for the transversal nonreversible relaxation time T2 of about 37 ns is found from the experimental dependence of the echo intensity on the time between the pulses. The dependence of the photon echo intensity on a stationary magnetic field is also investigated. When observing the photon echo in CaWO4:Nd3+, the resonance conditions are achieved with the help of a dye laser. A theory of the optical coherent response is developed for both the cases of Δt > T and of Δt < T (Δt pulse duration, T time of the transverse reversible relaxation). Satisfactory agreement of the experimental and theoretical results is found.
Optical and Electrooptical Properties of the Interfaces of Media
Authors: Timashev S. F. , Krykin M. A. 
Published: July 2007

AbstractIn terms of classical Maxwell electrodynamics using the nonlocal matter equation the reflection coefficient of a plane electromagnetic wave at the interface in presence of an arbitrary transition layer is obtained. Instead of traditional parameters of the layer (thickness, index of refraction) linear response functions to the electromagnetic perturbance are introduced.
Phonon Dispersion in V3Si
Authors: Achar B. N. N. , Barsch G. R. 
Published: July 2007

AbstractThe phonon frequencies of V3Si along the principal symmetry directions are calculated for a two band model from the microscopic dielectric screening approach. The d‐electrons are approximated by a δ2 band without interchain coupling. For the electron‐ion interaction a local Heine‐Abarenkov pseudopotential is used, with parameters very similar to those given before for V and Si. The results agree semiquantitatively with inelastic neutron scattering data and exhibit the observed softening of the TA mode in [110] and of the LA mode in [100]. In addition, LA mode softening along [110] and [111] is predicted. The Γ12 optic mode shows a small decrease with decreasing temperature.
Temperature Dependence of Nuclear Quadrupole Resonance in Layer‐Type Crystals
Authors: Lyfar D. L. , Goncharuk V. E. , Ryabchenko S. M. 
Published: July 2007

AbstractThe temperature dependences of the NQR frequencies of halides in layer‐type crystals of CdBr2, Cdl2(4H), Pbl2(2H, 4H)are studied within 10 to 300 K temperature range. The NQR frequency grows weakly with temperature in CdI2, and CdBr2, whereas it decreases in PbI2. For PbI2 the frequency depends linearly on the temperature beginning at T≧20K. The difference of the NQR frequencies of nonequivalent nuclei in (4H) crystals corresponds to the calculated differences of the ion contributions to the gradient, while the sum of the calculated ion and covalent contributions does not account for the observed values of the NQR frequencies. This discrepancy is suggested to arise from the strong polarization of the covalent bonds by the internal electric field in the layer crystals. The growth of the NQR frequency with increasing temperature in CdI2 and CdBr2 is shown to be due to the fact that in these crystals the ionic and covalent contributions to the gradient are of opposite sign. In PbI2 the temperature dependence of the NQR frequency is associated with the peculiarities of the phonon spectrum of the layer crystals.
Excitons and Electron‐Hole Plasma. A Ground State Calculation
Authors: Zimmermann R. 
Published: July 2007

AbstractFor calculating the energy per pair ℰo in a highly excited semiconductor a Green's function theory is presented which covers the whole density range from the exciton limit to the electron‐hole plasma at high densities. Extensive use is made of the analogy to the excitonic phase theory which is extended here to include correlation and dynamical screening. In a model with equal‐mass parabolic bands a rather shallow minimum of E, somewhat below the exciton energy is found at the density rs = 3.
Phase Transitions in Perovskite Crystals
Authors: Darlington C. N. W. 
Published: July 2007

AbstractAtomic displacements are analysed that occur at phase transitions in perovskites caused by the condensation of zone boundary modes. It is shown that for phases in which modes with wave vectors at the M‐point, ½ (a* + (a* + b*), and the R‐point, ½(a* + b* + c*), have condensed, static displacements associated with a normal mode with wave vector ½ + c* of the X‐point are also present. Such induced displacements are to be expected from Landau's theory of continuous phase transitions when terms that couple the parameters X, R, and M me included in the free energy expression. Such an expression is applied to (Na, K)NbO3, and the results derived from it are compared with the experiment.
Stress Dependence of the (El, El + Δ1), Transition in Germanium
Authors: Kühn W. 
Published: July 2007

AbstractThe influence of uniaxial stress on the optical δE1, E1 + Δ1), transition of Ge is reinvestigated theoretically and experimentally. Existing theoretical approaches are extended by explicitly considering the stress dependence of spin‐orbit interaction. For comparison of the results obtained the stress dependence of the (E1, E1, + Δ1), structure is measured by means of electroreflection, where special care is given to the potential stability at the electrolytesemiconductor interface. It can be shown that the combined influence of stress‐induced valence band coupling and modification of the spin‐orbit splitting allows to explain the measured stress dependences.
Heat Conductivity Measurement at V3Si in the Temperature Range 4.2 to 80 K
Authors: Hegenbarth E. , Schmidt B. 
Published: July 2007

AbstractThe heat conductivity of a V3Si‐crystal is measured in the temperature range from 4.2 to 80 K by a stationary method. The analysis of the measuring results shows, that the thermal conductivity of V3Si above the transition temperature can be described by the scattering of electrons on defects and phonons. From the behaviour of the thermal conductivity below the transition temperature the energy gap of V3Si can be determined.