bluuebunny/crossref_metadata_2025 huggingface dataset:
X‐Ray Double and Triple Crystal Diffractometry of Silicon Crystals with Small Defects
Authors: Holý V. , Kuběna J.
Published: July 2007

Intensities of X‐ray diffraction and X‐ray diffuse scattering are calculated on the basis of the kinematical approximation of the statistical dynamical diffraction theory for crystals with randomly distributed small defects. Various approaches are derived for defect parameter determination (the density, size, and strength of defects) and some of them are compared with previously published X‐ray methods. The theoretical results obtained for two various defect models are proved by double and triple crystal diffractometry measurements.
Size Changes of Lattice Parameters in Small Particles of Hexagonal Crystals
Authors: Gamarnik M. Ya.
Published: July 2007

The role of particle size in the change of lattice parameters is theoretically and experimentally studied. A monotonous increase of the lattice parameters a and c with decreasing particle size is established by means of X‐ray analysis for a series of fine‐dispersed powder samples of quartz obtained by fractionation. It is shown that the effect cannot be explained by impurity or intrinsic defects in the small quartz particles. Analytical expressions are obtained to calculate the size changes of the lattice parameters in crystals with hexagonal syngony and external shape of a hexagonal prism. The expressions derived are based on the size dependence of the internal stress – the intracrystalline pressure stipulated by the interaction of the elements of the crystal charge lattice. Calculations made for the ion charge lattice of quartz evidence the monotonous increase of the lattice parameters with decreasing particle size and agree with the results of the X‐ray experiment.
Microhardness of AN−1 BN + 1 C Chalcopyrite Semiconductors
Authors: Kumar V. , Prasad G. M. , Chandra D.
Published: July 2007

Microhardness of AN−1 BN + 1 C28−N (N = 2, 3) tetrahedrally coordinated chalcopyrite semiconductors are calculated using plasma oscillation theory of solids. The calculated values are in fair agreement with the values reported by several workers and the experimental values.
Green's Functions for Tight‐Binding Electrons on a Square Lattice in a Magnetic Field
Authors: Thamm R. , Kolley E. , Kolley W.
Published: July 2007

Tight‐binding electrons on a two‐dimensional lattice with a uniform magnetic flux per unit plaquette are described by the method of Green's functions. In analogy to Harper's equation for the energy dispersion a matrix equation for the Green's function is given. Analytical and numerical results for the Green's functions are presented at two special fluxes.
Practicable Method for Estimating Thermal Depths from Phonon‐Broadened Photoexcitation Cross‐Section Bands. II. Analytical Representation for Neutralized Centres
Authors: Pässler R.
Published: July 2007

The zero‐phonon threshold behaviour of thermally broadened absorption cross‐sections σ (hv, T) is examined for processes of photoexcitation of electrons and holes from deep acceptor and donor levels into unperturbed Bloch states of nearly parabolic conduction or valence band edge regions, respectively. In terms of parabolic cylinder functions, analytical descriptions are given for alternative photon energy dependences of the transformed excitation curves ϱ (hv, T) ≡ exp (− hv/2kT) σ (hv, T). Their vertex positions hv′ (T) are found to be shifted with respect to the thermal depths J (T) in question by amounts of about (1 + f) 2kT, where f = 0 or 1, for allowed or forbidden transitions. A remaining uncertainty up to ∓ kT, at maximum, is due to possible variations of the electron–phonon coupling from an extremely weak (vanishing) up to an extremely strong one. From our assessment of optical electron excitation data given by Wang et al. for the B level in GaAs and by Kullendorff et al. for the CuB level in InP we conclude that 1. both transitions are allowed (f = 0) in dipole approximation, 2. both levels are located at low temperatures by about 0.1 eV below the middles of the respective forbidden gaps, and 3. the actual distances of both levels from the respective conduction band edges decrease by about (0.08 ± 0.01) eV when the temperature increases from very low up to room temperature. This “depth shrinkage effect” is pointed out to have important consequences for theoretical interpretations of thermal electron capture properties on the basis of the nonradiative multiphonon (NMP) capture mechanism. The complementary processes of optical hole excitation at both centres appear to be markedly affected by Rydberg‐like states extending up to several dozens of meV above the respective valence band edges.
Effect of a Tilted Magnetic Field on Energy Quantization and the Einstein Relation in Quantum Wires of Semiconductors
Authors: Das N. R. , Chakravarti A. N.
Published: July 2007

A perturbational approach is used to investigate the effect of a tilted magnetic field on the energy eigenvalues and the diffusivity‐mobility ratio (the Einstein relation) of the electrons in a quantum wire (a quasi‐one‐dimensional structure) of an n‐type wide‐gap semiconductor having a rectangular cross‐section. The results computed corresponding to the temperature 4.2 K are presented, taking degenerate n‐GaAs as an example.
Optical Destruction and Restoration of {F, In2+} Pairs in KBrIn Crystals
Authors: Braslavets P. F. , Kalniņš A. , Pļaviņa I. , Popov A. I. , Rapoport B. I. , Tāle A.
Published: July 2007

Using photostimulated luminescence of {F, In+} defect pairs in KBr‐In crystals, their F‐light optical destruction under excitation has been investigated. It is shown that the effective absorption cross‐section of F centers manifests itself in the kinetics of photostimulated luminescence of {F, In2} pairs under constant F‐stimulation. It is shown also that the resulting products ‐ {va+, In+} pairs ‐ may be again converted to {F, In2+} under C‐absorption band illumination.
TEM and DLTS Investigations of the Electrical Activity of Σ17 and Σ41 Grain Boundaries in Germanium
Authors: Wang N. , Haasen P.
Published: July 2007

The electronic defect states of near Σ17 and Σ41 tilt grain boundaries (GBs) in germanium are determined by means of deep‐level‐transient spectroscopy (DLTS). The results indicate energy levels Ec − 0.56 ± 0.02 eV and Ec − 0.38 ± 0.02 eV for the Σ17 and Σ41 GBs, respectively. The structure of these GBs is further studied by transmission electron microscopy (TEM). Because of a small twist component of the misorientation in the Σ17 bicrystal, GB dislocations have jogs. These jogs may create dangling bonds which are considered to be at the origin of the electrical activity of the Σ17 GB. No structural feature has been found which may create jogs or dangling bonds in the small angle Σ41 GB.
Molecular Dynamics Simulations of Ion Beam Mixing Effects of the Interatomic Forces
Authors: Mazzone A. M.
Published: July 2007

Results are given of molecular dynamics simulations of the intermixing of a metallic bilayer taking place during the thermalization of the cascade. These simulations indicate a complex dependence of mixing on the type of bonding and on the elastic properties of the target.
Balance Equations for Hot Electron Transport in an Arbitrary Energy Band
Authors: Lei X. L.
Published: July 2007

By distinguishing the center‐of‐mass momentum from its velocity the hot‐electron balance‐equation transport theory developed by Lei and Ting for a parabolic band is extended to carriers moving in a single arbitrary energy band. An ensemble averaged inverse effective mass tensor is introduced to describe the collective response of the Bloch electrons to an external electric field. This allows to use a two‐parameter (the center‐of‐mass momentum and the electron temperature) description of nonlinear carrier transport in a uniform electric field. The resulting effective force and energy balance equations turn out to be so succinct that one can perform any hot‐carrier‐transport calculation for a realistic energy band with only a little more numerical effort than that in the parabolic band cases.
Temperature Dependence of the Acoustic Deformation Potentials in CuInSe2 and CuInTe2
Authors: Rincón C. , Fernández B.
Published: July 2007

From the analysis of the variation of the energy gap with temperature and pressure in the chalcopyrite semiconducting compounds CuInSe2 and CuInTe2, the valence and conduction band acoustic deformation potentials are calculated as a function of temperature between 50 and 300 K. It is found that these parameters increase with temperature for both compounds. The results indicate, specially for CuInTe2, that the variation is important enough to be taken into account in the fit to the mobility data.
Low‐Temperature Dependence of the EPR Spectra of Gd0.5RE0.5Ba2Cu3O7−δ Compounds in the Tetragonal Phase
Authors: Guskos N. , Likodimos V. , Londos C. A. , Windsch W. , Metz H. , Koufoudakis A. , Mitros C. , Gamari‐Seale H. , Niarchos D.
Published: July 2007

The EPR spectra of perovskite ceramics of Gd0.5RE0.5Ba2Cu3O7−δ type (RE  La, Pr, Nd, Sm, Eu, Dy, Ho, Y, Er, Tm, Yb, and Lu) in tetragonal (non‐HTC) phase are investigated as a function of temperature in the range 40 to 3 K. A signal arising from the isotropic exchange interaction of coupled pairs of Cu2+ ions is detected. The intensity of this signal is strongly dependent on the RE ions and its g‐factor varies with temperature. Magnetic measurements show the existence of ferromagnetic anomalies at these low temperature. In the orthorhombic phase, these materials exhibit a superconducting phase transition in the range of 85 to 95 K, apart from the samples with La ions where a TC ≈ 69 K is found and the samples with Prions where a superconducting state can not be observed.
Optical Properties of (F)H Centers in NaCl:SH− Crystals
Authors: Casalboni M. , de Viry D. , Grassano U. M. , Luci A. , Palummo M. , Scacco A.
Published: July 2007

The optical properties of the (F)H center are investigated in NaCl: SH−. The negative sulfur ion stabilizes the center in the same way as the oxygen ion does in the better known system NaCl: OH−. The spectral properties, absorption, emission, and excitation spectra, in the two cases are quite similar, but in NaCl: SH− the F centers are more easily bound to the sulfur ion and this fact yields a greater stability to these centers.
The Effect of Fractal Dimensionality on Self‐similar Grain Growth
Authors: Streitenberger P. , Förster D.
Published: July 2007

The modified LSW‐theory of second phase particle coarsening for irregularly shaped particles recently presented is extended to fractal‐like grain growth in polycrystalline solids, where the grain boundaries can be characterized by a fractal dimensionality D. It is shown that a fractal dimension D > 2 leads to a considerable broadening of the scaled grain size distribution function in the pure‐drift approximation.
Renormalization of Electron Mass by 2D Coulomb Interactions in a Laterally‐Microstructured Silicon Inversion Layer
Authors: Parish J. L.
Published: July 2007

The effect is considered of high‐momentum plasmons on electrons in a 2D EG. It is found that the high‐momentum, low‐velocity plasmons can form electron‐plasmon pairs. Renormalizing the mass due to Coulomb interactions of these paired electrons, it is possible to explain very accurately the large mass shift measured in far‐infrared transmission experiments.
Fluctuation Effect on the Excess Conductivity of Layered Superconductors
Authors: Ye Zhen
Published: July 2007

By using thermo field dynamics (TFD) and Ginzburg‐Landau theory, the fluctuation effects on conductivity are studied for layered superconductors above critical temperature. It is argued that the broadness of the resistivity is due to the decoupling of electron tunneling among layers. The results are compared with experiments.
Electromagnetic Properties of a Type‐II Superconducting Cylinder
Authors: Saif A. G.
Published: July 2007

The electromagnetic field distributions and solutions for an infinitely long superconducting cylinder placed parallel to an infinitely long normal conducting wire carrying an alternating current, are obtained. The propagation behavior of the fields within the cylinder is investigated in terms of the propagation constant as a function of the London penetration depth. The power loss on the cylinder surface is calculated.
Intergranular Diffusion in Real Polycrystals
Authors: Kondratev V. V. , Trachtenberg I. Sh.
Published: July 2007

An analysis of grain‐boundary diffusion for a real polycrystal is given. The non‐uniformity of diffusion permeability of the grain interface and the non‐uniform pumping of diffusants from the boundary into the bulk due to dislocations are taken into account. The meaning of the grain‐boundary and volume diffusion coefficients for different annealing regimes is elucidated. The conditions of the true C‐regime for a polycrystal is obtained.
The Influence of the Exchange Interaction on Atomic Ordering in Alloys with F.C.C. Lattice
Authors: Binnatov K. G. , Masharov S. I. , Rybalko A. F. , Safarov D. A.
Published: July 2007

The investigation of ordering in binary substitution alloys with f.c.c. lattice, in which magnetic ordering can exist along with the f.c.c. structure is performed in second approximation of the Kirkwood method. It is shown that the exchange interaction essentially effects the development of atomic ordering. The possible types of phase diagrams are investigated. It is shown that the correlation considered makes a correct description possible of the kind of order‐disorder phase transition in the alloys of L1 superstructure. The correlation contributions to the heat capacity of alloys are calculated.
Crystal Structure and Electronic Properties of (2,6‐DMbTN)3(ClO4)2
Authors: Söderholm S. , Von Schütz J. U. , Noreland J. , Olovsson I. , Hellberg J.
Published: July 2007

Crystal structure, conductivity, and magnetic properties of the monoclinic cation radical salt 2,6‐dimethoxy‐3,6‐bis(methylthio)naphthalene‐perchlorate [(2,6‐DMbTN)3(ClO4)2] are presented. The trimerization of the 2,6‐DMbTN stack causes the salt to be semiconducting, σ(300 K) ≈︁ 0.5 S cm−1, with a band gap of 0.5 eV. The susceptibility is quite low 0.02 spins/complex at 300 K, above 200 K the susceptibility originates from the thermal excitation of triplet spin excitons of the Wannier type from an antiferromagnetically coupled ground state with J = 0.11 eV. The electrical and magnetic properties below roughly 200 K are governed by deep defect levels in the band gap. The unusually narrow ESR linewidth for a sulphur containing ion‐radical salt, 0.8 × 10−4T at 300 K, implies that the electron structure is “one‐dimensional” and/or the spin density on the sulphur atoms is low.
Chemical Bond Approach to the Dielectric Constant of Semiconductors
Authors: Esperidião A. S. C. , Brum J. A. , Ramos J. G.
Published: July 2007

The extension of the Nucho, Ramos, and Wolff method for the non‐tetrahedral semiconductors was made by Brum and Ramos. This extended NRW method is applied to the elemental, group III‐V, and group II‐VI semiconductors and better accuracy is claimed. The ionicity for the II‐VI semiconductors (zinc‐blende structure) is also calculated and confirms the ionicity scale established by Phillips. Again, Coulson's definition is reconciled with that of Phillips.
Volume Dependence of Effective Charge Parameter and Photoelastic Behaviour of Alkali Halide Crystals
Authors: Dikshit U. C. , Kumar M.
Published: July 2007

An analysis of the volume dependence of effective charge parameter and electronic strain polarizability parameter is presented for alkali halide crystals. The deformation dipole model due to Hardy is used along with the seven different potential forms for the short‐range overlap repulsive energy and the modified form of the additivity rule for the crystalline state polarizabilities as proposed by Raghurama and Narayan. The effective charge model due to Hanlon and Lawson is adopted to obtain the strain polarizability parameter within the framework of Coker's theory. The results are compared with the available experimental data. It is found that the results are fairly insensitive to the form of the overlap repulsive potential used whereas they depend sensitively upon the choice of polarizability.
Polaron Pairing in Multi‐Layer Structures: II. Interlayer Bipolaron States in Structures with Quantum Wells
Authors: Pokatilov E. P. , Beril S. I. , Fomin V. M. , Ryabukhin G. Yu. , Goryachkovskii E. R.
Published: July 2007

A structure containing conductive planes separated by polar dielectric interlayers is shown to possess the possibility to create bipolaron states by charge carriers moving in the different planes. The attraction is due to the inertial polarization induced by similar charges in polar interlayers which separate them from each other (interlayer polaron pairing). The potential energy of electron‐electron interaction and the bipolaron binding energy are calculated as functions of material and geometric parameters of the structure.
On the Electrical Conductivity of PrREBaCuO (RE = Er, Yb, Lu, Tm): Compounds in Orthorhombic Phase
Authors: Triberis G. P. , Guskos N. , Svoboda P. , Vaŝek P. , Z̆emanova D. , Koufoudakis A. , Mitros C. , Gamari‐Seale H. , Niarchos D.
Published: July 2007

Experimental results showing the temperature dependence of the conductivity of a variety of PrREBaCuO (RE = Er, Yb, Lu, and Tm) compounds are presented. The semiconducting behavior of the conductivity observed is analysed using In σ ∼ T−2/5 and In σ ∼ T−1/4 laws suggested, using the small‐polaron hopping model by Triberis et al., for the high‐ and low‐temperature regime, respectively. The agreement of the experimental data with these models is very satisfactory, giving a meaningful interpretation of the transport mechanism of these materials.
Electrical Properties of Silver‐Doped As2S3
Authors: Goyal N. , Vohra A.
Published: July 2007

Temperature and frequency dependent ac conductivity studies are made for silver‐doped a‐As2S3. The temperature dependence of σac for the As2S3‐Ag system can be well explained by correlated barrier hopping model. Numerical calculations agree well with the experimental results. The energy levels and the concentration of charged defects are also estimated for these materials.
Configurational Energy Change Caused by Slip in Short‐Range Ordered Ni–Mo
Authors: Schwander P. , Schönfeld B. , Kostorz G.
Published: July 2007

Based on short‐range order parameters determined from X‐ray diffuse scattering measurements on a Ni–11.2 at% Mo single crystal and effective pair potentials obtained from applying the inverse Monte‐Carlo method, the configurational energy change caused by the successive slip of a variable number of unit dislocations on the same {111} plane is calculated. A fraction of 97% of the diffuse antiphase boundary energy of 25 mJ/M2 has to be provided by the first slip step. The glide of subsequent dislocations on the same glide plane is thus favored which leads to planar slip as observed by transmission electron microscopy. Monte‐Carlo simulations also show that the diffuse antiphase boundary energy depends more sensitively on the concentration of the alloy than on the temperature where thermodynamic equilibrium was established.
Internal Solid State Reactions
Authors: Schmalzried H.
Published: July 2007

Internal solid state reactions have first been studied as internal alloy oxidation reactions. The concept of this type of solid state is reactions is generalized and applied to nonmetallic systems. Firstly, interal oxidation reactions with (A,B)O solid solutions are discussed. Then it is shown how one can induce internal reduction reactions with systems such as (A,B)O or (A,B)3O4. A third type of internal solid state reactions is met if A and B diffuse in the solvent crystal C from opposite sides and form a stabel compound AB internally. Finally we discuss AO–BO interdiffusion in in extended solid solutions. It is shown that diffusion potentials can build up which change the local oxygen potential to such an extent that new phases form internally.
Pairing between Fast Diffusing Donors and Shallow Acceptors in p‐CdTe
Authors: Zoth G. , Ridel F. G. , Schröter W.
Published: July 2007

Defect reactions of mobile intersitial donors Di with immobile substitutional acceptors Acd in the spacecharge region of Schottky contacts on p‐type CdTe are observed at about room temperature. The results are discussed in terms of formation and dissociation of defect complexes (AcdDi). For copper diffused samples the dissociation kinetics of a complex, presumably (CuCdCui), is determined. The dissociation enthalpy of 0.74 eV leads to the rather low value of 0.2 to 0.3 eV for the migration enthalpy of Cui. Furthermore it is shown that thermally stimulated capacitance (TSCap) yields a step with a shape characteristic of defect complex dissociation and drift of the intersitial donor.
A Combined Cluster‐Dynamic and Deterministic Description of Decomposition Kinetics in Binary Alloys with a Tendency for Clustering
Authors: Kampmann R. , Ebel Th. , Haese M. , Wagner R.
Published: July 2007

A novel numerical model is devised to describe the decomposition kinetics in metastable binary alloys with a positive pair exchange parameter. The stochastic process of nucleation and growth of precipitates is simulated on grounds of a novel cluster‐dynamical model, which for a description of the later stages is linked to a deterministic model. The discussion of the precipitation kinetics in a Cu‐Co type alloy yields the interpretation of kinetic experiments in terms of classical theories to be inadequate.
Configuration Dynamics in a Simulated Finite‐Temperature Transition Metal Glass
Authors: Teichler H.
Published: July 2007

The dynamics of configurations and of ‘nearest‐neighbour’ relations are investigated for an amorphous metallic alloy by molecular dynamics computer simulations covering the whole range of temperatures up into the liquid state. The configuration dynamics are expressed in terms of nearest‐neighbour auto‐correlation functions, where at finite temperatures ‘nearest neighbours’ are defined with regard to the force‐free configuration closest to the actual atomic arrangement. Results are presented for an atomistic model adapted to the recent hybridized nearly‐free‐electron‐tight‐binding‐bond description of Ni0.5Zr0.5. The results agree well the predictions of the extended mode‐coupling theory for the time dependence of density fluctuations and related properties around the glass‐liquid transition in ‘real’ systems.
Optical Excitation and Bose Condensation of Excitons in Low‐Dimensional Systems
Authors: Glutsch S. , Bechstedt F. , Zimmermann R.
Published: July 2007

The linear optical response of excitons is studied in low‐dimensional semiconductor systems – bulk, quantum well, quantum wire – pumped near the energy of the lowest exciton transition under inclusion of many‐body interactions. Attention is focused especially to three aspects: (i) influence of dimensionality on the Coulomb interaction, (ii) different behaviour of the optical response for pump frequencies lower than or close to the nominal exciton resonance, and (iii) time dependence until reaching stationarity. For this purpose the two‐band Bloch equations and the generalized gap equations are simultaneously solved. Far from the absorption edge results for the excitonic dynamical Stark effect are confirmed, at least in the collision‐free limit. For pump frequencies near or above the nominal exciton resonance an additional stationary solution of the gap equation is observed that is not vanishing for zero pump field and, therefore, characterizes a Bose‐like condensation in the electron–hole system. In the fully time‐dependent regime it is studied how this so‐called “excitonic insulator” can be reached during the time evolution. The resulting optical spectra as well as the existence and observability of the condensed phase are discussed.
Lattice Dynamics of Methanol
Authors: Weng S. X. , Anderson A.
Published: July 2007

A simple ten parameter force constant model is described and applied to the vibrations of molecular methanol. The model is extended to crystalline methanol in its ordered α‐phase, with the use of twelve parameters describing the van der Waals intermolecular interactions, three representing those for the hydrogen bond and twelve for the intramolecular forces. Comparision with experimental frequencies from Raman and infrared studies of CH3OH, CH3OD and CD3OD shows acceptable agreement. The effects of hydrogen bonding on the intramolecular forces are briefly discussed.
Brillouin Instability in Longitudinally Magnetized Crystals with High Dielectric Constant
Authors: Gadkari P. , Ghosh S.
Published: July 2007

An analytical investigation is made of the stimulated Brillouin Scattering of a plane polarized intense electromagnetic wave propagating parallel to the applied magnetic field in a n‐type semiconducting plasma with high dielectric constant. The general dispersion relation is obtained and solved for both, the cases of scattered electromagnetic waves (i.e. in the case of left hand and right hand circularly polarized waves) to study the threshold condition and the growth rate of the unstable Brillouin mode at a pump amplitude well above the threshold. It is found that the threshold pump amplitude is independent of applied magnetic field and is the same for both types of polarization. The growth rate of the unstable Brillouin mode has linear dependence on the magnetic field. Numerical estimations are made for n‐BaTiO3irradiated with a pulsed 10.6 μm CO2 laser to obtain the necessary electric field.
Study of Precipitated Phases in Mn2+ Singly Doped KBr Crystals by Optical Spectroscopy and EPR
Authors: de Lucas M. C. Marco , Rodríguez F. , Moreno M.
Published: July 2007

The aggregation states of Mn2+ in KBr Mn2+ doped crystals are investigated using optical techniques. Excitation and luminescence spectra as well as lifetimes are obtained as a function of temperature of different crystals. It is demonstrated that at least three different Mn2+ precipitates are formed inside the KBr. Two of them contain MnBr64‐ units with slightly distorted octahedral symmetries, while the third one involves two non‐equivalent tetrahedral MnBr42‐ units. These centers exhibit a broad green luminescence band.
Time‐Resolved Four‐Wave Mixing in GaAs/AlAs Quantum Well Structures
Authors: Göbel E. O. , Koch M. , Feldmann J. , von Plessen G. , Meier T. , Schulze A. , Thomas P. , Schmitt‐Rink S. , Köhler K. , Ploog K.
Published: July 2007

Femtosecond nonlinear laser spectroscopy is applied to study the dynamics of coherent optical excitations in semiconductors. In particular, excitonic excitations in quantum wells and superlattice structures are considered. Two examples demonstrate that four‐wave mixing experiments with time‐resolved detection of the nonlinear signal are able to distinguish between quantum beats and polarization interference.
Coherent and Incoherent Exciton Dynamics in Al1−yGayAs/GaAs Multiple Quantum Wells
Authors: Oberhauser D. , Pantke K. H. , Langbein W. , Lyssenko V. G. , Kalt H. , Hvam J. M. , Weimann G. , Klingshirn C.
Published: July 2007

Picosecond lasers allow to detect the dynamics of excitons directly in the time domain. Here examples of such experiments are presented for high quality AlGaAs MQW samples at low temperatures and in the low and medium density regime. The phase relaxation time T2 is measured in FWM experiments and quantum beats are observed between the hh and lh exciton and between exciton and biexciton states. A value of the biexciton binding energy of 1.75 meV is deduced in agreement with our luminescence data. The T2 times are compared with scattering times τ obtained from diffusion measurements using ps and ns laser‐induced gratings. Interface‐roughness scattering occurs only for thermalized excitons. The influence of increasing exciton‐exciton scattering with density is demonstrated for (magneto‐) excitons and Landau levels. Finally the luminescence dynamics of excitons is investigated for resonant and nonresonant excitation. The influence of cooling (intraband relaxation) is discussed and a simple model is presented to explain the generally observed increase of the lifetime T1 with lattice temperature.
Real Time Approach to Relaxation and Dephasing Processes in Semiconductors
Authors: Hartmann M. , Schäfer W.
Published: July 2007

Starting from the kinetic equations for the nonequilibrium Green's functions, a practical method is proposed which allows to solve the set of equations for the two‐time propagators numerically. In contrast to the usual semiconductor Bloch equations with phenomenological damping relaxation and dephasing processes are treated microscopically for the case of coupling to a bath of LO‐phonons. Beside a standard approximation for the treatment of LO‐phonon scattering, no further approximations are performed. For the low‐density limit we compare exact results with approximate treatments, as the Marcovian and non‐Marcovian approximations resulting from a density matrix approach. First results concerning nonlinear optical properties are presented.
Monte‐Carlo Simulations of the Coulomb Boltzmann Kinetics in a Quasi‐Two‐Dimensional Electron–Hole Plasma for Various Screening Models
Authors: El Sayed K. , Haug H.
Published: July 2007

For carrier‐carrier scattering the relaxation kinetics of a quasi‐two‐dimensional degenerate electron‐hole plasma is studied in the framework of the Boltzmann equation using both a static and a dynamic RPA screening. The time development of an initial nonequilibrium distribution is calculated by ensemble Monte‐Carlo simulation. A comparison of the results for both screening models for the energy relaxation and the total scattering rate yields conditions for the validity of static screening.
Thermalization of Photoexcited Carriers in Bulk and Quantum Wire Semiconductors
Authors: Lugli P. , Rota L. , Rossi F.
Published: July 2007

A theoretical study of the internal thermalization of high‐energy carriers created by laser excitation in bulk and quantum wire semiconductors is presented. For the bulk case the results of our Monte Carlo simulation are compared to luminescence up‐conversion experiments used to monitor the spectral and temporal evolution of the photoexcited carrier distributions with a time resolution of about 100 fs. The agreement between theory and experiment is very good and shows that the Coulomb interaction among carriers is responsible for the initial ultrafast thermalization. On the contrary, a much slower thermalization is found theoretically in quantum wires, which can be mainly attributed to the reduced efficiency of intersubband processes and to the reduced effect of electron‐electron intrasubband scattering. Available experiments seem to confirm such findings.
Sum Rules for Nonlinear Optical Susceptibilities
Authors: Bassani F. , Scandolo S.
Published: July 2007

New sum rules are obtained for the dispersive and dissipative parts of the nonlinear susceptibilities, applicable to pump and probe experimental situations. A three‐level model shows how they clarify the nonlinear lineshapes. A detailed calculation on atomic hydrogen shows how the sum rules allow a global estimate of the magnitude of nonlinear effects.
Absorption Edge Singularities for Non‐Equilibrium Carrier Distributions
Authors: Foing J.‐P. , Joffre M. , Jackson M. K. , Oudar J.‐L. , Hulin D.
Published: July 2007

Non‐equilibrium carrier distributions are studied in GaAs by monitoring the induced bleaching in femtosecond pump–probe experiments. Unchirped pump pulses are required in order to avoid asymmetrical coherence oscillations. The observation of an induced absorption on the high‐energy side of the hole burning is explained in terms of absorption edge singularities, emphasizing the importance of many‐body effects in highly excited semiconductors.
Electron–Hole Plasma Decay Time in GaAs/(Ga, Al)As Quantum Wells
Authors: Bongiovanni G. , Staehli J. L.
Published: July 2007

A steady state photoluminescence experiment is reported investigating the lifetime of an electron–hole plasma confined in a quantum well. The results are compared with theoretical evaluations of the pure radiative decay rate and good agreement is found. The effects caused by the filling of electron and hole subbands are also discussed.
Tunneling of Charged Particles through a Crystal with Narrow Conduction Band in the Presence of an Electric Field
Authors: Sugakov V. I. , Yatskevich S. A.
Published: July 2007

The energy spectrum and tunneling probability are obtained for charged particle tunneling through a crystal with narrow conduction band in an electric field. It is also shown that the current–voltage characteristic of such a system has a region with negative differential resistance which does not arise in the usual way (due to a resonant level) but due to the small width of the conduction band. Also the role of surface interaction is investigated.
Electronic Structure and Optical Characteristics of Alkaline‐Earth Oxides
Authors: Stepanyuk V. S. , Szasz A. , Grigorenko A. A. , Katsnelson A. A. , Farberovich O. V. , Mikhalin V. V. , Hendry A.
Published: July 2007

The electronic structure of MgO, CaO, SrO, and BaO oxides is calculated by the linear augmented plane wave method. From the electronic structure of the compounds their optical parameters are derived. A satisfying correspondence with the experiments is observed.
Ti Deep‐Level Delta‐Doping in GaAs Schottky Diodes: Capacitance–Voltage Analysis
Authors: Piprek J. , Krispin P. , Kostial H. , Lange C. H. , Böer K. W.
Published: July 2007

Deep‐level Ti centers delta‐doped within Al/n‐GaAs Schottky diodes are investigated by CV‐analysis. CV‐profiles are modeled as a function of deep‐level energy and local center distribution using 1D device simulation. The deep‐level analysis is applied to MBE‐grown GaAs samples with almost delta‐like Ti doping (half‐width 12 nm) yielding a double peak structure of the GV‐profile. Besides the well‐known Ti acceptor level in GaAs a yet unknown deeper level at aboutEc–Er= 0.5 eV is observed.
Impurity‐Induced Core–Valence Luminescence in Halide Compounds
Authors: Voloshinovskii A. S. , Mikhailik V. B. , Syrotyuk S. V. , Rodnyi P. A.
Published: July 2007

Investigation of the impurity‐induced core–valence luminescence (IICVL) in doped RbCl:Cs, SrCl2:Cs, and RbCaCl3:Cs crystals is made under pulsed X‐ray excitation (δt ≈ 1 ns). It is shown that the spectral structure and emission intensity of IICVL is determined by the spatial structure of the nearest anion environment of the cesium ion. The model of an isolated emission centre (cluster [CsCln]) for the interpretation of the IICVL peculiarities is proposed.
Time‐Resolved Polarization Spectroscopy and Off‐Centre Position of Excited Ga+ Ions in NaCl:Ga Crystal
Authors: Nagirnyi V. , Stolovich A. , Zazubovich S. , Janson N.
Published: July 2007

In the temperature region from 0.35 to 4.2 K the decay kinetics of the triplet AT and Ax emission of NaCl:Ga crystals as well as the spectral and polarization charcteristics of the single decay components of the emission are investigated. An off‐centre displacement of the Ga+ ion in the tetragonal (T) minima of the relaxed excited state (RES) is found. No manifestations of the off‐centre displacement of the Ga+ ion in the trigonal (X) minima are observed. It is shown that fast tunnelling transitions between various equivalent off‐centre positions of an excited Ga+ ion in the plane normal to the tetragonal Jahn‐Teller distortion axis lead to the tunnel splitting of the metastable as well as of the radiative T minima of the triplet RES of the Ga+ centre. It is evident from the existence of two slow and two fast components in the low‐temperature decay kinetics of the AT emission. In case of NaCl:Ga the energy (δ) of tunnel splitting of the radiative minimum is assumed to be larger than the energy (2E) of the spin–orbit splitting of this minimum which is caused by the lowering of the RES symmetry due to the off‐centre displacements of the Ga+ ion. As a result, off‐centre effects are not observable in the ODMR spectra of Ga+ centres in NaCl:Ga crystals, unlike Ga+ centres in other alkali halides, where the energy δ is probably smaller than 2E.
Electroabsorption in Conjugated Polymers
Authors: Horváth Á. , Bässler H. , Weiser G.
Published: July 2007

Electroabsorption measurements are peformed with solution cast films of poly‐(dodecylthiophene) and poly‐(phenylphenylenevinylene). The spectra are described within the framework of second‐order Stark effect acting on an excitonic transitonic including field‐induced transfer of oscillator strength to higher lying states. The spatial extent of the exciton is two to three repeat units of the polymer backbone consistent with the notion that the absorbers are ordered subunits of the conjugated chain.
Hole trap analysis in SiO2/Si structures by electron tunneling
Authors: Schmidt M. , Köster H.,
Published: July 2007

Hole trap parameters in Si/SiO2 structures are investigated by their charge relaxation after a foregoing hole trap population. A concept based on tunneling between extended Si states and localized SiO2 states is developed to explain the logarithmic time dependence, gate voltage dependence, and temperature dependence of hole trap depopulation. A positively charged hole trap level with a trap energy of E = (6.3 ± 0.2) eV above the SiO2 valence band edge is found. The corresponding value of the wave vector is |K(E)| = 6.1 nm−1 according to the two‐band approximation for the K(E) relation in the SiO2 gap. The hole trap concentration is N = 2 to 5 × 1019 cm−3 inside a small (5 nm) near‐interface region. These results are in agreement with EPR experiments and confirm that the E'‐centre (oxygen vacancy) constitutes a dominating paramagnetic defect and a hole trap.
The electronic structure of single crystal YBa2Cu3O7–δ studied by polarized Cu Kβ2,5 X‐ray emission
Authors: Czolbe W. , Dick U. , Dräger G. , Fischer K.
Published: July 2007

The polarized Cu Kβ2,5 valence band spectra of an oriented YBa2Cu3O7–δ sample measured on a Johann‐type X‐ray spectrometer, are presented. The experimental spectra are aligned in a binding energy scale together with calculated partial Cu pη DOS (η = x, y, or z), taken from the literature. Due to a detailed comparison between the spectra and the DOS features the distribution of occupied Cu p‐like states along the valence band of this high‐Tc superconductor, their symmetry and contribution to different CuO bonds can be discussed and explained in principle.
Ruderman‐Kittel‐like interaction of electric dipoles in systems with carriers
Authors: Glinchuk M. D. , Kondakova I. V.
Published: July 2007

The energy of indirect interaction of off‐center ions via conduction electrons is derived. The calculations are carried out for ions described by double‐well potentials with the axis along a cubic axis of the lattice. Tunnel motion of ions is taken into account. It was shown that the obtained electric dipole‐dipole interaction Vdd depends on the values of the ion electric dipole moment, electron wave vector on the Fermi surface, and is an alternating sign function of the distance between the dipoles. The form of this function for the main part of Vdd looks like the well‐known Ruderman‐Kittel interaction of magnetic dipoles. Estimations show that the value of interaction is comparable with the conventional electric dipole‐dipole interaction.
Redistribution of electrons to the impurity level in Hg1–xMnxSe under pressure
Authors: Chudinov S. M. , Mancini G. , Stizza S.
Published: July 2007

The galvanomagnetic effect and the Shubnikov‐de Haas oscillation in n‐type Hg1–xMnxSe are investigated under hydrostatic pressure up to 1.6 GPa. The measurements are made at magnetic fields H ≦ 6.5 T in the range of helium temperatures for Mn concentrations x = 0.032, 0.07, 0.075. A significant decrease of concentration and Fermi energy of the Γ‐electrons under pressure are observed. Pressure induced electron redistribution between different bands in the energy spectrum of Hg1–xMnxSe is discussed.
Transfer matrix approach to the optical reflectance of non‐local periodic superlattices
Authors: Mochán W. L. , del Castillo‐Mussot M. , Vázques‐Nava R. A.
Published: July 2007

A generalized transfer matrix is applied to calculate the reflectance of a semi‐infinite periodic superlattice made up of spatially dispersive conducting layers, accounting for the propagation of plasma waves allowed by non‐locality. The field inside the superlattice is written as a superposition of the two outgoing solutions of the bulk dispersion relation, with amplitudes given by the electromagnetic boundary conditions supplemented by additional boundary conditions. The discrepancies of the used formalism with that of Trutschel et al. are discussed.
On effective diffusivity of periodic non‐homogeneous solid media
Authors: Mishin Yu. M.
Published: July 2007

Diffusion in a non‐homogeneous medium with periodic spatial variations of diffusivity and driving force is analyzed in detail. After a time t ≪ τ (τ being time to attain local quasi‐stationarity) diffusion proceeds like in a homogeneous medium with an effective diffusivity D̃ij. Correct procedure of D̃ij calculation includes solution of a stationary diffusion problem in a periodic unit cell with special boundary conditions. Methods of exact calculation and estimation of D̃ij are discussed, some simple estimates are presented. Possible applications of the theory are sketched.
Pressure dependence of Zr diffusion in the equiatomic B.C.C. TiZr alloy
Authors: Büscher M. , Herzig C. , Köhler U. , Mehrer H. , Lojkowski W.
Published: July 2007

Positive as well as negative activation volumes have been measured in the past for self‐diffusion in “anomalous” b.c.c. metals. To contribute to this problem and to obtain further information on the diffusion mechanism in b.c.c. transition metals the pressure dependence of diffusion of 95Zr is measured in the equiatomic b.c.c. TiZr alloy at 1003 and 1493 K in the pressure range up to 14.4 × 108 Pa. The rather small but positive activation volumes ΔV = 0.31 ± 0.06Ω and ΔV = 0.35 ± 0.10Ω, respectively, are obtained. The results are in conflict with negative ΔV values reported for this alloy by Smirnov et al., however, they agree very well with ΔV of self‐diffusion in β‐Ti. Within the limits of error ΔV is independent of temperature which indicates that only one diffusion mechanism is operating despite of the strongly curved Arrhenius relations of 95Zr and 44Ti diffusion in TiZr alloy. The present results confirm a monovacancy mechanism in group IVA b.c.c. metals.
Evaluation of the correlation length critical exponent of the N‐vector model
Authors: Silva P. R.
Published: July 2007

An extension of a heuristic treatment of the critical behavior of the Ising model (N = 1) developed by Thompson, to the general N‐vector model is made. Two approximations for the correlation length critical exponent v as a function of the lattice spatial dimension d and of the number of components of the order parameter N are obtained. Using the hyperscaling relation, the specific heat critical exponent α is also evaluated. Finally, diagrams are constructed displaying in the plane (d, N) various regions, namely: classical critical indexes, non‐classical critical indexes, and no‐phase transition regions or regions where the critical temperature goes to zero. These diagrams are also used to separate regions with divergent specific heat (α > 0) from the ones where it has a cusp‐like behavior (α < 0).
Spin waves in a semi‐infinite field‐induced metamagnet with three‐ion anisotropic interactions. I. Green function approach
Authors: Rudziński W. , Maciejewski W.
Published: July 2007

A quartic spin model with three‐ion anisotropic interactions is employed to study surface and bulk spin waves in a layered structure of a semi‐infinite field‐induced metamagnet with a (001) surface at low temperatures, T ≪ TN. The three‐ion surface anisotropy field is allowed to deviate from the bulk value. The calculations involve a derivation of expressions for the spin‐spin Green functions for the semi‐infinite medium. The Green functions are found to have poles at energies corresponding to bulk and surface spin‐wave modes. The theory includes an evolution of the dispersion relations for both the antiferromagnetic and paramagnetic phases.
Effective Pair Potential and Dynamic Correlations in Ni53B47 Melt
Authors: Gopala Rao R. V. , Debnath D.
Published: July 2007

Using experimental values the structure factor of the Ni53B47 melt measured by Nassif et al., the potential function is derived through the use of Rao‐Joarder's equation. The calculated potential function exhibits a repulsive hump corresponding to the first nearest‐neighbour distance. The potential function is also calculated using Percus‐Yevick (PY) and hyper‐netted chain (HNC) equations. In all the cases the repulsive hump appears. The nearest‐neighbour distances obtained by different methods are compared and found in fair agreement with each other. The potential function is used in the evaluation of phonon frequencies. From these phonon frequencies the elastic constants are computed. They are also calculated from I1 and I2 integrals of Schofield. Using the potential function, the characteristic frequency is also calculated and compared with the average frequency by other methods.
Influence of a Quantizing Magnetic Field on the Effective Electron Mass in HgTe/CdTe Superlattices with Graded Structures
Authors: Ghatak K. P. , Mondal M.
Published: July 2007

An attempt is made to study the influence of magnetic quantization on the effective electron mass at the Femi level in HgTe/CdTe superlattices with graded structures by formulating a new dispersion relation and to compare the same with that of the constituent materials. It is found that the effective mass increases with increasing electron concentration and oscillates with reciprocal quantizing magnetic field in all the cases due to the SdH effect. The effective mass also depends on the magnetic quantum number in addition to the Fermi energy which is the characteristic feature of such superlattices and the presence of the finite interface width enhances the effective mass. The numerical values of the same mass in superlattices are greater than those of the constituent materials. In addition, the corresponding well‐known results of relatively wide band gap materials are also obtained from our generalized formulations under certain limiting conditions.
Magnetic Properties of Layered Heisenberg Antiferromagnets
Authors: Wei G. Z. , Du A.
Published: July 2007

The method of double‐time—temperature spin Green's function is used to study the magnetic properties of layered antiferromagnets with arbitrary spin S within Tyablikov's decoupling approximation. According to the extent of suppression of interlayer coupling on the two‐dimensional spin fluctuations, we divide the low‐temperature region into two new regions, and give the asymptotic expressions of sublattice magnetization and parallel susceptibility for each of these temperature regions. The relation between Néel temperature and interlayer coupling strength is calculated and compared with other works.
Semi‐Classical Approximation to the Soliton Energy in a One‐Dimensional Antiferromagnet
Authors: Pires A. S. T.
Published: July 2007

The quantum‐statistical mechanics, at low temperature, of a one‐dimensional antiferromagnetic Heisenberg model with two anisotropies is studied in the weak‐coupling limit. The temperature dependence of the magnon masses, the soliton energy, and the soliton density are determined. Experimental data for TMMC are analysed.
Composition Dependence of Raman Bands in Amorphous AsxSe100−x
Authors: Mikla V. I. , Baganich A. A. , Sokolov A. P. , Shebanin A. P.
Published: July 2007

Raman scattering spectra are studied for Se‐rich amorphous semiconductors AsSe. By comparing peak width, peak position, and Raman intensity in the range of bond modes it is derived that the changes occur not monotonically with increasing As content. The change of the spectra is explained as due to cross linking Se chains.