bluuebunny/crossref_metadata_2025 huggingface dataset:
Semiempirical Crystal Field Calculations for fN‐Systems. III. Crystal Field Splitting and Jahn‐Teller Effect in Cubic (NEt4)4U(NCS)8 Crystal
Authors: Mironov V. S. , Rosov S. P.
Published: July 2007

The crystal field spectrum of 5f2‐configuration of cubic U(NCS) anion in (NEt4)4U(NCS)8 is calculated by using the new semiempirical many‐electron method, developed in previous papers. The Γ5‐triplet is found to be the ground state of U(NCS) anion. The T ⊗ (eg + t + t) Jahn‐Teller problem is studied quantitatively for U(NCS) anion. It is shown that tetragonal Jahn‐Teller minima in the adiabatic potential of U(NCS) are much deeper (EJT = 78 cm−1) than the trigonal ones. The temperature dependence of the effective magnetic moment of the U4+ ion in (NEt4)4U(NCS)8 is reproduced very well in the frame of modified Jahn‐Teller model suggesting simultaneous existence in (NEt4)4U(NCS)8 both, tetragonally compressed and elongated U(NCS) anions arising due to Jahn‐Teller distortions and influence of cation vacancies, respectively.
Magnetostatic Waves in a Thin Uniaxial Platelet with Stripe Domains Magnetized along the Easy Axis
Authors: Kostenko V. I. , Sigal M. A.
Published: July 2007

The theory of magnetostatic waves propagating along the domains is introduced. It is shown that only uniform‐surface and volume‐uniform waves may exist. In the case kx → 0 we obtain two FMR frequencies: the low‐frequency mode ωlf as a limiting case of volume‐uniform waves and the high‐frequency mode ωhf of uniform‐surface waves. Comparison with the theory of uniform resonance and experiment on barium ferrite platelets is carried out.
Curie Temperatures in Mixed Ferromagnetic Systems with Different Types of Interactions
Authors: Zheng Siming
Published: July 2007

The Curie temperature, TC, of a ferromagnetic system with different spins and different type interactions is considered – spin ½ with Ising‐like interaction JI, spin 1 with Heisenberg‐like interaction JH, and Ising‐like interaction JIH between unlike spin pair. The TC vs. p (concentration of Ising‐like spin) curves are drawn under different JH/JI and JIH/JI for square and s.c. lattices. The condition of the convex behaviour of the curves is discussed.
EPR of Triclinic Fe Centers in KCl and RbCl
Authors: Callens F. , Maes F. , Matthys P. , Moens P. , Boesman E.
Published: July 2007

A new paramagnetic Fe center with triclinic g‐tensor symmetry is reported. This type of defect is detected in both KCl and RbCl single crystals doped with iron, after they were quenched, irradiated at 77 K, and annealed at 160 K. The g‐tensors exhibit a very large g‐tensor anisotropy with g∥ ≈ 10 and g⟂ ≈ 5 in an axial approximation. Several models for this type of paramagnetic defect are discussed.
Excitons in BeO
Authors: Feldbach E. , Laasch W. , Varding D. , Zimmerer G.
Published: July 2007

BeO crystals in both orientations (⟂ c and ∥ c) are studied under polarized synchrotron radiation. The long‐wave shift (43 meV) and the splitting (29 meV) of the main excitonic reflection peak in the case of E ∥ c are detected at 10 K. The correspondence of the observed transitions to the selection rules is discussed. The excitation spectra of the luminescence band of self‐trapped excitons are measured in the excitonic region. It is found that in the case of E ∥ c the excitation spectrum is characteristic of localized excitations. Possible evidence of the coexistence of large‐ and small‐radius (self‐trapped) exciton states is discussed.
Comparative Study of the Polarized XANES at the Cu K Edge of Bi2Sr2Ca1Cu2O8, Bi2Sr2Ca2Cu3O10, and Y1Ba2Cu3O7 Thin Films
Authors: Poumellec B. , Cortes R. , Labdi S. , Raffy H. , Roas B. , Fretigny C.
Published: July 2007

The angular dependence of the Cu K edge in films of BiSrCaCuO (2212, 2223) and YBa2Cu3O7 are measured for both orientations (e ⟂ c, e ∥ c). A similar XANES is observed (but magnitudes and energy positions change slightly) although the compounds are quite different except the first shell. Specifically, anisotropy spectra are very close. After connecting the same XANES features to the local arrangement, it is found that the pyramidal site in 2223 is flattened compared to the one in 2212 by only (0.003 ± 0.001) nm. In addition, it is deduced that the electron density in the copper sphere is similar in all compounds whatever the site (CuII). Then, taken into consideration that pyramidal sites in 2212 and in YBaCuO are rather close to each other, an estimate is deduced of the absorption edge arising from the chain site following the two polarization directions. The major resonance on these spectra yields 0.009 nm of distance anisotropy, a result in very good agreement with the XRD data. In the course of the discussion, it is explained why the anisotropy spectrum is smaller in YBaCuO than in 2212. Lastly, the difference spectrum published by Tranquada et al. on oxygen reduction of YBa2Cu3Ox is revisited. It is deduced that the electron density increases in the copper sphere as soon as x = 7 when the copper atoms in the chains change from four to three oxygen coordination. Then, when the local structure evolves from three to two oxygen coordination, the local valence on the copper sphere does not change anymore but on the bridging oxygen.
Diffusion of 113Sn in the Intermetallic B82 Compounds of the System Ni/Sn
Authors: Schmidt H. , Frohberg G. , Miekeley W. , Wever H.
Published: July 2007

The direction dependence of the diffusion coefficient of 113Sn has been measured in single crystals of Ni61Sn39 by the layer grinding technique. The activation energies for diffusion parallel and perpendicular to the hexagonal c‐axis are 2.93 and 3.07 eV, respectively, the D0‐values 4.6 and 9.9 cm2/s. The experimental values of ϕ = D⊥/D| are between 0.56 and 0.75 depending on the temperature. The experimental values are compared with theoretical values calculated for the different possible diffusion mechanisms. The main mechanism should be a normal vacancy mechanism superimposed by minority mechanisms the increase of which with rising temperature explains the temperature dependence of ϕ.
Growth and Optical Characterization of CrSi2 Thin Films
Authors: Lange H. , Giehler M. , Henrion W. , Fenske F. , Sieber I. , Oertel G.
Published: July 2007

CrSi2 thin films are grown by dc magnetron sputtering of Cr single layers on Si and Cr/Si multi‐layer configurations on SiO2 and subsequent external annealing. In the optical spectra around the band edge the phonon emission and the absorption branches of the indirect transition at 0.5 eV and the direct transition around 1 eV can be discerned. Spectral features in the reflectivity spectrum at higher energies are discussed on the basis of published density of states calculations. The reflectivity spectrum in the IR region exhibits pronounced structures which can be fitted by a six‐oscillator model yielding the TO frequencies, oscillator strengths, and damping constants of the lattice modes. The Raman spectrum shows bands in the 310 to 410 cm−1 wave number region.
AC Conduction of Charge Density Waves in η‐Mo4O11 with Quasi‐Two‐Dimensional Structure
Authors: Koyano M. , Inoue M. , Hughes H. P. , Yoffe A. D.
Published: July 2007

The real and imaginary components of the resistive voltage (ZR) and Hall voltage (ZH) impedances of a quasi‐two‐dimensional η‐Mo4O11 crystal are measured over the temperature range 4.2 to 300 K and frequency range 100 Hz to 100 kHz under magnetic fields up to 4.6 T. The resistive impedance shows an appreciable ac response of electrical conduction due to an oscillating charge density wave (CDW) condensate below the first CDW transition temperature Tc1 (= 105 K), and below the second CDW transition temperature Tc2 (= 35 K) Shubnikov‐de Haas oscillations due to free conduction carriers that locate on the remaining Fermi surfaces produced by the CDW transitions. A qualitative discussion is given for these dynamic properties.
Low‐Field Magnetoresistance in Ferromagnetic Amorphous Alloys
Authors: Ziman J. , Zagyi B. , Novák L.
Published: July 2007

The low‐field longitudinal magnetoresistance in amorphous ribbons is measured. The contribution of “excess resistance” associated with the presence of the transverse domain walls is determined by transverse magnetoresistance measurements. The influence of the tensile and torsion stresses is discussed, taking into account the presence of the transverse domain walls and anomalous ferromagnetic anisotropy of the resistance.
Hopping Hall Conductivity in Disordered Systems from the Generalized Stochastic Liouville Equation Model
Authors: Čápek V.
Published: July 2007

As recently proved, equilibrium field‐independent hopping (transfer) rates disappear in the linearized (in the electric field) kinetic equations owing to cancellation on both sides as far as the theory is sufficiently rigorously formulated beyond the lowest‐order treatment of scattering. Thus, standard kinetic (rate) equation formulations have to be scrutinized, possibly avoiding the approximate kinetic equation stage. The present work shows how to perform the task treating the hopping Hall conduction in the site representation via the generalized stochastic Liouville equation model which has previously been used mostly for exciton transfer in condensates. Corrections to standard hopping Hall conductivity results are revealed.
The Perpendicular Magnetization Component in Single Crystals of Permanent Magnetic Materials
Authors: Hermann R. , Elk K. , Scholl R.
Published: July 2007

A systematical study of the perpendicular component of the magnetization in single crystals is carried out. Using the model of Stoner and Wohlfarth and restricting to the first‐order anisotropy constant K1, the perpendicular component m⊥ as a function of h for different angles 9 is calculated. A practicable alternative method to determine the anisotropy field Ha from the point of intersection of the second derivatives of different m⊥(h) curves is presented.
Intraband Luminescence in Ionic Crystals
Authors: Deich R. G. , Abdrakhmanov M. S.
Published: July 2007

Time‐resolved luminescence spectra of CsI, NaI, α‐Al2O3, and AgBr are investigated under sub‐ nanosecond electron pulse excitation. New intrinsic emission bands with a decay time less than 20 ps are revealed. It is proposed that the fast emission components originate from hot carrier intraband radiative transitions. The intraband luminescence spectra and light output are temperature independent and are not affected by crystal impurities. The correlation between the intraband luminescence spectra and crystal band structure is discussed.
Raman Scattering in ZnAs2 Monoclinic Crystals
Authors: Weszka J. , Balkanski M. , Jouanne M. , Pishchikov D. I. , Marenkin S. F.
Published: July 2007

Raman scattering in ZnAs2 monoclinic crystals is studied at room temperature in backward geometry. Features within the 190 to 270 cm−1 frequency interval are due to As‐chain modes. Low frequency features within the 10 to 100 cm−1 area are attributed to modes with considerable contribution from Zn atoms involved.
Why a Complete Solution Has not Been Found of a Mode I Crack in an Elastic Plastic Solid
Authors: Weertman J.
Published: July 2007

No demonstrated satisfactory solution, complete or partial, has ever been found for the plastic zone of a mode I crack in an elastic plastic solid. (By complete is meant a stress‐strain field solution of both the crack tip plastic zone and the elastic region which surrounds it.) In this paper it is shown, as well as a negative can be shown, that it is unlikely that a solution exists for this problem if a condition on the solution is that an active plastic zone touches the crack tip and/or the crack faces behind the crack tip. The specific mode I crack problem considered is the stationary mode I crack in plane strain in an elastic perfectly plastic and incompressible solid with a plastic zone size which is small compared with the crack length (small scale yielding).
Diffusion in Nanostructured Metals
Authors: Gleiter H.
Published: July 2007

Nanostructured materials are polycrystals with a crystal size of a few (typically 20 or less) nm and hence consist of about equal volume fractions of grain (interphase) boundary cores and crystals. Due to the large volume fractions of incherent interfaces, the diffusional processes in these materials seem to differ from the ones in single or coarse grained polycrystals with the same chemical composition. The diffusion data available today for nanostructured metals suggest that self and solute diffusion is enhanced relative to lattice as well as grain boundary diffusion, provided the solute atoms are substitutionally dissolved in the host lattice. This enhancement seems to originate from the free volume in the boundary cores. The diffusivities of atoms that are interstitially dissolved in the lattice of the host metals (e.g. B in Fe, H in Pd) are comparable to the corresponding lattice diffusivities.
Temperature and Volume Dependence of Positron Lifetime in Y1Ba2Cu3O6+x
Authors: Hentrich D. , Kluin J.‐E. , Hehenkamp Th.
Published: July 2007

Positron lifetimes in sintered Y1Ba2Cu3O6+x are determined between 80 and 1200 K using 22Na from 80 to 450 K and 58Co from 450 to 1200 K as a positron source. Below 450 K two lifetimes are detectable. A constant τ2 at 192 ps is attributable to a shallow trap supposed to be due to twin boundaries and growth steps, the binding energy of which can be estimated to be 0.16 eV. A second lifetime is increasing with temperature and understood as bulk lifetime. At higher temperatures only one mean lifetime is detectable. Taking the volume changes (T, p) properly into account it becomes apparent that the bulk lifetime is the only significant lifetime besides the shallow trap lifetime at low temperatures and that it increases uniformly with volume, whatever is the reason for its change, temperature, transformation, or oxygen partial pressure.
Monte Carlo Study of the Phase Stability of Ordered F.C.C. Phases under Irradiation
Authors: Haider F.
Published: July 2007

The phase stability of different ordered phases on the f.c.c. lattice in a binary Ising crystal with competing thermal and forced jumps is studied to model the behaviour of high energy particle irradiation. A special Monte Carlo technique is used to compute a so‐called stochastic potential, the equivalent of a thermodynamic potential under nonequilibrium conditions. Furthermore the case of irradiation‐induced phase separation is studied in a simple one‐dimensional model with a similar technique.
On Sequences of Alternate Stable and Unstable Regions along Tensile Deformation Curves
Authors: Kalk A. , Schwink Ch.
Published: July 2007

The occurrence and boundaries of (in‐)stabilities along stress–strain curves are investigated for f.c.c. solid solutions of CuMn. There are up to four temperature intervals where the σ(ϵ)‐curves over their whole extension are either smooth (stable deformation) or serrated (unstable deformation). In between, three transitional intervals, named α, β, and γ, are constituted. Here two or more regions of stable and unstable deformation alternate along σ(ϵ). In particular, inside interval β a region of stability is embedded between two regions of instability. For two Mn concentrations the variatin of the boundaries with temperature and strain rate is investigated for the intervals α and β. The results allow a comparison with theoretical “scenarios” developed by Kubin and Estrin. Possibilities for an improvement and/or extension of this model are discussed.
Reversibility and Hysteresis of Martensitic Transformations
Authors: Hornbogen E.
Published: July 2007

The term martensitic is used for a diffusionless structural phase transformation β → αM which is associated with a considerable amount of lattice variant shear: γβα ≈ 0.2. This reaction is defined as reversible if the reverse reaction αM → β faultlessly restores the original parent crystal β. Sets of factors are established which favour the reversible (or irreversible) mode. Hysteresis is due to lattice friction acting on the moving β/αM‐transformation interfaces during a transformation cycle. It can vary by several orders of magnitude. Analogies are indicated between ferromagnetic and martensitic hysteresis.
TEM and HREM Investigation of Secondary Dislocations in a Near‐Coincidence Σ19‐[011]‐Tilt Grain Boundary in Germanium
Authors: Möller H. J. , Griess M.
Published: July 2007

Germanium bicrystals with [011]‐tilt grain boundaries are grown by the two‐seed Czochralski method. Dissociated near‐coincidence grain boundaries with an orientation between Σ19 and Σ27 and asymmetric boundary planes contain networks of secondary dislocation which account for the deviation from the ideal coincidence orientation. Both conventional TEM and HREM are applied to determine the Burgers vectors and arrangements of the secondary dislocations. The complex dislocation structure can be analyzed and related to the Σ19 DSC lattice. The main result that uncommon and nonprimitive Burgers vectors are observed is discussed from the perspective of the total energy of the grain boundary.
The Dissociation of a Screw Superdislocation in the B2 Structure
Authors: Saada G. , Veyssière P.
Published: July 2007

Cross‐slip of a dissociated 〈111〉 screw superdislocation in the absence of an applied stress is studied within the frame of anisotropic elasticity in model B2 crystals. Lattice friction is ignored. The superdislocation can either be coplanar, and then confined to either of the {110} and {112} habit planes, or dihedral. The stability of these configurations is governed by the values of one elasticity parameter named N, by the ratio z between the antiphase boundary (APB) energies on these planes, and by the cross‐slip angle α. The different situations of interest are studied. By contrast to what occurs in L12 alloys, it is shown that besides two alternative planar configurations, one may encounter dihedral stable or metastable configurations which add to the variety of possible locking configurations in B2 alloys. The relative stability of these possible locking modes is discussed in details with respect to N and z, for the different cases of cross‐slip.
Model for Anomalous Power Dependence of Microwave Heating in Ceramic Materials
Authors: Kenkre V. M. , Kus M. , Katz J. D.
Published: July 2007

A model is presented to explain the anomalous power dependence of microwave heating in ceramic materials. The anomaly considered consists of a saturation effect wherein the dependence of the heating rate on microwave power becomes sublinear. Our proposed explanation is based on the idea of spatial confinement of the absorbing charges in grain boundary regions. The mechanism of saturation, more generally, of a decrease in absorption efficiency, arises from a mismatch of two lengths, one of which measures the maximum distance an absorbing charge travels under the combined action of damping and of the oscillating microwave field, and the other measures the spatial extent of the confining region. Explicit calculations for a simple model are presented and the results are examined in the light of observations on ferric oxide.
On the Instability Leading to the Martensitic Transformation of Cu–Zn–Al Single Crystals
Authors: Amazit Y. , Zarembowitch A.
Published: July 2007

Cu–Zn–Al alloys are known to exhibit shape memory properties when the temperature is varied. This behaviour is directly connected to the martensitic transformation that such materials undergo. If the general characteristics of this transformation are well understood, the nucleation mechanism of the transformation is not completely clear. However, it seems that the residual stresses play a major role in the process of nucleation. As a consequence, the measurement of the nonllnear elastic parameters as a function of temperture for shape memory single crystals may clarify the process of nucleation. Such data are reported in this paper; a model is proposed for the interpretation of the experimental results.
Structure Sensitive Electronic Properties of Quasi‐Two‐Dimensional η‐Mo4O11 Crystals
Authors: Ôhara S. , Negishi H. , Inoue M.
Published: July 2007

The electrical resistivity, Hall effect, and magnetoresistance are measured over the temperature range from 1.5 to 300 K for a number of η‐Mo4O11 crystals, that undergo two charge density wave transitions at Tc1 = 105 K and Tc2 = 35 K, prepared by a chemical vapor transport method using TeCl4 (10 and 2 mg/cm3) as a carrier gas. Good samples grown with 10 mg/cm3 TeCl4 have high carrier mobilities, showing quantum oscillations in the magnetoresistance and Hall coefficient at low temperatures. Poor samples grown with 2 mg/cm3 TeCl4 have rather low mobilities and anomalous temperature dependence of resistivity with a negative coefficient, together with a log T‐dependence. These behaviors may be due to microscopic structural imperfections, such as excess oxygen or vacancies. Based on these experimental results, a tentative band model for conduction electrons and holes, as well as some localized states lying near the Fermi energy, is proposed.
Evaluation of the Potential Function of Liquid Germanium from Experimental Structure Factors and Computation of Elastic Constants and Phonon Frequencies
Authors: Rao R. V. Gopala , Debnath D.
Published: July 2007

Rao‐Joarder's equation for the determination of potential function is used to derive the latter. The computed potential function of liquid Ge shows an inflection in the positive region and a negative minimum. The inflection is attributed to the first nearest‐neighbour distance and the negative minimum to the second nearest‐neighbour distance. The present computed potential function is found to be similar to that obtained by different methods. The derived potential function along with the measured values of g(r) is used in the evaluation of longitudinal and transverse phonon frequencies through Takeno and Goda's equations. The low‐k values of phonon frequencies are found to be nearly linear with k. From the slope of these frequencies the elastic constants are calculated. Schofield's equations are also used in the calculation of elastic constants. The values obtained by different methods are found to be interconsistent. The potential function is utilised in the evaluation of the characteristic frequency of the atoms in the liquid. The computed value is compared with that obtained from low temperature heat capacities and average frequencies obtained by different methods. They are found to be fairly interconsistent.
Vacancy‐Type Defects in Cd, Al, Si, and GaAs Studied Using Variable‐Energy Positron Beam
Authors: Takamura S. , Ito Y.
Published: July 2007

The S‐parameter versus incident positron energy is measured using variable‐energy positron beam method in Cd, Al, Si, and GaAs after irradiation with Ar or He ions at liquid nitrogen temperature. The fraction of positrons trapped in defects and in the bulk is calculated from the defect distribution based on the computer code TRIM and the fitting procedure is performed. It is suggested that the migration of vacancy‐type defects occurs below the irradiation temperature in Cd and n‐type Si.
Tunneling in Periodic and Quasi‐Periodic Superlattices
Authors: Singh M. , Tao Z. C. , Tong B. Y.
Published: July 2007

A comparative study of electron tunneling in periodic and quasi‐periodic systems is made. The quasi‐periodic systems considered are Fibonacci and Thue‐Morse superlattices. The transfer matrix method and Landauer formula are used to calculate the electron transmission coefficient and tunneling conductance, respectively. For the numerical calculation the superlattices are considered to be made from GaAs and GaAlAs semiconductors. Numerical calculations are performed for the tunneling conductance for different values of potential barrier height and width. The present calculations show that the peak positions of the tunneling conductance are at different energies for periodic, Fibonacci, and Thue‐Morse superlattices with equal number of layers. The height of peaks changes from one system to another. We also investigated the effect of temperature, damping, and magnetic field on the tunneling conductance in simple approximations. It is found that the peak height of the tunneling conductance decreases as the temperature increases and peak width increases at the same time. As magnetic field increases, the position of peaks of the tunneling conductance shifts to higher energy. The tunneling conductance decreases as the damping increases. Theses calculations can be extended to hole tunneling in these systems.
Compton Profile Anisotropy Studies in Hexagonal CdS
Authors: Panda B. K.
Published: July 2007

The Compton profile anisotropies are calculated by empirical pseudopotential method for [0001], [1010], and [1120] directions of CdS with wurtzite structure. The earlier theoretical anisotropy between [0001] and [1010] directions obtained by LCAO method could not explain the corresponding measured anisotropy. But the present calculation yields a better result.
Ab Initio Calculation of Electron Effective Mass in Crystals
Authors: Shilkova N. A. , Shirokovskii V. P. , Trubitsin V. Yu.
Published: July 2007

Using the Hellmann‐Feynman theorem an expression for the electron effective mass in crystal is derived. Calculations are carried out at some points of Fermi‐surface cross‐section of vanadium by the plane z = 0. The comparison with the data of numerical differentiation demonstrates the reliability of the results as well as the high accuracy of the technique presented. The method proposed for obtaining the effective mass is suitable for any calculation technique of electron spectra.
Interplay between Diagonal and Off‐Diagonal Disorders on the Density of States
Authors: Hamdache F. , Brezini A.
Published: July 2007

Calculations of the density of states of the Anderson model Hamiltonian are reported in presence of both, diagonal and off‐diagonal disorders for a Bethe lattice. In particular an analytical mean field treatement is developed in the dilute limit. Extensive results are presented displaying their characteristic features and discussed in the light of other theories.
Oxygen in InxGa1−xAsyP1–y Grown on GaAs
Authors: Zhu Q. S. , Hiramatsu K. , Sawaki N. , Akasaki I.
Published: July 2007

The measurement of DLTS on the alloy In xGa1–xAsyP1–y (0 ≦ y ≦ 0.3; 0.5 ≧ x ≧ 0.35) shows a new signal, labeled as E2, with an activation energy of Ec–0.61 eV and the SIMS signals show a large number of oxygen. To clarify is further, the energy of the deep level E2 is quantitatively calculated by using Vogl's tight‐binding theory and Hjalmarson's deep level theory. As a result, the deep A1‐symmetric level associated with an oxygen on the anion site of InxGa1–xAsyP1–y located deeply in the band gap. Thus, the level E2 is considered to be induced by the oxygen impurity.
Acoustoelectric Amplification in the Presence of Non‐Uniform Electric Field and Carrier Concentration in Solids
Authors: Misra K. D. , Pandey M. K.
Published: July 2007

Acoustoelectric amplification in a strain‐dependent, transversely magnetised semiconductor is studied in the presence of non‐uniform electric field and carrier concentration along the sample length. A generalised expression for the amplification coefficient in strain‐dependent and piezoelectric materials is also discussed for a single carrier semiconductor. Application to a multilayered system of different single carrier semiconductors coupled to one elastic wave is also suggested. The non‐uniformity in carrier concentration is found to have significant effect on the amplification coefficient in a multilayer system.
Coherent and Incoherent Exciton Dynamics in II–VI Semiconductors
Authors: Pantke K.‐H. , Lyssenko V. G. , Razbirin B. S. , Schwab H. , Erland J. , Hvam J. M.
Published: July 2007

Dynamics of free and bound excitons in CdSe and localized excitons in CdSexS1−x are studied by degenerate four‐wave mixing and light‐induced grating experiments. In the coherent range, the dephasing of excitons is determined as a function of temperature, density and energy. For incoherent excitons, recombination lifetimes and diffusion coefficients are determined. In particular, the mobility of excitons in CdSexS1−x near the mobility edge is studied.
Polarization Dependence in the Intraband Optical Stark Effect of Quantum Wells
Authors: Fröhlich D. , Spitzer S. , Uebbing B. , Zimmermann R.
Published: July 2007

Experiments and a theoretical description of the optical intraband Stark effect in multiple quantum wells for an in‐plane polarization of pump and test fields are presented. The nonlinear absorption spectra show a pronounced dependence on the angle between pump and test field polarization. For a quantitative agreement between theory and experimental results it is necessary to take the inhomogeneous distribution of the pump field into account, which is due to multiple reflections of the CO2 laser beam in the sample.
Nonlinear Quantum Beats of Excitons in CdSe
Authors: Pantke K.‐H. , Schillak P. , Erland J. , Lyssenko V. G. , Razbirin B. S. , Hvam J. M.
Published: July 2007

By observing quantum beats in the self‐diffracted signal from a nonlinear mixing process in a crystal, and by spectrally resolving this signal, information is obtained about solid state parameters, such as energy splittings and energy shifts. This technique is used to study bound excitons in CdSe with a strong, resonant third‐order susceptibility. Furthermore, in the region of free exciton propagation, effects on the third‐order nonlinear polarization are observed and explained by quantum beats between propagating polaritons.
Quantum Beats and Exciton Coherence in Time‐Resolved Resonant Light Scattering
Authors: Stolz H.
Published: July 2007

The time‐resolved resonant light scattering spectrum following short pulse excitation provides information on the various inelastic (energy relaxation) and elastic (phase relaxation) relaxation processes of an exciton system. Taking properly the implications of the measurement process into account this can be exploited to discriminate between Raman scattering processes, proceeding without loss of coherence, and hot luminescence, occurring after phase relaxation. The coherent nature of the states can be directly revealed by quantum beats in the time‐dependent scattering spectrum if the resonant states are energetically split. Such experiments are presented for bound excitons in CdS and the lowest exciton‐polariton in Cu2O. These measurements allow to deduce refined magneto‐optical parameters due to the high energy resolution of this method. The exciton dephasing times found are surprisingly long (up to 1 ns) and determined by energy relaxation due to phonon scattering. Pure dephasing due to elastic scattering gives only a minor contribution.
Microscopic Theory of Rabi Flopping, Photon Echo, and Resonant Pulse Propagation in Semiconductors
Authors: Koch S. W. , Knorr A. , Binder R. , Lindberg M.
Published: July 2007

The phenomena of Rabi oscillations, resonant pulse propagation, and photon echo in semiconductors are investigated by numerically solving the full semiconductor Bloch equations for the example of bulk CdSe. The calculations are done for resonant femtosecond excitation at the exciton resonance. The many‐body modifications of Rabi flopping and semiconductor photon echo are discussed for a thin medium, whereas the propagation of weak and strong pulses is analyzed for an extended material. Rabi flopping of the density can lead to field reamplification and thus to long propagation distances of sufficiently strong pulses and to pulse break‐up. The area theorem known from self‐induced transparency (SIT) in atomic systems is generalized for the case of phase modulated fields and shown to be modified significantly in semiconductors. Even though no ideal SIT occurs, the Coulomb interacting electron–hole system nevertheless seems to support extremely long propagation distances of so‐called π‐pulses.
Evolution Equations for the Nonequilibrium LO‐Phonon Dynamics in Laser Excited Polar Semiconductors
Authors: Wenschuh U. , Heiner E. , Fulde P.
Published: July 2007

Starting from an extensive model Hamiltonian a system of evolution equations is derived by means of a time‐dependent projection operator technique which describes the nonequilibrium dynamics of the LO‐phonon system in a laser excited polar semiconductor. The carrier system is described in a hydrodynamic stage, whereas the phonon system is treated in an athermal stage. Third‐order anharmonic interactions between optical and acoustic phonons as well as between the acoustic phonons are contained in the theory.
Effects of Trapped States on Spectrum of Four‐Wave Mixing
Authors: Hanamura E.
Published: July 2007

Excitons are sometimes trapped at impurities in crystals and at defects on interfaces in the confined systems, such as quantum wells and microcrystallites. These trapping effects on the third‐order optical responses are discussed: (i) excitonic and trapped‐state enhancement, (ii) nearly degenerate four‐wave mixing spectrum, and (iii) differential transmission spectrum under nearly resonant pumping of the lowest exciton. It will be shown how these and linear responses give information on the surface states in semiconductor microcrystallites.
A New Approach to the Study of Bose‐Einstein Condensation of Biexcitons
Authors: Hasuo M. , Nagasawa N. , Mysyrowicz A.
Published: July 2007

A new method is described that allows to detect the presence of a Bose condensate of biexcitons in a direct band gap material. It probes the accumulation of biexcitons in the coherent k = 0 state by measuring the increase of the optical phase conjugation signal of a crystal simultaneously pumped by an intense incoherent pump source. Experimental verification of the method is achieved in CuCl at low temperature. Results show an increase of the k = 0 biexciton mode occupation number by at least 8.6 × 105 when the randomly injected biexciton density exceeds the critical value for Bose condensation.
Quantum Confined Structures and Coherent Light Emitters in II–VI Heterostructures
Authors: Nurmikko A. V. , Gunshor R. L.
Published: July 2007

Recent developments in wide‐gap II–VI quantum wells and p–n‐junction heterostructures with emphasis on two‐dimensonal exciton physics are reported. Current device issues related to blue and green diode lasers and light emitting diodes in the ZnSe‐based configurations are briefly summarized. This is followed by experimental review of evidence which strongly suggests that exciton effects, which dominate interband absorption, are also of importance in both spontaneous and stimulated emission processes.
1D Charge Carrier Dynamics in GaAs Quantum Wires Carrier Capture, Relaxation, and Recombination
Authors: Christen J. , Kapon E. , Grundmann M. , Hwang D. M. , Joschko M. , Bimberg D.
Published: July 2007

Time‐ and spatially resolved luminescence of GaAs/AlGaAs quantum wires (QWRs) grown by organometallic chemical vapor deposition on nonplanar substrates is reported. The effective width of these nanoscopic wires is as small as 9.5 nm, giving rise to a separation of 45 meV for the quasi‐one‐dimensional conduction subbands. The QWRs exhibit high luminescence efficiency and the radiative recombination lifetime of the quantum confined carriers is found to be larger than 310 ps. Combined spectral‐time‐resolved experiments directly visualize the carrier capture process and the thermalization of the 1D carriers in the QWR structures.
Ultrafast Ligth Induced Transmission Changes and Laser Emission of Semiconductor Quantum Dots
Authors: Dneprovskii V. S. , Klimov V. I. , Okorokov D. K. , Vandyshev Yu. V.
Published: July 2007

Transitions between energy levels of carriers spatially confined within CdSe nanocrystals are observed as bleaching bands in time‐resolved nonlinear transmission spectra. Gain and laser generation at the transition between the lowest levels of size quantization are achieved.
Optical Bistability Induced by Charge Separation in Asymmetric Quantum Wells
Authors: Scandolo S. , Tassone F.
Published: July 2007

A novel mechanism for intrinsic optical bistability in quantum wells is proposed. In suitable structures the intrinsic electric field generated by optical excitation leads to the lowering of some interband transition energies. Therefore, increasing‐absorption bistability occurs for proper choice of operation conditions, without considering excitonic or many‐body effects. A numercal study of a InAs–GaSb quantum well is carried out and switch‐on intensities of the order of 102 kW/cm2 for a nanosecond response time are found.
Saturation of Band‐Tail Absorption in Multiple Quantum Wells and Its Link with the Nonlinear Index Saturation
Authors: Raj R. , Sfez B. G. , Pellat D. , Oudar J.‐L.
Published: July 2007

A pump and probe measurement of the nonlinear transmission reveals that the origin of the saturating behaviour of the nonlinear index δnNLobserved in the band tail of GaAs/GaAlAs multiple quantum wells is due to the saturation of the photoexcited carrier density.
Direct Summation of Coulomb Potentials over an Ionic Lattice
Authors: Fisenko I. G. , Kholopov E. V.
Published: July 2007

The possibility of performing the direct summation of Coulomb potentials over an ionic lattice is discussed. The regular way of enhancing the convergence is investigated, provided that artificial unit cells of high symmetry are introduced. The simplest ionic structures are considered as examples. The universal algorithm is proposed of calculating the appropriate surface contribution within the framework of the direct summation. As a result, Coulomb potentials for a boundless lattice can be reconstruced.
Unified Study of the Lattice Mechanics of Fluorites Indications of the Superionic Phase Transition
Authors: Makur M. , Ghosh S.
Published: July 2007

A unified study of the different lattice mechanical properties of the three fluorites CaF2, SrF2, and BaF2 is presented, using a semi‐microscopic model and a single set of parameters. The cohesive energy, elastic constants, dielectric constants, and phonon dispersion curves are calculated. The temperature dependence of the phonon frequencies is estimated from a simple quasi‐harmonic approach which indicated softening of a 〈100〉 zone boundary phonon mode which may lead to the diffuse transition to the superionic state.
Theory of Size Quantization of Exciton in Quasi‐Zero‐Dimensional Semiconductor Structures
Authors: Pokutnyi S. I.
Published: July 2007

A theory of size quantization of the exciton energy spectrum in a small semiconductor crystal is developed under condition where polarization interaction of electron and hole with the surface of a microcrystal plays an important role. From the comparison of the theory with experiments the effective mass of an exciton in a simple parabolic band microcrystal is determined.
Light‐Induced Ferrielectric Structures in Exciton Systems in Molecular Crystals
Authors: Izvekov S. V. , Sugakov V. I.
Published: July 2007

The possibility of spatial stratification of excitons and polarization in molecular crystals with traps for excitons at stationary pumping is considered. Such a structure can exist only under external irradiation and may be considered as an example of a dissipative structure which can arise in nonequilibrium systems. The numerical calculations are carried out for α‐benzophenon crystals. It is shown that at some critical values of pumping rate and crystal temperature the homogeneous density of excitons becomes unstable and spatial stratification of excitons arises in the crystal. The dipole moment of the elementary cell becomes uncompensated and a periodically changing dipole moment exists. The critical values of pumping rate and crystal temperature and the period of the spatial stratification of exciton density are calculated.
The Dependence of Tc on Pressure, Heat Capacity, and Thermal Expansion Coefficient Near Tc for Quasi‐Two‐Dimensional Superconductors with Paramagnetic Impurities
Authors: Zhernov A. P. , Chulkin E. P.
Published: July 2007

The phonon model of the high‐temperature superconductors is considered, taking into account the quasi‐two‐dimensional spectrum of carriers. The influence of paramagnetic impurities on the dependence of Tc on pressure, as well as on the value of jumps of the electron heat capacity and thermal expansion, is analyzed.
Microphysical Analysis of the Magnetooptical Interband Effects of Amorphous Semiconductor Films
Authors: Kessler F. R. , Schulz R. , Sievers B.
Published: July 2007

The polarization modulation spectroscopy of the (integral) Faraday and Kerr rotation of amorphous germanium and silicon films permits the separation of the magnetooptical contributions from various interband transitions with high sensitivity. The analysis is based primarily on a mathematical matrix calculation. This method yields the differential magnetooptical effects of arbitrary successions of multilayer systems including coherent and incoherent multiple reflections and magnetooptical interface effects. A quantum‐mechanical “density of states model” has been developed for the microphysical interpretation which is based on the optical constants and on the Mott‐CFO‐model of amorphous semiconductors. In this microphysical model the energy‐dependent integral sum‐g‐factor is obtained from the sign‐correct superposition of the magnetooptical contributions of the interband transitions at all critical points of higher energy. The weight of these magnetooptical contributions is significantly different for amorphous germanium and silicon.
A novel approach for constructing symmetry‐adapted basis sets for quantum‐chemical calculations. I. Real symmetry‐adapted orbitals
Authors: Kantorovich L. , Livshits A.
Published: July 2007

A novel method for generating symmetry‐adapted basis orbitals (widely applied in quantum chemistry) is proposed. It extensively combines the ideas of both the local‐group and projection‐operator methods. Some important improvements of the previous formalisms are made, namely: (i) the simple procedure for the construction of the transformation matrix (TM) while the matrix making this TM unitary is extracted individually for further usage, (ii) the partial reduction of the initial reducible representation results in the possibility of conserving all matrices real even in the cases when the group in question has complex irreducible representations. The procedure proposed in this paper is applied to the general finite group which could be both the usual point or large unit cell group. Our method is well suited for computer utilization.
Electronic structure of point defects in antiferromagnetic insulating cuprates
Authors: Loktev V. M. , Prosandeev S. A. , Tarasevich Yu. Yu.
Published: July 2007

The energies and wave functions of impurity (defect) states which appear in the energy gap of La2CuO4 due to doping are calculated by the unrestricted Hartree‐Fock method. It is shown that the state vectors have such a structure that localized carriers (holes) can occupy not only oxygen ions but also copper ones.
Polarization of dislocation absorption and luminescence in direct gap semiconductors with edge dislocations
Authors: Razumova M. A. , Khotyaintsev V. N.
Published: July 2007

The bound electron states at an edge dislocation in direct gap semiconductors are investigated in the deformation potential model taking into account the triple degeneracy of the valence band maximum. The experimentally observed polarization of dislocation absorption (DA) and dislocation luminescence along the Burgers vector is explained theoretically. The interaction between the anisotropic dislocation field and the anisotropic valence subbands gives rise to three series of hole dislocation levels of different depths, the DA polarization being determined by the polarization of the optical transitions from the deepest holes series to the isotropic states of the conduction band.
Electron paramagnetic resonance and optical absorption spectra of Gd3+ ions in alkali cadmium borosulphate glasses
Authors: Rao A. S. , Rao J. L. , Kumar V. V. R. K. , Jayasankar C. K. , Lakshman S. V. J.
Published: July 2007

Electron paramagnetic resonance (EPR) at 9.205 GHz is examined for a series of Gd3+‐doped alkali cadmium borosulphate (R2SO4 · CdSO4 · B2O3; R = Li, Na, K or Cs) glasses. The spectra are similar to the U‐spectra familiar in many oxide, fluoride, and sulphate glasses, indicating very low and disordered site symmetries with a broad distribution of crystal fields. Remarkable changes are observed in the spectrum with changes in temperature, glass composition, and concentration of Gd3+ ions. Optical absorption spectra of Gd3+‐doped cadmium borosulphate glass are also studied at 300 K.
Two‐stage relaxation of the electron‐phonon system in polar semiconductors
Authors: Král K. , Hejda B.
Published: July 2007

The effect of acoustic phonon bottleneck is studied theoretically in the hot electron‐phonon system in the approximation of dispersionless longitudinal optical phonons in polar semiconductors. Kinetic equations for electrons, LO‐phonons, and second generation acoustic phonons are solved numerically for CdSe. The fusion lifetime of acoustic phonons is estimated for the processes of anharmonic acoustic‐optical phonon coupling. The fusion processes are found to be critical for the occurrence of the acoustic phonon bottleneck.
Long‐range order of an exactly solvable model of a quantum antiferrormagnet
Authors: Chamati H. , Tonchev N. S.
Published: July 2007

The thermodynamics of a simple electron‐spin model for the paramagnetic‐antiferromagnetic phase transition is discussed. The spontaneous magnetization m and the root‐mean‐square order parameter mo for arbitrary temperature and band filling are obtained. For the ratio r(T, μ) = m/mo, where μ is the chemical potential, the value r(T, μ) = √3 is found.
Tunneling‐Assisted Photon Emission in MIS(p+) Junctions
Authors: Volkov R. A. , Chuiko A. F. , Dmitriev M. V.
Published: July 2007

Analytical expressions for the tunneling current and the intensity of light emission associated with the inelastic tunneling of electrons in sandwich structures are obtained for semi‐classical and quantummechanical descriptions of the electron gas in the electrodes. A metal—insulator—p+‐semiconductor (MIS(p+)) junction with a crystallographically oriented metal cathode is considered. To increase the spectral density of the tunneling‐assisted photon emission from the MIS(p+) junction, it is proposed to employ a cathode made of such transition metals as Ni, Co, Mn, Ti, Fe, as well as of Pt, β‐NiAl which exhibit peaks of the density of occupied states near the Fermi levels.
The Kinetics of Excitons in Krypton Cryocrystals with Impurity Traps
Authors: Belov A. G. , Ya. Fugol I. , Yurtaeva E. M.
Published: July 2007

The kinetics of energy transport by free (FE) and self‐trapped (STE) excitons is studied for krypton cryocrystals with impurity traps. The experimental data on the dependence of the intrinsic or sensitized luminescence spectrum on impurity concentration in Kr‐O2, Kr‐Ar‐O2, and Kr‐Xe solid solutions are considered. A model description of excitation motion and trapping in the presence of FE and STE is presented. The effect of different impurity centers on the balance relation between FE and STE in the matrix is determined. For small impurity concentrations the energy transport is shown to be realized by FE and forc> 10−1at% by STE.