bluuebunny/crossref_metadata_2025 huggingface dataset:
Intramolecular Excitation Dynamics in a Thermalized Chain. II. Formation of Autolocalized States in a Chain with Free Ends
Authors: Kadantsev V. N. , Lupichev L. N. , Savin A. V.
Published: July 2007

A numerical study of the Davydov soliton dynamics in a thermalized unclosed molecular chain with asymmetric exciton–phonon interaction is carried out. The effect of thermal vibrations in the scope of the quantum‐mechanical model is taken into account. When initiating the excitation at the free end of the chain, the soliton transport is shown to be able to occur at temperatures close to 300 K only. In this case the soliton formation is dependent on which of the end molecules in the chain is excited first.
Large Polarons in the Discrete SSH Model
Authors: Ruckh R. , Sigmund E.
Published: July 2007

Stationary polaron solutions are derived for the discrete version of the Su‐Schrieffer‐Heeger (SSH) model in two different ways. First Holstein's large polaron theory is applied. Second, the polaron solution is derived from Heisenberg's equations of motion. It turns out that the results obtained from the continuum version of the SSH Hamiltonian are different from those obtained from Holstein's molecular crystal model (MCM).
Influence of Bond‐Angle and Dihedral‐Angle Disorders on the Top of the Valence Band of Amorphous Germanium
Authors: Ratag V. M.
Published: July 2007

The electronic structure of amorphous germanium (a‐Ge) is considered using a tight‐binding scheme with all first‐neighbor couplings in a continuous random network. The dihedral‐angle and bond‐angle disorders are found to be important at the valence‐band‐edge and responsible for the observed features near the top of the valence band. At the top of the valence band of a‐Ge a tail of localized states ≈︁ 0.4 eV in width is found.
Band‐Gap Narrowing Due to Many‐Body Effects in n‐Type Degenerate GaAs Crystals
Authors: van Cong H. , Charar S. , Brunet S.
Published: July 2007

The band‐gap narrowing due to many‐body effects in n‐type degenerate GaAs crystals is analytically expressed in terms of the total donor concentration Nd, basing principally on the integral expressions of conduction‐and‐valence band edge shifts, established and numerically computed by Abram et al. in the plasmon pole approximation and a self‐energy approach. Here, the additional screened Coulomb hole effect is also considered. Then, the numerical results are compared with the corresponding results deduced from other works. Finally, it is suggested that, at both, very low and high concentrations, the result of band‐gap narrowing is reduced to that given by the traditional Hartree‐Fock exchange effect, and thus proportional to N.
Solitons and Polarons in Polyyne. Discrete Lattice and Broken Electron–Hole Symmetry Effects
Authors: Malek J. , Drechsler S. L. , Heiner E. , Kahnt R.
Published: July 2007

A SSH‐like Hamiltonian proposed by Rice et al. for polyyne extended by second neighbour hoppings is investigated. The discrete system of electron eigenvalue equations together with selfconsistency conditions for relative atomic displacements is numerically solved for a rich variety of soliton and polaron states. In particular, it is found that various degeneracies inhererent for the model mentioned above are lifted due to terms which break the charge conjugation symmetry. Discrete lattice effects enhance the symmetry breaking. For polarexcitons and neutral solitons (antisolitons) an internal charge structure is found.
Collective Excitation Effects on the Electrical Resistivity of Singlet Ground State Paramagnets of Metallic Conduction
Authors: Szukiel A. E. , Markowski P. J. , Durczewski K.
Published: July 2007

The possibility of the occurrence of a maximum in the temperature dependence of the electrical resistivity in the paramagnetic phase of singlet ground state crystal field materials is examined theoretically on the basis of an earlier developed formalism for a many‐level system (PrIn3) and a simple two‐level model. It is shown that the existence of a weak exchange interaction in paramagnetic PrIn3 improves the fitting of the theoretical results to experimental data but the occurrence of the maximum in the temperature dependence of the resistivity in ferromagnetic Pr3Tl in an earlier paper by using the same formalism may be questioned. Further investigations are required to explain the effect.
Model Calculations for Spin‐Density‐Wave Instabilities in Itinerant Magnets
Authors: Jelitto R. J.
Published: July 2007

By evaluation of a generalized Stoner criterion everywhere on the Brillouin zone the instability of the paramagnetic state (PS) vs. a first ordered state (FOS) of spin‐density‐wave type at T = 0 is investigated for a simple but non‐trivial two‐dimensional model of itinerant magnetism. As main results (i) the pitch q0 of the FOS varies continuously with the band‐filling leading to an incommensurate magnetic structure in the most cases, and (ii) the variation of q0 occurs along some symmetry axes of the Brillouin zone. By (ii) a long‐standing conjecture is proved for the model considered. (i) shows that it is not justified to restrict to fixed magnetic structures when setting up theories of itinerant magnetism. First discussions on the meaning of these findings for the results of a more complete description (including the fully developed ordered state and finite temperatures) are given.
Anderson Localization in Ferromagnetic Semiconductors Due to Spin Disorder I. Narrow Conduction Band
Authors: Kogan E. M. , Auslender M. I.
Published: July 2007

Theoretical investigation is made of the temperature induced metal‐insulator transition in doped ferromagnetic semiconductors, described by s‐d exchange model. The transition is a result of the mobility edge movement, the disorder being due to magnetic ions spin density fluctuations. The electrons are described in tight binding approximation. Using ideas and methods of Anderson localization theory simple formulas are obtained, which connect the mobility edge with shortrange order characteristics of the magnetic subsystem‐static spin correlators. The theory developed is applied to the description of the resistivity of CdCr2Se4. The theoretical results are in agreement with experiment in a wide temperature range.
Effect of Monoclinic Symmetry on the EPR Spectra of Gd3+‐Doped Hydrated Single Crystals of Rare‐Earth Trichlorides
Authors: Misra S. K. , Rudowicz C.
Published: July 2007

Room‐temperature X‐band EPR data for Gd3+‐doped isostructural rare‐earth trichloride hexahydrates are analyzed using a rigorous least‐squares‐fitting procedure. Discussion of the three forms of spin Hamiltonian corresponding to the orientations of the two‐fold monoclinic symmetry axis C2 parallel to the magnetic axes X, Y, and Z is provided. It is found that for, C2 ∥︁ Y, a significantly lower value of χ2, representing the goodness of fit, is obtained. A discussion of the determination of degree of monoclinicity, as revealed by the spin Hamiltonian parameters, is provided. Systematics of the spin Hamiltonian parameters b and b, as functions of the host–ion radius, are also studied.
Photoluminescence of GaSe
Authors: Dobynde I. I. , Bobrysheva A. I. , Razdobreev I. M. , Shekun Yu. G.
Published: July 2007

The photoluminescence and absorption in GaSe over the energy range 2.00 to 2.12 eV at 77 K is investigated under different conditions, namely with different excitation quantum energies and low and high pumping power. The observed features of the spectra are explained using the energetic band model proposed by Voitchovsky and Mercier.
Optical Bistability and Multistability in a System of High Density Excitons and Biexcitons in Semiconductors
Authors: Khadzhi P. I. , Shibarshina G. D. , Rotaru A. Kh. , Slavov Yu. D.
Published: July 2007

Results are given on the investigation of optical bistability and multistability phenomena both, in the exciton range of the spectrum due to exciton‐photon and exciton‐exciton interactions and in the range of two‐photon generation of the biexcitons (excitonic molecules). For each case the steady‐state equations are obtained.
Phase Transition in 1D KDP‐Type Ferroelectrics
Authors: Sajfert V. D. , Mašković Lj. D. , Djajić‐Jovanović R. P. , Kozmidis‐Luburić U. F.
Published: July 2007

The behaviour of quasi‐one‐dimensional ferroelectrics of the KDP‐family is investigated in the vicinity of the transition temperature. Three forms of the proton–proton potential are examined and it is concluded that long‐range forces are responsible for the second‐order phase transition in these materials.
Radiative and Non‐Radiative Processes of Localized Impurity Centers in Alkali Halides: KBr:Ag−
Authors: Shimanuki S.
Published: July 2007

The C and A emissions in KBr:Ag− are measured in detail between 11.2 and 300 K. At 11.2 K, excitation in the peak position of the C band exhibits the C emission but it hardly produces A emission. Changing the excitation position to the higher energy side of the C band peak, the A emission is produced a little. The intensity (emitted photon number) ratio of the A to C emission, η, shows an excitation photon‐energy dependence and changes over three orders within both sides of the C band. Increasing the temperature, η itself increases as a whole and its excitation photon‐energy dependence weakens and disappears at about 80 K. The values of η in the lower energy edge show an activation process of Arrhenius type over four orders above around 45 K, whose activation energy is 0.076 eV, and below 30 K, another activation process of the same type with an activation energy of 0.0025 eV. A few causes of the exciting energy dependence of η at low temperatures are considered and the mechanism due to the dynamical nonradiative transition is proposed.
Weak Impurity Metallic Behaviour of p‐Hg1−xCdxTe
Authors: Höschl P. , Moravec P. , Grill R. , Čápek V.
Published: July 2007

On samples of p‐Hg1−xCdxTe (x ≈ 0.2) with critical concentration of Hg‐vacancies at which shallower acceptor level disappears completely, the electric conductivity and Hall coefficient are measured. An impurity band model is reported for a simultaneous analysis of the Hall coefficient and Hall mobility data in the region of 4.2 to 100 K. Concentrations of Hg‐vacancies and of compensating donors are determined. It appears that there is no physical reason to distinguish between heavy hole and impurity band conduction.
A Molecular‐Dynamical Study of the Changes in the Atomic Structure of a Material Induced by a Shock Wave with a Distorted Front
Authors: Korostelev S. Yu. , Psakhe S. G. , Panin V. E.
Published: July 2007

Atomic structure changes of a crystal induced by shock waves are studied by molecular dynamic method. α‐Fe crystallites are used for simulation having a rectangular area in the (110) plane and containing about 104atoms. The radial distribution function of atoms is shown to lose its discrete form, typical of a crystal and to become continuous when the shock wave planar front is distorted. Microfragmentation in the corresponding areas of a crystal is shown at the same time to be the result of the shock wave distortion with a planar front.
A Variational Linear Approach for the Free Energy Applied to the Ising Model
Authors: Ferreira J. R. Faleiro
Published: July 2007

A recently proposed variational approach is applied to the Ising model. Former results may be improved when a sum of linear chain Hamiltonians is used as an auxiliary Hamiltonian instead of a sum of single particle Hamiltonians.
Quantum Fluctuations and Time Structures of High Density Polaritons
Authors: Moskalenko S. A. , Rotaru A. H. , Shvera Yu. M. , Zaloi V. A.
Published: July 2007

The quantum fluctuations of coherent polaritons in bulk samples are quenched due to the macroscopic occupation number of the single‐particle state. But in the microcrystallites at low levels of excitation antibunching of the quasiparticles can occur. The nonstationary behavior of the coherent polaritons is characterized by the periodic, quasiperiodic, and chaotic self‐pulsations. The phase trajectories are characterized by the limiting cycle, torus or strange attractor.
Nuclear Polaritons in the Mössbauer Absorber
Authors: Haas M. , Hizhnyakov V. , Realo E. , Jōgi J.
Published: July 2007

A transient pulse of the Mössbauer radiation in a thick resonant absorber in the case of a significant isomer shift is investigated theoretically and experimentally. It is shown that on transmission the incident pulse decomposes into 1. a fast pulse ‐ an analog of the Sommerfeld‐Brillouin precursive pulse in optics and 2. a slow pulse connected with the motion of a nuclear polariton packet. The group velocity of the polaritons is determined.
Manifestation of Hot Phonons in the Luminescence Spectra of Highly Photoexcited AIIBVI Crystals
Authors: Baltramiejūnas R. , Žukauskas A.
Published: July 2007

The quasi‐stationary and transient spectra of the nonequilibrium electron‐hole plasma (EHP) emission are investigated in CdSe crystals. A broadening of the low‐energy side of the luminescence band due to the heating of longitudinal‐optical (LO) phonons is established. A slow stage of the EHP energy relaxation with a time constant of 150 ps is observed and referred to the accumulation of decay products of nonequilibrium LO phonons in the narrow interval of the k‐space. A model of the two‐stage relaxation of the phonon subsystem in a photoexcited semiconductor is used to calculate the power of energy‐loss in hot EHP and the nonequilibrium LO phonon distribution.
Stable Configuration of an Interstitial Atom with Relaxation Effect in Si and Ge
Authors: Soma T. , Atarashi N. , Kagaya H.‐Matsuo
Published: July 2007

The stable configuration of an interstitial atom in Si and Ge are studied using the perturbational formalism presented earlier and considering the relaxation effect around the interstitial atom. The interstitial atom is most stable at the hexagonal site for Si and Ge. The calculated migration energy of the interstitial atom is smaller than 0.1 eV for Si and about 0.4 eV for Ge.
Investigation of the Thermodynamic Properties of Anharmonic Crystals by the Momentum Method. I. General Results for Face‐Centred Cubic Crystals
Authors: Tang Nguyen , van Hung Vu
Published: July 2007

By the moment method established previously on the basis of the statistical mechanics, the thermodynamic properties of a strongly anharmonic crystal with the f.c.c. structure are considered. The analytic expressions for the thermodynamic quantities as the thermal expansion coefficient, the specific heats Cv and Cp, the isothermal and adiabatic compressibilities, etc. are obtained.
Effect of Magneto‐Size Quantization on Electric‐Field‐Aided Photoemission from Quantum Wells in Ultrathin Films of Wide‐Gap Semiconductors
Authors: Maity A. B. , Majumdar C. , Chakravarti A. N.
Published: July 2007

An attempt is made to investigate the quantized photoemission from the magneto‐size‐quantized levels of quantum wells in ultrathin films of wide‐gap semiconductors in the presence of an electric field, taking n‐type GaAs as an example. It is found that the photocurrent changes with photon energy in a step‐like manner in which the number of steps increases for relatively large film thicknesses with consequent decrease of the step heights at given values of the electric and magnetic fields. Besides, it is seen that the photoemission becomes quantized for photon energies significantly lower than the electron affinity whereas it becomes oscillatory as the electron affinity is approached. The effects of continuous variation of either the electric or the magnetic field on the photocurrent corresponding to different film thicknesses are also discussed.
Up‐Conversion from Er3+–Er3+ Pairs in CdF2 Crystals
Authors: Jouart J. P. , Mary G.
Published: July 2007

The effect of preferential clustering on the properties of infrared to visible conversion of Er3+ (between 1 to 4 mol%) is investigated in CdF2 at 77 K. Without preferential clustering, the green emission is predominant. If there is preferential clustering, the red emission is predominant. This emission mainly results from the non‐resonant cross relaxation process in an Er3+–Er3+ pair doubly excited in the state (4I11/2, 4I13/2). The efficiency of this up‐conversion process is two orders of magnitude higher than that of the single Er3+ sites. It is suggested to use the up‐conversion as a technique to inquire about the degree of association of the Er3+ ions in the host lattices.
Excitons Bound to Te Impurities in CdS, ZnS, and Their Mixed Compounds with Wurtzite Structure
Authors: Tchakpele K. P. , Albert J. P.
Published: July 2007

A calculation is made of the binding energy of holes to isolated, pairs and clusters of three Te impurities in ZnS, CdS, and their mixed compound ZnxCd1−xS. The calculation takes into account the multiband nature of the bound state as well as the relaxation induced by the incorporation of the impurity, the growing importance of which is shown when going from CdS to ZnS. The only imput parameters are the electronegativity difference between the exchanged atoms modeled by the energy difference of the free corresponding atomic s and p levels and the relaxation of the nearest neighbour distance as previously determined by Martins and Zunger. A very good agreement is obtained between the calculated values and the experimental data.
Magnetism in Thin Chromium Films
Authors: Schröder K. , Hejase H.
Published: July 2007

Torque magnetometer measurements show that the magnetic moment of mica sheets with Cr films 0.3 to 28 nm thick is smaller than the magnetic moment of the mica sheet at room temperature. This may mean that the average susceptibilities of the Cr films are negative, and of the order of −10−3 to −10−4, or that small magnetic moments form perpendicular to the plane of the Cr film. Both explanations cannot be reconciled with accepted models of the magnetic structure of chromium.
Nonlinear Refraction Due to Dynamical Burstein‐Moss Effect in Lead Chalcogenides
Authors: Paskov P. P. , Pavlov L. I. , Atanasov P. A.
Published: July 2007

A theoretical analysis of the nonlinear refraction due to the dynamical Burstein‐Moss effect in IV–VI semiconductors, is developed. A nonlinear refractive cross‐section in lead chalcogenides at T = 100 K and various compositions is quantitatively evaluated. It is shown that the electronic optical nonlinearity in these ternary compounds is substantial for nonequilibrium carrier concentrations of the order of 1015 to 1016 cm−3. It is possible to use the obtained theoretical results in modeling nonlinear opto‐electronic elements.
Raman Determination of the Faust‐Henry Coefficient of GaAs in the 1.9 to 2.7 eV Range at Ordinary and Low Temperatures
Authors: Zekeng S. , Prevot B. , Schwab C.
Published: July 2007

The Faust‐Henry coefficient of GaAs is determined at ordinary and low (20 K) temperatures by using Raman scattering by the TO and LO first order mode vibrations excited by photons with energies located between 1.9 and 2.7 eV. After careful normalization of the experimental results with respect to optical as well as instrumental conditions, the Faust‐Henry coefficient is found to be nearly constant over the energy range investigated with a mean value of C = −0.48 ± 0.04 at 300 K and a slightly lower value (C = −0.53 ± 0.04) at low temperature. The pure electrooptic contribution to the Raman efficiency of GaAs is also deduced.
Thermoluminescence and Other Optical Studies on γ‐Irradiated RbCl: Gd2+
Authors: Sastry S. B. S. , Muralidharan G.
Published: July 2007

Thermoluminescence (TL) of RbCl:Gd2+ is discussed. Z1 centres are formed even in the irradiation process. Hydroxyl ion related centres are observed in the optical absorption. The TL process is identified to be due to the thermal release of F‐electrons and their subsequent recombination with the hole centres.
Coherent Nonlinear Optical Processes in Semiconductors. Physics and Applications
Authors: Schmitt‐Rink S.
Published: July 2007

The influence of monochromatic nonresonant laser fields is discussed on the elementary excitation spectrum of semiconductors. It is shown that the different renormalizations in inorganic and onedimensional organic materials can be understood in the framework of one and the same theory. Applications, such as generation of ultrashort voltage pulses in biased quantum wells, are discussed.
Theory of the Optical Stark Effect in Semiconductors
Authors: Schäfer W. , Schuldt K.‐H. , Binder R.
Published: July 2007

The light‐induced changes of the properties of a semiconductor caused by an ultra‐short strong laser pulse with a central frequency well below the fundamental absorption edge are investigated. Renormalization and time evolution of one‐ and two‐particle properties are discussed using a nonequilibrium Green's function formalism. Numerical results for the absorption spectra are presented.
Digital Circuits for All‐Optical Computing Based on Thin‐Film Semiconductor Interferometers
Authors: Sinitsyn G. V.
Published: July 2007

Results are reported of realizing optically programmable components of digital processors, a full adder, a space switch‐demultiplexer based on optically bistable vacuum‐deposited semiconductor interferometers (interference filters). Examples are given of arranging coupling between elements and synthesis of loop circuits, RS flip–flop, relaxation generators (clocks). The possibility is shown of performing logical operations with the aid of interference filters in nano‐ and picosecond times. Besides thermal nonlinearity, nonlinearities of electronic nature are considered which are due to size effects in semiconductor microcrystalline films.
The Theory of Nonlinear Slab Multistable Transmission
Authors: Khadzhi P. I. , Shibarshina G. D. , Rotaru A. Kh. , Slavov Yu. D.
Published: July 2007

A phenomenological theory of the nonlinear multistable light transmission by a semiconductor slab is presented taking into account the variation of the amplitude reflection coefficients of the interfaces versus the level of the excitation. It is shown, that the reflection coefficients of the front and back end faces of the slab are different. In the dispersive limit a generalized Airy formula for the multistable transmission is found.
Strong Dispersive Nonlinearity in the Excitonic Region of Wide‐Gap II–VI Compounds at Room Temperature
Authors: Spiegelber Ch. , Kretzschmar M. , Puls J. , Henneberger F.
Published: July 2007

The intensity‐dependent change of the refraction index of CdS at room temperature is deduced from degenerate four wave mixing experiments. For an application of the very strong dispersive nonlinearity optical switching, and bistability are demonstrated using the 514 nm output of an Ar+‐laser.
Beam Amplification by Transient Energy Transfer in GaAs and Si
Authors: Dubard J. , Smirl A. L. , Cui A. G. , Valley G. C. , Boggess T. F.
Published: July 2007

Two beam coupling experiments with 29‐ps (FWHM) pulses at a wavelength of 1.06 μm have been performed in GaAs and Si. By optimization of pump‐probe energy ratio, time delay and fluence we obtained optical gain up to 25 in GaAs at a fluence of 35 mJ/cm2 compared to roughly 140 mJ/cm2 required to obtain similar gain in Si.
The Effect of Pressure on the Thermodynamic Properties of Crystals near the Lattice Instability Point
Authors: Malinowska‐Adamska C.
Published: July 2007

Using the self‐consistent phonon theory based on the thermodynamic double‐time Green's function method the thermodynamical properties of the cubic crystals in the vicinity of the instability point are investigated. The results of calculations of the pressure dependence of the relative displacement of atoms, internal and free energy, free Gibbs energy obtained with the help of the generalized form of the Lennard‐Jones (n, m) renormalized potential are given and compared with experimental and other theoretical data for a number of cubic metals.
Electric Current Effect on Metal Surface Layers
Authors: Roshchupkin A. M. , Bataronov I. L. , Troitskii O. A. , Moiseenko M. M.
Published: July 2007

The electric current hydrodynamic effect on the metal surface layers due to the electronic liquid braking by the metal surface is considered in terms of the diffusivity parameter 𝒫 approximation. The relation between the coefficient of the electronic liquid kinematik friction against the surface and the 𝒫 parameter is obtained. It is shown that when current flows, in the near‐surface layer of the order of about the mean electron free path length the mechanical stresses ≈ 𝓅pFj/e arise, where pF stands for the Fermi momentum, j for the current density in the metal, e for the electron charge. It is noted that under the conditions of electroplastic deformation these stresses amount to a fairly significant value and can stimulate the dislocational processes in the nearsurface layer of metal.
Landau Levels and Cyclotron Masses in GaAsAlGaAs Quantum Wells
Authors: Del Valle J. Sab Ín , Gondar J. López , De Dios Leyva M.
Published: July 2007

Including the nonparabolicity and anisotropy effects the two‐dimensional cyclotron mass of electrons in GaAs is studied in the presence of a magnetic field parallel to the growth direction of the system. Numerical calculations for different quantum wells are reported and a comparison is made with recently reported experimental results.
Galvanomagnetic Properties of Graphite Fibers
Authors: Kulbachinskii V. A.
Published: July 2007

Using physical parameters extracted from the experimental studies of the negative magneto‐resistance the phenomena of resistivity rise with decreasing temperature and the subsequent onset of superconductivity at low temperature found in graphite fibers with NbC are explained by the electron‐electron interaction effect within the weak‐localization regime. It is shown that the galvanomagnetic properties of PAN‐, pitch‐, rayon‐based fibers before and after intercalation at low temperature can be explained completely by quantum corrections to the conductivity for the two‐dimensional case.
Two Valence Subbands in Single Crystals of Bismuth Telluride Doped with Lead and Its Electrical Properties
Authors: Biswas S. , Bhattacharya R.
Published: July 2007

Presence of two valence subbands in bismuth telluride has been suggested before from the study of transport properties under high magnetic fields. Results of experimental investigation of electrical conductivity and Hall effect with lead‐doped Bi2Te3 are reported. It is suggested that excitation from one valence subband to another is taking place such that the total number of carriers remains unchanged but the conductivity changes exponentially due to the difference of mobilities in the two subbands. This can account for the change in the electrical conductivity in the low temperature region which cannot be satisfactorily explained so far. Hall effect in the low temperature region can also be accounted for. The mobilities of holes in the two valence subbands and electrons in the conduction band are determined.
Effect of a Finite Voltage on Statistical Properties of Resistance in One‐Dimensional Disordered Systems
Authors: Heinrichs J.
Published: July 2007

Resistance fluctuations in the presence of an applied voltage are studied, using the Riccati equation for the reflection coefficient of a random conductor obtained by an invariant imbedding approach. As in the previously studied zero‐field case the conductor of length L is described by a Gaussian barrier, V(L), with a systematic part V, and the analysis is restricted to incident kinetic energies of the order of typical values of V(L)/2. The reflection coefficient is related to a resistance variable ϱ via the Landauer formula. Expressions for the distribution of the Landauer resistance and for its low order moments are obtained for arbitrary voltages and lengths L. The distribution of ϱ is exponential for L → 0 and log‐normal for L → ∞, with exponentially increasing mean‐ and rms‐values. The variable ln ϱ is self‐averaging at sufficiently low voltage. The properties of the reflection coefficient are discussed exactly in a different energy range, namely for low incident kinetic energies.
Electric Field Induced Splitting of the Stretching Vibration Lines of SH−, SeH−, and TeH− Centres in Alkali Halides
Authors: Otto J.
Published: July 2007

The stretching vibration lines of SH−, SeH−, and TeH− centres in various alkali halides are split into several components by an external electric field. No similar effects are observed yet for other centres. The splitting patterns are measured with high instrumental resolution for various directions of field and light polarization. From the number of componental lines the orientations turn out to be 〈111〉 for all investigated centres. Possible reasons for the splitting are discussed. Paraelectric alignment, however, well known for the homologeous OH− centres, is not observed.
Electron Transport in Amorphous (Ti, Nb)O2 Solid Solutions
Authors: Lagnel F. , Poumellec B. , Picard C.
Published: July 2007

Amorphous 1 μm thick (Ti, Nb)O2 films are prepared by r.f. sputtering in an argon atmosphere. Resistivity and thermopower measurements are carried out from room temperature up to 700 K in order to study the influence of the structural disorder on the electron transport properties. Experimental data are interpreted in the framework of Hurd's model, i.e. a hopping model including the temperature dependence of the tunnelling probability. Moreover, the analysis of the thermopower versus the Nb content indicates a partial electron transfer from Nb to Ti. On the other hand, these results suggest that, for the corresponding crystalline solid solutions at T > > 800K, the conduction is also caused by hopping and not delocalized.
On the Nature of the Self‐Trapping of Different Types of Polarons
Authors: Nagy P. , Markoš P.
Published: July 2007

A generalized Huybrechts approximation is developed and used for the calculation of the ground state energy, E, of the polaron as the function of the coupling constant α. A simple criterion for the continuity of ∂E/∂α is given, which enables to discuss the nature of the transition of the polaron from the free state to the localised state in the framework of different approximations.
Transformation of Excitonic Spectra of PbI2 Layer Crystals under Order‐Disorder‐Order Structural Transistions
Authors: Bibik V. A. , Davydova N. A.
Published: July 2007

The influence of ruby laser irradiation on the exciton spectra of PbI2 crystals of 2H and 4H poly‐types is studied. It is shown that the first pulses of laser irradiation lead to the creation of one‐dimensional disorder due to generation of stacking faults. The presence of stacking disorder leads to a localization of the excitonic wave functions in the direction perpendicular to the layer packets, causing a “fine” structure of the exciton band. The repeated irradiation leads to stacking faults ordering into the high‐symmetry 2H polytype, which has a shorter translational period in comparison with other polytypes and, thus requires less time for its ordering.
Investigation of (AlGa)As/GaAs quantum wells by optical and phototransport measurements
Authors: Ghezzi C. , Martin D. , Parisini A. , Staehli J. L. , Tarricone L.
Published: July 2007

An optical and phototransport characterization of (AlGa)As/GaAs multi‐quantum‐well (MQW) structures grown by molecular beam epitaxy (MBE) is reported. Photovoltage, photoluminescence, and optical transmission spectra are studied on p‐i‐n photodiodes in which the intrinsic (nominally undoped) region contains the wells. The thickness of GaAs and (AlGa)As layers is in the range 8 to 11 and 12 to 17 nm, respectively. The Al x‐molar fraction is in the interval 0.25 to 0.30. In the photovoltage spectra transitions between the fundamental and also the first excited state of electron and hole subbands in the quantum wells are observed. Agreement is found between these spectra and the results of luminescence and transmission measurements. Theoretical calculation fits the temperature dependence of the peaks in the spectra; it is possible to estimate the well width with an accuracy of about 10%.
The donor‐acceptor electron transfer with participation of short polymer chains
Authors: Ostapenko M. G. , Petrov E. G.
Published: July 2007

The rate dependence is investigated of activation and tunnel electron transfer between donor and acceptor separated by a chain of finite length on the energy and the relaxation characteristics of donor, chain, and acceptor. The chain length effect is shown to be especially pronounced at chain lengths not exceeding 5 to 7 links.
Photoconductivity in germanium selenide single crystals
Authors: Elkorashy A. M.
Published: July 2007

The photoconductivity spectral response is measured for GeSe single crystals at different temperatures from room temperature to near liquid nitrogen temperature. Measurements are carried out with plane polarized light with the plane of polarization parallel to the a‐ and c‐crystallographic axes which lie in the plane of cleavage. The photon energy range of measurements extends from 0.5 to 3.0 eV. The photoconductivity energy gap is determined from the spectral response by the Moss rule. The room temperature photoconductivity energy gap is found to be (1.144 ± 0.011) and (1.167 ± 0.025) eV for the a‐ and c‐axes, respectively. The temperature dependence of the photoconductivity energy gap is also deduced. It is found that it is linear between 93 and 250 K with a negative temperature coefficient equal to −(0.35 ± 0.01) meV/K for the a‐axis. The results for the c‐axis show a large discrepancy which is attributed to crystal lattice bending introduced during the cleavage process. The minority carrier lifetime is determined from the photoconductivity frequency response using square modulated illumination. The photoconductivity frequency response is found to be characterized by two different lifetimes.
Effective second‐order elastic constants and the second pressure derivatives of a hexagonal crystal
Authors: Rao R. Ramji , Padmaja A.
Published: July 2007

Expressions for the effective second‐order elastic constants of a hexagonal crystal and its second pressure derivatives are derived in terms of its natural second, third, and fourth order elastic constants using the finite strain theory. These formulae are employed to obtain the values for the combinations of the fourth order elastic constants of magnesium and zinc for which the experimental data are available. The combinations are positive in both of these metals. However, in zinc which is an example of extreme anisotropy among the h.c.p. metals, the combinations of the fourth order elastic constants are found to be larger compared to those in magnesium which has nearly the ideal c/a value.
Effective mass approximation for tunneling states with dissipation
Authors: Chen Hong , Wu Xiang
Published: July 2007

The dissipative tunneling in a symmetric double‐well potential is studied at low temperatures. Using effective mass approximation, the dissipation can be replaced by a temperature‐dependent effective mass. The effective mass increases with decreasing temperature and becomes infinite at T = 0. The partition function of the system is derived, which has the same form as that of a non‐dissipative tunneling system. Some possible applications in glasses and heavy Fermion systems are also discussed.
Hubbard systems with two channels for the localization
Authors: López‐Aguilar F. , Costa‐Quintana J. , Balle S.
Published: July 2007

A method is given for obtaining the electronic structure of the systems which present strong correlated bands arising from different orbitals, for instance f and d. This is performed by means of a non‐local potential deduced from the Green function of the interacting system. In addition, this method is applied to γ‐CeAl2 and the results agree reasonably with available experimental results.
Amplification of acoustic waves in magnetised piezoelectric semiconductors. Effect of nonlinearity in electron effective mass and collision frequency
Authors: Dey (Bose) M. , Ghosh S.
Published: July 2007

An attempt is made to provide a theoretical approach to experimental studies of the acoustoelectric amplification in magnetised piezoelectric semiconductors showing that the attenuation of an acoustic wave crosses over to its amplification when the electron drift velocity exceeds the acoustic wave velocity. First a phenomenological theory is developed to study the relative change in electron effective mass and collision frequency when an acoustic wave passes through piezoelectric semiconductors. Then, analytically, it is shown that this nonlinearity due to the electric vector of the field associated with the wave can also explain this behaviour. Numerical computations are made for n‐InSb at 78 K. It is found that the magnetic field does not have any remarkable effect on amplification.
Magnetic and Mössbauer studies of magnesium‐chromium ferrites
Authors: Balaev A. D. , Bayukov O. A. , Savitskii A. F.
Published: July 2007

The magnetization, Curie temperature, cation distribution, sublattice magnetization, and parameters of Mössbauer spectra are measured in MgFe2‐xCrxO4. The ligand‐cation 3d‐electron transfer parameters and the cation‐cation exchange interaction integrals are found using the superexchange bond model.
A Kinetic Theory for the Motion of Accelerated Particles in Solids. II. Dechanneling in Complex Crystals
Authors: Urmanov A. R. , Grigoriev V. G. , Puzanov A. A.
Published: July 2007

Models for the energy and angular distributions of the yield produced by Rutherford backscattering of fast ions and nuclear reaction products in the case of multiatomic crystals are suggested based on calculation of a generalized kinetic equation. Expressions for coefficients of this equation are obtained. The possibilities of the approach suggested are illustrated by interpreting experimental data on proton dechanneling in tungsten bronzes.
Temperature Effects of the Several Intense IR Absorption Peaks in NTD Si:H
Authors: Chen J. M. , Xei L. M. , Bai G. R. , Zhao Y. G.
Published: July 2007

Temperature effects of 2210, 1946, and 1831 cm−1 IR absorption peaks in NTD Si:H are measured and analysed. The phonon scattering process is the dominant factor of the temperature effects, while the phonon decomposition process can be neglected. The symmetry, Td, of defect‐complex corresponding to the 2210 cm−1 peak can be transformed into D2d symmetry as temperature rises above 200 K. The transition causes anomalous temperature effects which broaden the linewidth of the peak and destroy its symmetric peak shapes, i.e. shoulders appear on the low frequency sides of the peaks. The symmetric breaking mechanism strongly supports that the V + 4 H model is responsible for the 2210 cm−1 IR absorption peak. The correlation between the temperature‐dependent linewidth and the degeneracy of its vibration mode is discussed.
Semiempirical Construction of LCAO Parameters for II–VI Compound Semiconductors
Authors: Li Yuan , Lin‐Chung P. J.
Published: July 2007

A set of semiempirical expressions for the two‐center integrals which show the explicit dependence of these quantities on the atomic characteristics, on the dielectric constant, on the ionicity of the solid, and also on the interatomic separations is constructed for the II–VI compound semiconductors. The Slater‐Koster parameters are then constructed from these expressions for the two‐center integrals. The band structures of various zinc and cadmium compounds are obtained and are compared with other theoretical calculations and with experimental photoemission results.
Gyromagnetic Factor and Spin–Orbit Coupling of Ligands in CsVX3 (X = Cl, Br, I)
Authors: Du Mao‐Lu , Zhao Min‐Guang
Published: July 2007

A perturbation expression of the gyromagnetic factor of d3 ions is derived considering the spinorbit coupling of the ligand ions and taking into account all the excited states besides 4T2 state. The molecular orbital coefficients are determined by means of a semiempirical method in estimating the g‐shift. The results show that the contribution of the spin–orbit coupling of the ligand ions to the g‐shift must be considered for ligand ions possessing large spin–orbit coupling constants. The g‐shifts of CsVX3 are calculated and the results agree well with experimental data.
Electron Spin Resonance and Optical Absorption Spectra of Mn2+ Ions in K2SO4ZnSO4 Glasses
Authors: Rao J. L. , Narendra G. L. , Sreedhar B. , Lakshman S. V. J.
Published: July 2007

Electron spin resonance and optical absorption measurements are performed on K2SO4ZnSO4 glasses containing 1 to 5 mol% of MnSO4 · H2O. The ESR spectra of Mn2+ doped samples exhibit a sextet centered at g = 2.02. The optical absorption spectrum at room temperature shows six bands characteristic of Mn2+ ions in octahedral symmetry. The crystal field (Dq) and Racah interelectronic repulsion parameters (B and C) are evaluated. ESR and optical absorption studies reveal that the Mn2+ ions go substitutionally to zinc ion sites and the site symmetry around the transition metal ion is octahedral.
Mössbauer Spectroscopy of Cobalt in Germanium
Authors: Utzig J.
Published: July 2007

Mössbauer emission spectra of Co in Ge, introduced by high temperature diffusion, were recorded as a function of quenching rate and Fermi level. Two unbroadened single lines with different Debye‐Waller factors are found. One of the lines shows increasing intensity with decreasing quenchiing rates, and no shift with Fermi level. The other one exhibits a decreasing intensity with decreasing quenching rates and shows a small shift with Fermi level. Considering the Debye‐Waller factors a tentative attribution to Co on substitutional and on interstitial lattice is made.
Approximate Solution of the Peierls Equation for Screw Dislocations with Threefold Symmetry
Authors: Zhang H. T. , Mann E. , Seeger A.
Published: July 2007

Within the framework of the Peierls model an integral equation for the dislocation density distribution of 〈111〉 screw dislocations with threefold symmetry in b.c.c. crystals is derived. The dislocation core, described by a density of infinitesimal Burgers vectors, is extended along three {110} half‐planes intersecting at angles of 120° and acting as slip planes. The integral equation is approximately solved by an inversion procedure and by assuming appropriate functions with adjustable parameters representing the stresses along the slip planes. The results are in qualitative agreement with atomistic calculations reported in the literature. The total energy proves to be lower than in a planar Peierls model.
Comment on the Determination of the Midgap Density of States in Amorphous Semiconductors Using the Space‐Charge‐Limited‐Current Method
Authors: Chen Yang‐Fang
Published: July 2007

It is pointed out that the determination of the midgap density of states in amorphous semiconductors using the space‐charge‐limited‐current method is reliable only when the density of states is less than 1017 cm−3 eV−1 and the electrode spacing is in the order of μm. Beyond these restrictions the current will be affected by the Poole‐Frenkel effect which will distort the determination of density of states from current–voltage characteristics.
A Symmetry Adapted RPA: Dynamics of the Electronic Density Matrix in a Semiconductor
Authors: Schlösser J. , Stahl A.
Published: July 2007

The density matrix theory of electronic processes at the semiconductor band edge is developed in a symmetry adapted multi‐band version. A closed set of equations of motion describing coupled interband and intraband processes is derived on the basis of a Hartree‐Fock type RPA. From the general set of equations representing nonlinear crystal electrodynamics of the electronic edge a linearized subset is derived. The linearized set then forms the basis of linear crystal optics. As an example the polariton modes of the wurtzite type crystal CdS are discussed.
Study of Temperature Variation of EPR Linewidth of Gd3+‐Doped LiYbxY1−xF4 Single Crystals. Estimation of Yb3+ Spin–Lattice Relaxation Times
Authors: Misiak L. E. , Misra S. K. , Orhun U.
Published: July 2007

X‐band EPR linewidths of Gd3+‐doped LiYbxY1−xF4 crystals (0.0 ≤ x ≤ 1.0) are measured in the temperature range 4.2 to 290 K. It is found that the peak‐to‐peak linewidth of the first derivative lineshape (ΔHpp) can be fitted to T−n (T = tempeature), where generally n < 2, in the various temperature regions, for all the crystals. ΔHpp is, in general, found to be quite sensitive to the mole fraction (x) of Yb3+ ions. The Yb3+ spin–lattice relaxation times (τ′1) are estimated in these crystals, using the theories of Anderson‐Weiss and Mitsuma, taking into account the fact that there are present two different kinds of paramagnetic ions, Gd3+ and Yb3+. It is concluded that the dipole–dipole interactions play an important role in the spin–lattice relaxation of Yb3+ ions.
Photoionisation and Inter (Conduction) Band Absorption in S and Te Doped Gallium Phosphide
Authors: Goldys E.
Published: July 2007

The absorption coefficient of S and Te doped GaP i the region 0.25 to 1.4 eV is measured at 300 K. The experimental data are interpeted quantitatively. The spectrum is dominated by transitions from excited donor states. In the theoretical description the “mean” state concept is successfuly employed.
ESR Study of Silver Clusters in AgCl and AgBr. An Attempt to Detect Photographic Latent Images
Authors: Stevens A. D. , Symons M. C. R. , Eachus R. S.
Published: July 2007

ESR signals observed after the bandgap irradiation of AgCl and AgBr sols and primitive emulsions are reported. Their parameters are similar to those of small, pseudo‐metallic silver particles. An assignment to latent images is proposed on the basis of the results of empirical ESR/photographic crossover experiments. The size(s) of the latent image agglomerates seems to vary according to the halide, but there appears to be discrete size or small size distribution for a given system. Although it would appear that the clusters are larger than Ag7, precise particle size deductions from the ESR data are shown to await the development of a suitable electronic theory. Smaller, neutral or charged molecular silver aggregates have not, so far, been detected in pure silver halide systems. However, it is possible that signals are present but are too dispersed through a large field range for detection. The existing ESR data for silver clusters are reviewed and reviewed and the possible electronic structures and associated magnetic properties of latent images are discussed.
Long‐Wavelength Optical Phonons in GaAsAlAs Superlattices
Authors: Bechstedt F. , Gerecke H.
Published: July 2007

Optical phonons in polar semiconductor superlattices near the zone center are studied. The microscopic theory based on a rigid‐ion model is simplified by restriction to one elastic force constant per material and spatial average of the long‐range electric field. The theory allows much insight into the mode structure, particularly anisotropy and layer thickness dependence of frequencies. atomic displacements, and accompanying electric fields. Explicit results are given for short‐period GaAsAlAs superlattices. The relationship of confined and interface modes is discussed.
Magneto‐Optics of Holes in the Presence of Internal Strains and Temperature in Single Quantum Well Structures
Authors: Singh M. , Tang S. Y.
Published: July 2007

Expression are obtained for the frequency dependence conductivity tensor in the presence of a magnetic field and hole‐phonon interaction for p‐type single quantum well structures. From this one obtains the information about the transport and optical properties of single quantum well structures. The [4 × 4] Luttinger‐Kohn type of Hamiltonian is used to calculate Landau levels and Landau wave functions. Linear response theory is used to calculate the conductivity tensor and Green's function method is used to calculate the relaxation time (or Landau level broadening) due to hole‐phonon interaction. The [4 × 4] matrix interaction Hamiltonian is considered for the hole‐phonon interaction which includes the effects of internal strains arising from the mismatch of lattice constant or other defects. Selection rules Δn = ±3, Δn = ±1, are found and their corresponding probability amplitudes are obtained. Dispersional relations for magnetoplasma waves and elementary excitation are also given.