bluuebunny/crossref_metadata_2025 huggingface dataset:
Time Dependence of Wave‐Mixing Processes under Pulsed Resonant Excitation in Three‐Level Systems
Authors: Hönerlage B. , Frindi M. , Miletic J.
Published: July 2007

A theoretical analysis is given of wave‐mixing processes in three‐level systems excited by nano‐second pulses close to a two‐photon resonance. It is shown that the lifetime of the incoherent quasi‐particle population influences the time dependent generation of the coherent signal.
Applications of a Point Polarizable Ion Model to Suzuki Phase Precipitates in Alkali Halides
Authors: de Andrés A.
Published: July 2007

Lattice energy and phonon frequencies are calculated for the Suzuki phase (SP) precipitates in several materials. The dependences of the phonon frequencies versus pressure, anion relaxation, and the size of the precipitates are calculated in order to simulate the heat treatments effects on A1g and T2g, 1 modes observed by Raman spectroscopy. The observed changes seem to be due to the variation of some SP parameters and to the confinement of the phonons to an estimated size of the precipitates of about 4.5 nm.
Collective Plasmon Modes in Thin Semiconductor Films Including the Effect of the Image Charges
Authors: Aharonian K. H. , Erknapetian H. L. , Tilley D. R.
Published: July 2007

The dispersion of unretarded plasmons in quantized films in the presence of image charge forces is derived. It is shown that the lowest two‐dimensional plasma frequency increases strongly with decreasing film thickness and is independent of wave vector if the film dielectric constant is much larger than that of the substrate. The possibilities for experimental observation are discussed.
Binding Energies of Neutral Bound Excitons for Intermediate to High Magnetic Fields
Authors: Dujardin F. , Stébé B. , Munschy G.
Published: July 2007

The influence of an external constant magnetic field on the ground state energy of an exciton bound to a neutral shallow acceptor impurity is studied in the effective mass approximation using a variational non‐adiabatic method well‐adapted for intermediate to high magnetic field values. It is shown that the magnetic field enhances significantly the Coulomb interaction for any value of the electron‐to‐hole effective mass ratio. The results permit to discuss the validity of those ones previously obtained in the low and high field limits using respectively perturbation and adiabatic variational methods. They allow the estimation of the transition energy between a neutral acceptor A0 and an (A0, X)‐complex knowing the fundamental exciton transition energy in a magnetic field.
Uniaxial‐Stress Dependence of the NQR Frequency of 35Cl in Paradichlorobenzene
Authors: Zamar R. C. , Brunetti A. H.
Published: July 2007

Measurements of the NQR frequency (NQRF) of the 35Cl nucleus in single crystals of paradichlorobenzene are made as a function of isothermal uniaxial stress along several crystalline directions, in the range of 0 to 0.35 MPa, at a temperature of 0 °C. A linear and directional behaviour is observed. A theory is developed which includes the anharmonicity of the crystalline potential by means of the quasi‐harmonic approach. Starting from the Bayer‐Kushida theory an expression for the NQRF dependence with stress at constant temperature is deduced. Similarly, the first‐and second‐order derivatives with respect to the temperature at constant stress are obtained. In both the cases the derivatives are related with the Grüneisen parameters of the external modes and with the parameters which describe the thermodynamic state of the crystal (isothermal elastic constants and thermal expansion coefficients).
Theory of the Optical Stark Effect in Semiconductors under Ultrashort‐Pulse Excitation
Authors: Lindberg M. , Koch S. W.
Published: July 2007

The optical Stark effect is computed for femtosecond excitation of the electron–hole‐pair states in semiconductors. The theory is based on the effective Bloch equations for semiconductors and is evaluated for the large detuning, low intensity limit. For the example of CdS, good agreement with recent femtosecond experiments is found. It is shown that the coherent interaction of the pulses and the medium polarization dominates the semiconductor response as long as the pulse duration is of the order of, or less than, the coherence decay time of the medium.
Coherent Nonlinear Response of Excitons in Semiconductors
Authors: Balslev I. , Stahl A.
Published: July 2007

The nonlinear optical response of excitons in semiconductors is studied theoretically, particularly the so‐called nonresonant optical Stark effect. Results from a two‐band density matrix formalism and its reduction to a two‐level approximation are compared. It is found that the two models give similar results except at very high pumping levels. The calculated results agree well with experiments on quantum wells in (AlGa)As–GaAs.
All‐Epitaxial GaAs/AlAs Nonlinear Etalons. Towards Continuous and Parallel Operation
Authors: Kuszelewicz R. , Oudar J. L. , Azoulay R. , Michel J. C. , Brandon J. , Emile O.
Published: July 2007

The fabrication and optical characterization are reported of MOVPE‐grown GaAs bistable étalon with GaAs/AlAs integrated mirrors. A 4 mW threshold power and a 2:1 reflectivity contrast are observed. Quasi‐continuous operation is observed in relation with thermal dissipation improvement when the substrate is removed. Two‐dimensional arrays of individual devices are fabricated and buried with Al0.2Ga0.8As for electrical and optical confinement. A theoretical model is also described which calculates the temperature distribution by an iterative propagator method.
Cavityless Dispersive Bistability and Optical Addressing Using a Nonlinear Prism
Authors: Schülzgen A. , Henneberger F. , Rossmann H.
Published: July 2007

A new kind of cavityless dispersive optical bistability is analysed and demonstrated on ZnSe at room temperature. Switching intensities of about 10 W/cm2 are observed. Directing of light beams by all‐optical means is also performed.
Optical Bistability under Conditions of Total Reflection
Authors: Boiko B. B. , Petrov N. S. , Shakin V. A. , Zimin A. B.
Published: July 2007

Two simple, optically nonlinear plane, layered systems for bistability of intensity and phase of the reflected light wave are proposed and investigated in the plane‐wave approximation.
Optical Nonlinearity and Bistability in Silicon
Authors: Eichler H. J. , Brand T. , Glotz M. , Smandek B.
Published: July 2007

The large number of nonlinear optical effects observed in silicon is referenced. Special emphasis is given to laser‐induced absorption and refraction due to free‐carrier generation and optical heating produced by Nd: YAG‐lasers at 1.06 μm. Nonlinear silicon etalons exhibit high‐order optical bistability. Switching powers down to 1 mW are obtained with the injection of an additional photocurrent. Two‐wave mixing is discussed as an alternative switching scheme.
ps‐Laser Induced Transmission and Absorption in GaInAsP Epitaxial Layers
Authors: Rückmann I. , Kolenda J. , Henneberger F. , Petrauskas M.
Published: July 2007

For a possible application in optical signal processing in the infrared region the laser‐induced change of transmission and its temporal behaviour are studied (1) in 1.65 μm‐GaInAs using ps‐pulses at 1.06 μm (ħωL > Eg + Δso) and (2) – to realize a more near gap excitation – in 1.16 μm‐GaInAsP (Eg < ħωL < Eg + Δso). A strong induced transmission is found with a fast decay rate due to Auger recombination. The density dependence of the absorption coefficient including band filling and gap renormalization and thereby the transmittivity versus intensity and time are calculated and compared with the experiment. In GaInAsP an induced absorption is observed additionally due to free‐carrier intervalence band absorption.
Resonatorless Optical Bistability near the Excitonic Resonance of ZnSe
Authors: Gribkovskii V. P. , Zimin L. G. , Gaponenko S. V. , Malinovskii I. E. , Kuznetsov P. I. , Yakushcheva G. G.
Published: July 2007

Monocrystalline MOCVD‐grown ZnSe films are found to exhibit optical bistability and pulsation regimes at nitrogen temperatures. The peculiarities of nonlinear optical response are associated with the polaritonic mechanism of light propagation.
Time‐Resolved Light‐Induced Grating Spectroscopy in Semiconductors
Authors: Lévy R. , Hönerlage B. , Grun J. B.
Published: July 2007

Degenerate light‐induced grating spectroscopy experiments in semiconductors which are excited close to electronic resonances are discussed. Under these conditions, the finite lifetime of the quasiparticle states becomes important and governs the time dependence of the generated signal.
Transient Gratings in Metrology of Semiconductor Parameters and Optoelectronic Devices
Authors: Jarašiunas K. , Vaitkus J.
Published: July 2007

The origin of optical nonlinearity and its magnitude is investigated in different semiconductors and structures, as CdSe, GaAs, InSb, Si (pure, ion‐implanted, heavily doped or amorphous), MQWS. The usefulness of the transient grating technique is shown to study peculiarities of the non‐equilibrium processes in strong electric fields, at high excitation levels or to reveal the presence and transformation of defects. Some novel possibilities for the deflection of a laser beam and its modulation are demonstrated.
Thermoelectric Power of Dilute Magnetic Alloys in the Presence of CEF and Exchange Scattering
Authors: Kumar S. , Ahluwalia P. K. , Sharma K. C.
Published: July 2007

Self energy in third order and the thermo‐electric power, Q, of a metal containing magnetic impurities with crystal field split energy levels are calculated. The effect of exchange inelastic scattering on the thermo‐electric power is studied in the limit ω, T ≪ δ. The most dominant term in Q is found to be of the type sech2 (δ/2T) T ln T.
Internal Structure of Two‐Dimensional Domain Walls in Finite Bulk Crystals
Authors: Hartmann U.
Published: July 2007

The two‐dimensional structure of a 180° Bloch‐type interdomain boundary in the near‐surface region of bulk crystals of finite thickness is derived by an approximate analytic solution of the constitutive micromagnetic equations. The symmetrical equilibrium wall structure is characterized by an extensive flux closure internal to the crystal and a compressed charge distribution on the crystal surface. It is found that the magnetostatic energy term is the principal stabilizing factor that prevents a 180° Bloch wall, even in non‐magnetostrictive materials, from structural decomposition into two adjacent 90° walls.
The Selective Trapping of Arsenic Interstitial Atoms by Impurities in Gallium Arsenide
Authors: Collins J. D. , Gledhill G. A. , Murray R. , Nandhra P. S. , Newman R. C.
Published: July 2007

Carbon doped GaAs is irradiated either with fast neutrons (1 MeV) or2 electrons (2 MeV). Complexes previously labelled C(1) are produced and the fine structure of the two LVM lines is discussed. A comparison is made with the BGaAsi complex and it is concluded that the C(1) centre is a CAsAsi defect. The defect dissociates at about 150 °C.
Vibrational Spectra of Fibonacci Chains with Defects
Authors: Böttger H. , Bradl I. , Kasner G.
Published: July 2007

Using different methods the effect of defects on the vibrational spectrum of Fibonacci chains is studied. Mistakes can strongly affect high‐frequency vibrational modes. Replacements of every A(B) segment by two A(B) segments extend the low‐frequency band‐like behaviour up to higher frequencies and enlarge the gaps and narrow the bands in the high‐frequency part of the spectrum.
Electron Paramagnetic Resonance Study of Copper(II) Ions in Single Crystals of Thallium Magnesium Sulphate Hexahydrate
Authors: Krishna R. M. , Rao J. L. , Chand Prem , Lakshman S. V. J.
Published: July 2007

EPR investigations of the copper(II) ion is carried out in its paramagnetically dilute form in thallium magnesium sulphate hexahydrate (Tl2Mg(SO4)2 · 6 H2O) at 300 and 77 K at X‐band. Cu2+ ions are found to substitute for divalent cations exhibiting two symmetry related equivalent complexes. The g‐ and A‐tensors are found to be axially symmetric upto liquid nitrogen temperature (LNT) and are temperature dependent. The ground state is suggested to be mainly a d orbital. The molecular orbital coefficients are evaluated and indicate a moderate covalency of Cu2+ ion.
General Properties of Highly Excited Direct Semiconductors in the Athermal Stage
Authors: Heiner E.
Published: July 2007

Starting from first principles and prolonging a series of three previous papers some general and global properties of highly excited direct semiconductors are considered. The evolution of the lengths of the state vectors is considered and some properties are discussed. Furthermore the evolution of the total electron production rate and of the total energy are investigated. Thereby new terms are derived which describe the energy storage due to nonlinearities in the evolution equations. Finally, some aspects of ultrashort pulse duration are considered.
Pressure‐induced phase transitions and anharmonicity study of alkaline‐earth fluorides
Authors: Kourouklis G. A. , Anastassakis E.
Published: July 2007

Room temperature measurements of Raman scattering under hydrostatic pressure in a diamondanvil cell have shown that the cubic‐to‐orthorhombic (β → α) phase transition in CaF2 and BaF2 occurs at (8.5 ± 0.2) and (2.3 ± 0.2) GPa, respectively. The pressure dependence of the observed phonons in the α‐phase as well as their symmetry assignment based on a comparison to other fluorite‐structure crystals is presented. Anharmonic processes are shown to be small but not negligible and of the same nature in all alkaline‐earth fluorides. It is established that the β → α and α → β transition pressures show a linear dependence on the inverse volume of the unit cell for the alkaline‐earth fluorides CaF2, SrF2, and BaF2.
One‐dimensional dirac equation approach to the effective mass concept in diatomic crystals
Authors: Detcheva V. , Metodieva M. , Khristov T. , Kandilarov B. D.
Published: July 2007

A relativistic generalization of the effective‐mass concept is worked out on the basis of simplest one‐dimensional model of diatomic crystals and in the framework of the Dirac equation approach. The numerical analysis performed leads to reasonable values for both, the energy gaps and the effective masses of the carriers. The relativistic corrections thus obtained bring on a firm basis the justification of simpler calculations in the framework of the Schrödinger equation. The need of more powerful approaches beyond the Dirac equation is outlined, especially for crystals of very heavy atoms (Z > 68).
Hopping relaxation and photoconductivity of disordered semiconductors
Authors: Zvyagin I. P.
Published: July 2007

A method of evaluating the distribution function of hot localized carriers excited by an external electromagnetic wave is presented. The time‐dependent distribution function is used to find the hopping current relaxation for a quasi‐continuous distribution of states and the de photoconductivity at low temperatures.
Approximate solution for a two‐dimensional anisotropic ferromagnetic spin lattice with Bloch Wall and Bloch Line. II. Numeric Computation
Authors: Knoop U. , Heber G.
Published: July 2007

For a two‐dimensional classical spin lattice with local anisotropy or exchange anisotropy the nonlinear difference equations, which are derived from the Heisenberg Hamiltonian, are numerically solved. The spin configuration of a spin lattice containing a Bloch wall and a Bloch line is computed.
Low‐temperature IR and Raman scattering spectra of TlInS2 layered crystal
Authors: Allakhverdiev K. R. , Babaev S. S. , Tagiev M. M. , Shirinov M. M.
Published: July 2007

An investigation is made of the polarized far IR and Raman scattering spectra of layered single crystals of ternary compound TlInS2 in the 6 to 300 K temperature range including the series of the phase transition. New unknown low‐frequency Raman and IR active phonons are observed. It is shown that the interpretation of the vibrational properties of TlInS2 crystal are in good agreement with the C space group. A low‐frequency Raman active soft mode is observed and analysed in the ferroelectric phase of this crystal. The pecularities in changing of the spectra of optical phonons, connected with a transition of the crystal from the para to ferroelectric state, from modulating structure, and quadrupling along C‐axis are observed and analysed.
X‐Ray Emission Spectroscopy of the Electronic States in Pb1−xHgxS Thin Films
Authors: Acharya B. S. , Singh S. K. , Nayak B. B.
Published: July 2007

X‐ray emission specta in metals and alloys have been studied extensively. However, work on semiconductor alloy thin films is rather scarce. Here the sulphur Kα emission in Pb1−xHgxS thin films prepared by electroless deposition method is investigated. Films in the thickness range 0.2 to 0.3 μm are grown by this method. X‐ray emission spectra of SKα band in Pb1−xHgxS reveal the presence of three‐band structure at about 2, 5 to 7, and 17 to 18 eV from the Fermi level EF. The asymmetric index (x) is found to be higher than that of the pure SKα band. Alloying the film with Hg increases the α in the beginning and then decreases and finally increases it for HgS. It is observed that the density of states (DOS) at EF decreases for alloy thin films. From the shift in the Kα peak energy the charge associated with the S ion in these films is calculated. Besides, the bonding parameter (λ) is calculated from the charge and it is observed that for PbS, the bonding is homopolar and decreases with the increase in Hg concentration reaching a minimum nearer to neutral bonding value and thereafter it increases showing the bonding in HgS to be homopolar again. The satellite peaks are explained on the basis of hybridization of s– and p–d orbitals.
Bloch Oscillations of an Electron in a Crystal under the Action of an Electric Field
Authors: Nazareno H. N. , Gallardo J. C.
Published: July 2007

The dynamical localization problem of an electron in a ID crystal under the action of a uniform de electric field E is solved. Starting with a determined state at t = 0, it is solved for the Wannier amplitudes (propagator) and shown that they oscillate in time with the Bloch frequency. Moreover, it is shown the behaviour of the optical absorption coefficient as a function of the photon frequency ω which shows jumps whenever the photon energies equal multiples of eEa.
Fermi Level Density of States in AgAu Alloys. Nuclear Magnetic Spin‐Lattice Relaxation Rate and Low‐Temperature Specific Heat
Authors: Kokko K. , Ojala E. , Mansikka K.
Published: July 2007

The electronic structure of AgAu alloys is studied using LMTO self‐consistent band structure calculations. Both, the total and partial density of states as function of Au‐content at the Fermi level are presented. Some results of band structure calculations are analysed using a simple cluster model, which reveals that the behaviour of the partial density of states at the Fermi level can be explained by the s–d hybridisation strength and the position of the top of the d‐band. The results obtained for specific heat and spin–lattice relaxation are in general agreement with experimental observations. Furthermore, the calculations predict that the nuclear magnetic spin–lattice relaxation time of Au would decrease with increasing Ag‐content.
Ideal Two‐Dimensional Electron Gas in a Magnetic Field and at Non‐Zero Temperatures. An Alternative Approach
Authors: Wang L. , O'Connell R. F.
Published: July 2007

A new exact expression is derived for the free energy of an ideal two‐dimensional electron gas (2DEG) in a uniform magnetic field and at low, but finite, temperatures. The approach eliminates a 2D‐peculiarity that neither the weak nor the strong magnetic field limit can be easily taken in the result derived from the standard Sondheimer‐Wilson treatment. In the strong magnetic field limit, the result reduces to an existing one, for which a misunderstanding needs to be clarified. In the weak field limit, it agrees with a result obtainable through Euler's summation formula. The conditions are discussed under which the nondegenerate situation may occur.
EPR Linewidth in Pb1−xMnxTe
Authors: Korczak S. Z. , Korczak W. , Subotowicz M. , Wasiewicz H.
Published: July 2007

EPR in semimagnetic semiconductors Pb1−xMnxTe with x ≦ 0.017 is studied over the temperature range 3.5 to 300 K. The variations of linewidth with Mn content and temperature are analyzed. The analysis of the experimental results is based on the assumption of the presence in the system of both a Korringa‐type relaxation and a bottleneck effect. An approximate value of the exchange constant between localized magnetic ions and carriers, J = (0.08 ± 0.03) eV, is deduced.
Fundamental Solution of the Problem on the Relief Shape of a Kink on a Dislocation in an External Force Field Equal to the Barrier Reaction
Authors: Polyakov M. E.
Published: July 2007

A critical analysis is made of the double kink model of dislocations and its disadvantages are revealed. A modified model of single dislocation kink free of the disadvantages of the previous model is proposed. In the new model, the form of the single kink relief is analytically found from the variational problem of the effect of external stress on the dislocation for the case of Peierls sinusoidal barrier. The single kink is shown to be subjected to pseudobreakage. The pseudobreakage area is characterized by an imaginary solution. Analytical dependences are obtained of the single kink and pseudobreak relaxation areas on the barrier height, and the dislocation shift by the external force of the Peierls relief valley.
A Nonlocal Continuum Approach to the Dynamics of Crystals. I. S.C. and F.C.C. Lattices
Authors: Idiodi J. O. A.
Published: July 2007

With a view towards helping to bridge the gap between discrete and continuum models of crystalline elastic solids, a nonlocal continuum theory which describes exactly a class of lattice models that have been widely used for s.c. and f.c.c. lattices, is presented. Particular emphasis is placed on the treatment of the boundary conditions that arise in the presence of a surface defect.
Vibrational Spectra and Phase Transitions in Layered Semiconducting Ferroelectrics with TlGaSe2 Structure. I. Temperature Dependences of Parameters of the Raman Spectrum Lines of TlGaSe2
Authors: Durnev Yu. I. , Kulbuzhev B. S. , Malsagov A. U. , Rabkin L. M. , Torgashev V. I. , Yuzyk Yu. I.
Published: July 2007

Raman spectra of TlGaSe2 single crystals are obtained in the temperature range 85< T < 300 K, which includes the phase transition points. Fitting of the spectra, using the model of additive damped oscillators and a relaxator, yields quantitative information on parameters of the lines. No critical rise of the relaxation time is detected near phase transitions. On the other hand, the strength of the relaxator experiences an approximately twofold increase. An anomalous behavior of parameters of the lines is observed near phase transitions, which is thought to be associated with ordering processes. The lowest‐frequency lines in the 15 cm−1 region are interpreted as the lines which correspond to rigid‐layer modes.
Strain Effects on the Band Structure of Si/Si1–xGex (001) Superlattices
Authors: Rücker H. , Enderlein R. , Bechstedt F.
Published: July 2007

The electronic structure of strained layer Si/Si1–xGex superlattices is calculated by means of the empirical tight‐binding method. The influence of strain on band structure is treated by applying interatomic interaction parameters depending on bond lengths and bond angles. Strain effects on band discontinuities are studied. A strain induced type I‐type II transition in Si/Si0.5Ge0.5 superlattices on substrates with different lattice constants is obtained. Threshold energies and oscillator strengths of optical band‐to‐band transitions are reported for short period SinGen superlattices grown on Si1‐yGey in dependence on n and y.
On the Correct Use of Perturbation Theory for the Calculation of One‐Particle Green Functions
Authors: Monecke J.
Published: July 2007

Perturbation theory conventionally is applied either to one‐particle Green functions (or to equivalent quantities) directly or to their inverses, i.e. to the self‐energies. Both methods, however lead to wrong, in the first case even to negative and divergent, densities of states in the corresponding order. Here the system of equations for the Green functions is solved exactly in the standard form G = N/D and perturbation theory is applied to both, the nominator N and the denominator D separately. It is shown that this method is free of divergencies and results in the correct density of states in each order. This new perturbation theory is especially suited for the calculation of electron and phonon spectra of superlattices.
Effect of Laser Irradiation on Low‐Temperature Photoconductivity and Photoluminescence Spectra of Gallium Selenide
Authors: Mozol P. E. , Skubenko N. A. , Skubenko P. A. , Gnatenko Yu. P. , Salkov E. A. , Kovalyuk Z. D.
Published: July 2007

Features of photoluminescence (PL) and photoconductivity (PC) spectra, taken at 4.2 to 60 K, of GaSe crystals of stoichiometric composition and grown with an excess and with a lack of Ga atoms are analyzed. It is shown that the presence of excessive Se atoms in the matrix of GaSe crystals results in the formation of centres where triplet excitons are bound. Excessive Ga atoms formed in a crystal both, in the process of its growth and under the effect of laser irradiation give rise to regions with another polytype composition (γ‐modification). The presence of such (γ‐polytype) regions is shown to produce packing defects whre localization of excited electron states (n > 1) occurs. An effect of PC signal increase with increasing exposure dose is found, which is attributed to the formation of acceptor centres.
Diffusion Studies in CdTe and HgTe
Authors: Madhavan Y. , Ramachandran K. , Haridasan T. M.
Published: July 2007

Lattice dynamical calculations of the jump frequencies for various isotopes of cation and anion in both, CdTe and HgTe, are worked out following the general theory of diffusion for metals by Achar, assuming an active role for vacancy. The isotope effect, which is a measure of the diffusion rate in both the cases, is also deduced. The results indicate that the diffusion in these systems is governed prominently by a single vacancy mechanism as predicted from experiments.
Transmission Coefficient and Resonance Condition in Rectangular n‐Fold Barrier Structures
Authors: Yamamoto H. , Kanie Y. , Taniguchi K.
Published: July 2007

Resonant tunneling is studied theoretically in rectangular n‐fold barrier structures. Analytical expressions of the transmission coefficient and the resonance condition are derived for the first time. It is believed that the expressions are useful for evaluating the energy variations of the transmission coefficient and estimating the resonance energy, and for resonant tunneling device fabrications.
Conductivity and the Metal–Dielectric Transition in Compensated n‐ZnSe
Authors: Kasiyan V. A. , Nedeoglo D. D. , Simashkevich A. V. , Timchenko I. N.
Published: July 2007

In the temperature range from 1.6 to 300 K the dcresistivity of n‐ZnSe in the region of the metal–dielectric transition induced by compensation is investigated. Over the dielectric part of the transition the power dependence ϱ(T) with an exponent smaller than zero in the critical region is replaced at sufficiently low temperatures by a dependence of the form ϱ(T) = ϱ0 exp (T0/T)1/2. The observable exponential dependence of ϱ on T is treated in the frames of a model of hopping conductivity with a variable range hopping over the states of the Coulomb gap near by the Fermi level. Such parameters as the screening radius Rs, which determines the size of large‐scale fluctuations, the radius of localization of the electron wave function a, the dielectric constant x, and the coefficient g0 which determines the density of the localized states near the Fermi level are calculated. It is established that g0, χ, and a increase with the decrease of K = NA/ND, that is with the approach to the metal–dielectric transition. It is assumed that this is connected with the decrease of the width of the Coulomb gap and its possible disappearance in vicinity of the transition.
LCAO Recursion Approach for the Bound Electron States at the 90°‐Partial Dislocation in Silicon
Authors: Wang Yong‐Liang , Teichler H.
Published: July 2007

The evaluation of bound electron states at extended lattice defects in Si is studied within an LCAO scheme under full inclusion of s‐, p‐, and d‐states and their mutual non‐orthogonality. The levels are determined from a continuous‐fraction recursion treatment. Application to a supercell model of the unreconstructed 90°‐partial dislocation demonstrates that the method works well for basis sets of 2560 complex basis functions. The present results for the bound electron states at the 90°‐partial dislocation in Si appreciably differ from the earlier tight binding estimates due to the here included dangling bond interactions. Two one‐dimensional bands of bound states are found showing large band splitting inside the Brillouin zone and degeneracy of the bands at the zone boundary with an occupation limit between empty and full states 0.12 eV above the valence band edge.
VUV and UV Spectroscopy of Pb2+ and Bi3+ Centres in Alkaline‐Earth Fluorides
Authors: Oboth K. P. , Lohmeier F. J. , Fischer F.
Published: July 2007

Absorption and emission studies on Pb2+ and Bi3+ centres in different alkaline‐earth fluorides are reported. The A absorption is observed in the ultraviolet and the B and C absorption in the vacuum ultraviolet spectral range. The A band in CaF2 exhibits a vibrational structure. At low temperatures the symmetry‐forbidden B′ emission (3Eu, 3T2u → 1A1g) and the spin‐ and symmetryforbidden α emission (3A1u → 1A1g) are observed. In CaF2 and SrF2 the α emission exhibits a distinct vibrational structure. Its zero‐phonon line is due to a strong hyperfine coupling. At higher temperatures the α emission as well as the B′ emission vanish and only the A′ emission is left.
Calculation of Interband Tunneling in Inhomogeneous Fields
Authors: Schenk A. , Stahl M. , Wünsche H.‐J.
Published: July 2007

A new approach to the interband tunneling in semiconductor junctions is developed. It generalizes the traditional WKB and EMA theories, which fail in the limits of strong fields and inhomogeneous fields, respectively. The given expression for the energetic tunneling rate remains valid in both limits. In the strong field case it tends to the parabolic band EMA result, whereas for low fields with medium inhomogeneity any band structure model is possible. A Kane model is used to calculate tunneling and total I–U‐characteristics for narrow gap diodes. The breakdown behaviour yields information about the electrically active doping profile.
Electronic Properties of AmBn Complex Crystals
Authors: Kumar V. , Prasad G. M.
Published: July 2007

A simple model is proposed which relates the homopolar and heteropolar energy gaps, ionicity, dielectric constant, and other physical and chemical properties of AmBn complex crystals with their plasmon energy. Special care of d‐electrons is taken into account in the case of transition metal compounds. The calculated values are in fair agreement with the values reported by other workers.
Spin‐Correlated Crystal‐Field Analyses of the 4f2(Pr3+) Energy Levels in LiYF4:Pr3+ and LiBiF4:Pr3+
Authors: Jayasankar C. K. , Richardson F. S.
Published: July 2007

Comparative analyses of the 4f2(Pr3+) energy level structure in LiYF4:Pr3+ and LiBiF4:Pr3+ are reported. These analyses are based on a phenomenological crystal‐field model that includes some (partial) consideration of electron‐correlation effects in the 4f‐electron‐crystal‐field interactions. Parametric fits of calculated‐to‐experimental energy level data are reported for a 48‐level data set for LiYF4:Pr3+, a 38‐level data set for LiBiF4:Pr3+, and a data set that contains 37 levels assigned in common for the two systems. The 4f2 energy level structure is qualitatively similar in the two systems, but the LiYF4:Pr3+ system exhibits slightly stronger crystal‐field interactions. Inclusion of spin‐correlated crystal‐field (SCCF) interactions in the phenomenological energy level analyses produces slightly improved agreement between calculated and empirical results, but it does not resolve major discrepancies between calculated and observed crystal‐field splittings within the 1G4 and 1D2 multiplet manifolds.
Origin of Periodic Fluctuations with Temperature of Ultrasonic Attenuation in zinC, Aluminium, and Iron
Authors: Youssef Moheb H. , Roushdy M. , Gobran N. K.
Published: July 2007

The ultrasonic attenuation—temperature curves for zinc, aluminium, and iron are found to comprise a periodic fluctuation of the attenuation with temperature as measured at 2, 4, and 8 MHz. This fluctuation is superposed on the well‐known types of ultrasonic damping in metals. The periodicity of such fluctuations is explained in terms of successive thermal levels of atomic vibrations. This approach affords successfully the visualization of the responsible pair of levels which effectively predominates along a certain interval of temperature and furnishes a lossive two‐state model. The successive models are also supported by a previous work on nickel.
On Hydrogen Chemisorption on Simple Metals
Authors: Taranko E. , Gavrilenko G. M. , Taranko R. , Fedyanin V. K.
Published: July 2007

A theoretical study of the single hydrogen adatom chemisorption on free electron‐like metal surfaces is performed using an earlier published microscopic approach which includes the possibility of an influence of the adatom orbital occupancy on the hybridization of the adatom and substrate electron wave functions. The matrix elements inherent to the model are calculated from first principles using free‐electron wave functions for describing the substrate metal and treating the IS‐orbital of the adatom as an auxiliary orbital. The effect of the screening charge is considered phenomenologically in terms of an effective on‐adatom Coulomb integral. The self‐consistent Hartree‐Fock calculations show much more preference for hydrogen absorption rather than adsorption and slightly more adatom substrate—surface distance‐dependence of the chemisorption energy compared to results obtained within the local density functional method.
Quantum Calculation of Multiphonon Capture Cross‐Sections in the Model of Tangent Adiabatic Potentials
Authors: Belousov A. V. , Kovarskii V. A. , Chebotar V. N. , Pishkova T. V.
Published: July 2007

The multiphonon capture cross‐section of deep impurities in semiconductors is considered taking into account the tangency of the adiabatic potential. The calculation is based on the quantum description of the one‐coordinate vibrational model. A comparison with the quasiclassical approach is given.
Electronic States of a Hydrogen Atom Near a Hard Wall
Authors: Kovalenko A. F. , Sovyak E. N. , Golovko M. F.
Published: July 2007

An exact solution of the Schrödinger equation for a hydrogenic atom located near a hard wall is found. The discrete electronic spectrum and the set of semi‐bounded hydrogenic wave functions are obtained. The atom dipole moment which arises due to the presence of the wall is calculated.
Comparative Crystal‐Field Analyses of 4fN Energy Levels in LiYF4:Ln3+ Systems
Authors: Jayasankar C. K. , Reid M. F. , Richardson F. S.
Published: July 2007

Comparative analyses of the 4fN(Ln3+) energy level structures in a series of LiYF4:Ln3+ systems are reported. These analyses focus on empirical energy level data currently available in the literature for the Pr3+, Nd3+, Eu3+, Dy3+, Ho3+, Er3+, and Tm3+ systems, and they are based on the use of a phenomenological Hamiltonian that permits trans‐series comparisons of 4f‐electron/crystal‐field interactions. Spin‐correlated crystal‐field (SCCF) interactions are considered in the phenomenological energy level analyses of the Pr3+, Nd3+, Er3+, and Tm3+ systems. Differences between analyses carried out with and without inclusion of SCCF effects are most apparent for the Er3+ and Tm3+ systems. For these systems, inclusion of SCCF effects leads to significant improvement in agreement between calculated and observed crystal‐field splittings within certain multiplet manifolds. The calculated crystal‐field energy level structures within the 1G4 and 1D2 multiplets of Tm3+ and the lowest‐energy 2H11/2 multiplet of Er3+ exhibit particular sensitivity to the SCCF components of the model Hamiltonian. Comparisons between the present work and previously reported analyses of LiYF4:Ln3+ systems are presented.
Electron Energy Structure and Optical Properties of Microcrystalline Silicon
Authors: Gavrilenko V. I. , Humliček J. , Klyui N. I. , Litovchenko V. G.
Published: July 2007

Spectroellipsometry and electroreflectance are used to determine the dielectric functions and the direct optical transition energies in microcrystalline silicon produced by chemical vapor deposition and by recrystallization of amorphous silicon. Changes in the optical gaps depending on the average microcrystallite volume are found. These results are interpreted with a tight binding model which incorporates the virtual crystal approximation.
Relationship of Microscopic and Macroscopic Theories for Long‐Wavelength Optical Phonons in GaAs‐AlAs Superlattices
Authors: Bechstedt F. , Gerecke H.
Published: July 2007

A critical comparison is made of the lattice‐vibrational properties resulting within the macroscopic continuum model and a closely parallel microscopic force‐constant model. The elastic and electric forces are treated in the same manner. However, the atomic structure of matter and the bulk phonon dispersion are neglected in the continuum approach. For that reason it cannot explain the optical frequency spectrum of a superlattice even not in the long‐wavelength limit. On the other hand, the principal line shape of the atomic displacement patterns for optical phonons can be satisfactorily described by envelopes, at least, if the arbitrariness inherent any theory neglecting dispersion is utilized in an appropriate way. This holds particularly for modes which are not accompanied by long‐range electric fields. For the other modes — the p‐polarized symmetrical phonons propagating not parallel to the superlattice axis — the problem of identification appears additionally since the interface character restricted to four modes in the dispersionless continuum approach is distributed among all symmetrical phonons within the microscopic theory.
Optical Investigations of the Phase Transition in Pure and Cr3+‐Doped Potassium Alum
Authors: Brach D. , Glaremin P. , Weber H.‐J.
Published: July 2007

In chromium doped potassium alum photoluminescence and birefringence are measured. The material shows an unusual large number of R‐lines, which are. well resolved only below a phase transition at 59 K. At the same temperature the decay time of the R‐lines and the birefringence change. It is shown that the trielinic symmetry in the low temperature phase of doped crystals is caused by a mixture of an intrinsic trigonal and an additional chromium induced distortion. In the pure material the phase transition is established by a strong increase of the piezo‐optical coefficient q44.
Ising Model with Random Local Fields in a Two‐Dimensional Ferromagnet
Authors: Mielnicki J. , Balcerzak T. , Wiatrowski G.
Published: July 2007

A spin 1/2 Ising model with random fields is studied using a 3‐rd order Matsudaira approximation. The two‐dimensional ferromagnet with square lattice is considered only. The system is found to show a tricritical behaviour and the existence of two phase transitions, one of second order and another of first order, for certain values of the field strength. The behaviour of magnetization and spin‐correlation function between n.n. spins is discussed in detail.
Analysis of Self‐Diffusion Data in V and Nb
Authors: Eftaxias K. , Hadjicontis V.
Published: July 2007

Self‐diffusion studies have shown that V and Nb are characterized by a non‐linear Arrhenius behaviour. This behaviour is usually explained within the frame of the existence of two or three diffusion mechanisms and the relevant analysis requires the determination of a number of adjustable parameters. It is shown that the non‐linearity disappears when one plots In D vs. (BΩ)/(kT) instead of the usual In D vs. 1/T. Such an analysis does not need the introduction of any adjustable parameter and allows the determination of the activation entropy, enthalpy, and attempt frequency.
NMR Study of Internal Motions in a New Family of Ionic Conductors M2SbF5
Authors: Moskvich Yu. N. , Cherkasov B. I. , Polyakov A. M. , Sukhovskii A. A. , Davidovich R. L.
Published: July 2007

19F NMR spectra and relaxation times T,1, T2, T1e and electric conductivity in isostructural polycrystalline pentafluoroantimonates (III) M2SbF5 (M = Rb, K, Cs, Tl, NH4) are measured. It is found that the observed changes in the NMR spectra and relaxation times with temperature are determined by three types of motion: anisotropic and isotropic reorientations of [SbF5E]2− octahedra and diffusion motion of the fluorine ions. Parameters of these motions are found. All the compounds investigated show high conductivity at high temperature, amounting in the case of thallium and ammonium salts the values of σ = 101 to 100Ω−1 m−1 typical of superionic crystals. It is shown that the observed high conductivity is determined by fast diffusion motion of fluorine ions. On the basis of the data obtained a mechanism of fluorine ion diffusion is proposed. In (NH4)2SbF5 at 398 K a phase transition is found.
Stimulated Brillouin Scattering in a Transversely Magnetised Doped Centrosymmetric Semiconductor
Authors: Neogi A. , Ghosh S.
Published: July 2007

An analytical investigation is made for the stimulated Brillouin scattering (SBS) in a nearly centrosymmetric, doped semiconductor, which is subjected to a transverse magnetic field. The origin of this nonlinear interaction lies in the third order optical susceptibility arising due to the nonlinear current density and electrostriction of the medium. The threshold condition is obtained for the onset of the SBS. The effective Brillouin susceptibility and the resulting gain is determined using the coupled mode theory. The effect of magnetic field and doping on the Brillouin susceptibility and Brillouin gain is also studied. Numerical estimations are made for n‐type InSb at 77 K duly irradiated by a pulsed 10.6 μm CO2 laser. In the highly doped regime the transverse magnetic field effects a significant decrease in the threshold and an appreciable enhancement in the gain constant. The analysis established the possibility of occurrence of optical phase conjugation in a doped semiconductor.
Theoretical Method of Evaluation of Heats of Vaporisation of Fused Salts
Authors: Rao R. V. Gopala , Venkatesh R.
Published: July 2007

Starting with neutral hard spheres the entropy of vaporisation is calculated after mixing and charging them. The entropy contribution in overcoming the attraction is computed by the use of a potential function and the principle that the total entropy is a sum of the contributions from hard spheres and Coulomb interactions. The theoretical results are found to be in very good agreement with experiment except for lithium salts.
On Diffusion by the Dissociative Mechanism in the Case of a Finite Foreign‐Atom Source
Authors: Stolwijk N. A.
Published: July 2007

The dissociative mechanism describes long‐distance transport of mainly substitutionally dissolved impurities (Xs) by a small fraction of atoms in an interstitial configuration (Xi). Site interchange takes places with the aid of vacancies (V): Xi + V ⇌ Xs. Theoretical treatments of dissociative diffusion given in the literature deal with a virtually infinite foreign‐atom source producing a constant concentration at the boundary. Expressions of the dissociative diffusion coefficient and the corresponding solutions of Fick's second law are derived for finite source conditions under which all foreign atoms available get involved in the diffusion process.
Influence of Phonons on the Calculations of the Density of States in Thin Films
Authors: Stasiak J. W. , Kierul J. , Künne L.
Published: July 2007

The local densities of electronic states in Ni and Cu thin films are calculated including the influence of the electron–phonon interaction on the hopping integrals. The model gives the dependence of the density of states on temperature. The results however are affected to some extent by the detailed form of the electron–phonon interaction adopted for the calculations.
Excitation Propagation in the Presence of a Driven and Kicked Vibration
Authors: Esser B.
Published: July 2007

The propagation of excitons interacting with a driven and kicked vibration is considered using the generalized master equation (GME) approach. The amplitude of the vibration is described by a nonlinear evolution equation with a kicking term modelling a periodic perturbation. The connection between the GME kernel (memory function) and a map describing the dynamics of the vibration is established. This connection is used to discuss the behaviour of the GME kernel and resulting exciton motion following from the periodic and chaotic solutions of the map. For chaotic solutions an ensemble is introduced and the decay of the memory function calculated.
Intralattice Strain Analysis in Superconducting Crystals and Superconducting Electrons
Authors: Fukuhara M. , McKinstry H. A.
Published: July 2007

The bonding distances between atoms in five different structure types of superconductors, Nb3Sn, (La, Sr)2CuO4, YBa2Cu3O7–δ, Bi2Sr2CaCu2Ox and Tl2Ca2Ba2Cu3O8+x were calculated. There are many shrunken bonds in all the five compounds. Tc increases with the decrease in atomic distance of cations, rather than that of Cu–O. Structures which have large rhombicity tends to have higher density without the application of ultra‐high pressure owing to the existence of a large number of oxygen; that is, the more the number of oxygen anions surrounding the cations increases, the more the radii of cations shrink. In shrunken atoms, electron orbits are forced to condense toward to the nucleus, it is considered that superconducting electrons are apparently created and perfect diamagnetism is derived from inner filled bands.
Schrieffer‐Wolff Transformation of the Emery Model for Cu–O Superconductors
Authors: Kolley E. , Kolley W.
Published: July 2007

The minimal version of the Emery model – ascribed to a CuO2 layer in high‐Tc superconductors – is subject to a canonical transformation of the Schrieffer‐Wolff type up to the fourth order in the hybridization amplitude. An effective Hamiltonian is extracted systematically consisting of three parts: the kinetic energy of O‐holes, a Kondo‐like Cu–O exchange interaction, and the Heisenbergtype superexchange interaction between localized Cu spins with a modified coupling parameter in the undoped case. The assumptions and approximations underlying the derivation of the t̃–Jpd–Jdd model are pointed out.
Neutron Incoherent Inelastic Scattering from (NH4)xRb1−xH2PO4
Authors: Goyal P. S. , Chakravarthy R. , Dasannacharya B. A. , Kulshreshtha S. K. , Sastry M. S. , Tomkinson J.
Published: July 2007

Spectra of incoherently inelastically scattered neutrons from (NH4)xRb1−xH2PO4 (x = 0.0, 0.35, 1.0) in the energy transfer region of 2 to 200 meV are reported. All the peaks in the measured spectra are identified and compared with optical data. The energies (≈ 126 and ≈ 160 meV) associated with the motion of acidic protons are the same in the three salts at 6 K inspite of the fact that RDP (x = 0.0) is a ferroelectric, ADP (x = 1.0) is an antiferroelectric, and ARDP (x = 0.35) a dipolar glass. In the region (2 to 32 meV) of lattice mode RDP and ARDP spectra are similar, but they are considerably different from the ADP spectrum. Spectra corresponding to librational modes (≈ 38 meV) of NH are also different in ADP and ARDP. Reasons for these differences are discussed. The effect of temperature on the above spectra is studied between 6 and 160 K.
Excited‐State Molecular Superconductivity
Authors: Dunne L. J. , Kai A. Tang
Published: July 2007

The possibility of non‐equilibrium excited state molecular superconductivity is discussed and applied to the problem of photosynthetic electron transfer.