bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron Paramagnetic Resonance Spectra Observed in Single Crystals of YBa2Cu3O7–δ
Authors: Yu C. , Lai C. , Marshall S. A. , Yoder‐Short D. R. , Zhang Y. N.
Published: July 2007

An electron paramagnetic resonance absorption study is conducted on four separately prepared single crystal groups of the superconductor YBa2Cu3O7–δ, each group consisting of from four to six crystalline platelets. This investigation confirms conclusions reached by other regarding the EPR silence of this superconducting oxide while demonstrating that EPR spectroscopy can be used to determine certain properties of superconducting oxides. Such determinations include detection of paramagnetic impurity phases, noninvasive means for determining Tc, and estimating the depth of penetration of a microwave radiation field in these materials. Some comments are provided on the nonresonant, zero field absorption in both, the powders and single crystal platelets.
Functional Approaches to Superconductivity within an Extended Attractive Hubbard Model
Authors: Teubel A. , Kolley E. , Kolley W.
Published: July 2007

On the basis of an extended attractive Hubbard model with nearest‐neighbour interaction of opposite spins two functional methods are adopted for deriving a self‐consistent equation for the superconducting order parameter. The functional‐integral formalism is exploited in saddle‐point approximation. The functional‐derivative technique using Schwinger source fields leads via the self‐energy in Hartree‐Fock‐Bogolubov approximation to a nonlinear gap equation. The superconducting transition temperature is investigated for the case of s‐wave pairing.
Charge Fluctuation in the High Tc Superconductor YBaCuO in the Transition Temperature Region
Authors: Chen Ling , Wang Guanghou , Teng Mingkang
Published: July 2007

Positron annihilation rates in both, high and low Tc superconductors are calculated by considering the interaction between the thermalized positrons and electrons in superconducting and normal state in the framework the of KSS model. By properly chosen parameters (for instance, EF = 0.7 eV) the anomaly of positron annihilation rate in the high Tc superconductor YBaCuO at the transition temperature is predicted and is in agreement with recent experimental results.
Antiresonances in the Fundamental Spectra of CdI2–4H and CdBr2–6R Crystals
Authors: Dovgyi Ya. O. , Kityk I. V.
Published: July 2007

On the basis of the low temperature spectra measured for CdI2 (4H‐polytype) and CdBr2 (6R‐polytype) crystals in the region of UV radiation in the exciton resonance vicinity the distinct antiresonance are discovered. The exciton interaction with the continuous spectrum corresponding to interband transitions is determined to cause a self‐ionization and a band widening. That is the Fano effect manifestation. The exciton dynamical instability in the cadmium dihalide crystals is shown to be caused by energy band structure pecularities. According to the analysis of the absorption curve profiles and antiresonances parameters and types at T = 77 K the matrix element values of the exciton state hybridization with the continuous spectrum states are obtained. A significant dispersion of the antiresonance parameter being caused by the complex structure of the valence bands in cadmium dihalide is discovered.
Interstitial Atom in the Zincblende‐Type Lattice
Authors: Soma T. , Tahata S. , Kagaya H.‐Matsuo
Published: July 2007

The formation energy of an interstitial atom in the zincblende‐type lattice is investigated using the perturbational formalism presented earlier and considering the relaxation effect around the interstitial atom. The interstitial atom Ga or As in GaAs seems favourable at the tetrahedral site neglecting relaxation, but is found to be most stable at the hexagonal site considering the relaxation effect.
Energy Loss Distributions of Relativistic Charged Particles in Crystals
Authors: Kudryashov N. A. , Petrovskii S. V. , Samsonov V. M. , Strikhanov M. N.
Published: July 2007

The kinetic equations are derived which describe the energy loss for fast charged particles with account for their correlated motion in the field of atomic planes and multiple scattering. The numerical calculations for relativistic proton beams demonstrate that for large beam incident angles θ0 > θL (where θL is the Lindhard angle) the energy loss distributions coincide with the Landau theory with good accuracy. At small incidence angles θ0 < θL of a narrow positively charged particle beam the distribution maximum is shifted to anomalously small energy losses. The problem of particle selection according to the small energy loss criterion is considered. A comparison between the theory and experiment is made.
Lattice Dynamics of Binary Alloys
Authors: Gurskii Z. A. , Chushak Ya. G.
Published: July 2007

Lattice dynamics of binary alloys is investigated by the collective variables method. The relation between a developing technique and the well‐known virtual crystal and coherent potential approximations is analysed. The characteristic property of the theory presented consists in taking into account explicitly the short‐range order. The influence of pair correlations on the vibrational density of states and the Debye temperature for K–Rb alloys is calculated and discussed. The short‐range‐order effects on the dynamic properties are shown to be strong near the order–disorder phase transition temperature.
Calculation of Surface‐Induced Core‐Level Shifts for Covalent Semiconductors C, Si, Ge, and α‐Sn. II. (100) 2 × 1 Surfaces
Authors: Bechstedt F. , Reichardt D.
Published: July 2007

The theory developed for the surface‐induced core‐level shift is applied to (100) 2 × 1 surfaces of group IV materials. The initial‐state potential effect is related to ion charges (of the ground state), changes of the atomic hybridization state, and the polarity of chemical bonds accompanying the atomic configuration due to reconstruction. The many‐body relaxation mechanism is neglected. The atomic configurations are taken from total energy minimizations, dynamical LEED or ion scattering studies. In the case of Si and Ge the results are compared with experimental shifts. A tendency is observed for the confirmation of symmetric dimer formation during the reconstruction.
Determination of the Surface Magnetization of Fe(110) by Spin‐Polarized Low‐Energy Electron Diffraction
Authors: Tamura E. , Feder R. , Waller G. , Gradmann U.
Published: July 2007

Exchange and spin‐orbit scattering asymmetries of spin‐polarized low‐polarized low‐energy electrons from Fe(110) are measured at room temperature for several beams at constant energies as functions of the polar angle of incidence (“rocking curves”). Corresponding calculations by means of a relativistic multiple scattering formalism gave results in good agreement with the experimental data. The sensitivity of part of the calculated curves to changes in the layer magnetization implies an enhancement of the topmost layer magnetization by about 36 to 40% relative to the bulk and possibly a reduction in the second layer. Further, a dominance of spin‐down over spin‐up inelastic scattering is deduced associated with 3p → 3d excitation.
Hot LO‐Phonon Quasi‐Steady State Distribution Function in Polar Semiconductors
Authors: Král K. , Hejda B. , Khás Z.
Published: July 2007

The quasi‐steady state hot LO‐phonon distribution function, as it is produced by hot electron cooling in laser excitation experiments, is calculated induding the effect of electron energy collision broadening (EECB) and screeing. Numerical computations performed in several compound polar semiconductors reveal a rather strong effect of EECB.
Field Dependence of Dilute Ising Paramagnets
Authors: Taggart G. B.
Published: July 2007

Quenched and annealed dilute, spin one‐half, Ising paramagnets are studied under the influence of an external magnetic field of arbitrary magnitude. Both cases are considered for non‐interacting and interacting magnetic species. The short range magnetic interactions are taken into account by using, to the first order in the interaction strength, the exponential operator technique introduced by Honmura and Kaneyoshi. The results clearly show the differences between the behavior of the magnetization and susceptibility as magnetic interactions are turned on for the quenched and annealed cases. For the annealed dilute paramagnet, the magnetization and concentration are calculated for arbitrary external field and chemical potential and, as expected, exhibit phase behavior similar to a three‐state, spin‐one, Ising model.
Parametric Excitation of Laser in Magnetoactive Doped Piezoelectric Semiconductors
Authors: Aghamkar P. , Sen P. K.
Published: July 2007

Using the hydrodynamic model of a semiconductor plasma, a detailed analytical investigation of parametric excitation is made for a doped piezoelectric semiconductor in presence of a large transverse magnetostatic field. The analysis deals with the qualitative behaviour of the threshold condition and the gain mechanisms with respect to the doping level and the magnetostatic field. Numerical estimates are made for n‐InSb crystals at 77 K duly shined by a 10.6 μm CO2 laser. Efforts are directed towards optimizing the doping level and the applied magnetic field for the occurrence of parametric excitation using the lowest possible threshold to yield the maximum possible gain. The analysis suggests the possibility of observing parametric gain in a moderately doped magnetoactiv n‐InSb crystal even under continuous wave laser irradiation for an excitation intensity as low as 1 W cm−2.
The Relation between the Orientation of Streamer Discharges and Anisotropy of Thermal Vibrations of Atoms in II‐VI Semiconductors
Authors: Zubritskii V. V. , Purlys R. P.
Published: July 2007

For the first time the relationship is established between the thresholds (probabilities) of excitation of streamer discharges and anisotropy of thermal atomic vibrations of crystal lattices in semiconductors. By the example of cadmium sulphide for the first time the temperature dependence of streamer orientation at 294 K is modelled by doping the samples with lithium impurities; the mean‐square dynamic displacements of atoms are measured, and it is shown that the dynamic characteristics of crystal lattices of doped semiconductors cardinally differ from analogous parameters for undoped crystals, but as in purposely undoped samples, their anisotropy determines the temperature dependence of the excitation thresholds of streamers.
The Kinetic Theory of the Orientational Ordering Phase Transition. The Model and the Kinetic Behavior
Authors: Yang Zhengju , Ma Guangquen
Published: July 2007

Using a simple microscopic model, the kinetic equations of the orientational ordering phase transition are established and the general characters of the kinetic behavior of the first‐order and the second‐order phase transitions are obtained. The results about the equilibrium state obtained as a special case agree completely with those obtained from the phenomenological theories of the first‐order and the second‐order phase transitions. The different behavior of these two kinds of phase transitions is explained by a unified model. Meanwhile, the relation between them is given and the possibility of a system transiting from one to another caused by external influence is discussed. Lastly, it is shown that in such a non‐equilibrium process, a “potential function” corresponding to the thermodynamic force can be established which is equivalent to the free energy function in the thermodynamics in the equilibrium state and can be used to give the direction and speed of the phase transition in the whole non‐equilibrium process too.
Self‐Similarity of the Density of States of Electron and Phonon Models in Fibonacci Chains. An Analytic Study
Authors: Böttger H. , Kasner G.
Published: July 2007

For periodic one‐dimensional phonon and tight‐binding electron models, the densities of states are expressed through the trace of the dynamic transfer matrix. The resulting formulae are used for studying the self‐similarity of the spectrum of Fibonacci chains.
Diamagnetic Susceptibility of a Donor in a GaAs/Ga1−xAlxAs Quantum Well Heterostructure and Its Pressure Dependence
Authors: Sukumar B. , Navaneethakrishnan K.
Published: July 2007

The diamagnetic susceptibility (χdin) of a donor in the GaAs quanturn well of the GaAs/Ga1−xAlxAs heterostructure is calculated for a well of infinite strength. The binding energy of the donor electron in different magnetic fields is computed variationally and the χdia values estimated using the Langevin expression. It is found that as the well width increases the χdia values also increase, Using the pressure dependence of the effective mass, dielectric constant, and lattice constant of GaAs, the donor ionization energies are estimated for different pressures of 50 × 108, 150 × 108, and 200 × 108 Pa applied along the growth axis of the superlattice. It is shown that both, the ionization energies and χdia values increase with pressure for a given well width. Also the ionization energy decreases with increasing well width in the presence of both, pressure and magnetic field.
Quantum Oscillations of Galvanomagnetic Effects in Bismuth Bicrystals
Authors: Muntyanu F. M. , Onu M. I. , Kistol V. G.
Published: July 2007

The quantum oscillations of galvanomagnetic effects are studied in bismuth bicrystals with a crystallite boundary of an inclination or twisting type. The quantum oscillations of the Hall components of a the resistivity tensor, that occur due to a quanturn interaction between the charge carriers and intercrystallite boundary are observed. It is shown that the character of the quantum oscillations of galvanomagnetic effects in the Bi bicrystals depends essentially on the tilt angle of the crystallites.
Anisotropy of Magnetoresistance of the p‐Type Ferromagnetic Semiconductor HgCr2Se4
Authors: Kostylev V. A. , Gizhevskii B. A. , Samokhvalov A. A. , Auslender M. I. , Bebenin N. G.
Published: July 2007

An experimental investigation is made of the anisotropy of magnetoresistance of the ferromagnetic p‐type semiconductor HgCr2Se4 on specimens doped with intrinsic defects and Ag. A simple theory is developed dealing with the magnetoresistance of the p‐type ferromagnetic semiconductor with Γ8 symmetry of the valence band top split by a strong exchange field produced by localized spins. The theory provides a good description of the experimental data.
Anisotropic Reorientation of Octahedral Ions According to NMR Relaxation in Solid State
Authors: Kozlova S. G. , Kriger Yu. G. , Gabuda S. P.
Published: July 2007

The relationship between the temperature dependences of the nuclear spin lattice relaxation times and the anisotropic reorientation of octahedral ions is studied.
Generation of Picosecond Acoustic Pulses by Laser Radiation in Piezosemiconductors
Authors: Gusev V. E.
Published: July 2007

Theoretical description is proposed of the frequency spectrum of strain pulses, excited by optical generation of nonequilibrium charge carriers in piezocrystals. The acoustic pulses are excited by the inverse piezoeffect by electric fields induced by spatial separation of electrons and holes. The conditions necessary for picosecond strain pulse generation are determined. The possibility of acoustic pulse excitation with duration governed by Debye screening time is predicted. It is shown that piezomechanism of optoacoustic transformation may be more effective than thermoelastic and electronic mechanisms.
Coherent Signal Generation in CuCl by Light‐Induced Grating and Induced Biexciton Decay
Authors: Lévy R. , Gomes M. J. M. , Kippelen B. , Hönerlage B.
Published: July 2007

Using a picosecond light source and a streak camera as detection system, the temporal evolution of the signal generated throughout degenerate four‐wave mixing (DFWM) experiments is analysed in CuCl at low temperature. The signal emission shows different temporal behaviour depending on whether the test pulse reaches the surface of the sample before, during, or after the pump pulse. This enables one to distinguish the light‐induced grating contribution from the induced decay of quasi‐particles. The latter process can then be used to determine their dephasing time.
Tellurium Diffusion in the Mixed System CdxHg1−xTe
Authors: Madhavan Y. , Ramachandran K. , Haridasan T. M.
Published: July 2007

Lattice dynamical calculations on the jump frequencies of different isotopes of tellurium in the mixed system CdxHg1−x Te are worked out following the general theory of diffusion for metals by Achar, assuming an active role for the vacancy. An isotope effect, which is a measure of the diffusion rate is also deduced. The lattice dynamics of this mixed system is worked out in the virtual crystal approximation using the rigid ion model of Plumelle and Vandevyver. The results indicate that diffusion is carried out prominently by the single vacancy mechanism.
Lattice Dynamics of Ca and Sr in B. C. C. and F. C. C. Phases. A dynamic treatment
Authors: Singh N. , Singh S. P.
Published: July 2007

The pair potential defined for transition metals in a previous paper is extended to study the phonon dispersion of alkaline earths. The overlap between d states on different ions and the effect of sd hybridization are included in ion‐ion interaction as has been suggested by Wills and Harrison. The temperature dependence is also included through an asymptotic factor. The calculated dispersion curves do not differ from experimental values by more than 10% for both, Ca and Sr in both, f. c. c. and b. c. c. phases except at a few wave vectors near the zone boundary in the case of Ca(b. c. c). The binding energy and elastic constants are also calculated and compared with existing experimental data.
Vibrational Spectra of CuGaS2 and CuGaSe2 Compounds and CuGaS2xSe2(1−x) Solid Solutions2)
Authors: Bodnar I. V. , Smirnova G. F. , Karoza A. G. , Chernyakova A. P.
Published: July 2007

The phonon modes of the ternary compounds CuGaS2, CuGaSe2, and the CuGaS2xSe2(1−x) solid solution are investigated by means of IR and Raman Spectroscopy. The dependence of the frequences on the composition of the solid solutions is obtained. It is shown that the lattice vibrations of the solid solutions display mixed type behaviour.
A Field‐Theoretical Treatment of Third‐Sound Localization
Authors: Kleinert P.
Published: July 2007

The nonlinear σ model approach to Anderson localization is studied for third‐sound modes of a 4He film on a disordered substrate. The disorder is assumed to be spatially long‐range correlated. Gaussian and exponentially decaying correlations are considered. From the renormalization group equation a nonlinear equation is derived for the renormalized diffusion coefficient. The localization length and numerically calculated diffusion spectra are discussed.
Self‐Energy of an Electron in the Vacuum Gap between Two Metals
Authors: Thoai D. B. Tran , Zeitler E. , Nam Do
Published: July 2007

Expressions for the self‐energy of an electron at rest in the vacuum gap between two identical metals are derived using the formalism of Manson and Ritchie. Besides the quantum recoil effects, the effects of the spatial dispersion are included using the hydrodynamic model. The numerial results show that the effects of the spatial dispersion are even stronger than the quantum recoil effects. The corrections to the classical image potential due to the quantum recoil effects play a role only for small gap separation. However, the spatial dispersion increases the range of gap separation, at which the corrections to the classical image potential are non‐negligible.
The Functional Equations for the Kinetic and Exchange Energy Functionals
Authors: Baumann G. , Duscher R.
Published: July 2007

The Hohenberg‐Kohn theorem proves the existence of a universal energy functional. But still its explicit structure is unknown. Unknown parts of the energy functional, namely the kinetic and the exchange interaction energy are discussed. Based on a functional formulation of the virial theorem “functional equations” for these unknown quantities are deduced establishing a relation between the functional and its derivative. Though there is no general formalism for solving such equations they may be used to determine the structure of the unknown functionals. Starting with simple formal assumptions for the exchange energy, some well‐known approximations are derived for this functional thus proving the value of this new technique.
Antisite Defects in SiAlAs/GaAs Modulation‐Doped Superlattices
Authors: Wang En‐Ge , Zhang Li‐Yuan , Wang Huai‐Yu , Jin Wei‐Min
Published: July 2007

The electronic structure and binding energy of the antisite defects GaAs and ASGa in SiAlAs/GaAs modulation‐doped superlattices are calculated with use of the developed recursion technique. A detailed comparison between the antisites and impurities in the multilayer systems is presented based on the discussion of the local density of states and the electronic occupation possibilities on each defect site. The results are used to explain recent experiments.
Theory of the Residual Resistivity Dipole. Flow of Non‐Interacting Particles under a Weak Driving Force around a Fixed Point Scatterer
Authors: Lenk R.
Published: July 2007

Employing a superposition representation of the density matrix the perturbation of the stationary current flow by a fixed point scatterer is treated approximately. The current is driven by a weak constant force. The problem, the method, and the result parallel closely those of the corresponding force‐free diffusion case discussed previously. For all current‐proportional perturbation terms, the Einstein equivalence between a force and a density gradient is established for the inhomogeneous case under study. This equivalences, however, is broken by an additional current‐free term of the density matrix. This term can be identified as a relict of the inhomogeneous equilibrium state in the given static force field. This equilibrium state itself is also studied. In the ensemble the limitations of the concept of a local chemical potential become evident, even in the current‐free state.
Auger Recombination in Heavily Doped Semiconductors
Authors: Quang Doan Nhat
Published: July 2007

A theory is developed of the Auger recombination in a heavily doped semiconductor, taking into account the influence of the random inpurity field. For various recombination processes, general analytic expressions of the Auger coefficients are derived, which describe the effect of the random field in explicit dependence on the doping concentration, compensation, and excitation levels as well. The degeneracy and screening effects of non‐equilibrium carriers are shown to be of great importance. It is found that the Auger coefficients may be strongly enhanced due to the impurity field up to several orders of magnitude at high doping concentration and low excitation level. The numerical evaluation on p‐type GaAs indicates especially that in the case of low temperature (77 K) and high acceptor concentration (> 1019 cm−3) the random field effect on the valence band process is predominant over that caused by the phonon. Moreover, in a n‐type material the valence band process is predominant over that caused by the phonon. Moreover, in a n‐type material the valence band process might be competable with the conduction band one even at weak excitation.
Theory of R. F. Mössbauer Spectra of the Magnetics with Magnetic Anisotropy
Authors: Sadykov E. K. , Skvortsov A. I.
Published: July 2007

The theory of the r. f. collapse phenomenon in nuclear γ resonance (NGR) for magnetics is proposed. In contrast to the existing rotating‐field and stepwise‐field models this approach takes into consideration the detailed behaviour of the magnetization (consequently of hyperfine field (HF) under external r. f. field and magnetic anisotropy field. Using the Liouville superopetor technique the expression for NGR absorption cross‐section (57Fe) is obtained on the basis of time periodicity of the HF alone. The possibility of the chaotic evolution of the magnetization in some cases and the corresponding changes in the NGR spectra are shown. Numerical model calculations for a system of single domain particles with uniaxial anisotropy using the chain fraction method are carried out. So the matter under discussion is a quantitative to the r. f. collapse regime valid for an arbitrary time dependence of the HF.
Spin Glass Properties of Zn1−xMnxSe
Authors: Graf A. , Obermaier R. , Gebhardt W.
Published: July 2007

Faraday rotation is measured in Zn1−xMnxSe as a function of temperature and magnetic field in the range between 5 and 50 K and fields up to 5 T. Two characteristic temperatures, T1B) and T2B), are observed. T1 can be interpreted as transition temperature into the spin glass state which is supported by a plot of the Edwards‐Anderson order parameter qEA(T). The time dependent magnetization is investigated at T < T1. It is found that the decay of the themoremanent and isothermal remanent magnetization fits well a stretched exponential function.
NQR Measurements of 63Cu in YBa2Cu3O7−x
Authors: Grande S. , Lippold B. , Mayer G. , Schmiedel H.
Published: July 2007

63Cu‐NQR measurements are performed in powdered samples of superconducting YBa2Cu3O7−x (YBaCu) in weak external magnetic fields B0 < 2 mT at temperatures T > Tc (normal conducting state). The NQR‐echo amplitudes exhibit damped oscillations with increasing magnetic field B0. The theoretical evaluation of these oscillations allows the direct determination of the asymmetry parameter η of the electrical field gradient (EFG) tensor and the local magnetic field at the Cu‐sites in the lattice of YBaCu.
Stimulated Echo and Vibrational Dynamics
Authors: Hizhnyakov V.
Published: July 2007

Stimulated echo (SE), excited in an inhomogeneously‐broadened vibronic absorption band of impurity centres by three successive pulses, is considered. SE for a small delay time t2 of the second pulse, comparable with the characteristic time of vibrational relaxation, is essentially determined by the vibrational dynamics of the excited centres. The processes of a absorption and amplification of the third exciting pulse are differently modulated by vibrations, giving different contributions to the amplitude of SE. The absorption contribution is described by the function A(t3, t2, t2), while that of the amplification by A(t2, t3, t3), where A(μ, τ, τ′), is the correlation function of the resonance secondary emission of centres. For very short t2 and t3, i. e. essentially smaller than the periods of vibrations, the latter can play a role of inhomogeneity in the system, allowing one to observe the SE on homogeneous systems, such as ionic crystals with colour centres (F‐centres).
A Model Calculation of the Dielectric Band Structure of Diamond and Silicon
Authors: Castro J. D'Albuquerque E , Muniz R. Bechara , Oliveira L. E.
Published: July 2007

A model microscopic dielectric response previously proposed is used to calculate the dielectric band structure of diamond and silicon along several symmetry directions in the Brillouin zone. The most screened dielectric eigenpotentials at the Γ point are also obtained. Comparison of the present results with previous model calculations provides an indication that the model dielectric response used is satisfactorily accurate and realistically incorporates local‐field effects.
Ultrafast Nonlinear Optical Effects in Biased Semiconductor Quantum Wells
Authors: Chemla D. S. , Schmitt‐Rink S. , Knox W. H. , Miller D. A. B. , Goossen K. W. , Hasnain G.
Published: July 2007

The development of electronic and photonic devices operating in the many‐THz regime will require the invention of novel structure concepts and will implement physical processes different from those presently used in optoelectronics. An analysis is reviewed of the physical mechanisms that can be implemented to generate, propagate and detect femtosecond electrical signals in properly designed semiconductor quantum well structures. Also discussed are recent experimental investigations of these effects as well as the fundamental limitations caused by two photon absorption.
Screening of the Coulomb Potential in the Athermal Stage of Highly Excited Direct‐Gap Semiconductors
Authors: Heiner E.
Published: July 2007

Equations for the reduced density operator are evaluated. Investigating the subdynamics of nonrelevant observables a resolvent formulation is obtained selfconsistently for the screening part of the evolution superoperator in the athermal nonequilibrium. The analysis is taken in the limit of many collisional events and corresponds to an extension fo the RPA‐result. Thereby, the short memory approximation is improved.
Phonoriton Transient Phenomena
Authors: Gippius N. A. , Ivanov A. L. , Tikhodeev S. G. , Vygovskii S. G.
Published: July 2007

The modifications of exciton optical properties of a direct‐band semiconductor due to anti‐Stokes Raman scattering of intense polariton wave (phonoriton effect) are investigated theoretically. Special attention is paid to the role of ABC and to the reflection of short pulses. A possibility of processing light by means of total internal reflection from pumping beam boundary is proposed.
Kinetics of Partially Coherent Polaritons in Scmiconductors. Schwinger‐Dyson Equations
Authors: Beloussov I. V. , Shvera Yu. M.
Published: July 2007

A derivation of the equations is given describing kinetic and dynamic properties of a high density polariton system excited in direct‐gap semiconductors by an external laser field. Keldysh diagram technique for non‐equilibrium processes formulated in terms of functionals is used. Using functional Legendre transformation the Dyson equations are obtained for normal and abnormal connected Keldysh‐Green's functions, which describe quantum fluctuations in the system, as well as for a coherent amplitude of the polariton field. The vertices in the Dyson equations are calculated using perturbation theory.
Self‐Supporting Acoustoelectronic Pulses Driven by Optical Excitation of a Semiconductor in the Transparency Region
Authors: Kochelap V. A. , Kuznetsov A. V.
Published: July 2007

It is shown that the electron‐phonon interaction under certain circumstances can lead to the existence of moving self‐consistent distributions of lattice deformation and carrier density which are supported by an external optical excitation. They are moving towards the source of light with an intensity‐dependent velocity of the order of the sound velocity. The conditions of existence and the stability of such structures are investigated.
Passive and Active Q‐Switching in an AlGaAs/GaAs SQW Laser. Giant Stark Shift in the Laser p‐n SQW Heterostructure
Authors: Aleiner I. L. , Chistyakova L. M. , Kokhanovskii S. I. , Seisyan R. P. , Toropov A. A. , Shubina T. V.
Published: July 2007

A giant Stark shift is observed in AlGaAs/GaAs p‐n SQW heterostructures grown by MBE and LPE. The band edge shift determined from the photocurrent and transmission spectra is several times higher then predicted by the quantum confined Stark effect for pure QW in a pin structure. An efficient active Q‐switching is demonstrated in an injection laser with an intracavity monolithic loss modulator.
Possibilities for Intrinsic Optical Bistability in Semiconductor‐Doped Glasses
Authors: Peschel Th.
Published: July 2007

Equations of motion for the excitation rate of electron‐hole pairs in semiconductor microcrystals are derived for excitation with picosecond pulses. The inhomogeneous and nonlocal character of the polarization is taken into account. The result are equations describing the macroscopic susceptibility of a semiconductor‐doped glass. On this basis different possibilities for resonatorless optical bistability due to local field effects are discussed and necessary conditions for its appearence are derived.
The Pulsed Nonresonant Optical Stark Effect and the Urbach Tail in Semiconductors
Authors: Haug H. , Banyai L. , Liebler J. , Wicht T.
Published: July 2007

The theory of the intrinsic Urbach tail absorption in semiconductors is discussed. Some recent results of a cumulant expansion up to second order are presented which fit experimental observations excellently. The results can be simulated with a frequency‐dependent broadening which gives rise to memory effects for the optically induced polarization. The importance of these effects for the understanding of the pulsed nonresonant optical Stark effect is discussed.
Induced Absorption and the Dynamical Stark Shift Associated with Biexcitons
Authors: Balslev I.
Published: July 2007

The nonlinear optical phenomena associated with the formation of virtual biexcitons in direct gap semiconductors are studied. Relations are set up for real‐space two‐fermion operators using methods similar to those used when deriving a well known conservation law of k‐space operators for intraband and interband transitions. Restricting the analysis to the χ(3) level, or set of density matrix equations are obtained for the exciton‐biexciton system. Analytical solutions in the limit of small detuning show that the biexcitonic induced absorption and the dynamical Stark effect are closely related, and that the Stark shift as function of the pump frequency has a pole at the frequency of the exciton‐to‐biexciton transition.
The Origin of Third‐Order Optical Nonlinearity in Exciton Systems
Authors: Ishihara T.
Published: July 2007

The third‐order optical nonlinearity is estimated to be −10−4 e. s. u. at the exciton resonance in PbI2 evaporated films from the population oscillation effect. Such a large nonlinearity cannot be explained if the system is regarded as independent oscillators. The origin of the third‐order nonlinearity is examined in the density matrix formalism. The observed large nonlinearity is shown to come from the exciton‐exciton interaction.
Second Harmonic Generation on the Grating of Optical Inhomogeneities in ZnSe and ZnS
Authors: Gribkovskii V. P. , Zyulkov V. A. , Kazachenko A. E. , Shakin V. A.
Published: July 2007

Noncollinear second harmonic generation in ZnS and ZnSe crystals with quasi‐regular grating of optical inhomogeneities is discovered and investigated. The observed effects are described by a model in which the vector phase synchronism may be realized in the region of normal crystal dispersion due to the contribution of the wave vector of the grating.
Mott Transition in CdS Revealed by Picosecond Nonlinear Absorption Measurements
Authors: Egorov V. D. , Nguyen Hoang Xuan , Zimmermann R. , Dneprovskii V. S. , Kaschke M. , Khechinashvili D. S.
Published: July 2007

Optical spectra above the Mott transition of excitons are calculated from the inhomogeneous integral equation for the polarization function including self‐energy, screening, and band filling. Taking CdS parameters and adjusted values of the electron‐hole temperature and density, good agreement is achieved with the results of transmission measurements made at room temperature on a thin CdS crystal using two‐beam excite‐and‐probe technique with 5 ps time resolution.
Impact Excitation and Auger Decay of Bound Exciton Complexes in a Dense Exciton Gas
Authors: Kharchenko V. A. , Mikhailov G. V. , Nelson D. K. , Razbirin B. S.
Published: July 2007

Impact excitation processes due to free exciton collisions with bound exciton complexes D°X are studied in CdS. It is found that the excitation power increasing from 103 to 104 W/cm2 leads to a considerable decrease of the I2 line intensity followed by the disappearence of the I2 line at the upper limit, whereas the I1 line can be distinctly seen in the luminescence spectra in the whole range of the excitation power. A model explaining the quenching of the I2 emission line by the Auger‐decay of an excited state of the D°X complex at high concentrations of free excitons is proposed. Experimental data are in good agreement with predicted dependences.
The Statistical Properties of the Coherently Excited Polariton Wave in a Small Volume Crystal
Authors: Misko V. R. , Moskalenko S. A. , Rotaru A. H. , Shvera Yu. M.
Published: July 2007

A selfconsistent steady state solution is given of the Fokker‐Planck equation for polaritons with wave vector ko and the kinetic equations for the scattered ones are found. The statistics of the coherently excited polaritons ko differs in a most general case from the pure coherent state. It depends on the crystallite volume V, the frequency detuning Δω between the polariton bare frequency ω(ko) and the laser one ωL, and the driving field energy flow SL. In the case of small values of the product V · SL polariton bunching takes place when Δω < 0, and antibunching in the opposite case. Polariton squeezed states can arise in both the detuning cases.
A New Hole Centre in Neutron Irradiated LiF Crystals
Authors: Kamikawa T.
Published: July 2007

A new colour center is observed by ESR in low temperature γ‐irradiated and then heat treated LiF crystals which are heavily neutron irradiated previously. From the analysis of the ESR spectra this center is assigned to an asymmetric interstitial fluorine atom associated with a lattice fluoride ion. This center lies in one of the (110) planes and its molecular axis makes an angle of 23° with the [110] axis. The ESR parameters are determined by fitting the calculated resonance fields to the experimental ones.
Heavy Ion Channeling Implantation Processes in Single Crystals
Authors: Abbas M. , Kerkow H. , Wedell R.
Published: July 2007

Implantations of Ne‐, Mg‐ and Ca‐ions into silicon have been performed for different channeling orientations with energies ranging from 30 to 300 keV. The produced radiation damage distributions are investigated by using RBS, the depth distributions of implanted foreign atoms are measured by SIMS. A correlation between dechanneling, damage production, and depth distributions of the channeled ions could be observed. Simple models for dechanneling and energy losses allow an explanation of some experimental results.
Angular Force Models in the Lattice Dynamics of Cubic Systems
Authors: Upadhyaya J. C. , Lehri S. , Sharma D. K. , Upadhyaya S. C.
Published: July 2007

Angular force models of de Launay (DL) and Clark‐Gazis‐Wallis (CGW), as applied to study the lattice dynamics of b. c. c. and f. c. c. systems, are analysed and discussed. Removal of the deficiencies from the DL angular force model results in a scheme identical to the central pairwise potential procedure. As far as the calculation of phonon frequcncies in cubic metals is concerned, the CGW model gives dispersion results identical to those obtained by applying the general tensor force (GTF) model. It is observed that the GTF constants are composite quantities, expressible in terms of elementary CGW force constants. The two schemes are applied in the calculation of phonon dispersion curves of some b. c. c. and f. c. c. metals and numerical results are discussed. Finally the stability of the lattice is discussed.
Third‐Order Constants of a Cubic Quasi‐Isotropic Solid
Authors: Wasserbäch W.
Published: July 2007

The isotropic third‐order elastic constants l, m, n, and v1, v2, v3 as defined by Murnaghan and Toupin and Bernstein, respectively are evaluated from the single‐crystal data and are compared with experimental values as available in the literature.
Influence of Level Broadening in Mobility Calculations for Low‐Dimensional Semiconductor Heterostructures
Authors: Leon H. , Comas F. , Suhrke M.
Published: July 2007

The influence of constant level broadening in momentum‐relaxation rate and low‐field mobility calculations is investigated if acoustic‐phonon scattering, via deformation‐potential (DP) and piezoelectric (PZ) coupling, is present in low‐dimensional semiconductor heterostructures such as quantum wires or quantum wells. An approach is developed using the quasi‐particle approximation on the basis of the method of Kadanoff and Baym. Calculations are done in the low carrier concentration case, neglecting screening effects and using non‐degenerate carrier statistics. The role of PZ scattering is established and comparison with earlier results is made.
Determination of the Indirect Exciton Energy Gap of GaP Using Thermomodulation in the Temperature Range 80 to 350 K
Authors: Beckmann A. , Dettmer K. , Kessler F. R.
Published: July 2007

Thermoabsorption studies were performed of the first indirect band edge of n‐GaP in the temperature range from 80 to 350 K. The energy of the indirect exciton bandgap and its temperature dependence are determined from the thermoabsorption spectra. The temperature dependence is fitted to Varshni's equation as well as to an expression containing the Einstein‐Bose statistical factors for phonon absorption and emission. The energies of the involved phonons are deduced from the energy difference between the transitions with absorption and emission of a particular phonon: E(TA) = (13.2 ± 0.2) meV, E(LA) = (29.6 ± 0.7) meV, and E(TO) = (43.5 ± 0.3) meV.
Internal Conversion Coefficient for Valence Electrons of Metals. Theory and Application to 110Ag
Authors: Schober C. , Antonov V. N. , Ziesche P.
Published: July 2007

The ab‐initio electron theory of the internal conversion coefficient (ICC) is developed for the case of pure crystalline metals, the radioactive nuclei of which emit γ‐quanta with relatively small energy (≦ keV) exciting thus preponderantly valence electrons. In this case the photon wavelength is large in comparison with the radius of the nucleus and the ICC ( = energy spectrum of the emitted conversion electrons) becomes independent of the properties of the nucleus. For any electric or magnetic transition and using spherically symmetric muffin‐tin potentials the ICC is expressed by the characteristic quantitites of the electron making a transition from an occupied valence state to an unoccupied free electron state. The calculations for 110Ag show the ICC to reflect directly the density of states of the valence electrons.
Correlated Pair‐Transfer Mechanism for High Temperature Superconductivity. The Case of YBa2Cu3O6+x
Authors: Sinha K. P. , Singh M.
Published: July 2007

A theoretical study of high temperature superconductivity in the system YBa2Cu3O6+x is made using the correlated pair transfer mechanism suggested recently. The mechanism involves the role of electron pairs in the spin singlet state of species \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm X}^{\uparrow \downarrow } (= {\rm Cu}^{{\rm 1 + }},{\rm O}_{\rm 2}^{{\rm 2 - }} {\rm, or O}^{{\rm 2 - }} {\rm)} $\end{document} and their virtual excited state X0(Cu3+, O, or O0) in the pairing interaction of quasiholes. The two states of these species can be likened to a two‐level Bose system. The strength of the interaction depends on the hole and species concentration. The expression for the transition temperature is a combination of phononic and the electronic mechanism and the calculated trend as a function of hole concentration (in effect excess O−) is in agreement with the observed trend. The mechanism is applicable to other oxide systems where such variable species can exist (e. g. Bi3+, Bi5+ or Tl1+, Tl3+, etc.). The theory has electron and hole symmetry, therefore it can also be applied to n‐type high temperature superconductors.
Amplification of Acoustohelicon Waves in Piezoelectric Semiconductors. Effect of Nonlinearity in Electron Effective Mass and Collision Frequency
Authors: Dey (Bose) M. , Ghosh S.
Published: July 2007

It is shown that the attenuation of an acoustohelicon wave crosses over to its amplification at an electron drift velocity (Vd) exceeding the acoustic wave velocity (Vs). Firstly a phenomenological theory is developed to study the variation of the carrier temperature with wave amplitude and cyclotron frequency, and the corresponding relative change in electron effective mass and collision frequency, when the acoustohelicon wave passes through a piezoelectric semiconductor. It is inferred from the analysis that this change, due to carrier heating in high mobility semiconductors, is appreciable at high power flux of the acoustohelicon mode. Then, analytically, it is shown that this nonlinearity in carrier mass and collision frequency, arising due to the electric vector of the field associated with the wave, can also contribute to the acoustohelicon amplification 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 the crossover value of the velocity ratio (= Vd/Vs), but at very high magnetic fields (ωc ≧ 1013 S−1) the acoustohelicon mode propagate without attenuation.
Evaluation of Moments from the NMR Solid Echoes in Dipolar Solids
Authors: Kahol P. K.
Published: July 2007

Two‐pulse‐solid echo, formed by the pulse sequence Py(90°)‐τ‐P x(90°)‐t′, can be used as an alternative to the free‐induction‐decay to evaluate the lower order moments of the magnetic resonance lineshape in “molecular” solids, provided τ is small. The applicability of this method in determining the lower moments depends not only on the value of τ and hence the so‐called “error terms” but also on the distributional arrangement of the spins. These aspects of the solid echo method are quantified in this paper from the analytical treatment of a rectangular array of two equivalent pairs of spins‐1/2. Predictions of the rather simple model, when compared with the appropriate experimental data, are found to be in excellent agreement.
Magnetooptical Activity of Allowed Electron Transitions in Atoms in Free State and in Crystals
Authors: Malakhovskii A. V.
Published: July 2007

Magnetic circular birefringence (MCB) is rigorously decomposed into diamagnetic (A) and paramagnetic (B and C) terms without substantial restrictions on the values of the fields and interactions. Peculiarities of the magnetooptical properties of the 3d‐and 4f‐atoms both, in free state and in crystals are analyzed on the basis of most general properties of the atom wave functions, spin‐orbit interaction and interactions of the atom with the magnetic and crystal fields, and the values of MCB terms are evaluated. The crystal field is shown to be in all cases a condition necessary for the B‐terms (mixing term) to exist. The mechanisms of the B‐term occurrance are considered and its values at different mechanisms are evaluated. MCB due to magnetic ordering is considered.
Collective Excitations in a Liquid Alloy
Authors: Bhandari D. , Pratap A. , Saxena N. S. , Saksena M. P.
Published: July 2007

A theory of phonons in amorphous solids developed by Takeno and Goda using a self‐consistent method is extended for the numerical calculations of the longitudinal and transverse modes in equiatomic NaK alloy. The computed results are compared with recently reported molecular dynamics results. The longitudinal phonon mode agrees fairly well with the MD results.
On the Relation of Ideal Strength to Other Physical Properties
Authors: Pokluda J. , Šandera P.
Published: July 2007

Simple relations of the ideal cohesive strength of crystals with b.c.c., f.c.c., and diamond structures to other physical properties are derived. The interatomic potential is approximated by the generalized Morse and Lennard‐Jones functions. Generally, the calculated values and the data previously published by other authors are consistent within a factor of two.
Electronic Structure of TiB2 and ZrB2
Authors: Anishchik V. M. , Dorozhkin N. N.
Published: July 2007

Calculations of the electronic structure of titanium and zirconium diborides are performed using a self‐consistent version of LMTO method. The total as well as partial densities of states are calculated and some detail of the valence band structure are also analyzed, charge transfer and chemical bond in diborides are studied.
Three‐Photon Interband Process in Solids
Authors: Hassan A. R.
Published: July 2007

A perturbative approach is used to calculate the three‐photon absorption coefficients through the most energetically favourable band models. Both parabolic and non‐parabolic band approximations are considered. Numerical estimates for three different alkali iodides are compared with a recent experimental result and with a recent theoretical non‐perturbative approach. The results show that the transition mechanisms through the perturbative non‐parabolic four‐band model, in which all the transition matrix elements are allowed, give reasonable agreement with the experiment. The other perturbative models, parabolic as well as non‐parabolic and the non‐perturbative approach, greatly underestimate the experimental results.