bluuebunny/crossref_metadata_2025_split huggingface dataset:
Relationship of Microscopic and Macroscopic Theories for Long‐Wavelength Optical Phonons in GaAs‐AlAs Superlattices
Authors: Bechstedt F. , Gerecke H. 
Published: July 2007

AbstractA 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

AbstractIn 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

AbstractA 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

AbstractSelf‐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

Abstract19F 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

AbstractAn 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

AbstractStarting 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractSpectra 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.
Radial Energy Transfer Density Distribution around the Fast Ion Tracks in Silicon and Germanium
Authors: Khlupin S. I. , Akkerman A. F. 
Published: July 2007

AbstractThe radial energy transfer density distributions around the tracks of fast protons (p), α‐particles, and C‐ions in Si and Ge targets are obtained by the Monte‐Carlo method using individual collision scheme. A semiempirical formula for D(r) is proposed based on these calculations. Some aspects of its application are discussed.
Suppression of Transparency Effect and Enhancement of Particle Reflection in an Asymmetric Potential Well
Authors: Gevorgyan L. A. , Atsagortsyan K. Z. 
Published: July 2007

AbstractThe one‐dimensional motion of a particle in the field of the Pöschl‐Teller asymmetric potential well is considered. It is shown that at positive energies the transparency effect is suppressed, as compared to the symmetric well case. The reflection coefficient increases with the well asymmetry, while at high energies its exponential decrease versus energy is replaced by a power‐like dependence. The results can be useful to those concerned with particle channeling in complex crystals.
Electron–Phonon Interaction Induced Hybridisation in Mixed‐Valence Systems
Authors: Ghosh N. K. , Ghosh S. C. , Sarkar R. L. 
Published: July 2007

AbstractThe two‐site extended spinless Falicov‐Kimball model with electron phonon (EP) interaction is considered. The usual electron‐phonon interaction term is electron operators. Average occupation number of f‐electrons, the correlation function between f and d‐hands, and the correlation between sites are calculated. It is shown that the discontinuous change in f‐electron occupation number depends on EP interaction. Finite temperature specific heat is calculated. A comparison is also made with previously obtained results.
Magnetic Structure and Phase Transformations in Mn5Si3
Authors: Menshikov A. Z. , Vokhmyanin A. P. , Dorofeev Yu. A. 
Published: July 2007

AbstractA symmetry analysis of possible magnetic structures in crystals with the space group D is made for the wave vector of star {k12}. This analysis allows deciphering the neutron diffraction data on the character of antiferromagnetic ordering in the AF1 (below T1 = 66 K) and AF2 (below T2 = 99 K) phases. It is shown that the magnetic phase transitions arise mainly from the structural transformations that occur at T1 and T2.
Emission Processes in Thermally Stimulated Exoelectron Emission and Thermoluminescence of KCl:Ag Single Crystals
Authors: Tomita A. , Fukuda Y. , Kutomi Y. , Takeuchi N. 
Published: July 2007

AbstractThe emission processes in the TSEE and TL glow peaks of KCl:Ag single crystals are discussed from the results of simultaneous recording of TSEE and TL glow curves, the spectral measurements of the TL and the optical re‐excitation study. A large fraction of the exoelectron emissions at 400 K may be considered to arise from Auger‐like decays of the excited Ag+ centers induced by the recombination of electrons from Ag0 with Ag2+ via the conduction band. The 275 nm emission band of TL at 400 K may be due to competing radiative decays of the excited Ag+ centers. The electron emissions and the 275 nm component of TL at 250 K may originate from the excited Ag+ centers resulting from the recombination of F′ (or F) centers with Ag+ centers.
Hydrides and Deuterides of Lithium and Sodium. I. Model Potentials and Their Use in Perfect Lattice
Authors: Haque E. , Islam A. K. M. A. 
Published: July 2007

AbstractAn interionic potential model is developed for lighter and heavier alkali hydrides and deuterides. The method uses a combination of theoretical techniques, empirical fit, and a few plausible assumptions. An assessment of the derived potentials is made by calculating the lattice statics and dynamics of the crystals and by comparing both with experiment (where available) and with other calculations. The potentials are found to describe the elastic and dielectric properties reasonably well. The phonon dispersion curves of hydride and deuteride of sodium are compared with the calculations of Dyck and Jex based on force constant model approach and the results are discussed. The need for further experiments on heavier hydrides and deuterides is stressed.
Phase Transition in Rb3H(SeO4)2‐Type Crystals. II. The Molecular Field Approximation
Authors: Salejda W. , Dzhavadov N. A. 
Published: July 2007

AbstractThe microscopic model of the phase transitions in superionic Rb3H(SeO4)2‐type crystals is proposed. The phase diagrams are studied within the molecular field approximation. The phase diagram obtained is in agreement with the phase diagram of Rb3H(SeO4)2 crystals. Application of the proposed model to the description of phase transitions observed in Cs3H(SeO4)2 crystals is discussed.
The New Mechanism of Physical Adsorption on Solid Surface. I. Adsorption of Nonpolar Molecules
Authors: Lozovskii V. Z. , Khudik B. I. 
Published: July 2007

AbstractA phenomenological theory of a new type of physical adsorption is constructed. The mechanism of the adsorption is due to softening of one of the energy levels of an nonmonoatomic molecule. This mode‐softening occurs due to the interaction of the molecule with the adsorbent surface. It is shown, that the nonlinear polarizabilities of a molecule and solid can play an essential role in forming of the adsorption potential.
Non‐Resonant Energy Transfer between Donor and Acceptor Systems in Presence of Relaxation
Authors: Boulil B. , Giry M. , Henri‐Rousseau O. 
Published: July 2007

AbstractThe non‐resonant energy transfer at zero temperature between two interacting two‐level quantum systems in the presence of relaxation is studied by aid of the density matrix formalism in the framework of the Markoff approximation. This gives the time evolution of the four probabilities to find the system in each of the four levels owing to the fact that at the initial time, the energy is in the excited state of the donor. The results obtained lead to the Davydov and Serikov solutions for resonant transfer of energy, and to the special one for the indirect coupling between a single level and a continuous medium.
Fine Structure of the Luminescence of Solid Neon and Its Relation to Exciton Trapping and Desorption Induced by Excitonic Excitation
Authors: Laasch W. , Hagedorn H. , Kloiber T. , Zimmerer G. 
Published: July 2007

AbstractThe luminescence of Ne* centers in solid Ne is investigated. The samples are excited with synchrotron radiation in the excitonic range. By an appropriate selection of the excitation energy and by time correlated measurement of the emission spectra, overlapping luminescence bands are separated and assigned to Ne* centers in different electronic states (1P1, 3P1, 3P2) and different environments in the bulk and at the surface of the solid. Lifetime measurements and excitation spectra of spectrally selected luminescence bands support the assignments, Electronic relaxation from n ≧ 2 free excitons to the n = 1 state is found to be less effective than exciton trapping into Ne* centers. Resonance fluorescence of free Ne* atoms indicates a close connection between exciton trapping and desorption.
Kinetics of Screening in Optically Excited Semiconductors
Authors: Hartmann M. , Stolz H. , Zimmermann R. 
Published: July 2007

AbstractKinetic equations for the density correlation and density‐density response function are derived. Solving these equations numerically and comparing its solution with the adiabatic approximation the change in the transmitted spectrum of a single electronic transition is calculated where a red‐shift can be observed.
Single and Many Particle Effects in the Emission Spectra of Laterally Homogeneous 2D Plasmas
Authors: Lach E. , Kulakovskii V. D. , Forchel A. , Reinecke T. L. , Straka J. , Grützmacher D. , Weimann G. 
Published: July 2007

AbstractBy the use of homogeneously excited small mesa structures a highly uniform electron‐hole plasma (EHP) is created in InGaAs/InP and GaAlAs single quantum wells. The spectra obtained from mesa structures differ strongly from data of usually used inhomogeneously excited samples. The spectra clearly show band‐filling up to the of the potential well and strong renormalization effects. The emission spectra are analyzed by a lineshape model including the full coupled non‐parabolic valence bands for the dispersion of the holes and the transition matrix elements in the case of GaAs/GaAlAs and by the use of parabolic subbands for electron‐hole plasmas in InGaAs/InP quantum wells. A strong dependence is observed of the renormalization on the subband index for low plasma temperatures.
Optical Properties of Biexcitons in ZnSe/Zn1−xMnxSe Quantum Wells
Authors: Bobrysheva A. I. , Russu S. S. 
Published: July 2007

AbstractIt is shown that in ZnSe/(Zn, Mn)Sequantum wells six biexcitonic states can exist. The group‐theoretical classification of these states is performed. The probabilities of biexciton two‐photon absorption, hyper‐Raman scattering, and radiative recombination are calculated. The polarization properties of two‐photon absorption of two‐and three‐dimensional excitons are discussed.
Dynamics of Carrier‐Screened Photon‐Exciton Interaction
Authors: Mareyen M. , Schütte F. ‐J. , Tiebel R. 
Published: July 2007

AbstractThe exciton‐photon interaction in a semiconductor‐optical device is influenced by the carrier pair density. To take this into account, the coupling constant is corrected to the first order. This simple model shows remarkable dynamical behaviour as, for instance, up to five coexisting fixed points and chaos. Chaotic transients are found which are precursors of permanent chaotic motion.
Nonlinear Optical Properties of Quantum Confined Semiconductor Microcrystallites
Authors: Ricard D. , Roussignol P. , Hache F. , Flytzanis Ch. 
Published: July 2007

AbstractResults are given of recent studies on the nonlinear optical properties of quantum‐confined semiconductor (CdSSe) particles. The nonlinear mechanism is saturation of the first or 1s‐1s transition which behaves as a two‐level system. The intrinsic broadening of this transition by electron‐phonon coupling is very important and dominant at room temperature. The enhancement of the nonlinearity due to quantum confinement could however be observed, which may lead to important applications. Similarly, for hybrid devices, the static Kerr effect studied through the change in absorption spectrum induced by a defield shows a large nonlinearity which can be fully understood in terms of a Stark effect.
Experimental Investigation of Three‐Level Optical Stark Effect in Semiconductors
Authors: Fröhlich D. , Neumann Ch. , Uebbing B. , Wille R. 
Published: July 2007

AbstractThe three‐level optical Stark effect is due to a dynamical coupling of two excited levels. The coupling is achieved by a high intensity CO2 laser. Results of experiments on multiple quantum wells (MQW) and excitons in Cu2O are presented. In the MQW the coupling between the two lowest conduction band sublevels leads to characteristic changes in the one‐photon spectra of the lowest exciton transitions. In Cu2O the CO2 laser pulse couples 1 S and 2 P excitons. In this case the one‐photon spectrum of the 2 P exciton (dipole allowed) and also of the 1 S spectrum (quadrupole allowed) show drastic changes. Most of the experimental results can be explained quantitatively in terms of a nonlinear susceptibility.
Transverse Relaxation and Polarization Specifics in the Dynamical Stark Effect
Authors: Zimmermann R. 
Published: July 2007

AbstractThe transverse relaxation of excitons in the dynamical Stark effect is treated on a microscopic level within a density‐matrix approach explaining the strongly suppressed damping for the off‐resonant pump pulse. An expression for the differential transmission spectrum is derived including the dependence on pump‐ and test polarization. Calculations for CdS with its three exciton series are given.
Coupled Two‐Level Systems and the Dynamics of Semiconductor Electrons
Authors: Stahl A. 
Published: July 2007

AbstractA basic model in quantum optics is a system of interacting two‐level atoms. The simplest case where the interaction is only via macroscopic electromagnetic fields leads to the so called Maxwell‐Bloch dynamics. When a tunneling interaction and Coulomb potentials are added the model may by extended to yield the density matrix dynamics for a two‐band semiconductor. A typical property of this dynamics is the coherent coupling between electromagnetic and quantal waves. This is exploited in a solution of the ABC‐problem. The basic nonlinearities of the electron dynamics in the two‐band model are reviewed. Some of them are related to the various manifestations of the dynamical Stark effect.