bluuebunny/crossref_metadata_2025_split huggingface dataset:
Simulation of Plastic Deformation in a Flexible Chain Glassy Polymer
Authors: Argon A. S. , Mott P. H. , Suter U. W. 
Published: July 2007

AbstractA short report of a new computer simulation of plastic deformation is given in a typical flexible chain glassy polymer–polypropylene. The model which makes no arbitrary assumptions about structure and modes of molecular segment relaxations is based on systematic static energy minimizations in a small periodic cell representing a typical molecular environment. The simulation shows that because of the relatively inflexible nature of backbone bonds and bond angles, permitting conformational changes only through alterations in torsional degrees of freedom, the size of a coherent plastic relaxation unit is typically 8 to 10 nm in dimensions. Careful scrutiny of conformational alterations revels no recurring simple kinematical configurations such as dislocations, disclinations, rotons, strophons, or the like. The best emerging picture is that plastic relaxation is by the repeated nucleation of shear transformations in a very large group of segments in a very cooperative manner resulting, however, in transformation shear strains of only about 1.5%. While such transformations are dilatant in polypropylene, they are the opposite in polycarbonate which has a more structured and stiff molecule.
Spatial and Spatio‐Temporal Patterns in pnpn Semiconductor Devices
Authors: Niedernostheide F.‐J. , Arps M. , Dohmen R. , Willebrand H. , Purwins H.‐G. 
Published: July 2007

AbstractSpatial and spatio‐temporal patterns of the current density and the potential distribution in silicon pnpn devices are expermentally investigated. In dependence on various parameters we observe homogeneous current distributions and the spontaneous appearance of stationary standing, rocking, and travelling well‐localized current density filaments. Model calculations reproduce the main features of the experiments.
Superconductivity of Fullerenes Doped with Alkali Metals
Authors: Davydov A. S. 
Published: July 2007

AbstractThe properties of the alkali‐metal‐doped fullerene C60 are discussed. It was shown that a superconductivity of this crystal is determined by a quasi‐one‐dimensional spreading of a planar bisolation condensate. The condensate is formed when the energy of the electron–phonon interaction exceeds the energy of the one‐dimensional zeor vibrations caused by local pairing. Under very strong electronphonon interaction small‐radius, spherically symmetric bisolitons are created instead of planar bisolitons. Their formation needs overcoming of a potential barrier. The localized bisolitions are moving through the crystal by a hopping mechanism only.
Ambipolar Diffusion of Electron–Hole Pairs in β‐Rhombohedral Boron
Authors: Werheit H. , Kristen F. , Franz R. 
Published: July 2007

AbstractIn β‐rhombohedral boron the electronic transport is essentially determined by deep electron traps of different bonding energies. The ambipolar diffusion of optically excited electron–hole pairs on diffusion paths of up to 5.37 mm length was investigated. The diffusion length at stationary conditions is 21 mm and the mean diffusion velocity is 5.8 × 10−6 m s−1. From these data a lifetime of 3620 s results. By Fick's second law the diffusion coefficient was determined to increase from about 5 × 10−3 to 101 cm2 s−1 with the diffusion velocity increasing from 10−5 to 10−1 m s−1. The Einstein relation yields the corresponding mobilities, which increase accordingly from about 0.2 to about 50 cm2 V−1 s−1. These mobilities are mean values for electrons and holes. The relaxation of the excited state of the sample into the thermal equilibrium is determined by five relaxation times (6.6, 50, 514, 5650, 14700 s), which are immediately related to the thermal excitation of the electrons from the traps with the bond energies 0.18, 0.37, 0.58, 0.77, and 0.89 eV.
Magnetic Susceptibility of Electrons in Quantum Wires of Wide‐Gap Semiconductors
Authors: Das N. R. , Chakravarti A. N. 
Published: July 2007

AbstractA transverse perturbing magnetic field is assumed to be applied to a rectangular quantum wire of a wide‐gap semicondutor, to determine the magnetic susceptibilities of electrons. The computations are perfomed for a low value of the temperature, taking degenerate n‐GaAs as an example of such semiconductors, It is interesting to note that, not only the paramagnetic‐to‐diamagnetic susceptibility ratio for electrons in a semiconducting quantum wire deviates from that (⅓) in conventional semiconductors, but also that there is a critical region, governed by the concentration of the electrons and the transverse dimensions of the wire within quenching of the diamagnetism occurs.
Nucleation Field for Three‐Dimensional Rotation in Inhomogeneous Ferromagnets
Authors: Hu Jifan , Kronmüller H. 
Published: July 2007

AbstractBased on the method developed for the one‐dimensional rotation process the nucleation field for three‐dimensional rotation is deduced from the linearized micromagnetic equation for magnetic inhomogeneities due to a spatial variation of the magnetocrystalline anisotropy energy. Comparing the nucleation field of the one‐dimensional rotation with that of three‐dimensional rotation, it turns out that one‐dimensional rotation describes experimental data of sintered magnets much better than the latter one. The physical process of the nucleation is dominated by the low‐dimensional rotation – especially the one‐dimensional rotation – due to the limited freedom of magnetization state by the alignment of the magnetization within grains and the existence of an easy axis in uniaxial magnets.
The Effect of Germanium Overlayers on the Magnetization of Chromium Films
Authors: Schröder K. , Nayak S. 
Published: July 2007

AbstractThe magnetic moment of chromium films with and without germanium overlayers is determined in an ultra‐high vacuum oscillating torque magnetometer at room temperature. Films deposited with growth rates from 0.002 to 0.01 nm/s at pressures of about 10−5 to 10−6 Pa show frequently strong diamagnetism. This diamagnetism usually increases if Ge overlayers are deposited on the Cr film. It seems difficult to associate this increase with the magnetization of the germanium film. It is suggested that one should try to explain the observed susceptibility values with a kind of exciton mechanism.
Parametrization of S‐ and P‐Magnetoexcitons in Quantum Wells
Authors: Engbring J. , Zimmermann R. 
Published: July 2007

AbstractAn analytical formula is proposed that describes magneto‐optical data of heavy hole S‐ and P‐magnetoexcitons in quantum welis quite well. The open coefficients in this formula have been fitted to numerically derived magnetoexciton energies. Experimental results for S‐ and P‐magnetoexcitons are presented obtained from one‐ and two‐photon excitation spectroscopy. It is possible to extract parameters for conduction band and valence band separately, while band mixing is taken into account by nonparabolicity parameters. The investigation shows that the dominant features of the spectra can be quantitatively described in the frame of a two‐band model.
Propagation Beats of Quadrupole Polaritons in Cu2O
Authors: Fröhlich D. , Kulik A. , Uebbing B. , Langer V. , Stolz H. , von der Osten W. 
Published: July 2007

AbstractTime‐resolved measurements of transmission in resonance with the 1 S quadrupole polariton in Cu2O reveal a long‐lasting signal which is modulated by pronounced oscillations with time‐dependent period. These oscillations are interpreted as “propagation beats”, which are caused by superposition of excited states of both polariton branches which propagate coherently through the crystal. At higher excitation intensities the decay becomes faster and the beats disappear. Magneto‐quantum beats are also influenced significantly by propagation effects in transmission measurements.
Coherent Polariton Dynamics in Quantum Wells
Authors: Rappen T. , Mohs G. , Wegener M. 
Published: July 2007

AbstractIt is demonstrated that the exciton–photon coupling, or equivalently the polariton nature of the exciton, may lead to qualitative changes in the lineshape of time‐resolved four‐wave mixing. For small oscillator strength and hence small longitudinal–transverse splitting this coupling acts very similar to the recently discovered coherent exciton–exciton interaction, i.e. as an exponential rise of the time‐integrated signal for negative time delay. Here the exciton–photon coupling can be interpreted as a mediated exciton–exciton interaction. For larger coupling a distinct nonperiodic modulation evolves that reflects the oscillation between the two branches of the polariton dispersion. Spectrally and time‐resolved four‐wave mixing is shown to give direct access to the dynamics on the two branches. Corresponding femtosecond experiments on two‐dimensional magneto‐excitons are compared with numerical calculations.
Interband Quantum Kinetics with LO‐Phonon Scattering in a Laser‐Pulse‐Excited Semiconductor I. Theory
Authors: Haug H. 
Published: July 2007

AbstractNonequilibrium Green's function theory is employed to derive with the use the generalized Kadanoff‐Baym ansatz quantum kinetic equations for the interband polarization and the densities of electrons and holes. These equations are valid in the regime of ultrafast spectroscopy, where that usual Boltzmann kinetics with its individual energy‐conserving collisions no longer holds. The memory kernel of the retarded quantum kinetic equations is determined by retarded and advanced Green's functions. It is stressed and explicitly shown by numerical solutions in an adjacent paper that these Green's functions have to be approximated consistently with the coupling to phonons.
Electronic Coherence Effects in Absorption Saturation Experiments
Authors: Scholz R. , Stahl A. 
Published: July 2007

AbstractIn pump‐probe experiments with femtosecond pulses, a coherent peak is found in the absorption saturation for short pulse delays. This coherent peak is analysed within a density matrix model, including a realistic multi‐valence band structure and the optical matrix elements to leading order. Dissipative processes are treated in a relaxation time approximation. The parameters of the model are determined by comparison to experimental results.
Optical Excitations near the Fundamental Gap of Solid C60
Authors: Feldmann J. , Fischer R. , Göbel E. O. , Schmitt‐Rink S. 
Published: July 2007

AbstractOptical studies of solid C60 employing time‐integrated as well as time‐resolved photoluminescence spectroscopy are reported. At low excitation intensity, a richly structured luminescence spectrum is observed. Increasing the excitation intensity leads to pronounced carrier accumulation and to the emission of intense and spectrally broad luminescence. It is concluded that multiple‐step absorption processes lead to carrier recombination originating from higher conduction or lower valence band states.
Tunneling through Single AlGaAs Barriers
Authors: Heberle A. P. , Rühle W. W. , Köhler K. 
Published: July 2007

AbstractTunneling transfer through Al0.35Ga0.65As barriers is studied in asymmetric double quantum well structures by time‐resolved photoluminescence measurements in the pico‐ and femtosecond regime. A large variety of electron and hole resonances is detected when electric fields of both signs are externally applied. The ground state resonance shifts, when the electrons tunnel in the reverse direction, revealing the importance of excitonic effects. Longitudinal optical phonon assisted tunneling plays a minor role for narrow quantum wells in comparison to impurity or interface roughness assisted transfer. Resonant electron tunneling times depend exponentially on the integrated square root of the tunneling barrier height and are an order of magnitude faster than resonant hole tunneling times. The n = 2 to n = 1 electronic intersubband scattering time in a 10 nm quantum well is determined to be 550 fs measuring the transfer time through a thin barrier.
Quantum‐Confined Electron–Hole Droplets
Authors: Kalt H. , Nötzel R. , Ploog K. , Giessen H. 
Published: July 2007

AbstractThe liquid phase of quantum‐confined electron–hole plasma is observed in a type‐II GaAs/AlAs short‐period quantum‐well wire superlattice grown on a high‐index (311) GaAs substrate surface. This superlattice system is the first low‐dimensional structure to support the condensation of electron–hole droplets as is evidenced by the characteristic properties of photoluminescence spectra.
Luminescence Spectra and Lattice Dynamics of NaNO2 in the Ferroelectric Phase
Authors: Janssen W. B. J. M. , Van Avoird A. Der , Köhler J. , Schmid D. 
Published: July 2007

AbstractIn order to understand the results of recent luminescence experiments harmonic lattice dynamics calculations on the ferroelectric phase of NaNO2 are performed. Starting from an atom‐atom potential and including the polarization of the nitrite ions by means of the shell model the phonon and vibron dispersion curves and densities of states are calculated. The calculated phonon dispersion curves are in good agreement with the inelastic neutron scattering data. The phonon density of states is compared with the phonon side band measured by luminescence. Above 15 K the measured phonon side band resembies the density of states, but at lower temperatures this resemblance is lost because of the non‐uniforn distribution of the excited states over the Brillouin zone. The density of states of the v2‐vibron band is compared with the lineshape of the v2 line in luminescence spectra. It is shown that this lineshape can be explained by inclusion of the dispersion of the singlet exciton.
Positron Annihilation Studies in GaAs
Authors: Panda B. K. , Padhi H. C. , Viswanathan B. 
Published: July 2007

AbstractAngular correlation of positron annihilation radiation (ACPAR) along different crystallographic directions of GaAs are measured using the long‐slit geometry. The shapes of the profiles indicate in addition to the ideal sp3 orbitals contribution, the contribution of σ bonding along [111] bonding direction and an admixture of σ and π* bondings for the [100] and [110] directions. The ACPAR anisotropies in this case are somewhat less compared to that of Ge which is caused because of the presence of the ionicity in compound semiconductors. Although the zero points of the autocorrelation functions for ACPAR data are shifted from the actual lattice positions, their periodicity remains same as the actual lattice spacing.
On the Possibility of Space‐Charge Wave Amplification in Multielectrode MIS Microstructure
Authors: Tralle I. E. , Filonov A. B. 
Published: July 2007

AbstractThe beam instability in a metal–insulator–semiconductor microstructure with periodical field electrodes is considered. The instability appears as a consequence of the inversion of the electron distribution function in gallium arsenide in a strong electric field. The amplification of a space‐charge wave in such microstructures can be basis for working out of microwave amplifiers.
Tunneling current through a resonant structure
Authors: Lu X. J. , Horing N. J. M. 
Published: July 2007

AbstractTunneling transport through a resonant double‐barrier structure is examined, with attention focused on the suppression of transmission current due to Pauli exclusion arising from population growth of the resonant state. A quasi‐analytic theory is developed to self‐consistently determine the population of the resonant state and its exclusion suppression of tunneling. Furthermore, the role of resonant state population in inducing an electrostatic increase of the resonant level energy, which tends to enhace tunneling, is considered. Taking all these features in joint consideration, a quasi‐analytic description of intrinsic bistability in the regime of negative differential conductance is achieved, exhibiting hysteresis in the I‐U characteristic.
Electron‐molecular vibration coupling in the cation‐radical salt of a giant analogue of TTF with ClO anion
Authors: Graja A. , Yartsev V. M. , Garrigou‐Lagrange C. , Sallé M. , Gorgues A. 
Published: July 2007

AbstractSpectral studies of an ion‐radical salt DClO4 of a giant analogue (D) of TTF are performed in which the dihydro‐TTF core is bearing the two conjugated 1,4‐dithiafulvalen‐6‐yl side arms. Both IR absorption and electronic spectra of the donor molecule are measured; Raman scattering is also accomplished. Our main attention is paid to the electronmolecular vibration coupling in the DClO4 salt. It is shown that the idea of treating the donor molecules as an aggregate of two parts excited by the intramolecular charge transfer is an acceptable first approximation of the EMV coupling in the giant π‐donor. The EMV coupling constants are calculated and compared with those determined for the parent TTF molecule.
Inversion effect of the electric field in the quantum well in the presence of a single diamagnetic impurity
Authors: Monozon B. S. , Hüther P. , Wies P. , Gerlach H. , Schmidt M. 
Published: July 2007

AbstractThe effects of parallel magnetic and electric fields on a shallow donor state in thin semiconductor layers (quantum wells) are investigated. The magnetic field is strong in comparison with the Coulomb field of the electron. It is shown that in such systems the inversion effect appears, i.e. the eigenstates and energies of the impurity states change with the inversion of the direction of the electric field. Using the weak impurity potential approximation analytical expressions for the inversion shift of the energy levels are obtained. The results for the shift of the ground level for GaAs well are obtained in closed form. By measurement of the inversion energy shift it will be possible to determine the position of the impurity centre relative to the boundary of the layer.
Rhombic Fe3+ centers in KTaO3
Authors: Pechenyi A. P. , Glinchuk M. D. , Antimirova T. V. , Kleeman W. 
Published: July 2007

AbstractThe EPR spectrum of rhombic Fe3+ paramagnetic centers in KTaO3 both nominally pure and with Li and Nb impurities is investigated. The absolute values and signs of the parameters of the spin‐Hamiltonian describing the observed EPR spectrum are determined on the basis of the angular and temperature dependences of the mentioned spectrum. The model of a rhombic iron center involving the Fe3+ ion substituted for a Ta5+ ion with a nearby interstitial bivalent positive ion in [110] direction for charge compensation is suggested.
Magnetopolaron in a rectangular quantum wire under a strong magnetic field
Authors: Lü Tian‐Quan , Gu Shi‐Wei 
Published: July 2007

AbstractConsidering the interaction of an electron with the confined LO‐phonons, the ground state energy shift of a magnetopolaron in a rectangular quantum wire is calculated under a strong magnetic field using the effective mass approximation and the variational method. It is shown that when the external magnetic field keeps constant, the absolute value of the ground state energy shift of a magnetopolaron in a rectangular quantum wire as a function of the cross section of the wire increases monotonically with the increase of the size in one direction remaining constant in the other direction and increases very slowly when the size of the wire is very large. At constant cross section of the quantum wire the absolute value of the ground state energy shift increases approximately linarly with the increase of the external magnetic field.
Soliton excitations in the classical one‐dimensional antiferromagnet in a staggered field
Authors: Cassino F. S. L. , Pires A. S. T. 
Published: July 2007

AbstractSoliton excitations in an easy‐plane classical one‐dimensional antiferromagnet in the presence of a staggered magnetic field transverse to the chain direction are studied. The out‐of‐plane soliton solution is found to have an instability similar to the one found before by Magyari‐Thomas‐Kumar for the easy‐plane ferromagnetic chain in an external magnetic field. Also studied are the soliton stability and the phase shift of spin waves in the presence of the soliton.
Hole Distribution in YBCO Superconductor Ionic Model Studies
Authors: Gong X. G. , Pan Wei , Zheng Q. Q. 
Published: July 2007

AbstractThe total energy is approximated in an ionic model. By applying the obtained total energy formula to the YBa2Cu4O8 superconductor, the possible hole distributions are investigated. The results show that the hole is predominantly on the apical oxygen, only about 0.06 holes per CuO2 plane, which is in agreement with photoemission experiments indicating that the hole has oxygen character.
EPR Studies of Restricted Motion of NH3+ Radicals in X‐Irradiated ADA and Glassy RADA
Authors: Waplak S. , Trybula Z. , Drumheller J. E. , Schmidt V. H. 
Published: July 2007

AbstractThe electron paramagnetic resonance (EPR) spectra of NH3+ radicals in NH4H2AsO4 (ADA)and mixed Rb1−x(NH4)xH2AsO4 (RADA) (x = 0.35) in the temperature region 4.2 to 300 K are measured. It is shown that in ADA at room temperature (RT) NH3+ molecules undergo fast rotation around the principal axes of the crystal field tensor, and make rapid jumps between them. These motions provide nearly isotropic averaging of the spectra. Below TN = 216 K rapid rotation only around the a‐ and b‐axes is present. In contrast to ADA, in RADA the NH3+ groups rotate rapidly only around the a‐ and b‐axes even at RT and inversion tunneling of nitrogen is suggested. The hyperfine structure of the ammonia proton is isotropically averaged down to 115 K in ADA and down to 60 K in RADA.
Optical Absorption of the Fd Center in KCl: Ca2+
Authors: Cárdenas‐García D. , López‐Téllez E. R. , Ruíz‐Mejía C. 
Published: July 2007

AbstractThe optical absorption of the Fd center in KCl: Ca2+ is studied by using point‐ion and ion‐size correction models. For the calculations the Gourary and Adrian wave function types I, II, III and Gaussian functions are used. The principal axes are obtained for several positions of the impurity‐vacancy dipoles, and the corresponding ground and excited states of the Fd center are calculated. Good agreement with the experimental values is found for the Gourary and Adrian wave functions of type I.
Absorption, Emission, and Thermoluminescence of Copper Doped Alkali Sulphate Phosphors
Authors: Kishan Kumar V. S. , Acharyulu B. S. V. S. R. , Sastry S. B. S. 
Published: July 2007

AbstractResults of optical absorption (OA), fluorescence, and thermoluminescence (TL) studies made on copper doped sulphates of cesium, rubidium, and potassium are reported and discussed. Optical absorption spectra revealed that the impurity enters these lattices in both its mono‐ and divalent states. Fluorescent emission of Cu+ is detected in the three phosphors. Both Cu+ and Cu2+ are found to be acting as electron traps when the samples are exposed to γ‐rays. Absorption due to Cu0 thus formed is detected around 3.45 eV in the irradiated samples. TL glow curves show a three‐peak pattern and are assigned to the thermal mobilization of hole centres formed during irradiation. Unlike in alkali halides, the TL emission spectra do not always contain the characteristic Cu+ emission band.
Heavy Hole—Light Hole Splitting in GaAs/AlAs Superlattices
Authors: Munzar D. 
Published: July 2007

AbstractEppenga's six‐band k · p model for the description of the valence band structure of GaAs/AlAs quantum wells is modified for the k⟂ = 0 states in GaAs/AlAs superlattices. The model takes into account the coupling between the light holes and the spin—orbit split‐off holes and yields a very good agreement between experimental and calculated values of the splitting between the first heavy and light hole energy levels, which is not obtained in the simple effective mass approximation.