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Energy Spectrum (energy + spectrum)
Selected AbstractsChemical-Picture-Based Modeling of Thermodynamic Properties of Dense Multicharged-Ion Plasmas Using the Superconfiguration ApproachCONTRIBUTIONS TO PLASMA PHYSICS, Issue 10 2009P.A. Loboda Abstract Using the chemical-picture representation of plasmas as a mixture of various ions and free electrons, a consistent description of thermodynamics of dense multicharged-ion plasmas is being developed that involves the effects of Coulomb non-ideality and degeneracy of plasma electrons; contribution of the excited ion states (on the base of the superconfiguration approach) that may exist under an appropriate truncation of ion energy spectra due to plasma effects; hard-sphere-model representation of the finite-volume effects of plasma ions with the model parameters (effective ion sizes) corresponding to superconfigurations yielding the greatest contribution to partition functions. We present the calculated data for average ionization, Grüneisen coefficient, and specific heat of aluminum and iron plasmas at temperatures of 0.03,3 keV and densities 10,3 , 10,5 of their normal material densities. Calculated thermodynamic functions and shock Hugoniots are compared with other theoretical and experimental data (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Earthquake-resistant structural design through energy demand and capacityEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 14 2007Adang Surahman Abstract An energy-based earthquake-resistant structural design method is proposed. The proposed method uses specific input energy spectra, modal or time-history analyses, and energy distribution among structural members. For a given member strength and stiffness, a relationship between the energy attributable to damage absorbed by a member and its cumulative ductility demand can be determined. Member strength, stiffness and energy capacity are design parameters which are simultaneously used in the design. The method can avoid soft-storey design. The damage is measured based on a cumulative basis considering earthquake magnitude, frequency, and duration. Tests have been carried out to determine energy absorbing capacities of various structural components. More efforts are needed to make the energy-based earthquake-resistant structural design practical, but ssimple formulations for this method are possible. Copyright © 2007 John Wiley & Sons, Ltd. [source] Multivariate calibration of hyperspectral ,-ray energy spectra for proximal soil sensingEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2007R. A. Viscarra Rossel Summary The development of proximal soil sensors to collect fine-scale soil information for environmental monitoring, modelling and precision agriculture is vital. Conventional soil sampling and laboratory analyses are time-consuming and expensive. In this paper we look at the possibility of calibrating hyperspectral ,-ray energy spectra to predict various surface and subsurface soil properties. The spectra were collected with a proximal, on-the-go ,-ray spectrometer. We surveyed two geographically and physiographically different fields in New South Wales, Australia, and collected hyperspectral ,-ray data consisting of 256 energy bands at more than 20 000 sites in each field. Bootstrap aggregation with partial least squares regression (or bagging-PLSR) was used to calibrate the ,-ray spectra of each field for predictions of selected soil properties. However, significant amounts of pre-processing were necessary to expose the correlations between the ,-ray spectra and the soil data. We first filtered the spectra spatially using local kriging, then further de-noised, normalized and detrended them. The resulting bagging-PLSR models of each field were tested using leave-one-out cross-validation. Bagging-PLSR provided robust predictions of clay, coarse sand and Fe contents in the 0,15 cm soil layer and pH and coarse sand contents in the 15,50 cm soil layer. Furthermore, bagging-PLSR provided us with a measure of the uncertainty of predictions. This study is apparently the first to use a multivariate calibration technique with on-the-go proximal ,-ray spectrometry. Proximally sensed ,-ray spectrometry proved to be a useful tool for predicting soil properties in different soil landscapes. [source] On the use of high-order finite-difference discretization for LES with double decomposition of the subgrid-scale stressesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 4 2008J. Meyers Abstract Large eddy simulation (LES) with additional filtering of the non-linear term, also coined LES with double decomposition of the subgrid-scale stress, is considered. In the literature, this approach is mainly encountered in combination with pseudo-spectral discretization methods. In this case, the additional filter is a sharp cut-off filter, which appears in the eventual computational algorithm as the 2/3-dealiasing procedure. In the present paper, the LES approach with additional filtering of the non-linear term is evaluated in a spatial, finite-difference discretization approach. The sharp cut-off filter used in pseudo-spectral methods is then replaced by a ,spectral-like' filter, which is formulated and discretized in physical space. As suggested in the literature, the filter width , of this spectral-like filter corresponds at least to 3/2 times the grid spacing h to avoid aliasing. Furthermore, spectral-like discretization of the derivatives are constructed such that derivative-discretization errors are low in the wavenumber range resolved by the filter, i.e. 0,kh,2,/3. The resulting method in combination with a Smagorinsky model is tested for decaying homogeneous isotropic turbulence and compared to standard lower-order discretization methods. Further, an analysis is elaborated of the Galilean-invariance problem, which arises when LES in double decomposition approach is combined with filters, which do not correspond to an orthogonal projection. The effects of a Galilean coordinate transformation on LES results, are identified in simulations, and we demonstrate that a Galilean transformation leads to wavenumber-dependent shifts of the energy spectra. Copyright © 2007 John Wiley & Sons, Ltd. [source] Large eddy simulation of compressible turbulence using high-resolution methodsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 8-9 2005M. Hahn Abstract The paper presents a numerical investigation of high-resolution schemes for solving the compressible Euler and Navier,Stokes equations in the context of implicit large eddy simulation (ILES), also known as monotone integrated LES (MILES). We have employed three high-resolution schemes: a flux vector splitting (FVS), a characteristics-based (Godunov-type) and a hybrid total variation diminishing (TVD) scheme; and carried out computations of: (i) decaying turbulence in a triply periodic cube and (ii) compressible flow around open cavities for low and high Reynolds numbers, at transonic and supersonic speeds. The decaying turbulence simulations show that all high-resolution schemes employed here provide plausible solutions without adding explicit dissipation with the energy spectra being dependent on the numerics. Furthermore, the ILES results for cavity flows agree well with previously published direct numerical simulations and experimental data. Copyright © 2004 John Wiley & Sons, Ltd. [source] Exactly solvable effective mass Schrödinger equation with coulomb-like potentialINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010C. Pacheco-García Abstract Exactly solvable Schrödinger equation (SE) with a position-dependent mass distribution allowing Morse-like eigenvalues is presented. For this, the position-dependent mass Schrödinger equation is transformed into a standard SE, with constant mass, by means of the point canonical transformation scheme. In that method, the choice of potential for the position-dependent mass Schrödinger equation allows us to obtain the transformation that should be used to find the exactly solvable SE. As a useful application of the proposal, the equivalent of the Witten superpotential is chosen to be constant to find the position-dependent mass distribution and the exactly solvable potential V(m(x)) allowing Morse-type energy spectra. This V(m(x)) is shown to have a Coulomb potential structure and can be useful in the study of the electronic properties of materials in which the carrier effective mass depends on the position. Moreover, the worked example, the approach is general and can be applied in the search of new potentials suitable on the study of quantum chemical systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Monoenergetic electron beam generation in a laser-driven plasma accelerationLASER PHYSICS LETTERS, Issue 2 2006M. Adachi Abstract We obtained a 7-MeV monoenergetic electron beam from a plasma with the electron density ne of 1.5 × 1020 cm,3 produced by a 2-TW 50-fs laser pulse. In both higher and lower sides of the density region of 4 × 1019 ÷ 4 × 1020 cm,3, energy spectra of electrons were bi-Maxwellian distribution function whose maximum electron energy and effective electron temperature were 30 MeV and approximately MeV, respectively. Observed first Stokes satellites in the forward scattering light spectra, and the density dependences of maximum electron energy and the effective temperature suggest that electrons are first accelerated by SMLWFA and are further accelerated by direct laser acceleration (DLA) in the ne region of more than 2 × 1020 cm,3; a cascade acceleration by SMLWFA and DLA. A Stokes satellite peak observed with the monoenergetic beam suggests that the monoenergetic beam would be accelerated by SMLWFA. (© 2006 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Influence of electron correlations on strong field ionization of calciumLASER PHYSICS LETTERS, Issue 1 2004E. Charron Abstract Non-perturbative time-dependent calculations of single and double ionization of a one-dimensional model of atomic calcium by short and intense laser pulses were performed at various wavelengths. The comparison of the probabilities calculated within a two-active electron (TAE) approach with those obtained using a single-active electron (SAE) approximation clearly demonstrates the crucial role played by the electronic correlation and by doubly excited states within this model, even for the formation of Ca+. Experimental and calculated energy spectra of the singlet states of Ca. The experimental values (b) are taken from [20], and the calculated values (a) on the left and (c) on the right correspond to the present SAE and TAE models respectively. (© 2004 by HMS Consultants. Inc. Published exclusively by WILEY-VCH Verlag GmbH & Co.KGaA) [source] Hard electron energy distribution in the relativistic shocks of gamma-ray burst afterglowsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008L. Resmi ABSTRACT Particle acceleration in relativistic shocks is not a very well understood subject. Owing to that difficulty, radiation spectra from relativistic shocks, such as those in gamma-ray burst (GRB) afterglows, have been often modelled by making assumptions about the underlying electron distribution. One such assumption is a relatively soft distribution of the particle energy, which need not be true always, as is obvious from observations of several GRB afterglows. In this paper, we describe modifications to the afterglow standard model to accommodate energy spectra which are ,hard'. We calculate the overall evolution of the synchrotron and Compton flux arising from such a distribution. We also model two afterglows, GRB010222 and GRB020813, under this assumption and estimate the physical parameters. [source] Excitonic polaron in InAs/GaAs self-assembled quantum dot moleculesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2007M. Adames Abstract We study the excitonic polaron formation in two InAs/GaAs coupled quantum dots. We calculate the coupling between the exciton and the LO-phonon states by using the Fröhlich Hamiltonian, from which we determine the excitonic polaron states varying the quantum dot separation. We study the dependence on the excitonic polaron formation with the coupling between the dots. Polaron formation strongly modifies the energy spectra due to the appearance of several anticrossings in the excited states. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Numerical simulations of photon trapping in doped photonic crystals doped with multi-level atomsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 8 2005Mahi R. Singh Abstract A theory of photon trapping has been developed in photonic band-gap (PBG) and dispersive polaritonic band-gap (DPBG) materials doped with an ensemble of five-level atoms. These materials have gaps in their photon energy spectra. The atoms are prepared as coherent superpositions of the two lower states and interact with a reservoir and two photon fields. They also interact with each other by dipole-dipole interaction. The Schrödinger equation and the Laplace transform method are used to calculate the expressions for the number densities of the atomic states. Numerical simulations for a PBG material reveal that when the resonance energies lie away from the band edges and within the lower or upper bands, trapping is observed at certain values of the relative Rabi frequency associated with the two fields, which vary depending on the strength of the dipole-dipole interaction between the atoms. Also, if the photon fields are held constant, the population densities of the excited states of the atoms increase with increasing dipole-dipole interaction. These are very interesting phenomena. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Detection of red luminescence with improved photocathodesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2005R. Downey Abstract Measurement of luminescence from 700 to 1,000 nm has been improved both by retrofit designs on existing photomultiplier tubes and design of new cathode structures. Based on the S20 type multialkali photocathodes the enhanced systems offer improvements over conventional PM tubes from a factor of ,2 from 200 to 700 nm, increasing to ,25 times on progressing to longer wavelengths from 900 to beyond 1,100 nm. These gains offer major changes in sensitivity for detection of weak long wavelength emission bands, with improved signal to noise and reductions in problems of second order light. Indeed, previously ignored red emission bands may dominate the photon energy spectra in some cases. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Quasiperiodic impurity energy spectra of GaAs/GaxAl1,xAs superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2004M. S. Vasconcelos Abstract In this work we consider a generalized Fibonacci quasiperiodic superlattice (GFQPSL), within a tight-binding model, in which its nearest-neighbor-hopping matrix elements are distributed according to the generalized Fibonacci sequence. The electronic density of states (DOS) is then determined by using a Green function method based on Dyson's equation together with a transfer-matrix treatment. The resulting energy spectrum is then determined, considering initial physical parameters according to the scheme used in the experimental realization of a GFQPSL. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic structure of three-dimensional triangular torus-shaped quantum rings under external magnetic fieldsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2003Yiming Li Abstract In this paper, we calculate the electron,hole energy states and the magnetization for InAs/GaAs triangular torus-shaped (TTS) quantum rings in a magnetic field. Our three-dimensional (3D) model considers (i) the effective one-band Hamiltonian approximation, (ii) the position- and energy-dependent quasi-particle effective mass approximation, (iii) the finite hard wall confinement potential, and (iv) the Ben Daniel-Duke boundary conditions. This model is solved numerically with the nonlinear iterative method to obtain the "self-consistent" solutions. We investigate the electron-hole energy spectra versus magnetic field for two different ring widths: R0 = 20 and 50 nm, and find that they strongly depend on the ring shape and size. Since the magnetic field penetrates into the inside region of the nonsimply connected ring, the electron (hole) transition energy between the lowest states versus magnetic field oscillates nonperiodically and is different from that of quantum dots. We find the magnetization at zero temperature is a negative function, saturates, and oscillates nonperiodically when the magnetic field increases. [source] X-ray energy spectra of CAL87ASTRONOMISCHE NACHRICHTEN, Issue 2 2010K. Ebisawa Abstract We present X-ray spectral analysis of the super-soft source CAL87 using ASCA, Chandra, XMM-Newton observations. Early ASCA CCD spectrum reported a strong oxygen absorption edge, which is considered to originate in the an optically thick white-dwarf atmosphere. On the other hand, contemporaneous grating observations by Chandra and XMM-Newton indicate emission line dominated spectra, which obviously indicate the optically thin origin. Fitting all the available CCD (ASCA and XMM-Newton) and grating spectra (XMM-Newton and Chandra) simultaneously, we show that the CAL87 X-ray energy spectrum is in fact composed of both an optically thick component with deep absorption edges and an optically thin component with numerous emission lines. The current result supports the standard SSS model that the primary source of X-ray emission is nuclear burning in the white dwarf atmosphere, surrounded by a highly photoionised, optically thin corona (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Hysteretic energy spectrum and damage controlEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 12 2001Rafael Riddell Abstract The inelastic response of single-degree-of-freedom (SDOF) systems subjected to earthquake motions is studied and a method to derive hysteretic energy dissipation spectra is proposed. The amount of energy dissipated through inelastic deformation combined with other response parameters allow the estimation of the required deformation capacity to avoid collapse for a given design earthquake. In the first part of the study, a detailed analysis of correlation between energy and ground motion intensity indices is carried out to identify the indices to be used as scaling parameters and base line of the energy dissipation spectrum. The response of elastoplastic, bilinear, and stiffness degrading systems with 5 per cent damping, subjected to a world-wide ensemble of 52 earthquake records is considered. The statistical analysis of the response data provides the factors for constructing the energy dissipation spectrum as well as the Newmark,Hall inelastic spectra. The combination of these spectra allows the estimation of the ultimate deformation capacity required to survive the design earthquake, capacity that can also be presented in spectral form as an example shows. Copyright © 2001 John Wiley & Sons, Ltd. [source] Wavelet energy spectrum for time-frequency localization of earthquake energyINTERNATIONAL JOURNAL OF IMAGING SYSTEMS AND TECHNOLOGY, Issue 2 2003Ziqin Zhou Abstract The authors recently developed a method for time-frequency signal analysis of earthquake records using Mexican hat wavelets. Ground motions in earthquakes are postulated as a sequence of simple penny-shaped ruptures at different locations along a fault line and occurring at different times. In this article, a wavelet energy spectrum is proposed for time-frequency localization of the earthquake input energy. The ground acceleration generated by a simple penny-shaped rupture is used as the basis to form the mother wavelet. The symmetric Mexican hat wavelet is chosen as the mother wavelet. The spectrum is presented pictorially in a two-dimensional, time-frequency domain. The proposed wavelet energy spectrum can be used to observe the evolution of the frequency contents of earthquake energy over time and distance of the site from the epicenter in a more accurate manner than the traditional time series (accelerogram) or frequency domain (Fourier amplitude spectrum) representation. It can be viewed as a microscope for looking into the time-frequency characteristics of earthquake acceleration records. The wavelet energy spectrum provides frequency evolution information to be used in the structural design process. © 2003 Wiley Periodicals, Inc. Int J Imaging Syst Technol 13, 133,140, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ima.10038 [source] Positive width function and energy indeterminacies in ammonia moleculeINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 8 2007Theodosios G. Douvropoulos A recently published methodology based on the semiclassical path integral theory was applied in a double well structure and gave the analytic form of the system's Green's function. This type of potential can describe the ammonia molecule as far as the motion of the nitrogen atom perpendicular to the hydrogen plane is discussed. Because of the fact that a double well describes a bound system and correspondingly stationary states (constructed by the symmetric and antisymmetric superposition of the eigenstates of the two unperturbed wells), it was expected that the energy spectrum would be real, in a form of doublets due to the splitting effect that takes place. However, the result was a pair of complex poles, which had a clearly positive imaginary part for each member. The present work explains the role of the imaginary parts of the complex poles as the decay rate of quantities defined as the energy indeterminacies, which are directly related to the fact that energy is not well determined in a classically forbidden region of motion. These quantities come as a function of (d,)/dE, which is the derivative of the classical action inside the potential barrier, with respect to energy. The major contribution comes from the turning points, and then the imaginary parts are responsible, not only for the conservation of energy, but for the correct sign of time as well. In this way, a different approach for the tunneling process is adopted, in which the entry or exit of the particle from the potential barrier takes place inside a neighborhood of the turning point, as though the latter was broadened and fluctuating. The magnitude of the previously mentioned decay rate is equal to ,/,, where , is the frequency of the classical oscillations inside one well. In contrast, the inversion frequency is generated by the part of the complex pole that is unrelated to (d,)/dE and is much smaller in magnitude than the classical frequency, since it is given as ,/, exp(,,). In this way, the period of the energy fluctuations is much smaller than the internal period of the system produced by the oscillating communication of the two classically allowed regions of motion. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Discrete and continuum quantum states for the Kratzer oscillatorINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2002Adelio R. MatamalaArticle first published online: 2 JUL 200 Abstract Kratzer oscillator is a realistic zero-order model for describing the anharmonic ro-vibrational motion in diatomic molecules. Kratzer oscillator has an energy spectrum containing both discrete and continuum parts. Wavefunctions belonging to the continuum would be useful in the study of transitions to the continuum in molecular dissociation processes. In this article, bound and scattering wavefunctions of the Kratzer oscillator are reviewed and the bound,bound and the bound,free matrix elements are obtained. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] Study on the long-term thermal-oxidative aging behavior of polyamide 6JOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2008Ying Shu Abstract The long-term thermal-oxidative aging behavior of polyamide 6 (PA6) was studied by comparison with the stabilized sample in this work. The variation of mechanical properties of the pure and the stabilized samples of PA6 with aging time at 110°C, 130°C, and 150°C were investigated, respectively. The aging mechanism of PA6 under heat and oxygen was studied in terms of the reduced viscosity, crystallization behavior, dynamic mechanical behavior, and chemical composition through the methods of polarized light microscopy (PLM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X-ray photoelectron energy spectrum (XPS), and so on. The results indicated that at the initial stage of aging, the molecular crosslinking reaction of PA6 dominated resulting in the increase of the mechanical strength, reduced viscosity, and the glass transition temperature of the sample. And the molecular degradation dominated in the subsequent aging process resulting in the decrease of the melting temperature, the increase of the crystallinity, and the formation of the oxides and peroxides products. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Hydrogenic impurity in two-dimensional semiconductors with anisotropic energy spectrum of carriersLASER PHYSICS LETTERS, Issue 12 2005S. P. Andreev Abstract The binding energy and wave function of a ground state of a shallow hydrogenic impurity in two-dimensional semiconductors with isotropic and anisotropic effective mass of carriers m* = {m,, m,, m,} are derived. The calculations are performed by the variational method based on a two-parametric trial wave function. The dependence of binding energy and deformation of an impurity on 2D layer thickness and effective mass anisotropy parameter m,/m, is investigated. The obtained results are in a good agreement with experimental data and in the limiting cases coincide with the theoretical calculations of shallow impurity binding energy for bulk semiconductors [1] and two-dimensional semiconductors with isotropic effective mass of electrons [2]. (© 2005 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Neutron capture effects on samarium, europium, and gadolinium in Apollo 15 deep drill-core samplesMETEORITICS & PLANETARY SCIENCE, Issue 3 2000Hiroshi HIDAKA Large isotopic deviations of 150Sm/149Sm, 156Gd/155Gd, and 158Gd/157Gd derived from neutron capture effects were observed in all samples. Although neutron capture products in lunar samples were investigated extensively in the 1970s, our precise isotopic measurements resulted in several new findings. The neutron fluence in the Apollo 15 drill core is a function of depth with a symmetric peak at 190 g/cm2 depth from the surface, confirming the results of earlier investigations. Neutron fluence values calculated from the isotopic shifts by comparison to artificially irradiated standard reagents were (5.16,7.49) × 1016 n/cm2. These values are 1.3 to 1.4x larger than those previously reported. Variations of ,Sm/,Gd with depth are interpreted as being due to variations in the neutron energy spectrum. Here ,Sm and ,Gd are defined as in previous studies of lunar neutron stratigraphy. Our data suggest that the neutron is more thermalized at the lower layers than it is at the upper layers. In addition to large isotopic shifts for 149Sm, 150Sm, 155Gd, 156Gd, 157Gd, and 158Gd, isotopic enrichments of 152Gd and 154Gd derived from neutron capture for 151Eu and 153Eu, respectively, were also observed in all samples. [source] Analysing the atolls: X-ray spectral transitions of accreting neutron starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007Jeanette Gladstone ABSTRACT We systematically analyse all the available X-ray spectra of disc accreting neutron stars (atolls and millisecond pulsars) from the RXTE data base. We show that while all these have similar spectral evolution as a function of mass accretion rate, there are also subtle differences. There are two different types of hard/soft transition, those where the spectrum softens at all energies, leading to a diagonal track on a colour,colour diagram, and those where only the higher energy spectrum softens, giving a vertical track. The luminosity at which the transition occurs is correlated with this spectral behaviour, with the vertical transition at L/LEdd, 0.02 while the diagonal one is at ,0.1. Superimposed on this is the well-known hysteresis effect, but we show that classic, large-scale hysteresis occurs only in the outbursting sources, indicating that its origin is in the dramatic rate of change of mass accretion rate during the disc instability. We show that the long-term mass accretion rate correlates with the transition behaviour, and speculate that this is due to the magnetic field being able to emerge from the neutron star surface for low average mass accretion rates. While this is not strong enough to collimate the flow except in the millisecond pulsars, its presence may affect the inner accretion flow by changing the properties of the jet. [source] Coherent phases and magnetoexcitons in graphenePHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2009Yu. E. Lozovik Abstract Unique band structure peculiarities of graphene imply that near Fermi level electrons are described by two-dimensional Dirac equation for massless particles. We investigate how these peculiarities manifest in electron,hole pairing and properties of indirect magnetoexcitons in two spatially separated, independently gated graphene layers. For electron,hole pairing, we derive asymptotical expressions for the gap in energy spectrum and discuss system behavior at various controlling parameters. We derive dispersion relations for magnetoexcitons and their effective-mass decompositions, and also discuss a possibility of magnetoexcitonic superfluidity. The systems under consideration can reveal coherent properties, dissipationless currents and Josephson phenomena at room temperature. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of electric field on the probability of optical transitions in InGaAs/GaAs quantum wells observed by photo- and electroreflectance methodsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2005A. N. Pikhtin Abstract The influence of an electric field on the energy spectrum and the probability of optical transitions in InGaAs/GaAs single quantum wells (QWs) of different widths has been investigated with photo- and electroreflectance techniques. The electric field in the area of a QW is varied in a wide range and controlled by well-defined Franz,Keldysh oscillations. A quadratic red shift of electroreflectance features concerned with interband excitonic transitions in QWs is observed. The electric field dependence of the intensity of these features and calculated data for the probability of optical transitions are compared. There are some field values when transitions that are symmetry-forbidden in zero field are much stronger than symmetry-allowed transitions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Magnetic-field-driven quantum criticality and thermodynamics in trimerized spin-1/2 isotropic XY chain with three-spin interactionsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2010L. J. Ding Abstract The quantum criticality and thermodynamics for the trimerized spin-1/2 isotropic XY chain with three-spin interactions in an external magnetic field are investigated by means of the Green's function theory combined with Jordan,Wigner transformation. The ground-state phase diagrams are explored, in which various phases are identified and described by typical M,h curves. Therein, two cusps emerge for strong three-spin interactions in two gapless phases at low and high fields, respectively. Moreover, the spin correlations and two-site entanglement entropy are calculated for a further understanding of quantum phase transition (QPT). It is also found that the magnetic-field-driven quantum criticality is closely related to the energy spectrum, in which an energy gap responsible for the appearance of 1/3 magnetization plateau can be opened up by three-spin interactions. The critical behavior disappears when the temperature becomes nonzero, yielding only a crossover behavior. In addition, the gapped low-lying excitations are responsible for the observed thermodynamic behaviors, wherein a structure with three peaks in the temperature dependence of specific heat is unveiled. [source] Density functional study of graphene overlayers on SiCPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2008Alexander Mattausch Abstract Despite the ongoing "graphene boom" of the last three years our understanding of epitaxial graphene grown on SiC substrate is only beginning to emerge. Along with experimental methods such as low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES), ab initio calculations help to uncover the geometric and electronic structure of the graphene/SiC interface. In this chapter we describe the density-functional calculations we performed for single and double graphene layers on Si- and C-terminated 6H-SiC surfaces. Experimental data reveal a pronounced difference between the two surface terminations. On a Si-terminated surface the interface adopts a 6,3 × 6,3 unit cell whereas the C-face supports misoriented (turbostratic) graphene layers. It has been recently realized that, on the Si-face, the large commensurate cell is subdivided into patches of coherently matching to the substrate carbon atoms. In our calculations we assumed the "coherent match" geometry for the whole interface plane. This reduces the periodic unit to the ,3 × ,3R 30° cell but requires a substantial stretching of the graphene sheet. Although simplified, the model provides a qualitative picture of the bonding and of the interface electron energy spectrum. We find that the covalent bonding between the carbon layer and the substrate destroys the massless "relativistic" electron energy spectrum, the hallmark of a freestanding graphene. Hence the first carbon layer cannot be responsible for the graphene-type electron spectrum observed by ARPES and rather plays a role of a buffer between the substrate and the subsequent carbon sheets. The "true" graphene spectrum appears with the second carbon layer which exhibits a weak van der Waals bonding to the underlying structure. For Si-terminated substrate, we find that the Fermi level is pinned by the interface state at 0.45 eV above the graphene Dirac point, in agreement with experimental data. This renders the interface metallic. On the contrary, for a C-face the "coherent match" model predicts the Fermi level exactly at the Dirac point. However, this does not necessarily apply to the turbostratic graphene layers that normally grow on the C-terminated substrate. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Lack of universal conductance features in disordered graphene nanoribbonsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010Antonino La Magna Abstract Recent experimental characterisation of graphene flakes has demonstrated the existence of local structural alterations of the ideal honeycomb lattice whose influence on the conductance mechanism of this material has not yet been fully evaluated. In this study a numerical statistical analysis of the conductance distribution function in disordered graphene nanoribbons is presented. Calculations are performed in statistically equivalent replica large systems within the Non Equilibrium Green's Function formalism. Different kinds of local scattering centers have been considered. A characteristic general behavior of the conductance variance in these quasi one-dimensional systems is the linear scaling with the average of logarithm of the conductance, in the localization regime. However, in a broad class of realization of the local disorder, the slope is not a constant as the disorder degree varies in any region of the energy spectrum, i.e. the single parameters scaling hypothesis is not verified (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of external magnetic field and confinement on spectrum rearrangement and exciton polaritons in optical microcavityPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2009Natalia Kaputkina Abstract Influence of external magnetic field and confinement on direct and spatially-indirect excitons is studied. Exciton-photon interaction and exciton polariton formation are discussed for single and coupled quantum wells or quantum dots embedded in optical microcavity. Possibility to control polariton resonance, polariton splitting and polariton dispersion by magnetic field is studied analitically and numerically. Magnetic field changes effective mass of magnetoexciton. Magneticfield increases effective steepness of confining potential in quantum dots also. This leads to the transformation of exciton energy spectrum. At low temperatures spontaneous coherence and Kosterlitz-Thouless transition to superfluid state of exciton polaritons in the system of coupled quantum wells embedded in microcavity or Bose-Einstein condensation of exciton polaritons in the system of coupled quantum dots embedded in optical microcavity can take place. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Theory of trapped polaritons in patterned microcavitiesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2006Pierre Lugan Abstract We consider the system of a quantum well embedded in a planar semiconductor microcavity with a shallow circular mesa patterned on top of the cavity spacer. For this system we develop the linear coupling theory of polaritons. We then compute polariton eigenstates and the corresponding optical spectrum. The theory predicts the existence of laterally confined polariton states with a discrete energy spectrum, as well as continuum states above the finite mesa potential barrier. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||||||||||||||