Sharp Energy Research Articles

Structure inthe cosmic ray spectrum: an update

An update is given of the evidence from extensive airshowers relating toour claim that there is `structure' in the energy spectrum ofhigh-energy cosmic rays.The structure is explained, in our model, in terms of there havingbeen arecent nearby supernova, the remnant of which has accelerated cosmicraynuclei, with rather sharp energy cut-offs at ~ 3×106 GeV for oxygennuclei and ~ 1×107 GeV for iron nuclei.The addition of the later data on the shower size spectra whichamountsto a 250% increase and includes that of Cherenkov radiation, hadronsandmuons, is considered by us to enhance the probability of our modelbeingcorrect, although the case is still not `watertight'.

Poincaré section decomposition for quantum scatterers

We consider here scatterers that may be modeled as a cavity with an entrance lead. The decomposition is achieved by sectioning the cavity and adding new leads, thus generating two new scatterers. So a resonant scatterer, whose S -matrix has sharp energy peaks, can be resolved into a pair of scatterers with smooth energy dependence. The resonant behaviour is concentrated in a spectral determinant obtained from a dissipative section map. The semiclassical limit of this theory coincides with the orbit resummation previously proposed by Georgeot and Prange. A numerical example for a semiseparable scatterer is investigated, revealing the accurate portrayal of the Wigner time delay by the spectral determinant.

Synchrotron Radiation as the Source of Gamma‐Ray Burst Spectra

We investigate synchrotron emission models as the source of gamma-ray burst spectra. We show that including the possibility for synchrotron self-absorption, a ``smooth cutoff'' to the electron energy distribution, and an anisotropic distribution for the electron pitch angles produces a whole range of low energy spectral behavior. In addition, we show that the procedure of spectral fitting to GRB data over a finite bandwidth can introduce a spurious correlation between spectral parameters - in particular, the value of the peak of the nu F_nu spectrum, E_p, and the low energy photon spectral index alpha (the lower E_p is, the lower (softer) the fitted value of alpha will be). From this correlation and knowledge of the E_p distribution, we show how to derive the expected distribution of alpha. We show that optically thin synchrotron models with an isotropic electron pitch angle distribution can explain the distribution of alpha below alpha=-2/3. This agreement is achieved if we relax the unrealistic assumption of the presence of a sharp low energy cutoff in the spectrum of accelerated electrons, and allow for a more gradual break. We show that this low energy portion of the electron spectrum can be at most flat. We also show that optically thin synchrotron models with an anisotropic electron pitch angle distribution can explain all bursts with -2/3 < alpha <= 0$. The very few bursts with low energy spectral indices that fall above alpha=0 may be due the presence of a the synchrotron self-absorption frequency entering the lower end of the BATSE window. Our results also predict a particular relationship between alpha and E_p during the temporal evolution of a GRB. We give examples of spectral evolution in GRBs and discuss how the behavior are consistent with the above models.

Electron resonances and Savitzky–Golay methods: How can the tunneling current be properly calculated?

We deal with the calculation of the current vs. applied bias, I=I(V) , through a double-barrier resonant tunneling heterostructure in the approximation of one-electron states. Because this calculation implies to numerically integrate the quantum-mechanical electron-transmission probability function with its maxima associated to resonances, we face the general problem of computing the integral of a continuous and derivable physical function which presents sharp local maxima at a few locations within the integration-variable range. Adequate energy integration of the maxima of the transmission probability function, several orders of magnitude larger than the local average of the function itself, is absolutely necessary because they represent the principal contribution to the current. Our approach to the problem consists, firstly, in determining – to the desired precision – the energy interval where each maximum occurs. Savitzky–Golay derivation techniques with decreasing energy interval size are well suited for this purpose. In a second stage before energy integration, the transmission is evaluated at energy-intervals of increasing size around the centered maximum until the original energy-interval size is regained. To obtain the tunneling current through the heterostructure for each bias, energy integration is undertaken using the vector of variable size intervals previously determined when analyzing the shape of the transmission function at that bias. Following this procedure, the contribution of sharp energy resonances to the I(V) curve is accurately included. The here described method of integration of functions with very few but sharp extrema is universal and could be applied to other physical problems.

Wavepacket dynamics and predissociation of the [formula omitted] state of Rb 2

We present the first experimental investigation of the real-time dynamics of predissociation in a diatomic molecule involving more than two electronic states – the predissociation of the D 1Π u state of Rb 2. Our results show that the state is strongly predissociated, τ≈5 ps, above a sharp energy threshold. We propose that mainly the (1) 3 Δ u state causes the fast predissociation and that the fine-structure components of the products are mixed by coupling among molecular states at large internuclear distances. Furthermore, an outward–inward asymmetry of the wavepacket signal is attributed to autoionisation of the wavepacket evolving in the probe (Rydberg) state.

Reconsidering the inverted‐V particle signature: Relative frequency of large‐scale electron acceleration events

The shape of auroral zone large‐scale electron acceleration events is widely described as an inverted V, with the spectral peak gradually rising to and falling from the spatially central peak. We investigated the relative frequency of all sharp (≥3) electron energy flux enhancement events which are large‐scale enough to be resolvable in the DMSP F7 particle data set. The first three days of each month of 1984 were studied, amounting to 355 passes through the nightside oval (restricted to the range 2000–2300 magnetic local time (MLT)) with a total of 1049 such events. Only 42 of these 1049 events actually fit the inverted‐V morphology, with another 76 cases fitting an extremely loose definition of an inverted V. Thus, between 4.0 and 11% of all sharp energy flux enhancements are inverted Vs, depending on the strictness of the criteria. In that subset of events for which electrostatic acceleration is likely, namely those which have monoenergetic peaks and which extend over 20 km or more, inverted Vs are still just 10–29% of the total. More common are events which have relatively constant “monoenergetic peaks” (accelerating potentials). A crucial feature common to all non‐V forms with monoenergetic peaks is that the maximum accelerating potential (spectral peak) is observed near one edge or both. Thus very large gradients in the potential exist near the edges of large‐scale auroral forms, implying much larger electric fields than does an inverted V. The present study thus supports earlier evidence that the important physics of auroral arcs is associated with the edge of the arcs, rather than the central portion of the acceleration region. The most common type of energy flux enhancement event is a broad spectral enhancement (suprathermal burst). Contrary to popular perception, suprathermal bursts are not always narrowly confined in latitude. All categories of sharp electron energy flux enhancement events studied here exhibited a sharp high‐energy drop off near the spectral peak, suggesting acceleration of one type or another (either wave‐particle or electrostatic).

Low-energy electron beam focusing in self-organized porous alumina vacuum windows

Micromachined, micron-thick porous alumina membranes with closed pore endings show high electron transparency above an energy of 5 keV. This is due to the channeling of electrons along the negatively charged insulating pores after surmounting the thin entrance layer. We also find a sharp hightransparency energy window at energies as low as 2 keV which may be the result of a local maximum of channeling, as predicted by simulations, and positive charge up of the entrance layer causing electron electrostatic focusing. Applications for these membranes range from atmospheric electron spectroscopy to self-assembled, nanoscale, large-area electron collimators.

Surface conditions for the observation of metal–insulator transitions on Cr-doped V 2O 3

Chromium-doped vanadium sesquioxide, (V 1− x Cr x ) 2O 3, displays two metal–insulator transitions with temperature for 0.005< x<0.0179. The high-temperature (∼300 K) transition occurs in a temperature range that is of interest for catalysis; the physics of the transition is also of fundamental interest. However, so far that transition has been observed only on cleaved samples. Due to the difficulty in obtaining doped single crystals for cleaving, alternate preparation methods are required. (V 0.985Cr 0.015) 2O 3 (0001) was prepared both by scraping with a diamond file and by ion bombardment followed by annealing in oxygen; the resulting surfaces were examined by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) for evidence of the high-temperature metal–insulator transition. Although resistance measurements across the transition temperature showed that a transition had taken place in the bulk, no changes were observed in photoemission spectra of scraped samples; they appeared insulating both above and below the transition. Annealed samples, on the other hand, displayed a clear increase in the density of states at E F, as well as changes in core-level XPS consistent with a metallic surface, as the temperature was lowered through the transition. Sharp (1×1) low energy electron diffraction (LEED) patterns were obtained from annealed surfaces, and Auger spectra showed no evidence of chromium segregation following annealing.

A black hole spectral signature

An accreting black hole is, by definition, characterized by the drain. Namely, the matter falls into a black hole much the same way as water disappears down a drain — matter goes in and nothing comes out. As this can only happen in a black hole, it provides a way to see “a black hole”, an unique observational signature. The accretion proceeds almost in a free-fall manner close to the black hole horizon, where the strong gravitational field dominates the pressure forces. In this paper we present analytical calculations and Monte-Carlo simulations of the specific features of X-ray spectra formed as a result of upscattering of the soft (disk) photons in the converging inflow (CI) into the black hole. The full relativistic treatment has been implemented to reproduce these spectra. We show that spectra in the soft state of black hole systems (BHS) can be described as the sum of a thermal (disk) component and the convolution of some fraction of this component with the CI upscattering spread (Greens) function. The latter boosted photon component is seen as an extended power-law at energies much higher than the characteristic energy of the soft photons. We demonstrate the stability of the power spectral index over a wide range of the plasma temperature 0 – 10 keV and mass accretion rates (higher than 2 in Eddington units). We also demonstrate that the sharp high energy cutoff occurs at energies of 200–400 keV which are related to the average energy of electrons m e c 2 impinging upon the event horizon. The spectrum is practically identical to the standard thermal Comptonization spectrum when the CI plasma temperature is getting of order of 50 keV (the typical ones for the hard state of BHS). In this case one can see the effect of the bulk motion only at high energies where there is an excess in the CI spectrum with respect to the pure thermal one. Furthermore we demonstrate that the change of spectral shapes from the soft X-ray state to the hard X-ray state is clearly to be related with the temperature of the bulk flow. In other words the effect of the bulk Comptonization compared to the thermal one is getting stronger when the plasma temperature drops below 10 keV. We clearly demonstrate that these spectra emerging from the converging inflow are a inevitable stamp of the BHS where the strong gravitational field dominates the pressure forces.

The atomic displacements on surface generated by low-energy projectile

A simplified analytical model for ion–solid interactions, appropriate for low-energy beam depositions, is presented to determine the analytic solution of the deposited energy in the solid. The deposited energy in the surface and the energy deposited in the underlying bulk, correlative to the atomic displacement leaving the lattice damage, are dependent on the mass and incident energy of projectiles as well as the sharp displacement threshold energy. The determination of deposited energy of ion in subsurface yields a ceiling for the beam energy above which more defects are generated in the bulk resulting in defective films. The accuracy of the angular average over the scattering cross section is evaluated from comparison with experimental results. The analytic solution is applied to Ar + and N + projectiles in diamond, graphite and SiC materials, respectively.

Atomic Scale Sliding and Rolling of Carbon Nanotubes

A carbon nanotube is an ideal object for understanding the atomic scale aspects of interface interaction and friction. Using molecular statics and dynamics methods different types of motion of nanotubes on a graphite surface are investigated. We found that each nanotube has unique equilibrium orientations with sharp potential energy minima. This leads to atomic scale locking of the nanotube. The effective contact area and the total interaction energy scale with the square root of the radius. Sliding and rolling of nanotubes have different characters. The potential energy barriers for sliding nanotubes are higher than that for perfect rolling. When the nanotube is pushed, we observe a combination of atomic scale spinning and sliding motion. The result is rolling with the friction force comparable to sliding.

Decomposition of Resonant Scatterers by Surfaces of Section

Scattering on the energy shell is viewed here as the relation between the bound states of the Hamiltonian, restricted to sections on leads that are asymptotically independent, far away from the interaction region. The decomposition is achieved by sectioning this region and adding new leads, thus generating two new scatterers, so a resonant scatterer, whose S matrix has sharp energy peaks, can be resolved into a pair of scatterers with smooth energy dependence. The resonant behaviour is concentrated in a spectral determinant obtained from a dissipative section map. The semiclassical limit of this theory coincides with the orbit resummation previously proposed by Georgeot and Pranges (Phys. Rev. Lett.74, 4110 (1995)). A numerical example for a semiseparable scatterer is investigated, revealing the accurate portrayal of the Wigner time delay by the spectral determinant.

Low-Energy Electron Damage to Condensed-Phase Deoxyribose Analogues Investigated by Electron Stimulated Desorption of H- and Electron Energy Loss Spectroscopy

We report the 5−20 eV electron-stimulated desorption (ESD) yields of H- produced by dissociative electron attachment (DEA) to the DNA backbone sugar-like analogues tetrahydrofuran (I), 3-hydroxytetrahydrofuran (II), and α-tetrahydrofurfuryl alcohol (III) physisorbed on a polycrystalline Pt substrate. For the pure disordered solid films, we observe one peak in the H- yield function at an incident electron energy, Ei, of ∼10 eV, which is attributed to selective dissociation of endocyclic α-CH bonds via the formation of a core-excited shape resonance; no desorbing polyatomic fragments were detected. A second peak is also observed in the H- ESD yield function of II; it appears as a weak shoulder superimposed on the low-energy side of the 10 eV structure displaying a sharp vertical onset near 6.7 eV and a peak maximum around 7.3 eV. The sharp onset and narrow energy width are characteristic of a core-excited Feshbach resonance; it is attributed to H- produced via DEA to the OH substituent whose corresponding parent state may be similar to that observed in CH3OH, where the 7.3 eV resonance originating from the hydroxyl group was assigned to a 2A‘ ‘ Feshbach resonance. High-resolution electron energy loss (HREEL) spectra recorded with 11 and 14 eV incident electrons are also reported for solid I. They show that, with increasing electron dose, degradation of the solid leads to formation of a C−O π-bonded product believed to arise from fragmentation of an α-cleaved transient intermediate following direct electronic excitation of the parent molecule. The HREEL spectra suggest a rather complex fragmentation pathway following low-energy electron bombardment. In vacuo kinetic energy (Ek) distribution measurements of desorbed H- in the 10 eV resonance suggest that for I, II, and III a dissociation mechanism similar to that proposed for the 14 eV HREELS spectra occurs, whereby α-cleavage of the C−O bond occurs within the lifetime of the dissociative negative ion state.

Energy distribution of hydrogen sites for MgNi0.86M10.03 (M1=Cr, Fe, Co, Mn) alloys desorbing hydrogen at low temperature

The energy distribution of hydrogen sites for MgNi0.86M10.03 (M1=Cr, Fe, Co, Mn) alloys prepared by the ball milling technique was investigated. From the results of DSC, TG analysis and the calculation of the enthalpy of hydrogenation, we derived that the Cr and Fe-substituted alloys have a broad energy distribution and the averaged enthalpy of hydrogenation is about −82 kJ mol−1 H2−1. Because of the broad distribution, the hydride of larger enthalpy decomposes at low temperature. However, the Co and Mn-substituted alloys have sharp energy distributions though the averaged value of hydrogenation is about −67 kJ mol−1 H2−1. Therefore, these alloys do not desorb hydrogen at low temperature. In order to develop Mg-based alloys that desorb hydrogen at low temperature, the dependence of substitution element on the energy distribution should be considered and substitution elements that make the energy distribution broad must be selected.

Optimum spectroscopic performance from CZT γ- and X-ray detectors with pad and strip segmentation

We describe a method for obtaining optimum spectroscopic performance from CZT γ- and X-ray detectors with segmented readout. This method allows to circumvent the effects of incomplete charge collection and to obtain energy resolution approaching the limitation due to detector and electronic noise. It is insensitive to hole trapping over most of the detector volume, while a modest amount of electron trapping can be tolerated. The method requires segmentation of the positive electrode. Signals from the common electrode, taken in coincidence with those of the segmented positive electrode, provide a measure of the depth of interaction. A distinct correlation profile exists between the signals of the segmented element and the common electrode. These profiles can be linearized to yield very sharp energy lines with high photopeak efficiency. Additional cuts can be made to improve peak-to-valley ratio, with small loss in efficiency. We present measurements taken with CZT detectors with pad segmentation. Illuminating with a 133Ba source, we obtain for the 356 keV line a FWHM of 1.0–1.5%, with a peak-to-valley ratio of ∼100/1.

Energy deposition of electrons in low-, medium- and high-Z material: Comparison of the Monte Carlo transport code EGS4 with experiment

The Monte Carlo electron transport code EGS4 was benchmark tested against early experimental results derived by Freyberger. These consist of absolute depth ionization and depth dose curves measured at a pencil beam with sharp energy definition of nominally 4, 10 and 20 MeV electrons extracted from a Betatron. The Freyberger precision measurements have been made with a wide plane-parallel ionization chamber in a slab phantom for the materials PMMA, C, Al, Cu, and Pb. The bremsstrahlung and, in some experiments, the forward and backward directed contributions had been determined separately. The pencil-beam/wide-chamber geometry is equivalent, in respect to the measurement of depth ionization and depth dose curves, to the more common wide-beam/point-detector geometry. However, it requires the simulation of merely one pencil beam position and practically all particle histories contribute to the ionization in the wide air cavity. Thus a considerable amount of computing time is saved. We applied a prototype of the new electron transport code PRESTA II. The results of our simulation generally agree very well, even in absolute terms, with experiment. Small deviations are found at lower energies and high-Z materials. For low energies they may in part be explained by contamination with bremsstrahlung from the beam guide system and an overestimation of bremsstrahlung-production in the experiment. The simulation of a gas-filled chamber within a high-Z absorber block seems to produce small errors in the calculation of ionization for electrons of approximately 4 MeV. Larger deviations for high-Z materials are attributed to the employment of screened Rutherford cross sections which lead to an underprediction of ionization from backscattered particles. Backscatter was found to be sufficiently accurate in the simulation of the PMMA absorber.

Collective modes in high-temperature superconductors

The role of collective modes in various experiments on the cuprates is investigated. We calculate the neutron scattering, photoemission (ARPES), and Raman scattering intensities below $T_c$ within the fluctuation-exchange (FLEX) approximation for the two-dimensional Hubbard model. It is shown that the large peak in the dynamical spin susceptibility arises from a weakly damped spin-density-wave collective mode. This gives rise to a dip between the sharp low energy peak and the higher binding energy hump in the ARPES spectrum. Furthermore, we show that the collective mode of the amplitude fluctuation of the d-wave gap yields a broad peak above the pair-breaking threshold in the $B_{1g}$ Raman spectrum.

An Atomic-Scale Simulation of Lubricated Motion

Alkyl chain monolayers and bilayers are investigated by static atomistic simulation. Optimisation of the chain conformations by imposing sets of random torsions leads mainly to all-trans structures. The relaxations of Y-configuration bilayers are investigated after imposing mutual displacements to the interface. Energy profiles for sliding motions of the bilayer show periodic behaviour from which the friction associated with the motion is related to the maximum force in each cycle of the profile. This is consistent with the observation that the lubricating action of an alkyl chain increases with the length of the chain. Derived shear strength values are reasonably close to experimental measurement. For small displacement intervals the profiles showed sharp energy spikes that are analogous to the ‘stick-slip’ discontinuities found in low-speed surface force studies of molecular layers. These are discussed in terms of low-lying metastable states from which, on further displacement, the system relaxes to the ground state.

Collective modes and the superconducting-state spectral function ofBi2Sr2CaCu2O8

Photoemission spectra of the high temperature superconductor Bi2212 near (pi,0) show a dramatic change when cooling below Tc: the broad peak in the normal state turns into a sharp low energy peak followed by a higher binding energy hump. Recent experiments find that this low energy peak persists over a significant range in momentum space. We show in this paper that these data are well described by a simple model of electrons interacting with a collective mode which appears only below Tc.

Distinction between bulk and interface states in CuInSe2/CdS/ZnO by space charge spectroscopy

We present a detailed study of admittance spectroscopy and deep level transient spectroscopy on CuInSe2/CdS/ZnO thin film solar cells. The admittance spectra reveal an emission from a distribution of hole traps centered at an activation energy of 280 meV and a shallower level with a sharp activation energy of ∼ 120 meV. After repetitive annealing of the device in air at 200 °C, the activation energy of the latter level increases continuously from 120 to 240 meV, while the 280 meV hole traps remain unaffected. Deep level transient spectroscopy with optical excitation reveals an emission of minority carriers with time constants comparable to those observed for the shallow level in admittance spectroscopy. The shift of the activation energy after annealing also occurs in deep level transient spectroscopy and ascertains that the emissions observed in both techniques have the same origin. The magnitude and continuous shift of the activation energy of the minority carrier emission indicates a distribution of levels in the vicinity of the CdS/CuInSe2 heterointerface. In the case of interface states, the activation energy deduced from admittance spectroscopy corresponds to the position of the electron quasi-Fermi level at the interface, pointing to an inversion of the carrier type at the absorber surface. Measurements with an applied dc bias indicate that the electron Fermi level is pinned at the interface.