Continuum Energy Research Articles

Investigation of electronic bound states of a radially modulated nanowire: Level anti-crossing and bound states in continuum

In this paper the electronic confinement energy states of a radially modulated cylindrical nanowire are numerically calculated, assuming that the modulated section has finite length and its radius is greater than the remaining section of the nanowire. The Schrodinger equation is solved within the effective mass approximation using the finite element method to specify the necessary conditions for emersion of 3-dimensionally confined states and to investigate size and magnetic field dependence of the confinement energies. Our results show that for any local increment of nanowire radius, there is at least one bound state for each channel of angular momentum. The energies of bound states of higher angular momentum are shown to lie in the energy continuum of lower angular momentums. In addition, there is some level crossing and anti-crossing in the size dependence of confinement energies which can be explained on the basis of symmetry. It is observed that in the vicinity of the level anti-crossing, the oscillator strengths associated to intersubband transitions from ground state change dramatically and vanish for some specific width of modulation.

Discrete minimisers are close to continuum minimisers for the interaction energy

Under suitable technical conditions we show that minimisers of the discrete interaction energy for attractive-repulsive potentials converge to minimisers of the corresponding continuum energy as the number of particles goes to infinity. We prove that the discrete interaction energy $\Gamma$-converges in the narrow topology to the continuum interaction energy. As an important part of the proof we study support and regularity properties of discrete minimisers: we show that continuum minimisers belong to suitable Morrey spaces and we introduce the set of empirical Morrey measures as a natural discrete analogue containing all the discrete minimisers.

Antiproton-impact ionization of hydrogen atom with Yukawa interaction

The process of ionization of hydrogen atom by antiproton impact is studied when the interparticle interactions in the system are described by screened interactions of Yukawa type. The collision dynamics is described by the semiclassical atomic-orbital close-coupling method in which the bound atomic states and positive energy continuum pseudostates are determined by diagonalization of target Hamiltonian in a sufficiently large even-tempered basis to ensure convergence of the results at each value of the screening length λ of the interaction. With decreasing the screening length, the bound states in the Yukawa potential become unbound, thus increasing the number of continuum pseudostates. At low collision energies, this leads to the increase of the ionization cross section. It is observed that the energies of pseudostates, generated by the exit of nl bound states in the continuum, at certain critical values λ nl c exhibit series of avoided crossings when λ is varied. The avoided crossings appear between the (n + k) l and (n + k + 1) l (n = 1, 2, 3, … ; k = 0, 1, 2, …) states at screening lengths close to the critical screening length λ nl c . The avoided crossings become increasingly less pronounced with increasing n, k and l. The matrix elements for the (n + k) l - (n + k + 1) l transitions at the avoided crossings λ x,(n+k)l (n+k+1)l exhibit maxima and are reflected in the structure of the cross sections for population of the lower nl pseudostates. These structures are, however, smeared out in the total ionization cross section.

Quasi-stationary states and fermion pair creation from a vacuum in supercritical Coulomb field

Creation of charged fermion pair from a vacuum in so-called supercritical Coulomb potential is examined for the case when fermions can move only in the same (one) plane. In which case, quantum dynamics of charged massive or massless fermions can be described by the two-dimensional Dirac Hamiltonians with an usual [Formula: see text] Coulomb potential. These Hamiltonians are singular and require the additional definition in order for them to be treated as self-adjoint quantum-mechanical operators. We construct the self-adjoint two-dimensional Dirac Hamiltonians with a Coulomb potential and determine the quantum-mechanical states for such Hamiltonians in the corresponding Hilbert spaces of square-integrable functions. We determine the scattering amplitude in which the self-adjoint extension parameter is incorporated and then obtain equations implicitly defining possible discrete energy spectra of the self-adjoint Dirac Hamiltonians with a Coulomb potential. It is shown that this quantum system becomes unstable in the presence of a supercritical Coulomb potential which manifests in the appearance of quasi-stationary states in the lower (negative) energy continuum. The energy spectrum of those states is quasi-discrete, consists of broadened levels with widths related to the inverse lifetimes of the quasi-stationary states as well as the probability of creation of charged fermion pair by a supercritical Coulomb field. Explicit analytical expressions for the creation probabilities of charged (massive or massless) fermion pair are obtained in a supercritical Coulomb field.

A method to predict fatigue crack initiation in metals using dislocation dynamics

AbstractA theory is proposed to predict the initiation of fatigue cracks using cyclic dislocation dynamics (DD) simulations. The evolution of dislocation networks in a grain is simulated over several cycles. It is shown that the dislocation density and the energy stored in the dislocation networks increase with the number of cycles. The results of the DD simulations are used to construct an energy balance expression for crack initiation. A hypothetical crack is inserted into the grain, and the Gibbs energy consisting of the energy of the dislocation structure, the surface energy of the hypothetical crack, and the reduction in continuum energy is evaluated. Once the Gibbs energy attains a maximum, the dislocation structure becomes unstable, and it becomes energetically more favorable to form a real crack. The proposed method is applied to oxygen-free high conductivity copper, and the results are compared against experiments. Finally, it is shown how the method can be amended to account for environmental effects.

Monitoring observations of 6.7 GHz methanol masers

We report results of 6.7 GHz methanol maser monitoring of 139 star-forming sites with the Torun 32 m radio telescope from June 2009 to February 2013. The targets were observed at least once a month, with higher cadences of 2-4 measurements per week for circumpolar objects. Nearly 80 percent of the sources display variability greater than 10 per cent on a time-scale between a week and a few years but about three quarters of the sample have only 1-3 spectral features which vary significantly. Irregular intensity fluctuation is the dominant type of variability and only nine objects show evidence for cyclic variations with periods of 120 to 416 d. Synchronised and anti-correlated variations of maser features are detected in four sources with a disc-like morphology. Rapid and high amplitude bursts of individual features are seen on 3-5 occasions in five sources. Long (>50 d to 20 months) lasting bursts are observed mostly for individual or groups of features in 19 sources and only one source experienced a remarkable global flare. A few flaring features display a strong anti-correlation between intensity and line-width that is expected for unsaturated amplification. There is a weak anti-correlation between the maser feature luminosity and variability measure, i.e. maser features with low luminosity tend to be more variable than those with high luminosity. The analysis of the spectral energy distribution and continuum radio emission reveals that the variability of the maser features increases when the bolometric luminosity and Lyman flux of the exciting object decreases. Our results support the concept of a major role for infrared pumping photons in triggering outburst activity of maser emission.

Low-temperature biological activation of methane: structure, function and molecular interactions of soluble and particulate methane monooxygenases

Mechanistic aspects of oxidation of methane to methanol by methanotrophic bacteria via methane monooxygenase (MMO) is still not well understood. Elucidating how various molecules pertinent to methane oxidation interact with each other at the MMO active site offers critical insights on low-temperature activation of methane, which is one of the greatest technical challenges in hydrocarbon chemistry. In this review, most recent contributions to the area are analyzed comparing soluble (sMMO) and particulate (pMMO) forms. Initially, the taxonomical, morphological and physiological differences of different methanotrophs are discussed. Then, the structural and functional differences of sMMO and pMMO are analyzed while considering substrate/product-cofactor-active site interactions. A docking analysis was performed using Autodock Vina to uncover interactions between cofactors and corresponding enzymes. With natural gas becoming a significant contributor to the energy continuum, this literature analysis and molecular simulations conducted brings new insights to low-temperature activation of methane.

Analytical Property of Scattering Matrix:Spectroscopy Phenomena and Sharp Overlapping Autoionization Resonances

An extended atomic data base with sufficiently high precision is required in astrophysics studies and the energy researches. For example, there are “infinite” energy levels in discrete energy region as well as overlapping resonances in autoionization region. We show in this paper the merits of our relativistic eigenchannel R-matrix method R-R-Eigen based on the analytical continuation properties of scattering matrices for the calculations of the energy levels, overlapping resonances and the related transitions. Using Ne+ as an illustration example, the scattering matrices of Ne+ in both discrete and continuum energy regions are calculated by our R-R-Eigen method directly. Based on our proposed projected high dimensional quantum-defect graph (symmetrized), one can readily determine the accuracies of the calculated scattering matrices using the experimental energy levels in a systematical way. The calculated resonant photoionization cross sections in the autoionization region are in excellent agreement with the benchmark high resolution experiments. With the scattering matrices checked/calibrated against spectroscopy data in both discrete and continuum energy regions, the relevant dynamical processes should be calculated with adequate accuracies. It should then satisfy the needs of the astrophysical and energy researches.

Analytical treatment for the asymptotic analysis of microscopic impenetrability constraints for atomistic systems

In this paper we provide rigorous statements and proofs for the asymptotic analysis of discrete energies defined on a two-dimensional triangular lattice allowing for fracture in presence of a microscopic impenetrability constraint. As the lattice parameter goes to 0, we prove that any limit deformation with finite energy is piecewise rigid and we prove a general lower bound with a suitable Griffith-fracture energy density which reflects the anisotropies of the underlying triangular lattice. For such a continuum energy we also provide a class of (piecewise rigid) deformations satisfying “opening-crack” conditions on which the lower bound is sharp. Relying on these results, some consequences have been already presented in the companion paper [A. Braides et al. , J. Mech. Phys. Solids 96 (2016) 235–251] to validate models in Computational Mechanics in the small-deformation regime.

Asymptotic analysis of a ferromagnetic Ising system with “diffuse” interfacial energy

We give an example of a one-dimensional scalar Ising-type energy with long-range interactions not satisfying standard decay conditions and which admits a continuum approximation finite for all functions u in $$BV((0,L),[-1,1])$$ and taking into account the total variation of u. The optimal discrete arrangements show a periodic pattern of interfaces. In this sense, the continuum energy is generated by “diffuse” microscopic interfacial energy. We also show that related minimum problems show boundary and size effects in dependence of L.

(Invited) Macroscopic Models of Ion-Nafion Interations: Influence of Counter Ion Density on Network Morphology

We develop a continuum energy formulation for an ionomer membrane with a multicomponent counter-ion electrolye, such as occur in Vanadium redox flow batteries. At low hydration levels the high electrolyte molarity induces strong interactions between tethered and counter ions, as well as competition for solvation among ionic groups. Through-membrane variation in composition of the counter-ion density can lead to changes in the ionomer membrane network morphology, which in turn couple to the counter ion mobility. We consider a gradient flow model which couples electrolyte motion, solvation, ionomer network morphology and present both analytical and numerical simulations.

Bound states emerging from below the continuum in a solvablePT-symmetric discrete Schrödinger equation

At the lower edge of the energy continuum the birth of an isolated quantum bound state is studied as caused by an infinitesimally small change of the interaction. In our model a single, asymptotically free massive quantum particle is assumed moving along a discretized real line of coordinates, $x \in \mathbb{Z}$. The motion is assumed controlled by a weakly nonlocal 2J-parametric external potential which is non-Hermitian but PT-symmetric. Mathematically, the bound states are then reinterpreted as Sturmians, i.e., the bound-state energy is treated as a variable real parameter while the value of one of the couplings (responsible for the existence of the bound state) is determined via the standard secular equation. It is found that in such an arrangement the model is exactly solvable at all of the finite counts J of the couplings. For illustration, the explicit closed bound-state formulae are presented up to J=7.

Theoretical study of Charge Transfer simulation At Fe Metal with Ge and ZnO semiconductors Nano devices material

Focuses of ours paper is on the describing and investigation of the simulation of charge transition at Fe metal with Ge and ZnO semiconductors at electronic Nano devices system. Theoretical treatment of Fe / Ge and Fe/ZnOdevices system taken account to be continuum energy levels for two solid state material.Quantum theory had been used to describe the interface between two system Nano devices Fe / Ge or ZnOtechnology largely applied on the Metal/Semiconductor field system, whose many coefficient should be effected on the quantum transfer of charge due to the interface such that:transition energy Δmet/sem, work function σmet, electronic negatively of semiconductor σsem, coupling transition coefficient 〈|CT(0)¯|2〉, concentration of electron nin, volume of unit cell for semiconductor Vsem, penetration factor γ, and temperature T(K). The quantum charge transfer simulation could be able to selection the most devices that suitable used in nanotechnology applied and science studied. Flow charge current of charge transfer βCT has been estimated due to Δmet/sem using a MATLAP program. Theoretical data show that flow charge current is more probable with decreasing the Δmet/sem and increasing the 〈|CT(0)¯|2〉. Data of flow charge current results of transition are increases with decreasing potential at interface, this indicate, increasing potential forbidden more electrons to cross the tunneling at interface at the metal/semiconductor system and vice versa.

Stability of dislocation networks of low angle grain boundaries using a continuum energy formulation

Low angle grain boundaries can be modeled as arrays of line defects (dislocations) in crystalline materials. The classical continuum models for energetics and dynamics of curved grain boundaries are mainly based on those with equilibrium dislocation structures without the long-range elastic interaction, leading to a capillary force proportional to the local curvature of the grain boundary. The new continuum model recently derived by Zhu and Xiang (J. Mech. Phys. Solids, 69,175-194,2014) incorporates both the long-range dislocation interaction energy and the local dislocation line energy, and enables the study of low angle grain boundaries with non-equilibrium dislocation structures that involves the long-range elastic interaction. Using this new energy formulation, we show that the orthogonal network of two arrays of screw dislocations on a planar twist low angle grain boundary is always stable subject to both perturbations of the constituent dislocations within the grain boundary and the perturbations of the grain boundary itself.

Charge transfer dynamics between MPA capped CdTe quantum dots and methyl viologen

Abstract The understanding of charge carrier dynamics in hybrid materials based on colloidal semiconductor nanocrystals (or quantum dots) and organic moieties is fundamental for the design of efficient photonic and photovoltaic devices. In the present work, we investigate the interactions occurring between CdTe quantum dots, capped with a strong capping agent such as 3-mercaptopropionic acid, and a well known electron acceptor such as methylviologen molecule. The nature of the interactions and of exciton dynamics is investigated by stationary and time-resolved spectroscopies. Luminescence data recorded in presence of increasing methylviologen concentrations, indicate that the organic molecule is able to statically interact with the surface sites of CdTe quantum dots; a biphasic interaction behavior is evidenced by determining the apparent association constants. These latter are obtained through the analysis of luminescence data, and values in the range 103–104 are determined. The nature of the interactions is characterized by nanosecond and femtosecond transient absorption spectroscopies, to clarify the dynamics and the conditions able to foster charge mobility. Nanosecond flash photolysis measurements, carried out upon quantum dots excitation, shows the absorption of methylviologen radical cation specie at 605 nm, suggesting the occurrence of electron transfer from CdTe nanocrystals to the organic acceptor; the relatively long decay time of the transient signal (10.3 μs) indicates that back electron transfer processes are negligible. Ultrafast transient absorption measurements confirm the occurrence of an ultrafast electron transfer process; spectral and kinetic analysis of the transient data show that methylviologen radical cation is formed almost instantaneously on the ps-time scale but mainly when the samples are pumped in the energy continuum at 400 nm. This finding suggests that electron mobility from the nanocrystals to the organic units is achieved mainly when the excitonic states possess an excess of energy. The comparison of the kinetic behaviour of the signals at increasing methylviologen concentrations indicates that the electron transfer process competes with the radiative exciton recombination. In addition, the kinetic data suggests that surface trapping and Auger recombination processes might slow down the charge mobility.

CXO J004318.8+412016, a steady supersoft X-ray source in M 31

We obtained an optical spectrum of a star we identify as the optical counterpart of the M31 Chandra source CXO J004318.8+412016, because of prominent emission lines of the Balmer series, of neutral helium, and a He II line at 4686 Angstrom. The continuum energy distribution and the spectral characteristics demonstrate the presence of a red giant of K or earlier spectral type, so we concluded that the binary is likely to be a symbiotic system. CXO J004318.8+412016 has been observed in X-rays as a luminous supersoft source (SSS) since 1979, with effective temperature exceeding 40 eV and variable X-ray luminosity, oscillating between a few times 10(35) erg/s and a few times 10(37) erg/s. The optical, infrared and ultraviolet colors of the optical object are consistent with an an accretion disk around a compact object companion, which may either be a white dwarf, or a black hole, depending on the system parameters. If the origin of the luminous supersoft X-rays is the atmosphere of a white dwarf that is burning hydrogen in shell, it is as hot and luminous as post-thermonuclear flash novae, yet no major optical outburst has ever been observed, suggesting that the white dwarf is very massive (m>1.2 M(sol)) and it is accreting and burning at the high rate (mdot>10(-8)M(sol)/year) expected for type Ia supernovae progenitors. In this case, the X-ray variability may be due to a very short recurrence time of only mildly degenerate thermonuclear flashes.

The effect of oxide layer thickness on the quantification of 1.5 MeV γ–radiation induced interface traps in the Ag/SiO2/Si MOS devices

This work presents the effect of varied thickness of oxide layer and radiation dose on electrical characteristics of Ag/SiO2/Si MOS devices irradiated by 1.5MeV γ–radiations of varied doses. SiO2 layers of 50, 100, 150 and 200nm thickness were grown on Si substrates using dry oxidation and exposed to radiation doses of 1, 10 and 100kGy. The exposure to radiation resulted in generation of fixed charge centers and interface traps in the SiO2 and at the Si/SiO2 interface. Capacitance-conductance-voltage (C-G-V) and capacitance-conductance-frequency (C-G-f) measurements were performed at room temperature for all MOS devices to quantify the active traps and their lifetimes. It is shown that accumulation and minimum capacitances decreased as the thickness of SiO2 layer increased. For the unexposed MOS devices, the flat band voltage VFB decreased at a rate of −0.12V/nm, density of active traps increased by 4.5 times and depletion capacitance CDP, increased by 2.5 times with the increase of oxide layer thickness from 50 to 200nm. The density of active traps showed strong dependence on the frequency of the applied signal and the thickness of the oxide layer. The MOS device with 200nm thick oxide layer irradiated with 100kGy showed density of active interface traps was high at 50kHz and was 3.6×1010eV−1cm−2. The relaxation time of the interface traps also increased with the exposure of γ–radiation and reached to 9.8µs at 32kHz in 200nm thick oxide MOS device exposed with a dose of 100kGy. It was inferred that this was due to formation of continuum energy states within the band gap and activation of these defects depended on the thickness of oxide layer, applied reverse bias and the working frequency. The present study highlighted the role of thickness of oxide layer in radiation hard environments and that only at high frequency, radiation induced traps remain passivated due to long relaxation times.

Euler elastica as a Γ-limit of discrete bending energies of one-dimensional chains of atoms

In the 1920s, Hencky proposed a discrete elastica model describing a chain of identical rigid bars connected by torsional springs. Hencky observed that this discrete elastica model converges to Euler’s elastica as the number of bars increases while their lengths decrease, and Hencky’s bar-chain model has been used primarily as an approximation of Euler’s elastica. A Hencky-type bar-chain model can also be incorporated into a Frenkel–Kontorova-type discrete atomistic model, where the joints and bars represent the atoms and interatomic bonds, respectively, while the entire chain of atoms interacts with either a substrate or other chains. The energy of a continuum system corresponding to this Frenkel–Kontorova-type model can then be recovered by taking an appropriate discrete-to-continuum limit. Developing a correct limiting procedure for the discrete elastica establishes the bending component of this continuum energy. In this paper we use Γ-convergence to rigorously show that as the bar length in the discrete elastica model we consider goes to 0, the bending energies of the chain Γ-converge to the continuum bending energy associated with Euler’s elastica.

The SAGE-Spec Spitzer Legacy program: the life-cycle of dust and gas in the Large Magellanic Cloud. Point source classification – III

The Infrared Spectrograph (IRS) on the {\em Spitzer Space Telescope} observed nearly 800 point sources in the Large Magellanic Cloud (LMC), taking over 1,000 spectra. 197 of these targets were observed as part of the Sage-Spec Spitzer Legacy program; the remainder are from a variety of different calibration, guaranteed time and open time projects. We classify these point sources into types according to their infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership, and variability information, using a decision-tree classification method. We then refine the classification using supplementary information from the astrophysical literature. We find that our IRS sample is comprised substantially of YSO and H\,{\sc ii} regions, post-Main Sequence low-mass stars: (post-)AGB stars and planetary nebulae and massive stars including several rare evolutionary types. Two supernova remnants, a nova and several background galaxies were also observed. We use these classifications to improve our understanding of the stellar populations in the Large Magellanic Cloud, study the composition and characteristics of dust species in a variety of LMC objects, and to verify the photometric classification methods used by mid-IR surveys. We discover that some widely-used catalogues of objects contain considerable contamination and others are missing sources in our sample.

The energy and identification continua of burnout and work engagement: Developmental profiles over eight years

Understanding of the mutual developmental dynamics between burnout and work engagement is limited due to the lack of longitudinal studies with long follow-ups and multi-wave data. This study sought to identify subgroups of employees characterized by long-term exhaustion-vigor (energy continuum) and cynicism-dedication (identification continuum). A further important aim was to investigate differences between the identified subgroups in their experiences of progress in their personal work goals. Five-wave, eight-year follow-up data among Finnish white-collar professionals (n=168) were studied using Latent Profile Analysis (LPA). The analysis yielded three exhaustion-vigor subgroups: 1) “Low stable exhaustion – high stable vigor” (n=141), 2) “Fluctuating exhaustion and vigor” (n=19), and 3) “Stable average exhaustion – decreasing vigor” (n=8). Three subgroups were also found for cynicism-dedication: 1) “Low stable cynicism – high stable dedication” (n=124), 2) “Increasing cynicism – decreasing dedication” (n=27), and 3) “Decreasing cynicism – increasing dedication” (n=17). Exhaustion and vigor were found to be stable and mutually exclusive experiences for the great majority of the participants. However, mean changes were also detected – especially in vigor – but these were rare. A notable finding was that the levels of and changes in cynicism and dedication showed opposite trends in each subgroup: among the majority of the participants (74%), the levels of cynicism and dedication were stable and inversely related, while among one-third their levels simultaneously changed in the reverse direction. The most successful progress in personal work goals was found in the groups described by the identification continuum, i.e., in the groups of “Low stable cynicism – high stable dedication” and “Decreasing cynicism – increasing dedication”.