Positive Binding Energy Research Articles

Dirac Born Infeld (DBI) cosmic strings

Motivated by brane physics, we consider the non-linear Dirac-Born-Infeld (DBI) extension of the Abelian-Higgs model and study the corresponding cosmic string configurations. The model is defined by a potential term, assumed to be of the mexican hat form, and a DBI action for the kinetic terms. We show that it is a continuous deformation of the Abelian-Higgs model, with a single deformation DBI parameter depending on a dimensionless combination of the scalar coupling constant, the vacuum expectation value of the scalar field at infinity, and the brane tension. By means of numerical calculations, we investigate the profiles of the corresponding DBI-cosmic strings and prove that they have a core which is narrower than that of Abelian-Higgs strings. We also show that the corresponding action is smaller than in the standard case suggesting that their formation could be favoured in brane models. Moreover we show that the DBI-cosmic string solutions are non-pathological everywhere in parameter space. Finally, in the limit in which the DBI model reduces to the Bogomolnyi-Prasad-Sommerfield (BPS) Abelian-Higgs model, we find that DBI cosmic strings are no longer BPS: rather they have positive binding energy. We thus argue that, when they meet, two DBI strings will not bind with the corresponding formation of a junction, and hence that a network of DBI strings is likely to behave as a network of standard cosmic strings. On the other hand, we also find that, if the BPS condition is no longer satisfied and the coupling constant is less than twice the charge squared of the scalar field, DBI strings can change their behaviour from type I to type II depending on the DBI parameter.

ENDOHEDRAL FULLERENES C60 AND C82 WITH SILVER

The models of endofullerenes C 60 and C 82 with silver atom or diatomic silver are calculated with ab initio methods including the full geometry optimization. Ag @ C 60 is the bound system with positive binding energy while Ag 2@ C 60 is not bound because of strong geometrical strain. In Ag @ C 60 silver atom has the off-center position for the lowest energy model, and the whole structure reduces the symmetry from I h down to C 1. The endo-structures with the C 82 cage can exist with both mono- and diatomic silver interiors. Electronic charge is transferred from the carbon cage to silver.

Observing the Creation of Electronic Feshbach Resonances in Soft X-ray–Induced O 2 Dissociation

When an atom or molecule is ionized by an x-ray, highly excited states can be created that then decay, or autoionize, by ejecting a second electron from the ion. We found that autoionization after soft x-ray photoionization of molecular oxygen follows a complex multistep process. By interrupting the autoionization process with a short laser pulse, we showed that autoionization cannot occur until the internuclear separation of the fragments is greater than approximately 30 angstroms. As the ion and excited neutral atom separated, we directly observed the transformation of electronically bound states of the molecular ion into Feshbach resonances of the neutral oxygen atom that are characterized by both positive and negative binding energies. States with negative binding energies have not previously been predicted or observed in neutral atoms.

Structures and stabilities of endohedral fullerenes: X@C 24H 24 (X = H, Li, Li +, Na, Na +, K, K +, Be, Mg, Mg 2+, Ca, Ca 2+, B, Al, C, Si, N and P)

Based on the D 6 d -symmetrical C 24H 24 fullerene, the equilibrium geometries, electronic structures, vibrational frequencies, energy gaps and BSSE-corrected binding energies of the endohedral fullerenes X@C 24H 24 (X = H, Li, Li +, Na, Na +, K, K +, Be, Mg, Mg 2+, Ca, Ca 2+, B, Al, C, Si, N and P) have been calculated using the DFT-B3LYP/6-31G(d) level. The results suggest that all the endohedral fullerenes are slightly enlarged along the same groups, such as IA, IIA, IIIA, IVA and VA, respectively. The endohedral fullerenes containing C, Si, N and P atoms have their high-spin states, and containing Li +, Mg 2+ and Ca 2+ cations are energetically favorable. Additionally, some of the investigated compounds have high energy gap, but they are unlikely to show high thermodynamic stability because of the positive BSSE-corrected binding energies.

Dependence of Positive Binding Energies on Side ChainsA Theoretical Prediction on the Origin of Regular Ordering for the Amino Acid Residues in the Selectivity Filter

The side-chain effects of metalated and protonated dipeptides, including GGH(+)M(+), GAH(+)M(+), AGH(+)M(+), AAH(+)M(+), GWH(+)M(+), GSH(+)M(+), GTH(+)M(+), GFH(+)M(+), GYH(+)M(+), and GVH(+)M(+) (G = glycine, A = alanine, W = tryptophan, S = serine, T = threonine, F = phenylalanine, and V = valine; M = Li, Na, and K), are theoretically explored in this paper on their positive binding energies (PBEs), which are derived from interactions of M+ with the carboxyl oxygen(s). The B3LYP/6-311++G(**)// B3LYP/6-31G(*) calculations suggest that the PBEs of dipeptides with side chain(s) are much smaller than those with no side chain (GGH(+)M(+)). Generally, larger side chains and smaller M(+) radii would lead to fewer PBEs for the M(+) involved systems. On the basis of the direct dependence of PBE on the electrostatic repulsion between two kinds of cations (H(+) and M(+)) in these dipeptide models, it could be reasonably expected that the side-chain effect on the electrostatic repulsion and consequently on the PBEs could offer one good insight, on a chemical-physical basis, into the origin of regular ordering of the amino acids when they form a filter in the K(+) channel protein (MacKinnon, et al. Science 1998, 280, 106).

Stability properties of Q-stars

We present the evolution of Q-star configurations using numerical methods. We solve the full Einstein–Klein–Gordon system of equations and show that: Q-stars can be stable and unstable. The unstable branch is two fold: configurations with negative binding energy that collapse and form black holes, and others with positive binding energy that explode and release the scalar field.

The Bipolaron in the Strong Coupling Limit

The bipolaron are two electrons coupled to the elastic deformations of an ionic crystal. We study this system in the Fr\"{o}hlich approximation. If the Coulomb repulsion dominates, the lowest energy states are two well separated polarons. Otherwise the electrons form a bound pair. We prove the validity of the Pekar-Tomasevich energy functional in the strong coupling limit, yielding estimates on the coupling parameters for which the binding energy is strictly positive. Under the condition of a strictly positive binding energy we prove the existence of a ground state at fixed total momentum $P$, provided $P$ is not too large.

Interaction of helium–vacancy clusters with edge dislocations in α-Fe

Molecular static calculations were performed to evaluate the formation energies and binding energies of helium–vacancy (He–V) clusters in and near the core of an a/2<111>{110} edge dislocation in α-Fe with empirical potentials. The formation energies of these He–V clusters and their binding energies to the dislocation depend on the helium-to-vacancy ratio of the clusters. For the ratio equal to or larger than 1, the helium–vacancy clusters have negative binding energies on the compression side of the dislocation and strong positive binding energy on the tension side. However, for the ratio less than 1, the He–V clusters have positive binding energy on the both sides near the dislocation core. On the slip plane, the binding energies of the He–V clusters to the dislocation depend on not only the helium-to-vacancy ratio, but also the cluster size.

Optimal promoter edge decoration of CoMoS catalysts: A combined theoretical and experimental study

The present work combines X-ray Photoelectron Spectroscopy (XPS), catalytic tests, and density functional theory (DFT) modeling for determining the optimized Co edge decoration according to the stability and reactivity of the CoMoS active phase. Using the phase diagram and morphology of the CoMoS nano-crystallites determined by DFT calculations [Krebs et al. Catal. Today, 130 (2007) 160], the values of the Co/Mo ratio and the number of mixed Co–Mo sites are evaluated. DFT calculations of the 2p core level shift of Co at the edges with respect to the Co 9S 8 phase are compatible with the experimental positive binding energy shift measured by XPS on CoMoS catalysts. Moreover, the decomposition of the XPS spectral envelope is used to quantify the proportion of CoMoS phase and to normalize the catalytic activity in toluene-hydrogenation per edge site as a function of the Co/Mo ratio. The optimal Co/Mo ratio for the catalytic activity is found to reflect the presence of mixed Co–Mo sites at Mo-edge when considering the measured average size of particles and the calculated morphology. These results are helpful for an improved optimization of the promoter content in the CoMoS phase.

XPS Investigation of Plasma‐Deposited Polysiloxane Films Irradiated with Helium Ions

AbstractThis work describes an XPS investigation of plasma‐deposited polysiloxane films irradiated with 170 keV He+ ions at fluences, Φ, ranging from 1 × 1014 to 1 × 1016 cm−2. Modifications in the atomic concentrations of the surface atoms with Φ were revealed by changes in the [O]/[Si], [O]/[C] and [C]/[Si] atomic ratios. Surface chemical structure modifications were evidenced by the increasing C1s peak width and asymmetry as Φ was increased, due to the formation of ether and carboxyl functionalities. Moreover, structural transformations were indicated by the positive binding energy shift of the Si2p peaks, due to the increasing Si oxidation. Correlations of the XPS data with other results from previous work on polysiloxanes illustrate the role of ion beam‐induced bond breaking on the structural modifications.magnified image

Atomistic modeling of helium interacting with screw dislocations in α-Fe

Formation energies, binding energies, and migration energies of interstitial He atoms in and near the core of an a/2〈1 1 1〉 screw dislocation in α-Fe are determined in atomistic simulations using molecular statics employing conjugate gradient relaxation and the dimer method for determining saddle point energies. The set of interatomic potentials employed is the same as used in many recent static and dynamic He–Fe simulations. Interstitial He atoms have a maximum positive binding energy of about 1 eV to the screw dislocation core relative to interstitial He atoms in the perfect crystal, which is about half that of binding energies of He to the edge dislocation. However, interstitial He atoms diffuse along the screw dislocation core with a migration energy of 0.4–0.5 eV, which is the same range as migration energies of interstitial He atoms along the edge dislocation.

Ethynylbenzene Monolayers on Gold: A Metal-Molecule Binding Motif Derived from a Hydrocarbon

Exposure of a Au(111) surface to ethynylbenzene in solution leads to the formation of a bound monolayer. A chemisorption process occurs to give a stable layer consisting of oxygen-containing hydrocarbon species. Ethynylbenzene itself does not oxidize under the deposition conditions indicating that the gold surface facilitates the oxidation process. Calculations show that ethynylbenzene and its oxidation products phenylacetic acid and phenyloxirane have positive binding energies to the gold surface. 1,4-Diethynylbenzene also binds to Au(111) and anchors gold nanoparticles deposited from solution to form dense, semiregular arrays.

Multiple Coordination of CO on Molybdenum Nanoparticles: Evidence for Intermediate Mox(CO)ySpecies by XPS and UPS

CO chemisorption on the metallic molybdenum nanoparticles supported on the thin alumina film was investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). A binary compound of molybdenum and CO is found to be formed on the surface upon CO dose, accompanied with a positive binding energy shift of the Mo 3d doublet and a localized Mo 4d valence band. A loose packing of the metallic molybdenum favors the formation of this intermediate Mox(CO)y species. The formation of the Mox(CO)y species implies that the property of the metallic molybdenum nanoparticles on the thin alumina film is much different from that of the bulk molybdenum, indicating a significant nanometer size effect.

Interaction of helium atoms with edge dislocations in α-Fe

Formation energies, binding energies, and migration energies of interstitial He atoms in and near the core of an a/2〈1 1 1〉{1 1 0} edge dislocation in α-Fe are determined in atomistic simulations using conjugate gradient relaxation and the Dimer method for determining saddle point energies. Results are compared as a function of the proximity of the He to the dislocation core and the excess interstitial volume in regions around the dislocation. Interstitial He atoms have negative binding energy on the compression side of the dislocation and strong positive binding energy on the tension side. Even at low temperatures, interstitial He atoms in the vicinity of the dislocation easily migrate to the dislocation core, where they form crowdion interstitials oriented along the close-packed slip direction, with binding energies in excess of 2 eV. Crowdion interstitial He atoms diffuse along the dislocation core, transverse to the crowdion direction, with a migration energy of 0.4–0.5 eV.

Theoretical Study of the Stability of Lithium Atoms in α-Rhombohedral Boron

The stability of lithium atoms in alpha-rhombohedral boron was investigated by first-principles calculations of total energies and molecular dynamics (MD) simulations. In the case of a low concentration (1.03 at. %), Li at the center of the icosahedral B12 site (the I-site) had a negative binding energy, which suggests Li at the I-site is unstable. However, MD simulations at temperatures below 750 K indicated that Li is still confined in the B12 cage under these conditions, which means Li at the I-site is metastable. Over 800 K, Li began to move away from the B12 site and settled at the tetrahedral site (the T-site) or at the octahedral site (the O-site). Li at the T-site also had a negative binding energy, but MD simulations indicated it was metastable up to 1400 K and did not move to other sites. Li at the O-site was energetically the most favorable, having a positive binding energy. In the case of a high concentration (7.69 at. %), the I-site changed to an unstable saddle point. At this concentration, the T-site was metastable and the O-site became the most stable. In MD simulations at 1400 K, Li atoms at the O-site never jumped to other sites regardless of concentration. Considering these facts, the diffusion coefficient of Li in alpha-rhombohedral boron would have to be very small below 1400 K.

Changes in the electronic structure of gold particles upon thiol adsorption as a function of the mean particle size

We studied the interaction of adsorbed thiol molecules with gold nanoparticles as a function of the mean particle size. The results obtained from MXPS (monochromated X-Ray Photoelectron Spectroscopy) measurements showed that attachment of the thiol sulfur headgroup onto the cluster surface leads to a positive binding energy shift in the Au 4f core-level. The absence of line width broadening upon adsorption indicates that these changes affect the whole particle and not only the particle surface, where the actual Au–S bond is located. The positive binding energy shift depends on the cluster size and increases with decreasing diameter. A maximum shift of 0.41 eV could be measured for the smallest particles (∼1 nm). The valence band exhibited positive binding energy shifts similar to the Au 4f core-levels, but smaller in absolute values. Changes in the valence band shape were interpreted as re-hybridization of Au 5d electrons due to the creation of Au–S bonds. Furthermore, we observed a disappearance of the Fermi edge upon thiol adsorption, which we attribute to a sulfur-induced metal-insulator-transition of the gold cluster.

Modified embedded-atom method interatomic potential for theFe−Cualloy system and cascade simulations on pure Fe andFe−Cualloys

A modified embedded-atom method (MEAM) interatomic potential for the $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Cu}$ binary system has been developed using previously developed MEAM potentials of Fe and Cu. The $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Cu}$ potential was determined by fitting to data on the mixing enthalpy and the composition dependencies of the lattice parameters in terminal solid solutions. The potential gives a value of $0.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the dilute heat of solution and reproduces the increase of lattice parameter of Fe with addition of Cu in good agreement with experiments. The potential was used to investigate the primary irradiation defect formation in pure Fe and $\mathrm{Fe}\text{\ensuremath{-}}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%\phantom{\rule{0.3em}{0ex}}\mathrm{Cu}$ alloy by a molecular dynamics cascade simulation study with a PKA energy of $2\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ at $573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A tendency for self-interstitial atom-Cu binding, the formation of mixed $(\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Cu})$ dumbbells and even $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Cu}$ dumbbells was observed. Given a positive binding energy between Cu atoms and self-interstitials, Cu transport by an interstitial diffusion mechanism could be proposed to contribute to the formation of Cu-rich precipitates and irradiation-induced embrittlement in nuclear structural steels.

Vapor-Phase Photo-Oxidation of Methanol Over Nanosize Titanium Dioxide Clusters Dispersed in MCM-41 Host Material Part 1: Synthesis and Characterization

Nanosize clusters of titania were dispersed in mesoporous MCM-41 silica matrix with the help of the incipient wet-impregnation route, using an isopropanol solution of titanium isopropoxide as precursor. The clusters thus formed were of pure anatase phase and their size depended upon the titania loading. In the case of low (< 15 wt %) loadings, the TiO2 particles were X-ray and laser-Raman amorphous, confirming very high dispersion. These particles were mostly of < or = 2 nm size. On the other hand, larger size clusters (2-15 nm) were present in a sample with a higher loading of approximately 21 wt %. These particles of titania, irrespective of their size, exhibited an absorbance behavior similar to that of bulk TiO2. Powder X-ray diffraction, N2-adsorption and transmission electron microscopy results showed that while smaller size particles were confined mostly inside the pore system, the larger size particles occupied the external surface of the host matrix. At the same time, the structural integrity of the host was maintained even though some deformation in the pore system was noticed in the case of the sample having highest loading. The core level X-ray photoelectron spectroscopy results revealed a + 4 valence state of Ti in all the samples. A positive binding energy shift and the increase of the width of Ti 2p peaks were observed, however, with the decrease in the particle size of supported titania crystallites, indicative of a microenvironment for surface sites that is different from that of the bulk.

Vinyl halides adsorbed on TiO 2 surface: FTIR spectroscopy studies and ab initio calculations

The adsorption at room temperature of vinyl fluoride and vinyl chloride on TiO 2 was studied by Fourier-transform infrared spectroscopy. The surface acidity was preventively investigated through the adsorption at room temperature of carbon monoxide. Vibrational spectra of adsorbed vinyl halides were compared with those of the compounds in the gas-phase and the most interesting differences consist in a large shift of the C–H and C–F stretching vibrations and in a small displacement of the C C stretching mode. The obtained data led to a model of adsorbed vinyl halides on the TiO 2 surface through the occurrence of an acid–base interaction between the halogen atom and the surface Lewis acid site (Ti 4+), and an H-bonding between one hydrogen of the CH 2 group and a surface basic site. A model where an oxygen ion (O 2−) represents the surface basic site was considered and ab initio periodic calculations at DFT/B3LYP level were performed on the system CH 2CHF/TiO 2 using the rutile (110) surface. The optimization of the system gave rise to a positive binding energy and to an increase of the bond lengths C–F and C–H, involved in the H-bonding, in agreement with the results from the infrared data.

QUANTUM BOUND STATES FOR A DERIVATIVE NONLINEAR SCHRÖDINGER MODEL AND NUMBER THEORY

A derivative nonlinear Schrödinger model is shown to support localized N-body bound states for several ranges (called bands) of the coupling constant η. The ranges of η within each band can be completely determined using number theoretic concepts such as Farey sequences and continued fractions. For N≥3, the N-body bound states can have both positive and negative momenta. For η&gt;0, bound states with positive momentum have positive binding energy, while states with negative momentum have negative binding energy.