Search NASA⌕ Search

SEARCH · Search NASA

Results for “CLUSTER MODEL”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Structure of Se{sub 95}As{sub 5} Chalcogenide Glassy Semiconductor Doped by EuF{sub 3} Impurity

The local structure of film samples of the chalcogenide glassy semiconductors Se{sub 95}As{sub 5} and Se{sub 95}As{sub 5}(EuF{sub 3}){sub x} (x = 0.01–1 at %) are studied by X-ray diffraction and Raman scattering. The “quasiperiod” of the structure, the correlation length, the structural elements, and the chemical bonds, which form the amorphous matrix of the investigated materials, are determined. The obtained results are interpreted within the framework of the Elliott void–cluster model taking into account the chemical activity of europium ions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Excitation of surface and volume plasmons in a metal nanosphere by fast electrons

Collective multipole oscillations (surface and volume plasmons) excited in a metal nanosphere by moving electron and corresponding inelastic scattering spectra are studied based on the hydrodynamic approach. Along with the bulk (dielectric) losses traditionally taken into account, the surface and radiative ones are also considered as the physical mechanisms responsible for the plasmon damping. The second and third mechanisms are found to be essential for the surface plasmons (at small or large cluster radii, respectively) and depend very differently on the multipole mode order. The differential equations are obtained which describe the temporal evolution of every particular mode as that one of a linear oscillator excited by the given external force, and the electron energy loss spectra are calculated. The changes in spectrum shape with the impact parameter and with the electron passage time are analyzed; the first of them is found to be in good enough agreement with the data of scanning transmission electron microscopy experiments. It is shown that, in the general case, a pronounced contribution to the formation of the loss spectrum is given by the both surface and volume plasmons with low and high multipole indices. In particular, at long electron passage time, the integral (averaged over the impact parameter) loss spectrum which is calculated for the free-electron cluster model contains two main peaks: a broad peak from merging of many high-order multipole resonances of the surface plasmons and a narrower peak of nearly the same height from merged volume plasmons excited by the electrons that travel through the central region of the cluster. Comparatively complex dependences of the calculated excitation coefficients and damping constants of various plasmons on the order of the excited multipole result in wide diversity of possible types of the loss spectrum even for the same cluster material and should be taken into account in interpretation of corresponding electron energy loss spectroscopy experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Investigating the Surface Structure of γ-Al 2 O 3 Supported WO X Catalysts by High Field 27 Al MAS NMR and Electronic Structure Calculations

The metal-support interaction in γ-Al2O3 supported WOX catalysts is investigated by a combination of high field quantitative single pulse (SP) 27Al MAS NMR spectroscopy, 2D MQMAS, 1H-27Al CP/MAS, and electronic structure calculations. NMR allows the observation of at least seven different Al sites, including a pentahedral Al site, three different tetrahedral Al sites, and three octahedral Al sites. It is found that the penta-coordinated Al (AlP) site density decreases monotonically with an increased WOX loading while the octahedral Al (AlO) site density increases concurrently. This suggests that the Alp sites are the preferred surface anchoring positions for the WOX species. Importantly, the AlP site isotropic chemical shift observed for the unsupported γ-Al2O3 at about 38 ppm migrates into the octahedral region with a new isotropic chemical shift value appearing near 7 ppm when the Alp site is anchored by WOX species. Density functional theory (DFT) computational modeling of the NMR parameters on proposed cluster models is carried out to accurately interpret the dramatic chemical shift changes from which the detailed anchoring mechanisms are obtained. It is found that tungsten dimers and monomers are the preferred supported surface species on γ-Al2O3, wherein one monomeric and several dimeric structures are identified as the most likely surface anchoring structures.

27Al MAS NMR↗
DOE OSTI2022

International Photovoltaic Modeling Intercomparison [Slides]

Irradiance transposition models seem to perform well, except the Isotropic with -11.25 W/m 2 underestimation. Most temperature models could not capture behavior when ΔΤ between module and ambient is negative. Uncertainties due to derate factors: modelers overbudgeted resulting in significant power underestimation; maybe ~10% is appropriate for commercial systems but not lab-scale? Most software and models cluster together showing good reproducibility among participants. Modeler’s skills seem to be more important than the PV model itself (flat efficiency with irradiance, positive power temperature coefficients, etc.). Results and best practices will be communicated in a journal article.

14 SOLAR ENERGY↗
DOE OSTI2016

Alignment of CH{sub 3}F in para-H{sub 2} crystal studied by IR quantum cascade laser polarization spectroscopy

In order to investigate the alignment of CH{sub 3}F in para-H{sub 2} crystals, high resolution polarization spectroscopy of the ν{sub 3} vibrational band is studied using a quantum cascade laser at 1040 cm{sup −1}. It is found that the main and satellite series of peaks in the ν{sub 3} vibrational band of CH{sub 3}F have the same polarization dependence. This result supports the previously proposed cluster model with ortho-H{sub 2} in first and second nearest neighbor sites. The observed polarization dependence function is well described by a simple six-axis void model in which CH{sub 3}F is not aligned along the c-axis of the crystal but tilted to 64.9(3)° from it.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Post-Hartree-Fock studies of the He/Mg(0001) interaction: Anti-corrugation, screening, and pairwise additivity

The adsorption of noble gases on metallic surfaces represents a paradigmatic case of van-der-Waals (vdW) interaction due to the role of screening effects on the corrugation of the interaction potential [J. L. F. Da Silva et al., Phys. Rev. Lett. 90, 066104 (2003)]. The extremely small adsorption energy of He atoms on the Mg(0001) surface (below 3 meV) and the delocalized nature and mobility of the surface electrons make the He/Mg(0001) system particularly challenging, even for state-of-the-art vdW-corrected density functional-based (vdW-DFT) approaches [M. P. de Lara-Castells et al., J. Chem. Phys. 143, 194701 (2015)]. In this work, we meet this challenge by applying two different procedures. First, the dispersion-corrected second-order Möller-Plesset perturbation theory (MP2C) approach is adopted, using bare metal clusters of increasing size. Second, the method of increments [H. Stoll, J. Chem. Phys. 97, 8449 (1992)] is applied at coupled cluster singles and doubles and perturbative triples level, using embedded cluster models of the metal surface. Both approaches provide clear evidences of the anti-corrugation of the interaction potential: the He atom prefers on-top sites, instead of the expected hollow sites. This is interpreted as a signature of the screening of the He atom by the metal for the on-top configuration. The strong screening in the metal is clearly reflected in the relative contribution of successively deeper surface layers to the main dispersion contribution. Aimed to assist future dynamical simulations, a pairwise potential model for the He/surface interaction as a sum of effective He–Mg pair potentials is also presented, as an improvement of the approximation using isolated He–Mg pairs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Astrophysical S-Factor of p{sup 7}Bе Capture at Low Energies

In the modified potential cluster model, the possibility of describing the astrophysical S-factor of radiative p{sup 7}Ве→{sup 8}Bγ capture to the ground state of the {sup 8}B nucleus at energies from 10 keV to 1 MeV is considered. Potentials of intercluster interactions, matched to the spectra of the {sup 8}B nucleus for scattering processes, and the potential of the bound 3Р2 ground state in the p7Ве cluster channel are constructed. The resonance in the 3P1 scattering wave at the energy 0.722 MeV, which leads to an М1-transition to the ground state, is considered. Total cross sections and the reaction rate of p7Ве capture are calculated in the temperature range from 0.01·Т9 to 5·Т9.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

In silico simulations of tunneling barrier measurements for molecular orbital-mediated junctions: A molecular orbital theory approach to scanning tunneling microscopy

A new simulator for scanning tunneling microscopy (STM) is presented based on the linear combination of atomic orbitals molecular orbital (LCAO-MO) approximation for the effective tunneling Hamiltonian, which leads to the convolution integral when applied to the tip interaction with the sample. This approach intrinsically includes the structure of the STM tip. Through this mechanical emulation and the tip-inclusive convolution model, dI/dz images for molecular orbitals (which are closely associated with apparent barrier height, ϕ{sub ap}) are reported for the first time. For molecular adsorbates whose experimental topographic images correspond well to isolated-molecule quantum chemistry calculations, the simulator makes accurate predictions, as illustrated by various cases. Distortions in these images due to the tip are shown to be in accord with those observed experimentally and predicted by other ab initio considerations of tip structure. Simulations of the tunneling current dI/dz images are in strong agreement with experiment. The theoretical framework provides a solid foundation which may be applied to LCAO cluster models of adsorbate–substrate systems, and is extendable to emulate several aspects of functional STM operation.

42 ENGINEERING↗
DOE OSTI2021

Spin Quantization in Heavy Ion Collision

We analyzed recent experimental data on the disassembly of 28Si into 7α in terms of a hybrid α-cluster model. We calculated the probability of breaking into several α-like fragments for high l-spin values for identical and non-identical spin zero nuclei. Resonant energies were found for each l-value and compared to the data and other theoretical models. Toroidal-like structures were revealed in coordinate and momentum space when averaging over many events at high l. The transition from quantum to classical mechanics is highlighted.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Does the Excited Cluster {sup 14}N{sup *} Exist in the {sup 15}O Nucleus?

On the basis of the modified potential two-cluster model with forbidden state which effectively take the Pauli principle into account, calculations have been performed of the astrophysical S-factor of radiative p{sup 14}N capture to the ground state of the {sup 15}O nucleus at proton energies up to 5 MeV in the center-of-mass system (c.m.s.) with allowance for wide resonances up to 3.4 MeV in the c.m.s. For an acceptable explanation of the available experimental data it is necessary to allow the existence of the {sup 14}N cluster in the excited state {sup 14}N{sup *} at an excitation energy of 5.69 MeV and angular momentum J{sup π} = 1{sup –}. It is assumed that in such a case it is possible to use the wave function of the {sup 4}D{sub 1/2} state with respect to the relative motion of the p{sup 14}N{sup *} clusters. It is also shown that a description of the S-factor of p{sup 14}N capture in the resonance region is possible only under the assumption that all of the low-lying resonances at 260(1/2{sup +}), 987(3/2{sup +}), 1447(1/2{sup +}), 2187(3/2{sup +}), and 3211(3/2{sup +}) keV in the c.m.s. are {sup 4}D{sub 1/2} and {sup 4}D{sub 3/2} scattering waves.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Implementation of density functional embedding theory within the projector-augmented-wave method and applications to semiconductor defect states

We report a new implementation of the density functional embedding theory (DFET) in the VASP code, using the projector-augmented-wave (PAW) formalism. Newly developed algorithms allow us to efficiently perform optimized effective potential optimizations within PAW. The new algorithm generates robust and physically correct embedding potentials, as we verified using several test systems including a covalently bound molecule, a metal surface, and bulk semiconductors. We show that with the resulting embedding potential, embedded cluster models can reproduce the electronic structure of point defects in bulk semiconductors, thereby demonstrating the validity of DFET in semiconductors for the first time. Compared to our previous version, the new implementation of DFET within VASP affords use of all features of VASP (e.g., a systematic PAW library, a wide selection of functionals, a more flexible choice of U correction formalisms, and faster computational speed) with DFET. Furthermore, our results are fairly robust with respect to both plane-wave and Gaussian type orbital basis sets in the embedded cluster calculations. This suggests that the density functional embedding method is potentially an accurate and efficient way to study properties of isolated defects in semiconductors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

THE REACTION {sup 8}Li(n,γ){sup 9}Li AT ASTROPHYSICAL ENERGIES AND ITS ROLE IN PRIMORDIAL NUCLEOSYNTHESIS

The possibility of describing available experimental data for the total cross sections of neutron radiative capture on {sup 8}Li at thermal and astrophysical energies was considered within the framework of the modified potential cluster model with the state classification of nuclear particles according to the Young tableaux. Our approach allows one to transmit available data in the energy range 0.1–1.0 MeV quite well, and predicts the behavior of the total cross sections at super-low energies, down to 25.3 × 10{sup −9} MeV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

On Decoration of a Zigzag Edge of Epitaxial Graphene

Two approaches are proposed to the problem of the coupling of adsorbed particles with atoms of a zigzag edge of graphene formed on a metal substrate. The first approach is based on the Kalkstein and Soven scheme, which makes it possible to determine the electronic structure of a semi-infinite graphene sheet. The second approach is based on a cluster model of a zigzag edge. Analytical expressions are obtained for the local densities of the system’s states and the occupation numbers of a carbon adatom and an adparticle. The case of an isolated adparticle is considered in detail, and a method of taking into account the dipole–dipole interaction of particles aligned along the edge is proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of Electron–Phonon Interaction on the Conductivity and Work Function of Epitaxial Graphene

In the context of the cluster model of epitaxial graphene, the condition for a stepwise change in the charge of carbon atoms under the action of graphene–substrate electron–phonon interaction and an external electric field is derived. Such charge transfer brings about corresponding steps of the static conductivity and work function of epitaxial graphene. Numerical estimates for the case of weak coupling of quasi-free graphene with metal and semiconductor substrates are given.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Radiative р{sup 14}N Capture to the First Excited State of the {sup 15}O Nucleus

Within the framework of the modified potential cluster model, the possibility of describing the available experimental data on the astrophysical S-factor for the p{sup 14}N capture reaction to the first excited state of the {sup 15}O nucleus at 5.18 MeV for proton energies up to 1 MeV is considered. Under the assumption that this excited state is a D wave, we have successfully described the experimental data for the astrophysical S-factor.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Radiative p{sup 14}N Capture to the Third Excited State of the {sup 15}O Nucleus at 6.17 MeV

Within the framework of the modified potential cluster model, results of a description of available experimental data in terms of the astrophysical S-factor for the reaction of p{sup 14}N capture to the third excited state of the 15O nucleus at 6.17 MeV with J = 3/2– for proton energies up to 1 MeV are presented. Under the assumption that this excited state is a P{sub 3/2} level and the two resonances at 260 and 987 keV are D scattering waves, we have on the whole successfully reproduced the experimental data for the astrophysical S-factor.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Exotic structure in light neutron-rich nuclei

In this presentation I discussed two subjects. One is the persistence of threefold symmetry in the ground state of {sup 12}C. Recently D{sub 3h} symmetry has been established in {sup 12}C, which reflects the geometric symmetry of the three α particles. Although the spin-orbit interaction plays a significant role and this interaction breaks the α clusters, we show that threefold symmetry of {sup 12}C is still there. We use AQCM approach and discuss that inclusion of spin-orbit interaction, which is absent in the conventional microscopic α cluster models, is possible keeping the threefold symmetry. The second subject is the appearance of rod shape in C isotopes, which has been investigated in the framework of the cranking covariant density functional theory. The relationship between the stability of such states and the spin and isospin degrees of freedom is discussed; adding valence neutrons and rotating the system. These two effects stabilize the rod shape, and in addition, their coherent effect has been found; the σ-orbits (parallel to the symmetry axis) of the valence neutrons, which enhances the rod shape, is lowered by the rotation of the system, and this σ-orbit pulls down the single particle energies of protons with linear configuration owing to the proton-neutron interaction effect.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗