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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.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Geometry and electronic structure of iridium adsorbed on graphene

We report investigation of the geometry and electronic structure of iridium atoms adsorbed onto graphene through a combined experimental and theoretical study. Ir atoms were deposited onto a flake of graphene on a Pt(111) surface and found to form clusters even at low temperatures. The areal density of the observed clusters on the graphene flake suggests the clusters are most likely pairs of Ir atoms. Theoretical ab initio density functional (DFT) calculations indicate that these Ir dimers are oriented horizontally, near neighboring "bridge" sites of the graphene lattice, as this configuration has the strongest adsorption energy of all high-symmetry configurations for the Ir dimer. A large peak in the local density of states (LDOS) at the Dirac point energy was measured via scanning tunneling spectroscopy, and this result is reproduced by a DFT calculation of the LDOS. The peak at the Dirac point energy is found to be from the Ir s and p states. Finally, the LDOS in the monomer case was also calculated, and is found to significantly differ from the experimentally determined data, further supporting the hypothesis of low-temperature clustering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural, electronic and magnetic properties of carbon doped boron nitride nanowire: Ab initio study

Using spin-polarized density functional theory calculations, we demonstrated that carbon doped boron nitride nanowire (C-doped BNNW) has diverse electronic and magnetic properties depending on position of carbon atoms and their percentages. Our results show that only when one carbon atom is situated on the edge of the nanowire, C-doped BNNW is transformed into half-metal. The calculated electronic structure of the C-doped BNNW suggests that doping carbon can induce localized edge states around the Fermi level, and the interaction among localized edge states leads to semiconductor to half-metal transition. Overall, the bond reconstruction causes of appearance of different electronic behavior such as semiconducting, half-metallicity, nonmagnetic metallic, and ferromagnetic metallic characters. The formation energy of the system shows that when a C atom is doped on surface boron site, system is more stable than the other positions of carbon impurity. Our calculations show that C-doped BNNW may offer unique opportunities for developing nanoscale spintronic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Electronic structure, optical properties and bonding in alkaline earth halo-fluoride scintillators: BaClF, BaBrF and BaIF

We report first-principles studies of the structural, electronic, and optical properties of the alkaline-earth halofluorides, BaXF (X = Cl, Br, and I), including pressure dependence of structural properties. The band structures show clear separation of the halogen p derived valence bands into higher binding energy F and lower binding energy X derived manifolds reflecting the very high electronegativity of F relative to the other halogens. Implications of this for bonding and other properties are discussed. We find an anisotropic behavior of the structural parameters especially of BaIF under pressure. The optical properties on the other hand are almost isotropic, in spite of the anisotropic crystal structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

An ab initio study of the electronic structure and optical properties of CdS{sub 1-x}Te{sub x} alloys

The structural, electronic and optical properties of cubic CdS{sub 1-x}Te{sub x} alloys, with Te-concentrations varying from 0% up to 100% are investigated. The calculations are based on the total-energy calculations using the full potential-linearized augmented plane wave (FP-LAPW) method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA) for the total-energy calculations, while for electronic properties in addition to that the Engel-Vosko (EV-GGA) formalism was also applied. The ground state properties for all Te-concentrations are presented. The optical dielectric constant is also determined for both the binary and their related ternary alloys. (author)

36 MATERIALS SCIENCE↗
DOE OSTI2014

Atomic and electronic structures of lattice mismatched Cu 2 O/TiO 2 interfaces

Heterojunction interfaces between metal oxides are often highly lattice mismatched. The atomic and electronic structures of such interfaces, however, are not well understood. Here, we have synthesized Cu 2 O/TiO 2 heterojunction thin films with 13% lattice mismatch and studied the interface via experimental methods and large-scale density function theory calculations of supercells containing 1300 atoms. We find that an interface of epitaxial quality is formed via a coincidence site lattice of 8 Cu 2 O unit cells matching 9 TiO 2 unit cells. Calculations reveal the existence of a dislocation core of the O sublattices at the interface and a random arrangement of one layer of interfacial Cu atoms. The interfacial electronic structure is found to be mostly determined by the interfacial Cu distribution, rather than by the O dislocation core. The conduction band minimum and valence band maximum states are spatially separated, and there is no strongly localized state near the core.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Magnetism and electronic structure of YTiO{sub 3} thin films

High-quality (001)-oriented (pseudo-cubic notation) ferromagnetic YTiO{sub 3} thin films were epitaxially synthesized in a layer-by-layer way by pulsed laser deposition. Structural, magnetic, and electronic properties were characterized by reflection-high-energy-electron-diffraction, X-ray diffraction, vibrating sample magnetometry, and element-resolved resonant soft X-ray absorption spectroscopy. To reveal ferromagnetism of the constituent titanium ions, X-ray magnetic circular dichroism spectroscopy was carried out using four detection modes probing complementary spatial scale, which overcomes a challenge of probing ferromagnetic titanium with pure Ti{sup 3+}(3d{sup 1}). Our work provides a pathway to distinguish between the roles of titanium and A-site magnetic rare-earth cations in determining the magnetism in rare-earth titanates thin films and heterostructures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Electronic Structure, Optical Properties, and Pressure Behavior of the CdB{sub 4}O{sub 7} and HgB{sub 4}O{sub 7} Compounds

Ab initio calculations of the structural, electronic, and optical properties of the CdB{sub 4}O{sub 7} and HgB{sub 4}O{sub 7} tetraborate compounds in three structural modifications with the Pbca, Cmcm, and Pmn2{sub 1} symmetry have been performed in the framework of the density functional theory using the VASP package. The calculations of the electronic band structure showed that these compounds in all the investigated modifications are dielectrics with a band gap of 2–4 eV. The calculation of the structural properties of the tetraborates under pressure showed that the phase transition between the Pbca and Pmn2{sub 1} structures in cadmium and mercury tetraborates occurs under pressures of 4.8 and 4.7 GPa, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Surrogate Hessian accelerated structural optimization for stochastic electronic structure theories

In this work, we present an efficient energy-based method for structural optimization with stochastic electronic structure theories, such as diffusion quantum Monte Carlo (DMC). This method is based on robust line-search energy minimization in reduced parameter space, exploiting approximate but accurate Hessian information from a surrogate theory, such as density functional theory. The surrogate theory is also used to characterize the potential energy surface, allowing for simple but reliable ways to maximize statistical efficiency while retaining controllable accuracy. We demonstrate the method by finding the minimum DMC energy structures of the selected flake-like aromatic molecules, such as benzene, coronene, and ovalene, represented by 2, 6, and 19 structural parameters, respectively. In each case, the energy minimum is found within two parallel line-search iterations. The method is near-optimal for a line-search technique and suitable for a broad range of applications. It is easily generalized to any electronic structure method where forces and stresses are still under active development and implementation, such as diffusion Monte Carlo, auxiliary-field Monte Carlo, and stochastic configuration interaction, as well as deterministic approaches such as the random-phase approximation. Accurate and efficient means of geometry optimization could shed light on a broad class of materials and molecules, showing high sensitivity of induced properties to structural variables.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Recent Developments in the General Atomic and Molecular Electronic Structure System

A discussion of many of the recently implemented features of GAMESS (General Atomic and Molecular Electronic Structure System) and LibCChem (the C++ CPU/GPU library associated with GAMESS) is presented. These features include fragmentation methods like the fragment molecular orbital, effective fragment potential and effective fragment molecular orbital methods, hybrid MPI/OpenMP approaches to Hartree-Fock and resolution of the identity second order perturbation theory. Many new coupled cluster theory methods have been implemented in GAMESS, as have multiple levels of density functional/tight binding theory. The role of accelerators, especially graphical processing units, is discussed in the context of the new features of LibCChem, as is the associated problem of power consumption as the power of computers increases dramatically. The process by which a complex program suite like GAMESS is maintained and developed is considered. Future developments are briefly summarized.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Electronic structure, magnetic properties, and magnetostructural transformations in rare earth dialuminides

We report electronic structure, magnetic properties, and magnetostructural transformations of selected rare earth dialuminides calculated by using local spin density approximation (LSDA), including the Hubbard U parameter (LSDA + U) approach. Total energy calculations show that CeAl{sub 2} and EuAl{sub 2} adopt antiferromagnetic (AFM) ground states, while dialuminides formed by other magnetic lanthanides have ferromagnetic (FM) ground states. The comparison of theoretical and experimental magnetic moments of CeAl{sub 2} indicates that the 4f orbital moment of Ce in CeAl{sub 2} is quenched. Theoretical calculations confirm that Eu in EuAl{sub 2} and Yb in YbAl{sub 2} are divalent. PrAl{sub 2} exhibits a tetragonal distortion near FM transition. HoAl{sub 2} shows a first order magnetostructural transition while DyAl{sub 2} shows a second order transformation below magnetic transition. The dialuminides formed by Nd, Tb, and Er are simple ferromagnets without additional anomalies in the FM state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Characterization of ferredoxins involved in electron transfer pathways for nitrogen fixation implicates differences in electronic structure in tuning 2[4Fe 4S] Fd activity

Ferredoxins (Fds) are small proteins which shuttle electrons to pathways like biological nitrogen fixation. Physical properties tune the reactivity of Fds with different pathways, but knowledge on how these properties can be manipulated to engineer new electron transfer pathways is lacking. Recently, we showed that an evolved strain of Rhodopseudomonas palustris uses a new electron transfer pathway for nitrogen fixation. This pathway involves a variant of the primary Fd of nitrogen fixation in R. palustris, Fer1, in which threonine at position 11 is substituted for isoleucine (Fer1 T11I ). To understand why this substitution in Fer1 enables more efficient electron transfer, we used in vivo and in vitro methods to characterize Fer1 and Fer1 T11I . Electrochemical characterization revealed both Fer1 and Fer1 T11I have similar redox transitions (–480 mV and – 550 mV), indicating the reduction potential was unaffected despite the proximity of T11 to an iron-sulfur (Fe—S) cluster of Fer1. Additionally, disruption of hydrogen bonding around an Fe—S cluster in Fer1 by substituting threonine with alanine (T11A) or valine (T11V) did not increase nitrogenase activity, indicating that disruption of hydrogen bonding does not explain the difference in activity observed for Fer1 T11I . Electron paramagnetic resonance spectroscopy studies revealed key differences in the electronic structure of Fer1 and Fer1 T11I , which indicate changes to the high spin states and/or spin-spin coupling between the Fe—S clusters of Fer1. Finally, our data implicates these electronic structure differences in facilitating electron flow and sets a foundation for further investigations to understand the connection between these properties and intermolecular electron transfer.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Low-rank factorization of electron integral tensors and its application in electronic structure theory

In this letter, we introduce the reverse Cuthill-McKee (RCM) algorithm, which is often used for the bandwidth reduction of sparse tensors, to transform the two-electron integral tensors to their block diagonal forms. By further applying the pivoted Cholesky decomposition (CD) on each of the diagonal blocks, we are able to represent the high-dimensional two-electron integral tensors in terms of permutation matrices and low-rank Cholesky vectors. This representation facilitates the low-rank factorization of the high-dimensional tensor contractions that are usually encountered in post-Hartree-Fock calculations. In this letter, we discuss the second-order Møller-Plesset (MP2) method and linear coupled- cluster model with doubles (L-CCD) as two simple examples to demonstrate the efficiency of the RCM-CD technique in representing two-electron integrals in a compact form.

electronic structure theory↗