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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 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Investigation of the Unoccupied Electronic Structure of UO2 with Bermstrahlung Isochromat Spectroscopy and X-Ray Absorption Spectroscopy

UO{sub 2} is an important nuclear fuel for electrical power generation. Global goal : Actinides (5f electron systems) exhibit fascinating physical and chemical properties, due to 5f electron correlation, including the highly radioactive systems such as Pu. Onsite Instrumentation: A spectroscopic system containing spin resolved photoelectron spectroscopy (SRPES) and bremsstrahlung isochromat spectroscopy (BIS) has been built and commissioned at LLNL. ALS Instrumentation: The XAS was done on Beamline 8. Both Total Electron Yield (TEY) and Total Fluorescence Yield (TFY) were used. TFY is less surface sensitive than TEY. A combined experimental and theoretical study of Uranium Dioxide has been performed, including XAS, BIS, XPS and spectral simulations. The Conduction Bands or Unoccupied Density of States (UDOS) of UO{sub 2} are shown to be divided into two parts, the lower region being U5f-O2p and the upper region U6d-O2p. This means that UO{sub 2} is an f-f Mott Insulator, electron-correlated system. The keys to success with the XAS were the (1) the utilization of both TEY and TFY and (2) the accurate co-location of the uranium and oxygen states, which in turn hinged upon a proper calibration of the gratings of the beamline monochromator. The calibration of the gratings was greatly aided by the availibility of the O1s XAS from 1st, 2nd and 3rd order light. The success of this approach to differentiation of the Uranium UDOS into U5f and U6d components is of great importance and bodes well for its application to other actinide systems. Our ultimate goal remains Pu and its electron correlation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Strain-controlled evolution of electronic structure indicating topological phase transition in the quasi-one-dimensional superconductor TaSe 3

In this work, we report the signature of a strain-controlled topological phase transition in the electronic structure of a quasione-dimensional superconductor TaSe 3 . Using angle-resolved photoemission spectroscopy and first-principles calculation, TaSe 3 is identified to be in a weak topological insulator phase which has topologically nontrivial surface states only at the allowed planes. Under uniaxial tensile strain, a Dirac point and the topological surface state emerge on the originally forbidden ($10\overline{1}$) plane, which demonstrates the transition to a strong topological insulator phase. Our results accomplish the experimental realization of possible topological insulating phases in TaSe 3 and highlight the possibility of coupling the superconductivity with two distinct topological insulating phases in a controllable manner.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Topological Electronic Structure Evolution with Symmetry Breaking Spin Reorientation in (Fe 1–x Co x )Sn

Topological materials hosting kagome lattices have drawn considerable attention due to the interplay between topology, magnetism, and electronic correlations. The (Fe 1–x Co x )Sn system not only hosts a kagome lattice but has a tunable symmetry-breaking magnetic moment with temperature and doping. In this study, angle-resolved photoemission spectroscopy and first-principles calculations are used to investigate the interplay between the topological electronic structure and varying magnetic moment from the planar to axial antiferromagnetic phases. Here, evidence for a theoretically predicted gap at the Dirac point is revealed in the low-temperature axial phase, but no gap opening is observed across a temperature-dependent magnetic phase transition. However, topological surface bands are observed to shift in energy as the surface magnetic moment is reduced or becomes disordered over time during experimental measurements. The shifting surface bands may preclude the determination of a temperature-dependent bulk gap, but this highlights the intricate connections between magnetism and topology with a surface/bulk dichotomy that can affect material properties and their interrogation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Accelerating Atomic Orbital-based Electronic Structure Calculation via Pole Expansion plus Selected Inversion

We describe how to apply the recently developed pole expansion plus selected inversion (PEpSI) technique to Kohn-Sham density function theory (DFT) electronic structure calculations that are based on atomic orbital discretization. We give analytic expressions for evaluating charge density, total energy, Helmholtz free energy and atomic forces without using the eigenvalues and eigenvectors of the Kohn-Sham Hamiltonian. We also show how to update the chemical potential without using Kohn-Sham eigenvalues. The advantage of using PEpSI is that it has a much lower computational complexity than that associated with the matrix diagonalization procedure. We demonstrate the performance gain by comparing the timing of PEpSI with that of diagonalization on insulating and metallic nanotubes. For these quasi-1D systems, the complexity of PEpSI is linear with respect to the number of atoms. This linear scaling can be observed in our computational experiments when the number of atoms in a nanotube is larger than a few hundreds. Both the wall clock time and the memory requirement of PEpSI is modest. This makes it even possible to perform Kohn-Sham DFT calculations for 10,000-atom nanotubes on a single processor. We also show that the use of PEpSI does not lead to loss of accuracy required in a practical DFT calculation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Electronic structure factors and the importance of adsorbate effects in chemisorption on surface alloys

The chemisorption energy is an integral aspect of surface chemistry, central to numerous fields such as catalysis, corrosion, and nanotechnology. Electronic-structure-based methods such as the Newns-Anderson model are therefore of great importance in guiding the engineering of material surfaces with optimal properties. However, existing methods are inadequate for interpreting complex, multi-metallic systems. Herein, we introduce a physics-based chemisorption model for alloyed transition metal surfaces employing primarily metal d-band properties that accounts for perturbations in both the substrate and adsorbate electronic states upon interaction. Importantly, we show that adsorbate-induced changes in the adsorption site interact with its chemical environment leading to a second-order response in chemisorption energy with the d-filling of the neighboring atoms. We demonstrate the robustness of the model on a wide range of transition metal alloys with O, N, CH, and Li adsorbates yielding a mean absolute error of 0.13 eV versus density functional theory reference chemisorption energies.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Probing the interplay between geometric and electronic-structure features via high-harmonic spectroscopy

We present molecular-frame measurements of the recombination dipole matrix element (RDME) in CO 2 , N 2 O, and carbonyl sulfide (OCS) molecules using high-harmonic spectroscopy. Both the amplitudes and phases of the RDMEs exhibit clear imprints of a two-center interference minimum, which moves in energy with the molecular alignment angle relative to the laser polarization. Here, we find that whereas the angle dependence of this minimum is consistent with the molecular geometry in CO 2 and N 2 O, it behaves very differently in OCS; in particular, the phase shift which accompanies the two-center minimum changes sign for different alignment angles. Our results suggest that two interfering structural features contribute to the OCS RDME, namely, (i) the geometrical two-center minimum and (ii) a Cooper-like, electronic-structure minimum associated with the sulfur end of the molecule. We compare our results to ab initio calculations using time-dependent density functional theory and present an empirical model that captures both the two-center and the Cooper-like interferences. We also show that the yield from unaligned samples of two-center molecules is, in general, reduced at high photon energies compared to aligned samples, due to the destructive interference between molecules with different alignments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Electronic structure and charge-density wave transition in monolayer $\mathrm{VS}_2$

Vanadium disulfide (VS 2 ) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS 2 monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Here, our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Magnetism and electronic structures of novel layered CaFeAs{sub 2} and Ca{sub 0.75}(Pr/La){sub 0.25}FeAs{sub 2}

The magnetic and electronic properties of the parent material CaFeAs{sub 2} of new superconductors are investigated using first-principles calculations. We predict that the ground state of CaFeAs{sub 2} is a spin-density-wave (SDW)-type striped antiferromagnet driven by Fermi surface nesting. The magnetic moment around each Fe atom is about 2.1 μ{sub B}. We also present electronic and magnetic structures of electron-doped phase Ca{sub 0.75}(Pr/La){sub 0.25}FeAs{sub 2}, the SDW order was suppressed by La/Pr substitution. The As in arsenic layers is negative monovalent and acts as blocking layers enhancing two-dimensional character by increasing the spacing distance between the FeAs layers. This favors strong antiferromagnetic fluctuations mediated pairing, implying higher T{sub c} in Ca{sub 0.75}(Pr/La){sub 0.25}FeAs{sub 2} than Ca{sub 0.75}(Pr/La){sub 0.25}Fe{sub 2}As{sub 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Beyond real: alternative unitary cluster Jastrow models for molecular electronic structure calculations on near-term quantum computers

Near-term quantum devices require wavefunction ansätze that are expressive while also of shallow circuit depth in order to both accurately and efficiently simulate molecular electronic structure. While the unitary coupled cluster ansatz (e.g., UCCSD) has become a standard, the high gate count associated with the implementation of this limits its feasibility on noisy intermediate-scale quantum (NISQ) hardware. k -Fold unitary cluster Jastrow (uCJ) ansätze mitigate this challenge by providing O( kN 2 ) circuit scaling and favorable linear depth circuit implementation. Previous work has focused on the real orbitalrotation (Re-uCJ) variant of uCJ, which allows an exact (Trotter-free) implementation. Here we extend and generalize the k -fold uCJ framework by introducing two new variants, Im-uCJ and g-uCJ, which incorporate imaginary and fully complex orbital rotation operators, respectively. Similar to Re-uCJ, both of the new variants achieve quadratic gate-count scaling. Our results focus on the simplest k = 1 model, and show that the uCJ models frequently maintain energy errors within chemical accuracy (∼1 kcal mol −1 ). Both g-uCJ and Im-uCJ are more expressive in terms of capturing electron correlation and are also more accurate than the earlier Re-uCJ ansatz. We further show that Im-uCJ and g-uCJ circuits can also be implemented exactly, without any Trotter decomposition. Numerical tests using k = 1 on H 2 , H 3 + , Be 2 , C 2 H 4 , C 2 H 6 and C 6 H 6 in various basis sets confirm the practical feasibility of these shallow Jastrow-based ansätze for applications on near-term quantum hardware.

Tkachenko, Nikolay V. [University of California, B↗
DOE OSTI2014

Electronic structure and reactivity in water splitting of the iron oxide dimers and their hexacarbonyls: A density functional study

The iron oxide dimers (FeO){sub 2} and their peroxide isomers are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. Among the bare clusters the planar four-member ring structures are more stable than the non-planar ones and the rhombic dioxide Fe{sub 2}O{sub 2} with antiferromagnetically ordered electrons on iron centers is the global minimum. Water adsorption on the bare diiron dioxide is exothermic, but dissociation does not occur. Carbonylation favors a non-planar Fe{sub 2}O{sub 2} ring for both the dioxides and the peroxides and high electron density at the Fe centers is induced, evidenced by the natural charge distribution, the high proton affinity, and the values of global electronegativity and hardness. The iron dioxide hexacarbonyl Fe{sub 2}O{sub 2}(CO){sub 6} is diamagnetic in the state of the global minimum. It is separated from the next low-lying triplet state by a small energy gap of 0.22 eV. Time-dependent density functional theory methods were applied to examine electron excitations from the ground state to the low-lying triplet states in the hexacarbonyls and their adsorption complexes with water. Singlet-to-triplet state excitations occur via ligand-to-metal charge transfer in the hexacarbonyls; in the adsorption complexes excitations from the oxygen lone pairs to the adsorption center also occur and they appear in the IR-visible region. The lowest energy singlet and triplet state reaction paths for water splitting were followed. On the singlet potential energy surface (PES), water splitting is spontaneous, while for the triplet PES an activation barrier of 14.1 kJ mol{sup −1} was determined.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Correlating Crystallography, Magnetism, and Electronic Structure Across Anhysteretic First-Order Phase Transition in Pr 2 In

Here, temperature-dependent powder X-ray diffraction and magnetization measurements of Pr 2 In conclusively prove that the unusual anhysteretic first-order paramagnetic-ferromagnetic phase transition in the compound is related to concurrent changes in both the magnetic and crystallographic lattices. At the same time, the hexagonal Ni 2 In-type structure is stable at least between 6 and 298 K, including at T C = ~57 K. From the density functional theory calculations, the electronic structure of the compound is extraordinarily sensitive to minor changes in lattice parameters that occur across the phase transition, revealing the origin of strong magnetoelastic coupling. In the vicinity of T C , the maximum entropy change, ΔS Max = -16 J Kg -1 K -1 induced by a moderate magnetic field change of 20 kOe (ΔS Max = -20 J Kg -1 K -1 for 50 kOe magnetic field change) is comparable to other known potentially functional materials demonstrating large cryogenic magnetocaloric effect.

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

Density functional theory investigation of the electronic structure and defect chemistry of Sr1-xKxFeO3

Solid oxide fuel cells (SOFCs) efficiently generate electricity, but high operating temperatures (T op > 800 °C) limit their utility. Reducing T op requires mixed ion–electron conducting (MIEC) cathode materials. Density functional theory is used here to investigate the role of potassium substitutions in the MIEC material Sr1-x K x FeO3 (SKFO). We predict that such substitutions are endothermic. SrFeO3 and SKFO have nearly identical metallic electronic structures. Oxygen vacancy formation energies decrease by ~0.2 eV when x K increases from 0 to 0.0625. SKFO is a promising SOFC MIEC cathode material; however, further experimental investigations must assess its long-term stability at the desired operating temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Synthesis, crystal structure, and optical properties of Ba{sub 2}Cu{sub 2}ThS{sub 5}, and electronic structures of Ba{sub 2}Cu{sub 2}ThS{sub 5} and Ba{sub 2}Cu{sub 2}US{sub 5}

The compound Ba{sub 2}Cu{sub 2}ThS{sub 5} has been synthesized by the reaction of BaS, S, Cu, and Th at 1173 K. It crystallizes in the monoclinic space group C{sub 2h}{sup 3}−C2/m with two formula units in a cell of dimensions a=13.5616(8) Å, b=4.1159(2) Å, c=9.3548(6) Å, β=115.89(1)° (100 K). The structure comprises Ba{sup 2+} cations and {sup 2}{sub ∞}[Cu{sub 2}ThS{sub 5}{sup 4−}] layers. The two dimensional layers are formed by the connection of ThS{sub 6} octahedra and CuS{sub 4} tetrahedra along [001] with a sequence…. oct tet tet oct tet tet…. Optical measurements performed on Ba{sub 2}Cu{sub 2}ThS{sub 5} indicate an indirect band gap of 1.86 eV. DFT calculations performed with the HSE06 functional yield band gaps of 1.7 eV for Ba{sub 2}Cu{sub 2}ThS{sub 5} and 1.5 eV for the isostructural compound Ba{sub 2}Cu{sub 2}US{sub 5}. - Graphical abstract: View of the Ba{sub 2}Cu{sub 2}ThS{sub 5} structure and its indirect band gap. DFT calculation gives 1.7 eV. Highlights: ► The new solid-state compound Ba{sub 2}Cu{sub 2}ThS{sub 5} has been synthesized from BaS, S, Cu, and Th at 1173 K. ► Single-crystal optical measurements indicate of indirect band gap of 1.86 eV for Ba{sub 2}Cu{sub 2}ThS{sub 5}. ► DFT with the HSE06 functional predict band gaps of 1.7 and 1.5 eV for Ba{sub 2}Cu{sub 2}ThS{sub 5} and Ba{sub 2}Cu{sub 2}US{sub 5}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural, electronic and magnetic properties of the series of double perovskites (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6}

Polycrystalline samples of the series of double perovskites Sr{sub 2−x}La{sub x}FeIrO{sub 6} were synthesized. Their structural, electronic and magnetic properties were investigated by X-ray powder diffraction, Mössbauer spectroscopy, magnetic susceptibility, heat capacity and electrical resistivity experiments. The compounds crystallize in a monoclinic structure and were fitted in space group P2{sub 1}/n, with a significant degree of Fe/Ir cationic disorder. As in Ca{sub 2−x}La{sub x}FeIrO{sub 6} the Sr-based system seems to evolve from an antiferromagnetic ground state for the end members (x=0.0 and x=2.0) to a ferrimagnetic order in the intermediate regions (x∼1). Since Mössbauer spectra indicate that Fe valence remains 3+ with doping, this tendency of change in the nature of the microscopic interaction could be attributed to Ir valence changes, induced by La{sup 3+} electrical doping. Upon comparing both Ca and Sr series, Sr{sub 2−x}La{sub x}FeIrO{sub 6} is more structurally homogenous and presents higher magnetization and transition temperatures. Magnetic susceptibility measurements at high temperatures on Sr{sub 1.2}La{sub 0.8}FeIrO{sub 6} indicate a very high ferrimagnetic Curie temperature T{sub C}∼700K. For the Sr{sub 2}FeIrO{sub 6} compound, electrical resistivity experiments under applied pressure suggest that this material might be a Mott insulator. - Graphical abstract: The Weiss constant as a function of La doping for the (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6} series, indicating changes in Fe–Ir magnetic coupling on both families. - Highlights: • The double perovskite series (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6} were synthesized. • Changes in the Fe-Ir magnetic coupling due to La doping on both series. • Evidence of high T{sub C} on Sr{sub 1.2}La{sub 0.8}FeIrO{sub 6}. • Indication of Mott insulator behavior on Sr{sub 2}FeIrO{sub 6}.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structural, electronic and magnetic properties of the MnGa(111)-1 $\times$ 2 and 2 $\times$ 2 reconstructions: Spin polarized first principles total energy calculations

In this work, using first principles total energy calculations within the periodic spin polarized density functional theory, we have investigated the structural, electronic, and magnetic properties of manganese gallium (MnGa) alloys. Specifically, we explore the MnGa(111)-1 $\times$ 2 and 2 $\times$ 2 reconstructions. The surface formation energies reveal that selected substitutions occur under Mn (Ga) rich growth conditions. Structures with top layers missing all Mn (Ga) atoms and two layers deep substitutions are also investigated. However, the formation energy shows that these structures are less favorable. For the stable structures, the magnetic properties per layer are proportional to the Mn:Ga ratio. Also, the density of states shows that the MnGa surfaces are metallic. The projected density of states shows that the electronic states in the vicinity of the Fermi level are due mainly to the manganese 3d orbitals. However charge density plots reveal that Mn 3d electrons are closer to the nucleus than Ga sp electrons. Consequently, experimental scanning tunneling microscopy images reveal periodically-arranged bright features, corresponding to the Ga atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of Size Effects on the Electronic Structure of Hexagonal Gallium Telluride

Using methods of the density functional theory, the electronic band structure of a hexagonal modification of the layered GaTe semiconductor has been calculated. The structural parameters of a bulk crystal with the β-polytype symmetry have been determined taking into account van der Waals interactions and agree with experimental data for polycrystalline films within 2%. Estimates for the position of extrema of the upper valence band and the lower conduction band have been obtained with respect to the vacuum level for bulk β-GaTe and for ultrathin plates with the number of elementary layers ranging from 1 to 10, which corresponds to a thickness range of 0.5–8 nm. The calculations demonstrate that hexagonal GaTe is an indirect band gap semiconductor with a forbidden band width varying from 0.8 eV in the bulk material to 2.3 eV in the monolayer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Quaternary Pavonites A 1+ x S 2– x Bi 5+ x S 10 (A + = Li + , Na + ): Site Occupancy Disorder Defines Electronic Structure

The field of mineralogy represents an area of untapped potential for the synthetic chemist, as there are numerous structure types that can be utilized to form analogues of mineral structures with useful optoelectronic properties. In this work, we describe the synthesis and characterization of two novel quaternary sulfides A 1+x Sn 2–x Bi 5+x S 10 (A = Li + , Na + ). Though not natural minerals themselves, both compounds adopt the pavonite structure, which crystallizes in the C2/m space group and consists of two connected, alternating defect rock-salt slabs of varying thicknesses to create a three-dimensional lattice. Despite their commonalities in structure, their crystallography is noticeably different, as both structures have a heavy degree of site occupancy disorder that affects the actual positions of the atoms. The differences in site occupancy alter their electronic structures, with band gap values of 0.31(2) eV and 0.07(2) eV for the lithium and sodium analogues, respectively. LiSn 2 Bi 5 S 10 exhibits ultralow thermal conductivity of 0.62 W m –1 K –1 at 723 K, and this result is corroborated by phonon dispersion calculations. Here, this structure type is a promising host candidate for future thermoelectric materials investigation, as these materials have narrow band gaps and intrinsically low thermal conductivities.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗