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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Electronic structure of single-layer superlattices (GeC){sub 1}(SiC){sub 1}, (SnC){sub 1}(SiC){sub 1}, and (SnC){sub 1}(GeC){sub 1}

The crystal and electronic structures of GeC, SiC, and SnC crystals and superlattices based on them, GeC/SiC, SnC/SiC, and SnC/GeC, are modeled within density functional theory. The equilibrium lattice constants are obtained, the band spectra and densities of states are calculated, and the specific features of the formation of the valence band and chemical bonding in these crystals and superlattices are studied.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Quantum Fluctuations in Fe{sub x}Mn{sub 1 –} {sub x}Si with Inclusion of the LDA + U + SO Calculations of the Electronic Structure

Quantum spin fluctuations are studied in chiral helicoidal Fe{sub x}Mn{sub 1 –} {sub x}Si ferromagnets on the basis of the direct calculations of the electronic structure. The disappearance of the helicoidal long-range order is shown to be accompanied by the appearance of the crossover of the thermodynamic and quantum transitions with a sharp decrease in the local magnetization and the zero fluctuation amplitude. Suppression of the local magnetization by quantum spin fluctuations leads to the concentration–temperature magnetic transition with the disappearance of the helicoidal short-range order. The calculations of the magnetization of the Fe{sub x}Mn{sub 1 –} {sub x}Si compositions show that at 0.12 < x < 0.20 the magnetic state in the thermodynamic limit is characterized by the short-range (not long-range) helicoidal order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Toward Accurate Spin–Orbit Splittings from Relativistic Multireference Electronic Structure Theory

Most nonrelativistic electron correlation methods can be adapted to account for relativistic effects, as long as the relativistic molecular spinor integrals are available, from either a four-, two-, or one-component mean-field calculation. Furthermore, relativistic multireference correlation methods remain a relatively unexplored area, with mixed evidence regarding the improvements brought by perturbative treatments. We report, for the first time, the implementation of state-averaged four-component relativistic multireference perturbation theories to second and third order based on the driven similarity renormalization group (DSRG). With our methods, named 4c-SA-DSRG-MRPT2 and 3, we find that the dynamical correlation included on top of 4c-CASSCF references can significantly improve the spin-orbit splittings in p-block elements and potential energy surfaces when compared to 4c-CASSCF and 4c-CASPT2 results. We further show that 4c-DSRG-MRPT2 and 3 are applicable to these systems over a wide range of the flow parameter, with systematic improvement from second to third order in terms of both improved error statistics and reduced sensitivity with respect to the flow parameter.

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

Probing electronic structure in berkelium and californium via an electron microscopy nanosampling approach

Due to their rarity and radioactive nature, comparatively little is known about the actinides, particularly those with atomic numbers higher than that of plutonium, and their compounds. In this work, we describe how transmission electron microscopy can provide comprehensive, safe, and cost-effective characterization using only single nanogram amounts of highly-radioactive, solid compounds. Chlorides of the rare elements berkelium and californium are dropcast and then converted in situ to oxides using the electron beam. The f-band occupancies are probed using electron energy loss spectroscopy and an unexpectedly weak spin-orbit-coupling is identified for berkelium. In contrast, californium follows a jj coupling scheme. These results have important implications for the chemistries of these elements and solidify the status of californium as a transitional element in the actinide series.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Electronic structure and lattice dynamics of 1⁢𝑇−VSe 2 : Origin of the three-dimensional charge density wave

To characterize in detail the charge density wave (CDW) transition of 1⁢𝑇−VSe 2 , its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, muon spectroscopy, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering, and state-of-the-art first-principles density functional theory calculations. Resonant elastic x-ray scattering does not show any resonant enhancement at either V or Se, indicating that the CDW peak at the 𝐾 edges describes a purely structural modulation of the electronic ordering. ARPES experiments identify (i) a pseudogap at 𝑇 > 𝑇⁢ CDW , which leads to a depletion of the density of states in the ML-M'L' plane at 𝑇 < 𝑇⁢ CDW , and (ii) anomalies in the electronic dispersion reflecting a sizable impact of phonons on it. A diffuse scattering precursor, characteristic of soft phonons, is observed at room temperature (RT) and leads to the full collapse of the low-energy phonon (𝜔 1 ) with propagation vector (0.25 0 −0.3) r.l.u. Here, we show that the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI), hence demonstrating that the origin of the CDW is, to a much larger extent, due to the momentum dependent EPI with a small contribution from nesting. The pressure dependence of the 𝜔 1 soft mode remains nearly constant up to 13 GPa at RT, with only a modest softening before the transition to the high-pressure monoclinic C2/m phase. The wide set of experimental data is well captured by our state-of-the art first-principles anharmonic calculations with the inclusion of van der Waals corrections in the exchange-correlation functional. The comprehensive description of the electronic and dynamical properties of VSe 2 reported here adds important pieces of information to the understanding of the electronic modulations in the family of transition-metal dichalcogenides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

The Reactivity and Structural Dynamics of Supported Metal Nanoclusters Using Electron Microscopy, in-situ X-Ray Spectroscopy, Electronic Structure Theories, and Molecular Dynamics Simulations

This research will provide new methods to determine the structures and compositions of heterogeneous catalysts as they exist in reaction conditions. The new tools will make it possible to understand how extremely small material structures—ones in a size range spanning from single atoms to larger polyatomic clusters—make it possible to carry out highly selective chemical transformations required to synthesize useful materials as well as for efficient energy production and utilization. The tools will make it possible to see how the atomic structures of catalytic materials evolve over time under the conditions of their use, and in this way provide understandings to guide the design of new energy-efficient catalysts. The work will make it possible to answer long standing questions that include the following. How does catalyst structure help guide molecules along specific reaction pathways? What are the important intermediates involved in these transformations? How do the structures and reactivity of catalytic materials, as well as the chemistry of their interfaces, vary with reaction conditions? Is it possible to develop new catalytic materials using new knowledge of the active structures and reaction mechanisms? The work will provide heretofore unavailable experimental tools that can be applied in advanced programs of research addressing such questions.

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

Impact of Radiation on the Electronic Structure of MoS 2

Electrons in a semiconductor occupy states within certain energy ranges, called energy bands. The position of the Fermi level with respect to these energy bands determines the charge carrier type of the semiconductor. Molybdenum disulfide (MoS 2 ) is a two-dimensional, n-type semiconductor with potential applications in flexible electronics, transparent electronics, and optoelectronics. Electronic devices containing MoS 2 could be used in environments where radiation affects device performance. Thus, it is important to determine the impact of radiation on MoS 2 . A one-molecule-thick layer of MoS 2 (monolayer) and a two-molecule-thick layer of MoS 2 (bilayer) were placed onto different areas of a gold (Au) substrate containing 1.2-µm-deep holes. The MoS 2 was suspended over these holes but supported by the Au elsewhere on the substrate. This sample configuration was used to determine the effect of He + radiation on the electronic properties of the suspended MoS 2 and the Au-supported MoS 2 . The MoS 2 was irradiated by He+ ions in two stages. The energy bands of the MoS 2 were measured with respect to the Fermi level via photoelectron emission microscopy before irradiation and after each irradiation stage. From each measurement, the charge carrier type of the MoS 2 after the corresponding irradiation stage was determined. The Fermi levels of the suspended monolayer and bilayer decreased by ≈0.15 eV with respect to the bands during the first irradiation stage During the second irradiation stage, however, the Fermi levels didn’t change significantly. This lack of change supports the existence of a radiation threshold, above which the electronic properties of suspended MoS 2 remain the same. The Fermi levels of the supported monolayer and bilayer increased over the cumulative irradiation and didn’t show evidence of a threshold. Thus, suspended MoS 2 becomes less n-type as it is irradiated. Supported MoS 2 , however, becomes more n-type as it is irradiated. These results could inform the development of radiation tolerance standards for MoS 2 , and thus, radiation-tolerant MoS 2 -based electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Probing the Electronic Structure of [B 10 H 10 ] 2– Dianion Encapsulated by an Octamethylcalix[4]pyrrole Molecule

Despite being an important closo-borate in the condensed phase boron chemistry, isolated B 10 H 10 2- is electronically unstable and has never been detected in the gas phase. Herein, we report a successful capture of this fleeting species through binding with an octamethylcalix[4]pyrrole (omC4P) molecule to form a stable gaseous omC4P·[B 10 H 10 ] 2- complex and its characterizations utilizing negative ion photoelectron spectroscopy (NIPES). The recorded NIPE spectrum, contributed from both omC4P and [B 10 H 10 ] 2- , is deconvoluted by subtracting the omC4P contribution to yield a [B 10 H 10 ] 2- spectrum. The obtained [B 10 H 10 ] 2- spectrum consists of four major bands spanning electron binding energy (EBE) range from 1 to 5 eV with the EBE gaps matching excellently with the energy intervals of computed highly lying occupied molecular orbitals of the B 10 H 10 2- dianion. Finally, this study showcases a generic method to utilize omC4P to capture unstable multiply charged anions in the gas phase for experimental determination of their electronic structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Application of High-Frequency Electron Paramagnetic Resonance/Electron Spin Echo for the Identification of the Impurity Composition and Electronic Structure of Ceramics Based Garnets

Electron paramagnetic resonance (EPR) spectra of Ce{sup 3+}, Yb{sup 3+}, Cr{sup 3+}, and Gd{sup 3+} impurity ions in yttrium aluminum garnet Y{sub 3}Al{sub 5}O{sub 12} (YAG) ceramics were detected and identified at frequency of 94 GHz. The advantage of measuring the EPR spectra in the high-frequency range compared to the standard EPR technique is shown, which makes it possible to separate the EPR spectra characterized by different anisotropic g-factors and also to isolate the EPR signals due to the splitting of the fine structure for centers with high-spin states. In ceramics with a high content of magnetic Gd{sup 3+} ions, EPR and electron spin echo (ESE) spectra of multi-ionic gadolinium complexes were observed, and EPR spectra of complexes with the maximum number of exchange-coupled gadolinium ions are seen at low temperatures. The temperature dependences of the EPR spectra indicates ferromagnetic ordering of exchange-coupled complexes of gadolinium.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Stochastic Vector Techniques in Ground-State Electronic Structure

Herein we review a suite of stochastic vector computational approaches for studying the electronic structure of extended condensed matter systems. These techniques help reduce algorithmic complexity, facilitate efficient parallelization, simplify computational tasks, accelerate calculations, and diminish memory requirements. While their scope is vast, we limit our study to ground-state and finite temperature density functional theory (DFT) and second-order many-body perturbation theory. More advanced topics, such as quasiparticle (charge) and optical (neutral) excitations and higher-order processes, are covered elsewhere. We start by explaining how to use stochastic vectors in computations, characterizing the associated statistical errors. Next, we show how to estimate the electron density in DFT and discuss effective techniques to reduce statistical errors. Finally, we review the use of stochastic vectors for calculating correlation energies within the second-order Møller-Plesset perturbation theory and its finite temperature variational form. Example calculation results are presented and used to demonstrate the efficacy of the methods.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Electronic structure and mechanical properties of crystalline precipitate phases M 23 C 6 (M=Cr, W, Mo, Fe) in Ni-based superalloys

The presence of precipitate phases M 23 C 6 (M=Cr, Fe, Mo, or W) plays a significant role in the mechanical strength of Ni-based superalloys. The crystal structure of these complex carbides consists of 116 atoms in the cubic cell of an fcc lattice. Here we report the investigation of the electronic structure, interatomic bonding and mechanical properties for four binary phases of M 23 C 6 , and M 23 N 6 , and six ternary phases of M1 23–x M2 x C 6 . Based on the results of our comprehensive calculations and the evaluation of a quantum mechanical metric called the total bond order density (TBOD), we conclude that Fe tends to make the intermetallic structure less stable whereas W makes the crystal more compressible. We trace the root of these observations to the size of the metal ions and the complex interatomic bonding between C atoms and the metal d electrons. In addition, we correlate the TBOD with calculated mechanical properties leading to confirmation that Cr 23 C 6 is the most stable precipitate phase with superior overall physical properties. On the other hand, replacing C with N in these crystals does not result in discernable differences in their properties. Furthermore, our detailed investigation sheds much light on the use of carbides precipitates as a stabilization strategy for Ni-based superalloys.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electronic structure, magnetic properties, and exchange splitting of gadolinium intermetallics

Here, we report here the electronic structure calculations of the magnetic moment, magneto-crystalline anisotropy energy (MAE), and density of states (DOS) of Gd-metal, GdAl 2 , GdAl 3 , GdCo 2 , GdCo 3 , and GdCo 5 . From the DOS, the exchange splitting energy is derived to map ferrimagnetic to paramagnetic phase transition (Curie temperature, T C ). The employed theoretical method here is based on advanced density functional theory, specifically the linearized augmented plane wave method (LAPW) within the local spin density approximation (LSDA), including spin–orbit coupling (SOC) and Hubbard model parameter (U). DOS analysis of both Gd-Al and Gd-Co systems reveals a strong correlation between exchange splitting energy and T C . The Gd-Co system exhibits an enormous exchange splitting energy among Co-3d electrons, vastly overwhelming the vanishing Gd 5d exchange splitting. Finally, these findings suggest that exchange splitting of Co-3d electrons plays a key role in the ground state magnetism and magnetic transition temperatures in these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Layer-dependent spin-resolved electronic structure of ferromagnetic triple-layered ruthenate Sr4Ru3O10

High-resolution angle- and spin-resolved photoemission spectroscopy (ARPES) of the triple-layered ruthenate Sr4Ru3O10 reveals features of the electronic structure that extend our understanding of the layered strontium ruthenates. The spectra near the Fermi energy are very different from the nonmagnetic analogues Sr2RuO4 and Sr3Ru2O7 with distinct Fermi surfaces for wide electronlike minority spin bands around the zone center and narrow holelike majority spin Fermi surface contours around the zone corners. The most dramatic results are two narrow spectral peaks ∼30 meV below the Fermi level, a spin-minority holelike band at the Brillouin zone center, and a spin-majority saddle-band van Hove singularity at the zone edge, which exhibits almost 100% spin polarization at low temperature, and a strong temperature dependent coherence-incoherence crossover attributed to Hund metal correlations. Quantitative comparison of the ARPES to spin-polarized density functional theory (DFT) calculations identify the specific antibonding and nonbonding orbital origins of the narrow bands, with a prediction of different spatial localization in the central and outer layers. This is shown to be consistent with experimental ARPES multizone matrix element intensity variations, and implicates outer-layer-specific control of the in-plane metamagnetism. The renormalization of the bands relative to the mean-field DFT, the demonstration of spin-polarized oxygen bands, and of spin-minority and spin-majority band-crossing hybridization provide a more complete picture of the magnetism which displays aspects of both delocalized and local moment behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electronic Structures, Magnetic Properties and Half-Metallicity of Heusler Compounds Hf{sub 2}VZ (Z = Ga, In, Tl, Si, Ge, Sn and Pb): First-Principle Calculations

We investigate the electronic structures, magnetic properties and half-metallicity of Heusler compounds Hf{sub 2}VZ (Z = Ga, In, Tl, Si, Ge, Sn and Pb) with Hg{sub 2}CuTi-type structure by using the first-principle calculations based on density-functional theory. It is found that the equilibrium lattice constants of Hf{sub 2}VZ (Z = Ga, In, Tl, Si, Ge, Sn and Pb) compounds are 6.68, 6.88, 6.91, 6.57, 6.63, 6.85 and 6.94 Å, respectively. Hf{sub 2}VZ (Z = Ga, In, Tl, Si and Sn) compounds exhibit half-metallicity at their respective equilibrium lattice constants. Hf{sub 2}VGe and Hf{sub 2}VPb do not exhibit half-metallicity at their equilibrium lattice constants but can present the half-metallicity when the lattice constant decreases to 6.51 and 6.71 Å, respectively. In case of the lattice thermal expansion, the half-metallicity can be kept in the range of 6.30∼ 7.27, 6.36∼ 7.80, 6.46∼ 7.35, 6.00∼ 6.70 and 6.10∼ 6.95 Å (lattice constant) for Hf{sub 2}VZ (Z = Ga, In, Tl, Si and Sn). Under the unaxial strain, the half-metallicity can still be kept for Hf{sub 2}VZ (Z = Ga, In, Tl) when the lattice distortion is up to ± 10% (c/a) and be kept for Hf{sub 2}VZ (Z = Si, Sn) when the lattice distortion is 0.88∼ 1.09, 0.91∼ 1.09 (c/a). For the Hf{sub 2}VZ compounds with half-metallicity, the total magnetic moment is 2 μ{sub B} when Z is the third main group elements and is 1 μ{sub B} when Z is the fourth main group elements. The Hf{sub 2}VZ compounds with half-metallicity obey the Slater-Pauling formula: M{sub t} = 18 − Z{sub t} rule.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Interplay of electronic structure, magnetism, strain, and defects in carbide $\mathrm{MX}$enes

We report the two-dimensional (2D) magnetic and semiconducting materials, including carbide MXenes, are in high demand for magneto-electronic devices and applications. Here, we report a first-principles study of electronic structure and magnetism of a Ti 2 C MXene, its derivatives Ti 2 CT 2 (T = -F, -O, -OH, -H), and the effect of single vacancy defects (25% C, 12.5% O, and 12.5% and 5.5% Ti) and transition metal (V, Cr, Mn, and Co) dopings. The MAX phase Ti 2 AlC is a non-magnetic (NM) metal which upon removal of Al layer forms a pristine Ti 2 C MXene with A-type antiferromagnetic (AFM) semiconductor having reliable transition temperature (T N = 41K) predicted by the Heisenberg model. All the functionalized MXenes are stable as NM metal except for Ti 2 CO 2 which remains semiconducting with NM ground state. The significant effect of spin–orbit coupling (SOC) is evident in the band structure forming Dirac-like cones and band inversions near the Fermi level. Depending upon the type of vacancy defects Ti 2 CO 2 is NM metal and ferromagnetic (FM) metal. The transition metal (TM) doped MXenes fulfill FM Stoner criterion. The V- and Co-doped MXenes are metals, whereas Cr- and Mn-doped are half-metal and semiconductor.

2D magnetism↗
DOE OSTI2018

Phase Formation and Electronic Structure Peculiarities in the Al{sub 1–} {sub x}Si{sub x} Film Composites under the Conditions of Magnetron and Ion-Beam Sputtering

The peculiarities of the phase composition and electronic structure of aluminum–silicon composite films near the Al{sub 0.75}Si{sub 0.25} composition obtained by the magnetron and ion-beam sputtering methods on a Si(100) silicon substrate are studied using the X-ray diffraction techniques and ultrasoft X-ray emission spectroscopy. In addition to silicon nanocrystals of about 25 nm in size, an ordered solid solution corresponding to the previously unknown Al{sub 3}Si phase is formed in magnetron sputtering on a polycrystalline Al matrix. Films obtained by ion-beam sputtering of the composite target are found to be monophasic and contained only one phase of an ordered solid solution of aluminum silicide Al{sub 3}Si of the Pm3m cubic system with the primitive cell parameter a = 4.085 Å. However, subsequent pulsed photon annealing of the composite with different radiation doses from 145 to 216 J/cm{sup 2} gives rise to the partial decomposition of the Al{sub 3}Si phase with the formation of free metallic aluminum and silicon nanocrystals with sizes in the range from 50 to 100 nm, depending on the pulsed photon radiation dose.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗