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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Water Interactions with Actinide Oxides from First Principles: A Computational Study - 16470

The interactions between water and the actinide oxides UO{sub 2} and PuO{sub 2} are important when considering the long-term storage of spent nuclear fuel. However, experimental studies in this area are severely limited by plutonium's intense radioactivity, and hence we have recently begun to investigate these interactions computationally. In this contribution we report the results of first principles calculations of the electronic and geometric structures of AnO{sub 2} bulk, AnO{sub 2} clusters, and AnO{sub 2} surfaces (An = U, Np, Pu), as well as the interaction of water with the latter. Strongly-correlated effects are taken into account using a Hubbard corrected potential, which enables us to perform efficacious plane-wave density functional calculations of extended systems. In particular, we compare results of water adsorption on UO{sub 2} (111) with the corresponding results on CeO{sub 2}, focusing on the energetics and structural properties of molecular versus dissociative adsorption, for both partially and fully covered surfaces. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2025

Toward an Understanding of Linear Scaling Relations through Energy Decomposition Analysis

The discovery of linear scaling relations has fundamentally changed the field of heterogeneous catalysis. The scaling relations have been rationalized based on the d-band theory, specifically a separation of sp and d electron contributions to adsorption energies. Within the framework of energy decomposition analysis, a full understanding of such a separation would require one to further break down the adsorption energy into distinct energy components such as electrostatics, polarization, charge transfer, and van der Waals interactions, and to examine the sp and d contributions to each of them. As a step in this direction, we analyzed the interaction energy between CH x (x = 1–4) adsorbates and fcc(100) transition metal surfaces (M = Cu, Ag, Au, Rh, and Pt), with the surfaces represented both as slabs in plane-wave density functional theory (pw-DFT) calculations and as atomic clusters in atomic-orbital basis density functional theory (ao-DFT) calculations. Through an absolutely localized molecular orbital (ALMO) based energy decomposition analysis of the ao-DFT adsorption energy, each of the interaction energy components (electrostatics, polarization, van der Waals, and charge transfer) was found to follow its own scaling relations, with an intricate interplay among these energy components yielding the overall scaling relations for the total adsorption energies. Using the recently introduced ALMO-based polarization and charge-transfer analysis schemes, we further dissected polarization into metal surface and adsorbate contributions, and charge transfer into metal → adsorbate and adsorbate → metal contributions. The contributions from the sp and d electrons of the metal to these terms were further quantified, and the dominant role of the metal d electrons was reaffirmed. These results shed light on how CHx adsorbates interact with metal surfaces and further reveal the physical origin of the scaling relations.

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

Investigation of mode activity in NBI-heated experiments of Wendelstein 7-X

The 2018 operation phase (OP 1.2b) of the stellarator Wendelstein 7-X (W7-X) included, for the first time, neutral beam injection (NBI) to heat the plasma. Since the injection geometry at W7-X is not parallel, this generates both passing and trapped fast particles. During longer phases of NBI injection, with the primary purpose to study the heating efficiency of this system, Alfvén eigenmodes (AEs) were observed by a number of diagnostics, including the phase contrast imaging (PCI) system, the magnetic pick-up coils (Mirnov coils), and the soft x-ray multi-camera tomography system (XMCTS). Alfvén eigenmodes are of great interest for future fusion reactors as it has been shown that the resonant interaction of fast ions with self-excited AEs can lead to enhanced transport of fast ions and potentially to energy losses. This is especially true for so-called gap-modes, Alfvén eigenmodes with frequencies in gaps of the continuous spectrum, since they lack continuum damping. These modes are commonly known to be excited by fast ions, but other destabilizing mechanisms, e.g. the electron-pressure gradient are also possible. In this article we present a first analysis of the experimentally observed frequencies from the theoretical side. The calculation of shear Alfvén wave continua for selected cases and the assignment of observed frequencies to the gaps of the continuous spectra are presented. Using the ideal-MHD code CKA (Könies A. 2007 10th IAEA TM on Energetic Particles in Magnetic Confinement System), we find gap modes that match the experimental measurements in terms of the observed frequencies. We emphasize the crucial roles played by the coupling of sound and Alfvén waves as well as of the Doppler shift arising as a consequence of the radial electric field in W7-X. We employ the perturbative gyrokinetic code CKA-EUTERPE (Feh´er 2013 Simulation of the interaction between Alfv´en waves and fast particles), using a slowing-down distribution function for the fast ions as calculated by the Monte-Carlo particle following code ASCOT (Hirvijoki et al 2014 Comput. Phys. Commun. 185 1310–21) to assess the fast-ion drive. Here we find that the fast-ion drive is insufficient to overcome the background-plasma damping. The fact that unstable modes were observed experimentally may point to problems with the modelling or indicate the existence of other destabilizing mechanisms, e.g. associated with the electron-pressure gradient (Windisch et al 2017 Plasma Phys. Control. Fusion 59 105002) that sensitively depend on the profiles of the background plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Magneto-electronic and Thermodynamic Properties of Quaternary NiFeMnZ (Z = In, Sn) New Spin Gapless Semiconductors

This paper deals with investigation of magneto-electronic and thermodynamic properties of new spin gapless semiconductor quaternary NiFeMnZ (Z = In, Sn) Heusler alloys. In addition, electronic properties such as band structure and density of states were studied in detail. The obtained results are based on first-principles calculations, using the full potential linearized augmented plane wave (FP-LAPW) method within well-known generalized gradient approximation (GGA) coupling with spin-orbit calculation (SOC) in framework of the density functional theory (DFT) implemented in the WIEN2k package. As results, the alloys were found to be new spin gapless semiconductors (SGSs), and both compounds in LiMgPdSn-type crystal structure are stable. The magnetic moment of NiFeMnIn and NiFeMnSn was 4.39 and 4.99 μ{sub B} per formula unit, respectively. This result is in agreement with the Slater-Pauling rule (M{sub tot} = 24 −Z{sub tot}). Furthermore, the thermodynamic properties were also predicted. All these multifunctionalities lead to crucial interest for developing future spintronics device generations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Maser radiation from collisionless shocks: application to astrophysical jets

This paper describes a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetized collisionless shocks. It is motivated by the work of Begelman, Ergun and Rees [Astrophys. J. 625 , 51 (2005)] who argued that the cyclotron-maser instability occurs in localized magnetized collisionless shocks such as those expected in blazar jets. We report on recent research carried out to investigate electron acceleration at collisionless shocks and maser radiation associated with the accelerated electrons. We describe how electrons accelerated by lower-hybrid waves at collisionless shocks generate cyclotron-maser radiation when the accelerated electrons move into regions of stronger magnetic fields. The electrons are accelerated along the magnetic field and magnetically compressed leading to the formation of an electron velocity distribution having a horseshoe shape due to conservation of the electron magnetic moment. Under certain conditions the horseshoe electron velocity distribution function is unstable to the cyclotron-maser instability [Bingham and Cairns, Phys. Plasmas 7 , 3089 (2000); Melrose, Rev. Mod. Plasma Phys. 1 , 5 (2017)].

Optics↗
DOE OSTI2020

Ionization wave propagation in a He plasma jet in a controlled gas environment

Characterizing ionization wave propagation in low temperature plasma jets is critical to predicting production of reactive species and plasma–surface interactions for biomedical applications and surface functionalization. In this work, results from optical emission and laser induced fluorescence measurements of the ionization wave in a He plasma jet operating in a controlled gas environment are discussed and used for comparison with numerical modeling. Additionally, the ionization wave was observed using ICCD (Intensified Charge Coupled Device) imaging and characterized by time and spatially resolved electron density measurements using laser-collision-induced fluorescence. The plasma jet was initially characterized using pure He (nominally at 200 Torr), while varying pressure and voltage. When operating in pure He, the ionization wave broadly expands exiting the plasma tube. Increasing the operating pressure reduces the speed and isotropic expansion of the ionization wave. Furthermore, the jet operated with a humid He shroud was also studied. The humid He shroud results in the electron density increasing and having an annular profile due to the lower ionization potential of H2O compared to He and localized photoionization in the mixing region. Numerical modeling highlighted the importance of resonance radiation emitted by excited states of He, photoelectron emission from the quartz tube, and the kinetic behavior of the electrons produced by photoionization ahead of the ionization front.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Simulation of adiabatic quantum computing for molecular ground states

Quantum computation promises to provide substantial speedups in many practical applications with a particularly exciting one being the simulation of quantum many-body systems. Adiabatic state preparation (ASP) is one way that quantum computers could recreate and simulate the ground state of a physical system. In this paper, we explore a novel approach for classically simulating the time dynamics of ASP with high accuracy and with only modest computational resources via an adaptive sampling configuration interaction scheme for truncating the Hilbert space to only the most important determinants. We verify that this truncation introduces negligible error and use this new approach to simulate ASP for sets of small molecular systems and Hubbard models. Furthermore, we examine two approaches to speeding up ASP when performed on quantum hardware: (i) using the complete active space configuration interaction (CASCI) wave function instead of the Hartree–Fock initial state and (ii) a nonlinear interpolation between the initial and target Hamiltonians. We find that starting with a CASCI wave function with a limited active space yields substantial speedups for many of the systems examined, while nonlinear interpolation does not. In additional, we observe interesting trends in the minimum gap location (based on the initial state) as well as how state preparation time can depend on certain molecular properties, such as the number of valence electrons. Importantly, we find that the required state preparation times do not show an immediate exponential wall that would preclude an efficient run of ASP on actual hardware.

Kremenetski, Vladimir↗
DOE OSTI2023

Majorana bound states in a d -wave superconductor planar Josephson junction

We study phase-controlled planar Josephson junctions comprising a two-dimensional electron gas with strong spin-orbit coupling and d-wave superconductors, which have an advantage of a high critical temperature. We show that a region between the two superconductors can be tuned into a topological state by the in-plane Zeeman field, and can host Majorana bound states. The phase diagram as a function of the Zeeman field, chemical potential, and the phase difference between superconductors exhibits the appearance of Majorana bound states for a wide range of parameters. We further investigate the behavior of the topological gap and its dependence on the type of d-wave pairing, i.e., d, d+is, or d+id', and note the difficulties that can arise due to the presence of gapless excitations in pure d-wave superconductors. On the other hand, the planar Josephson junctions based on superconductors with d+is and d+id' pairings can potentially lead to realizations of Majorana bound states. Furthermore, our proposal can be realized in cuprate superconductors, e.g., in a twisted bilayer, combined with the layered semiconductor Bi 2 O 2 Se.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Electrostatic fluctuations in collisional plasmas

Here, we present a theory of electrostatic fluctuations in two-component plasmas where electrons and ions are described by Maxwellian distribution functions at unequal temperatures. Based on the exact solution of the Landau kinetic equation, that includes electron-electron, electron-ion, and ion-ion collision integrals, the dynamic form factor, S( → k,ω), is derived for weakly coupled plasmas. The collective plasma responses at ion-acoustic, Langmuir, and entropy mode resonances are described for arbitrary wave numbers and frequencies in the entire range of plasma collisionality. The collisionless limit of S( → k,ω) and the strong-collision result based on the fluctuation-dissipation theorem and classical transport at T e = T i are recovered and discussed. Results of several Thomson scattering experiments in the broad range of plasma parameters are described and discussed by means of our theory for S( → k,ω).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Dirac Fermion Hierarchy of Composite Fermi Liquids

Composite Fermi liquids (CFLs) are compressible states that can occur for 2D interacting fermions confined in the lowest Landau level at certain Landau level fillings. They have been understood as Fermi seas formed by composite fermions which are bound states of electromagnetic fluxes and electrons as reported by Halperin, Lee, and Read [Phys. Rev. B 47, 7312 (1993)]. At half filling, an explicitly particle-hole symmetric theory based on Dirac fermions was proposed by Son [Phys. Rev. X 5, 031027 (2015)] as an alternative low energy description. In this work, we investigate the Berry curvature of CFL model wave functions at a filling fraction of one-quarter, and observe that it is uniformly distributed over the Fermi sea except at the center where an additional π phase was found. Motivated by this, we propose an effective theory which generalizes Son’s half filling theory, by internal gauge flux attachment, to all filling fractions in which fermionic CFLs can occur. As a result, the numerical results support the idea of internal gauge flux attachment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Nonadiabatic effects in (1~2){sup 1}Π–X{sup 1}Σ{sup +} rovibronic transitions of KRb molecules

Numerical simulation of the effect of intramolecular electrostatic interactions on redistribution of relative intensities in the vibrational structure of (1 ~ 2){sup 1}Π–X{sup 1}Σ{sup +} rotationally resolved transitions of the KRb molecule is performed within the precision nonadiabatic model of coupled vibrational channels. It is established that mutual perturbation of electronically excited states modifies in a nontrivial way a nodal structure of nonadiabatic wavefunction of the (1 ~ 2){sup 1}Π complex, which is possible to use for rising efficiency of twostep laser synthesis and stabilization of ultracold ensembles of KRb molecules in the ground electronic state.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Effect of Impurities on the Oxygen Adsorption Properties on the NiTi(110) Surface

The effect of 3d–5d elements on the oxygen adsorption energy on the NiTi(110) surface has been studied by the projector augmented-waves method within density functional theory. It is shown that almost all elements, except for a few elements of the end of d periods, lead to an increase in the adsorption energy if they substitute for nickel. On the contrary, the substitutional impurities in the titanium sublattice lower this energy. Based on the analysis of the electronic characteristics of the surface with impurities, it has been found that an increase/decrease in the oxygen adsorption energy on NiTi(110) correlates with a change in the ionic contribution to the mechanism of oxygen bonding with the surface.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Symmetries of a reduced fluid-gyrokinetic system

Symmetries of a fluid-gyrokinetic model are investigated using Lie group techniques. Specifically the nonlinear system constructed by Zocco and Schekochihin (Zocco & Schekochihin 2011), which combines nonlinear fluid equations with a drift-kinetic description of parallel electron dynamics, is studied. Significantly, this model is fully gyrokinetic, allowing for arbitrary $k_{\bot \rho I}$ where $k_\bot$ is the perpendicular wave vector of the fluctuations and $\rho I}$ is the ion gyroradius. The model includes integral operators corresponding to gyroaveraging as well as the moment equations relating fluid variables to the kinetic distribution function. A large variety of exact symmetries is uncovered, some of which have unexpected form. Using these results, new nonlinear solutions are constructed, including a helical generalization of the Chapman-Kendall solution for a collapsing current sheet.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Nitrogen-Doped Graphene Oxide Electrocatalysts for the Oxygen Reduction Reaction

Platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) often exhibit a complex functionalized graphitic structure. Because of this complex structure, limited understanding exists about the design factors for the synthesis of high-performing materials. Graphene, a two-dimensional hexagonal structure of carbon, is amenable to structural and functional group modifications, making it an ideal analogue to study crucial properties of more complex graphitic materials utilized as electrocatalysts. In this paper, we report the synthesis of active nitrogen-doped graphene oxide catalysts for the ORR in which their activity and four-electron selectivity are enhanced using simple solvent and electrochemical treatments. The solvents, chosen based on Hansen’s solubility parameters, drive a substantial change in the morphology of the functionalized graphene materials by (i) forming microporous holes in the graphitic sheets that lead to edge defects and (ii) inducing 3D structure in the graphitic sheets that promotes ORR. Additionally, the cycling of these catalysts has highlighted the multiplicity of the active sites, with different durability, leading to a highly selective catalyst over time, with a minimal loss in performance. High ORR activity was demonstrated in an alkaline electrolyte with an onset potential of ~1.1 V and half-wave potential of 0.84 V vs RHE. Furthermore, long-term stability potential cycling showed minimal loss in half-wave potential (<3%) in both N 2 - and O 2 -saturated solutions with improved selectivity toward the four-electron reduction after 10000 cycles. Lastly, the results described in this work provide additional understanding about graphitic electrocatalysts in alkaline media that may be utilized to further enhance the performance of PGM-free ORR electrocatalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Structural, Electronic, Magnetic, and Optical Properties of Mn{sub 2}ZrGa{sub 1−x}Ge{sub x} Heusler Alloys: First-Principles Calculations

The structural, electronic, magnetic, and optical properties of Mn-based Heusler Mn{sub 2}ZrGa{sub 1−x}Ge{sub x} (x = 0, 0.25, 0.5, 0.75, and 1) alloys are calculated by first-principles full-potential linearized augmented plane wave plus local orbital (FP-LAPW+lo) method. It is found that Mn{sub 2}ZrGa{sub 1−x}Ge{sub x} alloys are half-metallic ferrimagnetic and their magnetic moments are in good agreement with the Slater–Pauling relationship. The calculations reveal that all alloys have a pseudo-gap in the majority-spin channel except for x = 0.75 and x = 1 which have a real gap in the majority-spin channel. The spin polarization is quite large for whole series, and it reaches to a value of 100% for x = 0.75 and x = 1. Our calculated results clearly show that with the Ge substitution, the lattice parameter linearly decreases while the bulk modulus increases. Moreover, the calculated formation energies confirm that these alloys are stable chemically. The optical properties, including the real and imaginary parts of the dielectric function, reflectivity, and absorption spectra, were also investigated. The results show that the dominant behavior, at energy below 1 eV, is due to interactions of free electrons in the system. The increment of Ge concentration leads to an increase in the plasma frequency of metallic spin channel.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Tailoring the Weight of Surface and Intralayer Edge States to Control LUMO Energies

Abstract The energies of the frontier molecular orbitals determine the optoelectronic properties in organic films, which are crucial for their application, and strongly depend on the morphology and supramolecular structure. The impact of the latter two properties on the electronic energy levels relies primarily on nearest‐neighbor interactions, which are difficult to study due to their nanoscale nature and heterogeneity. Here, an automated method is presented for fabricating thin films with a tailored ratio of surface to bulk sites and a controlled extension of domain edges, both of which are used to control nearest‐neighbor interactions. This method uses a Langmuir–Schaefer‐type rolling transfer of Langmuir layers (rtLL) to minimize flow during the deposition of rigid Langmuir layers composed of π‐conjugated molecules. Using UV–vis absorption spectroscopy, atomic force microscopy, and transmission electron microscopy, it is shown that the rtLL method advances the deposition of multi‐Langmuir layers and enables the production of films with defined morphology. The variation in nearest‐neighbor interactions is thus achieved and the resulting systematically tuned lowest unoccupied molecular orbital (LUMO) energies (determined via square‐wave voltammetry) enable the establishment of a model that functionally relates the LUMO energies to a morphological descriptor, allowing for the prediction of the range of accessible LUMO energies.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electronic Structure and Related Band Parameters of Hexagonal Wurtzite Zn{sub 1−x}Mn{sub x}O Magnetic Semiconducting Alloys

The electronic band structure and related band parameters such as, lattice parameters, energy band gaps, density of states and magnetic moment of Zn{sub 1−x}Mn{sub x}O ternary system in the hypothetical hexagonal wurtzite structure have been investigated. The calculations are performed using a full-potential linearized augmented plane wave method based on density functional theory in the generalized gradient approximation of Perdew-Burke-Ernzerhof. The evolution of the features of interest with increasing the Mn doping concentration from 0 to 1 in the ZnO structure has been examined and discussed. Our findings show that the behaviour of all studied band parameters with respect to the alloy composition x is consistent with those reported experimentally in the literature. The present study may be useful for spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Spin excitations and dynamics in 2D magnets: An overview of magnons and magnetic skyrmions

van der Waals magnetic materials open up exciting possibilities to investigate fundamental spin properties in low-dimensional systems and to build compact functional spintronic structures. This review focuses on the recent progress in two-dimensional (2D) magnets that explore beyond the homogenous magnetically ordered state, including magnons (spin waves), magnetic skyrmions, and complex magnetic domains. Properties of these spin and topology excitations in 2D magnets provide insights into spin-orbit interactions and other forms of coupling between electrons, phonons, and spin-dependent excitations. Such spin-based quasiparticles can also serve as information carriers for next-generation high-speed computing elements. Furthermore, we will first lay out the general theoretical basis of dynamical responses in magnetic systems, followed by detailed descriptions of experimental progress in magnons and spin textures (including magnetic domains and skyrmions). Discussion on the experimental techniques and future perspectives are also included.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗