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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 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Resistivity Minimum in Highly Frustrated Itinerant Magnets

We study here the transport properties of frustrated itinerant magnets comprising localized classical moments, which interact via exchange with the conduction electrons. Strong frustration stabilizes a liquidlike spin state, which extends down to temperatures well below the effective Ruderman-Kittel-Kasuya-Yosida interaction scale. The crossover into this state is characterized by spin structure factor enhancement at wave vectors smaller than twice the Fermi wave vector magnitude. Finally, the corresponding enhancement of electron scattering generates a resistivity upturn at decreasing temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Organic doping of rotated double layer graphene

Charge transfer techniques have been extensively used as knobs to tune electronic properties of two- dimensional systems, such as, for the modulation of conductivity \ mobility of single layer graphene and for opening the bandgap in bilayer graphene. The charge injected into the graphene layer shifts the Fermi level away from the minimum density of states point (Dirac point). In this work, we study charge transfer in rotated double-layer graphene achieved by the use of organic dopant, Tetracyanoquinodimethane. Naturally occurring bilayer graphene has a well-defined A-B stacking whereas in rotated double-layer the two graphene layers are randomly stacked with different rotational angles. This rotation is expected to significantly alter the interlayer interaction. Double-layer samples are prepared using layer-by-layer assembly of chemical vapor deposited single-layer graphene and they are identified by characteristic resonance in the Raman spectrum. The charge transfer and distribution of charges between the two graphene layers is studied using Raman spectroscopy and the results are compared with that for single-layer and A-B stacked bilayer graphene doped under identical conditions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS2012

FERMI Observations of Gamma -Ray Emission From the Moon

We report on the detection of high-energy γ -ray emission from the Moon during the first 24 months of observations by the Fermi Large Area Telescope (LAT). This emission comes from particle cascades produced by cosmicray (CR) nuclei and electrons interacting with the lunar surface. The differential spectrum of the Moon is soft and can be described as a log-parabolic function with an effective cutoff at 2-3 GeV, while the average integral flux measured with the LAT from the beginning of observations in 2008 August to the end of 2010 August is F(greater than100 MeV) = (1.04 plus or minus 0.01 [statistical error] plus or minus 0.1 [systematic error]) × 10(sup −6) cm(sup −2) s(sup −1). This flux is about a factor 2-3 higher than that observed between 1991 and 1994 by the EGRET experiment on board the Compton Gamma Ray Observatory, F(greater than100 MeV)≈5×10(sup −7) cm(sup −2) s(sup −1), when solar activity was relatively high. The higher gamma -ray flux measured by Fermi is consistent with the deep solar minimum conditions during the first 24 months of the mission, which reduced effects of heliospheric modulation, and thus increased the heliospheric flux of Galactic CRs. A detailed comparison of the light curve with McMurdo Neutron Monitor rates suggests a correlation of the trends. The Moon and the Sun are so far the only known bright emitters of gamma-rays with fast celestial motion. Their paths across the sky are projected onto the Galactic center and high Galactic latitudes as well as onto other areas crowded with high-energy gamma-ray sources. Analysis of the lunar and solar emission may thus be important for studies of weak and transient sources near the ecliptic.

fermi↗
DOE OSTI2026

Probing the γ -Ray Emission Origin of Two Star-forming Galaxies NGC 2403 and NGC 3424 with the Fermi-LAT

Star-forming galaxies (SFGs) are a subclass of γ-ray emitters, and a correlation between their γ-ray luminosity (L γ ) and the total infrared (IR) luminosity (L IR ) has been established based on the Fermi Large Area Telescope (LAT) data. NGC 2403 and NGC 3424 have been reported as outliers in the L γ –L IR correlation with light curves showing significant variability, which contrasts with the temporally stable γ-ray emission in other SFGs, originating primarily from cosmic rays interacting with interstellar medium. In this study, we reanalyze the γ-ray emission in the directions of NGC 2403 and NGC 3424 using more than 16.5 yr Fermi-LAT data. NGC 3424 is found to be spatially coincident with the detected γ-ray source, while NGC 2403 is significantly offset from the nearest γ-ray source, suggesting an implausible association. We confirm the previously reported variability of both γ-ray sources and the significant deviation from the L γ –L IR correlation when assuming an association of both γ-ray sources with the two galaxies. Our findings lend further support to the interpretation that their γ-ray emission is driven primarily by alternative radiative processes—rather than by star formation activity—such as the ejecta of the Type IIP supernova SN 2004dj in NGC 2403 interacting with a surrounding high-density shell and an obscured active galactic nucleus in NGC 3424.

Liu, Linjie [Chinese Academy of Sciences (CAS), Ku↗
DOE OSTI2011

Carbon nanotunnels form from single-walled carbon nanotubes interacting with a diamond (100)-(2 X 1) surface.

A quantum chemical study of the interaction of (5,5), (7,7), (9,9) and (8,0) single-walled carbon nanotubes with a clean (100)-(2 x 1) diamond surface is reported. Stable structures with covalent bonds at the interface were found for carbon nanotubes oriented parallel or perpendicular to the dimer rows on the reconstructed (100) surface. The binding energy of the most stable (5,5) nanotube-diamond structure is 1.7 eV/{angstrom}, and is attributed to strong covalent bonds formed between the carbon nanotube and the diamond surface. The structure of the nanotube is distorted by adsorption on the surface such that it adopts a tunnel-like geometry. Two other nanotunnel geometries were found for the (5,5) nanotube, with binding energies of 1.39 and 1.09 eV/{angstrom}. In the most stable (5,5) nanotube-diamond structure the interaction between the nanotube and the diamond surface produces a 0.6 eV band gap near the Fermi level, but the metallic character of the nanotube is maintained in the two other, less strongly bound nanotunnel structures. No charge transfer occurs between the diamond surface and the nanotunnels in any of the three orientations. Binding energies decrease with increases in tube diameter, to the extent that one of the three nanotunnel structures is not formed by (9,9) carbon nanotubes.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Tuning electrocatalytic activity of Pt monolayer shell by bimetallic Ir-M (M=Fe, Co, Ni or Cu) cores for the oxygen reduction reaction

Here, platinum monolayer electrocatalyst are known to exhibit excellent oxygen reduction reaction (ORR) activity depending on the type of substrate used. Here we demonstrate a relationship between the ORR electrocatalytic activity and the surface electronic structure of Pt monolayer shell induced by various IrM bimetallic cores (M=Fe, Co, Ni or Cu). The relationship is rationalized by comparing density functional theory calculations and experimental results. For an efficient Pt monolayer electrocatalyst, the core should induce sufficient contraction to the Pt shell leading to a downshift of the d-band center with respect to the Fermi level. Depending on the structure of the IrM, relative to that of pure Ir, this interaction not only alters the electronic and geometric structure but also induces segregation effects. Combined these effects significantly enhance the ORR activities of the Pt monolayer shell on bimetallic Ir cores electrocatalysts.

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

Magic-angle semimetals

Abstract Breakthroughs in two-dimensional van der Waals heterostructures have revealed that twisting creates a moiré pattern that quenches the kinetic energy of electrons, allowing for exotic many-body states. We show that cold atomic, trapped ion, and metamaterial systems can emulate the effects of a twist in many models from one to three dimensions. Further, we demonstrate at larger angles (and argue at smaller angles) that by considering incommensurate effects, the magic-angle effect becomes a single-particle quantum phase transition (including in a model for twisted bilayer graphene in the chiral limit). We call these models “magic-angle semimetals”. Each contains nodes in the band structure and an incommensurate modulation. At magic-angle criticality, we report a nonanalytic density of states, flat bands, multifractal wave functions that Anderson delocalize in momentum space, and an essentially divergent effective interaction scale. As a particular example, we discuss how to observe this effect in an ultracold Fermi gas.

Materials Science↗
DOE OSTI2018

Dichotomy between frustrated local spins and conjugated electrons in a two-dimensional metal–organic framework

Dichotomy between local spins and conjugated electrons spawns various exotic physical phenomena, however, it has mostly been reported in three-dimensional (3D) inorganic systems. We demonstrate, for the first time, that a rare 2D metal–organic framework exhibits intriguing dichotomy behavior, which can be directly identified through scanning tunneling microscopy/spectroscopy (STM/STS). In a newly synthesized Cu-hexaiminobenzene [Cu 3 (HAB) 2 ], on the one hand, the Cu 2+ ions form an ideal S –1/2 antiferromagnetic (AFM) kagome lattice; on the other hand, the conjugated-electrons from the organic ligands produce a frustrated π x , y model on a honeycomb lattice, giving rise to completely dispersionless energy bands around the Fermi level that favour the ferromagnetic (FM) state. Remarkably, the frustrated local spins and conjugated electrons interact through a strong FM Hund's coupling, giving rise to a wide range of intriguing quantum phases. Furthermore, we propose that this dichotomy can be directly characterized through STM/STS measurements due to its special 2D nature, which provides a unique exciting platform to investigate the dichotomy of frustrated spins and electrons in a single lattice.

42 ENGINEERING↗
DOE OSTI2013

Analysis of charged-current neutrino-nucleus cross section

A study of the cross section for chaged-current quasielastic (CCQE) scattering on nuclei has been performed using a description of nuclear dynamics based on the Relativistic Fermi Gas model (RFG). The role played by different parametrizations for the weak nucleon form factors is analyzed taking into account the relevance of the axial mass value. The results obtained are compared with the recent data for neutrinos measured by the MiniBooNE Collaboration.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion quantum Monte Carlo

We review recent advances in the capabilities of the open source ab initio Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for greater efficiency and reproducibility. The auxiliary field QMC (AFQMC) implementation has been greatly expanded to include k-point symmetries, tensor-hypercontraction, and accelerated graphical processing unit (GPU) support. These scaling and memory reductions greatly increase the number of orbitals that can practically be included in AFQMC calculations, increasing the accuracy. Advances in real space methods include techniques for accurate computation of bandgaps and for systematically improving the nodal surface of ground state wavefunctions. Results of these calculations can be used to validate application of more approximate electronic structure methods, including GW and density functional based techniques. To provide an improved foundation for these calculations, we utilize a new set of correlation-consistent effective core potentials (pseudopotentials) that are more accurate than previous sets; these can also be applied in quantum-chemical and other many-body applications, not only QMC. These advances increase the efficiency, accuracy, and range of properties that can be studied in both molecules and materials with QMC and QMCPACK.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Fermionic quantum processing with programmable neutral atom arrays

Simulating the properties of many-body fermionic systems is an outstanding computational challenge relevant to material science, quantum chemistry, and particle physics.-5.4pc]Please note that the spelling of the following author names in the manuscript differs from the spelling provided in the article metadata: D. González-Cuadra, D. Bluvstein, M. Kalinowski, R. Kaubruegger, N. Maskara, P. Naldesi, T. V. Zache, A. M. Kaufman, M. D. Lukin, H. Pichler, B. Vermersch, Jun Ye, and P. Zoller. The spelling provided in the manuscript has been retained; please confirm. Although qubit-based quantum computers can potentially tackle this problem more efficiently than classical devices, encoding nonlocal fermionic statistics introduces an overhead in the required resources, limiting their applicability on near-term architectures. In this work, we present a fermionic quantum processor, where fermionic models are locally encoded in a fermionic register and simulated in a hardware-efficient manner using fermionic gates. We consider in particular fermionic atoms in programmable tweezer arrays and develop different protocols to implement nonlocal gates, guaranteeing Fermi statistics at the hardware level. We use this gate set, together with Rydberg-mediated interaction gates, to find efficient circuit decompositions for digital and variational quantum simulation algorithms, illustrated here for molecular energy estimation. Finally, we consider a combined fermion-qubit architecture, where both the motional and internal degrees of freedom of the atoms are harnessed to efficiently implement quantum phase estimation as well as to simulate lattice gauge theory dynamics.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Superconductivity in the presence of spin-orbit interactions stabilized by Hund coupling

The intraorbital repulsive Hubbard interaction cannot lead to attractive superconducting pairing states, except through the Kohn-Luttinger mechanism. This situation may change when we include additional local interactions such as the interorbital repulsion U' and Hund interactions J . Adding these local interactions, we study the nature of the superconducting pairs in systems with tetragonal crystal symmetry including the d xz and d yz orbitals, and in octahedral systems including all three of d xz , d yz , and d xy orbitals. In the tetragonal case, spin-orbit interactions can stabilize attractive pairing channels containing a spin triplet, orbital singlet character. Here, depending on the form of spin-orbit coupling, pairing channels belonging to degenerate, nontrivial irreducible representations may be stabilized. In the octahedral case, the pairing interactions of superconducting channels are found to depend critically on the number of bands crossing the Fermi energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Global fit of electron and neutrino elastic scattering data to determine the strange quark contribution to the vector and axial form factors of the nucleon

We present a global fit of neutral-current elastic (NCE) neutrino-scattering data and parity-violating electron-scattering (PVES) data with the goal of determining the strange quark contribution to the vector and axial form factors of the proton. Previous fits of this form included data from a variety of PVES experiments (PVA4, HAPPEx, G0, SAMPLE) and the NCE neutrino and anti-neutrino data from BNL E734. These fits did not constrain the strangeness contribution to the axial form factor G A s ( Q 2 ) at low Q 2 very well because there was no NCE data for Q 2 < 0.45 GeV 2 . Our new fit includes for the first time MiniBooNE NCE data from both neutrino and antineutrino scattering; this experiment used a hydrocarbon target and so a model of the neutrino interaction with the carbon nucleus was required. Three different nuclear models have been employed: a relativistic Fermi gas model, the superscaling approximation model, and a spectral function model. We find a tremendous improvement in the constraint of G A s ( Q 2 ) at low Q 2 compared to previous work, although more data is needed from NCE measurements that focus on exclusive single-proton final states, for example from MicroBooNE. Published by the American Physical Society 2024

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Magic Gap Ratio for Optimally Robust Fermionic Condensation and Its Implications for High- $T_c$ Superconductivity

Bardeen-Schrieffer-Cooper (BCS) and Bose-Einstein condensation (BEC) occur at opposite limits of a continuum of pairing interaction strength between fermions. A crossover between these limits is readily observed in a cold atomic Fermi gas. Whether it occurs in other systems such as the high temperature superconducting cuprates has remained an open question. We uncover here unambiguous evidence for a BCS-BEC crossover in the cuprates by identifying a universal magic gap ratio 2Δ/k B T c ≈ 6.5 (where Δ is the pairing gap and T c is the transition temperature) at which paired fermion condensates become optimally robust. At this gap ratio, corresponding to the unitary point in a cold atomic Fermi gas, the measured condensate fraction N0 and the height of the jump δγ(T c ) in the coefficient γ of the fermionic specific heat at T c are strongly peaked. In the cuprates, δγ(T c ) is peaked at this gap ratio when Δ corresponds to the antinodal spectroscopic gap, thus reinforcing its interpretation as the pairing gap. We find the peak in δγ(T c ) also to coincide with a normal state maximum in γ, which is indicative of a pairing fluctuation pseudogap above T c .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2002

Atomic Data and Spectral Line Intensities for Ne III

A number of satellites and rockets have been launched to observe radiation from the Sun and other astrophysical objects. Line radiation is emitted when the electron impact excited levels decay to the lower levels by photon emission. From this radiation, the physical parameters such as electron temperature and density of the astrophysical plasma, elemental abundance, and opacity can be inferred. Ne III lines have been observed in H II regions, Ne-rich filaments in supernovae, and planetary nebulae. The allowed line at 489.50 Angstroms due to the transition 2s(sup 2) 2p(sup 5) (sup 3) P2 (goes to) 2s(sup 2)2p(sup 4)(sup 3)P2 has been identified in the solar spectrum by Vernazza and Reeves using Skylab observations. Other Ne III lines in the solar EUV spectrum have been reported by Thomas and Neupert based on observations from the Solar EUV Rocket Telescope and Spectrograph (SERTS) instrument. Atomic data for Ne III have been calculated by using a set of programs developed at, University College, London. The Superstructure and Distorted Wave (DW) programs have been updated over the years. In the Superstructure program, configuration interaction can be taken into account and radial functions are calculated in a modified Thomas-Fermi-Amaldi potential. This is a statistical potential and depends on parameters lambda 1 which are determined by optimizing the weighted sum of term energies. They are found to be lambda(sub 0)=1.2467, lambda(sub 1)=1.1617, and lambda(sub 2)=1.0663. The relativistic corrections are included by using the Breit-Pauli Hamiltonian as a perturbation to the nonrelativistic Hamiltonian. The same potential is used to calculate reactance matrices in the DW approximation in LS coupling. Collision strengths in intermediate coupling are obtained by using term coupling coefficients obtained from the Superstructure program. In this calculation, the configurations used are 2s(sup 2)2p(sup 4), 2s2p(sup 5), 2s(sup 2)2p(sup 3)3s, 2s(sup 2)p(sup 3)3d giving rise to 57 fine-structure levels in intermediate coupling.

Bhatia, A. K.↗
DOE OSTI2013

INVERSE-COMPTON CONTRIBUTION TO THE STAR-FORMING EXTRAGALACTIC GAMMA-RAY BACKGROUND

Fermi has resolved several star-forming galaxies, but the vast majority of the star-forming universe is unresolved, and thus contributes to the extragalactic gamma-ray background (EGB). Here, we calculate the contribution of star-forming galaxies to the EGB in the Fermi range from 100 MeV to 100 GeV due to inverse-Compton (IC) scattering of the interstellar photon field by cosmic-ray electrons. We first construct one-zone models for individual star-forming galaxies assuming that supernovae power the acceleration of cosmic rays. We develop templates for both normal and starburst galaxies, accounting for differences in the cosmic-ray electron propagation and in the interstellar radiation fields. For both types of star-forming galaxies, the same IC interactions leading to gamma rays also substantially contribute to the energy loss of the high-energy cosmic-ray electrons. Consequently, a galaxy's IC emission is determined by the relative importance of IC losses in the cosmic-ray electron energy budget ({sup p}artial calorimetry{sup )}. We calculate the cosmological contribution of star-forming galaxies to the EGB using our templates and the cosmic star formation rate distribution. For all of our models, we find that the IC EGB contribution is almost an order of magnitude less than the peak of the emission due to cosmic-ray ion interactions (mostly pionic p{sub cr} p{sub ism} {yields} {pi}{sup 0} {yields} {gamma}{gamma}); even at the highest Fermi energies, IC is subdominant. The flatter IC spectrum increases the high-energy signal of the pionic+IC sum, bringing it closer to the EGB spectral index observed by Fermi. Partial calorimetry ensures that the overall IC signal is relatively well constrained, with only uncertainties in the amplitude and spectral shape for plausible model choices. We conclude with a brief discussion on how the pionic spectral feature and other methods can be used to measure the star-forming component of the EGB.

79 ASTRONOMY AND ASTROPHYSICS↗