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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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ATS-6 - Synchronous orbit trapped radiation studies with an electron-proton spectrometer
The paper discusses the University of Minnesota experiment on ATS-6 designed to study the origin and dynamics of high-energy electrons and protons in the outer radiation belt and in the near-earth plasma sheet. The experiment consists of two nearly identical detector assemblies, each of which is a magnetic spectrometer containing four gold-silicon surface barrier detectors. The instrument provides a clean separation between protons and electrons by the combination of pulse height analysis and magnetic deflection.
X2000 power system electronics development
The Europa Orbiter mission, currently baselined for launch in March 2008, is intended to follow up on tantalizing results from the Gulileo spacecraft, data from which suggests that there may be a global Ocean underneath the jovian satellite Europa's water ice crust. Scientific objectives for the mission are to ascertain whether or not Europa has a liquid water ocean, to characterize ice layers beneath the surface, and to find the most scientifically enticing landing sites for future missions.
The Magnetic Electron Ion Spectrometer: A Review of On-Orbit Sensor Performance, Data, Operations, and Science
Measurements from NASA’s Van Allen Probes have transformed our understanding of the dynamics of Earth’s geomagnetically-trapped, charged particle radiation. The Van Allen Probes were equipped with the Magnetic Electron Ion Spectrometers (MagEIS) that measured energetic and relativistic electrons, along with energetic ions, in the radiation belts. Accurate and routine measurement of these particles was of fundamental importance towards achieving the scientific goals of the mission. We provide a comprehensive review of the MagEIS suite’s on-orbit performance, operation, and data products, along with a summary of scientific results. The purpose of this review is to serve as a complement to the MagEIS instrument paper, which was largely completed before flight and thus focused on pre-flight design and performance characteristics. As is the case with all space-borne instrumentation, the anticipated sensor performance was found to be different once on orbit. Our intention is to provide sufficient detail on the MagEIS instruments so that future generations of researchers can understand the subtleties of the sensors, profit from these unique measurements, and continue to unlock the mysteries of the near-Earth space radiation environment.
Hopping frustration-induced flat band and strange metallicity in a kagome metal
The introduction of localized electronic states into a metal can alter its physical properties, for example enabling exotic metal physics including heavy fermion and strange metal behaviour. A common source of localized states in such systems are partially filled 4f and 5f shells because of the inherently compact nature of those orbitals. The interaction of electrons in these orbitals with the conduction sea is well described by the Kondo framework. However, there have also been observations of Kondo-like behaviour in 3d transition metal oxides and in 4d- and 5d-containing van der Waals heterostructures. This calls for a broader consideration of the physical requirements for Kondo systems. Here we show transport and thermodynamic hallmarks of heavy fermion and strange metal behaviour that arise in the kagome metal Ni3In, wherein the source of localized states is destructive interference-induced band flattening of partially filled Ni 3d states. With magnetic field and pressure tuning, here we also find evidence that the system is proximate to quantum criticality, extending the analogy to f-electron Kondo lattices. These observations highlight the role of hopping frustration in metallic systems as a potential source for strong correlations. Additionally, this suggests a lattice-driven approach to realizing correlated metals with non-trivial band topology.
Orbital stability in combined uniform axial and three-dimensional wiggler magnetic fields for free-electron lasers
Zachary Phys. Rev. A 29 (6), 3224 (1984) recently analyzed the instability of relativistic-electron helical trajectories in combined uniform axial and helical wiggler magnetic fields when the radial variation of the wiggler field is taken into account. It is shown here that the type 2 instability comprised of secular terms growing linearly in time, identified by Zachary and earlier by Diament Phys. Rev. A 23 (5), 2537 (1981), is an artifact of simple perturbation theory. A multiple-time-scale perturbation analysis reveals a nonsecular evolution on a slower time scale which accommodates an arbitrary initial perturbation. It is shown that, in the absence of exponential instability, the electron seeks a modified helical orbit more appropriate to its perturbed state and oscillates stably about it. Thus, the perturbed motion is oscillatory but nonsecular, and hence the helical orbits are stable.
Utilizing Essential Symmetry Breaking in Auxiliary-Field Quantum Monte Carlo: Application to the Spin Gaps of the C 36 Fullerene and an Iron Porphyrin Model Complex
We present three distinct examples where phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) can be reliably performed with a single-determinant trial wave function with essential symmetry breaking. Essential symmetry breaking was first introduced by Lee and Head-Gordon [ Phys. Chem. Chem. Phys. 2019, 21, 4763-4778, 10.1039/C8CP07613H]. We utilized essential complex and time-reversal symmetry breaking with ph-AFQMC to compute the triplet–singlet energy gap in the TS12 set. We found statistically better performance of ph-AFQMC with complex-restricted orbitals than with spin-unrestricted orbitals. We then showed the utilization of essential spin symmetry breaking when computing the singlet–triplet gap of a known biradicaloid, C 36 . ph-AFQMC with spin-unrestricted Hartree–Fock (ph-AFQMC+UHF) fails catastrophically even with spin-projection and predicts no biradicaloid character. With approximate Brueckner orbitals obtained from regularized orbital-optimized second-order Møller–Plesset perturbation theory (κ-OOMP2), ph-AFQMC quantitatively captures strong biradicaloid character of C 36 . Lastly, we applied ph-AFQMC to the computation of the quintet–triplet gap in a model iron porphyrin complex where brute-force methods with a small active space fail to capture the triplet ground state. We show unambiguously that neither triplet nor quintet is strongly correlated using UHF, κ-OOMP2, and coupled-cluster with singles and doubles (CCSD) performed on UHF and κ-OOMP2 orbitals. There is no essential symmetry breaking in this problem. By virtue of this, we were able to perform UHF+ph-AFQMC reliably with a cc-pVTZ basis set and predicted a triplet ground state for this model geometry. The largest ph-AFQMC in this work correlated 186 electrons in 956 orbitals. Our work highlights the utility, scalability, and accuracy of ph-AFQMC with a single-determinant trial wave function with essential symmetry breaking for systems mainly dominated by dynamical correlation with little static correlation.
Electronic Structure Correspondence of Singlet-Triplet Scale Separation in Strained Sr2RuO4
At a temperature of roughly 1 K, Sr 2 RuO 4 undergoes a transition from a normal Fermi liquid to a superconducting phase. Even while the former is relatively simple and well understood, the superconducting state has not even been understood after 25 years of study. More recently, it has been found that critical temperatures can be enhanced by the application of uniaxial strain, up to a critical strain, after which it falls off. In this work, we take an “instability” approach and seek divergences in susceptibilities. This provides an unbiased way to distinguish tendencies to competing ground states. We show that in the unstrained compound, the singlet and triplet instabilities of the normal Fermi liquid phase are closely spaced. Under uniaxial strain, electrons residing on all orbitals contributing to the Fermiology become more coherent, while the electrons of the Ru-dxy character become heavier, and the electrons of the Ru d x z , y z characters become lighter. In the process, Im χ(q,ω) increases rapidly around q = ( 0.3 , 0.3 , 0 ) 2 π / a and q = ( 0.5 , 0.25 , 0 ) 2 π / a , while it gets suppressed at all other commensurate vectors, in particular at q = 0, which is essential for spin-triplet superconductivity. We observe that the magnetic anisotropy under strain drops smoothly, which is concomitant with the increment in singlet instability. Thus, the triplet superconducting instability remains the lagging instability of the system, and the singlet instability enhances under strain, leading to a large energy-scale separation between these competing instabilities. However, since this happens even without spin-orbit coupling, we believe it is primarily the enhancement in the spin fluctuation glue around quasi-anti-ferromagnetic vectors that drives the Cooper pairing instead of the magnetic anisotropy. At large strain, an instability to a spin density wave overtakes the superconducting one. The analysis relies on a high-fidelity, ab initio description of the one-particle properties and two-particle susceptibilities, based on the quasiparticle self-consistent GW approximation augmented by dynamical mean field theory. This approach is described and its high fidelity confirmed by comparing to observed one- and two-particle properties.
3D magnetic imaging using electron vortex beam microscopy
Abstract Electron vortex beams are free-electron waves that carry orbital angular momentum. There has been growing theoretical and experimental interest in the use of electron vortex beams as a tool for the investigation of magnetic materials. However, due to the complex wavefront of the propagating waves, a deeper understanding of the interaction of electron vortex beams and the magnetic sample is needed. Here we calculate the magnetic phase shift that an electron vortex beam obtains upon transmitting through a magnetic sample. We show that this magnetic phase shift is influenced by the out-of-plane magnetization, which is a unique characteristic of incident electron vortex beams and is proportional to their orbital angular momentum. Finally, we develop a phase retrieval methodology to retrieve the out-of-plane component of magnetization. Based on our theory, we discuss suitable experimental conditions that would enable this imaging capability for magnetic materials and further extend to non-magnetic chiral materials.
Mechanical Design and Configuration of Penetrations for the Europa Clipper Avionics Vault Structure
The main purpose of the Avionics Vault is to shield radiation sensitive electronics for the Europa Clipper Spacecraft. The vault is a box structure made out of aluminum panels. The panels are roughly 10 mm thick in order to shield the electronics from the orbital total ionizing radiation around Jupiter. The vault requires an electromagnetic interference (EMI) shielding effectiveness (SE) of at least 70 dB in order to mitigate EMI with the spacecraft radar receiver. Overall, the vault accommodates four main types of penetrations: receptacle connectors, pass-through cables, fluid lines, and vent holes. More than 150 cables penetrate the vault panels to connect to electronic boxes inside. Fluid pipes enter and exit the vault to transfer heat to the rest of the spacecraft. Vent holes provide a path for air to escape from the vault during launch. Several novel penetrations designs were created to meet EMI and radiation shielding requirements. Receptacle connectors interface to the vault panels using 1.3 mm thick Ta10W plates. Pass-through cables penetrate the vault using aluminum clamshells after being wrapped with Teflon cushion tape, Kapton tape, and copper tape. Vent hole penetrations consist of a copper mesh for EMI shielding and an aluminum radiation shield bracket to direct air out of the vault during launch. Fluid lines terminate at the vault wall using mechanical fittings that resemble a nut and bolt interface. In addition, most mechanical seams and penetrations utilize EMI gaskets to ensure proper EMI shielding. To reduce risk and confirm that the vault penetration designs were appropriate for EMI shielding, an EMI chamber at the Jet Propulsion Laboratory (JPL) was used to test a mock-up vault panel with multiple variations of all four main types of vault penetrations. This EMI SE test also incorporated different methods for bundling pass-through cables, and a comparison of flange mounted connectors versus jam nut connectors. A low noise preamplifier and a Rohde & Schwarz spectrum analyzer measured E-field levels transmitting through the mock-up vault panel. The results showed a shielding effectiveness of 77 dB for the mock-up vault panel, which exceeds the 70 dB target for Europa Clipper. Both the flange mounted connectors and jam nut connectors exhibited similar EMI SE results at the measured frequencies, and all variations of vault penetrations showed favorable EMI SE levels. Since the flight panels will be much larger and include many more penetrations, there will be testing of the flight vault to confirm its EMI SE is compliant with environmental requirements.
Efficient charge to spin conversion in iridium oxide thin films
Many 5d transition metal oxides have a unique electronic structure, where the density of states near the Fermi level is dominated by only 5d electrons with strong spin–orbit coupling. IrO 2 , a Dirac nodal line semi-metal, is the simplest of these oxides. The presence of 5d electrons and gap opening of Dirac nodal lines via strong spin–orbit coupling allows for the hybridization of the 5d electrons of the oxide with the itinerant d electrons of a ferromagnet, while simultaneously increasing the intrinsic spin Hall effect. We report large charge-to-spin conversion in thin films of this material using spin-torque ferromagnetic resonance experiments. Finally, by independently performing line shape analysis and linewidth modulation experiments, we conclusively determine the spin Hall angle of optimized IrO 2 films to be ∼8 times larger than that of Pt.
Electronic structure of α - RuCl 3 by fixed-node and fixed-phase diffusion Monte Carlo methods
Layered material α-RuCl 3 has caught wide attention due to its possible realization of Kitaev's spin liquid and its electronic structure that involves the interplay of electron-electron correlations and spin-orbit effects. Several DFT+U studies have suggested that both electron-electron correlations and spin-orbit effects are crucial for accurately describing the band gap. This work studies the importance of these two effects using fixed-node and fixed-phase diffusion Monte Carlo calculations both in spin-averaged and explicit spin-orbit formalisms. In the latter, the Slater-Jastrow trial function is constructed from two-component spin orbitals using our recent quantum Monte Carlo (QMC) developments and thoroughly tested effective core potentials. Our results show that the gap in the ideal crystal is already accurately described by the spin-averaged case, with the dominant role being played by the magnetic ground state with significant exchange and electron correlation effects. We find qualitative agreement between hybrid DFT, DFT+U, and QMC. In addition, QMC results agree very well with available experiments, and we identify the values of exact Fock exchange mixing that provide comparable gaps. Explicit spin-orbit QMC calculations reveal that the effect of spin-orbit coupling on the gap is minor, of the order of 0.2 eV, which corresponds to the strength of the spin orbit of the Ru atom.
Theoretical examination of covalency in berkelium(IV) carbonate complexes
Experimental studies on the speciation of berkelium in carbonate media have shown that complexation of berkelium(III) by carbonate results in spontaneous oxidation to berkelium(IV) and that multiple species can be present in solution. We studied two proposed structures present in solution based on theoretical comparisons with spectroscopic data previously reported for Bk(IV) carbonate solutions. The multiconfigurational character of the ground and low-lying excited states in both complexes is demonstrated to result from the strong spin-orbit coupling. Although bonding in Bk(IV) carbonate and carbonate-hydroxide complexes is dominated by strong Coulombic forces, the presence of non-negligible covalent character is supported by ligand-field theory, natural localized orbitals, topological studies of the electron density, and energy transition state natural orbitals for chemical valence. Bond orders based on natural localized molecular orbitals show that Bk-OH bonds possess enhanced orbital overlap, which is reflected in the bond strength. This is also observed in the decomposition of the orbital interaction energy into individual deformation density pairs.
Mechanistic Insights into Photocatalyzed H 2 Dissociation on Au Clusters
Localized surface plasmon resonances (LSPRs) have attracted much recent attention for their potential in promoting chemical reactions with light. However, the mechanism of LSPR-induced chemical reactions is still not clear, even for H 2 dissociation on metal nanoparticles. Here, we investigate the mechanism for photoinduced H 2 dissociation using a simple H 2 @Au 6 model. Our time-dependent density functional theory calculations indicate that the initial excitation is largely restricted to the metal cluster, involving intraband excitation that produces hot electrons (HEs). However, diabatization via overlapping orbitals reveals two types of nested electronic states, one involving excitations of the metallic electrons, namely, the HE states, and the other concerned with charge transfer (CT) to the adsorbate antibonding σ* orbital. Dissociation of H 2 thus takes place by transitions from the former to the latter. Quantum dynamics simulations on the diabatic CT states suggest rapid dissociation of H 2 , while no such dissociation occurs on diabatic HE states. Our research provides a clear physical picture of photoinduced H 2 dissociation on Au clusters, which has important implications in plasmonic facilitated photocatalysis.
Pitch Angle Dependence of Electron and Ion Flux Changes During Local Magnetic Dipolarization Inside Geosynchronous Orbit
In this study we have statistically examined flux changes of 1–1,000-keV electrons, and hydrogen, helium, and oxygen ions during local magnetic dipolarization inside geosynchronous orbit, with a focus on their pitch angle dependence. Here, using 144 dipolarization events that were selected in a previous study with Van Allen Probes observations in 2012–2016, we have performed a superposed epoch analysis of differential flux changes after the dipolarization onset for each species. On average the electron flux increases primarily around pitch angle (α) = 90° at >80 keV and almost isotropically at 10–50 keV. The electron flux at <5 keV increases at α = 0° and 180° but slightly decreases (or remains unchanged) at α = 90°. On the other hand, the ion flux at >80 keV increases around α = 90°, while at <30 keV it decreases nearly independent of pitch angle. Only the low-energy (<5 keV) helium and oxygen ion flux changes indicate a strong field-aligned enhancement, which could be attributed to their outflows from the topside ionosphere. After the dipolarization onset, >5-keV electron and >30-keV ion fluxes exhibit a perpendicular anisotropy, which is most pronounced for 50–200-keV electrons. Both distributions become more isotropic with decreasing energy. A noticeable field-aligned anisotropy is seen for <5-keV ion fluxes. These statistical pitch angle-dependent electron and ion properties during dipolarizations may be explained by combining various processes, including adiabatic acceleration and/or transport, wave-particle interactions, and ion outflow.
X-ray induced electron and ion fragmentation dynamics in IBr
Characterization of the inner-shell decay processes in molecules containing heavy elements is key to understanding x-ray damage of molecules and materials and for medical applications with Auger-electron-emitting radionuclides. The 1s hole states of heavy atoms can be produced by absorption of tunable x rays and the resulting vacancy decays characterized by recording emitted photons, electrons, and ions. The 1s hole states in heavy elements have large x-ray fluorescence yields that transfer the hole to intermediate electron shells that then decay by sequential Auger-electron transitions that increase the ion’s charge state until the final state is reached. In molecules, the charge is spread across the atomic sites, resulting in dissociation to energetic atomic ions. We have used x-ray/ion coincidence spectroscopy to measure charge states and energies of Iq+ and Brq′+ atomic ions following 1s ionization at the I and Br K-edges of IBr. We present the charge states and kinetic energies of the two correlated fragment ions associated with core-excited states produced during the various steps of the cascades. Here, to understand the dynamics leading to the ion data, we develop a computational model that combines Monte-Carlo/Molecular-Dynamics (MC/MD) simulations with a classical over-the-barrier model to track inner-shell cascades and redistribution of electrons in valence orbitals and nuclear motion of fragments.
Molecular orbital studies in oxidation: Sulfate formation and metal-metal oxide adhesion
The chemical mechanisms for sulfate formation from sodium chloride and sulfur trioxide, which is a product of jet fuel combustion was determined. Molten sodium sulfate leads to hot corrosion of the protective oxide layers on turbine blades. How yttrium dopants in nidkel-aluminum alloys used in turbine blades reduce the spalling rate of protective alumina films and enhance their adhesion was also determined. Two other fulfate mechanisms were deduced and structure of carbon monoxide on a clean chronium and clean platinum-titanium alloys surfaces was determined. All studies were by use of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. Seven studies were completed. Their titles and abstracts are given.
Tracing Long-Lived Atomic Coherences Generated via Molecular Conical Intersections
Accessing coherences is key to fully understand and control ultrafast dynamics of complex quantum systems like molecules. Most photochemical processes are mediated by conical intersections, which generate coherences between electronic states in molecules. We show with accurate calculations performed on gas-phase methyl iodide that electronic coherences of spin-orbit-split states persist in atomic iodine after dissociation. Our simulation predicts a maximum magnitude of vibronic coherence in the molecular regime of 0.75% of the initially photoexcited state population. Upon dissociation, one-third of this coherence magnitude is transferred to a long-lived atomic coherence where vibrational decoherence can no longer occur. To trace these dynamics, we propose a tabletop experimental approach—heterodyned attosecond four-wave-mixing spectroscopy. This technique can temporally resolve small electronic coherence magnitudes and reconstruct the full complex coherence function via phase cycling. Hence, heterodyned attosecond four-wave-mixing spectroscopy leads the way to a complete understanding and optimal control of spin-orbit-coupled electronic states in photochemistry.