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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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Estimation of mean electron energy in helium surface ionization waves on dielectric substrates

Abstract The determination of basic plasma parameters in atmospheric pressure discharges is critical to advancing their use in applications. Atmospheric pressure plasma jets have found use in the fields of medicine, agriculture, material modification and others. Atmospheric pressure plasma jets often generate plasma surface ionization waves (SIW) which interact with and propagate over surfaces. Electrical diagnostics are challenging in SIW due to high collision frequencies and small scale of the plasma discharge. This work employs a passive optical emission line ratio technique to estimate the mean electron energy in SIW over planar dielectric substrates. The method uses an intensity ratio of two helium triplet lines: He( 3 3 S ) at 706.5 nm and He( 3 3 D ) at 587.56 nm. A collisional-radiative model is used to correlate line ratio to mean electron energy and determine dependencies on electron density and He/air gas mixture. Mean electron energies ranging from 3–8 eV are determined in He/air mixtures and are found to remain constant as the surface wave propagates radially. This work provides a 2D, time-resolved, mean electron energy diagnostic for surface ionization wave propagation and validation of numerical modeling in atmospheric pressure systems with spatially varying He/air gas mixtures. The model in question is designed for use with any He line ratio in the n = 3 excitation level.

Morsell, Joshua (ORCID:0000000331073002)↗
DOE OSTI2024

Reversible to irreversible transitions for ac driven skyrmions on periodic substrates

Abstract Using atomistic simulations, we investigate the dynamical behavior of magnetic skyrmions in dimer and trimer molecular crystal arrangements, as well as bipartite lattices at 3/2 and 5/2 fillings, under ac driving over a square array of anisotropy defects. For low ac amplitudes, at all fillings reversible motion appears in which the skyrmions return to their original positions at the end of each ac drive cycle and the diffusion is zero. We also identify two distinct irreversible regimes. The first is a translating regime in which the skyrmions form channels of flow in opposing directions and translate by one substrate lattice constant per ac drive cycle. The translating state appears in the dimer and trimer arrangements, and produces pronounced peaks in the diffusivity in the direction perpendicular to the external drive. For larger ac amplitudes, we find chaotic irreversible motion in which the skyrmions can randomly exchange places with each other over time, producing long-time diffusive behavior both parallel and perpendicular to the ac driving direction.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Differences in substrate use linked to divergent carbon flow during litter decomposition

ABSTRACT Discovering widespread microbial processes that create variation in soil carbon (C) cycling within ecosystems may improve soil C modeling. Toward this end, we screened 206 soil communities decomposing plant litter in a common garden microcosm environment and examined features linked to divergent patterns of C flow. C flow was measured as carbon dioxide (CO2) and dissolved organic carbon (DOC) from 44-days of litter decomposition. Two large groups of microbial communities representing ‘high’ and ‘low’ DOC phenotypes from original soil and 44-day microcosm samples were down-selected for fungal and bacterial profiling. Metatranscriptomes were also sequenced from a smaller subset of communities in each group. The two groups exhibited differences in average rate of CO2 production, demonstrating that the divergent patterns of C flow arose from innate functional constraints on C metabolism, not a time-dependent artefact. To infer functional constraints, we identified features – traits at the organism, pathway or gene level – linked to the high and low DOC phenotypes using RNA-Seq approaches and machine learning approaches. Substrate use differed across the high and low DOC phenotypes. Additional features suggested that divergent patterns of C flow may be driven in part by differences in organism interactions that affect DOC abundance directly or indirectly by controlling community structure.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Asymmetric dimerization of adenosine deaminase acting on RNA facilitates substrate recognition

Abstract Adenosine deaminases acting on RNA (ADARs) are enzymes that convert adenosine to inosine in duplex RNA, a modification that exhibits a multitude of effects on RNA structure and function. Recent studies have identified ADAR1 as a potential cancer therapeutic target. ADARs are also important in the development of directed RNA editing therapeutics. A comprehensive understanding of the molecular mechanism of the ADAR reaction will advance efforts to develop ADAR inhibitors and new tools for directed RNA editing. Here we report the X-ray crystal structure of a fragment of human ADAR2 comprising its deaminase domain and double stranded RNA binding domain 2 (dsRBD2) bound to an RNA duplex as an asymmetric homodimer. We identified a highly conserved ADAR dimerization interface and validated the importance of these sequence elements on dimer formation via gel mobility shift assays and size exclusion chromatography. We also show that mutation in the dimerization interface inhibits editing in an RNA substrate-dependent manner for both ADAR1 and ADAR2.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2026

Compressive strain turns 𝑠 ± - into 𝑑-wave pairing in a one-unit-cell La 3 ⁢Ni 2 ⁢O 7 thin film via substrate-induced hole doping

Motivated by recent reports of ambient-pressure superconductivity in La 3 ⁢Ni 2⁢ O 7 films grown on LaSrAlO 4 , we investigate the superconducting instability in a one-unit-cell (1UC) thin film using ab initio and random-phase approximation techniques. Compared to the high-pressure bulk system, the ratio of interlayer 𝑑 3⁢𝑧 2 −𝑟 2 hopping to intralayer 𝑑 𝑥 2 −𝑦 2 hopping is suppressed in the 1UC thin film, and the crystal-field splitting of the 𝑒 𝑔 orbitals is increased. Here, our calculation indicates that spin-fluctuation-driven pairing correlations are weak for the stoichiometric case at ambient pressure, but increase significantly under hole doping. The leading pairing symmetry is also found to change by hole doping. Specifically, we obtain a leading 𝑑 𝑥 2 −𝑦 2 pairing state at moderate hole doping, followed by a 𝑑 𝑥⁢𝑦 state at higher doping. These states are driven by intraband spin-fluctuation scattering within the 𝛾 hole pocket centered around the 𝑀 point, and arise primarily from states in the Ni layer farther from the substrate. These results strongly suggest that the thin-film superconducting samples are hole-doped and that pairing in this system predominantly arises in the layer, as opposed to the interlayer pairing in the pressurized bulk system.

Zhang, Yang [Oak Ridge National Laboratory (ORNL),↗
DOE OSTI2020

Similarities and differences between nickelate and cuprate films grown on a SrTiO 3 substrate

The recent discovery of superconductivity in Sr-doped NdNiO 2 films grown on SrTiO 3 started a novel field within unconventional superconductivity. Therefore, to understand the similarities and differences between nickelate and cuprate layers on the same SrTiO 3 substrate, here based on the density functional theory we have systematically investigated the structural, electronic, and magnetic properties of NdNiO 2 / SrTiO 3 and CaCuO 2 / SrTiO 3 systems. Our results revealed a strong lattice reconstruction in the case of NdNiO 2 / SrTiO 3 , resulting in a polar film, with the surface and interfacial NiO 2 layers presenting opposite displacements. To avoid the “polar catastrophe,” the NiO 2 surface to the vacuum reconstructs as well. However, for CaCuO 2 / SrTiO 3 , the distortions of those same two CuO 2 layers were in the same direction. In addition, we found this distortion to be approximately independent of the studied range of film thickness for the nickelate films. Furthermore, we also observed a two-dimensional electron gas at the interface between NdNiO 2 and SrTiO 3 , caused by the polar discontinuity, in agreement with recent literature. For NdNiO 2 / SrTiO 3 the two-dimensional electron gas extends over several layers, while for CaCuO 2 / SrTiO 3 this electronic rearrangement is very localized at the interface between CaCuO 2 and SrTiO 3 . The electronic reconstruction found at the interface involves a strong occupation of the Ti 3 d x y state. In both cases, there is a significant electronic charge transfer from the surface Ni or Cu layers to the Ti interface layer. The interfacial Ni and Cu layers are hole and electron doped, respectively. By introducing magnetism and electronic correlation, we observed that the d 3 z 2 - r 2 orbital of Ni becomes itinerant while the same orbital for Cu remains doubly occupied, establishing a clear two- vs one-orbital active framework for the description of these systems. Furthermore, we also observed a strong magnetic reconstruction at the NdNiO 2 surface to vacuum layer where magnetism is basically suppressed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Tuning Dzyaloshinskii-Moriya interactions in magnetic bilayers with a ferroelectric substrate

Our group performed systematic first-principles calculations to explore the possibility of manipulating Dzyaloshinskii-Moriya interactions (DMIs) in heavy metal/ferromagnetic metal bilayers with ferroelectric (FE) polarization of a PbTiO 3 (PTO) substrate. Significantly, both the sign and magnitude of the DMI can be tailored by the FE polarization reversal in PTO. Moreover, the DMIs of Ir/Co/PTO and Ir/Fe/PTO can achieve large values (|d| > 10 meV) under the influence of an upward electric polarization of PTO. Our findings provided useful strategies for enhancing and tailoring DMIs in complex heterostructures and paved the way for the development of DMI-based devices with ultralow power consumption.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Clean quantum point contacts in an InAs quantum well grown on a lattice-mismatched InP substrate

Strong spin-orbit coupling, the resulting large g factor, and small effective mass make InAs an attractive material platform for inducing topological superconductivity. The surface Fermi level pinning in the conduction band enables highly transparent ohmic contact without excessive doping. In this study, we investigate electrostatically defined quantum point contacts (QPCs) in a deep-well InAs two-dimensional electron gas. Despite the 3.3% lattice mismatch between the InAs quantum well and the InP substrate, we report clean QPCs with up to eight pronounced quantized conductance plateaus at zero magnetic field. Source-drain dc bias spectroscopy reveals a harmonic confinement potential with a nearly 5 meV subband spacing. We find a many-body exchange interaction enhancement for the out-of-plane g factor |$g^{*}_{⊥}$|=27±1 , whereas the in-plane g factor is isotropic |$g^{*}_{x}$|=|$g^{*}_{y}$|=12±2 , close to the bulk value for InAs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Pattern formation and flocking for particles near the jamming transition on resource gradient substrates

Here, we numerically examine a bidisperse system of active and passive particles coupled to a resource substrate. The active particles deplete the resource at a fixed rate and move toward regions with higher resources, while all of the particles interact sterically with each other. We show that at high densities, this system exhibits a rich variety of pattern-forming phases along with directed motion or flocking as a function of the relative rates of resource absorption and consumption as well as the active to passive particle ratio. These include partial phase separation into rivers of active particles flowing through passive clusters, strongly phase separated states where the active particles induce crystallization of the passive particles, mixed jammed states, and fluctuating mixed fluid phases. For higher resource recovery rates, we demonstrate that the active particles can undergo motility-induced phase separation, while at high densities, there can be a coherent flock containing only active particles or a solid mixture of active and passive particles. The directed flocking motion typically shows a transient in which the flow switches among different directions before settling into one direction, and there is a critical density below which flocking does not occur. We map out the different phases as function of system density, resource absorption and recovery rates, and the ratio of active to passive particles.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2024

Spin-Dependent $\pi$$\pi$* Gap in Graphene on a Magnetic Substrate

Here, we present a detailed analysis of the electronic properties of graphene/Eu/Ni(111). By using angle- and spin-resolved photoemission spectroscopy and ab initio calculations, we show that the intercalation of Eu in the graphene/Ni(111) interface gives rise to a gapped freestanding dispersion of the $\pi$$\pi$* Dirac cones at the K point with an additional lifting of the spin degeneracy due to the mixing of graphene and Eu states. The interaction with the magnetic substrate results in a large spin-dependent gap in the Dirac cones with a topological nature characterized by a large Berry curvature and a spin-polarized Van Hove singularity, whose closeness to the Fermi level gives rise to a polaronic band.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Valley relaxation of resident electrons and holes in a monolayer semiconductor: Dependence on carrier density and the role of substrate-induced disorder

Using time-resolved optical Kerr rotation, here we measure the low-temperature valley dynamics of resident electrons and holes in exfoliated WSe 2 monolayers as a systematic function of carrier density. In an effort to reconcile the many disparate timescales of carrier valley dynamics in monolayer semiconductors reported to date, we directly compare the doping-dependent valley relaxation in two electrostatically gated WSe 2 monolayers having different dielectric environments. In a fully encapsulated structure ( hBN / WSe 2 / hBN , where hBN is hexagonal boron nitride), valley relaxation is found to be monoexponential. The valley relaxation time τ v is quite long ( ~ 10 μ s ) at low carrier densities, but decreases rapidly to less than 100 ns at high electron or hole densities ≳ 2 × 10 12 cm – 2 . In contrast, in a partially encapsulated WSe 2 monolayer placed directly on silicon dioxide ( hBN / WSe 2 / SiO 2 ) , carrier valley relaxation is multiexponential at low carrier densities. The difference is attributed to environmental disorder from the SiO 2 substrate. Unexpectedly, very small out-of-plane magnetic fields can increase τ v , especially in the hBN / WSe 2 / SiO 2 structure, suggesting that localized states induced by disorder can play an important role in depolarizing spins and mediating the valley relaxation of resident carriers in monolayer transition-metal dichalcogenide semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Buffer-layer-controlled nickeline vs zinc-blende/wurtzite-type MnTe growths on c -plane Al 2 O 3 substrates

In the recent past, MnTe has proven to be a crucial component of the intrinsic magnetic topological insulator (IMTI) family [MnTe] m [Bi2Te 3 ] n , which hosts a wide range of magneto-topological properties depending on the choice of m and n. However, bulk crystal growth allows only a few combinations of m and n for these IMTIs due to the strict limitations of the thermodynamic growth conditions. One way to overcome this challenge is to utilize atomic layer-by-layer molecular beam epitaxy (MBE) technique, which allows arbitrary sequences of [MnTe]m and [Bi 2 Te 3 ] n to be formed beyond the thermodynamic limit. For such MBE growth, finding optimal growth templates and conditions for the parent building block, MnTe, is a key requirement. Here, we report that two different hexagonal phases of MnTe - nickeline (NC) and zinc-blende/wurtzite (ZB-WZ) structures, with distinct in-plane lattice constants of 4.20 ± 0.04 Å and 4.39 ± 0.04 Å, respectively - can be selectively grown on c-plane Al 2 O 3 substrates using different buffer layers and growth temperatures. Moreover, we provide the first comparative studies of different MnTe phases using atomic-resolution scanning transmission electron microscopy and show that ZB and WZ-like stacking sequences can easily alternate between the two. Surprisingly, In 2 Se 3 buffer layer, despite its lattice constant (4.02 Å) being closer to that of the NC phase, fosters the ZB-WZ instead, whereas Bi 2 Te 3 , sharing the same lattice constant (4.39 Å) with the ZB-WZ phase, fosters the NC phase. Furthermore, these discoveries suggest that lattice matching is not always the most critical factor determining the preferred phase during epitaxial growth. Overall, this will deepen our understanding of epitaxial growth modes for chalcogenide materials and accelerate progress toward new IMTI phases as well as other magneto-topological applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Near-half-metallic state in the half-Heusler PtMnSb film on a III-V substrate

The interplay between half-metallic ferromagnetism and spin-orbit coupling within the inversion-symmetry-broken structure of half Heuslers provides an ideal platform for various spintronics functionalities. Taking advantage of good lattice matching, it is highly desired to epitaxially integrate promising Heuslers into III-V semiconductor-based devices. PtMnSb is one of the first half Heuslers predicted to be an above-room-temperature half-metal with large spin orbit coupling, however, its half-metallicity and potential as a spintronics material has remained elusive due to lack of high quality samples. Here, in this study, we demonstrate epitaxial growth of single crystal PtMnSb(001) film on GaSb(001) substrates using molecular beam epitaxy. Direct observation of the band structure via angle-resolved photoemission spectroscopy and many-body perturbation theory within the quasi-particle self-consistent GW approximation (QPGW) reveal that PtMnSb hosts rather a near-half-metallic state with both spin bands crossing the Fermi level and with high spin polarization over 90%. Temperature dependence of magnetization also shows an anomalous enhancement below 60 K, which can be associated with the development of such a near-half-metallic state at low temperatures. Epitaxial growth of high crystalline PtMnSb on a III-V paves the way for systematic clarification of its spin transport properties with fine-tuning of strain in heterostructure devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Improving Performance of III-V Solar Cells Grown on Spalled Germanium with Ex Situ Substrate Planarization

Controlled spalling allows removal of devices and provides an opportunity for cost reduction through substrate reuse. However, the fracture-based process can leave behind morphological surface features, notably river lines, that can disrupt epitaxial growth and degrade device performance. We investigate the viability of various wet etch chemistries to planarize river lines to ensure high-quality device growth and performance without mechanical repolishing, and so maintain a route towards cost-effective reuse. Etching in a HF: HNO 3 :CH 3 COOH solution effectively planarizes river lines and produces a surface that yields devices with equivalent performance to those grown on epi-ready Ge wafer surfaces. Further studies will focus on optimizing etch composition, temperature, and time to minimize material removal while maintaining a suitable surface for high-quality epitaxy.

germanium↗
DOE OSTI2022

Enzyme kinetics by GH7 cellobiohydrolases on chromogenic substrates is dictated by non‐productive binding: insights from crystal structures and MD simulation

Cellobiohydrolases (CBHs) in the glycoside hydrolase family 7 (GH7) ( EC3.2.1.176 ) are the major cellulose degrading enzymes both in industrial settings and in the context of carbon cycling in nature. Small carbohydrate conjugates such as p‐ nitrophenyl‐β‐ d ‐cellobioside (pNPC), p‐ nitrophenyl‐β‐ d ‐lactoside (pNPL) and methylumbelliferyl‐β‐ d ‐cellobioside have commonly been used in colorimetric and fluorometric assays for analysing activity of these enzymes. Despite the similar nature of these compounds the kinetics of their enzymatic hydrolysis vary greatly between the different compounds as well as among different enzymes within the GH7 family. Through enzyme kinetics, crystallographic structure determination, molecular dynamics simulations, and fluorometric binding studies using the closely related compound o‐nitrophenyl‐β‐ d ‐cellobioside (oNPC), in this work we examine the different hydrolysis characteristics of these compounds on two model enzymes of this class, TrCel7A from Trichoderma reesei and PcCel7D from Phanerochaete chrysosporium . Protein crystal structures of the E212Q mutant of TrCel7A with pNPC and pNPL, and the wildtype TrCel7A with oNPC, reveal that non‐productive binding at the product site is the dominating binding mode for these compounds. Enzyme kinetics results suggest the strength of non‐productive binding is a key determinant for the activity characteristics on these substrates, with PcCel7D consistently showing higher turnover rates ( k cat ) than TrCel7A, but higher Michaelis–Menten ( K M ) constants as well. Furthermore, oNPC turned out to be useful as an active‐site probe for fluorometric determination of the dissociation constant for cellobiose on TrCel7A but could not be utilized for the same purpose on PcCel7D, likely due to strong binding to an unknown site outside the active site.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Substrate availability and not thermal acclimation controls microbial temperature sensitivity response to long-term warming

Microbes are responsible for cycling carbon (C) through soils, and predicted changes in soil C stocks under climate change are highly sensitive to shifts in the mechanisms assumed to control the microbial physiological response to warming. Two mechanisms have been suggested to explain the long-term warming impact on microbial physiology: microbial thermal acclimation and changes in the quantity and quality of substrates available for microbial metabolism. Yet studies disentangling these two mechanisms are lacking. To resolve the drivers of changes in microbial physiology in response to long-term warming, we sampled soils from 13- and 28-year-old soil warming experiments in different seasons. We performed short-term laboratory incubations across a range of temperatures to measure the relationships between temperature sensitivity of physiology (growth, respiration, carbon use efficiency, and extracellular enzyme activity) and the chemical composition of soil organic matter. We observed apparent thermal acclimation of microbial respiration, but only in summer, when warming had exacerbated the seasonally-induced, already small dissolved organic matter pools. Irrespective of warming, greater quantity and quality of soil carbon increased the extracellular enzymatic pool and its temperature sensitivity. We propose that fresh litter input into the system seasonally cancels apparent thermal acclimation of C-cycling processes to decadal warming. Our findings reveal that long-term warming has indirectly affected microbial physiology via reduced C availability in this system, implying that earth system models including these negative feedbacks may be best suited to describe long-term warming effects on these soils.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Ultrawide bandgap vertical β-(Al x Ga1 −x )2O3 Schottky barrier diodes on free-standing β-Ga2O3 substrates

Ultrawide bandgap β-(AlxGa1−x)2O3 vertical Schottky barrier diodes on (010) β-Ga2O3 substrates are demonstrated. The β-(AlxGa1−x)2O3 epilayer has an Al composition of 21% and a nominal Si doping of 2 × 1017 cm−3 grown by molecular beam epitaxy. Pt/Ti/Au has been employed as the top Schottky contact, whereas Ti/Au has been utilized as the bottom Ohmic contact. The fabricated devices show excellent rectification with a high on/off ratio of ∼109, a turn-on voltage of 1.5 V, and an on-resistance of 3.4 mΩ cm2. Temperature-dependent forward current-voltage characteristics show effective Schottky barrier height varied from 0.91 to 1.18 eV while the ideality factor from 1.8 to 1.1 with increasing temperatures, which is ascribed to the inhomogeneity of the metal/semiconductor interface. The Schottky barrier height was considered a Gaussian distribution of potential, where the extracted mean barrier height and a standard deviation at zero bias were 1.81 and 0.18 eV, respectively. A comprehensive analysis of the device leakage was performed to identify possible leakage mechanisms by studying temperature-dependent reverse current-voltage characteristics. At reverse bias, due to the large Schottky barrier height, the contributions from thermionic emission and thermionic field emission are negligible. By fitting reverse leakage currents at different temperatures, it was identified that Poole–Frenkel emission and trap-assisted tunneling are the main leakage mechanisms at high- and low-temperature regimes, respectively. Electrons can tunnel through the Schottky barrier assisted by traps at low temperatures, while they can escape these traps at high temperatures and be transported under high electric fields. This work can serve as an important reference for the future development of ultrawide bandgap β-(AlxGa1−x)2O3 power electronics, RF electronics, and ultraviolet photonics.

Materials Science↗
DOE OSTI2025

Atomic layer deposition of nanofilms on porous polymer substrates: Strategies for success

Atomic layer deposition (ALD) is a versatile technique for engineering the surfaces of porous polymers, imbuing the flexible, high-surface-area substrates with inorganic and hybrid material properties. Previously reported enhancements include fouling resistance, electrical conductance, thermal stability, photocatalytic activity, hydrophilicity, and oleophilicity. However, there are many poorly understood phenomena that introduce challenges in applying ALD to porous polymers. In this paper, we address five common challenges and ways to overcome them: (1) entrapped precursor, (2) embrittlement, (3) film fracture, (4) deformation, and (5) pore collapse. These challenges are often interrelated and can exacerbate one another. To investigate these phenomena, we applied various ALD chemistries to porous polymers including polyethersulfone, polysulfone, polyvinylidene fluoride, and polycarbonate track-etched membranes. Reaction-diffusion modeling revealed why certain precursors and processing conditions result in embrittling subsurface material growth, entrapment of unreacted precursors, and nongrowth. We quantify the limits of ALD processing temperatures that are dictated by thermal expansion mismatch and can lead to fractured ALD films. The results herein allow us to make recommendations to avoid, mitigate, or overcome the difficulties encountered when performing ALD and plasma-enhanced ALD on porous polymers. We intend this article to serve as a “lessons learned” guide informed by previous experience to provide a better understanding of the difficulties and limitations of ALD on porous polymers and knowledge-based guidelines for successful depositions. This knowledge can accelerate future research and help experimentalists navigate and troubleshoot as they expose porous polymers to reactive precursor vapors.

36 MATERIALS SCIENCE↗