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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 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Biexciton and trion energy transfer from CdSe/CdS giant nanocrystals to Si substrates

Observation of energy transfer (ET) from multiexcitonic (MX) complexes in nanocrystal quantum dots (NQDs) has been severely restricted due to efficient nonradiative Auger recombination leading to very low MX emission quantum yields. Here we employed “giant” CdSe/CdS NQDs with suppressed Auger recombination to study ET of biexcitons (BX) and charged excitons (trions) into Si substrate. Photoluminescence (PL) measurements of (sub)monolayers of gNQDs controllably assembled on various interacting surfaces and augmented by single gNQD's imaging reveal appearance of BX spectral signatures and progressive acceleration of PL lifetimes of all excitonic species on Si substrates. From statistical analysis of a large number of PL lifetime traces, representative exciton, trion and BX ET efficiencies are measured as ~75%, 55% and 45% respectively. Detailed analysis of the MX's radiative rates demonstrate the crucial role of the radiative (waveguide) ET in maintaining high overall transfer efficiency despite the prevalent Auger recombination. Furthermore, our observations point towards practical utilization of MX-bearing nanocrystals in future optoelectronics architectures.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Interaction-induced backscattering in short quantum wires

We study interaction-induced backscattering in clean quantum wires with adiabatic contacts exposed to a voltage bias. Particle backscattering relaxes such systems to a fully equilibrated steady state only on length scales exponentially large in the ratio of bandwidth of excitations and temperature. Here in this paper we focus on shorter wires in which full equilibration is not accomplished. Signatures of relaxation then are due to backscattering of hole excitations close to the band bottom which perform a diffusive motion in momentum space while scattering from excitations at the Fermi level. This is reminiscent to the first passage problem of a Brownian particle and, regardless of the interaction strength, can be described by an inhomogeneous Fokker-Planck equation. From general solutions of the latter we calculate the hole backscattering rate for different wire lengths and discuss the resulting length dependence of interaction-induced correction to the conductance of a clean single channel quantum wire.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Aluminum hydroxide, bayerite, boehmite, and gibbsite ToF-SIMS spectra in the negative ion mode. I

We report time-of-flight secondary ion mass spectrometry (ToF-SIMS) was performed for boehmite (AOH-60) and its potential products of oxidation including pseudo-boehmite (AOH-180), α- and γ-Al 2 O 3 , and α- and γ-Al(OH) 3 . Since boehmite is often incorporated on cladding materials to prevent corrosion, surface analysis techniques are performed to determine the amount of oxidation present. This ToF-SIMS spectral library is of significance because it includes boehmite and its potential oxidation products (i.e., aluminum oxide and hydroxide), which can be used to compare to spectra obtained for real-world samples containing boehmite. Furthermore, ToF-SIMS is often used as a complementary technique to x-ray photoelectron spectroscopy due to its surface sensitivity and ability to compare spectra via a multivariate analysis, therefore establishing that the molecular signatures of boehmite and relevant compounds are essential for peak identification. The SIMS spectra shown are acquired from commercially available powders, which were deposited onto a silicon wafer substrate via liquid slurry drop casting. This library of SIMS mass spectra will serve as a comparison of boehmite [γ-AlO(OH)], pseudo-boehmite [AlOOH∙nH 2 O], α- and γ-Al 2 O 3 aluminum oxide, and α- and γ-Al 2 O 3 aluminum hydroxide in the negative ion mode, which compliments those reported in the positive ion mode.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Identification of the Major Expressed S-Layer and Cell Surface-Layer-Related Proteins in the Model Methanogenic Archaea: Methanosarcina barkeri Fusaro and Methanosarcina acetivorans C2A

Many archaeal cell envelopes contain a protein coat or sheath composed of one or more surface exposed proteins. These surface layer (S-layer) proteins contribute structural integrity and protect the lipid membrane from environmental challenges. To explore the species diversity of these layers in the Methanosarcinaceae, the major S-layer protein in Methanosarcina barkeri strain Fusaro was identified using proteomics. The Mbar_A1758 gene product was present in multiple forms with apparent sizes of 130, 120, and 100 kDa, consistent with post-translational modifications including signal peptide excision and protein glycosylation. A protein with features related to the surface layer proteins found in Methanosarcina acetivorans C2A and Methanosarcina mazei Goel was identified in the M. barkeri genome. These data reveal a distinct conserved protein signature with features and implied cell surface architecture in the Methanosarcinaceae that is absent in other archaea. Paralogous gene expression patterns in two Methanosarcina species revealed abundant expression of a single S-layer paralog in each strain. Respective promoter elements were identified and shown to be conserved in mRNA coding and upstream untranslated regions. Prior M. acetivorans genome annotations assigned S-layer or surface layer associated roles of eighty genes: however, of 68 examined none was significantly expressed relative to the experimentally determined S-layer gene.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

The Role of Back Pressure in the Atmospheric Response to Surface Stress Induced by the Kuroshio

Abstract The effect of ocean current drag on the atmosphere is of interest as a test case for the role of back pressure, because the response is independent of the thermally induced modulation of the boundary layer stability and hydrostatic pressure. The authors use a regional atmospheric model to investigate the impact of drag induced by the Kuroshio in the East China Sea on the overlying winter atmosphere. Ocean currents dominate the wind stress curl compared to the impacts of sea surface temperature (SST) fronts. Wind stress convergences and divergences are weakly enhanced even though the ocean current is almost geostrophic. These modifications change the linear relationships (coupling coefficients) between the wind stress curl/divergence and the SST Laplacian, crosswind, and downwind gradients. Clear signatures of the ocean current impacts are found beyond the sea surface: sea surface pressure (back pressure) decreases near the current axis, and precipitation increases over the downwind region. However, these responses are very small despite strong Ekman pumping due to the current. A linear reduced gravity model is used to explain the boundary layer dynamics. The linear vorticity equation shows that the oceanic influence on wind stress curl is balanced by horizontal advection decoupling the boundary layer from the interior atmosphere. Spectral transfer functions are used to explain the general response of back pressure to geostrophic ocean currents and sea surface height.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Selection signatures in four lignin genes from switchgrass populations divergently selected for in vitro dry matter digestibility

Switchgrass is undergoing development as a dedicated cellulosic bioenergy crop. Fermentation of lignocellulosic biomass to ethanol in a bioenergy system or to volatile fatty acids in a livestock production system is strongly and negatively influenced by lignification of cell walls. This study detects specific loci that exhibit selection signatures across switchgrass breeding populations that differ in in vitro dry matter digestibility (IVDMD), ethanol yield, and lignin concentration. Allele frequency changes in candidate genes were used to detect loci under selection. Out of the 183 polymorphisms identified in the four candidate genes, twenty-five loci in the intron regions and four loci in coding regions were found to display a selection signature. All loci in the coding regions are synonymous substitutions. Selection in both directions were observed on polymorphisms that appeared to be under selection. Genetic diversity and linkage disequilibrium within the candidate genes were low. The recurrent divergent selection caused excessive moderate allele frequencies in the cycle 3 reduced lignin population as compared to the base population. As a result, this study provides valuable insight on genetic changes occurring in short-term selection in the polyploid populations, and discovered potential markers for breeding switchgrass with improved biomass quality.

09 BIOMASS FUELS↗
DOE OSTI2024

Multi-track reconstruction algorithm in the Mu2e experiment

The Mu2e experiment, under construction at Fermilab, will search for the neutrino-less coherent $\mu^-N\rightarrow e^-N$ conversion in the field of a $^{27}$Al nucleus. Such a process violates lepton flavor conservation. About $60\%$ of muons stopped by an $^{27}$Al nucleus will undergo nuclear capture, while about $40\%$ will decay in orbit. To quantify the conversion probability, we define $R_{\mu e}$, which is given by the ratio between the $\mu^-\rightarrow e^-$ conversion rate and the nuclear capture rate [1]: \begin{equation} R_{\mu e}= \frac{\Gamma\left(\mu^- + N\left(Z,A\right)\rightarrow e^- + N\left(Z,A\right)\right)}{\Gamma\left(\mu^- + N\left(Z,A\right)\rightarrow \nu^-_\mu + N\left(Z-1,A\right)\right)}\,. \end{equation} The upper limit on $R_{\mu e}$ is $7\cdot 10^{-13}$ at $90\%$ CL, set by the SINDRUM II experiment~\cite{SINDRUM II:limit}. The goal of the Mu2e experiment is to reach a sensitivity on $R_{\mu e}$ of $8\cdot 10^{-17}$ at $90\%$ CL. This represents a four-order of magnitude improvement over the current experimental limit. Mu2e will take its first data in 2027. The signature for the muon conversion is a monochromatic electron of $104.97$~\si{\mega\eV}/c, an energy slightly below the muon rest mass. While the main experiment goal is to reconstruct the conversion electron, i.e., an event with a single track, there are motivations to develop an efficient tracking algorithm for reconstructing more simultaneous tracks. This could better constrain the background generated by $p\bar{p}$-annihilation in the Al target and to search for other Beyond the Standard Model processes. In this paper, we present an algorithm designed to reconstruct multi-particle events.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

Molecular simulations and NMR reveal how lipid fluctuations affect membrane mechanics

Lipid bilayers form the main matrix of functional cell membranes, and their dynamics underlie a host of physical and biological processes. Here we show that elastic membrane properties and collective molecular dynamics (MD) are related by the mean-square amplitudes (order parameters) and relaxation rates (correlation times) of lipid acyl chain motions. We performed all-atom MD simulations of liquid-crystalline bilayers that allow direct comparison with carbon-hydrogen (CH) bond relaxations measured with NMR spectroscopy. Previous computational and theoretical approaches have assumed isotropic relaxation, which yields inaccurate description of lipid chain dynamics and incorrect data interpretation. Instead, the new framework includes a fixed bilayer normal (director axis) and restricted anisotropic motion of the CH bonds in accord with their segmental order parameters, enabling robust validation of lipid force fields. Simulated spectral densities of thermally excited CH bond fluctuations exhibited well-defined spin-lattice (Zeeman) relaxations analogous to those in NMR measurements. Their frequency signature could be fit to a simple power-law function, indicative of nematic-like collective dynamics. Moreover, calculated relaxation rates scaled as the squared order parameters yielding an apparent K C modulus for bilayer bending. Our results show a strong correlation with K C values obtained from solid-state NMR studies of bilayers without and with cholesterol as validated by neutron spin-echo measurements of membrane elasticity. The simulations uncover a critical role of interleaflet coupling in membrane mechanics and thus provide important insights into molecular sites of emerging elastic properties within lipid bilayers.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2018

Iron isotope fractionation during pyrite formation in a sulfidic Precambrian ocean analogue

We present that the chemical response of the Precambrian oceans to rising atmospheric O 2 levels remains controversial. The iron isotope signature of sedimentary pyrite is widely used to trace the microbial and redox states of the ocean, yet the iron isotope fractionation accompanying pyrite formation in nature is difficult to constrain due to the complexity of the pyrite formation process, difficulties in translating the iron isotope systematics of experimental studies to natural settings, and insufficient iron isotope datasets for natural euxinic (i.e. anoxic and sulfidic) marine basins where pyrite formation occurs. Herein we demonstrate, that a large, permil-level shift in the isotope composition of dissolved iron occurs in the Black Sea euxinic water column during syngenetic pyrite formation. Specifically, iron removal to syngenetic pyrite gives rise to an iron isotope fractionation factor between Fe(II) and FeS 2 of 2.75 permil (‰), the largest yet reported for reactions under natural conditions that do not involve iron redox chemistry. These iron isotope systematics offer the potential to generate permil-level shifts in the sedimentary pyrite iron isotope record due to partial drawdown of the oceanic iron inventory. The implication is that the iron stable isotope signatures of sedimentary pyrites may record fundamental regime shifts between pyrite formation under sulfur-limited conditions and pyrite formation under iron-limited conditions. To this end, the iron isotope signatures of sedimentary pyrite may best represent the extent of euxinia in the past global ocean, rather than its oxygenation state. On this basis, the reinterpreted sedimentary pyrite Fe isotope record suggests a fundamental shift towards more sulfidic oceanic conditions coincident with the ‘Great Oxidation Event’ around 2.3 billion years ago. Importantly, this does not require the chemical state of the ocean to shift from mainly de-oxygenated to predominantly oxygenated in parallel with the permanent rise in atmospheric oxygen, contrary to other interpretations based on iron isotope systematics.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Multi-omics of NET formation and correlations with CNDP1, PSPB, and L-cystine levels in severe and mild COVID-19 infections

We report that we performed a multi-omics analysis of an immunologically naïve SARS-CoV-2 clinical cohort to characterize overall changes in plasma among control (uninfected), mild, and severe infections. A comparison of healthy controls and patient samples showed activation of neutrophil degranulation pathways. Consistent with this observation, we characterized neutrophil extracellular trap (NET) complexes that were partially initiated in a subset of the mild infections (showing partially formed NETs) and fully-formed NETs in a subset of severe infections (containing multiple NET proteins in individual patient samples). As a potential mechanism to suppress NET formation, multiple redox enzymes were elevated in the mild and severe population. Analysis of metabolites from the same cohort showed a 24 and 60-fold elevation in plasma L-cystine, the oxidized form of cysteine and substrate of the powerful antioxidant glutathione in mild and severe patients, respectively. Unique to patients with mild infections, the carnosine dipeptidase modifying enzyme (CNDP1) was up-regulated. The strong protein and metabolite oxidation signatures suggest multiple compensatory pathways working to suppress both free radical and NET formation in SARS-CoV-2 infections.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Dynamic cyanobacterial response to hydration and dehydration in a desert biological soil crust

Biological soil crusts (BSCs) cover extensive portions of the earth’s deserts. In order to survive desiccation cycles and utilize short periods of activity during infrequent precipitation, crust microorganisms must rely on the unique capabilities of vegetative cells to enter a dormant state and be poised for rapid resuscitation upon wetting. To elucidate the key events involved in the exit from dormancy, we performed a wetting experiment of a BSC and followed the response of the dominant cyanobacterium, Microcoleus vaginatus, in situ using a whole-genome transcriptional time course that included two diel cycles. Immediate, but transient, induction of DNA repair and regulatory genes signaled the hydration event. Recovery of photosynthesis occurred within 1 h, accompanied by upregulation of anabolic pathways. Onset of desiccation was characterized by the induction of genes for oxidative and photo-oxidative stress responses, osmotic stress response and the synthesis of C and N storage polymers. Early expression of genes for the production of exopolysaccharides, additional storage molecules and genes for membrane unsaturation occurred before drying and hints at preparedness for desiccation. We also observed signatures of preparation for future precipitation, notably the expression of genes for anaplerotic reactions in drying crusts, and the stable maintenance of mRNA through dormancy. These data shed light on possible synchronization between this cyanobacterium and its environment, and provides key mechanistic insights into its metabolism in situ that may be used to predict its response to climate, and or, land-use driven perturbations.

biological soil crust↗
DOE OSTI2016

Structure of CC chemokine receptor 2 with orthosteric and allosteric antagonists

CC chemokine receptor 2 (CCR2) is one of 19 members of the chemokine receptor subfamily of human class A G-protein-coupled receptors. CCR2 is expressed on monocytes, immature dendritic cells, and T-cell subpopulations, and mediates their migration towards endogenous CC chemokine ligands such as CCL2 (ref. 1). CCR2 and its ligands are implicated in numerous inflammatory and neurodegenerative diseases2 including atherosclerosis, multiple sclerosis, asthma, neuropathic pain, and diabetic nephropathy, as well as cancer3. These disease associations have motivated numerous preclinical studies and clinical trials4 (see http://www.clinicaltrials.gov) in search of therapies that target the CCR2–chemokine axis. To aid drug discovery efforts5, here we solve a structure of CCR2 in a ternary complex with an orthosteric (BMS-681 (ref. 6)) and allosteric (CCR2-RA-[R]7) antagonist. BMS-681 inhibits chemokine binding by occupying the orthosteric pocket of the receptor in a previously unseen binding mode. CCR2-RA-[R] binds in a novel, highly druggable pocket that is the most intracellular allosteric site observed in class A G-protein-coupled receptors so far; this site spatially overlaps the G-protein-binding site in homologous receptors. CCR2-RA-[R] inhibits CCR2 non-competitively by blocking activation-associated conformational changes and formation of the G-protein-binding interface. The conformational signature of the conserved microswitch residues observed in double-antagonist-bound CCR2 resembles the most inactive G-protein-coupled receptor structures solved so far. Like other protein–protein interactions, receptor–chemokine complexes are considered challenging therapeutic targets for small molecules, and the present structure suggests diverse pocket epitopes that can be exploited to overcome obstacles in drug design.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

The Marchantia polymorpha pangenome reveals ancient mechanisms of plant adaptation to the environment

Plant adaptation to terrestrial life started 450 million years ago and has played a major role in the evolution of life on Earth. The genetic mechanisms allowing this adaptation to a diversity of terrestrial constraints have been mostly studied by focusing on flowering plants. Here, we gathered a collection of 133 accessions of the model bryophyte Marchantia polymorpha and studied its intraspecific diversity using selection signature analyses, a genome-environment association study and a pangenome. We identified adaptive features, such as peroxidases or nucleotide-binding and leucine-rich repeats (NLRs), also observed in flowering plants, likely inherited from the first land plants. The M. polymorpha pangenome also harbors lineage-specific accessory genes absent from seed plants. We conclude that different land plant lineages still share many elements from the genetic toolkit evolved by their most recent common ancestor to adapt to the terrestrial habitat, refined by lineage-specific polymorphisms and gene family evolution.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2014

Epitaxy of polar semiconductor Co{sub 3}O{sub 4} (110): Growth, structure, and characterization

The (110) plane of Co{sub 3}O{sub 4} spinel exhibits significantly higher rates of carbon monoxide conversion due to the presence of active Co{sup 3+} species at the surface. However, experimental studies of Co{sub 3}O{sub 4} (110) surfaces and interfaces have been limited by the difficulties in growing high-quality films. We report thin (10–250 Å) Co{sub 3}O{sub 4} films grown by molecular beam epitaxy in the polar (110) direction on MgAl{sub 2}O{sub 4} substrates. Reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction, and transmission electron microscopy measurements attest to the high quality of the as-grown films. Furthermore, we investigate the electronic structure of this material by core level and valence band x-ray photoelectron spectroscopy, and first-principles density functional theory calculations. Ellipsometry reveals a direct band gap of 0.75 eV and other interband transitions at higher energies. A valence band offset of 3.2 eV is measured for the Co{sub 3}O{sub 4}/MgAl{sub 2}O{sub 4} heterostructure. Magnetic measurements show the signature of antiferromagnetic ordering at 49 K. FTIR ellipsometry finds three infrared-active phonons between 300 and 700 cm{sup −1}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Quasiballistic quantum transport through Ge/Si core/shell nanowires

We study signatures of ballistic quantum transport of holes through Ge/Si core/shell nanowires at low temperatures. We observe Fabry–Pérot interference patterns as well as conductance plateaus at integer multiples of 2e 2 /h at zero magnetic field. Magnetic field evolution of these plateaus reveals relatively large effective Landé g-factors. Ballistic effects are observed in nanowires with silicon shell thickness of 1–3 nm, but not in bare germanium wires. These findings inform the future development of spin and topological quantum devices which rely on ballistic sub-band-resolved transport.

42 ENGINEERING↗
DOE OSTI2018

Interplay of magnetism and transport in HoBi

Here, we report the observation of an extreme magnetoresistance (XMR) in HoBi with a large magnetic moment from Ho f electrons. Neutron scattering is used to determine the magnetic wave vectors across several metamagnetic transitions on the phase diagram of HoBi. Unlike other magnetic rare-earth monopnictides, the field dependence of resistivity in HoBi is nonmonotonic and reveals clear signatures of every metamagnetic transition in the low-temperature and low-field regime, at T < 2 K and H < 2.3 T. The XMR appears at H > 2.3 T after all the metamagnetic transitions are complete and the system is spin polarized by the external magnetic field. The existence of an onset field for XMR and the intimate connection between magnetism and transport in HoBi are unprecedented among the magnetic rare-earth monopnictides. Therefore, HoBi provides a unique opportunity to understand the electrical transport in magnetic XMR semimetals.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Pinch point singularities of tensor spin liquids

Recently, a new class of three-dimensional spin liquid models have been theoretically discovered, which feature generalized Coulomb phases of emergent symmetric tensor U(1) gauge theories. These “higher rank” tensor models are particularly intriguing due to the presence of quasiparticles with restricted mobility, such as fractons. We investigate universal experimental signatures of tensor Coulomb phases. Most notably, we show that tensor Coulomb spin liquids (both quantum and classical) feature characteristic pinch point singularities in their spin-spin correlation functions, accessible via neutron scattering, which can be readily distinguished from pinch points in conventional U(1) spin liquids. These pinch points can thus serve as a crisp experimental diagnostic for such phases. Here, we also tabulate the low-temperature heat capacity of various tensor Coulomb phases, which serves as a useful additional diagnostic in certain cases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗