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DOE OSTI2020

Electronic structure calculations of twisted multi-layer graphene superlattices

Quantum confinement endows two-dimensional (2D) layered materials with exceptional physics and novel properties compared to their bulk counterparts. Although certain two- and few-layer configurations of graphene have been realized and studied, a systematic investigation of the properties of arbitrarily layered graphene assemblies is still lacking. We introduce theoretical concepts and methods for the processing of materials information, and as a case study, apply them to investigate the electronic structure of multi-layer graphene-based assemblies in a high-throughput fashion. We provide a critical discussion of patterns and trends in tight binding band structures and we identify specific layered assemblies using low-dispersion electronic bands as indicators of potentially interesting physics like strongly correlated behavior. A combination of data-driven models for visualization and prediction is used to intelligently explore the materials space. This work more generally aims to increase confidence in the combined use of physics-based and data-driven modeling for the systematic refinement of knowledge about 2D layered materials, with implications for the development of novel quantum devices.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Producing a BOSS CMASS sample with DES imaging

We present a sample of galaxies with the Dark Energy Survey (DES) photometry that replicates the properties of the BOSS CMASS sample. The CMASS galaxy sample has been well characterized by the Sloan Digital Sky Survey (SDSS) collaboration and was used to obtain the most powerful redshift-space galaxy clustering measurements to date. A joint analysis of redshift-space distortions (such as those probed by CMASS from SDSS) and a galaxy-galaxy lensing measurement for an equivalent sample from DES can provide powerful cosmological constraints. Unfortunately, the DES and SDSS-BOSS footprints have only minimal overlap, primarily on the celestial equator near the SDSS Stripe 82 region. Using this overlap, we build a robust Bayesian model to select CMASS-like galaxies in the remainder of the DES footprint. The newly defined DES-CMASS (DMASS) sample consists of 117,293 effective galaxies covering $1,244 {\rm deg}^2$. Through various validation tests, we show that the DMASS sample selected by this model matches well with the BOSS CMASS sample, specifically in the South Galactic cap (SGC) region that includes Stripe 82. Combining measurements of the angular correlation function and the clustering-z distribution of DMASS, we constrain the difference in mean galaxy bias and mean redshift between the BOSS CMASS and DMASS samples to be $\Delta b = 0.010^{+0.045}_{-0.052}$ and $\Delta z = \left( 3.46^{+5.48}_{-5.55} \right) \times 10^{-3}$ for the SGC portion of CMASS, and $\Delta b = 0.044^{+0.044}_{-0.043} $ and $\Delta z= ( 3.51^{+4.93}_{-5.91}) \times 10^{-3}$ for the full CMASS sample. These values indicate that the mean bias of galaxies and mean redshift in the DMASS sample is consistent with both CMASS samples within $1\sigma$.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2025

Layer-dependent spin-resolved electronic structure of ferromagnetic triple-layered ruthenate Sr4Ru3O10

High-resolution angle- and spin-resolved photoemission spectroscopy (ARPES) of the triple-layered ruthenate Sr4Ru3O10 reveals features of the electronic structure that extend our understanding of the layered strontium ruthenates. The spectra near the Fermi energy are very different from the nonmagnetic analogues Sr2RuO4 and Sr3Ru2O7 with distinct Fermi surfaces for wide electronlike minority spin bands around the zone center and narrow holelike majority spin Fermi surface contours around the zone corners. The most dramatic results are two narrow spectral peaks ∼30 meV below the Fermi level, a spin-minority holelike band at the Brillouin zone center, and a spin-majority saddle-band van Hove singularity at the zone edge, which exhibits almost 100% spin polarization at low temperature, and a strong temperature dependent coherence-incoherence crossover attributed to Hund metal correlations. Quantitative comparison of the ARPES to spin-polarized density functional theory (DFT) calculations identify the specific antibonding and nonbonding orbital origins of the narrow bands, with a prediction of different spatial localization in the central and outer layers. This is shown to be consistent with experimental ARPES multizone matrix element intensity variations, and implicates outer-layer-specific control of the in-plane metamagnetism. The renormalization of the bands relative to the mean-field DFT, the demonstration of spin-polarized oxygen bands, and of spin-minority and spin-majority band-crossing hybridization provide a more complete picture of the magnetism which displays aspects of both delocalized and local moment behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Utilizing the maximum likelihood estimator for flow analysis

We explore the possibility of evaluating flow harmonics by employing the maximum likelihood estimator (MLE). For a given finite multiplicity, the MLE simultaneously furnishes estimations for all the parameters of the underlying distribution function while efficiently suppressing the variance of measures. Also, the method provides a means to assess a specific class of mixed harmonics, which is not straightforwardly feasible by the approaches primarily based on particle correlations. The results are analyzed using the Wald, likelihood ratio, and score tests of hypotheses. Besides, the resultant flow harmonics obtained using MLE are compared with those derived using particle correlations and event plane methods. Here, the dependencies of extracted flow harmonics on the multiplicity of individual events and the total number of events are analyzed. It is shown that the proposed approach works efficiently to deal with the deficiency in detector acceptability. Moreover, we elaborate on a fictitious scenario where the event plane is not a well-defined quantity in the distribution function. For the latter case, the MLE is shown to largely perform better than the two-particle correlation estimator. In this regard, one concludes that the MLE furnishes a meaningful alternative to the existing approaches for flow analysis.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2019

Guided Optimization of Phase-Unstable III–V Compositionally Graded Buffers by Cathodoluminescence Spectrum Imaging

Compositionally graded buffers (CGBs) are essential components of high-efficiency III-V inverted metamorphic solar cells, and high-quality CGBs with low defect densities are a requirement for high device efficiencies. Here, we use plan-view and cross-sectional cathodoluminescence spectrum imaging (CLSI) to investigate the effect of growth conditions on the microstructure of Al y Ga 1-x-y In x AS CGBs with x In > 0.30. CLSI reveals significant compositional fluctuation in these materials that correlates with the distribution of threading dislocations in the CGBs. Threading dislocations are observed to collect in regions between abrupt transitions in emission energy, implying that concomitant strain fluctuations in these regions restrict dislocation motion. Thus, the compositional fluctuation leads to elevated threading dislocation densities that degrade CGB quality. We also correlate compositional fluctuation in the CGB with compositional fluctuation in subsequently grown device layers. The effectiveness of specific growth conditions at suppressing compositional fluctuation and limiting threading dislocation density is evaluated. CGBs grown at high temperatures with high Al content exhibit the lowest defect densities. The use of a high V/III ratio suppresses compositional fluctuation and defect density. In conclusion, these insights lead directly to improvements in the efficiency of metamorphic Ga 1-x In x As solar cells grown on AlyGa 1-x-y In x As CGBs.

14 SOLAR ENERGY↗

DOE OSTI2017

Quantitative trait locus mapping of Populus bark features and stem diameter

Background: Bark plays important roles in photosynthate transport and storage, along with physical and chemical protection. Bark texture varies extensively among species, from smooth to fissured to deeply furrowed, but its genetic control is unknown. This study sought to determine the main genomic regions associated with natural variation in bark features and stem diameter. Quantitative trait loci (QTL) were mapped using an interspecific pseudo-backcross pedigree ( Populus trichocarpa x P. deltoides and P. deltoides ) for bark texture, bark thickness and diameter collected across three years, two sites and three biological replicates per site. Results: QTL specific to bark texture were highly reproducible in shared intervals across sites, years and replicates. Significant positive correlations and co-localization between trait QTL suggest pleiotropic regulators or closely linked genes. A list of candidate genes with related putative function, location close to QTL maxima and with the highest expression level in the phloem, xylem and cambium was identified. Conclusion: Candidate genes for bark texture included an ortholog of Arabidopsis ANAC104 (PopNAC128), which plays a role in lignified fiber cell and ray development, as well as Pinin and Fasciclin (PopFLA) genes with a role in cell adhesion, cell shape and migration. The results presented in this study provide a basis for future genomic characterization of genes found within the QTL for bark texture, bark thickness and diameter in order to better understand stem and bark development in Populus and other woody perennial plants. The QTL mapping approach identified a list of prime candidate genes for further validation using functional genomics or forward genetics approaches.

Populus trichocarpa↗

DOE OSTI2016

Oxide Interfaces: emergent structure and dynamics

This Final Report describes the scientific accomplishments that have been achieved with support from grant DE-FG02-06ER46273 during the period 6/1/2012– 5/31/2016. The overall goals of this program were focused on the behavior of epitaxial oxide heterostructures at atomic length scales (Ångstroms), and correspondingly short time-scales (fs -ns). The results contributed fundamentally to one of the currently most active frontiers in condensed matter physics research, namely to better understand the intricate relationship between charge, lattice, orbital and spin degrees of freedom that are exhibited by complex oxide heterostructures. The findings also contributed towards an important technological goal which was to achieve a better basic understanding of structural and electronic correlations so that the unusual properties of complex oxides can be exploited for energy-critical applications. Specific research directions included: probing the microscopic behavior of epitaxial interfaces and buried layers; novel materials structures that emerge from ionic and electronic reconfiguration at epitaxial interfaces; ultrahigh-resolution mapping of the atomic structure of heterointerfaces using synchrotron-based x-ray surface scattering, including direct methods of phase retrieval; using ultrafast lasers to study the effects of transient strain on coherent manipulation of multi-ferroic order parameters; and investigating structural ordering and relaxation processes in real-time.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Refined Outgassing Model for the LiH/LiOH System

Previous models for H 2 and H 2 O outgassing from moisture exposed LiH specimens were based on temperature programmed desorption/decomposition (TPD) experiments and analyses that yielded kinetic descriptions which were specific to defined sets of initial conditions. Subsequently the inputs to the kinetic models were refined to correlate with infrared spectra of the adsorbed species. In this current enhancement, better agreement between prediction and experiment was obtained through: 1) refining the reaction mechanisms for some of the decomposition reactions; 2) developing the potential range of kinetic parameters by simultaneous fitting of all the TPD outgassing curves obtained at different heating rates; and, 3) limiting the scope of possible kinetic parameters to an optimized subset by matching the outgassing prediction curves at 3 different temperatures to the outcomes from the accurate but cumbersome, non-intuitive, and effort demanding isoconversional analysis.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Achieving high-ZT in Te/Se-based thermoelectric materials via volume-constrained shear-induced plastic deformation

This study investigated the correlation between severe plastic deformation and microstructural evolution, along with the corresponding response in the thermoelectric performance. Specifically, severe plastic deformation introduced into intrinsically brittle thermoelectric materials like Bi 2 Te 3 - x Se x leads to microstructural changes ranging from nano- to micro- scales, including phase transformation, phase segregation, and stacking faults. The resultant residual strain and lattice defects in the Bi 2 Te 3-x Se x materials led to engineering of electronic bands, which increases the effective mass of electrons while controlling their concentration, thereby enhancing the power factor.

36 MATERIALS SCIENCE↗

NASA NTRS1974

A dual frequency radar for ocean roughness sampling

A microwave technique for determining the roughness (rms wave height) of a randomly varying air-water interface has been developed theoretically, verified with laboratory wavetank studies and is currently being implemented for ocean surface measurements in a series of applications flight experiments. These aircraft observations will be near the Chesapeake Light Tower and will include a range of altitudes and sea conditions. The measurement concept involves cross-correlating the envelope fluctuations on two received carriers that are monochromatic when transmitted in a normal direction to the interface and are observed in a backscatter direction after reflection by the large number of randomly distributed specular points on the surface. The measured correlation coefficient (normalized covariance) as a function of carrier frequency separation will depend on the surface roughness, specifically the probability density function of the specular point height. Details of the flight system are discussed, and preliminary flight results are presented.

Weissman, D. E.↗

NASA NTRS1991

Elevated temperature fracture of RS/PM aluminum alloy 8009

The fracture behavior of advanced powder metallurgy Al-Fe-V-Si alloy 8009 (previously called FVS0812) is being characterized under monotonic loads, as a function of temperature. Particular attention is focused on contributions to the fracture mechanism from the fine grained dispersoid strengthened microstructure, dissolved solute from rapid solidification, and the moist air environment. Time-dependent crack growth is characterized in advanced aluminum alloys at elevated temperatures with the fracture mechanics approach, and cracking mechanisms are examined with a metallurgical approach. Specific tasks were to obtain standard load crack growth experimental information from a refined testing system; to correlate crack growth kinetics with the j-integral and time dependent C(sub t)(t); and to investigate the intermediate temperature embrittlement of 8009 alloy in order to understand crack growth mechanisms.

Porr, William C., Jr.↗

NASA NTRS1994

Soot Volume Fraction Imaging

A new technique is described for the full-field determination of soot volume fractions via laser extinction measurements. This technique differs from previously reported point-wise methods in that a two-dimensional array (i.e., image) of data is acquired simultaneously. In this fashion, the net data rate is increased, allowing the study of time-dependent phenomena and the investigation of spatial and temporal correlations. A telecentric imaging configuration is employed to provide depth-invariant magnification and to permit the specification of the collection angle for scattered light. To improve the threshold measurement sensitivity, a method is employed to suppress undesirable coherent imaging effects. A discussion of the tomographic inversion process is provided, including the results obtained from numerical simulation. Results obtained with this method from an ethylene diffusion flame are shown to be in close agreement with those previously obtained by sequential point-wise interrogation.

Greenberg, Paul S.↗

NASA NTRS1996

Development of Low Density Titanium Alloys for Structural Applications

In this report the results of a program designed to reduce the density of titanium by adding magnesium are presented. Because these two elements are immiscible under conventional ingot metallurgy techniques, two specialized powder metallurgy methods namely, mechanical alloying (MA) and physical vapor deposition (PVD) were implemented. The mechanical alloying experiments were done both at the University of Idaho and at the Defense Research Agency in UK. Since titanium is reactive with interstitial elements, a secondary goal of this research was to correlate solubility extensions with interstitial contamination content, especially oxygen and nitrogen. MA was carried out in SPEX 8000 shaker mils and different milling containers were utilized to control the level of contamination. Results showed that solubilities of Mg in Ti were obtained up to 28 at.% (16.4 wt. %) Mg in Ti for Ti-39.6 at. % (25 wt. %) Mg alloys, which greatly exceed those obtained under equilibrium conditions. This reflects a density reduction of approximately 26 %. Contamination of oxygen and nitrogen seemed to increase the solubility of magnesium in titanium in some cases; however, we were not able to make a clear correlation between contamination levels with solubilities. Work at the DRA has emphasized optimization of present PVD equipment, specifically composition and temperature control. Preliminary PVD data has shown Ti-Mg deposits have successfully been made up to 2 mm thick and that solubility extensions were achieved. The potential for density reduction of titanium by alloying with magnesium has been demonstrated; however, this work has only scratched the surface of the development of such low density alloys. Much research is needed before such alloys could be implemented into industry. Further funding is required in order to optimize the MA/PVD processes including contamination control, determination of optimal alloy compositions, microstructure development, and mechanical property determination.

Froes, F. H.↗

NASA NTRS2001

Familial orthostatic tachycardia due to norepinephrine transporter deficiency

Orthostatic intolerance (OI) or postural tachycardia syndrome (POTS) is a syndrome primarily affecting young females, and is characterized by lightheadedness, palpitations, fatigue, altered mentation, and syncope primarily occurring with upright posture and being relieved by lying down. There is typically tachycardia and raised plasma norepinephrine levels on upright posture, but little or no orthostatic hypotension. The pathophysiology of OI is believed to be very heterogeneous. Most studies of the syndrome have focused on abnormalities in norepinephrine release. Here the hypothesis that abnormal norepinephrine transporter (NET) function might contribute to the pathophysiology in some patients with OI was tested. In a proband with significant orthostatic symptoms and tachycardia, disproportionately elevated plasma norepinephrine with standing, impaired systemic, and local clearance of infused tritiated norepinephrine, impaired tyramine responsiveness, and a dissociation between stimulated plasma norepinephrine and DHPG elevation were found. Studies of NET gene structure in the proband revealed a coding mutation that converts a highly conserved transmembrane domain Ala residue to Pro. Analysis of the protein produced by the mutant cDNA in transfected cells demonstrated greater than 98% reduction in activity relative to normal. NE, DHPG/NE, and heart rate correlated with the mutant allele in this family. CONCLUSION: These results represent the first identification of a specific genetic defect in OI and the first disease linked to a coding alteration in a Na+/Cl(-)-dependent neurotransmitter transporter. Identification of this mechanism may facilitate our understanding of genetic causes of OI and lead to the development of more effective therapeutic modalities.

Review, Tutorial↗

NASA NTRS2009

Sensitive, Rapid Detection of Bacterial Spores

A method of sensitive detection of bacterial spores within delays of no more than a few hours has been developed to provide an alternative to a prior three-day NASA standard culture-based assay. A capability for relatively rapid detection of bacterial spores would be beneficial for many endeavors, a few examples being agriculture, medicine, public health, defense against biowarfare, water supply, sanitation, hygiene, and the food-packaging and medical-equipment industries. The method involves the use of a commercial rapid microbial detection system (RMDS) that utilizes a combination of membrane filtration, adenosine triphosphate (ATP) bioluminescence chemistry, and analysis of luminescence images detected by a charge-coupled-device camera. This RMDS has been demonstrated to be highly sensitive in enumerating microbes (it can detect as little as one colony-forming unit per sample) and has been found to yield data in excellent correlation with those of culture-based methods. What makes the present method necessary is that the specific RMDS and the original protocols for its use are not designed for discriminating between bacterial spores and other microbes. In this method, a heat-shock procedure is added prior to an incubation procedure that is specified in the original RMDS protocols. In this heat-shock procedure (which was also described in a prior NASA Tech Briefs article on enumerating sporeforming bacteria), a sample is exposed to a temperature of 80 C for 15 minutes. Spores can survive the heat shock, but nonspore- forming bacteria and spore-forming bacteria that are not in spore form cannot survive. Therefore, any colonies that grow during incubation after the heat shock are deemed to have originated as spores.

Kern, Roger G.↗

NASA NTRS2010

Interactions of Ku70/80 with Double-Strand DNA: Energetic, Dynamics, and Functional Implications

Space radiation is a proficient inducer of DNA damage leading to mutation, aberrant cell signaling, and cancer formation. Ku is among the first responding proteins in nucleus to recognize and bind the DNA double strand breaks (DSBs) whenever they are introduced. Once loaded Ku works as a scaffold to recruit other repair factors of non-homologous end joining and facilitates the following repair processes. The crystallographic study of the Ku70/80 heterodimer indicate the core structure of this protein shows virtually no conformational change after binding with DNA. To investigate the dynamical features as well as the energetic characteristics of Ku-DNA binding, we conduct multi-nanosecond molecular dynamics simulations of a modeled Ku70/80 structure and several complexes with two 24-bp DNA duplexes. Free energy calculations show significant energy differences between the complexes with Ku bound at DSBs and those with Ku associated at an internal site of a chromosome. The results also reveal detailed interactions between different nucleotides and the amino acids along the DNA-binding cradle of Ku, indicating subtle binding preference of Ku at specific DNA sequences. The covariance matrix analyses along the trajectories demonstrate the protein is stimulated to undergo correlated motions of different domains once bound to DNA ends. Additionally, principle component analyses identify these low frequency collective motions suitable for binding with and translocation along duplex DNA. It is proposed that the modification of dynamical properties of Ku upon binding with DSBs may provide a signal for the further recruitment of other repair factors such as DNA-PKcs, XLF, and XRCC4.

Hu, Shaowen↗

NASA NTRS2020

Recent NASA Wind Tunnel Free-Flight Testing of a Multirotor Unmanned Aircraft System Vehicle

Wind tunnel free-flight tests of a small quadrotor vehicle were completed in the NASA Langley 12-Foot Low Speed Tunnel and 20-Foot Vertical Spin Tunnel. The primary objective of these tests was to acquire flight data in forward and descending flight for the purpose of assessing the fidelity of an all-attitude flight dynamics simulation previously developed from static and dynamic wind tunnel data. A secondary objective was to demonstrate and mature free-flight test methods specific to the emerging class of small unmanned aircraft and urban air mobility vehicles. Experimental flight data correlated well with simulation predictions which included steady trim conditions consistent with straight and level flight and quasi-steady conditions associated with descent velocities prone to vortex ring state development. The test method was shown to be well suited to these classes of vehicles and several unique infrastructure requirements, including tether systems and data acquisition requirements were identified. In addition, unique operational test techniques and safety procedures were developed to accommodate the testing. Results of these tests will be used to support air traffic management studies and research into safety assurance concepts that enable the integration of these classes of vehicles into the National Airspace System.

John V Foster↗

NASA NTRS2020

Multidisciplinary Systems Analysis of a Six Passenger Quadrotor Urban Air Mobility Vehicle Powertrain

There is a high degree of research interest in the design space for electric vertical takeoff and landing (eVTOL) vehicles, because these vehicles are seen as key enablers for urban air mobility (UAM). This work further explores the eVTOL design space, by presenting analysis of a six-passenger eVTOL quadrotor powertrain, with integrated power, propulsion, and thermal management systems modeled using the Numerical Propulsion System Simulation (NPSS) and the NPSS Power System Library. Four architectures are modeled at the same design point and compared over a design mission. Results from an architecture comparison study show that a hybrid architecture performs best in terms of cruise range, however, the hybrid needs batteries with high specific energy and specific power in order to obtain a benefit vs turboelectric architectures. Sensitivity studies are conducted to show correlations between component parameters and system metrics. This data provides useful indicators for further technological improvement. Lastly, an initial TMS model is presented, and sensitivity studies on TMS design parameters are presented as well, to show high level TMS design trends.

electrified propulsion↗