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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Room temperature spin-polarizations of Mn-based antiferromagnetic nanoelectrodes

Antiferromagnets produce no stray field, and therefore, a tip electrode made of antiferromagnetic material has been considered to be the most suitable choice to measure such as magnetoresistance (MR) through single isolated magnetic nanoparticles, molecules, and ultrathin films. Spin polarizations (P) of antiferromagnetic 3-nm, 6-nm, and annealed 3-nm Mn films grown on W tips with a bcc(110) apex as well as bulk-NiMn tips were obtained at 300 K by measuring MR in ultrahigh vacuum by means of spin-polarized scanning tunneling microscopy using a layerwise antiferromagnetically stacking bct-Mn(001) film electrode. The Mn-coated tips with coverages of 3 and 6 nm exhibited P values of 1 ± 1% and 3 ± 2%, respectively, which tips likely contain α- or strained Mn. With a thermal assist, the crystalline quality and the magnetic stability of the film could increase. The annealed tip exhibited P = 9 ± 2%. The bulk-NiMn tips exhibit spin polarizations of 0 or 6 ± 2% probably depending on the chemical species (Mn or Ni) present at the apex of the tip. Fe-coated W tips were used to estimate the bct-Mn(001) film spin polarization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structure, strain, and composition profiling of InAs/GaAs(211)B quantum dot superlattices

The morphology, nanostructure, and strain properties of InAs quantum dots (QDs) grown on GaAs(211)B, uncapped or buried, are explored by transmission electron microscopy and related quantitative techniques. Besides the built-in piezoelectric field, other differences of (211) growth compared to (100)-oriented growth are discussed in terms of the (211) surface non-singularity, leading to anisotropic shape of the QDs and local chemical inhomogeneity of the wetting layer. The shape of the uncapped QDs was precisely defined as truncated pyramidal, elongated along the 〈111〉 direction, and bounded by the (110), (100), and (213) facets. Local strain measurements showed that large surface QDs were almost unstrained due to plastic relaxation, exhibiting small residual elastic strain at the interface that gradually diminished toward their apex. Conversely, buried QDs were pseudomorphically grown on GaAs. By postulating a plane stress state, we have established a systematic increase of the local strain from the base toward the apex region of the QDs. Using Vegard's law, their chemical composition profiles were calculated, revealing an indium content gradient along the growth direction and compositional variants among different QDs. Photoluminescence measurements showed variations in emission energy between the QDs and consistency with a graded In-content, which complied with the quantitative strain analysis.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Strong-coupling quantum dot microscope

Here, we report the development and fabrication of a novel cryogenic instrument, a scanning quantum dot microscope (SQDM), capable of positioning a quantum dot to within tunneling range of a sample surface. The microscope is strong-coupling in the sense that electron wavefunctions in the quantum dot can directly overlap with electron wavefunctions in the sample, in contrast to probes based on electric-field coupling. The SQDM consists of a sharp glass probe with two asymmetric electrodes leading to an aluminum quantum dot at the apex. One electrode is capacitively coupled to the quantum dot, while the other lead is connected to the dot via a tunneling junction. A charge sensing circuit constructed from a high-electron-mobility transistor is attached to the capacitance lead to measure the charge state of the dot. The last key feature of the design is a tilted-apex geometry. This design feature, in conjunction with the capacitive sensing scheme, results in a robust quantum-dot probe capable of operating within angstrom distances of the sample surface while protecting the single tunnel junction responsible for electron transport onto the quantum dot. We demonstrate that our instrument can detect single-electron tunneling events and can also be operated as a standard scanning tunneling microscope capable of nanometer-scale resolution of the surface topography.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Design and crystal structure of a native-like HIV-1 envelope trimer that engages multiple broadly neutralizing antibody precursors in vivo

Induction of broadly neutralizing antibodies (bNAbs) by HIV-1 envelope glycoprotein immunogens would be a major advance toward an effective vaccine. A critical step in this process is the activation of naive B cells expressing germline (gl) antibody precursors that have the potential to evolve into bNAbs. Here, we reengineered the BG505 SOSIP.664 glycoprotein to engage gl precursors of bNAbs that target either the trimer apex or the CD4-binding site. The resulting BG505 SOSIP.v4.1-GT1 trimer binds multiple bNAb gl precursors in vitro. Immunization experiments in knock-in mice expressing gl-VRC01 or gl-PGT121 show that this trimer activates B cells in vivo, resulting in the secretion of specific antibodies into the sera. A crystal structure of the gl-targeting trimer at 3.2-Å resolution in complex with neutralizing antibodies 35O22 and 9H+109L reveals a native-like conformation and the successful incorporation of design features associated with binding of multiple gl-bNAb precursors.

Medina-Ramírez, Max↗
DOE OSTI2013

SPATIALLY RESOLVED SPECTROSCOPY OF A PULSAR WIND NEBULA IN MSH 15-56

We report the study of a pulsar wind nebula (PWN) in the middle-aged supernova remnant (SNR) MSH 15-56. High-resolution X-ray imaging observations using XMM-Newton and Chandra provided clear images of its comet-like structure, as seen in other PWNe moving rapidly through interstellar mediums. At the PWN apex, Chandra detected a point source emitting a power-law spectrum with a photon index of {Gamma} = 1.3. The photon index of the PWN steepens from 1.7 to 2.5 along the flow line from the apex toward the tail, implying that the PWN is powered by the point source. The opening angle of the tail implies a pulsar velocity of v{sub PSR} = 1900 km s{sup -1}. We also discovered a thin X-ray filament at the edge of the SNR and just near the PWN. Assuming that the SNR is in the Sedov phase, the shell is expanding at 860 km s{sup -1}, which is consistent with the existence of the non-thermal filament. Based on the physical relationship, the PWN will run through the blast wave in the near future.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

X-ray emission from η Carinae near periastron in 2009. I. A two-state solution

X-ray emission from the supermassive binary system η Car declines sharply around periastron. This X-ray minimum has two distinct phases—the lowest flux phase in the first ∼3 weeks and a brighter phase thereafter. In 2009, the Chandra X-ray Observatory monitored the first phase five times and found the lowest observed flux at ∼1.9 × 10{sup –12} erg cm{sup –2} s{sup –1} (3-8 keV). The spectral shape changed such that the hard band above ∼4 keV dropped quickly at the beginning and the soft band flux gradually decreased to its lowest observed value in ∼2 weeks. The hard band spectrum had begun to recover by that time. This spectral variation suggests that the shocked gas producing the hottest X-ray gas near the apex of the wind-wind collision (WWC) is blocked behind the dense inner wind of the primary star, which later occults slightly cooler gas downstream. Shocked gas previously produced by the system at earlier orbital phases is suggested to produce the faint residual X-ray emission seen when the emission near the apex is completely blocked by the primary wind. The brighter phase is probably caused by the re-appearance of the WWC plasma, whose emissivity significantly declined during the occultation. We interpret this to mean that the X-ray minimum is produced by a hybrid mechanism of an occultation and a decline in the emissivity of the WWC shock. We constrain timings of superior conjunction and periastron based on these results.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Isotopologues of dense gas tracers in NGC 1068

We present observations of isotopic lines of dense gas tracers toward the nuclear region of nearby Seyfert 2 galaxy NGC 1068 with the IRAM 30 m telescope and the Atacama Pathfinder Experiment (APEX) 12 m telescope. We detected four isotopic lines (H{sup 13}CN 1-0, H{sup 13}CO{sup +} 1-0, HN{sup 13}C 1-0, and HC{sup 18}O{sup +} 1-0) at the 3 mm band with the IRAM 30 m telescope and obtained upper limits of other lines. We calculated optical depths of dense gas tracers with the detected isotopic lines of HCN 1-0, HCO{sup +} 1-0, and HNC 1-0. We find that the {sup 14}N/{sup 15}N abundance ratio is greater than 420 if we adopt the upper limit of HC{sup 15}N(1-0) emission. Combining this with fluxes of 1-0 lines from IRAM 30 m observations and the upper limit of 3-2 lines from APEX 12 m observations, we also estimated the excitation condition of molecular gas in the nuclear region of NGC 1068, which is less dense than that in the extreme starburst regions of galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Statistical evidence for the existence of Alfvénic turbulence in solar coronal loops

Recent observations have demonstrated that waves capable of carrying large amounts of energy are ubiquitous throughout the solar corona. However, the question of how this wave energy is dissipated (on which timescales and length scales) and released into the plasma remains largely unanswered. Both analytic and numerical models have previously shown that Alfvénic turbulence may play a key role not only in the generation of the fast solar wind, but in the heating of coronal loops. In an effort to bridge the gap between theory and observations, we expand on a recent study by analyzing 37 clearly isolated coronal loops using data from the Coronal Multi-channel Polarimeter instrument. We observe Alfvénic perturbations with phase speeds which range from 250 to 750 km s{sup –1} and periods from 140 to 270 s for the chosen loops. While excesses of high-frequency wave power are observed near the apex of some loops (tentatively supporting the onset of Alfvénic turbulence), we show that this excess depends on loop length and the wavelength of the observed oscillations. In deriving a proportional relationship between the loop length/wavelength ratio and the enhanced wave power at the loop apex, and from the analysis of the line widths associated with these loops, our findings are supportive of the existence of Alfvénic turbulence in coronal loops.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The Atacama Cosmology Telescope: The LABOCA/ACT survey of clusters at all Redshifts

We present a multi-wavelength analysis of 11 Sunyaev–Zel’dovich effect (SZE)-selected galaxy clusters (10 with new data) from the Atacama Cosmology Telescope (ACT) southern survey. We have obtained new imaging from the Large APEX Bolometer Camera (345 GHz; LABOCA) on the Atacama Pathfinder EXperiment (APEX) telescope, the Australia Telescope Compact Array (2.1 GHz; ATCA), and the Spectral and Photometric Imaging Receiver (250, 350, and 500 μm; SPIRE) on the Herschel Space Observatory.{sup 24} Spatially resolved 345 GHz SZE increments with integrated signal-to-noise ratio (S/N)>5 are found in six clusters. We compute 2.1 GHz number counts as a function of cluster-centric radius and find significant enhancements in the counts of bright sources at projected radii θ<θ{sub 2500c}. By extrapolating in frequency, we predict that the combined signals from 2.1 GHz-selected radio sources and 345 GHz-selected submillimeter galaxies (SMGs) contaminate the 148 GHz SZE decrement signal by ∼5% and the 345 GHz SZE increment by ∼18%. After removing radio source and SMG emission from the SZE signals, we use ACT, LABOCA, and (in some cases) new Herschel SPIRE imaging to place constraints on the clusters’ peculiar velocities. The sample’s average peculiar velocity relative to the cosmic microwave background is 〈v{sub p}〉=153±383 km s{sup −1}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SOLAR MICRO-TYPE III BURST STORMS AND LONG DIPOLAR MAGNETIC FIELD IN THE OUTER CORONA

Solar micro-type III radio bursts are elements of the so-called type III storms and are characterized by short-lived, continuous, and weak emissions. Their frequency of occurrence with respect to radiation power is quite different from that of ordinary type III bursts, suggesting that the generation process is not flare-related, but due to some recurrent acceleration processes around the active region. We examine the relationship of micro-type III radio bursts with coronal streamers. We also explore the propagation channel of bursts in the outer corona, the acceleration process, and the escape route of electron beams. It is observationally confirmed that micro-type III bursts occur near the edge of coronal streamers. The magnetic field line of the escaping electron beams is tracked on the basis of the frequency drift rate of micro-type III bursts and the electron density distribution model. The results demonstrate that electron beams are trapped along closed dipolar field lines in the outer coronal region, which arise from the interface region between the active region and the coronal hole. A 22 year statistical study reveals that the apex altitude of the magnetic loop ranges from 15 to 50 R{sub S}. The distribution of the apex altitude has a sharp upper limit around 50 R{sub S} suggesting that an unknown but universal condition regulates the upper boundary of the streamer dipolar field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

{sup 13}C-METHYL FORMATE: OBSERVATIONS OF A SAMPLE OF HIGH-MASS STAR-FORMING REGIONS INCLUDING ORION-KL AND SPECTROSCOPIC CHARACTERIZATION

We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic center for methyl formate, HCOOCH{sub 3}, and its isotopologues H{sup 13}COOCH{sub 3} and HCOO{sup 13}CH{sub 3}. The observations were carried out with the APEX telescope in the frequency range 283.4-287.4 GHz. Based on the APEX observations, we report tentative detections of the {sup 13}C-methyl formate isotopologue HCOO{sup 13}CH{sub 3} toward the following four massive star-forming regions: Sgr B2(N-LMH), NGC 6334 IRS 1, W51 e2, and G19.61-0.23. In addition, we have used the 1 mm ALMA science verification observations of Orion-KL and confirm the detection of the {sup 13}C-methyl formate species in Orion-KL and image its spatial distribution. Our analysis shows that the {sup 12}C/{sup 13}C isotope ratio in methyl formate toward the Orion-KL Compact Ridge and Hot Core-SW components (68.4 ± 10.1 and 71.4 ± 7.8, respectively) are, for both the {sup 13}C-methyl formate isotopologues, commensurate with the average {sup 12}C/{sup 13}C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with the {sup 12}C/{sup 13}C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H{sup 13}COOCH{sub 3} and HCOO{sup 13}CH{sub 3} species. New spectroscopic data for both isotopomers H{sup 13}COOCH{sub 3} and HCOO{sup 13}CH{sub 3}, presented in this study, have made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Quantification of in-contact probe-sample electrostatic forces with dynamic atomic force microscopy

Here, atomic force microscopy (AFM) methods utilizing resonant mechanical vibrations of cantilevers in contact with a sample surface have shown sensitivities as high as few picometers for detecting surface displacements. Such a high sensitivity is harnessed in several AFM imaging modes. Here, we demonstrate a cantilever-resonance-based method to quantify electrostatic forces on a probe in the probe-sample junction in the presence of a surface potential or when a bias voltage is applied to the AFM probe. We find that the electrostatic forces acting on the probe tip apex can produce signals equivalent to a few pm of surface displacement. In combination with modeling, the measurements of the force were used to access the strength of the electrical field at the probe tip apex in contact with a sample. We find an evidence that the electric field strength in the junction can reach ca. 1 V nm –1 at a bias voltage of a few volts and is limited by non-ideality of the tip-sample contact. This field is sufficiently strong to significantly influence material states and kinetic processes through charge injection, Maxwell stress, shifts of phase equilibria, and reduction of energy barriers for activated processes. Besides, the results provide a baseline for accounting for the effects of local electrostatic forces in electromechanical AFM measurements as well as offer additional means to probe ionic mobility and field-induced phenomena in solids.

36 MATERIALS SCIENCE↗
DOE OSTI2014

TIME EVOLUTION OF PLASMA PARAMETERS DURING THE RISE OF A SOLAR PROMINENCE INSTABILITY

We present high-spatial resolution spectropolarimetric observations of a quiescent hedgerow prominence taken in the He I 1083.0 nm triplet. The observation consisted of a time series in sit-and-stare mode of ∼36 minutes duration. The spectrograph's slit crossed the prominence body and we recorded the time evolution of individual vertical threads. Eventually, we observed the development of a dark Rayleigh-Taylor plume that propagated upward with a velocity, projected onto the plane of the sky, of 17 km s{sup –1}. Interestingly, the plume apex collided with the prominence threads pushing them aside. We inferred Doppler shifts, Doppler widths, and magnetic field strength variations by interpreting the He I Stokes profiles with the HAZEL code. The Doppler shifts show that clusters of threads move coherently while individual threads have oscillatory patterns. Regarding the plume we found strong redshifts (∼9-12 km s{sup –1}) and large Doppler widths (∼10 km s{sup –1}) at the plume apex when it passed through the prominence body and before it disintegrated. We associate the redshifts with perspective effects while the Doppler widths are more likely due to an increase in the local temperature. No local variations of the magnetic field strength associated with the passage of the plume were found; this leads us to conclude that the plumes are no more magnetized than the surroundings. Finally, we found that some of the threads' oscillations are locally damped, what allowed us to apply prominence seismology techniques to infer additional prominence physical parameters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Turning points in the age–metallicity relations – created by late satellite infall and enhanced by radial migration

ABSTRACT The present-day age–metallicity relation (AMR) is a record of the star formation history of galaxies, as this traces the chemical enrichment of the gas over time. We use a zoomed-in cosmological simulation that reproduces key signatures of the Milky Way (MW), g2.79e12 from the NIHAO-UHD project, to examine how stellar migration and satellite infall shape the AMR across the disc. We find in the simulation, similar to the MW, the AMR in small spatial regions (R, z) shows turning points that connect changes in the direction of the relations. The turning points in the AMR in the simulation are a signature of late satellite infall. This satellite infall has a mass radio similar as that of the Sagittarius dwarf to the MW (∼0.001). Stars in the apex of the turning points are young and have nearly not migrated. The late satellite infall creates the turning points via depositing metal-poor gas in the disc, triggering star formation of stars in a narrow metallicity range compared to the overall AMR. The main effect of radial migration on the AMR turning points is to widen the metallicity range of the apex. This can happen when radial migration brings stars born from the infallen gas in other spatial bins, with slightly different metallicities, into the spatial bin of interest. These results indicate that it is possible that the passage of the Sagittarius dwarf galaxy played a role in creating the turning points that we see in the AMR in the Milky Way.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Numerical exploration of spontaneous broken symmetries in multiorbital Hubbard models

Here, we study three proposals for broken symmetry in the cuprate pseudogap-oxygen antiferromagnetism, Theta(II) orbital loop currents, and circulating currents involving apex oxygens-through numerical exploration of multiorbital Hubbard models. Our numerically exact results show no evidence for the existence of oxygen antiferromagnetic order or the Theta(II) phase in the three-orbital Hubbard model. The model also fails to sustain an ordered current pattern even with the presence of additional apex oxygen orbitals. Thus, we conclude that it is difficult to stabilize the aforementioned phases in the multiorbital Hubbard models for parameters relevant to cuprate superconductors. However, the Theta(II) phase might be stabilized through explicit flux terms. We also found an enhanced propensity for circulating currents with such terms in calculations simulating applied stress or strain, which skew the copper-oxygen plane to resemble a kagome lattice. We propose an experimental viewpoint to shed additional light on this problem.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Visualization and Quantum Computation of Moire Superconductivity in Bilayer Graphene, Carbon Nanocones and Nanostrips

The purpose of this project is to visualize and study the electronic properties of carbon nanostructures. The principal structures studied are AA and AB (Bernal) stacked graphene bilayers, an 80C nanocone with pentagonal defects at its apex, and a 16C nanoring. Graphene and other based nanostructures have attracted much attention due to their unique electronic properties. For example, bilayer graphene shows superconductivity at certain magic angles of rotation. Open source molecular modeling and visualization programs like SAMSON, Avogadro, and VMD are utilized to generate the structures above. Using bond information in order to find nearest neighbors, an adjacency matrix can be generated from which the energy eigenvalues are solvable and are used to visualize the dispersion relation, free energy, persistent current, Green’s function, and entanglement entropy. The goal of this project is to study the effect of rotation on the electronic properties of bilayer graphene, particularly as it pertains to the magic angle where superconductivity occurs, as well as the electronic properties of the bilayer Moire patterns which emerge due to the superposition of the identical lattices at different angles. An important goal is to incorporate phonon interactions into the tight-binding model for carbon structures, and to utilize quantum based computing as a tool to find the ground state properties of these structures, particularly in the terminating case where boundary conditions have an effect.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2018

The response of boreal peatland community composition and NDVI to hydrologic change, warming, and elevated carbon dioxide

Abstract Widespread changes in arctic and boreal Normalized Difference Vegetation Index ( NDVI ) values captured by satellite platforms indicate that northern ecosystems are experiencing rapid ecological change in response to climate warming. Increasing temperatures and altered hydrology are driving shifts in ecosystem biophysical properties that, observed by satellites, manifest as long‐term changes in regional NDVI . In an effort to examine the underlying ecological drivers of these changes, we used field‐scale remote sensing of NDVI to track peatland vegetation in experiments that manipulated hydrology, temperature, and carbon dioxide ( CO 2 ) levels. In addition to NDVI , we measured percent cover by species and leaf area index ( LAI ). We monitored two peatland types broadly representative of the boreal region. One site was a rich fen located near Fairbanks, Alaska, at the Alaska Peatland Experiment ( APEX ), and the second site was a nutrient‐poor bog located in Northern Minnesota within the Spruce and Peatland Responses Under Changing Environments ( SPRUCE ) experiment. We found that NDVI decreased with long‐term reductions in soil moisture at the APEX site, coincident with a decrease in photosynthetic leaf area and the relative abundance of sedges. We observed increasing NDVI with elevated temperature at the SPRUCE site, associated with an increase in the relative abundance of shrubs and a decrease in forb cover. Warming treatments at the SPRUCE site also led to increases in the LAI of the shrub layer. We found no strong effects of elevated CO 2 on community composition. Our findings support recent studies suggesting that changes in NDVI observed from satellite platforms may be the result of changes in community composition and ecosystem structure in response to climate warming.

McPartland, Mara Y.↗
DOE OSTI2023

Shape tuning of large area silicon nanotip arrays through reactive ion etching

Nanostructures formed in silicon form an important class of structures that span a broad spectrum of application areas. Of these, columnar structures of silicon featuring tiplike apexes have their own niche applications. Here, the ability to afford shape tunability for these structures further enhances their application potential. In this paper, we present our findings on the large area fabrication of silicon nanotips defined through microsphere lithography and shape tuned through a combination of different reactive ion etching (RIE) techniques. The self-sharpening mechanism of the tips when using nonplanar etch masks (microspheres) under anisotropic etching conditions is elucidated. We further show that depending on the manner of etching (continuous versus discrete multistep etch), identical anisotropic etching recipes produce vastly different tip morphologies. Hourglass-shaped silicon tips were obtained when silicon was subjected to anisotropic followed by isotropic etching conditions. Sharp silicon tips with tip apex radii on the order of 2 nm have been successfully realized when the RIE shape tuned tips were subjected to a series of oxidative sharpening steps.

74 ATOMIC AND MOLECULAR PHYSICS↗