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At least 451 records · Page 25

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NASA NTRS1988

Essentially non-oscillatory shock capturing methods applied to turbulence amplification in shock wave calculations

ENO (essentially non-oscillatory) schemes can provide uniformly high order accuracy right up to discontinuities while keeping sharp, essentially non-oscillatory shock transitions. Recently, an efficient implementation of ENO schemes was obtained based on fluxes and TVD Runge-Kutta time discretizations. The resulting code is very simple to program for multi-dimensions. ENO schemes are especially suitable for computing problems with both discontinuities and fine structures in smooth regions, such as shock interaction with turbulence, for which results for 1-D and 2-D Euler equations are presented. Much better resolution is observed by using third order ENO schemes than by using second order TVD schemes for such problems.

Osher, Stanley↗

NASA NTRS2003

Quartz Microbalance Study of 400-angstrom Thick Films near the lambda Point

In a recent measurement we observed the thinning of an adsorbed helium film induced by the confinement of critical fluctuations a few millikelvin below the lambda point. A capacitor set-up was used to measure this Casimir effect. In this poster we will present our measurement of an adsorbed helium film of 400 angstroms near the lambda point with a quartz microbalance. For films this thick, we must take into account the non-linear dynamics of the shear waves in the fluid. In spite of the added complications, we were able to confirm the thinning of the film due to the Casimir effect and the onset of the superfluid transition. In addition, we observe a sharp anomaly at the bulk lambda point, most likely related to critical dissipation of the first sound. This work is carried out in collaboration with Rafael Garcia, Stephen Jordon and John Lazzaretti. This work is funded by NASA's Office of Biological and Physical Research under grant.

Chan, Moses H. W.↗

NASA NTRSDate not supplied

Isochemical Characteristics of the Clay-Sulfate Transition in Gale Crater, Mars: APXS Results From Mont Mercou to the Marker Band Valley

During sols 3052-3572 (March 2021 to August 2022), Curiosity traversed a region in Gale crater where orbital observations predicted a vertical stratigraphic transition from a phyllosilicate unit to the overlying layered sulfates of Mt. Sharp [1, 2]. This region was informally named the clay-sulfate transition (CST). The Alpha Particle X-ray Spectrometer (APXS) was systematically deployed to sedimentary bedrock and selected diagenetic features to analyze possible chemical changes with increasing elevation (a proxy for increasing stratigraphic position). The occurrence of Ca- and Mg-sulfate was a particular focus. Here, we present results from the APXS investigation of the CST, as well as results from the first targets of the sulfate unit located in Marker Band Valley (MBV).

J.A. Berger↗

NASA NTRSDate not supplied

Mars Science Laboratory Mastcam Multispectral Investigation of Drill Targets From the Beginning of the Clay Sulfate Transition to Marker Band Valley

In March, 2021 the Mars Science Laboratory Curiosity rover officially started exploring the Clay Sulfate Transition (CST). This area of Mt. Sharp has been interpreted to have hydrated Mg-sulfate spectral signatures in CRISM orbital data, and it no longer has the strong phyllosilicate spectral signature as the previous Glen Torridon region. Recently, the Curiosity rover has also encountered the marker band that was also identified in orbital data [1,3]. This shift from phyllosilicates to hydrated Mg-sulfates suggests a major change in the environmental conditions during which these rocks were deposited. This abstract focuses on changes seen in the Mastcam multispectral data between the CST drill targets.

S R Jacob↗

DOE OSTI2022

Charge-neutral fermions and magnetic field-driven instability in insulating YbIr 3 Si 7

Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr 3 Si 7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr 3 Si 7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.

36 MATERIALS SCIENCE↗

NASA NTRS1961

Effects of Cone Angle, Mach Number, and Nose Blunting on Transition at Supersonic Speeds

An investigation has been made to determine the transition characteristics of a group of blunt cones which varied in included apex angle from 27 deg to 60 deg over a Mach number range from 1.61 to 2.20 and a range of tunnel Reynolds number per foot from about 1.5 x 10(exp 6) to 8.0 x 10(exp 6). The tests were made at zero angle of attack and with zero heat transfer. The results indicate that the general level of transition Reynolds number based on boundary-layer momentum thickness and local flow conditions just outside the boundary layer varied between 600 and 1,100. Changes in Mach number had little effect on transition distance and transition Reynolds number for the near-sharp or very small bluntnesses. The effect of Mach number variation on the larger hemispherical bluntnesses was much stronger, with the strongest Mach number effect occurring for Mach numbers between 1.61 and 1.82. With an increase in nose radius, there was a strong decrease in transition distance and transition Reynolds number at the lower Mach numbers. This adverse effect tended to become weaker with increase in Mach number. An increase in cone angle at a constant Mach number caused a reduction in transition distance and transition Reynolds number for the blunt configurations which had approximately the same values of nose radius.

Czarnecki, K. R.↗

DOE OSTI2013

Phase transitions in the assembly of multivalent signalling proteins

Cells are organized on length scales ranging from angstrom to micrometers. However, the mechanisms by which angstrom-scale molecular properties are translated to micrometer-scale macroscopic properties are not well understood. Here we show that interactions between diverse synthetic, multivalent macromolecules (including multi-domain proteins and RNA) produce sharp liquid-liquid-demixing phase separations, generating micrometer-sized liquid droplets in aqueous solution. This macroscopic transition corresponds to a molecular transition between small complexes and large, dynamic supramolecular polymers. The concentrations needed for phase transition are directly related to the valency of the interacting species. In the case of the actin-regulatory protein called neural Wiskott-Aldrich syndrome protein (N-WASP) interacting with its established biological partners NCK and phosphorylated nephrin1, the phase transition corresponds to a sharp increase in activity towards an actin nucleation factor, the Arp2/3 complex. The transition is governed by the degree of phosphorylation of nephrin, explaining how this property of the system can be controlled to regulatory effect by kinases. The widespread occurrence of multivalent systems suggests that phase transitions may be used to spatially organize and biochemically regulate information throughout biology.

59 BASIC BIOLOGICAL SCIENCES↗

NASA NTRS1972

Polarized absorption spectra of single crystals of lunar pyroxenes and olivines.

Measurements have been made of the polarized absorption spectra (360-2200 nm) of compositionally zoned pyroxene minerals in rocks 10045, 10047 and 10058 and olivines in rocks 10020 and 10022. The Apollo 11 pyroxenes with relatively high Ti/Fe ratios were chosen initially to investigate the presence of crystal field spectra of Fe(2+) and Ti(3+) ions in the minerals. Broad intense bands at about 1000 and 2100 nm arise from spin-allowed, polarization-dependent transitions in Fe(2+) ions in pyroxenes. Several weak sharp peaks occur in the visible region. Peaks at 402, 425, 505, 550, and 585 nm represent spin-forbidden transitions in Fe(2+) ions, while broader bands at 460-470 nm and 650-660 nm are attributed to Ti(3+) ions. Charge transfer bands, which in terrestrial pyroxenes often extend into the visible region, are displaced to shorter wavelengths in lunar pyroxenes. This feature correlates with the absence of Ti(3+) ions in these minerals.

Burns, R. G.↗

NASA NTRS1981

Some anomalies between wind tunnel and flight transition results

A review of environmental disturbance influence and boundary layer transition measurements on a large collection of reference sharp cone tests in wind tunnels and of recent transonic-supersonic cone flight results have previously demonstrated the dominance of free-stream disturbance level on the transition process from the beginning to end. Variation of the ratio of transition Reynolds number at onset-to-end with Mach number has been shown to be consistently different between flight and wind tunnels. Previous correlations of the end of transition with disturbance level give good results for flight and large number of tunnels, however, anomalies occur for similar correlation based on transition onset. Present cone results with a tunnel sonic throat reduced the disturbance level by an order of magnitude with transition values comparable to flight.

Harvey, W. D.↗

DOE OSTI2015

Gaps and pseudogaps in perovskite rare earth nickelates

We report on tunneling measurements that reveal the evolution of the quasiparticle state density in two rare earth perovskite nickelates, NdNiO{sub 3} and LaNiO{sub 3}, that are close to a bandwidth controlled metal to insulator transition. We measure the opening of a sharp gap of ∼30 meV in NdNiO{sub 3} in its insulating ground state. LaNiO{sub 3}, which remains a correlated metal at all practical temperatures, exhibits a pseudogap of the same order. The results point to both types of gaps arising from a common origin, namely, a quantum critical point associated with the T = 0 K metal-insulator transition. The results support theoretical models of the quantum phase transition in terms of spin and charge instabilities of an itinerant Fermi surface.

36 MATERIALS SCIENCE↗

DOE OSTI2012

QATAR-2: A K DWARF ORBITED BY A TRANSITING HOT JUPITER AND A MORE MASSIVE COMPANION IN AN OUTER ORBIT

We report the discovery and initial characterization of Qatar-2b, a hot Jupiter transiting a V = 13.3 mag K dwarf in a circular orbit with a short period, P{sub b} = 1.34 days. The mass and radius of Qatar-2b are M{sub P} = 2.49 M{sub J} and R{sub P} = 1.14 R{sub J}, respectively. Radial-velocity monitoring of Qatar-2 over a span of 153 days revealed the presence of a second companion in an outer orbit. The Systemic Console yielded plausible orbits for the outer companion, with periods on the order of a year and a companion mass of at least several M{sub J}. Thus, Qatar-2 joins the short but growing list of systems with a transiting hot Jupiter and an outer companion with a much longer period. This system architecture is in sharp contrast to that found by Kepler for multi-transiting systems, which are dominated by objects smaller than Neptune, usually with tightly spaced orbits that must be nearly coplanar.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers

Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature T g , whether there is an underlying thermodynamic transition at some finite temperature below T g , what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section discusses the specific case of polymeric glass-forming liquids, which usually have extremely high fragility. We emphasize the apparent difference between the glass transitions in polymers and small molecules. We also discuss the possible role of quantum effects in the glass transition of light molecules and highlight the recent discovery of the unusually low fragility of water. At the end, we formulate the major challenges and questions remaining in this field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2012

Bifurcation theory for the L-H transition in magnetically confined fusion plasmas

The mathematical field of bifurcation theory is extended to be applicable to 1-dimensionally resolved systems of nonlinear partial differential equations, aimed at the determination of a certain specific bifurcation. This extension is needed to be able to properly analyze the bifurcations of the radial transport in magnetically confined fusion plasmas. This is of special interest when describing the transition from the low-energy-confinement state to the high-energy-confinement state of the radial transport in fusion plasmas (i.e., the L-H transition), because the nonlinear dynamical behavior during the transition corresponds to the dynamical behavior of a system containing such a specific bifurcation. This bifurcation determines how the three types (sharp, smooth, and oscillating) of observed L-H transitions are organized as function of all the parameters contained in the model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2016

Evolution of OH and CO-dark molecular gas fraction across a molecular cloud boundary in Taurus

We present observations of {sup 12}CO J = 1-0, {sup 13}CO J = 1-0, H i, and all four ground-state transitions of the hydroxyl (OH) radical toward a sharp boundary region of the Taurus molecular cloud. Based on a photodissociation region (PDR) model that reproduces CO and [C i] emission from the same region, we modeled the three OH transitions, 1612, 1665, and 1667 MHz successfully through escape probability non-local thermal equilibrium radiative transfer model calculations. We could not reproduce the 1720 MHz observations, due to unmodeled pumping mechanisms, of which the most likely candidate is a C-shock. The abundance of OH and CO-dark molecular gas is well-constrained. The OH abundance [OH]/[H{sub 2}] decreases from 8×10{sup −7} to 1×10{sup −7} as A{sub v} increases from 0.4 to 2.7 mag following an empirical law: which is higher than PDR model predictions for low-extinction regions by a factor of 80. The overabundance of OH at extinctions at or below 1 mag is likely the result of a C-shock. The dark gas fraction (DGF, defined as the fraction of molecular gas without detectable CO emission) decreases from 80% to 20% following a Gaussian profile: This trend of the DGF is consistent with our understanding that the DGF drops at low visual extinction due to photodissociation of H{sub 2} and drops at high visual extinction due to CO formation. The DGF peaks in the extinction range where H{sub 2} has already formed and achieved self-shielding but {sup 12}CO has not. Two narrow velocity components with a peak-to-peak spacing of ∼1 km s{sup −1} were clearly identified. Their relative intensity and variation in space and frequency suggest colliding streams or gas flows at the boundary region.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2018

A vibrational spectroscopic study of kernite to 25 GPa: Implications for the high-pressure stability of borate polyhedra

The Raman and infrared spectra of kernite [Na 2 B 4 O 6 (OH) 2 ·3H 2 O] have been characterized up to ~25 and 23 GPa, respectively, to explore pressure-induced changes in a structurally novel mineral that contains mixed coordination borate groups (threefold and fourfold), as well as both hydroxyl groups and water molecules. BO 3 and BO 4 vibrational modes are characterized in both the Raman and infrared spectra of kernite, and reassignments of some modes are made based on observed pressure shifts. Under compression to ~25 GPa, kernite undergoes three phase transitions: one initiates near ~2.5 GPa, the second occurs at ~7.0 GPa, and the third near 11.0 GPa. The first transition is characterized by a loss of both a subset of the Na-associated modes and a sharp OH peak, and it is fully reversible. The second transition is characterized by the loss of most of the BO 3 modes, some of the BO 4 modes at ~7 GPa, and further broadening of the H 2 O and OH peaks. This transition is partially reversible on decompression, but the Raman spectra indicate that the high-pressure structure and its reversion products are likely disordered. The third transition is characterized by the loss of most of the Raman and infrared modes, and it is not reversible on decompression. The decompression products from ~25 GPa have markedly different infrared and Raman spectra from the starting material, and appear to involve different bonding environments of boron than are present in the starting materials: thus, pressure treatment of boron-rich materials may generate novel quenchable structures. Notably, the BO 3 groups in kernite are destabilized under compression, with the BO 4 groups appearing to be more stable under compression. This shift in coordination may be endemic among borate glasses and crystalline borates under compression. The coordination conversion (and its products on decompression) is likely to highly depend on the detailed topology of the starting materials. Commonalities and differences between the threefold to fourfold transitions in borate systems relative to those in carbonate systems are presented. Lastly our results, combined with prior shock data, indicate that kernite, which may be representative of low-temperature borate minerals, can persist during impact events with peak shock pressures less than ~33 GPa, and the primary polyborate structural units involved in early ribose stabilization would thus survive most impact conditions.

36 MATERIALS SCIENCE↗

DOE OSTI2013

Color tunable phosphor CaMoO{sub 4}:Eu{sup 3+},Li{sup +} via energy transfer of MoO{sub 4}{sup 2−}–Eu{sup 3+} dependent on morphology and doping concentration

Graphical abstract: Nanostructured phosphor CaMoO{sub 4}:Eu{sup 3+},Li{sup +} with various delicate morphologies has been obtained by molten salt synthesis. The luminescent color and the morphology of the phosphor are tuned by controlling the sintering temperature and the molar ratio of salt KCl to molybdates CaMoO{sub 4}. Highlights: ► Phosphor CaMoO{sub 4}:Eu{sup 3+},Li{sup +} with various delicate morphologies has been obtained. ► Energy transfer MoO{sub 4}{sup 2−} → Eu{sup 3+} is dependent on morphology and doping concentration. ► The luminescent color can be tuned by controlling energy transfer of MoO{sub 4}{sup 2−}–Eu{sup 3+}. - Abstract: The cooling rate and the solubility of molybdate in molten salt play crucial roles on the microscopic morphologies and photoluminescence (PL) properties of the phosphor CaMoO{sub 4}:Eu{sup 3+},Li{sup +}. Undoped CaMoO{sub 4} exhibits self-activated green PL with a broad emission band which is assigned to the spin forbidden {sup 3}T{sub 1,2}–{sup 1}A{sub 1} transition of the MoO{sub 4}{sup 2−} complex. The strong UV absorption of undoped CaMoO{sub 4} is originated from the charge transfer from O{sup 2−} ligands to the central Mo{sup 6+} ions inside MoO{sub 4}{sup 2−} group. The emission of the phosphor CaMoO{sub 4}:Eu{sup 3+},Li{sup +} obtained by molten salt synthesis (MSS) at 900 °C is composed of a broad green emission from MoO{sub 4}{sup 2−} and a group of sharp red emission peaks from f–f transitions of Eu{sup 3+}. Efficient energy transfer from MoO{sub 4}{sup 2−} to Eu{sup 3+} is responsible for the increased red PL of the phosphor. The dependence of energy transfer efficiency on the processing conditions and doping concentration has been investigated.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Samarium: from a distorted-fcc phase to melting under dynamic compression using in-situ x-ray diffraction

Abstract Lattice and electronic structure interactions for f -electrons are fundamental challenges for lanthanide equation of state development. Difficulties in first-principles calculations, such as density functional theory (DFT), emphasize the need for well-characterized experimental data. Here, we measure in-situ x-ray diffraction of shocked samarium (Sm) and temperature along the Hugoniot for the first time, providing direct evidence for phase transitions. We report direct evidence of a distorted fcc (dfcc) phase at 23 GPa. Shocked samarium melts from the dfcc phase starting at 33 GPa (1333 K), with complete melt at 40 GPa (1468 K). Previous work indicated shock melt at 27 GPa (1200 K), underscoring the significance of x-ray measurements for detecting phase transitions. Interestingly, our observed melting is in sharp contrast with the melting reported by a diamond anvil cell study. These experimental data can tightly constrain first principles calculations and serve as key touchstones for equation of state modeling.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Nature of exciton transitions in hexagonal boron nitride

In contrast to other III-nitride semiconductors GaN and AlN, the intrinsic (or free) exciton transition in hexagonal boron nitride ( h -BN) consists of rather complex fine spectral features (resolved into six sharp emission peaks) and the origin of which is still unclear. In this work, the free exciton transition (FX) in h -BN bulk crystals synthesized by a solution method at atmospheric pressure has been probed by deep UV time-resolved photoluminescence (PL) spectroscopy. Based on the separations between the energy peak positions of the FX emission lines, the identical PL decay kinetics among different FX emission lines, and the known phonon modes in h -BN, we suggest that there is only one principal emission line corresponding to the direct intrinsic FX transition in h -BN, whereas all other fine features are a result of phonon-assisted transitions. The identified phonon modes are all associated with the center of the Brillouin zone. Our results offer a simple picture for the understanding of the fundamental exciton transitions in h -BN.

36 MATERIALS SCIENCE↗