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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Excited-state quantum phase transitions in systems with two degrees of freedom: Level density, level dynamics, thermal properties

Quantum systems with a finite number of freedom degrees f develop robust singularities in the energy spectrum of excited states as the system’s size increases to infinity. We analyze the general form of these singularities for low f, particularly f=2, clarifying the relation to classical stationary points of the corresponding potential. Signatures in the smoothed energy dependence of the quantum state density and in the flow of energy levels with an arbitrary control parameter are described along with the relevant thermodynamical consequences. The general analysis is illustrated with specific examples of excited-state singularities accompanying the first-order quantum phase transition. -- Highlights: •ESQPTs found in infinite-size limit of systems with low numbers of freedom degrees f. •ESQPTs related to non-analytical evolutions of classical phase–space properties. •ESQPT signatures analyzed for general f, particularly f=2, extending known case f=1. •ESQPT signatures identified in smoothened density and flow of energy spectrum. •ESQPTs shown to induce a new type of thermodynamic anomalies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Bottlebrush Polymers at Liquid Interfaces: Assembly Dynamics, Mechanical Properties, and All-Liquid Printed Constructs

Bottlebrush polymer surfactants (BPSs), formed by the interfacial interactions between bottlebrush polymers (BPs) with poly(acrylic acid) side chains dissolved in an aqueous phase and amine-functionalized ligands dissolved in the oil phase, assemble and bind strongly to the fluid-fluid interface. The ratio between N BB (backbone degree of polymerization) and N SC (side chain degree of polymerization) defines the initial assembly kinetics, interface packing efficiency, and stress relaxation. The equilibrium interfacial tension (γ) increases when N BB < N SC , but decreases when N BB $\gg$ N SC , correlating to a pronounced change in the effective shape of the BPs from being spherical to worm-like structures. The apparent surface coverage (ASC), i.e., the interfacial packing efficiency, decreases as N BB increases. The dripping-to-jetting transition of an injected polymer solution, as well as fluorescence recovery after photobleaching experiments, revealed faster initial assembly kinetics for BPs with higher N BB . Furhter, Euler buckling of BPS assemblies with different N BB values was used to characterize the stress relaxation behavior and bending modulus. The stress relaxation behavior was directly related to the ASC, reflecting the strong influence of macromolecular shape on packing efficiency. The bending modulus of BPSs decreases for N BB < N SC , but increased when N BB $\gg$ N SC , showing the effect of molecular architecture and multisite anchoring. All-liquid printed constructs with lower N BB BPs yielded more stable structured liquids, underscoring the importance of macromolecular packing efficiency at fluid interfaces. Overall, this work elucidates fundamental relationships between nanoscopic structures and macroscopic properties associated with various bottlebrush polymer architectures, which translate to the stabilization of all-fluidic printed constructs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

A new layered kagome strip structure Na 2 Co 3 (AsO 4 ) 2 (OH) 2 : static and dynamic magnetic properties

One-dimensional kagome strip chains share much of the same frustrated structural motif as two-dimensional kagome antiferromagnets, making them valuable for deepening our understanding of kagome lattice magnetism. In this paper, we report the hydrothermal synthesis and detailed structural and property characterization of Na 2 Co 3 (AsO 4 ) 2 (OH) 2 , a striped kagome system. The crystal structure was characterized using single crystal X-ray diffraction which reveals that Na 2 Co 3 (AsO 4 ) 2 (OH) 2 crystallizes in monoclinic crystal system C2/m. The structure features a one-dimensional kagome strip lattice built from Co 2+ ions and undergoes an antiferromagnetic transition at T N = 14 K. The magnetic ground state at zero field was characterized using neutron powder diffraction. Below the magnetic transition, Na 2 Co 3 (AsO 4 ) 2 (OH) 2 orders into an antiferromagnetic structure with a k-vector (0.5, 0.5, 0.5). In the proposed model, the Co1 moment is predominantly confined to the ac-plane while the Co2 moment is primarily aligned along the b-axis. Two flat bands were observed in the inelastic neutron spectra below the magnetic transition, 5 and 10 meV. Inelastic neutron spectra were modeled with a Heisenberg Hamiltonian including three nearest-neighbor exchange interactions (J 1 , J 2 , J 3 ) and strong single-ion anisotropy to stabilize the observed magnetic structure. Our study highlights the complexity of Co 2+ -based kagome strip magnetic lattice compound Na 2 Co 3 (AsO 4 ) 2 (OH) 2 which provides an excellent platform to broaden our understanding of the frustrated kagome magnetic lattice space.

Liurukara, Duminda S. [Oak Ridge National Laborato↗
DOE OSTI2015

Surface effects on the mechanical elongation of AuCu nanowires: De-alloying and the formation of mixed suspended atomic chains

We report here an atomistic study of the mechanical deformation of Au{sub x}Cu{sub (1−x)} atomic-size wires (nanowires (NWs)) by means of high resolution transmission electron microscopy experiments. Molecular dynamics simulations were also carried out in order to obtain deeper insights on the dynamical properties of stretched NWs. The mechanical properties are significantly dependent on the chemical composition that evolves in time at the junction; some structures exhibit a remarkable de-alloying behavior. Also, our results represent the first experimental realization of mixed linear atomic chains (LACs) among transition and noble metals; in particular, surface energies induce chemical gradients on NW surfaces that can be exploited to control the relative LAC compositions (different number of gold and copper atoms). The implications of these results for nanocatalysis and spin transport of one-atom-thick metal wires are addressed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Thin yttrium iron garnet films grown by pulsed laser deposition: Crystal structure, static, and dynamic magnetic properties

Pulsed laser deposition has been used to grow thin (10–84 nm) epitaxial layers of Yttrium Iron Garnet Y{sub 3}Fe{sub 5}O{sub 12} (YIG) on (111)–oriented Gadolinium Gallium Garnet substrates at different growth conditions. Atomic force microscopy showed flat surface morphology both on micrometer and nanometer scales. X-ray diffraction measurements revealed that the films are coherent with the substrate in the interface plane. The interplane distance in the [111] direction was found to be by 1.2% larger than expected for YIG stoichiometric pseudomorphic film indicating presence of rhombohedral distortion in this direction. Polar Kerr effect and ferromagnetic resonance measurements showed existence of additional magnetic anisotropy, which adds to the demagnetizing field to keep magnetization vector in the film plane. The origin of the magnetic anisotropy is related to the strain in YIG films observed by XRD. Magneto-optical Kerr effect measurements revealed important role of magnetization rotation during magnetization reversal. An unusual fine structure of microwave magnetic resonance spectra has been observed in the film grown at reduced (0.5 mTorr) oxygen pressure. Surface spin wave propagation has been demonstrated in the in-plane magnetized films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Static and dynamic spin properties in the quantum triangular lattice antiferromagnet Ag 2 CoO 2

In Ag 2 CoO 2 , Co forms triangular lattice layers, which are separated by the metallic (Ag 2 ) block. The magnetic susceptibility and heat capacity measurements show that this material exhibits an antiferromagnetic transition at T N =17.5 K and the Weiss temperature (T Θ ) and the effective moment are -274 K and 1.62μ B , respectively, indicating that the Co ion carries spin (S) 1/2 and has a strongly frustrated state with T Θ /T N =15.7. A density functional theory calculation confirmed that the valence state of the Co ions is 2+ and the low-spin state with S=1/2 is realized at reduced on-site Coulomb interaction on Co. We performed elastic and inelastic neutron scattering experiments in a powder sample of Ag 2 CoO 2 . Although no noticeable magnetic Bragg peaks were observed below T N , distinct magnetic excitations were observed in the inelastic neutron scattering experiments. The excitations are consistent with those expected for the S=1/2 Heisenberg triangular lattice antiferromagnet. These results suggest that the ordered moment is reduced due to the quantum effect, which explains the absence of the magnetic Bragg peaks. Our results thus suggest that Ag 2 CoO 2 is a good candidate to realize a quantum Heisenberg triangular lattice antiferromagnet.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Static and dynamical magnetic properties of the extended Kitaev-Heisenberg model with spin vacancies

Motivated by the potential to suppress antiferromagnetic long-range order in favor of the long-sought-after Kitaev quantum spin liquid state, we study the effect of spin vacancies in the extended Kitaev-Heisenberg model. In particular, we focus on a realistic model obtained from fitting inelastic neutron scattering on α–RuCl 3 . We observe that the long-range zigzag magnetic ordered state only survives when the doping concentration is smaller than 5%. Upon further increasing the spin vacancy concentration, the ground state becomes a short-range ordered state at low temperatures. Compared with experiments, our classical solution overstabilizes the zigzag correlation in the presence of spin vacancies. Here, our theoretical results provide guidance toward interpreting inelastic neutron scattering experiments on magnetically diluted Kitaev candidate materials

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Collaborative Research: Properties and Dynamics of the Shallow Crust (Final Report)

Ground motions recorded at one location are often extrapolated to nearby regions within a given radius. Here, we explore the appropriateness of spatial extrapolation using data from seven small aperture seismic network deployments in southern California. Six of these deployments are linear arrays of 4-13 stations, and one is a 2D array of 13 stations at Pinyon Flats Observatory. The spatial footprint array diameters are 3 km or less, and each array was operational for a year or more. From our base catalog (M2.5+ earthquakes; 4038 events; September 2010 - June 2023), automated methods remove temporally overprinted waveforms from nearby events (< 5 km) in quick succession (< 5 min) and data with nonviable waveforms. These 200 samples per second data are filtered at 0.5-25 Hz and must have signal-tonoise ratios (SNRs) of 2.5+. Peak ground acceleration (PGA) and peak ground velocity (PGV) are derived individually from the maximum absolute values of each of the 3-component waveforms (vertical, northsouth, and east-west). Five of the seven arrays traverse the San Jacinto fault, and two do not. Ground motion observations are compared with theoretical estimates from Abrahamson et al., 2014. On average, arrays deployed within and across fault zones consistently record ground motions above theoretical expectations, whereas off-fault arrays record ground motions at or slightly below theoretical expectations. We attribute these differences to site conditions because these trends prevail for the full data suites. For each network and each individual channel, the coefficient of variation indicates that the standard deviations are ~30±6% of the mean. Exploring relative ground motion contributions from all three channels (ternary plots), as expected, most data show that vertical ground motions are attenuated compared to horizontal ones. However, this is not always the case for RA array data, where vertical motions can be ~2-3 times larger than horizontal motions for select events near Cahuilla, CA. These anomalously high vertical motions are focal mechanism-related. These results suggest that ternary plots created using only a small amount of data can be used as a data quality metric and a tool to find anomalous features in three-component data.

58 GEOSCIENCES↗
DOE OSTI2023

Influence of Temperature on Properties and Dynamics of Gas-Solid Flow in Fluidized-Particle Tubular Solar Receiver

A fluidized particle single-tube solar receiver has been tested for investigating the gas-particle characteristics that enable the best operating conditions in a commercial-scale plant. The principle of the solar receiver is to fluidize the particles in a vessel – the dispenser – in which the receiver tube is plunged. The particles are flowing upward in the tube, irradiated over 1-meter height, by applying an overpressure in the dispenser. Experiments with a concentrated solar flux varying between 188 and 358 kW/m² are carried out, and the particle mass flux varied from 0 to 72 kg/(m²s). The mean particles and external tube wall temperatures in the irradiated zone are heated from the ambient to respectively 700°C and 940°C. It is shown that the temperature rise leads to a decrease of the particle volume fraction. Furthermore, a self-regulation of the system is evidenced with a short transient regime. This characteristic is essential from the operational viewpoint. The thermal efficiency of the receiver increases with the particle flow rate, reaching between 60 and 75% above 30 kg/(m²s). Several fluidization regimes are identified thanks to pressure signal analyses, like slugging, turbulent and fast fluidization, showing that regimes transitions are strongly affected by the temperature.

Gueguen, Ronny (ORCID:0000000252368638)↗