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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Elucidation of Active Sites in Aldol Condensation of Acetone over Single-Facet Dominant Anatase TiO 2 (101) and (001) Catalysts

Aldol condensations of carbonyl compounds for C–C bond formation are a very important class of reactions in organic synthesis and upgrading of biomass-derived feedstocks. However, the atomic level understanding of reaction mechanisms and structure-activity correlation on widely used transition metal oxide catalysts are limited due to the high degree of struc-tural heterogeneity of catalysts such as commercial TiO 2 powders. Here, we provide a deep understanding of the reaction mechanisms, kinetics, and structure–function relationships for vapor phase acetone aldol condensation through the con-trolled synthesis of two catalysts with high surface areas and clean, dominant facets, coupled with detailed characterization and kinetic studies that are further assisted by density functional theory (DFT) calculations. Temperature-dependent diffuse reflectance infrared Fourier transform spectroscopy showed the existence of abundant acetone bonded to surface hydroxyl groups (acetone-O s H) and acetone bonded to Lewis acid sites (acetone-Ti 5c ) on surface of both {101} and {001} facet dom-inant TiO 2 . Intermolecular C–C coupling of enolate intermediate from acetone-Ti 5c and a vicinal acetone-O s H is a kinetically relevant step, which is consistent with kinetic and isotopic studies and DFT calculations. The {001} facet showed a lower apparent activation energy (or higher reactivity) than the {101} facet. This is likely caused by the weaker Lewis acid and Brønsted base strengths of the {001} facet which favors the reprotonation–desorption of the coupled intermediate, making the C–C coupling step more exothermic on the {001} facet and resulting in an earlier transition state with a lower activation barrier. It is also possible that the {001} facet has a smoother surface configuration and less steric hindrance during intermo-lecular C–C bond formation than the {101} facet.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Copper adparticle enabled selective electrosynthesis of n-propanol

Here, the electrochemical reduction of carbon monoxide is a promising approach for the renewable production of carbon-based fuels and chemicals. Copper shows activity toward multi-carbon products from CO reduction, with reaction selectivity favoring two-carbon products; however, efficient conversion of CO to higher carbon products such as n-propanol, a liquid fuel, has yet to be achieved. We hypothesize that copper adparticles, possessing a high density of under-coordinated atoms, could serve as preferential sites for n-propanol formation. Density functional theory calculations suggest that copper adparticles increase CO binding energy and stabilize two-carbon intermediates, facilitating coupling between adsorbed *CO and two-carbon intermediates to form three-carbon products. We form adparticle-covered catalysts in-situ by mediating catalyst growth with strong CO chemisorption. The new catalysts exhibit an n-propanol Faradaic efficiency of 23% from CO reduction at an n-propanol partial current density of 11 mA cm –2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Block excitonic condensate at n = 3.5 in a spin-orbit coupled t 2 g multiorbital Hubbard model

Theoretical studies recently predicted the condensation of spin-orbit excitons at momentum $q={\pi}$ in ${t}_{2g}^{4}$ spin-orbit coupled three-orbital Hubbard models at electronic density $n=4$. In parallel, experiments involving iridates with noninteger valence states for the Ir ions are starting to attract considerable attention. Using the density matrix renormalization group technique we present evidence for the existence of an excitonic condensate at $n=3.5$ in a one-dimensional Hubbard model with a degenerate ${t}_{2g}$ sector, when in the presence of spin-orbit coupling. At intermediate Hubbard $U$ and spin-orbit ${\lambda}$ couplings, we found an excitonic condensate at the unexpected momentum $q={\pi}/2$ involving ${j}_{\mathrm{eff}}=3/2,m=\pm{}1/2$, and ${j}_{\mathrm{eff}}=1/2,m=\pm{}1/2$ bands in the triplet channel, coexisting with an also unexpected block magnetic order. We also present the entire ${\lambda}$ vs $U$ phase diagram, at a fixed and robust Hund coupling. Interestingly, this new “block excitonic phase” is present even at large values of ${\lambda}$, unlike the $n=4$ excitonic phase discussed before. Our computational study helps to understand and predict the possible magnetic phases of materials with ${d}^{3.5}$ valence and robust spin-orbit coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Reducing orbital eccentricity of precessing black-hole binaries

Building initial conditions for generic binary black-hole evolutions which are not affected by initial spurious eccentricity remains a challenge for numerical-relativity simulations. This problem can be overcome by applying an eccentricity-removal procedure which consists of evolving the binary black hole for a couple of orbits, estimating the resulting eccentricity, and then restarting the simulation with corrected initial conditions. The presence of spins can complicate this procedure. As predicted by post-Newtonian theory, spin-spin interactions and precession prevent the binary from moving along an adiabatic sequence of spherical orbits, inducing oscillations in the radial separation and in the orbital frequency. For single-spin binary black holes these oscillations are a direct consequence of monopole-quadrupole interactions. However, spin-induced oscillations occur at approximately twice the orbital frequency, and therefore can be distinguished and disentangled from the initial spurious eccentricity which occurs at approximately the orbital frequency. Taking this into account, we develop a new eccentricity-removal procedure based on the derivative of the orbital frequency and find that it is rather successful in reducing the eccentricity measured in the orbital frequency to values less than 10{sup -4} when moderate spins are present. We test this new procedure using numerical-relativity simulations of binary black holes with mass ratios 1.5 and 3, spin magnitude 0.5, and various spin orientations. The numerical simulations exhibit spin-induced oscillations in the dynamics at approximately twice the orbital frequency. Oscillations of similar frequency are also visible in the gravitational-wave phase and frequency of the dominant l=2, m=2 mode.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Addressing the spin question in gravitational-wave searches: Waveform templates for inspiralling compact binaries with nonprecessing spins

This paper presents a post-Newtonian (PN) template family of gravitational waveforms from inspiralling compact binaries with nonprecessing spins, where the spin effects are described by a single 'reduced-spin' parameter. This template family, which reparametrizes all the spin-dependent PN terms in terms of the leading-order (1.5PN) spin-orbit coupling term in an approximate way, has very high overlaps (fitting factor >0.99) with nonprecessing binaries with arbitrary mass ratios and spins. We also show that this template family is 'effectual' for the detection of a significant fraction of generic spinning binaries in the comparable-mass regime (m{sub 2}/m{sub 1} < or approx. 10), providing an attractive and feasible way of searching for gravitational waves from spinning low-mass binaries. We also show that the secular (nonoscillatory) spin-dependent effects in the phase evolution (which are taken into account by the nonprecessing templates) are more important than the oscillatory effects of precession in the comparable-mass (m{sub 1}{approx_equal}m{sub 2}) regime. Hence the effectualness of nonspinning templates is particularly poor in this case, as compared to non-precessing-spin templates. For the case of binary neutron stars observable by Advanced LIGO, even moderate spins (L-circumflex{sub N}{center_dot}S/m{sup 2}{approx_equal}0.015-0.1) will cause considerable mismatches ({approx}3%-25%) with nonspinning templates. This is contrary to the expectation that neutron-star spins may not be relevant for gravitational wave detection.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Majorana Fermions in Equilibrium and in Driven Cold-Atom Quantum Wires

We introduce a new approach to create and detect Majorana fermions using optically trapped 1D fermionic atoms. In our proposed setup, two internal states of the atoms couple via an optical Raman transition--simultaneously inducing an effective spin-orbit interaction and magnetic field--while a background molecular BEC cloud generates s-wave pairing for the atoms. The resulting cold-atom quantum wire supports Majorana fermions at phase boundaries between topologically trivial and nontrivial regions, as well as ''Floquet Majorana fermions'' when the system is periodically driven. We analyze experimental parameters, detection schemes, and various imperfections.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Compact intermediates in RNA folding

Large noncoding RNAs fold into their biologically functional structures via compact yet disordered intermediates, which couple the stable secondary structure of the RNA with the emerging tertiary fold. The specificity of the collapse transition, which coincides with the assembly of helical domains, depends on RNA sequence and counterions. It determines the specificity of the folding pathways and the magnitude of the free energy barriers to the ensuing search for the native conformation. By coupling helix assembly with nascent tertiary interactions, compact folding intermediates in RNA also play a crucial role in ligand binding and RNA-protein recognition.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2011

The Dwell Time of Electron Tunneling Through a Double Barrier in the Presence of Rashba SOI

Some aspects related to the influence of the Rashba spin-orbit interaction (SOI) on the dwell time spent by the electrons in an asymmetric double barrier are analyzed. It is revealed that in the presence of the Rashba SOI, a difference between the dwell times associated to the spin-up and spin-down species can be obtained. This opens the way to a spin filtration in the time domain.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Angular momenta and spin-orbit interaction of nonparaxial light in free space

We give an exact self-consistent operator description of the spin and orbital angular momenta, position, and spin-orbit interactions of nonparaxial light in free space. Both quantum-operator formalism and classical energy-flow approach are presented. We apply the general theory to symmetric and asymmetric Bessel beams exhibiting spin- and orbital-dependent intensity profiles. The exact wave solutions are clearly interpreted in terms of the Berry phases, quantization of caustics, and Hall effects of light, which can be readily observedexperimentally.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Quantum teleportation in the spin-orbit variables of photon pairs

We propose a polarization to orbital angular momentum teleportation scheme using entangled photon pairs generated by spontaneous parametric down-conversion. By making a joint detection of the polarization and angular momentum parity of a single photon, we are able to detect all the Bell states and perform, in principle, perfect teleportation from a discrete to a continuous system using minimal resources. The proposed protocol implementation demands experimental resources that are currently available in quantum optics laboratories.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Ballistic spin-dependent transport of Rashba rings with multi-leads

Research Highlights: > Transmission coefficients of each outgoing lead in multi-lead mesoscopic Rashba rings. > Spin polarizations of each outgoing lead in multi-lead mesoscopic Rashba rings. > Resonant and antiresonant conditions of spin polarization in multi-lead Rashba rings. > Symmetries of conductance and spin polarization of symmetric multi-lead Rashba rings. - Abstract: Using the Landauer-Buettiker formula with the transfer matrix technique, we develop a formalism of the ballistic spin-dependent electron transport in the multi-lead Rashba rings. We give analytic formulas of the total conductance G{sub j}, spin-{sigma} conductance g{sub j}{sup {sigma}} and spin polarization P{sub j} of each outgoing lead j, and their resonant and antiresonant conditions. Analytic studying with numerical investigating Rashba rings with several symmetric and asymmetric leads, we find that G{sub j}, g{sub j}{sup {sigma}} and P{sub j} oscillate with the incoming electron energy and the spin-orbit interaction (SOI) strength, and their antiresonances depend on the incoming electron energy, the SOI strength and the outgoing-lead angle with the incoming lead. For the symmetric-lead rings, G{sub j}, g{sub j}{sup {sigma}} and P{sub j} have some symmetries, G{sub j}=G{sub N-j},g{sub j}{sup {sigma}}=g{sub N-j}{sup -{sigma}}, and P{sub j} = -P{sub N-j} for symmetric leads, j and N - j, where the angles between the symmetric outgoing leads j and N - j and the incoming lead are {gamma}{sub N-j} = 2{pi} - {gamma}{sub j}. The spin polarization of the outgoing lead with {gamma}{sub j} = {pi} is exactly zero for even-N-symmetric-lead rings. These symmetries originate from the lead symmetry and time reversal invariance. For asymmetry-lead rings these symmetries vanish.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Topology-induced phase transitions in quantum spin Hall lattices

Physical phenomena driven by topological properties, such as the quantum Hall effect, have the appealing feature that they are robust with respect to external perturbations. Lately, a new class of materials has emerged that manifests topological properties at room temperature and without the need of external magnetic fields. These topological insulators are band insulators with large spin-orbit interactions and exhibit the quantum spin-Hall (QSH) effect. Here we investigate the transition between QSH and normal insulating phases under topological deformations of a two-dimensional lattice. We demonstrate that the QSH phase present in the honeycomb lattice loses its robustness as the occupancy of extra lattice sites is allowed. Furthermore, we propose a method for verifying our predictions with fermionic cold atoms in optical lattices. In this context, the spin-orbit interaction is engineered via an appropriate synthetic gauge field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Spin-orbit-path hybrid Greenberger-Horne-Zeilinger entanglement and open-destination teleportation with multiple degrees of freedom

We propose a scheme to generate hybrid Greenberger-Horne-Zeilinger (GHZ) entanglement where multiple photons are entangled in different degrees of freedom of spin, orbital angular momentum (OAM), and path (linear momentum). The generation involves mapping the preliminary OAM entanglement of photon pairs onto their spin-orbit and spin-path degrees of freedom, respectively. Based on the hybrid GHZ entanglement, we demonstrate an open-destination teleportation with multiples degrees of freedom, via which a spin state of a single photon is teleported onto a superposition of multiple photons with the postselection technique and the original information could be read out at any photon in individual spin, OAM, or the linear-momentum state. Our scheme holds promise for asymmetric optical quantum network.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Bose-Einstein condensation in the presence of artificial spin-orbit interaction

Bose-Einstein condensation in the presence of a synthetic spin-orbit interaction is considered, focusing on the case where a Dirac or Rashba potential is generated via a tripod scheme. We found that the ground states can be either plane-wave states or superpositions of them, each characterized by their unique density distributions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Spectrum of baryons with one charm quark

Diquark is employed to study the spectrum of baryons with one charm quark and two light quark within the mass loaded flux tube model. The spin-orbit interaction is taken into account based on the heavy quark symmetry. Some observed {Lambda}{sub c} and {Xi}{sub c} baryons are systematically classified with suggested J{sup P} quantum numbers.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Next-to-leading order spin-orbit and spin(a)-spin(b) Hamiltonians for n gravitating spinning compact objects

We derive the post-Newtonian next-to-leading order conservative spin-orbit and spin(a)-spin(b) gravitational interaction Hamiltonians for arbitrary many compact objects. The spin-orbit Hamiltonian completes the knowledge of Hamiltonians up to and including 2.5 post Newtonian for the general relativistic three-body problem. The new Hamiltonians include highly nontrivial three-body interactions, in contrast to the leading order consisting of two-body interactions only. This may be important for the study of effects like Kozai resonances in mergers of black holes with binary black holes. The derivation was done via two independent methods giving fully consistent results.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Radiative Improvement of the Lattice Nonrelativistic QCD Action Using the Background Field Method and Application to the Hyperfine Splitting of Quarkonium States

We present the first application of the background field method to nonrelativistic QCD (NRQCD) on the lattice in order to determine the one-loop radiative corrections to the coefficients of the NRQCD action in a manifestly gauge-covariant manner. The coefficients of the {sigma}{center_dot}B term in the NRQCD action and the four-fermion spin-spin interaction are computed at the one-loop level; the resulting shift of the hyperfine splitting of bottomonium is found to bring the lattice predictions in line with experiment.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Bismuth in gallium arsenide: Structural and electronic properties of GaAs{sub 1-x}Bi{sub x} alloys

The structural and electronic properties of cubic GaAs{sub 1-x}Bi{sub x} alloys with bismuth concentration 0.0, 0.25, 0.50, 0.75 and 1.0 are studied using the 'special quasi-random structures' (SQS) approach of Zunger along with the generalized gradient approximation (GGA) and the Engel-Vosko generalized gradient approximation (EV-GGA). The lattice constant, bulk modulus, derivative of bulk modulus and energy gap vary with bismuth concentration nonlinearly. The present calculations show that the band gap decreases substantially with increasing bismuth concentration and that spin-orbit coupling influences the nature of bonding at high Bi concentrations. - Graphical abstract: Bowing effect of spin-orbit split-off band values versus Bi content with and without spin-orbit coupling for GaAs{sub 1-x}Bi{sub x} (at x=0.25, 0.50 and 0.75). Calculations are done with GGA. Highlights: Black-Right-Pointing-Pointer Structural and electronic properties of GaAs{sub 1-x}Bi{sub x} alloys were studied. Black-Right-Pointing-Pointer We present results of lattice constant, energy gap, bulk modulus and derivative. Black-Right-Pointing-Pointer The band gap decreases substantially with increasing Bi concentration. Black-Right-Pointing-Pointer Calculations of the density of states and charge densities are also presented. Black-Right-Pointing-Pointer We have performed calculations without and with spin-orbit coupling.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗