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At least 1,027 records · Page 57

All sources NASA NTRS DOE OSTI

DOE OSTI2016

Structural and electrical properties of T′-type Ln{sub 2}CuO{sub 4} (Ln = Pr, Nd, Sm, Eu and Gd) ceramics

T′-type Ln{sub 2}CuO{sub 4} (Ln = Pr, Nd, Sm, Eu and Gd) were successfully synthesized in phase pure form using conventional solid state reaction and sintering route. For all the Ln{sub 2}CuO{sub 4} samples, the solid state reaction temperature was found to be 950 °C and pure phase was realized only after 2-3 intermediate milling and solid state reaction cycles, irrespective of the lanthanide ion radius. Effect of lanthanide ion contraction on the structural properties was clearly revealed by the powder X-ray diffraction, with the XRD peaks observed to be shifting towards higher 2θ values with the decrease in Ln ionic radii. The optimum sintering temperature of these solid state reacted Ln{sub 2}CuO{sub 4} powder was found to be 1100 °C except for Pr{sub 2}CuO{sub 4} powder. The conductivity of these samples were tested over a wide temperature range (−100 to 150 °C), an anomaly was observed in the conductivity of all the Ln{sub 2}CuO{sub 4} samples near the antiferromagnetic ordering temperature of Cu (~ 30 °C), thus suggesting the bearing of ordering of magnetic moments on the electrical properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1989

Sequential design of discrete linear quadratic regulators via optimal root-locus techniques

A sequential method employing classical root-locus techniques has been developed in order to determine the quadratic weighting matrices and discrete linear quadratic regulators of multivariable control systems. At each recursive step, an intermediate unity rank state-weighting matrix that contains some invariant eigenvectors of that open-loop matrix is assigned, and an intermediate characteristic equation of the closed-loop system containing the invariant eigenvalues is created.

Shieh, Leang S.↗

DOE OSTI2015

Out-of-equilibrium dynamics of photoexcited spin-state concentration waves

The spin crossover compound [Fe II H 2 L 2-Me ][PF 6 ]2 presents a two-step phase transition. In the intermediate phase, a spin state concentration wave (SSCW) appears resulting from a symmetry breaking (cell doubling) associated with a long-range order of alternating high and low spin molecular states. Lastly, by combining time-resolved optical and X-ray diffraction measurements on a single crystal, we study how such a system responds to femtosecond laser excitation and we follow in real time the erasing and rewriting of the SSCW

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1989

Theoretical studies of the transition metal-carbonyl systems MCO and M(CO)2, M = Ti, Sc, and V

Large Gaussian basis sets and an electron correlation treatment are used in ab initio calculations on the transition metal-carbonyl systems MCO and N(CO)2 for M = Ti, Sc, and V. The results show that high-spin ground states are favored for the monocarbonyl molecules, and that for the dicarbonyl molecules there is a competition between high, intermediate, and low spin states which are very close in energy. Dissociation energies of 0.62 eV for Ti-CO and 1.02 for Ti(CO)2 are found, relative to the ground state Ti atomic asymptote and CO 1Sigma(+). The binding energy per carbonyl for the three metal atoms is shown to be significantly lower for the dicarbonyl than for the the monocarbonyl molecules.

Barnes, Leslie A.↗

NASA NTRS1972

Transient-forced convection film boiling on an isothermal flat plate.

Development of a new approach for the solution of transient-forced convection film boiling on an isothermal flat plate using the boundary layer model. The similarity variables are used to convert the governing partial differential equations to ordinary ones. The results of numerical solutions of these ordinary equations indicate that the transient process can be classified as one-dimensional conduction, intermediate, and the steady-state regions. The time required for the one-dimensional conduction and the time necessary to attain a steady-state condition are obtained. The use of local similarity approximations for the intermediate regime facilitates prediction of complete boundary layer growth. Using the ratio of time at any instant to the steady-state time as abscissa, the curves representing the boundary layer growth can be merged into a single mean curve within 5%. Further, the analysis shows that the average rate of heat transfer during transient is 50 to 100% higher than those at steady state. The average rate of vapor convected away is 10 to 15% lower than at steady state while the average rate of accumulation to form the vapor layer is 1 to 14 times larger.

Nagendra, H. R.↗

DOE OSTI2016

Photoproduction of the f 1 ( 1285 ) meson

The f 1 (1285) meson with mass 1281.0 ± 0.8 MeV/c 2 and width 18.4 ± 1.4 MeV (full width at half maximum) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the ηπ + π - , K + K 0 π - , and K - K 0 π + decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the ηπ + π - final-state Dalitz distribution are consistent with the axial-vector J P = 1 + f 1 (1285) identity, rather than the pseudoscalar 0-η (1295). The production mechanism is more consistent with s -channel decay of a high-mass N * state and not with t -channel meson exchange. Decays to η π π go dominantly via the intermediate a$^±_0$ (980) π ∓ states, with the branching ratio Γ [a 0 π (no $\overline{K}K$) ] / Γ [ηππ (all) ] = 0.74 ± 0.09 . The branching ratios Γ ($\overline{K}K$ π )/ Γ (ηππ) = 0.216 ± 0.033 and Γ (γρ 0 ) / Γ (ηππ) = 0.047 ± 0.018 were also obtained. The first is in agreement with previous data for the f 1 (1285), while the latter is lower than the world average.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Electrified Operando -Freezing of Electrocatalytic CO 2 Reduction Cells for Cryogenic Electron Microscopy

The ability to freeze and stabilize reaction intermediates in their metastable states and obtain their structural and chemical information with high spatial resolution would be very powerful to unravel the fundamentals in many important materials technologies such as catalysis and batteries. Here, we develop an electrified operando-freezing methodology for the first time to preserve these metastable states under electrochemical reaction conditions for cryogenic electron microscopy (cryo-EM) imaging and spectroscopy. Using Cu catalysts for CO 2 reduction as a model system, we observe restructuring of the Cu catalyst in a CO 2 atmosphere while the same catalyst remains intact in an air atmosphere at the nanometer scale. Furthermore, we discover the existence of single valance Cu (1+) state and C-O bonding at the electrified liquid-solid interface of the operando-frozen samples, which are key reaction intermediates that traditional ex situ measurements fail to detect. Finally, this work highlights our novel technique to study the local structure and chemistry of electrified liquid-solid interfaces, which has broad impact for many electrochemical reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Solar energy conversion by photosystem II: principles and structures

Photosynthetic water oxidation by Photosystem II (PSII) is a fascinating process because it sustains life on Earth and serves as a blue print for scalable synthetic catalysts required for renewable energy applications. The biophysical, computational, and structural description of this process, which started more than 50 years ago, has made tremendous progress over the past two decades, with its high-resolution crystal structures being available not only of the dark-stable state of PSII, but of all the semi-stable reaction intermediates and even some transient states. Here, we summarize the current knowledge on PSII with emphasis on the basic principles that govern the conversion of light energy to chemical energy in PSII, as well as on the illustration of the molecular structures that enable these reactions. The important remaining questions regarding the mechanism of biological water oxidation are highlighted, and one possible pathway for this fundamental reaction is described at a molecular level.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2020

Protein folding from heterogeneous unfolded state revealed by time-resolved X-ray solution scattering

One of the most challenging tasks in biological science is to understand how a protein folds. In theoretical studies, the hypothesis adopting a funnel-like free-energy landscape has been recognized as a prominent scheme for explaining protein folding in views of both internal energy and conformational heterogeneity of a protein. Despite numerous experimental efforts, however, comprehensively studying protein folding with respect to its global conformational changes in conjunction with the heterogeneity has been elusive. Here we investigate the redox-coupled folding dynamics of equine heart cytochrome c (cyt-c) induced by external electron injection by using time-resolved X-ray solution scattering. A systematic kinetic analysis unveils a kinetic model for its folding with a stretched exponential behavior during the transition toward the folded state. With the aid of the ensemble optimization method combined with molecular dynamics simulations, we found that during the folding the heterogeneously populated ensemble of the unfolded state is converted to a narrowly populated ensemble of folded conformations. These observations obtained from the kinetic and the structural analyses of X-ray scattering data reveal that the folding dynamics of cyt-c accompanies many parallel pathways associated with the heterogeneously populated ensemble of unfolded conformations, resulting in the stretched exponential kinetics at room temperature. This finding provides direct evidence with a view to microscopic protein conformations that the cyt-c folding initiates from a highly heterogeneous unfolded state, passes through still diverse intermediate structures, and reaches structural homogeneity by arriving at the folded state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS2001

Quantum Dots Investigated for Solar Cells

The NASA Glenn Research Center has been investigating the synthesis of quantum dots of CdSe and CuInS2 for use in intermediate-bandgap solar cells. Using quantum dots in a solar cell to create an intermediate band will allow the harvesting of a much larger portion of the available solar spectrum. Theoretical studies predict a potential efficiency of 63.2 percent, which is approximately a factor of 2 better than any state-of-the-art devices available today. This technology is also applicable to thin-film devices--where it offers a potential four-fold increase in power-to-weight ratio over the state of the art. Intermediate-bandgap solar cells require that quantum dots be sandwiched in an intrinsic region between the photovoltaic solar cell's ordinary p- and n-type regions (see the preceding figure). The quantum dots form the intermediate band of discrete states that allow sub-bandgap energies to be absorbed. However, when the current is extracted, it is limited by the bandgap, not the individual photon energies. The energy states of the quantum dot can be controlled by controlling the size of the dot. Ironically, the ground-state energy levels are inversely proportional to the size of the quantum dots. We have prepared a variety of quantum dots using the typical organometallic synthesis routes pioneered by Ba Wendi et al., in the early 1990's. The most studied quantum dots prepared by this method have been of CdSe. To produce these dots, researchers inject a syringe of the desired organometallic precursors into heated triocytlphosphine oxide (TOPO) that has been vigorously stirred under an inert atmosphere (see the following figure). The solution immediately begins to change from colorless to yellow, then orange and red/brown, as the quantum dots increase in size. When the desired size is reached, the heat is removed from the flask. Quantum dots of different sizes can be identified by placing them under a "black light" and observing the various color differences in their fluorescence (see the photograph).

Bailey, Sheila G.↗

DOE OSTI2020

Fission of Relativistic Nuclei with Fragment Excitation and Reorientation

Experimental studies of fission induced in relativistic nuclear collisions show a systematic enhancement of the excitation energy of the primary fragments by a factor of ~2, before their decay by fission and other secondary fragments. Although it is widely accepted that by doubling the energies of the single-particle states may yield a better agreement with fission data, it does not prove fully successful, since it is not able to explain yields for light and intermediate mass fragments. State-of-the-art calculations are successful to describe the overall shape of the mass distribution of fragments, but fail within a factor of 2–10 for a large number of individual yields. In this work, we present a novel approach that provides an account of the additional excitation of primary fragments due to final state interaction with the target. Our method is applied to the 238 U + 208 Pb reaction at 1GeV/nucleon (and is applicable to other energies), an archetype case of fission studies with relativistic heavy ions, where we find that the large probability of energy absorption through final state excitation of giant resonances in the fragments can substantially modify the isotopic distribution of final fragments in a better agreement with data. Finally, we demonstrate that large angular momentum transfers to the projectile and to the primary fragments via the same mechanism imply the need of more elaborate theoretical methods than the presently existing ones.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2010

Probing the Structure of {sup 74}Ge Nucleus with Coupled-channels Analysis of {sup 74}Ge+{sup 74}Ge Fusion Reaction

We study the fusion reaction of the {sup 74}Ge+{sup 74}Ge system in term of the full order coupled-channels formalism. We especially calculated the fusion cross section as well as the fusion barrier distribution of this reaction using transition matrix suggested by recent Coulomb excitation experiment. We compare the results with the one obtained by coupling matrix based on pure vibrational and rotational models. The present coupled-channels calculations for the barrier distributions obtained using experiment coupling matrix is in good agreement with the one obtained with vibrational model, in contrast to the rotational model. This is indicates that {sup 74}Ge nucleus favor a spherical shape than a deformed shape in its ground state. Our results will resolve the debates concerning the structure of this nucleus.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

NASA NTRS1971

Transient-forced convection film boiling on an isothermal flat plate

An approach for the solution of transient-forced convection film boiling on an isothermal flat plate using the boundary layer model is developed. The similarity variables are used to convert the governing partial differential equations to ordinary ones. The results of numerical solutions of these ordinary equations indicate that the transient process can be classified as one-dimensional conduction, intermediate, and the steady state regions. The time required for the one-dimensional conduction and the time necessary to attain a steady state condition are obtained. The influence of interfacial shear is seen to be negligible while the Prandtl Number and the ratio (C sub p delta T divided by h sub fg times Pr) have major influence. The use of local similarity approximations for the intermediate regime facilitates prediction of complete boundary layer growth. Using the ratio of time at any instant to the steady state time as abscissa, the curves representing the boundary layer growth can be merged into a single mean curve within 5 percent. Further, the analysis shows that the average rate of heat transfer during transient is 50 to 100 percent higher than those at steady state. The average rate of vapor convected away is 10 to 15 percent lower than at steady state while the average rate of accumulation to form the vapor layer is 1 to 14 times larger. Further, the total heat transferred during transient increases and the evaporation decreases for increasing values of C sub p delta T divided by h sub fg times Pr

Nagendra, H. R.↗

DOE OSTI2022

Dissipative Reactions with Intermediate-Energy Beams: A Novel Approach to Populate Complex-Structure States in Rare Isotopes

A novel pathway for the formation of multiparticle-multihole excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of 38 Ca on a Be target. The negative-parity, complex-structure final states in 38 Ca are observed following the in-beam γ-ray spectroscopy of events in the 9 Be( 38 Ca, 38 Ca + γ)X reaction in which the scattered projectile loses longitudinal momentum of order Δp || = 700 MeV/c. Here, the characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly energetic projectile-target collision. Unlike the far-less-dissipative, surface-grazing reactions usually exploited for the in-beam γ-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multiparticle configurations in rare isotopes—final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

NASA NTRS2011

Accretion States of the Galactic Micro Quasar GRS 1758-258

We present the results of a radio and X-ray study of the Galactic micro quasar GRS 1758-258, using unpublished archival data and new observations. We focus in particular on the 2000-2002 state transitions, and on its more quiet behaviour in 2008-2009. Our spectral and timing analysis of the XMM-Newton data shows that the source was in the canonical intermediate, soft and hard states in 2000 September 19,2001 March 22 and 2002 September 28, respectively. We estimate the disk size, luminosity and temperature, which are consistent with a black hole mass approx.10 Solar Mass, There is much overlap between the range of total X-ray luminosities (on average approx. 0.02L(sub Edd)) in the hard and soft states, and probably between the corresponding mass accretion rates; in fact, the hard state is often more luminous. The extended radio lobes seen in 1992 and 1997 are still present in 2008-2009. The 5-GHz radio core flux density has shown variability between approx. 0.1-0.5 mJy over the last two decades. This firmly places GRS 1758-258 in the radio-quiet sequence of Galactic black holes, in the radio/X-ray plane. We note that this dichotomy is similar to the dichotomy between the radio/X-ray sequences of Seyfert and radio galaxies. We propose that the different radio efficiency of the two sequences is due to relativistic electron/positron jets in radio-loud black holes, and sub-relativistic, thermally dominated outflows in radio-quiet sources.

Soria, Roberto↗

DOE OSTI2024

Scalable Experimental Bounds for Entangled Quantum State Fidelities

Estimating the state preparation fidelity of highly entangled states on noisy intermediate-scale quantum (NISQ) devices is important for benchmarking and application considerations. Unfortunately, exact fidelity measurements quickly become prohibitively expensive, as they scale exponentially as O(3 N for N-qubit states, using full state tomography with measurements in all Pauli bases combinations. However, Somma et al.established that the complexity could be drastically reduced when looking at fidelity lower bounds for states that exhibit symmetries, such as Dicke states and GHZ states. These bounds must still be tight enough for larger states to provide reasonable estimations on NISQ devices. For the first time and more than 15 years after the theoretical introduction, we report meaningful lower bounds for the state preparation fidelity of all Dicke states up to N=10 and all GHZ states up to N=20 on Quantinuum H1 ion-trap systems using efficient implementations of recently proposed scalable circuits for these states. Our achieved lower bounds match or exceed previously reported exact fidelities on superconducting systems for much smaller states. Furthermore, we provide evidence that for large Dicke states |$D^{N}_{N/2}\rangle$, we may resort to a GHZ-based approximate state preparation to achieve better fidelity. This work provides a path forward to benchmarking entanglement as NISQ devices improve in size and quality.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2014

Anomalous temperature dependence in valence band spectra: A resonant photoemission study of layered perovskite Sr{sub 2}CoO{sub 4}

Valence band spectra (VBS) and its modification across Curie temperature (T{sub C}) of Sr{sub 2}CoO{sub 4} thin film are studied using resonant photoemission spectroscopy. It is found that VBS mainly consists of hybridized states of Co-3d t{sub 2g}e{sub g} and O-2p; however, Co-3d e{sub g} states show its prominence only in the ferromagnetic temperature regime. Below T{sub C}, spectral weight transfer takes place anomalously from high binding energy (B.E.) region to low B.E. region, signifying the enhanced intermediate or low spin state Co{sup 4+} ions. It is suggested that spin-lattice coupling and many-body effects in Sr{sub 2}CoO{sub 4} derived from the strong electron correlations lead to such temperature dependence of VBS.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2018

Nucleon Associations in Direct Nuclear Reactions

Based on the model of nucleon associations, the two-nucleon transfer reaction (p, t) is considered in intermediate nuclei. The interaction between the nucleons is local and short-range, and the bineutron association already exists in the nucleon. The internal states of the incident proton, departing triton, and any intermediate association states are assumed to be completely symmetric S-states. Investigations are performed with distorted plane waves of the proton and triton, and the assumption is made that there is no exchange by nucleons among nucleon fragments. The differential cross sections calculated on the basis of this approach are presented, and these results demonstrate that the association model allows one to explain qualitatively the experimental data on particle scattering. Some possible directions of further study of direct nuclear reactions on the basis of the model are discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗