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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 1,441 records · Page 80
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Diffusion in Coulomb crystals

Diffusion in Coulomb crystals can be important for the structure of neutron star crusts. We determine diffusion constants D from molecular dynamics simulations. We find that D for Coulomb crystals with relatively soft-core 1/r interactions may be larger than D for Lennard-Jones or other solids with harder-core interactions. Diffusion, for simulations of nearly perfect body-centered-cubic lattices, involves the exchange of ions in ringlike configurations. Here ions ''hop'' in unison without the formation of long lived vacancies. Diffusion, for imperfect crystals, involves the motion of defects. Finally, we find that diffusion, for an amorphous system rapidly quenched from Coulomb parameter {Gamma}=175 to Coulomb parameters up to {Gamma}=1750, is fast enough that the system starts to crystalize during long simulation runs. These results strongly suggest that Coulomb solids in cold white dwarf stars, and the crust of neutron stars, will be crystalline and not amorphous.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Electron-Hole Transitions in Multiply Charged Ions for Precision Laser Spectroscopy and Searching for Variations in {alpha}

We consider transitions of electron holes (vacancies in otherwise filled shells of atomic systems) in multiply charged ions that, due to level crossing of the holes, have frequencies within the range of optical atomic clocks. Strong E1 transitions provide options for laser cooling and trapping, while narrow transitions can be used for high-precision spectroscopy and tests of fundamental physics. We show that hole transitions can have extremely high sensitivity to {alpha} variation and propose candidate transitions that have much larger {alpha} sensitivities than any previously seen in atomic systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Novel correlation of Schottky constants with lattice energies for II-VI and I-VII compounds

Correlations of computed Schottky constants (K{sub S}=[V''{sub Zn}][V{sub S}{sup ..}]) with structural and thermodynamic properties showed linear dependences of log K{sub S} on the lattice energies for the Zn-, Cd-, Hg-, Mg-, and Sr-chalcogenides and for the Na- and K-halides. These findings suggest a basic relation between the Schottky constants and the lattice energies for these families of compounds from different parts of the Periodic Table, namely, {Delta}H{sub T,L}{sup o}=-(2.303nRT log K{sub S})+2.303nRm{sub b}+2.303nRTi{sub b}. {Delta}H{sub T,L}{sup o} is the experimental (Born-Haber) lattice energy (enthalpy), n is a constant approximately equal to the formal valence (charge) of the material, m{sub b} and i{sub b} are the slope and intercept, respectively, of the intercept b (of the log K{sub S} versus {Delta}H{sub L}{sup o} linear relation) versus the reciprocal temperature. The results of this work also provide an empirical correlation between the Gibbs free energy of vacancy formation and the lattice energy. - Graphical abstract: For the Zn-chalcogenides, the quantities n and I{sub e} are 2.007 and 650.3 kcal (2722 kJ), respectively. For the other groups of compounds, they are approximately equal to the formal valences and ionization energies of the metals: Log K{sub S{approx}}-(2.303nRT){sup -1} (0.99{Delta}H{sup o}{sub T,L}-I{sub e}).

36 MATERIALS SCIENCE↗
DOE OSTI2010

Quantum corrections to resistance of microblock tellurium at ultralow temperatures under phonon freezing conditions

The low-temperature magnetoresistance of bulk tellurium samples with a microcrystalline structure is investigated. At ultralow temperatures T {<=} 1 K, an anomalous positive magnetoresistance (APMR), viz. the antilocalization effect, is observed. It is shown that this effect can be explained using the weak localization theory. The characteristic parameters of the theory are determined. It is concluded that charge carriers produce a predominant effect on the phase breakdown time in the APMR mode of elastic scattering from structure defects, which leads to intervalley transitions without spin flip.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Interaction of copper impurity with radiation defects in silicon doped with boron

The spectrum of deep levels formed in boron-doped Czochralski-grown silicon single crystals as a result of interaction of radiation defects with copper impurity is studied. It is shown that, irrespective of the order of introduction of defects (both in the case of low-temperature copper diffusion into crystals preliminarily irradiated with electrons and in the case of irradiation of the samples contaminated with copper), the same set of deep levels appears. In addition to conventional radiation defects, three types of levels have been detected in the band gap of copper-containing crystals. These levels include the level E{sub v} + 0.49 eV (already mentioned in available publications), the level E{sub v} + 0.51 eV (previously not related to copper), and a level close to the donor level of a vacancy. Based on the analysis of concentration profiles, the interstitial carbonoxygen pair is excluded from possible precursors of the copper-containing center with level E{sub v} + 0.49 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Interaction between water and defective silica surfaces

We use the density functional theory method to study dry (1 x 1) {alpha}-quartz (0001) surfaces that have Frenkel-like defects such as oxygen vacancy and oxygen displacement. These defects have distinctively different effects on the water-silica interface depending on whether the adsorbent is a single water molecule, a cluster, or a thin film. The adsorption energies, bonding energies, and charge transfer or redistributions are analyzed, from which we find that the existence of a defect enhances the water molecule and cluster surface interaction by a large amount, but has little or even negative effect on water thin film-silica surface interaction. The origin of the weakening in film-surface systems is the collective hydrogen bonding that compromises the water-surface interaction in the process of optimizing the total energy. For clusters on surfaces, the lowest total energy states lower both the bonding energy and the adsorption energy.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Electron spin decoherence in nuclear spin baths and dynamical decoupling

We introduce the quantum theory of the electron spin decoherence in a nuclear spin bath and the dynamical decoupling approach for protecting the electron spin coherence. These theories are applied to various solid-state systems, such as radical spins in molecular crystals and NV centers in diamond.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Effect of the V{sub As}V{sub Ga} complex defect doping on properties of the semi-insulating GaAs

The different position V{sub As}V{sub Ga} cluster defect doping in semi-insulating (SI) GaAs has been studied by first-principles calculation based on hybrid density functional theory. Our calculated results show that EL6 level is formed due to the V{sub As}V{sub Ga} complex defect, which is very close to the experimental result. It provides the explanation of the absorption of laser with the wavelength beyond in semi-insulating GaAs. The formation energy of V{sub As}V{sub Ga} complex defect is found to decrease from surface to interior gradually. The conduction band minima and valence band maxima of GaAs (001) surface with the V{sub As}V{sub Ga} complex defect are all located at Γ point, and some defect levels are produced in the forbidden band. In contrast, the conduction band minima and valence band maxima of GaAs with the interior V{sub As}V{sub Ga} complex defect are not located at the same k-point, so it might involve the change of momentum in the electron transition process. The research will help strengthen the understanding of photoelectronic properties and effectively guide the preparation of the SI-GaAs materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Quantum-information transfer with nitrogen-vacancy centers coupled to a whispering-gallery microresonator

We propose an efficient scheme for the realization of quantum information transfer and entanglement with nitrogen-vacancy (N-V) centers coupled to a high-Q whispering-gallery mode (WGM) microresonator. We show that based on the effective dipole-dipole interaction between the N-V centers mediated by the WGM, quantum information can be transferred between the N-V centers through Raman transitions combined with laser fields. This protocol may open up promising possibilities for quantum communications with the solid-state quantum electrodynamic cavity system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

The effect of disorder in Ba{sub 2}YTaO{sub 6} on the tetragonal to cubic phase transition

Synchrotron X-ray diffraction and Raman spectroscopy have been used to study the structure of the complex perovskite Ba{sub 2}YTaO{sub 6}, at temperatures down to 100 K. Where the Ta and Y cations exhibit long-range rock-salt like ordering, Ba{sub 2}YTaO{sub 6} displays a continuous phase transition from a high temperature cubic structure, described in space group Fm3-bar m, to a tetragonal, I4/m, structure near 260 K. This transition is inhibited if extensive disorder and/or vacancies are/is present in the sample. -- Graphical abstract: The tetragonal-cubic phase transition observed in the cation ordered double perovskite Ba{sub 2}YTaO{sub 6} is inhibited when these are disordered. Display Omitted Highlights: {yields} Double perovskite Ba{sub 2}YTaO{sub 6} characterised by XRD and Raman spectroscopy. {yields} Cubic-tetragonal transition in Ba{sub 2}YTaO{sub 6} studied. {yields} Impact of disorder on the structure and phase transitions established.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Electron Screening in Metals

Electron screening enhances cross sections for nuclear reactions at low energies. We have studied the {sup 1}H({sup 7}Li,{alpha}){sup 4}He reaction in inverse kinematics and observed large electron screening only when the metallic targets were under tensile stress. Hydrogen was loaded into Pd and PdAg alloy foils from gas phase and concentrations reached about 0.6 protons per metallic atom. Mechanical pressure creates stress induced states in the metal. Without pressure hydrogen occupies octahedral interstitial positions in the fcc Pd lattice. Under stress some protons move to dislocated octahedral (dis-O) sites in the lattice. We believe that the magnitude of screening in various metals depends on the electron density on dis-O sites in case of fcc lattices or dislocated tetrahedral sites in bcc metals. We are currently trying to confirm the above by determining the electron density at the sites occupied by protons via the measurement of proton Knight's shift with {sup 1}H nuclear magnetic resonance technique.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Entangling separate nitrogen-vacancy centers in a scalable fashion via coupling to microtoroidal resonators

We propose a potentially practical scheme to entangle negatively charged nitrogen-vacancy (N-V) centers in distant diamonds. Each diamond is supposed to be fixed on the exterior surface of a microtoroidal resonator, and the single-photon input-output process - a currently available technique - could entangle separate N-V centers in a scalable fashion. The feasibility of our scheme and the experimental challenge are discussed by considering currently available techniques for qualified N-V centers and cavities.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Controlled Coupling of a Single Nitrogen-Vacancy Center to a Silver Nanowire

We report on the controlled coupling of a single nitrogen-vacancy (NV) center to a surface plasmon mode propagating along a chemically grown silver nanowire (NW). We locate and optically characterize a single NV center in a uniform dielectric environment before we controllably position this emitter in the close proximity of the NW. We are thus able to control the coupling of this particular emitter to the NW and directly compare the photon emission properties before and after the coupling. The excitation of single plasmonic modes is witnessed and a total rate enhancement by a factor of up to 4.6 is demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Defect Complexes in CrSBr Revealed Through Electron Microscopy and Deep Learning

Atomic defects underpin the properties of van der Waals materials, and their understanding is essential for advancing quantum and energy technologies. Scanning transmission electron microscopy is a powerful tool for defect identification in atomically thin materials, and extending it to multilayer and beam-sensitive materials would accelerate their exploration. Here, we establish a comprehensive defect library in a bilayer of the magnetic quasi-1D semiconductor CrSBr by combining atomic-resolution imaging, deep learning, and calculations. We apply a custom-developed machine learning work flow to detect, classify, and average point vacancy defects. This classification enables us to uncover several distinct Cr interstitial defect complexes, combined Cr and Br vacancy defect complexes, and lines of vacancy defects that extend over many unit cells. We show that their occurrence is in agreement with our computed structures and binding energy densities, reflecting the intriguing layer interlocked crystal structure of CrSBr. Our ab initio calculations show that the interstitial defect complexes give rise to highly localized electronic states. These states are of particular interest due to the reduced electronic dimensionality and magnetic properties of CrSBr and are, furthermore, predicted to be optically active. Our results broaden the scope of defect studies in challenging materials and reveal new defect types in bilayer CrSBr that can be extrapolated to the bulk and to over 20 materials belonging to the same FeOCl structural family.

deep learning↗
DOE OSTI2011

Structural and catalytic properties of lanthanide (La, Eu, Gd) doped ceria

Ce{sub 0.9}M{sub 0.1}O{sub 2-{delta}} mixed oxides (M=La, Eu and Gd) were synthesized by coprecipitation. Independent of the dopant cation, the obtained solids maintain the F-type crystalline structure, characteristic of CeO{sub 2} (fluorite structure) without phase segregation. The ceria lattice expands depending on the ionic radii of the dopant cation, as indicated by X-ray diffraction studies. This effect also agrees with the observed shift of the F{sub 2g} Raman vibrational mode. The presence of the dopant cations in the ceria lattice increases the concentration of structural oxygen vacancies and the reducibility of the redox pair Ce{sup 4+}/Ce{sup 3+}. All synthesized materials show higher catalytic activity for the CO oxidation reaction than that of bare CeO{sub 2}, being Eu-doped solid the one with the best catalytic performances despite of its lower surface area. - Graphical abstract: In this work, Ce{sub 0.9}M{sub 0.1}O{sub 2-{delta}} mixed oxides (M=La, Eu and Gd) were synthesized by coprecipitation. Independent of the dopant cation, the obtained solids maintain the F-type crystalline structure, characteristic of CeO{sub 2} (fluorite structure) without phase segregation. The ceria lattice expands depending on the ionic radii of the dopant cation, as indicated by X-ray diffraction studies. This effect also agrees with the observed shift of the F{sub 2g} Raman vibrational mode. The presence of the dopant cations in the ceria lattice increases the concentration of structural oxygen vacancies and the reducibility of the redox pair Ce{sup 4+}/Ce{sup 3+}. All synthesized materials show higher catalytic activity for the CO oxidation reaction than that of bare CeO{sub 2}, being Eu-doped solid the one with the best catalytic performances despite of its lower surface area. Highlights: > Lanthanide doped ceria as catalytic supports for CO oxidation reaction. > A higher concentration of oxygen vacancies promotes a higher catalytic activity. > Eu-doped ceria shows improved catalytic properties for the CO oxidation reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Preparation of anisotropic Nd(Fe,Mo){sub 12}N{sub 1.0} magnetic materials by strip casting technique and direct nitrogenation for the strips

The Nd(Fe,Mo){sub 12}-type alloys are prepared by strip casting technique, and direct nitrogenation of the strips without precrushing is executed in this paper. It is found that 6 h annealing treatment at 1050 deg. C for the strips is enough to obtain the single-phase Nd(Fe,Mo){sub 12} compounds. The strips can be directly nitrogenated at 620 deg. C to obtain interstitial Nd(Fe,Mo){sub 12}N{sub 1.0} materials, and a spontaneous pulverization phenomenon in the strips induced by nitrogenation is found. The results indicate that the nitrogenation process of the strips can be used to prepare Nd(Fe,Mo){sub 12}N{sub 1.0} interstitial nitrides and pulverize the casted strips into fine particles simultaneously. Base on this, we propose a new technical route of preparing Nd(Fe,Mo){sub 12}N{sub X} magnetic powders without precrushing and obtain anisotropic NdFe{sub 10.5}Mo{sub 1.5}N{sub 1.0} powders with a remanence of B{sub r} = 1.08 T, a coercivity of {sub i}H{sub c} = 400 kA/m, and a maximum energy product of (BH){sub max} = 144 kJ/m{sup 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Fibreoptic diffuse-light irradiators of biological tissues

We report techniques for the fabrication of laser radiation diffusers for interstitial photodynamic therapy. Using chemical etching of the distal end of silica fibre with a core diameter of 200 - 600 {mu}m, we have obtained long (up to 40 mm) diffusers with good scattering uniformity. Laser ablation has been used to produce cylindrical diffusers with high emission contrast and a scattering uniformity no worse than {approx}10 % in their middle part. The maximum length of the diffusers produced by this method is 20 - 25 mm. (laser applications and other topics in quantum electronics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Effect of localization on photoluminescence and zero-field splitting of silicon color centers

The study of defect centers in silicon has been recently reinvigorated by their potential applications in optical quantum information processing. A number of silicon defect centers emit single photons in the telecommunication O-band, making them promising building blocks for quantum networks between computing nodes. The two-carbon G-center, self-interstitial W-center, and spin-1/2 T-center are the most intensively studied silicon defect centers, yet despite this, there is no consensus on the precise configurations of defect atoms in these centers, and their electronic structures remain ambiguous. Here, in this work, we employ ab initio density functional theory to characterize these defect centers, providing insight into the relaxed structures, band structures, and photoluminescence spectra, which are compared to experimental results. Motivation is provided for how these properties are intimately related to the localization of electronic states in the defect centers. In particular, we present the calculation of the zero-field splitting for the excited triplet state of the G-center defect as the structure is linearly interpolated from the A-configuration to the B-configuration, showing a sudden increase in the magnitude of the D zz component of the zero-field-splitting tensor. By performing projections onto the local orbital states of the defect, we analyze this transition in terms of the symmetry and bonding character of the G-center defect, which sheds light on its potential application as a spin-photon interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗