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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 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Electronic structure of YbGa1.15Si0:85 and YbGaxGe2-x probed by resonant x-ray emission and photoelectron spectroscopies

We performed an x-ray spectroscopic study combining resonant x-ray emission spectroscopy (RXES) and photoelectron spectroscopy on the superconducting ternary silicide YbGa 1.15 Si 0.85 and nonsuperconducting ternary germanide YbGa x Ge 2-x (x = 1.0 and 1.1). The Yb valence for all three compounds is found to be about 2.3. In YbGa 1.15 Si 0.85 no temperature dependence of the Yb valence is observed in the RXES spectra in the temperature range of 7–300 K, while the valence shows a drastic increase under pressure from the Yb 2+ state partially including itinerant electrons to the localized Yb 3+ state. Differences are observed in the valence-band spectra of the photoelectron spectroscopy between YbGa 1.15 Si 0.85 and YbGa x Ge 2-x , which may be attributed to the difference of crystal structure. We conclude that both the crystal structure of the planar GaSi layer in YbGa 1.15 Si 0.85 and the resultant electronic structure may have a crucial role in the occurrence of superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Control of octahedral rotations in (LaNiO{sub 3}){sub n}/(SrMnO{sub 3}){sub m} superlattices.

Oxygen octahedral rotations have been measured in short-period (LaNiO{sub 3}){sub n}/(SrMnO{sub 3}){sub m} superlattices using synchrotron diffraction. The in-plane and out-of-plane bond angles and lengths are found to systematically vary with superlattice composition. Rotations are suppressed in structures with m > n, producing a nearly unrotated form of LaNiO{sub 3}. Large rotations are present in structures with m < n, leading to reduced bond angles in SrMnO{sub 3}. The metal-oxygen-metal bond lengths decrease as rotations are reduced, in contrast to behavior previously observed in strained, single-layer films. This result demonstrates that superlattice structures can be used to stabilize nonequilibrium octahedral behavior in a manner distinct from epitaxial strain, providing a novel means to engineer the electronic and ferroic properties of oxide heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Spatial inhomogeneity in RFeAs(O,F)(R = Pr, Nd) determined from rare earth crystal field excitations.

We report inelastic neutron-scattering measurements of crystal-field transitions in PrFeAsO, PrFeAsO{sub 0.87}F{sub 0.13}, and NdFeAsO{sub 0.85}F{sub 0.15}. Doping with fluorine produces additional crystal-field excitations, providing evidence that there are two distinct charge environments around the rare-earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr{sup 3+} and Nd{sup 3+} ions have nonmagnetic and magnetic ground states, respectively, indicating that the enhancement of T{sub c} compared to LaFeAsO{sub 1-x}F{sub x} is not due to rare-earth magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Irradiation damage in Gd{sub 2}Ti{sub 2}O{sub 7} single crystals: Ballistic versus ionization processes

The structural transformations induced in Gd{sub 2}Ti{sub 2}O{sub 7} single crystals irradiated at high energies (870-MeV Xe), where ionization processes (electronic stopping) dominate, and at low energies (4-MeV Au), where ballistic processes (nuclear stopping) dominate, have been studied via the combination of Rutherford backscattering spectrometry and channeling (RBS/C), Raman spectroscopy, and transmission electron microscopy (TEM) experiments. At high energy, amorphization occurs directly in individual ion tracks from the extreme electronic-energy deposition, and full amorphization results from the overlapping of these tracks as described by a direct impact model. The track diameters lie in the range 6-9 nm. At low energy, amorphization occurs via indirect processes, driven by ballistic nuclear energy deposition from the ions, that is accounted for in the framework of both direct-impact/defect-stimulated and multi-step damage accumulation models. The ion fluence for total amorphization of the irradiated layer is much higher at low energy (0.5 ion nm{sup -2}) than at high energy (0.05 ion nm{sup -2}), consistent with the nuclear stopping at low energy (5.2 keV/nm) compared to the electronic stopping at high energy (29 keV/nm).

36 MATERIALS SCIENCE↗
DOE OSTI2011

Epitaxial Fe3-xTixO4 films from magnetite to ulvöspinel by pulsed laser deposition

Epitaxial films along the Fe3-xTixO4 (titanomagnetite) compositional series from pure end-members magnetite (Fe3O4) to ulvöspinel (Fe2TiO4) were successfully grown by pulsed laser deposition on MgO(100) substrates. Spectroscopic characterization including high resolution x-ray diffraction, x-ray photoelectron spectroscopy, and synchrotron-based x-ray absorption and magnetic circular dichroism consistently shows that Ti(IV) substitutes for Fe(III) in the inverse spinel lattice with a proportional increase in lattice Fe(II) concentration. No evidence of Ti interstitials, spinodal decomposition, or secondary phases was found in the bulk of the grown films. At the uppermost few nanometers of the Ti-bearing film surfaces, evidence suggests that Fe(II) is susceptible to facile oxidation, and that an associated lower Fe/Ti ratio in this region is consistent with surface compositional incompleteness or alteration to a titanomaghemite-like composition and structure. The surface of these films nonetheless appear to remain highly ordered and commensurate with the underlying structure despite facile oxidation, a surface condition that is found to be reversible to some extent by heating in low oxygen environments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Simulating complex atomistic processes: On-the-fly kinetic Monte Carlo scheme with selective active volumes

An accelerated atomistic kinetic Monte Carlo (KMC) approach for evolving complex atomistic structures has been developed. The method incorporates on-the-fly calculations of transition states (TSs) with a scheme for defining active volumes (AVs) in an off-lattice (relaxed) system. In contrast to conventional KMC models that require all reactions to be predetermined, this approach is self-evolving and any physically relevant motion or reaction may occur. Application of this self-evolving atomistic kinetic Monte Carlo (SEAK-MC) approach is illustrated by predicting the evolution of a complex defect configuration obtained in a molecular dynamics (MD) simulation of a displacement cascade in Fe. Over much longer times, it was shown that interstitial clusters interacting with other defects may change their structure, e.g., from glissile to sessile configuration. The direct comparison with MD modeling confirms the atomistic fidelity of the approach, while the longer time simulation demonstrates the unique capability of the model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Diffusive-to-ballistic transition in grain boundary motion studied by atomistic simulations

An adapted simulation method is used to systematically study grain boundary motion at velocities and driving forces across more than five orders of magnitude. This analysis reveals that grain boundary migration can occur in two modes, depending upon the temperature ( T ) and applied driving force ( P ). At low P and T , grain boundary motion is diffusional, exhibiting the kinetics of a thermally activated system controlled by grain boundary self-diffusion. At high P and T , grain boundary migration exhibits the characteristic kinetic scaling behavior of a ballistic process. A rather broad transition range in both P and T lies between the regimes of diffusive and ballistic grain boundary motion, and is charted here in detail. In conclusion, the recognition and delineation of these two distinct modes of grain boundary migration also leads to the suggestion that many prior atomistic simulations might have probed a different kinetic regime of grain boundary motion (ballistic) as compared to that revealed in most experimental studies (diffusional).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Pressure-induced transformations in amorphous Si-Ge alloy

The pressure behavior of an amorphous Si-rich SiGe alloy ({alpha}-Si{sub x}Ge{sub 1-x}, x = 0.75) has been investigated up to about 30 GPa, by a combination of Raman spectroscopy, x-ray absorption spectroscopy, and x-ray diffraction measurements. The trends of microscopic structural properties and of the Raman-active phonon modes are presented in the whole pressure range. Nucleation of nanocrystalline alloy particles and metallization have been observed above 12 GPa, with a range of about 2 GPa of coexistence of amorphous and crystalline phases. Transformations from the amorphous tetrahedral, to the crystalline tetragonal ({beta}-Sn) and to the simple hexagonal structures have been observed around 13.8 and 21.8 GPa. The recovered sample upon depressurization, below about 4 GPa, shows a local structure similar to the as-deposited one. Inhomogeneities of the amorphous texture at the nanometric scale, probed by high-resolution transmission electron microscopy, indicate that the recovered amorphous sample has a different ordering at this scale, and therefore the transformations can not be considered fully reversible. The role of disordered grain boundaries at high pressure and the possible presence of a high-density amorphous phase are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Distinct oxygen hole doping in different layers of Sr₂CuO 4-δ /La₂CuO₄ superlattices

X-ray absorption in Sr₂CuO 4-δ /La₂CuO₄ (SCO/LCO) superlattices shows a variable occupation with doping of a hole state different from holes doped for x≲x optimal in bulk La 2-x Sr x CuO₄ and suggests that this hole state is on apical oxygen atoms and polarized in the a-b plane. Considering the surface reflectivity gives a good qualitative description of the line shapes of resonant soft x-ray scattering. The interference between superlattice and surface reflections was used to distinguish between scatterers in the SCO and the LCO layers, with the two hole states maximized in different layers of the superlattice.

Smadici, S.↗
DOE OSTI2012

Probing the connections between superconductivity, stripe order, and structure in La₁.₉₀₅Ba₀.₀₉₅Cu 1-y Zn y O₄

The superconducting system La 2-x Ba x CuO₄ is known to show a minimum in the transition temperature T c at x=1/8 where maximal stripe order is pinned by the anisotropy within the CuO₂ planes that occurs in the low-temperature-tetragonal (LTT) crystal structure. For x=0.095, where T c reaches its maximum value of 32 K, there is a roughly coincident structural transition to a phase that is very close to LTT. Here, we present a neutron scattering study of the structural transition, and demonstrate how features of it correlate with anomalies in the magnetic susceptibility, electrical resistivity, thermal conductivity, and thermoelectric power. We also present measurements on a crystal with 1% Zn substituted for Cu, which reduces T c to 17 K, enhances the spin stripe order, but has much less effect on the structural transition. We make the case that the structural transition correlates with a reduction of the Josephson coupling between the CuO₂ layers, which interrupts the growth of the superconducting order. We also discuss evidence for two-dimensional superconducting fluctuations in the normal state, analyze the effective magnetic moment per Zn impurity, and consider the significance of the anomalous thermopower often reported in the stripe-ordered phase.

Wen, Jinsheng↗
DOE OSTI2012

Anomalous grain growth in the surface region of a nanocrystalline CeO2 film under low-temperature heavy ion irradiation

Grain growth and phase stability of nanocrystalline ceria are investigated under ion irradiation at different temperatures. Irradiations at temperatures of 300 and 400 K result in uniform grain growth throughout the film. Anomalous grain growth is observed in thin films of nanocrystalline ceria under 3 MeV Au+ irradiation at 160 K. At this low temperature, significant grain growth is observed within 100 nm from the surface, no obvious growth is detected in the rest of the films. While the grain growth is attributed to a defect-stimulated mechanism at room temperature and above, a defect diffusion-limited mechanism is significant at low temperature with the primary defect responsible being the oxygen vacancy. The nanocrystalline grains remain in the cubic phase regardless of defect kinetics.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2012

Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO₁₋ x F x and Sr(Fe₁₋ x Co x )₂As₂

A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO₁₋ x F x (x = 0, 0.11) and Sr(Fe₁₋ x Co x )2As2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10⁻¹⁵ s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3μ B in CeFeAsO and 2.1μB in SrFe₂As₂. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9μ B in CeFeAsO₀.₈₉F₀.₁₁ and 1.3μ B in Sr(Fe₀.₉Co₀.₁)₂As₂. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity.

Vilmercati, Paolo↗
DOE OSTI2012

Ab Initio Molecular Dynamics Simulations of Low-Energy Recoil Events in ThO2, CeO2, and ZrO2

Ab initio molecular dynamics simulations of low-energy recoil events in ThO2, CeO2, and ZrO2 have been carried out to determine the threshold displacement energies, resulting defect configurations, dynamics of defect generation, and role of charge transfer during the process. The results reveal that, in most cases, these fluorite structure oxides exhibit a similar response to low-energy recoils. A variety of different defect configurations are created, consisting mainly of vacancies and interstitials. Charge transfer occurs during the dynamic displacement process. Local charge redistribution leads to cation and O vacancies being negatively and positively charged, respectively. Likewise, due to charge redistribution, the cation and O interstitials are less positively and negatively charged, respectively, than the ions on lattice sites in perfect MO2.

Environmental Molecular Sciences Laboratory↗