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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Variation of Physical Properties in the Nominal Sr4V2O6Fe2As2

We show using a combination of powder X-ray and neutron diffraction, first-principles calculations, temperature- and field-dependent magnetization, heat capacity and resistivity data that the superconducting behavior of Sr{sub 4}V{sub 2}O{sub 6}Fe{sub 2}As{sub 2} is dependent on synthesis conditions, particularly, heating profiles result in unintentional chemical doping. This compound can be tuned from a state in which the vanadium electrons are itinerant with a high electronic density of states, to a state where the vanadium-oxide layers are insulating and presumably magnetic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Quantum Critical Behavior in a Concentrated Ternary Solid Solution

The face centered cubic (fcc) alloy NiCoCr x with x ≈ 1 is found to be close to the Cr concentration where the ferromagnetic transition temperature, T c , goes to 0. Near this composition these alloys exhibit a resistivity linear in temperature to 2 K, a linear magnetoresistance, an excess –TlnT (or power law) contribution to the low temperature heat capacity and excess low temperature entropy. All of the low temperature electrical, magnetic and thermodynamic properties of the alloys with compositions near x ≈ 1 are not typical of a Fermi liquid and suggest strong magnetic fluctuations associated with a quantum critical region. The limit of extreme chemical disorder in this simple fcc material thus provides a novel and unique platform to study quantum critical behavior in a highly tunable system.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Research Update: Spatially resolved mapping of electronic structure on atomic level by multivariate statistical analysis

Atomic level spatial variability of electronic structure in Fe-based superconductor FeTe0.55Se0.45 (Tc = 15 K) is explored using current-imaging tunneling-spectroscopy. Multivariate statistical analysis of the data differentiates regions of dissimilar electronic behavior that can be identified with the segregation of chalcogen atoms, as well as boundaries between terminations and near neighbor interactions. Subsequent clustering analysis allows identification of the spatial localization of these dissimilar regions. Similar statistical analysis of modeled calculated density of states of chemically inhomogeneous FeTe1 x Sex structures further confirms that the two types of chalcogens, i.e., Te and Se, can be identified by their electronic signature and differentiated by their local chemical environment. This approach allows detailed chemical discrimination of the scanning tunneling microscopy data including separation of atomic identities, proximity, and local configuration effects and can be universally applicable to chemically and electronically inhomogeneous surfaces.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Strong spin-lattice coupling in CrSiTe{sub 3}

CrSiTe{sub 3} has attracted recent interest as a candidate single-layer ferromagnetic semiconductor, but relatively little is known about the bulk properties of this material. Here, we report single-crystal X-ray diffraction, magnetic properties, thermal conductivity, vibrational, and optical spectroscopies and compare our findings with complementary electronic structure and lattice dynamics principles calculations. The high temperature paramagnetic phase is characterized by strong spin-lattice interactions that give rise to glassy behavior, negative thermal expansion, and an optical response that reveals that CrSiTe{sub 3} is an indirect gap semiconductor with indirect and direct band gaps at 0.4 and 1.2 eV, respectively. Measurements of the phonons across the 33 K ferromagnetic transition provide additional evidence for strong coupling between the magnetic and lattice degrees of freedom. The Si-Te stretching and Te displacement modes are sensitive to the magnetic ordering transition, a finding that we discuss in terms of the superexchange mechanism. Spin-lattice coupling constants are also extracted.

36 MATERIALS SCIENCE↗
DOE OSTI2015

High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe{sub 12}O{sub 19}

High quality single crystals of BaFe{sub 12}O{sub 19} were grown using the floating zone technique in 100 atm of flowing oxygen. Single crystal neutron diffraction was used to determine the nuclear and magnetic structures of BaFe{sub 12}O{sub 19} at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe{sup 3+} ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90 to 300 K. The inverse dielectric permittivity, 1/ε, along the c-axis shows a T{sup 2} temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in 1/ε. These features resemble those of classic quantum paraelectrics such as SrTiO{sub 3}. The presence of the upturn in 1/ε indicates that BaFe{sub 12}O{sub 19} is a critical quantum paraelectric system with Fe{sup 3+} ions involved in both magnetic and electric dipole formation.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Extended magnetic exchange interactions in the high-temperature ferromagnet MnBi

The high-temperature ferromagnet MnBi continues to receive attention as a candidate to replace rare-earth-containing permanent magnets in applications above room temperature. This is due to a high Curie temperature, large magnetic moments, and a coercivity that increases with temperature. The synthesis of MnBi also allows for crystals that are free of interstitial Mn, enabling more direct access to the key interactions underlying the physical properties of binary Mn-based ferromagnets. In this work, we use inelastic neutron scattering to measure the spin waves of MnBi in order to characterize the magnetic exchange at low temperature. Consistent with the spin reorientation that occurs below 140 K, we do not observe a spin gap in this system above our experimental resolution. A Heisenberg model was fit to the spin wave data in order to characterize the long-range nature of the exchange. It was found that interactions up to sixth nearest neighbor are required to fully parametrize the spin waves. Surprisingly, the nearest-neighbor term is antiferromagnetic, and the realization of a ferromagnetic ground state relies on the more numerous ferromagnetic terms beyond nearest neighbor, suggesting that the ferromagnetic ground state arises as a consequence of the long-ranged interactions in the system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Calculation of integrated intensities in aberration-corrected Z-contrast images

Inclusion of spatial incoherence has been shown to give quantitative agreement between non-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy images and theoretical simulations. Here we show that, using the same approach, a significant improvement in the correlation between calculated and experimental normalized integrated intensities is obtained in the InAsP ternary semiconductor alloy, but residual discrepancies remain. We have demonstrated, in good agreement with experimental intensities obtained in calibrated samples, that normalized integrated intensities show a low dependence on the sample thickness over a wide range of thickness values. This behaviour does not occur in conventional (non-aberration-corrected) images and constitutes a powerful tool for straightforward interpretation of high-resolution images in terms of atomic column-resolved compositional maps.

36 MATERIALS SCIENCE↗
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 OSTI2012

Lattice dynamics and anomalous softening in the YbFe[subscript 4]Sb[subscript 12] skutterudite

The lattice dynamics of the filled skutterudite YbFe{sub 4}Sb{sub 12} was studied by resonant ultrasound spectroscopy and an anomalous softening in the temperature dependence of the elastic constants at {approx} 50 K was observed. This anomaly can not be explained by the dynamics of the filler, in contrast to other filled skutterudites. We have further investigated the origin of this anomaly using macroscopic and microscopic measurements. A rearrangement of the spectral weight of the Yb phonon states was observed in the temperature dependence of the density of phonon states, obtained by inelastic neutron scattering. We suggest that the anomaly is due to a change of the Yb valence state and that the anomaly and the phonon spectral weight rearrangement have the same origin.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗