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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 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Anisotropic impurity states, quasiparticle scattering and nematic transport in underdoped Ca(Fe1-xCo x )2As2

Iron-based high-temperature superconductivity develops when the ‘parent’ antiferromagnetic/orthorhombic phase is suppressed, typically by introduction of dopant atoms1. But their impact on atomic-scale electronic structure, although in theory rather complex2,3,4,5,6,7,8,9,10,11,12,13, is unknown experimentally. What is known is that a strong transport anisotropy14,15,16,17,18,19,20,21,22,23,24,25 with its resistivity maximum along the crystal b axis14,15,16,17,18,19,20,21,22,23,24,25, develops with increasing concentration of dopant atoms14,20,21,22,23,24,25; this ‘nematicity’vanishes when the parent phase disappears near the maximum superconducting Tc. The interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom and the transport nematicity has therefore become a pivotal focus7,8,12,22,23 of research into these materials. Here, by directly visualizing the atomic-scale electronic structure, we show that substituting Co for Fe atoms in underdoped Ca(Fe1-xCox)2As2 generates a dense population of identical anisotropic impurity states. Each is ~ 8 Fe–Fe unit cells in length, and all are distributed randomly but aligned with the antiferromagnetic a axis. By imaging their surrounding interference patterns, we further demonstrate that these impurity states scatter quasiparticles in a highly anisotropic manner, with the maximum scattering rate concentrated along the b axis. These data provide direct support for the recent proposals7,8,12,22,23 that it is primarily anisotropic scattering by dopant-induced impurity states that generates the transport nematicity; they also yield simple explanations for the enhancement of the nematicity proportional to the dopant density14,20,21,22,23,24,25 and for the occurrence of the highest resistivity along the b axis14,15,16,17,18,19,20,21,22,23,24,25.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

High-pressure X-ray diffraction, Raman, and computational studies of MgCl 2 up to 1 Mbar: Extensive pressure stability of the β-MgCl 2 layered structure

We studied magnesium chloride (MgCl 2 ) with the rhombohedral layered CdCl 2 -type structure (α-MgCl 2 ), experimentally, using synchrotron angle-dispersive powder x-ray diffraction and Raman spectroscopy using a diamond-anvil cell up to 100 GPa at room temperature and theoretically using first-principles density functional calculations. Our results reveal a pressure-induced second-order structural phase transition to a hexagonal layered CdI 2 -type structure (β-MgCl 2 ) at 0.7 GPa: the stacking sequence of the Cl anions are altered resulting in a reduction of the c-axis length. Theoretical calculations confirm this phase transition sequence and the calculated transition pressure is in excellent agreement with the experiment. Lattice dynamics calculations also reproduce the experimental Raman spectra measured for the ambient and high-pressure phase. According to our experimental results MgCl 2 remains in a 2D layered phase up to 100 GPa and further, the 6-fold coordination of Mg cations is retained. Theoretical calculations of relative enthalpy suggest that this extensive pressure stability is due to a low enthalpy of the layered structure ruling out kinetic barrier effects. Our observation is unusual, as it contradicts with the general structural behavior of highly compressed AB 2 compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Superconductivity bordering Rashba type topological transition

Strong spin orbital interaction (SOI) can induce unique quantum phenomena such as topological insulators, the Rashba effect, or p-wave superconductivity. Combining these three quantum phenomena into a single compound has important scientific implications. Here we report experimental observations of consecutive quantum phase transitions from a Rashba type topological trivial phase to topological insulator state then further proceeding to superconductivity in a SOI compound BiTeI tuned via pressures. The electrical resistivity measurement with V shape change signals the transition from a Rashba type topological trivial to a topological insulator phase at 2 GPa, which is caused by an energy gap close then reopen with band inverse. Superconducting transition appears at 8 GPa with a critical temperature T C of 5.3 K. Structure refinements indicate that the consecutive phase transitions are correlated to the changes in the Bi–Te bond and bond angle as function of pressures. As a result, the Hall Effect measurements reveal an intimate relationship between superconductivity and the unusual change in carrier density that points to possible unconventional superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural and Na-ion conduction characteristics of Na 3 PS x Se 4–x

The recent discovery of the isostructrual cubic Na 3 PS 4 and Na 3 PSe 4 as fast Na-ion conductors provided a general structural framework for the exploration of new sodium superionic conductors. In this work, we systematically investigated the structures and ionic conduction characteristics of a series of compounds with the general chemical formula of Na 3 PS x Se 4–x . Synthesis of Na 3 PS 4 under different conditions (e.g., temperature, reaction vessel, mass of the precursors) reveals the reactivity of the precursors with the reaction tubes, producing different polymorphs. X-ray diffraction studies on the solid solution phases Na 3 PS x Se 4–x identified a tetragonal-to-cubic phase transition with increasing Se concentration. This observation is consistent with the computed stability of the tetragonal and cubic polymorphs, where the energy difference between the two polymorphs becomes very close to zero in Se-rich compositions. Furthermore, ab initio molecular dynamic simulations suggest that the fast Na-ion conduction in Na 3 PS x Se 4–x may not be causally related with the symmetry or the composition of these phases. The formation of defects, instead, enables fast Na-ion conduction in this class of materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

X-ray diffraction in the pulsed laser heated diamond anvil cell

We have developed in situ x-ray synchrotron diffraction measurements of samples heated by a pulsed laser in the diamond anvil cell at pressure up to 60 GPa. We used an electronically modulated 2-10 kHz repetition rate, 1064-1075 nm fiber laser with 1-100 {micro}s pulse width synchronized with a gated x-ray detector (Pilatus) and time-resolved radiometric temperature measurements. This enables the time domain measurements as a function of temperature in a microsecond time scale (averaged over many events, typically more than 10,000). X-ray diffraction data, temperature measurements, and finite element calculations with realistic geometric and thermochemical parameters show that in the present experimental configuration, samples 4 {micro}m thick can be continuously temperature monitored (up to 3000 K in our experiments) with the same level of axial and radial temperature uniformities as with continuous heating. We find that this novel technique offers a new and convenient way of fine tuning the maximum sample temperature by changing the pulse width of the laser. This delicate control, which may also prevent chemical reactivity and diffusion, enables accurate measurement of melting curves, phase changes, and thermal equations of state.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

The Nature of Electron Lone Pairs in BiVO4

The electronic structure of BiVO{sub 4} has been studied by x-ray photoelectron, x-ray absorption, and x-ray emission spectroscopies, in comparison with density functional theory calculations. Our results confirm both the direct band gap of 2.48 eV and that the Bi 6s electrons hybridize with O 2p to form antibonding 'lone pair' states at the top of the valence band. The results highlight the suitability of combining s{sup 2} and d{sup 0} cations to produce photoactive ternary oxides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Synchrotron radiation infrared microscopic study of non-bridging oxygen modes associated with laser-induced breakdown of fused silica

Nanosecond pulse laser-driven optical breakdown at SiO{sub 2} surfaces as probed by synchrotron-based Fourier transform infrared (SRFTIR) and photoluminescence (PL) microscopies is presented. SRFTIR mapping of laser damage identified localized non-bridging Si-O vibrational modes at {approx}950 cm{sup -1} which became stiffer as 355 nm laser pulse lengths were increased from 5 to 20 ns. The bridging Si-O-Si transverse optic mode frequency varied significantly across damaged regions indicating a wide range of average bond angles, softening slightly with increasing pulse length. 355 nm-excited PL images of laser modified regions could be directly correlated with the structural modifications identified through SRFTIR.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Electronic and thermal transport in GeTe: A versatile base for thermoelectric materials

GeTe is a narrow-band gap semiconductor, where Ge vacancies generate free charge carriers, holes, forming a self-dopant degenerate system with p-type conductivity, and serves as a base for high-performance multicomponent thermoelectric materials. There is a significant discrepancy between the electronic and thermal transport data for GeTe-based materials reported in the literature, which obscures the baseline knowledge and prevents a clear understanding of the effect of alloying GeTe with various elements. A comprehensive study including XRD, SEM, EDS, Seebeck coefficient, electrical resistivity, thermal conductivity, and {sup 125}Te NMR of several GeTe samples was conducted. Similar Seebeck coefficient and electrical resistivity are observed for all GeTe samples used showing that the concentration of Ge vacancies generating charge carriers is constant along the ingot. Very short {sup 125}Te NMR spin-relaxation time agrees well with high carrier concentration obtained from the Hall effect measurements. Our data show that at ∼700 K, GeTe has a very large power factor, 42 μWcm{sup −1}K{sup −2}, much larger than that of any high efficiency thermoelectric telluride at these temperatures. Electronic and thermal properties of GeTe are compared to PbTe, another well-known thermoelectric material, where free charge carriers, holes or electrons, are generated by vacancies on Pb or Te sites, respectively. Discrepancy in the data for GeTe reported in literature can be attributed to the variation in the Ge:Te ratio of solidified samples as well as to different conditions of measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Aspects of the inverse problem for the Toda chain

We generalize Babelon's approach to equations in dual variables so as to be able to treat new types of operators which we build out of the sub-constituents of the model's monodromy matrix. Further, we also apply Sklyanin's recent monodromy matrix identities so as to obtain equations in dual variables for yet other operators. The schemes discussed in this paper appear to be universal and thus, in principle, applicable to many models solvable through the quantum separation of variables.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Nonlinear luminescence response of CaF{sub 2}:Eu and YAlO{sub 3}:Ce to single-ion excitation

Understanding scintillation physics and nonproportionality is essential to accelerate materials discovery that has been restricted due to the difficulties inherent to large crystal growth and complex nature of gamma-solid interaction. Taking advantage of less restrictive growth and deposition techniques for smaller crystal sizes or thin films and better fundamental understanding of ion-solid interactions, a unique ion approach is demonstrated to effectively screen candidate scintillators with relatively small size and evaluate their nonlinear scintillation response. Response of CaF{sub 2}:Eu and YAlO{sub 3}:Ce scintillators to single ions of H{sup +}, He{sup +}, and O{sup 3+} are measured by the corresponding pulse height over a continuous energy range using a time-of-flight–scintillator–photoelectric multiplier tube apparatus. Nonlinear response of the scintillators under ionizing ion irradiation is quantitatively evaluated by considering the energy partitioning process. In a differential energy deposition region with negligible displacement damage, the low, medium and high excitation energy deposition density (D{sub exci}) can be produced by energetic H{sup +}, He{sup +} and O{sup 3+} ions, respectively, and significantly different impacts on the response characteristics of these two benchmark scintillators are observed. For CaF{sub 2}:Eu, the scintillation efficiency under ion irradiation monotonically decreases with increasing excitation-energy density. In contrast, the response efficiency of YAlO{sub 3}:Ce scintillation initially increases with excitation-energy density at low excitation-energy densities, goes through a maximum, and then decreases with further increasing excitation-energy density. The fundamental mechanism causing these different response behaviours in the scintillators is based on the competition between the scintillation response and the nonradiative quenching process under different excitation densities, which is also the main origin of the nonlinear response of the scintillators to irradiation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Thermoelectric transport in indium and aluminum-doped lead selenide

We present galvanomagnetic and thermomagnetic properties of bulk PbSe doped by substituting the donor elements In and Al for Pb. Although prominent resonant level effects are not seen, lightly doped samples display a high thermoelectric figure of merit (zT) in excess of 1.2 at 600 K, a temperature corresponding well to automotive waste heat recovery applications. This material's high zT is achieved without the use of nanostructuring or the relatively rare element Te. Phonon drag contributions to thermopower appear at temperatures below 30 K in Al-doped samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Formation and evolution of ripples on ion-irradiated semiconductor surfaces

We have examined the formation and evolution of ripples on focused-ion-beam (FIB) irradiated compound semiconductor surfaces. Using initially normal-incidence Ga + FIB irradiation of InSb, we tuned the local beam incidence angle (θ eff ) by varying the pitch and/or dwell time. For single-pass FIB irradiation, increasing θ eff induces morphological evolution from pits and islands to ripples to featureless surfaces. Multiple-pass FIB irradiation of the rippled surfaces at a fixed θ eff leads to island formation on the ripple crests, followed by nanorod (NR) growth. Finally, this ripple-NR transition provides an alternative approach for achieving dense arrays of NRs.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Phonon-magnon interactions in body centered cubic iron: A combined molecular and spin dynamics study

Combining an atomistic many-body potential with a classical spin Hamiltonian parameterized by first principles calculations, molecular-spin dynamics computer simulations were performed to investigate phonon-magnon interactions in body centered cubic iron. Results obtained for spin-spin and density-density dynamic structure factors show that noticeable softening and damping of magnon modes occur due to the presence of lattice vibrations. Furthermore, as a result of the phonon-magnon coupling, additional longitudinal spin wave excitations are observed, with the same frequencies as the longitudinal phonon modes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Band gap of epitaxial in-plane-dimerized single-phase NbO{sub 2} films

We demonstrate the epitaxial growth of high quality crystalline phase-pure NbO{sub 2} films on various oxide substrates using molecular beam epitaxy. Using a combination of reflection high-energy electron diffraction and x-ray diffraction we show that the films grow with the pseudo-rutile (100) orientation out of plane. The band gap of the NbO{sub 2} films is determined to be at least 1.0 eV using a combination of x-ray photoelectron spectroscopy and inverse photoelectron spectroscopy, in conjunction with hybrid density functional calculations of the density of states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗