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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 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Hybridization and suppression of the superconductivity in CeFeAsO 1-y . Pressure and temperature dependences of the electronic structure

Pressure and temperature dependence of the electronic structure of superconducting (SC) CeFeAsO 1-y and non-SC CeFeAsO 1-y have been investigated using two complementary hard x-ray spectroscopic probes at the Ce L 3 edge, partial fluorescence yield x-ray absorption spectroscopy and resonant x-ray emission spectroscopy. With increasing pressure, the ratio between the intensity of the peak related to the f 0 (Ce 4+ ) state and that of the f 1 (Ce 3+ ) state, I(f 0 )/I(f 1 ), is found to increase continuously for both compounds, indicating a continuous increase in the Ce valence. The valence of non-SC CeFeAsO 1-y is found to be slightly higher than that of SC CeFeAsO 1-y in the entire pressure and temperature ranges of this study. The valence of CeFeAsO 1-y around 6 GPa, where the superconductivity breaks down, is estimated to be ~3.0 , but no change in the valence is observed upon cooling. The dependence of the interatomic distances on the concentration of oxygen vacancies is studied via extended absorption fine structure spectroscopy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Distinct superconducting states in the pressure-induced metallic structures of the nominal semimetal Bi[subscript 4]Te[subscript 3]

The end members, Bi and Bi{sub 2}Te{sub 3}, of the infinitely adaptive (Bi{sub 2})m(Bi{sub 2}Te{sub 3}){sub n} series of compounds have not only been revealed to be topological insulators under the appropriate conditions, but have also been shown to be superconductors under pressure, suggesting the potential for bulk superconductor-topological-insulator interfaces and associated quantum computing applications. Herein, we report the pressure-dependent evolution of the structure and electrical transport of the nominal semimetal Bi4Te{sub 3}, a member of the (Bi{sub 2})m(Bi{sub 2}Te{sub 3}){sub n} series. Under pressure, Bi4Te{sub 3} undergoes several structural phase transformations, ultimately yielding a metallic body-centered-cubic structure exhibiting superconductivity with a maximum T{sub c} = 8.4 K at 16.2 GPa. The occurrence of structure-dependent superconductivity in Bi{sub 4}Te{sub 3} is remarkably similar to the end members of the (Bi{sub 2})m(Bi{sub 2}Te{sub 3}){sub n} series, intimating a convergence to high-pressure universal behavior that may expose the subtle variations that lead to the topological insulating and superconducting states in these systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetic structure and spin excitations in BaMn 2 Bi 2

We present a single crystal neutron scattering study of BaMn 2 Bi 2 , a recently synthesized material with the same ThCr 2 Si 2 type structure found in several Fe-based unconventional superconducting materials. We show long range magnetic order, in the form of a G-type antiferromagnetic structure, to exist up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering we observe a spin-gap of 16 meV, with spin-waves extending up to 55 meV. We find these magnetic excitations are well fit to a J 1 -J 2 -J c Heisenberg model and present values for the exchange interactions. The spin wave spectrum appears to be unchanged by the 100 K structural phase transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Electronic structure and x-ray magnetic circular dichroism in YbAgCu4 and YbInCu4

X-ray absorption and magnetic circular dichroism spectra at the L-3 edge of Yb have been studied theoretically in the mixed-valent compounds YbAgCu4 and YbInCu4 using the fully relativistic Dirac linear muffin-tin orbital band structure method. The effect of the spin-orbit (SO) interaction and Coulomb repulsion U on the electronic structure of YbAgCu4 and YbInCu4 is examined in the frame of the local spin-density approximation (LSDA)+SO+U method. The core-hole effect in the final states as well as the effect of the electric quadrupole E-2 and magnetic dipole M-1 transitions have been investigated. The calculated results are compared with available experimental data.

Antonov, VN↗
DOE OSTI2017

Tuning the electronic and the crystalline structure of LaBi by pressure: From extreme magnetoresistance to superconductivity

Here, extreme magnetoresistance (XMR) in topological semimetals is a recent discovery which attracts attention due to its robust appearance in a growing number of materials. To search for a relation between XMR and superconductivity, we study the effect of pressure on LaBi. By increasing pressure, we observe the disappearance of XMR followed by the appearance of superconductivity at P ≈ 3.5 GPa. We find a region of coexistence between superconductivity and XMR in LaBi in contrast to other superconducting XMR materials. The suppression of XMR is correlated with increasing zero-field resistance instead of decreasing in-field resistance. At higher pressures, P ≈ 11 GPa, we find a structural transition from the face-centered cubic lattice to a primitive tetragonal lattice, in agreement with theoretical predictions. The relationship between extreme magnetoresistance, superconductivity, and structural transition in LaBi is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Predicting the ground-state structure of sodium boride

Binary borides has been a subject of extensive research. However, the exact compositions and crystal structures of sodium borides remained controversial. Furthermore, using the ab initio variable-composition evolutionary algorithm, a new stable Na 2 B 30 with I2 1 2 1 2 1 symmetry (I2 1 2 1 2 1 –Na 2 B 30 ) is found, which is –7.38 meV / atom lower in energy than the Imma–Na 2 B 30 structure reported by experimentalists. Interestingly, the Imma–Na 2 B 30 structure is predicted to be a topological nodal line semimetal, which may result in superior electronic transport. In contrast, I2 1 2 1 2 1 –Na 2 B 30 is an ultrahard semiconductor with an unprecedented open-framework structure, whose interstitial helical boron sublattice enhances its hardness and energetic stability.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Polarized Structure Function {sigma}LT from {pi}0 p Electroproduction Data in the Resonance Region at 0.4 GeV2 < Q2 < 1.0 GeV2

The first results on the sigma(LT') structure function in exclusive pi(0)p electroproduction at invariant masses of the final state of 1.5 GeV < W < 1.8 GeV and in the range of photon virtualities 0.4 GeV2 < Q(2) < 1.0 GeV2 were obtained from data on beam spin asymmetries and differential cross sections measured with the CLAS detector at Jefferson Lab. The Legendre moments determined from the sigma(LT') structure function have demonstrated sensitivity to the contributions from the nucleon resonances in the second and third resonance regions. These new data on the beam spin asymmetries in pi(0)p electroproduction extend the opportunities for the extraction of the nucleon resonance electro-excitation amplitudes in the mass range above 1.6 GeV.

Isupov, E. L.↗
DOE OSTI2010

Multi-quasiparticle structures up to spin {approx}44({h_bar}/2{pi}) in the odd-odd nucleus {sup 168}Ta

High-spin states in the odd-odd nucleus {sup 168}Ta have been populated in the {sup 120}Sn({sup 51}V,3n) reaction. Two multi-quasiparticle structures have been extended significantly from spin {approx}20({h_bar}/2{pi}) to above 40({h_bar}/2{pi}). As a result, the first rotational alignment has been fully delineated and a second band crossing has been observed for the first time in this nucleus. Configurations for these strongly coupled rotational bands are proposed based on signature splitting, B(M1)/B(E2) ratio information, and observed rotation-alignment behavior. Properties of the observed bands in {sup 168}Ta are compared to related structures in the neighboring odd-Z, odd-N, and odd-odd nuclei and are discussed within the framework of the cranked shell model.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

Structure of three-quasiparticle isomers in {sup 169}Ho and {sup 171}Tm.

A three-quasiparticle isomer with {tau}=170(8) {micro}s and K{sup {pi}} = (19/2{sup +}) has been identified in the neutron-rich isotope {sup 169}Ho. The isomer decays with K-forbidden transitions to members of a band associated with the 7/2-[523] proton configuration, whose structure is characterized through analysis of the in-band {gamma}-ray branching ratios. In the isotone {sup 171}Tm, the rotational band based on the known 19/2{sup +}, three-quasiparticle isomer has also been observed. Alternative one-proton two-neutron configurations for the isomer in {sup 169}Ho are discussed in terms of multiquasiparticle calculations and through a comparison with the structures observed in {sup 171}Tm.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Influence of the vg{sub 9/2} orbital on level structures of neutron-rich {sup 61,62} Mn{sub 36,37}.

Level structures in {sup 61,62}Mn{sub 36,37} were studied with Gammasphere in the reaction of a 430-MeV {sup 64}Ni beam and a thick {sup 238}U target. The newly identified levels decrease in excitation energy compared to the analogous structures in the lighter Mn isotopes and behave similarly to states in the corresponding Fe isotones that involve g{sub 9/2} neutron excitations. This behavior illustrates the importance of the inclusion of the ?g{sub 9/2} orbital in any realistic shell-model calculations in this region.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Resonance region structure functions and parity-violating deep inelastic scattering

The primary motive of parity-violating deep inelastic scattering experiments has been to test the standard model, particularly the axial couplings to the quarks, in the scaling region. The measurements can also test for the validity of models for the off-diagonal structure functions ${F}_{1,2,3}^{γZ}$($x,Q^2$) in the resonance region. The off-diagonal structure functions are important for the accurate calculation of the $γZ$-box correction to the weak charge of the proton. Currently, with no data to determine ${F}_{1,2,3}^{γZ}$($x,Q^2$) directly, models are constructed by modifying existing fits to electromagnetic data. Here, we present the asymmetry value for deuteron and proton targets predicted by several different ${F}_{1,2,3}^{γZ}$($x,Q^2$) models and demonstrate that there are notable disagreements.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Pion structure function from leading neutron electroproduction and SU(2) flavor asymmetry

In this paper, we examine the efficacy of pion exchange models to simultaneously describe leading neutron electroproduction at HERA and the $\bar{d}-\bar{u}$ flavor asymmetry in the proton. A detailed $\chi^2$ analysis of the ZEUS and H1 cross sections, when combined with constraints on the pion flux from Drell-Yan data, allows regions of applicability of one-pion exchange to be delineated. The analysis disfavors several models of the pion flux used in the literature, and yields an improved extraction of the pion structure function and its uncertainties at parton momentum fractions in the pion of $4 \times 10^{-4} \lesssim x_\pi \lesssim 0.05$ at a scale of $Q^2$=10 GeV$^2$. Also, we provide estimates for leading proton structure functions in upcoming tagged deep-inelastic scattering experiments on the deuteron with forward protons, based on the fit results, at Jefferson Lab.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Structural and magnetic phase transitions near optimal superconductivity in BaFe 2 (As 1-x P x ) 2

In this study, we use nuclear magnetic resonance (NMR), high-resolution x-ray and neutron scattering to study structural and magnetic phase transitions in phosphorus-doped BaFe 2 (As 1-x P x ) 2 . Thus, previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x = 0.3. However, we show that the tetragonal-to-orthorhombic structural (T s ) and paramagnetic to antiferromagnetic (AF, T N ) transitions in BaFe 2 (As 1-x P x ) 2 are always coupled and approach to T N ≈ T s ≥ T c (≈ 29 K) for x = 0.29 before vanishing abruptly for x ≥ 0.3. These results suggest that AF order in BaFe 2 (As 1-x P x ) 2 disappears in a weakly first order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Real-Time Examination of Atomistic Mechanisms during Shock-Induced Structural Transformation in Silicon

Experimental determination of atomistic mechanisms linking crystal structures during a compression driven solid-solid phase transformation is a long standing and challenging scientific objective. Also, when using new capabilities at the Dynamic Compression Sector at the Advanced Photon Source, the structure of shocked Si at 19 GPa was identified as simple hexagonal and the lattice orientations between ambient cubic diamond and simple hexagonal structures were related. Furthermore, this approach is general and provides a powerful new method for examining atomistic mechanisms during stress-induced structural changes.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Structure, electronic and magnetic characterization, and calculated electronic structures of two oxyhalide hexagonal perovskites

Herein we report the crystal structures, initial magnetic and charge transport characterization, and calculated electronic structures of the hexagonal oxyhalide perovskites, Ba 7 Ru 4 O 15 Cl 2 and Ba 7 Ru 4 O 15 Br 2 . The experimental information is obtained through the study of single crystals. Face-sharing Ru O 6 octahedra form Ru 2 O 9 dimers in a layered triangular geometry in these materials; minor amounts of off-magnetic-site structural disorder are present in our crystals. Both are magnetically isotropic, with 2.5 μB/mol-Ru, Curie-Weiss theta –185 K, and 2.9 μB/mol-Ru, Curie-Weiss theta –168 K, respectively. Broad features in the magnetic susceptibility and heat capacity associated with magnetic ordering (at 35 and 37 K, respectively) are observed. The charge transport band gaps are 0.03 eV for the oxychloride and 0.006 eV for the oxybromide. There is no gap at the Fermi level for either of these compounds in density-functional theory electronic structure calculations.

36 MATERIALS SCIENCE↗