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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Bandwidth and Electron Correlation-Tuned Superconductivity in Rb 0.8 Fe 2 ( Se 1 - z S z ) 2

Here, we present a systematic angle-resolved photoemission spectroscopy study of the substitution dependence of the electronic structure of Rb 0.8 Fe 2 (Se 1-z S z ) 2 (z = 0, 0.5, 1), where superconductivity is continuously suppressed into a metallic phase. Going from the nonsuperconducting Rb 0.8 Fe 2 S 2 to superconducting Rb 0.8 Fe 2 Se 2 , we observe little change of the Fermi surface topology, but a reduction of the overall bandwidth by a factor of 2. Hence, for these heavily electron-doped iron chalcogenides, we have identified electron correlation as explicitly manifested in the quasiparticle bandwidth to be the important tuning parameter for superconductivity, and that moderate correlation is essential to achieving high T C .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Erratum: Creation of X-Ray Transparency of Matter by Stimulated Elastic Forward Scattering [Phys. Rev. Lett. 115 , 107402 (2015)]

X-ray absorption by matter has long been described by the famous Beer-Lambert law. Here we show how this fundamental law needs to be modified for high-intensity coherent x-ray pulses, now available at x-ray free electron lasers, due to the onset of stimulated elastic forward scattering. We present an analytical expression for the modified polarization-dependent Beer-Lambert law for the case of resonant core-to-valence electronic transitions and incident transform limited x-ray pulses. Upon transmission through a solid, the absorption and dichroic contrasts are found to vanish with increasing x-ray intensity, with the stimulation threshold lowered by orders of magnitude through a super-radiative coherent effect. Our results have broad implications for the study of matter with x-ray lasers.

MATSCI↗
DOE OSTI2016

NMR evidence for inhomogeneous nematic fluctuations in BaFe 2 ( As 1 - x P x ) 2

We present evidence for nuclear spin-lattice relaxation driven by glassy nematic fluctuations in isovalent P-doped BaFe 2 As 2 single crystals. Both the 75 As and 31 P sites exhibit a stretched-exponential relaxation similar to the electron-doped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. Lastly, these results suggest that glassy nematic dynamics are a common phenomenon in the iron-based superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Correlation-driven insulator-metal transition in near-ideal vanadium dioxide films

We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction, and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO 2 . We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO 2 films are preceded by the purely electronic softening of Coulomb correlations within V-V singlet dimers. Furthermore, this process starts 7 K (±0.3 K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically promising material.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Picosecond electric-field-induced threshold switching in phase-change materials [THz-induced threshold switching and crystallization of phase-change materials]

Many chalcogenide glasses undergo a breakdown in electronic resistance above a critical field strength. Known as threshold switching, this mechanism enables field-induced crystallization in emerging phase-change memory. Purely electronic as well as crystal nucleation assisted models have been employed to explain the electronic breakdown. Here, picosecond electric pulses are used to excite amorphous Ag 4 In 3 Sb 67 Te 26 . Field-dependent reversible changes in conductivity and pulse-driven crystallization are observed. The present results show that threshold switching can take place within the electric pulse on subpicosecond time scales—faster than crystals can nucleate. As a result, this supports purely electronic models of threshold switching and reveals potential applications as an ultrafast electronic switch.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Examining Electron-Boson Coupling Using Time-Resolved Spectroscopy

Nonequilibrium pump-probe time-domain spectroscopies can become an important tool to disentangle degrees of freedom whose coupling leads to broad structures in the frequency domain. Here, using the time-resolved solution of a model photoexcited electron-phonon system, we show that the relaxational dynamics are directly governed by the equilibrium self-energy so that the phonon frequency sets a window for “slow” versus “fast” recovery. The overall temporal structure of this relaxation spectroscopy allows for a reliable and quantitative extraction of the electron-phonon coupling strength without requiring an effective temperature model or making strong assumptions about the underlying bare electronic band dispersion.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Retrofitting the 5045 Klystron for Higher Efficiency

The 5045 klystron has been in production and accelerating particles at SLAC National Accelerator Laboratory for over 25 years. Although the design has undergone some changes there are still significant opportunities for improvement in performance. Retrofitting the 5045 for higher efficiencies and a more mono-energetic spent beam profile is presented.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Evidence for a gapped spin-liquid ground state in a kagome Heisenberg antiferromagnet

Here, the kagome Heisenberg antiferromagnet is a leading candidate in the search for a spin system with a quantum spin-liquid ground state. The nature of its ground state remains a matter of active debate. We conducted oxygen-17 single-crystal nuclear magnetic resonance (NMR) measurements of the spin-1/2 kagome lattice in herbertsmithite [ZnCu 3 (OH) 6 Cl 2 ], which is known to exhibit a spinon continuum in the spin excitation spectrum. We demonstrated that the intrinsic local spin susceptibility χkagome, deduced from the oxygen-17 NMR frequency shift, asymptotes to zero below temperatures of 0.03J, where J ~ 200 kelvin is the copper-copper superexchange interaction. Combined with the magnetic field dependence of χ kagome that we observed at low temperatures, these results imply that the kagome Heisenberg antiferromagnet has a spin-liquid ground state with a finite gap.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Method for computationally efficient design of dielectric laser accelerator structures

Here, dielectric microstructures have generated much interest in recent years as a means of accelerating charged particles when powered by solid state lasers. The acceleration gradient (or particle energy gain per unit length) is an important figure of merit. To design structures with high acceleration gradients, we explore the adjoint variable method, a highly efficient technique used to compute the sensitivity of an objective with respect to a large number of parameters. With this formalism, the sensitivity of the acceleration gradient of a dielectric structure with respect to its entire spatial permittivity distribution is calculated by the use of only two full-field electromagnetic simulations, the original and ‘adjoint’. The adjoint simulation corresponds physically to the reciprocal situation of a point charge moving through the accelerator gap and radiating. Using this formalism, we perform numerical optimizations aimed at maximizing acceleration gradients, which generate fabricable structures of greatly improved performance in comparison to previously examined geometries.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

LDRD FY2013 Annual Report

The Department of Energy (DOE) and the SLAC National Accelerator Laboratory (SLAC) encourage innovation, creativity, originality and quality to maintain the Laboratory’s research activities and staff at the forefront of science and technology. To further advance the Laboratory’s scientific research capabilities, the Laboratory allocates a portion of its funds for the Laboratory Directed Research and Development (LDRD) program. With DOE guidance, the LDRD program enables SLAC scientists to make rapid and significant contributions to seeding new strategies for solving important national science and technology problems. The LDRD program is conducted using existing research facilities.

ACCPHY↗
DOE OSTI2015

SLAC Laboratory Directed Research and Development Program: FY2014 Annual Report

The Department of Energy (DOE) and the SLAC National Accelerator Laboratory (SLAC) encourage innovation, creativity, originality and quality to maintain the Laboratory’s research activities and staff at the forefront of science and technology. To further advance the Laboratory’s scientific research capabilities, the Laboratory allocates a portion of its funds for the Laboratory Directed Research and Development (LDRD) program. With DOE guidance, the LDRD program enables SLAC scientists to make rapid and significant contributions to seeding new strategies for solving important national science and technology problems. The LDRD program is conducted using existing research facilities. The project proposals can be characterized as: Small-scale research and development activities or pilot projects; Bench-scale research projects; and Information gathering, computer modeling, conceptual design and feasibility studies. In addition to building new core competencies that support the DOE missions, the LDRD project proposals may also conduct scientific research and development that support the missions of other federal agencies and/or non-federal sponsors. In FY2014, SLAC continued to see some of the previously funded LDRD programs transition to follow on funding with the Office of Science, (including an Early Career Award), EERE and other agencies such as DARPA and NIH. This external validation shows that the projects selected were important to continuing forefront science and use-inspired applications with mission relevance and demonstrates they were delivering significant results. Annual reports for the FY14 project activities are included in this report.

25 ENERGY STORAGE↗