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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Quantum phase transitions between a class of symmetry protected topological states

The subject of this paper is the phase transition between symmetry protected topological states (SPTs). We consider spatial dimension d and symmetry group G so that the cohomology group, Hd+1(G,U(1)), contains at least one Z2n or Z factor. We show that the phase transition between the trivial SPT and the root states that generate the Z2n or Z groups can be induced on the boundary of a (d+1)-dimensional View the MathML source-symmetric SPT by a View the MathML source symmetry breaking field. Moreover we show these boundary phase transitions can be “transplanted” to d dimensions and realized in lattice models as a function of a tuning parameter. The price one pays is for the critical value of the tuning parameter there is an extra non-local (duality-like) symmetry. In the case where the phase transition is continuous, our theory predicts the presence of unusual (sometimes fractionalized) excitations corresponding to delocalized boundary excitations of the non-trivial SPT on one side of the transition. This theory also predicts other phase transition scenarios including first order transition and transition via an intermediate symmetry breaking phase.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Comparative evaluation of deep learning workloads for leadership-class systems

Deep learning (DL) workloads and their performance at scale are becoming important factors to consider as we design, develop and deploy next-generation high-performance computing systems. Since DL applications rely heavily on DL frameworks and underlying compute (CPU/GPU) stacks, it is essential to gain a holistic understanding from compute kernels, models, and frameworks of popular DL stacks, and to assess their impact on science-driven, mission-critical applications. At Oak Ridge Leadership Computing Facility (OLCF), we employ a set of micro and macro DL benchmarks established through the Collaboration of Oak Ridge, Argonne, and Livermore (CORAL) to evaluate the AI readiness of our next-generation supercomputers. In this paper, we present our early observations and performance benchmark comparisons between the Nvidia V100 based Summit system with its CUDA stack and an AMD MI100 based testbed system with its ROCm stack. We take a layered perspective on DL benchmarking and point to opportunities for future optimizations in the technologies that we consider.

Yin, Junqi↗
DOE OSTI2020

Mercury Chalcogenide Nanoplatelet–Quantum Dot Heterostructures as a New Class of Continuously Tunable Bright Shortwave Infrared Emitters

Despite broad applications in imaging, energy conversion, and telecommunications, few nanoscale moieties emit light efficiently in the shortwave infrared (SWIR, 1000–2000 nm or 1.24–0.62 eV). We report quantum-confined mercury chalcogenide (HgX, where X = Se or Te) nanoplatelets (NPLs) can be induced to emit bright (QY > 30%) and tunable (900–1500+ nm) infrared emission from attached quantum dot (QD) “defect” states. Here, we demonstrate near unity energy transfer from NPL to these QDs, which completely quench NPL emission and emit with a high QY through the SWIR. This QD defect emission is kinetically tunable, enabling controlled midgap emission from NPLs. Spectrally resolved photoluminescence demonstrates energy-dependent lifetimes, with radiative rates 10–20 times faster than those of their PbX analogues in the same spectral window. Coupled with their high quantum yield, midgap emission HgX dots on HgX NPLs provide a potential platform for novel optoelectronics in the SWIR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Measuring 8–250 ps short pulses using a high-speed streak camera on kilojoule, petawatt-class laser systems

Short-pulse measurements using a streak camera are sensitive to space-charge broadening, which depends on the pulse duration and shape, and on the uniformity of photocathode illumination. An anamorphic-diffuser-based beam-homogenizing system and a space-charge-broadening calibration method were developed to accurately measure short pulses using an optical streak camera. This approach provides a more-uniform streak image and enables one to characterize space-charge-induced pulse-broadening effects.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

High-energy gamma-ray emission from solar flares: Summary of Fermi large area telescope detections and analysis of two M-class flares

We present the detections of 18 solar flares detected in high-energy γ-rays (above 100 MeV) with the Fermi Large Area Telescope (LAT) during its first 4 yr of operation. This work suggests that particle acceleration up to very high energies in solar flares is more common than previously thought, occurring even in modest flares, and for longer durations. Interestingly, all these flares are associated with fairly fast coronal mass ejections (CMEs). We then describe the detailed temporal, spatial, and spectral characteristics of the first two long-lasting events: the 2011 March 7 flare, a moderate (M3.7) impulsive flare followed by slowly varying γ-ray emission over 13 hr, and the 2011 June 7 M2.5 flare, which was followed by γ-ray emission lasting for 2 hr. We compare the Fermi LAT data with X-ray and proton data measurements from GOES and RHESSI. We argue that the γ-rays are more likely produced through pion decay than electron bremsstrahlung, and we find that the energy spectrum of the proton distribution softens during the extended emission of the 2011 March 7 flare. This would disfavor a trapping scenario for particles accelerated during the impulsive phase of the flare and point to a continuous acceleration process at play for the duration of the flares. CME shocks are known for accelerating the solar energetic particles (SEPs) observed in situ on similar timescales, but it might be challenging to explain the production of γ-rays at the surface of the Sun while the CME is halfway to the Earth. A stochastic turbulence acceleration process occurring in the solar corona is another likely scenario. Detailed comparison of characteristics of SEPs and γ-ray-emitting particles for several flares will be helpful to distinguish between these two possibilities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

New class of planar ferroelectric Mott insulators via first-principles design

which is not common in known materials. Here we use first-principles calculations to design layered double perovskite oxides AABBO6 which achieve the aforementioned properties in the context of Mott insulators. In our design rules, the gap is dictated by B/B electronegativity difference in a Mott state, while the polarization is obtained via nominal d0 filling on the B-site, A-type cations bearing lone-pair electrons, and A = A size mismatch. Successful execution is demonstrated in BaBiCuVO6, BaBiNiVO6, BaLaCuVO6, and PbLaCuVO6.

Kim, Chanul↗