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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Influence of temperature, doping, and amorphization on the electronic structure and magnetic damping of iron

Hybrid magnonic quantum systems have drawn increased attention in recent years for coherent quantum information processing, but too large magnetic damping is a persistent concern when metallic magnets are used. Their intrinsic damping is largely determined by electron-magnon scattering induced by spin-orbit interactions. In the low scattering limit, damping is dominated by intra-band electronic transitions, which has been theoretically shown to be proportional to the electronic density of states at the Fermi level. In this work, we focus on body-centered-cubic iron as a paradigmatic ferromagnetic material. We comprehensively study its electronic structure using first-principles density functional theory simulations and account for finite lattice temperature, boron (B) doping, and structure amorphization. Our results indicate that temperature induced atomic disorder and amorphous atomic geometries only have a minor influence. Instead, boron doping noticeably decreases the density of states near the Fermi level with an optimal doping level of 6.25%. In addition, we show that this reduction varies significantly for different atomic geometries and report that the highest reduction correlates with a large magnetization of the material. Furthermore, this may suggest materials growth under external magnetic fields as a route to explore in experiment.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Spin squeezing with itinerant dipoles: A case for shallow lattices

Entangled spin squeezed states generated via dipolar interactions in lattice models provide unique opportunities for quantum enhanced sensing and are now within reach of current experiments. A critical question in this context is which parameter regimes offer the best prospects under realistic conditions. Light scattering in deep lattices can induce significant decoherence and strong Stark shifts, while shallow lattices face motional decoherence as a fundamental obstacle. Here we analyze the interplay between motion and spin squeezing in itinerant fermionic dipoles in one dimensional chains using exact matrix product state simulations. We demonstrate that shallow lattices can achieve more than 5 dB of squeezing, outperforming deep lattices by up to more than 3 dB, even in the presence of low filling, loss, and decoherence. We relate this finding to SU(2)-symmetric superexchange interactions, which keep spins aligned and protect collective correlations. We show that the optimal regime is achieved for small repulsive off-site interactions, with a trade-off between maximal squeezing and optimal squeezing time. Published by the American Physical Society 2024

Wellnitz, David (ORCID:0000000349788938)↗
DOE OSTI2019

First Annual Progress Report on the Procurement and Post-Irradiation Examination of the Selected Samples of Alloy 800H and Grade 92 and 91 Steels

Ferritic-martensitic steel Grade 92, with Grade 91 as a reference, together with austenitic stainless steel (or Incoloy Alloy) 800H and its grain-boundary-engineered version 800H-TMP (Thermo-Mechanical Processing), is investigated in this project. A total of fifteen Grade 92 samples from two or three heats, four T91 samples from two heats, and six 800H and six 800H-TMP samples from one heat were selected, which were primarily irradiated in the Advanced Test Reactor (ATR) of Idaho National Laboratory (INL), with the rest in the High Flux Isotope Reactor (HFIR) of Oak Ridge National Laboratory (ORNL), BOR-60 of Russia, and Phénix reactor of France. The samples were irradiated in the temperature range of 241 to 720°C and a dose range of 1.28 to ~70 displacements per atom. All the INL and ORNL samples and part of the LANL (Los Alamos National Laboratory) samples have been received at the Irradiated Materials Examination and Testing (IMET) hot cell facility. The selected ORNL samples include six Grade 92 and two 800H samples, which were irradiated in the HFIR to 0.46–14.66 dpa at 400 to ~720°C. The Grade 92 is an optimized heat and named as G92-2b. The two 800H samples have one sample as the standard solution-annealed condition and the other sample as a TMP condition with a maximized fraction of low-Σ coincidence site lattice (CSL) boundaries. Tensile test of the G92-2b samples were conducted at room temperature in the IMET hot cell. The lower temperature irradiation at 400–496.7°C to 0.52–14.66 dpa resulted in minimal changes in tensile properties. However, the higher temperature irradiation at ~683.3–720°C to 0.46–14.63 dpa tended to reduce yield and tensile strength by about –136 to –311 MPa and plastic elongation by –1 to –3% with the increasing dose. The 800H and 800H-TMP samples were tensile-tested at their irradiation temperature 580°C in the IMET hot cell, which showed >40% increase in yield and tensile strength with nearly 9% reduction in elongation for 800H-TMP compared with 800H. Three of the G92-2b samples, i.e., GB04 (7.44 dpa at ~490°C), GB05 (14.66 dpa at 496.7°C), and GB12 (14.63 dpa at ~720°C), were examined at the LAMDA for Vickers hardness measurements and microstructural characterization by SEM and TEM. Vickers hardness was successfully measured on GB12, which was reduced by is – (112.4 ± 12.3) HV1 from the control sample. However, the Vickers hardness measurements were failed on GB04 and GB05 because the samples were polished too thin to prevent the interference from the substrate (epoxy). The other half of the GB04 and GB05 will be polished for the hardness measurements, as well as nanoindentation studies. Fractography by SEM indicated increased numbers of ductile dimples (or voids) with the increasing irradiation dose, while significantly increased numbers and sizes of dimples with the increasing irradiation temperature. TEM characterization exhibited the presence of Cr 23 C 6 , VN, and Laves precipitates at boundaries and matrix, which showed pronounced precipitate-dislocation interactions (pinning effect). Dense dislocations but limited dislocation loops were observed in the samples, which noticeably reduced in GB12. Additionally, some tiny partially amorphized spherical domains were observed in GB12, which had a normal body-centered cubic lattice parameter of 9Cr steels at the surround regions but ~11% smaller than the lattice parameter of the matrix away from the amorphized spherical domains. Further characterization will be pursued to clarify the new findings.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2015

Excited-state spectrum, and the low-energy degrees of freedom of QCD

A major challenge in the study of the structure of hadrons is eliminating the systematic uncertainty due to the presence of excited states whilst avoiding the exponential degradation of the signal-to-noise ratio that arises as the source and sink hadrons are placed at increasing temporal separations. In this talk, I describe how the use of ?distillation? combined with a basis of non-local operators and the variational method facilitates the construction of an ?optimal? interpolating operator for the ground state, and hence the hadronic matrix elements to be computed at shorter separations. I apply the method to the calculation of the moments of the quark distribution amplitude in the pion, using isotropic clover lattices.

Richards, David↗
NASA NTRS1990

A class of systolizable IIR digital filters and its design for proper scaling and minimum output roundoff noise

A class of infinite impulse response (IIR) digital filters with a systolizable structure is proposed and its synthesis is investigated. The systolizable structure consists of pipelineable regular modules with local connections and is suitable for VLSI implementation. It is capable of achieving high performance as well as high throughput. This class of filter structure provides certain degrees of freedom that can be used to obtain some desirable properties for the filter. Techniques of evaluating the internal signal powers and the output roundoff noise of the proposed filter structure are developed. Based upon these techniques, a well-scaled IIR digital filter with minimum output roundoff noise is designed using a local optimization approach. The internal signals of all the modes of this filter are scaled to unity in the l2-norm sense. Compared to the Rao-Kailath (1984) orthogonal digital filter and the Gray-Markel (1973) normalized-lattice digital filter, this filter has better scaling properties and lower output roundoff noise.

Lei, Shaw-Min↗
DOE OSTI2016

Delivery Ring Lattice Modifications for Transitionless Deceleration

A portion of the remnant Tevatron program infrastruc- ture at Fermilab is being reconfigured to be used for the generation and delivery of proton and muon beams for new high-precision particle physics experiments. With the 8 GeV Booster as its primary source, the Mu2e exper- iment will receive 8.9 GeV/c bunched beam on target, after being stored and slow spilled from the Delivery Ring (DR) -- a refurbished debuncher ring from Tevatron anti- proton production. For the Muon g-2 experiment, the DR will be tuned for 3.1 GeV/c to capture muons off of a target before sending them to this experiment's Storage Ring. The apertures in the beam transport systems are optimized for the large muon beams of this lower-energy experiment. In order to provide further flexibility in the operation of the DR for future possible low-energy, high- intensity particle physics experiments (REDTOP[1], for example) and detector development, investigations are underway into the feasibility of decelerating beams from its maximum kinetic energy of 8 GeV level to lower en- ergies, down to 1-2 GeV. In this paper we look at possi- ble lattice modifications to the DR to avoid a transition crossing during the deceleration process. Hardware re- quirements and other operational implications of this scheme will also be discussed.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

First-Principles Study of Carbon and Vacancy Structures in Niobium

The interstitial chemical impurities hydrogen, oxygen, nitrogen, and carbon are important for niobium metal production and particularly for the optimization of niobium SRF technology. These atoms are present in refined sheets and can be absorbed into niobium during processing treatments, resulting in changes to the residual resistance and the performance of SRF cavities. A first-principles approach is taken to study the properties of carbon in niobium, and the results are compared and contrasted with the properties of the other interstitial impurities. The results indicate that C will likely form precipitates or atmospheres around defects rather than strongly bound complexes with other impurities. On the basis of the analysis of carbon and hydrogen near niobium lattice vacancies and small vacancy chains and clusters, the formation of extended carbon chains and hydrocarbons is not likely to occur. Association of carbon with hydrogen atoms can, however, occur through the strain fields created by interstitial binding of the impurity atoms. Calculated electronic densities of states indicate that interstitial C may have a similar effect as interstitial O on the superconducting transition temperature of Nb.

Ford, Denise C.↗
DOE OSTI2015

Optimization of a Non-arsenic Iron-based Superconductor for Wire Fabrication

Here we report on the optimization of synthesis of iron selenide-based superconducting powders and the fabrication of selenide-based wire. The powders were synthesized by an ammonothermal method, whereby Ba is intercalated between FeSe layers to produce Ba x (NH 3 ) y Fe 2 Se 2 , with tetragonal structure similar to AFe 2 X 2 (X: As, Se), '122', superconductors. The optimal T c (up to 38 K) and Meissner and shielding superconducting fractions are obtained from the shortest reaction time (t) of reactants in liquid ammonia (30 min). With the increase of t, a second crystalline 122 phase, with a smaller unit cell, emerges. A small amount of NH 3 is released from the structure above ~200 °C, which results in loss of superconductivity. However, in the confined space of niobium/Monel tubing, results indicate there is enough pressure for some of NH 3 to remain in the crystal lattice, and thermal annealing can be performed at temperatures of up to 780 °C, increasing wire density and yielded a reasonable T c ≈ 16 K. Here, we report of the first successful wire fabrication of non-arsenic high-T c iron-based superconductor. We find that although bulk materials are estimated to carry critical current densities >100 kA cm ₋2 (4 K, self-field), the current transport within wires need to be optimized (J c ~ 1 kA cm ₋2 ).

36 MATERIALS SCIENCE↗
DOE OSTI2023

The Role of SnO2 Processing on Ionic Distribution in Double-Cation-Double Halide Perovskites

Moving toward a future of efficient, accessible, and less carbon-reliant energy devices has been at the forefront of energy research innovations for the past 30 years. Metal-halide perovskite (MHP) thin films have gained significant attention due to their flexibility of device applications and tunable capabilities for improving power conversion efficiency. Serving as a gateway to optimize device performance, consideration must be given to chemical synthesis processing techniques. Therefore, how does common substrate processing techniques influence the behavior of MHP phenomena such as ion migration and strain? Here, we demonstrate how a hybrid approach of chemical bath deposition (CBD) and nanoparticle SnO2 substrate processing significantly improves the performance of (FAPbI3)0.97(MAPbBr3)0.03 by reducing micro-strain in the SnO2 lattice, allowing distribution of K+ from K-Cl treatment of substrates to passivate defects formed at the interface and produce higher current in light and dark environments. X-ray diffraction reveals differences in lattice strain behavior with respect to SnO2 substrate processing methods. Through use of conductive atomic force microscopy (c-AFM), conductivity is measured spatially with MHP morphology, showing higher generation of current in both light and dark conditions for films with hybrid processing. Additionally, time-of-flight secondary ionization mass spectrometry (ToF-SIMS) observed the distribution of K+ at the perovskite/SnO2 interface, indicating K+ passivation of defects to improve the power conversion efficiency (PCE) and device stability. We show how understanding the role of ion distribution at the SnO2 and perovskite interface can help reduce the creating of defects and promote a more efficient MHP device.

conductive atomic force microscopy↗
DOE OSTI2021

Davis Computational Spectroscopy Workflow—From Structure to Spectra

Here, we describe an automated workflow that connects a series of atomic simulation tools to investigate the relationship between atomic structure, lattice dynamics, materials properties, and inelastic neutron scattering (INS) spectra. Starting from the atomic simulation environment (ASE) as an interface, we demonstrate the use of a selection of calculators, including density functional theory (DFT) and density functional tight binding (DFTB), to optimize the structures and calculate interatomic force constants. We present the use of our workflow to compute the phonon frequencies and eigenvectors, which are required to accurately simulate the INS spectra in crystalline solids like diamond and graphite as well as molecular solids like rubrene. We have also implemented a machine-learning force field based on Chebyshev polynomials called the Chebyshev interaction model for efficient simulation (ChIMES) to improve the accuracy of the DFTB simulations. We then explore the transferability of our DFTB/ChIMES models by comparing simulations derived from different training sets. We show that DFTB/ChIMES demonstrates ~100× reduction in computational expense while retaining most of the accuracy of DFT as well as yielding high accuracy for different materials outside of our training sets. The DFTB/ChIMES method within the workflow expands the possibilities to use simulations to accurately predict materials properties of increasingly complex structures that would be unfeasible with ab initio methods.

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

Molecular beam epitaxy growth of high electron mobility InAs/AlSb deep quantum well structure

InAs/AlSb deep quantum well (QW) structures with high electron mobility were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. AlSb and Al{sub 0.75}Ga{sub 0.25}Sb buffer layers were grown to accommodate the lattice mismatch (7%) between the InAs/AlSb QW active region and GaAs substrate. Transmission electron microscopy shows abrupt interface and atomic force microscopy measurements display smooth surface morphology. Growth conditions of AlSb and Al{sub 0.75}Ga{sub 0.25}Sb buffer were optimized. Al{sub 0.75}Ga{sub 0.25}Sb is better than AlSb as a buffer layer as indicated. The sample with optimal Al{sub 0.75}Ga{sub 0.25}Sb buffer layer shows a smooth surface morphology with root-mean-square roughness of 6.67 A. The electron mobility has reached as high as 27 000 cm{sup 2}/Vs with a sheet density of 4.54 Multiplication-Sign 10{sup 11}/cm{sup 2} at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Buffer-layer-controlled nickeline vs zinc-blende/wurtzite-type MnTe growths on c -plane Al 2 O 3 substrates

In the recent past, MnTe has proven to be a crucial component of the intrinsic magnetic topological insulator (IMTI) family [MnTe] m [Bi2Te 3 ] n , which hosts a wide range of magneto-topological properties depending on the choice of m and n. However, bulk crystal growth allows only a few combinations of m and n for these IMTIs due to the strict limitations of the thermodynamic growth conditions. One way to overcome this challenge is to utilize atomic layer-by-layer molecular beam epitaxy (MBE) technique, which allows arbitrary sequences of [MnTe]m and [Bi 2 Te 3 ] n to be formed beyond the thermodynamic limit. For such MBE growth, finding optimal growth templates and conditions for the parent building block, MnTe, is a key requirement. Here, we report that two different hexagonal phases of MnTe - nickeline (NC) and zinc-blende/wurtzite (ZB-WZ) structures, with distinct in-plane lattice constants of 4.20 ± 0.04 Å and 4.39 ± 0.04 Å, respectively - can be selectively grown on c-plane Al 2 O 3 substrates using different buffer layers and growth temperatures. Moreover, we provide the first comparative studies of different MnTe phases using atomic-resolution scanning transmission electron microscopy and show that ZB and WZ-like stacking sequences can easily alternate between the two. Surprisingly, In 2 Se 3 buffer layer, despite its lattice constant (4.02 Å) being closer to that of the NC phase, fosters the ZB-WZ instead, whereas Bi 2 Te 3 , sharing the same lattice constant (4.39 Å) with the ZB-WZ phase, fosters the NC phase. Furthermore, these discoveries suggest that lattice matching is not always the most critical factor determining the preferred phase during epitaxial growth. Overall, this will deepen our understanding of epitaxial growth modes for chalcogenide materials and accelerate progress toward new IMTI phases as well as other magneto-topological applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Metamorphic Tunnel Junctions Grown Via MOCVD Designed for GaAs 0.75 P 0.25 /Si Tandem Solar Cells

A high-performance metamorphic Al 0.2 Ga 0.8 As 0.75 P 0.25 /GaAs 0.75 P 0.25 heterojunction tunnel junction structure was developed for application to monolithic epitaxial GaAs 0.75 P 0.25 /Si 1.7 eV/1.1 eV bandgap tandem solar cells produced via metal-organic chemical vapor deposition. Doping optimization focused on both bulk doping concentrations and mitigating the transient incorporation effects that otherwise causes non-abrupt doping profiles in the Te doped layer. Furthermore, these efforts resulted in a fully-relaxed metamorphic tunnel diode at the target lattice constant having a peak tunneling current density (JP) of 279.1 A·cm –2 and a zero-bias resistance area product (RA) of 3.0×10 –4 Ω·cm 2 . After post-growth annealing to emulate the thermal load of the GaAs 0.75 P 0.25 top cell growth and elimination of the performance enhancing carbon activation anneal, the device achieved JP = 13.1 A·cm –2 and RA = 1.5×10 –3 Ω·cm 2 . These worst-case values are very promising as they enable the operation of GaAs 0.75 P 0.25 /Si tandems under one-sun AM1.5G illumination with negligible series resistance and at AM1.5D concentrations up to 240 suns with only 0.1% absolute efficiency loss.

14 SOLAR ENERGY↗
DOE OSTI2022

LDRD 226360 Final Project Report: Simulated X-ray Diffraction and Machine Learning for Optimizing Dynamic Experiment Analysis

This report is the final documentation for the one-year LDRD project 226360: Simulated X-ray Diffraction and Machine Learning for Optimizing Dynamic Experiment Analysis. As Sandia has successfully developed in-house X-ray diffraction tools for study of atomic structure in experiments, it has become increasingly important to develop computational analysis methods to support these experiments. When dynamically compressed lattices and orientations are not known a priori, the identification requires a cumbersome and sometimes intractable search of possible final states. These final states can include phase transition, deformation and mixed/evolving states. Our work consists of three parts: (1) development of an XRD simulation tool and use of traditional data science methods to match XRD patterns to experiments; (2) development of ML-based models capable of decomposing and identifying the lattice and orientation components of multicomponent experimental diffraction patterns; and (3) conducting experiments which showcase these new analysis tools in the study of phase transition mechanisms. Our target material has been cadmium sulfide, which exhibits complex orientation-dependent phase transformation mechanisms. In our current one-year LDRD, we have begun the analysis of high-quality c-axis CdS diffraction data from DCS and Thor experiments, which had until recently eluded orientation identification.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Ensemble Quasi-Newton HMC

We present a modification of the Hybrid Monte Carlo algorithm for tackling the critical slowing down of generating Markov chains of lattice gauge configurations towards the continuum limit. We propose a new method to exchange information within an ensemble of Markov chains, and use it to construct an approximate inverse Hessian matrix of the action inspired from quasi-Newton algorithms for optimization. The kinetic term of the molecular dynamics evolution includes the approximate Hessian for long distance couplings among the momenta. We show the result of applying the new algorithm to the U(1) gauge theory in two dimensions, and discuss our future plans.

Jin, Xiao-Yong↗
NASA NTRS2018

Performance Enhancements for the Lattice-Boltzmann Solver in the LAVA Framework

Performance enhancements in NASA's recently developed Lattice Boltzmann solver within the Launch Ascent and Vehicle Aerodynamics (LAVA) framework are presented. Two key algorithmic developments are highlighted. A coarse-fine interface treatment that discretely conserves mass and momentum has been implemented and successfully verified and validated. Code optimizations targeting improved serial and parallel performance were presented. For a simple turbulent Taylor-Green Vortex problem, we were able to demonstrate a 2.3 times speedup over the baseline code for a single Skylake-SP node containing 40 physical cores, and a 2.14 times speedup for 64 nodes containing 2560 physical cores. In addition, we were able to show that the optimizations enabled us to scale the code almost perfectly to 20480 physical cores where, including ghost cells, the problem size was 10 billion cells.

Barad, Michael↗
DOE OSTI2017

Structural, Chemical, and Dynamical Frustration: Origins of Superionic Conductivity in closo -Borate Solid Electrolytes

Li 2 B 12 H 12 , Na 2 B 12 H 12 , and their closo-borate relatives exhibit unusually high ionic conductivity, making them attractive as a new class of candidate electrolytes in solid-state Li- and Na-ion batteries. However, further optimization of these materials requires a deeper understanding of the fundamental mechanisms underlying ultrafast ion conduction. To this end, we use ab initio molecular dynamics simulations and density-functional calculations to explore the motivations for cation diffusion. We find that superionic behavior in Li 2 B 12 H 12 and Na 2 B 12 H 12 results from a combination of key structural, chemical, and dynamical factors that introduce intrinsic frustration and disorder. A statistical metric is used to show that the structures exhibit a high density of accessible interstitial sites and site types, which correlates with the flatness of the energy landscape and the observed cation mobility. Furthermore, cations are found to dock to specific anion sites, leading to a competition between the geometric symmetry of the anion and the symmetry of the lattice itself, which can facilitate cation hopping. Finally, facile anion reorientations and other low-frequency thermal vibrations lead to fluctuations in the local potential that enhance cation mobility by creating a local driving force for hopping. In conclusion, we discuss the relevance of each factor for developing new ionic conductivity descriptors that can be used for discovery and optimization of closo-borate solid electrolytes, as well as superionic conductors more generally.

25 ENERGY STORAGE↗