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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 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Determination of electronic band structure by electron holography of etched-and-regrown interfaces in GaN p-i-n diodes

The electrostatic potential variation across etched-and-regrown GaN p-i-n diodes for power electronics has been studied using electron holography in a transmission electron microscope. The potential profiles have been correlated with the composition profiles of Mg, Si, and O obtained by secondary ion mass spectroscopy. Electronic charges obtained from the potential profiles correlate well with the presence of Si and O impurities at regrown interfaces. The overlap of Mg and Si when Mg doped GaN is grown directly over an etched undoped GaN surface results in the formation of a highly doped p-n junction. The introduction of a thin undoped layer over the etched GaN surface prevents the formation of such a junction as the regrowth interface is moved away from the Mg-doped GaN, and results in diodes with improved reverse leakage currents, close to the best values of continuously grown p-i-n diodes. Potential profiles of continuously grown (not etched) p-i-n diodes are compared to those of etched-and-regrown diodes.

Alugubelli, Shanthan R. (ORCID:000000020202727X)↗
DOE OSTI2020

Dalton Project: A Python platform for molecular- and electronic-structure simulations of complex systems

The Dalton Project provides a uniform platform access to the underlying full-fledged quantum chemistry codes Dalton and LSDalton as well as the PyFraME package for automatized fragmentation and parameterization of complex molecular environments. The platform is written in Python and defines a means for library communication and interaction. Intermediate data such as integrals are exposed to the platform and made accessible to the user in the form of NumPy arrays, and the resulting data are extracted, analyzed, and visualized. Complex computational protocols that may, for instance, arise due to a need for environment fragmentation and configuration-space sampling of biochemical systems are readily assisted by the platform. The platform is designed to host additional software libraries and will serve as a hub for future modular software development efforts in the distributed Dalton community.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Efficient electronic structure theory via hierarchical scale-adaptive coupled-cluster formalism: I. Theory and computational complexity analysis

In this work, a novel reduced-scaling, general-order coupled-cluster approach is formulated by exploiting hierarchical representations of many-body tensors, combined with the recently suggested formalism of scale-adaptive tensor algebra. Inspired by the hierarchical techniques from the renormalization group approach, H/H 2 -matrix algebra and fast multipole method, the computational scaling reduction in our formalism is achieved via coarsening of quantum many-body interactions at larger interaction scales, thus imposing a hierarchical structure on many-body tensors of coupled-cluster theory. In our approach, the interaction scale can be defined on any appropriate Euclidean domain (spatial domain, momentum-space domain, energy domain, etc.). Additionally, we show that the hierarchically resolved many-body tensors reduce the storage requirements to O(N), where N is the number of simulated quantum particles. Subsequently, we prove that any connected many-body diagram with arbitrary-order tensors, e.g., an arbitrary coupled-cluster diagram, can be evaluated in O(NlogN) floating-point operations. On top of that, we elaborate an additional approximation to further reduce the computational complexity of higher-order coupled-cluster equations, i.e., equations involving higher than double excitations, which otherwise would introduce a large prefactor into formal O(NlogN) scaling.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Training models using forces computed by stochastic electronic structure methods

Abstract Quantum Monte Carlo (QMC) can play a very important role in generating accurate data needed for constructing potential energy surfaces. We argue that QMC has advantages in terms of a smaller systematic bias and an ability to cover phase space more completely. The stochastic noise can ease the training of the machine learning model. We discuss how stochastic errors affect the generation of effective models by analyzing the errors within a linear least squares procedure, finding that there is an advantage to having many relatively imprecise data points for constructing models. We then analyze the effect of noise on a model of many-body silicon finding that noise in some situations improves the resulting model. We then study the effect of QMC noise on two machine learning models of dense hydrogen used in a recent study of its phase diagram. The noise enables us to estimate the errors in the model. We conclude with a discussion of future research problems.

Ceperley, David M. (ORCID:0000000150826271)↗
DOE OSTI2025

Electronic structure, and magnetic and superconducting pairing tendencies of the alternating single layer–bilayer stacking nickelate La 5 ⁢Ni 3 ⁢O 11 under pressure

Nickelates have continued to surprise since their unconventional superconductivity was discovered. Recently, the layered nickelate La 5 ⁢Ni 3 ⁢O 11 with hybrid single-layer and bilayer stacking showed superconductivity under high pressure. This compound combines features of single-layer La 2 ⁢NiO 4 and bilayer La 3 ⁢Ni 2 ⁢O 7 , but its pairing mechanism remains to be understood. Motivated by this finding, here we report a comprehensive theoretical study of this system. Our density functional theory calculations reveal that the undistorted P4/mmm phase without pressure is unstable due to three distortion modes. As pressure increases, these modes are suppressed, leading to the high-symmetry P4/mmm phase without NiO 6 octahedron tilting. Moreover, the “charge transfer” between the single-layer and bilayer sublattices was observed, leading to hole doping in the single-layer blocks. Our random-phase approximation calculations indicate a leading 𝑑 𝑥 2 −𝑦 2 -wave pairing state that arises from spin-fluctuation scattering between Fermi surface states mainly originating from the single-layer blocks and additional weaker contributions from the bilayer blocks. These spin fluctuations could be detected by inelastic neutron scattering as a strong peak at 𝐪 = (𝜋,𝜋). Our findings distinguish La 5 ⁢Ni 3 ⁢O 11 from other nickelate superconductors discovered so far and the high-𝑇 𝑐 cuprates. We also discuss both similarities and differences between La 5⁢ Ni 3 ⁢O 11 and other hybrid stacking nickelates.

Zhang, Yang [Univ. of Tennessee, Knoxville, TN (Un↗