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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Time-dependent integral transport in one-dimensional infinite media using dimensionless variables and the reduced collision formulation

A new method for calculating time-dependent integral neutron transport single collision fluxes has been developed for one-dimensional infinite media problems using dimensionless variables and the reduced collision formulation. By transforming the integral scalar flux equation into dimensionless variables, one may immediately integrate over the time domain and reduce the problem complexity to integrations strictly over the spatial domain, significantly reducing the computational work required to obtain a solution. The results have been compared to published benchmark solutions for validation. Further, reformulating the scalar flux as a Green’s function has allowed for the determination of an analytical expression for the relative error of any collision level scalar flux to the summed scalar flux. This provides the absolute number of collisions required for convergence at any given mean free time, using previous work in one-dimensional Cartesian transport and derived work herein for the one-dimensional spherical transport.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Triple Excitations in Green’s Function Coupled Cluster Solver for Studies of Strongly Correlated Systems in the Framework of Self-Energy Embedding Theory

Embedding theories became important approaches used for accurate calculations of both molecules and solids. In these theories, a small chosen subset of orbitals is treated with an accurate method, called an impurity solver, capable of describing higher correlation effects. Ideally, such a chosen fragment should contain multiple orbitals responsible for the chemical and physical behavior of the compound. Handling a large number of chosen orbitals presents a very significant challenge for the current generation of solvers used in the physics and chemistry community. Here, we develop a Green’s function coupled cluster singles doubles and triples (GFCCSDT) solver that can be used for a quantitative description in both molecules and solids. This solver allows us to treat orbital spaces that are inaccessible to other accurate solvers. At the same time, GFCCSDT maintains high accuracy of the resulting self-energy. Moreover, in conjunction with the GFCCSD solver, it allows us to test the systematic convergence of computational studies. Developing the CC family of solvers paves the road to fully systematic Green’s function embedding calculations in solids. In this paper, we focus on the investigation of GFCCSDT self-energies for a strongly correlated problem of SrMnO 3 solid. Subsequently, we apply this solver to solid MnO showing that an approximate variant of GFCCSDT is capable of yielding a high accuracy orbital resolved spectral function.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Tracking vegetation phenology across diverse biomes using Version 2.0 of the PhenoCam Dataset

Monitoring vegetation phenology is critical for quantifying climate change impacts on ecosystems. We present an extensive dataset of 1783 site-years of phenological data derived from PhenoCam network imagery from 393 digital cameras, situated from tropics to tundra across a wide range of plant functional types, biomes, and climates. Most cameras are located in North America. Every half hour, cameras upload images to the PhenoCam server. Images are displayed in near-real time and provisional data products, including timeseries of the Green Chromatic Coordinate (Gcc), are made publicly available through the project web page (https://phenocam.sr.unh.edu/webcam/gallery/). Processing is conducted separately for each plant functional type in the camera field of view. The PhenoCam Dataset v2.0, described here, has been fully processed and curated, including outlier detection and expert inspection, to ensure high quality data. This dataset can be used to validate satellite data products, to evaluate predictions of land surface models, to interpret the seasonality of ecosystem-scale CO 2 and H 2 O flux data, and to study climate change impacts on the terrestrial biosphere.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2014

Comparative analysis of quantum cascade laser modeling based on density matrices and non-equilibrium Green's functions

We study the operation of an 8.5 μm quantum cascade laser based on GaInAs/AlInAs lattice matched to InP using three different simulation models based on density matrix (DM) and non-equilibrium Green's function (NEGF) formulations. The latter advanced scheme serves as a validation for the simpler DM schemes and, at the same time, provides additional insight, such as the temperatures of the sub-band carrier distributions. We find that for the particular quantum cascade laser studied here, the behavior is well described by simple quantum mechanical estimates based on Fermi's golden rule. As a consequence, the DM model, which includes second order currents, agrees well with the NEGF results. Both these simulations are in accordance with previously reported data and a second regrown device.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Quantum optimal control theory in the linear response formalism

Quantum optimal control theory (QOCT) aims at finding an external field that drives a quantum system in such a way that optimally achieves some predefined target. In practice, this normally means optimizing the value of some observable, a so-called merit function. In consequence, a key part of the theory is a set of equations, which provides the gradient of the merit function with respect to parameters that control the shape of the driving field. We show that these equations can be straightforwardly derived using the standard linear response theory, only requiring a minor generalization: the unperturbed Hamiltonian is allowed to be time dependent. As a result, the aforementioned gradients are identified with certain response functions. This identification leads to a natural reformulation of QOCT in terms of the Keldysh contour formalism of the quantum many-body theory. In particular, the gradients of the merit function can be calculated using the diagrammatic technique for nonequilibrium Green's functions, which should be helpful in the application of QOCT to computationally difficult many-electron problems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Analytical solution for partial heating on the exterior of the pipe with application to measuring fluid flow rate

An analytical model using Green’s functions for partial external heating of a pipe is developed, which results in an exact mathematical solution for the radial and axial temperature distribution in the pipe wall. Partial heating consists of a constant heat flux function imposed over a small section of the exterior of a pipe and for a limited time duration. The solution comprises steady-state and transient parts, and an algebraic identity is used to decrease the number of summation terms in the slowly-converging steady- state part. Intrinsic verification principles are used to verify the solution. As an example application, this transient solution is applied toward the development of a simple, noninvasive method for in-field measurement of the flow rate in pipes. Furthermore, to simulate this application, a pulse of energy is imposed to the wall of the pipe, and the developed mathematical solution is used to find the flow rate of the fluid inside the pipe. An optimal experiment is designed to find the best measurement location and time.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2018

The Relative Importance of Assumed Infrasound Mechanisms on the Linear Inversion of Infrasound Time Series at the Source Physics Experiment

We invert far field infrasound data for the equivalent seismo-acoustic time domain moment tensor to assess the relative importance of two assumed seismoacoustic source mechanisms. The infrasound data were produced by a four of the underground chemical explosions that were conducted during the Source Physics Experiment (SPE). For each SPE event that we invert, we produce three set of atmospheric Green's functions: an average model based on ten years of atmospheric data, as well as two extrema models designed to maximize the variability of atmospheric conditions for the given time-of-day and day-of-year for each SPE event. To parameterize the inversion, we assume that the source of infrasonic energy results from the linear combination of explosion-induced surface spall and linear seismic-to-elastic mode conversion at the Earth's free surface. We find that the inversion yields relatively repeatable results for the estimated spall source whereas the estimated isotropic explosion source is highly variable. This suggests that the majority of the observed acoustic energy is produced by the spall source and/or our modeling of the elastic energy propagation, and data are subsequent conversion to acoustic energy via linear elastic-to-acoustic coupling at the surface, is too simplistic.

58 GEOSCIENCES↗
DOE OSTI2014

A generalized Debye source approach to electromagnetic scattering in layered media

The standard solution to time-harmonic electromagnetic scattering problems in homogeneous layered media relies on the use of the electric field dyadic Green's function. However, for small values of the governing angular frequency ω, evaluation of the electric field using this Green's function exhibits numerical instability. In this short note, we provide an alternative approach which is immune from this low-frequency breakdown as ω → 0. Our approach is based on the generalized Debye source representation of Maxwell fields. Using this formulation, the electric and magnetic fields gracefully decouple in the static limit, a behavior similar to that of the classical Lorenz-Debye-Mie representation of Maxwell fields in spherical geometries. We derive extensions of both the generalized Deybe source and Lorenz-Debye-Mie representations to planar geometries, as well as provide equations for the solution of scattering from a perfectly conducting half-space and in layered media using a Sommerfeld-like approach. These formulas are stable as ω tends to zero, and offer alternatives to the electric field dyadic Green's function.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Space-time in the SYK model

We consider the question of identifying the bulk space-time of the SYK model. Focusing on the signature of emergent space-time of the (Euclidean) model, we explain the need for non-local (Radon-type) transformations on external legs of n-point Green’s functions. This results in a dual theory with Euclidean AdS signature with additional legfactors. As a result, we speculate that these factors incorporate the coupling of additional bulk states similar to the discrete states of 2d string theory.

2D Gravity↗
DOE OSTI2023

Gradient synthesis of carbon quantum dots and activated carbon from pulp black liquor for photocatalytic hydrogen evolution and supercapacitor

Black liquor (BL) is a by-product of the chemical pulping industry and is mainly used as a low-value fuel; however, its potential to produce high-value products has not been fully exploited. In this study, a green and simple strategy is reported for the gradient production of Na + -functionalized carbon quantum dots (Na + -CQDs) for the first time, N and S co-doped CQDs (N/S-CQDs), and N and S co-doped KOH-activated carbon (N/S-KAC) from BL by dialysis, hydrothermal carbonization and activation-carbonization, respectively. Due to the good electron trapping ability, photoluminescence and promising up-conversion luminescence of CQDs, the hydrogen evolution efficiency of Na + -CQDs/TiO 2 and N/S-CQDs/TiO 2 photocatalysts was improved by 2.45 and 1.46 times, respectively, compared with pure TiO 2 . N/S-KAC with a high specific surface area of 2294 m 2 g -1 provides an excellent specific capacitance of 253 F g -1 at 0.5 A g -1 and a promising energy density of 26.92 Wh kg -1 under a power density of 566 W kg -1 for the fabricated symmetrical supercapacitor. Moreover, the electrode material has good cycling stability with a capacitance retention of ~ 93.91% after 5000 cycles. In conclusion, this pathway provides a versatile and scalable approach for the construction and co-production of nanostructured materials, photocatalysts and energy storage devices.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Improving Spectral Resolution from Real-time Evolution for Correlated Systems

Abstract The quality of numerically simulated spectra using real-time evolution methods for strongly correlated systems is affected by both the length of simulation time and the system size, limiting resolution in both frequency and momentum. In this work, we propose a computationally cheap, linear autoregressive machine learning-based framework to extend short-time and short-distance results over a wider range. We use the proposed method to extend the lesser Green’s function for both the Hubbard model and the much more computationally challenging Hubbard-extended Holstein model. This technique significantly improves both the frequency and momentum resolution of the single-particle removal spectrum $${\mathcal{A}}(k,\omega )$$ A ( k , ω ) , allowing the observation of otherwise obscured spectral features due to electron-phonon coupling.

Tang, Ta↗
DOE OSTI2015

Electron- and photon-impact ionization of furfural

The He(I) photoelectron spectrum of furfural has been investigated, with its vibrational structure assigned for the first time. The ground and excited ionized states are assigned through ab initio calculations performed at the outer-valence Green’s function level. Triple differential cross sections (TDCSs) for electron-impact ionization of the unresolved combination of the 4a″ + 21a′ highest and next-highest occupied molecular orbitals have also been obtained. Experimental TDCSs are recorded in a combination of asymmetric coplanar and doubly symmetric coplanar kinematics. The experimental TDCSs are compared to theoretical calculations, obtained within a molecular 3-body distorted wave framework that employed either an orientation average or proper TDCS average. The proper average calculations suggest that they may resolve some of the discrepancies regarding the angular distributions of the TDCS, when compared to calculations employing the orbital average.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Excitation spectra of quantum matter without quasiparticles. II. Random t - J models

Here, we present numerical solutions of the spectral functions of large dimension t -J models with random nearest neighbor exchange and global SU(M) spin rotation symmetry. The solutions are obtained from the saddle-point equations of the large dimension limit, followed by the large M limit. The same saddle point equations also apply to the model with all-to-all and random hopping and exchange. Such a t -J model realizes a deconfined critical state proposed as a model for the optimally doped cuprates. The large M theory involves Green’s functions for fractionalized spinons and holons carrying emergent U(1) gauge charges, obeying relations similar to those of the Sachdev-Ye-Kitaev (SYK) models. The low frequency spectral functions are compared with an analytic analysis of the operator scaling dimensions with good agreement. We also compute the low frequency and temperature behavior of experimentally observable gauge-invariant observables: the electron Green’s function, the local spin susceptibility and the optical conductivity, along with the temperature dependence of the d.c. resistivity. The time reparameterization soft mode (equivalent to the boundary graviton in holographically dual models of two-dimensional quantum gravity) makes important contributions to all observables and provides a linear-in-temperature contribution to the d.c. resistivity.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Vacuum polarization in the field of a multidimensional global monopole

An approximate expression for the Euclidean Green function of a massless scalar field in the spacetime of a multidimensional global monopole has been derived. Expressions for the vacuum expectation values 〈ϕ{sup 2}〉{sub ren} and 〈T{sub 00}〉{sub ren} have been derived by the dimensional regularization method. Comparison with the results obtained by alternative regularization methods is made.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Tensor train continuous time solver for quantum impurity models

The simulation of strongly correlated quantum impurity models is a significant challenge in modern condensed matter physics that has multiple important applications. Thus far, the most successful methods for approaching this challenge involve Monte Carlo techniques that accurately and reliably sample perturbative expansions to any order. However, the cost of obtaining high precision through these methods is high. Recently, tensor train decomposition techniques have been developed as an alternative to Monte Carlo integration. In this study, we apply these techniques to the single-impurity Anderson model at equilibrium by calculating the systematic expansion in power of the hybridization of the impurity with the bath. Furthermore, we demonstrate the performance of the method in a paradigmatic application, examining the first-order phase transition on the infinite-dimensional Bethe lattice, which can be mapped to an impurity model through dynamical mean field theory. Our results indicate that using tensor train decomposition schemes allows the calculation of finite-temperature Green's functions and thermodynamic observables with unprecedented accuracy. The methodology holds promise for future applications to frustrated multiorbital systems, using a combination of partially summed series with other techniques pioneered in diagrammatic and continuous time quantum Monte Carlo.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Distributed non-equilibrium Green’s function algorithms for the simulation of nanoelectronic devices with scattering

Through the non-equilibrium Green’s function (NEGF) formalism, quantum-scale device simulation can be performed with the inclusion of electron-phonon scattering. Yet, the simulation of realistically sized devices under the NEGF formalism typically requires prohibitive amounts of memory and computation time. Two of the most demanding computational problems for NEGF simulation involve mathematical operations with structured matrices called semiseparable matrices. In this work, we present parallel approaches for these computational problems which allow for efficient distribution of both memory and computation based upon the underlying device structure. This is vital when simulating realistically sized devices due to the aforementioned computational burdens. First, we consider determining a distributed compact representation for the retarded Green’s function matrix G R . This compact representation is exact and allows for any entry in the matrix to be generated through the inherent semiseparable structure. The second parallel operation allows for the computation of electron density and current characteristics for the device. Specifically, matrix products between the distributed representation for the semiseparable matrix G R and the self-energy scattering terms in Σ < produce the less-than Green’s function G < . As an illustration of the computational efficiency of our approach, we stably generate the mobility for nanowires with cross-sectional sizes as large as 4.5 nm, assuming an atomistic model with scattering.

47 OTHER INSTRUMENTATION↗
DOE OSTI2014

B-spline algebraic diagrammatic construction: Application to photoionization cross-sections and high-order harmonic generation

We present the first implementation of the ab initio many-body Green's function method, algebraic diagrammatic construction (ADC), in the B-spline single-electron basis. B-spline versions of the first order [ADC(1)] and second order [ADC(2)] schemes for the polarization propagator are developed and applied to the ab initio calculation of static (photoionization cross-sections) and dynamic (high-order harmonic generation spectra) quantities. We show that the cross-section features that pose a challenge for the Gaussian basis calculations, such as Cooper minima and high-energy tails, are found to be reproduced by the B-spline ADC in a very good agreement with the experiment. We also present the first dynamic B-spline ADC results, showing that the effect of the Cooper minimum on the high-order harmonic generation spectrum of Ar is correctly predicted by the time-dependent ADC calculation in the B-spline basis. The present development paves the way for the application of the B-spline ADC to both energy- and time-resolved theoretical studies of many-electron phenomena in atoms, molecules, and clusters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗