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Results for “Greens function”

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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Sensitivity of the ITCZ Location to Ocean Forcing Via Q-Flux Green's Function Experiments

The sensitivity of the Intertropical Convergence Zone (ITCZ) position to forcing patterns is important for understanding changes in tropical rainfall. A set of Green’s function experiments reveals that this sensitivity is asymmetric depending on which hemisphere (northern or southern) the forcing is applied. Northern hemisphere forcings produce a much larger response than southern hemisphere forcings of similar magnitude. The response of the ITCZ position to forcing can be broken into linear and nonlinear components, and it is shown that the asymmetry arises from the nonlinear component. The linear and nonlinear response components have similar magnitudes, but the nonlinear component is insensitive to the location of the forcing, such that it amplifies the response to northern hemisphere forcings and dampens the response to southern hemisphere forcings. Furthermore, this asymmetry hints at an intrinsic mode of the climate system such as the ITCZ response to forcing depends on the current climate state.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2026

Global anomalies of Green's function zeros.

We study global anomalies of nonlocal effective theories proposed to describe symmetry-preserving Luttinger surfaces, i.e., the momentum-space manifolds of Green’s function zeros (GFZs) at zero energy, in strongly interacting fermionic systems. In particular, we focus on the simplest possible cases associated with a gapless Dirac zero, which is the counterpart of the gapless Dirac quasiparticle in weakly interacting systems. These theories may be derived by integrating out low-energy degrees of freedom that do not couple to the relevant gauge field. We discuss the global anomaly, the bulk-boundary correspondence, and the constraint on phases consistent with the anomaly, such as non-Fermi liquids and emergent gapless quasiparticles on Luttinger surfaces. Failing to avoid spontaneous symmetry breaking in the thermodynamical limit inevitably leads to unstable GFZs. We also provide some perspective on why the related nonlocal fermionic effective theory studied recently is not a suitable starting point for a symmetrically gapped phase

Su, Lei↗
DOE OSTI2019

Photon Radiation Scatter from Heterogeneous Shields using Green's Functions

The effect of shielding on ionizing photon radiation can be estimated using radiation transport simulations. This report covers the methodology and implementation of using Green's Functions to pre-compute this effect, which allows the radiation field exiting a variety of shielding configurations to be quickly computed. It also covers a weighting function that makes a relatively small pre-computed library applicable to a large variety of heterogeneous shields. The method enables rapid computation of the intensity versus energy for scattered radiation exiting a variety of shield materials and thicknesses without running a full transport simulation.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2017

Currents and Green's functions of impurities out of equilibrium: Results from inchworm quantum Monte Carlo

Here, we generalize the recently developed inchworm quantum Monte Carlo method to the full Keldysh contour with forward, backward, and equilibrium branches to describe the dynamics of strongly correlated impurity problems with time-dependent parameters. We introduce a method to compute Green's functions, spectral functions, and currents for inchworm Monte Carlo and show how systematic error assessments in real time can be obtained. We then illustrate the capabilities of the algorithm with a study of the behavior of quantum impurities after an instantaneous voltage quench from a thermal equilibrium state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Metallic behavior and negative differential resistance properties of (InAs){sub n} (n = 2 − 4) molecule cluster junctions via a combined non–equilibrium Green's function and density functional theory study

In this present work, the geometric structures and electronic transport properties of (InAs){sub n} (n = 2, 3, 4) molecule cluster junctions are comparatively investigated using NEGF combined with DFT. Results indicate that all (InAs){sub n} molecule cluster junctions present metallic behavior at the low applied biases ([−2V, 2V]), while NDR appears at a certain high bias range. Our calculation shows that the current of (InAs){sub 4} molecule cluster–based junction is almost the largest at any bias. The mechanisms of the current–voltage characteristics of all the three molecule cluster junctions are proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Green’s Function Coupled-Cluster Approach: Simulating Photoelectron Spectra for Realistic Molecular Systems

In this paper we present an efficient implementation for the analytical energy- dependent coupled cluster Green’s function with singles and doubles (GFCCSD) approach with our first practice being computing spectral functions of real molecular systems. Due to its algebraic structure, the presented method is highly scalable, and is capable of computing spectral function for a given molecular system in any energy region. Several typical examples have been given to demonstrate its capability of computing spectral functions not only in the valence band, but also in the core-level energy region. Satellite peaks have been observed in the inner valence band and core-level energy region where many-body effect becomes significant and single particle picture of ionization often breaks down. The accuracy test has been carried out by extensively comparing our GFCCSD results with experimental results as well as the theoretical results from other methods. It turns out GFCCSD method is able to provide qualitative or semi-quantitative level of description of ionization processes in both the core and valence regimes. To significantly improve the GFCCSD results for the main states, larger basis set can usually be employed, whereas the improvement of the GFCCSD results for the satellite states needs higher order many-body terms to be included in the GFCC implementation.

electronic structure theory↗
DOE OSTI2018

On the zero-crossing of the three-gluon Green's function from lattice simulations

We report on some efforts recently made in order to gain a better understanding of some IR properties of the 3-point gluon Green’s function by exploiting results from large-volume quenched lattice simulations. These lattice results have been obtained by using both tree-level Symanzik and the standard Wilson action, in the aim of assessing the possible impact of effects presumably resulting from a particular choice for the discretization of the action. The main resulting feature is the existence of a negative log-aritmic divergence at zero-momentum, which pulls the 3-gluon form factors down at low momenta and, consequently, yields a zero-crossing at a given deep IR momentum. The results can be correctly explained by analyzing the relevant Dyson-Schwinger equations and appropriate truncation schemes.

Athenodorou, Andreas↗
DOE OSTI2019

Hubbard Nonequilibrium Green’s Function Analysis of Photocurrent in Nitroazobenzene Molecular Junction

We present a combined experimental and theoretical study of photoinduced current in molecular junctions consisting of monolayers of nitroazobenzene oligomers chemisorbed on carbon surfaces and illuminated by ultraviolet-visible light through a transparent electrode. Experimentally observed dependence of the photocurrent on light frequency, temperature, and monolayer thickness is analyzed within first-principles simulations employing the Hubbard nonequilibrium Green’s function diagrammatic technique. We reproduce qualitatively correct behavior and discuss mechanisms leading to the characteristic behavior of dark and photoinduced currents in response to changes in bias, frequency of radiation, temperature, and thickness of molecular layer.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

A full-potential approach to the relativistic single-site Green's function

One major purpose of studying the single-site scattering problem is to obtain the scattering matrices and differential equation solutions indispensable to multiple scattering theory (MST) calculations. On the other hand, the single-site scattering itself is also appealing because it reveals the physical environment experienced by electrons around the scattering center. In this study, we demonstrate a new formalism to calculate the relativistic full-potential single-site Green's function. We implement this method to calculate the single-site density of states and electron charge densities. Lastly, the code is rigorously tested and with the help of Krein's theorem, the relativistic effects and full potential effects in group V elements and noble metals are thoroughly investigated.

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

Real-space Green's function approach for x-ray spectra at high temperature

There has been considerable interest in properties of condensed matter at high temperature, including nonequilibrium behavior and extreme conditions up to the warm dense matter regime. Such behavior is encountered, e.g., in experimental time-resolved x-ray absorption spectroscopy in the presence of intense laser fields. In an effort to simulate such behavior, we present an approach for calculations of high-temperature x-ray absorption spectra in arbitrary materials, using a generalization of the real-space Green's function formalism. Here, the method is incorporated as an option in the core-level x-ray spectroscopy code FEFF10. To illustrate the approach, we present calculations for several materials together with comparisons to experiment and with other methods.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Augmenting machine learning of Grad–Shafranov equilibrium reconstruction with Green's functions

This work presents a method for predicting plasma equilibria in tokamak fusion experiments and reactors. The approach involves representing the plasma current as a linear combination of basis functions using principal component analysis of plasma toroidal current densities (J t ) from the EFIT-AI equilibrium database. Then utilizing EFIT's Green's function tables, basis functions are created for the poloidal flux (ψ) and diagnostics generated from the toroidal current (J t ). Similar to the idea of a physics-informed neural network (NN), this physically enforces consistency between ψ, J t , and the synthetic diagnostics. First, the predictive capability of a least squares technique to minimize the error on the synthetic diagnostics is employed. The results show that the method achieves high accuracy in predicting ψ and moderate accuracy in predicting J t with median R 2 = 0.9993 and R 2 = 0.978, respectively. A comprehensive NN using a network architecture search is also employed to predict the coefficients of the basis functions. The NN demonstrates significantly better performance compared to the least squares method with median R 2 = 0.9997 and 0.9916 for J t and ψ, respectively. The robustness of the method is evaluated by handling missing or incorrect data through the least squares filling of missing data, which shows that the NN prediction remains strong even with a reduced number of diagnostics. Additionally, the method is tested on plasmas outside of the training range showing reasonable results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Discrete Green’s functions and spectral graph theory for computationally efficient thermal modeling

Here, this work concerns solutions of the heat equation with the spectral graph method, for which the temperature is defined at discrete points in the domain and the spatial relationship among the points is described by a graph. The heat equation on the graph is solved using matrix techniques involving the eigenvectors and eigenvalues of the Laplacian matrix. The spectral graph approach precludes the computationally intensive meshing and numerous time-integration steps of the finite element method. In the present work, the spectral graph method is extended to include heat loss at the boundaries with a generalized boundary condition, and physics-based edge weights are introduced which simplify the calibration process. From this approach a discrete Green’s function is defined which allows for solutions under a variety of heating conditions including: space-varying initial conditions; time-and-space varying internal heating; and, time-and-space-varying heating at boundaries of type 1 (Dirichlet), type 2 (Neumann) and type 3 (Robin). Results are provided for benchmark heat transfer problems in one spatial dimension and in three spatial dimensions, and verification is provided by comparison with exact analytical solutions and finite difference solutions. The spectral graph method converges within 0.4% error of the analytical solution. The practical utility of the approach is demonstrated by thermal simulation of a multilayer additive manufacturing process. The spectral graph results are compared to experimentally-obtained temperature data for two metal parts, with error less than 5% of the experimental measurements, with computation time less than one minute on a desktop computer.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characterization of the dynamic densification behavior of cerium dioxide powders as a function of green density

The uniaxial strain dynamic densification behavior of cerium dioxide powders as a function of powder compact green density is investigated in this work. Cerium dioxide powders of two particle morphologies (rod-like and equiaxed) and two green densities (55% and 62.5% TMD) are shock compressed using the gas gun with specifically designed fixtures and their particle and shock wave velocities are measured using VISAR and multiple PDV probes. The obtained data describe the Hugoniot of the shocked cerium dioxide powders. Finally, the method used to produce this high fidelity Hugoniot data for cerium dioxide powders is discussed and preliminary data are shown.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Evaluating the exfoliation of two-dimensional materials with a Green's function surface model

Previous methods for the evaluation of the exfoliation of two-dimensional (2D) layered materials have drawbacks in computational efficiency and are unable to describe cases with semi-infinite substrates. Based on a Green's function surface (GFS) model, here we develop an approach to efficiently determine the tendency of exfoliation of 2D materials from their bulk crystals or semi-infinite substrates. By constructing appropriate surface configurations, we may calculate the exfoliation energy more precisely and quickly than the traditional way with the slab model. Furthermore, the GFS approach can provide angle-resolved photoemission spectroscopy of surface systems for direct comparison with experimental data. In this work, our findings indicate that the GFS approach is powerful for studies of 2D materials and various surface problems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Effect of Time Window and Spectral Measurement Options on Empirical Green’s Function Analysis Using DAS Array and Seismic Stations

The recorded seismic waveform is a convolution of event source term, path term, and station term. Removing high-frequency attenuation due to path effect is a challenging problem. Empirical Green’s function (EGF) method uses nearly collocated small earthquakes to correct the path and station terms for larger events recorded at the same station. However, this method is subject to variability due to many factors. Here, we focus on three events that were well recorded by the seismic network and a rapid response distributed acoustic sensing (DAS) array. Using a suite of high-quality EGF events, we assess the influence of time window, spectral measurement options, and types of data on the spectral ratio and relative source time function (RSTF) results. Increased number of tapers (from 2 to 16) tends to increase the measured corner frequency and reduce the source complexity. Extended long time window (e.g., 30 s) tends to produce larger variability of corner frequency. The multitaper algorithm that simultaneously optimizes both target and EGF spectra produces the most stable corner-frequency measurements. The stacked spectral ratio and RSTF from the DAS array are more stable than two nearby seismic stations, and are comparable to stacked results from the seismic network, suggesting that DAS array has strong potential in source characterization.

58 GEOSCIENCES↗
DOE OSTI2019

Fermi surface volume of interacting systems

Three Fermion sumrules for interacting systems are derived at T=0, involving the number expectation N-bar (μ), canonical chemical potentials μ(m), a logarithmic time derivative of the Greens function γ{sub k-vector σ} and the static Greens function. In essence we establish at zero temperature the sumrules linking: N-bar (μ)↔∑{sub m}Θ(μ−μ(m))↔∑{sub k-vector ,σ}Θ(γ{sub k-vector σ})↔∑{sub k-vector ,σ}Θ(G{sub σ}( k-vector ,0)). Connecting them across leads to the Luttinger and Ward sumrule, originally proved perturbatively for Fermi liquids. Our sumrules are nonperturbative in character and valid in a considerably broader setting that additionally includes non-canonical Fermions and Tomonaga–Luttinger models. Generalizations are given for singlet-paired superconductors, where one of the sumrules requires a testable assumption of particle–hole symmetry at all couplings. The sumrules are found by requiring a continuous evolution from the Fermi gas, and by assuming a monotonic increase of μ(m) with particle number m. At finite T a pseudo-Fermi surface, accessible to angle resolved photoemission, is defined using the zero crossings of the first frequency moment of a weighted spectral function.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Theoretical description of spin-selective reactions of radical pairs diffusing in spherical 2D and 3D microreactors

In this work, we treat spin-selective recombination of a geminate radical pair (RP) in a spherical “microreactor,” i.e., of a RP confined in a micelle, vesicle, or liposome. We consider the microreactor model proposed earlier, in which one of the radicals is located at the center of the micelle and the other one undergoes three-dimensional diffusion inside the micelle. In addition, we suggest a two-dimensional model, in which one of the radicals is located at the “pole” of the sphere, while the other one diffuses on the spherical surface. For this model, we have obtained a general analytical expression for the RP recombination yield in terms of the free Green function of two-dimensional diffusion motion. In turn, this Green function is expressed via the Legendre functions and thus takes account of diffusion over a restricted spherical surface and its curvature. The obtained expression allows one to calculate the RP recombination efficiency at an arbitrary magnetic field strength. We performed a comparison of the two models taking the same geometric parameters (i.e., the microreactor radius and the closest approach distance of the radicals), chemical reactivity, magnetic interactions in the RP and diffusion coefficient. Significant difference between the predictions of the two models is found, which is thus originating solely from the dimensionality effect: for different dimensionality of space, the statistics of diffusional contacts of radicals becomes different altering the reaction yield. We have calculated the magnetic field dependence of the RP reaction yield and chemically induced dynamic nuclear polarization of the reaction products at different sizes of the microreactor, exchange interaction, and spin relaxation rates. Interestingly, due to the intricate interplay of diffusional contacts of reactants and spin dynamics, the dependence of the reaction yield on the microreactor radius is non-monotonous. Our results are of importance for (i) interpreting experimental data for magnetic field effects on RP recombination in confined space and (ii) for describing kinetics of chemical reactions, which occur predominantly on the surfaces of biomembranes, i.e., lipid peroxidation reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

A Test Problem for Codes Solving the Discretized Diffusion Equation in Cartesian Geometry Derived Via Discrete Green’s Functions

We obtain here a solution to a zone-centered discretization of the one dimensional time-dependent diffusion equation with arbitrary initial conditions and source, constant absorption and scattering opacity, and constant zone size and time step. The solution is obtained using the discrete Green’s functions of the discretized equation. The solution of the discretized equation is useful in the testing of computer codes, because the code can be expected to agree with the solution to the discrete equation, to within small errors caused by roundoff. This is in contrast to solutions of the differential equation, with which the code results only approximately agree. Finally, the usefulness of the solution for tests of an inertial confinement fusion code is demonstrated.

97 MATHEMATICS AND COMPUTING↗