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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Global centroid moment tensor solutions in a heterogeneous earth: the CMT3D catalogue

SUMMARY For over 40 yr, the global centroid-moment tensor (GCMT) project has determined location and source parameters for globally recorded earthquakes larger than magnitude 5.0. The GCMT database remains a trusted staple for the geophysical community. Its point-source moment-tensor solutions are the result of inversions that model long-period observed seismic waveforms via normal-mode summation for a 1-D reference earth model, augmented by path corrections to capture 3-D variations in surface wave phase speeds, and to account for crustal structure. While this methodology remains essentially unchanged for the ongoing GCMT catalogue, source inversions based on waveform modelling in low-resolution 3-D earth models have revealed small but persistent biases in the standard modelling approach. Keeping pace with the increased capacity and demands of global tomography requires a revised catalogue of centroid-moment tensors (CMT), automatically and reproducibly computed using Green's functions from a state-of-the-art 3-D earth model. In this paper, we modify the current procedure for the full-waveform inversion of seismic traces for the six moment-tensor parameters, centroid latitude, longitude, depth and centroid time of global earthquakes. We take the GCMT solutions as a point of departure but update them to account for the effects of a heterogeneous earth, using the global 3-D wave speed model GLAD-M25. We generate synthetic seismograms from Green's functions computed by the spectral-element method in the 3-D model, select observed seismic data and remove their instrument response, process synthetic and observed data, select segments of observed and synthetic data based on similarity, and invert for new model parameters of the earthquake’s centroid location, time and moment tensor. The events in our new, preliminary database containing 9382 global event solutions, called CMT3D for ‘3-D centroid-moment tensors’, are on average 4 km shallower, about 1 s earlier, about 5 per cent larger in scalar moment, and more double-couple in nature than in the GCMT catalogue. We discuss in detail the geographical and statistical distributions of the updated solutions, and place them in the context of earlier work. We plan to disseminate our CMT3D solutions via the online ShakeMovie platform.

58 GEOSCIENCES↗
DOE OSTI2015

Methods for calculating X-ray diffuse scattering from a crystalline medium with spheroidal quantum dots

Two independent approaches to calculate the angular distribution of X-ray diffusion scattering from a crystalline medium with spheroidal quantum dots (QDs) have been proposed. The first method is based on the analytical solution involving the multipole expansion of elastic strain fields beyond QDs. The second approach is based on calculations of atomic displacements near QDs by the Green’s function method. An analysis of the diffuse scattering intensity distribution in the reciprocal space within these two approaches shows that both methods yield similar results for the chosen models of QD spatial distribution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Simple Models of Lateral Heterostructures

General analytical expressions for densities of states of a lateral heterostructure, formed by the contact of two square semi-infinite lattices with single-band and two-band spectra, were obtained in the tight-binding approximation by the Green’s function method. The semi-elliptical density of states was used for numerical estimates, and the model of two interacting dimers was proposed to estimate the charge transfer. Application of this approach to description of lateral epitaxial and graphene-like heterostructures is discussed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS2015

The Prediction of Scattered Broadband Shock-Associated Noise

A mathematical model is developed for the prediction of scattered broadband shock-associated noise. Model arguments are dependent on the vector Green's function of the linearized Euler equations, steady Reynolds-averaged Navier-Stokes solutions, and the two-point cross-correlation of the equivalent source. The equivalent source is dependent on steady Reynolds-averaged Navier-Stokes solutions of the jet flow, that capture the nozzle geometry and airframe surface. Contours of the time-averaged streamwise velocity component and turbulent kinetic energy are examined with varying airframe position relative to the nozzle exit. Propagation effects are incorporated by approximating the vector Green's function of the linearized Euler equations. This approximation involves the use of ray theory and an assumption that broadband shock-associated noise is relatively unaffected by the refraction of the jet shear layer. A non-dimensional parameter is proposed that quantifies the changes of the broadband shock-associated noise source with varying jet operating condition and airframe position. Scattered broadband shock-associated noise possesses a second set of broadband lobes that are due to the effect of scattering. Presented predictions demonstrate relatively good agreement compared to a wide variety of measurements.

Miller, Steven A. E.↗
DOE OSTI2016

Hopf-algebraic renormalization of QED in the linear covariant gauge

In the context of massless quantum electrodynamics (QED) with a linear covariant gauge fixing, the connection between the counterterm and the Hopf-algebraic approach to renormalization is examined. The coproduct formula of Green’s functions contains two invariant charges, which give rise to different renormalization group functions. All formulas are tested by explicit computations to third loop order. The possibility of a finite electron self-energy by fixing a generalized linear covariant gauge is discussed. An analysis of subdivergences leads to the conclusion that such a gauge only exists in quenched QED.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

An efficient atomistic quantum mechanical simulation on InAs band-to-band tunneling field-effect transistors

We have presented a fully atomistic quantum mechanical simulation method on band-to-band tunneling (BTBT) field-effect transistors (FETs). Our simulation approach is based on the linear combination of bulk band method with empirical pseudopotentials, which is an atomist method beyond the effective-mass approximation or k.p perturbation method, and can be used to simulate real-size devices (∼10{sup 5} atoms) efficiently (∼5 h on a few computational cores). Using this approach, we studied the InAs dual-gate BTBT FETs. The I-V characteristics from our approach agree very well with the tight-binding non-equilibrium Green's function results, yet our method costs much less computationally. In addition, we have studied ways to increase the tunneling current and analyzed the effects of different mechanisms for that purpose.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Coherent thermoelectric transport in single, double, and U-bend structures

Coherent, i.e., ballistic, thermoelectric transport in electron waveguide structures containing right-angle bends in single, double, and U-bend configurations is investigated. A theory based on Green's functions is used to derive the transmission function (and from that the transport coefficients) and allows for the inclusion of realistic models of spatially distributed imperfections. The results for the single and double-bend structures are presented in more detail than elsewhere in the literature. In the U-bend structure, sharp resonances in the stop-band region of the transmission function lead to large-magnitude peaks in the thermopower and consequently a large thermoelectric figure of merit (of order ten in some instances). These properties are still readily apparent even in the presence of moderate edge roughness or Anderson disorder.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Phonon dispersion and local density of states in NiPd alloy using modified embedded atom method potential

A modified embedded atom method (MEAM) potential model up to second neighbours has been used to calculate the phonon dispersions for Ni{sub 0.55}Pd{sub 0.45} alloy in which Pd is introduced as substitutional impurity. Using the force-constants obtained from MEAM potential, the local vibrational density of states in host Ni and substitutional Pd atoms using Green’s function method has been calculated. The calculation of phonon dispersions of NiPd alloy shows a good agreement with the experimental results. Condition of resonance mode has also been investigated and resonance mode in the frequency spectrum of impurity atom at low frequency is observed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Thermal conductivity of InN with point defects from first principles

We present ab initio calculations of thermal conductivity of InN with vacancies and substitutional defects using a full solution of the Peierls-Boltzmann transport equation. Our parameter-free calculations are in good agreement with experimental measurements demonstrating the predictive power of this approach. Phonon-defect scattering rates are computed from a Green's function methodology that is nonperturbative and includes interatomic force constant variance induced near the defects. Restricting calculations to first-order perturbation approaches can overestimate optic phonon scattering rates by nearly three orders of magnitude. On the other hand, neglecting the force variance weakens the scattering rates by about an order of magnitude, mostly in the low-frequency region below 2 THz. Here, this work elucidates important properties of phonon-defect scattering in thermal transport and demonstrates the predictive power of the coupling of Peierls-Boltzmann transport, Green's function methods, and density functional theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Mott insulators with boundary zeros

The topological classification of electronic band structures is based on symmetry properties of Bloch eigenstates of single-particle Hamiltonians. In parallel, topological field theory has opened the doors to the formulation and characterization of non-trivial phases of matter driven by strong electron-electron interaction. Even though important examples of topological Mott insulators have been constructed, the relevance of the underlying non-interacting band topology to the physics of the Mott phase has remained unexplored. Here, we show that the momentum structure of the Green’s function zeros defining the “Luttinger surface" provides a topological characterization of the Mott phase related, in the simplest description, to the one of the single-particle electronic dispersion. Considerations on the zeros lead to the prediction of new phenomena: a topological Mott insulator with an inverted gap for the bulk zeros must possess gapless zeros at the boundary, which behave as a form of “topological antimatter” annihilating conventional edge states. Placing band and Mott topological insulators in contact produces distinctive observable signatures at the interface, revealing the otherwise spectroscopically elusive Green’s function zeros.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Large on/off current ratio in hybrid graphene/BN nanoribbons by transverse electric field-induced control of bandgap

We propose an approach to design efficient electronic switches based on Graphene/BN heterostructures. By means of atomistic Tight Binding simulations, we investigate heterostructures made of an armchair BN nanoribbon sided by two armchair graphene ribbons where a significant bandgap can be opened. In particular, we show that a bandgap of about 0.55 eV can be strongly suppressed by applying a relatively weak transverse electric field of 10 mV/A. Additionally, by using non-equilibrium Green's function simulation, we show that this effect can be used to control the current in electron devices even at small bias and small length. An on/off current ratio higher than 10{sup 4} is achieved at room temperature.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

On a Homogenization Problem

We study the average Green’s function of stochastic, uniformly elliptic operators of divergence form on Zdℤ{sup d}. When the randomness is independent and has small variance, we prove regularity of the Fourier transform of the self-energy. The proof relies on the Schur complement formula and the analysis of singular integral operators combined with a Steinhaus system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Charge separation at nanoscale interfaces: Energy-level alignment including two-quasiparticle interactions

The universal and fundamental criteria for charge separation at interfaces involving nanoscale materials are investigated. In addition to the single-quasiparticle excitation, all the two-quasiparticle effects including exciton binding, Coulomb stabilization, and exciton transfer are considered, which play critical roles on nanoscale interfaces for optoelectronic applications. We propose a scheme allowing adding these two-quasiparticle interactions on top of the single-quasiparticle energy level alignment for determining and illuminating charge separation at nanoscale interfaces. Employing the many-body perturbation theory based on Green's functions, we quantitatively demonstrate that neglecting or simplifying these crucial two-quasiparticle interactions using less accurate methods is likely to predict qualitatively incorrect charge separation behaviors at nanoscale interfaces where quantum confinement dominates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

On electric field in anti-de Sitter spacetime

In this paper we calculate the electromagnetic field produced using retarded Green's function in Anti-de Sitter spacetime (AdS). Since this spacetime is non-globally hyperbolic and has no Cauchy surface, we only consider the field originated from a charge moving along its geodesic in the region consists of points covered by future null geodesic of the charge.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

The effect of a magnetic field on the spin-selective transport in double-stranded DNA

Spin-polarization in double-stranded DNA is studied in the presence of a magnetic field applied along its helix axis using the non-equilibrium Green's function method. The spin-polarization could be tuned by changing the magnetic field. In some special cases, the double-stranded DNA behaved as a perfect spin-filter. Furthermore, the dependency of the spin-polarization on the spin-orbit strength and dephasing strength is studied.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Tunneling current noise spectra of biased impurity with a phonon mode

We report the results of theoretical investigations of the tunneling current noise spectra through a single-level impurity both in the presence and in the absence of electron–phonon interaction based on the nonequilibrium Green’s functions formalism. We show that due to the quantum nature of tunneling, the Fano factor is dramatically different from the Poisson limit both in the presence and in the absence of inelastic processes. The results are demonstrated to be sensitive to the tunneling contact parameters.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

The effect of electron-electron interaction induced dephasing on electronic transport in graphene nanoribbons

The effect of dephasing induced by electron-electron interaction on electronic transport in graphene nanoribbons is theoretically investigated. In the presence of disorder in graphene nanoribbons, wavefunction of electrons can set up standing waves along the channel and the conductance exponentially decreases with the ribbon's length. Employing the non-equilibrium Green's function formalism along with an accurate model for describing the dephasing induced by electron-electron interaction, we show that this kind of interaction prevents localization and transport of electrons remains in the diffusive regime where the conductance is inversely proportional to the ribbon's length.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

A caveat on building nonlocal models of cosmology

Nonlocal models of cosmology might derive from graviton loop corrections to the effective field equations from the epoch of primordial inflation. Although the Schwinger-Keldysh formalism would automatically produce causal and conserved effective field equations, the models so far proposed have been purely phenomenological. Two techniques have been employed to generate causal and conserved field equations: either varying an invariant nonlocal effective action and then enforcing causality by the ad hoc replacement of any advanced Green's function with its retarded counterpart, or else introducing causal nonlocality into a general ansatz for the field equations and then enforcing conservation. We point out here that the two techniques access very different classes of models, and that neither one of them may represent what would actually arise from fundamental theory.

79 ASTRONOMY AND ASTROPHYSICS↗