SEARCH · Engineering Papers

Results for “GREEN 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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 577 records · Page 32

All sources NASA NTRS DOE OSTI

DOE OSTI2016

On the spectral function of carriers in the pseudogap state

We consider the evolution of the spectral function of charge carriers for a 2D Kondo lattice depending on the parameters of the model. A self-consistent solution is obtained for the spectral function using the formalism of irreducible Green’s functions. In the low doping level regime, the behavior of the spectral function exhibits suppression of the spectral weight of carriers in the low-frequency range, which is typical of the pseudogap state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS2012

Prediction of Broadband Shock-Associated Noise Including Propagation Effects Originating NASA

An acoustic analogy is developed based on the Euler equations for broadband shock-associated noise (BBSAN) that directly incorporates the vector Green s function of the linearized Euler equations and a steady Reynolds-Averaged Navier-Stokes solution (SRANS) to describe the mean flow. The vector Green s function allows the BBSAN propagation through the jet shear layer to be determined. The large-scale coherent turbulence is modeled by two-point second order velocity cross-correlations. Turbulent length and time scales are related to the turbulent kinetic energy and dissipation rate. An adjoint vector Green s function solver is implemented to determine the vector Green s function based on a locally parallel mean flow at different streamwise locations. The newly developed acoustic analogy can be simplified to one that uses the Green s function associated with the Helmholtz equation, which is consistent with a previous formulation by the authors. A large number of predictions are generated using three different nozzles over a wide range of fully-expanded jet Mach numbers and jet stagnation temperatures. These predictions are compared with experimental data from multiple jet noise experimental facilities. In addition, two models for the so-called fine-scale mixing noise are included in the comparisons. Improved BBSAN predictions are obtained relative to other models that do not include propagation effects.

Miller, Steven↗

DOE OSTI2014

Quantum transport through the system of parallel quantum dots with Majorana bound states

We study the tunneling transport properties through a system of parallel quantum dots which are coupled to Majorana bound states (MBSs). The conductance and spectral function are computed using the retarded Green's function method based on the equation of motion. The conductance of the system is 2e{sup 2}/h at zero Fermi energy and is robust against the coupling between the MBSs and the quantum dots. The dependence of the Fermi energy on the spectral function is different for the first dot (dot1) than for the second dot (dot2) with fixed dot2-MBSs coupling. The influence of the Majorana bound states on the spectral function was studied for the series and parallel configurations of the system. It was found that when the configuration is in series, the Majorana bound states play an important role, resulting in a spectral function with three peaks. However, the spectral function shows two peaks when the system is in a parallel configuration. The zero Fermi energy spectral function is always 1/2 not only in series but also in the parallel configuration and robust against the coupling between the MBSs and the quantum dots. The phase diagram of the Fermi energy versus the quantum dot energy levels was also investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2012

Excitation of a slow wave structure

The Green's function on a slow wave structure is constructed. The Green's function includes all radial modes, and for each radial mode, all space harmonics. We compare the analytic solution of the frequency response on the slow wave structure with that obtained from a particle-in-cell code. Favorable comparison is obtained when the first few lower order modes are resonantly excited. This gives some confidence in the prediction of converting a pulse train into radiation using a slow wave structure.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2015

Study of ultrasonic attenuation in f-electron systems in the paramagnetic limit of Coulomb interaction

We report here a microscopic model study of ultrasonic attenuation in f-electron systems based on Periodic Anderson Model in which Coulomb interaction is considered within a mean-field approximation for a weak interaction. The Phonon is coupled to the conduction band and f-electrons. The phonon Green's function is calculated by Zubarev's technique of the Green's function method. The temperature dependent ultrasonic attenuation co-efficient is calculated from the imaginary part of the phonon self-energy in the dynamic and long wave length limit. The f-electron occupation number is calculated self-consistently in paramagnetic limit of Coulomb interaction. The effect of the Coulomb interaction on ultrasonic attenuation is studied by varying the phonon coupling parameters to the conduction and f-electrons, hybridization strength, the position of f-level and the Coulomb interaction Strength. Results are discussed on the basis of experimental results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2017

Understanding the Effect of Twisting Graphene Sheet on Its Magnetoresistance and Spin Filtration Properties

Spin-dependent quantum transport properties in twisted graphene is calculated using density functional theory (DFT) and non-equilibrium Green’s function (NEGF) formulation. Twisting increases the band gap and reduces the spin transport in twisted graphene. At low bias, twisting restricts any spin-up current in antiparallel configuration (APC) which results in higher magnetoresistance (MR), and at higher bias, twisting increases the spin-up current slightly, resulting in lower MR. The MR obtained in twisted graphene at low biases is greater than that of pristine graphene. High spin filtration is observed in parallel configuration and APC for twisted graphene at higher biases which is greater than that of pristine graphene, but at low voltages, pristine gives higher spin filtration than twisted graphene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Enhanced Magnetoresistance in In-Plane Monolayer MoS{sub 2} with CrO{sub 2} Electrodes

Magnetoresistance of monolayer MoS{sub 2} is reported to increase when used in in-plane configuration with CrO{sub 2} half-metal ferromagnet (HMF) electrodes. Density functional theory (DFT)- and non-equilibrium Green’s function (NEGF)-based simulations show that high magnetoresistance (MR) values of ∼860% can be achieved in in-plane monolayer MoS{sub 2} with CrO{sub 2} electrodes, which is much higher than the MR value of ∼300% for nine-layer and the MR value of ∼70% for single-layer out-of-plane MoS{sub 2} reported in Dolui et al., Phys. Rev. B 90(R), 041401 (2014) past by other researchers. High spin-injection efficiency ∼100% is also obtained at high bias voltages. High MR and perfect spin filtration suggests the importance of this configuration in spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Spin-filtering, giant magnetoresistance, rectifying and negative differential resistance effects in planar four-coordinate Fe complex with graphene nanoribbon electrodes

By using the nonequilibrium Green's function formalism combined with the density functional theory, we have investigated the spin-polarized transport properties of a planar four-coordinate Fe complex sandwiched between two zigzag-edge graphene nanoribbon (ZGNR) electrodes, where the ZGNRs are modulated by external magnetic field. The results show that the system can exhibit perfect dual spin-filtering and spin-rectifying effects at a wide bias range, giant magnetoresistance effect with large magnetoresistance ratio at small bias, and obvious negative differential resistance behavior. The mechanisms are proposed for these phenomena.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2011

Integration of Kaluza-Klein modes in Yang-Mills theories

A five-dimensional pure Yang-Mills theory, with the fifth coordinate compactified on the orbifold S{sup 1}/Z{sub 2} of radius R, leads to a four-dimensional theory which is governed by two types of infinitesimal gauge transformations, namely, the well-known standard gauge transformations (SGT) dictated by the SU{sub 4}(N) group under which the zero Fourier modes A{sub {mu}}{sup (0)a} transform as gauge fields, and a set of nonstandard gauge transformations (NSGT) determining the gauge nature of the Kaluza-Klein (KK) excitations A{sub {mu}}{sup (m)a}. By using a SGT-covariant gauge-fixing procedure for removing the degeneration associated with the NSGT, we integrate out the KK excitations and obtain a low-energy effective Lagrangian expansion involving all of the independent canonical-dimension-six operators that are invariant under the SGT of the SU{sub 4}(N) group and that are constituted by light gauge fields, A{sub {mu}}{sup (0)a}, exclusively. It is shown that this effective Lagrangian is invariant under the SGT, but it depends on the gauge-fixing of the gauge KK excitations. Our result shows explicitly that the one-loop contributions of the KK excitations to light (standard) Green's functions are renormalizable.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2014

Modulation of the electron transport properties in graphene nanoribbons doped with BN chains

Using density-functional theory and the non-equilibrium Green's function method, the electron transport properties of zigzag graphene nanoribbons (ZGNRs) doped with BN chains are studied by systematically calculating the energy band structure, density of states and the transmission spectra for the systems. The BN chains destroyed the electronic transport properties of the ZGNRs, and an energy gap appeared for the ZGNRs, and displayed variations from a metal to a wide-gap semiconductor. With an increase in the number of BN chains, the band gap increased gradually in the band structure and the transmission coefficient decreased near the Fermi surface. Additionally, the doping position had a significant effect on the electronic properties of the ZGNRs.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Theoretical study of electronic transport properties of a graphene-silicene bilayer

Electronic transport properties of a graphene-silicene bilayer system are studied using density-functional theory in combination with the nonequilibrium Green's function formalism. Depending on the energy of the electrons, the transmission can be larger in this system as compared to the sum of the transmissions of separated graphene and silicene monolayers. This effect is related to the increased electron density of states in the bilayer sample. At some energies, the electronic states become localized in one of the layers, resulting in the suppression of the electron transmission. The effect of an applied voltage on the transmission becomes more pronounced in the layered sample as compared to graphene due to the larger variation of the electrostatic potential profile. Our findings will be useful when creating hybrid nanoscale devices where enhanced transport properties will be desirable.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

Crossing resonance of wave fields in a medium with an inhomogeneous coupling parameter

The dynamic susceptibilities (Green’s functions) of the system of two coupled wave fields of different physical natures in a medium with an arbitrary relation between the mean value ɛ and rms fluctuation Δɛ of the coupling parameter have been examined. The self-consistent approximation involving all diagrams with noncrossing correlation lines has been developed for the case where the initial Green’s function of the homogeneous medium describes the system of coupled wave fields. The analysis has been performed for spin and elastic waves. Expressions have been obtained for the diagonal elements G{sub mm} and G{sub uu} of the matrix Green’s function, which describe spin and elastic waves in the case of magnetic and elastic excitations, and for the off-diagonal elements G{sub mu} and G{sub um}, which describe these waves in the case of cross excitation. Change in the forms of these elements has been numerically studied for the case of one-dimensional inhomogeneities with an increase in Δɛ and with a decrease in ɛ under the condition that the sum of the squares of these quantities is conserved: two peaks in the frequency dependences of imaginary parts of G{sub mm} and G{sub uu} are broadened and then joined into one broad peak; a fine structure appears in the form of narrow resonance at the vertex of the Green’s function of one wave field and narrow antiresonance at the vertex of the Green’s function of the other field; peaks of the fine structure are broadened and then disappear with an increase in the correlation wavenumber of the inhomogeneities of the coupling parameter; and the amplitudes of the off-diagonal elements vanish in the limit ɛ → 0.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

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 OSTI2011

Kubo formulas for relativistic fluids in strong magnetic fields

Magnetohydrodynamics of strongly magnetized relativistic fluids is derived in the ideal and dissipative cases, taking into account the breaking of spatial symmetries by a quantizing magnetic field. A complete set of transport coefficients, consistent with the Curie and Onsager principles, is derived for thermal conduction, as well as shear and bulk viscosities. It is shown that in the most general case the dissipative function contains five shear viscosities, two bulk viscosities, and three thermal conductivity coefficients. We use Zubarev's non-equilibrium statistical operator method to relate these transport coefficients to correlation functions of the equilibrium theory. The desired relations emerge at linear order in the expansion of the non-equilibrium statistical operator with respect to the gradients of relevant statistical parameters (temperature, chemical potential, and velocity.) The transport coefficients are cast in a form that can be conveniently computed using equilibrium (imaginary-time) infrared Green's functions defined with respect to the equilibrium statistical operator. - Highlights: > Strong magnetic fields can make charged fluids behave anisotropically. > Magnetohydrodynamics for these fluids contains 5 shear, 2 bulk viscosities, and 3 heat conductivities. > We derive Kubo formulas for these transport coefficients.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Negative differential resistance devices by using N-doped graphene nanoribbons

Recently, extensive efforts have been devoted to the investigations of negative differential resistance (NDR) behavior in graphene. Here, by performing fully self-consistent density functional theory calculations combined with non-equilibrium Green's function technique, we investigate the transport properties of three molecules from conjugated molecule, one-dimension alkane chain, and single molecule magnet, which are sandwiched between two N-doped zigzag and armchair graphene nanoribbons (GNRs). We observe robust NDR effect in all examined molecular junctions including benzene, alkane, and planar four-coordinated Fe complex. Through the analyses of the calculated electronic structures and the bias-dependent transmission coefficients, we find that the narrow density of states of N-doped GNRs and the bias-dependent effective coupling between the discrete frontier molecular orbitals and the subbands of N-doped GNRs are responsible for the observed NDR phenomenon. These theoretical findings imply that N-doped GNRs hold great potential for building NDR devices based on various molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRSDate not supplied

Predictions of the Azimuthal Variation of the Noise from Chevron Jets using an Acoustic Analogy

An existing acoustic analogy formulation is used to make predictions of the turbulent mixing noise from a series of chevron jets tested at NASA Glenn Research Center. Reynolds-averaged Navier-Stokes solutions are used as input to provide the mean flow and turbulence quantities needed to compute the Green’s function and parameterize the source model. The Green’s function for the non-axisymmetric mean flows generated by these nozzles is computed using a second-order finite volume method. A slightly modified version of an existing source model originally developed for round jets is used to assess its capability in these non-axisymmetric flows. Prediction results are compared with experimental data for a range of azimuthal orders of the nozzle geometry and flow speeds. The results show that the RANS-based acoustic analogy prediction method is able to capture the observed azimuthal variation of the sound field fairly well at relatively low jet exit velocities. At higher speeds, the prediction method still captures the observed trends in azimuthal directivity but not the absolute levels, particularly at higher frequencies. Areas for potential improvement of the method are suggested.

Jet Noise↗

NASA NTRSDate not supplied

Predictions of the Azimuthal Variation of the Noise from Chevron Jets using an Acoustic Analogy

Jet Noise↗

DOE OSTI2011

Kinetics of spin relaxation in quantum wires and channels: Boundary spin echo and formation of a persistent spin helix

In this paper we use a spin kinetic equation to study spin-polarization dynamics in one-dimensional (1D) wires and 2D channels. The spin kinetic equation is valid in both diffusive and ballistic spin transport regimes and therefore is more general than the usual spin drift-diffusion equations. In particular, we demonstrate that in infinite 1D wires with Rashba spin-orbit interaction the exponential spin-relaxation decay can be modulated by an oscillating function. In the case of spin relaxation in finite length 1D wires, it is shown that an initially homogeneous spin polarization spontaneously transforms into a persistent spin helix. We find that a propagating spin-polarization profile reflects from a system boundary and returns back to its initial position similarly to the reflectance of sound waves from an obstacle. The Green's function of the spin kinetic equation is derived for both finite and infinite 1D systems. Moreover, we demonstrate explicitly that the spin relaxation in specifically oriented 2D channels with Rashba and Dresselhaus spin-orbit interactions of equal strength occurs similarly to that in 1D wires of finite length. Finally, a simple transformation mapping 1D spin kinetic equation into the Klein-Gordon equation with an imaginary mass is found thus establishing an interesting connection between semiconductor spintronics and relativistic quantum mechanics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗