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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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

Radiative interaction of atmosphere and surface: write up with elements of code

In passive satellite remote sensing of the Earth, separation of the path radiance (atmosphere-only contribution) from the surface reflection remains a “significant challenge”. Recent literature names it among the gaps in radiative transfer (RT) topics that “require continued research in the near future”. The challenge comes from multiple reflections (bouncing) between the atmosphere and surface – radiative interaction. In this paper we use a known RT technique, the matrix-operator method (MOM), and a new modification of the monochromatic vector RT (vRT) code IPOL (Intensity and POLarization) to simulate the interaction of a plane-parallel atmosphere and a few widely used surface reflection models. Following the idea of the Green’s function method, IPOL no longer takes the surface model parameters on input. Instead, it provides the path radiance, and the atmospheric reflection and transmission matrices as output. Despite many RT codes use the MOM formalism, this output does not seem common. The surface reflection matrix is computed externally. Therefore, this paper extends the Green’s function atmospheric correction technique to the case of polarized light. Aiming clarity rather than performance, we explain in Python the structure of the surface matrices for the isotropic (Lambertian), directional unpolarized, and polarized ocean reflection models. We then combine these surface matrices and the precomputed IPOL output to get numerically accurate signal at the top of atmosphere (TOA) and test it vs. published benchmarks. Then, for each benchmark scenario we show how to get the surface from the TOA signal, i.e. perform the RT-based atmospheric correction.

radiative transfer↗
DOE OSTI2015

Infrared conductivity of cuprates using Yang-Rice-Zhang ansatz: Review of our recent investigations

A review of our recent investigations related to the ac transport properties in the psedogapped state of cuprate high temperature superconductors is presented. For our theoretical calculations we use a phenomenological Green’s function proposed by Yang, Rice and Zhang (YRZ). This is based upon the renormalized mean-field theory of the Hubbard model and takes into account the strong electron-electron interaction present in Cuprates. The pseudogap is also taken into account through a proposed self energy. We have tested the form of the Green’s function by computing ac conductivity of cuprates and then compared with experimental results. We found agreement between theory and experiment in reproducing the doping evolution of ac conductivity but there is a problem with absolute magnitudes and their frequency dependence. This shows a partial success of the YRZ ansatz. The ways to rectify it are suggested and worked out.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electronic Coupling Calculations for Bridge-Mediated Charge Transfer Using Constrained Density Functional Theory (CDFT) and Effective Hamiltonian Approaches at the Density Functional Theory (DFT) and Fragment-Orbital Density Functional Tight Binding (FODFTB) Level

In this paper, four methods to calculate charge transfer integrals in the context of bridge-mediated electron transfer are tested. These methods are based on density functional theory (DFT). We consider two perturbative Green's function effective Hamiltonian methods (first, at the DFT level of theory, using localized molecular orbitals; second, applying a tight-binding DFT approach, using fragment orbitals) and two constrained DFT implementations with either plane-wave or local basis sets. To assess the performance of the methods for through-bond (TB)-dominated or through-space (TS)-dominated transfer, different sets of molecules are considered. For through-bond electron transfer (ET), several molecules that were originally synthesized by Paddon-Row and co-workers for the deduction of electronic coupling values from photoemission and electron transmission spectroscopies, are analyzed. The tested methodologies prove to be successful in reproducing experimental data, the exponential distance decay constant and the superbridge effects arising from interference among ET pathways. For through-space ET, dedicated p-stacked systems with heterocyclopentadiene molecules were created and analyzed on the basis of electronic coupling dependence on donor-acceptor distance, structure of the bridge, and ET barrier height. The inexpensive fragment-orbital density functional tight binding (FODFTB) method gives similar results to constrained density functional theory (CDFT) and both reproduce the expected exponential decay of the coupling with donor-acceptor distances and the number of bridging units. Finally, these four approaches appear to give reliable results for both TB and TS ET and present a good alternative to expensive ab initio methodologies for large systems involving long-range charge transfers.

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

Bias-dependent local structure of water molecules at a metallic interface

Understanding the local structure of water at the interfaces of metallic electrodes is a key issue in aqueous-based electrochemistry. Nevertheless a realistic simulation of such a setup is challenging, particularly when the electrodes are maintained at different potentials. To correctly compute the effect of an external bias potential applied to truly semi-infinite surfaces, we combine Density Functional Theory (DFT) and Non-Equilibrium Green’s Function (NEGF) methods. This framework allows for the out-of-equilibrium calculation of forces and dynamics, and directly correlates to the chemical potential of the electrodes, which is introduced experimentally. In this work, we apply this methodology to study the electronic properties and atomic forces of a water molecule at the interface of a gold surface. We find that the water molecule tends to align its dipole moment with the electric field, and it is either repelled or attracted to the metal depending on the sign and magnitude of the applied bias, in an asymmetric fashion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Mechanical, electronic, and transport properties of functionalized graphene monolayers from ab initio studies

We present exemplary results of extensive studies of mechanical, electronic and transport properties of covalent functionalization of graphene monolayers (GML). We report new results of ab initio studies for covalent functionalization of GML with −NH{sub 2} groups up to 12.5 % concentration. Our studies are performed in the framework of the density functional theory (DFT) and non-equilibrium Green’s function (NEGF). We discuss the stability (adsorption energy), elastic moduli, electronic structure, band gaps, and effective electron masses as a function of the density of the adsorbed molecules. We also show the conductance and current – voltage I(V) characteristics for these systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Single particle spectral function in iron pnictide superconductors within two band model

The newly discovered iron based superconductors have shown a great deal of interest in the condensed matter physics community around the world to explore the experimental and theoretical aspects of electronic properties of these materials to underline the technological potential. These materials have a layered tetragonal crystal structure and generally show small anisotropy. Based on ARPES data to understand the band structure of these systems, we present a theoretical tight binding model and numerical computation of the single particle spectral function within two orbital per site for iron pnictide superconductors. The two band tight binding model Hamiltonian containing various orbitals hopping energies, intra- and inter- band electronic correlations and Hund’s coupling energy in Fe 3d orbitals has been used. The expressions of single particle spectral function have been obtained by employing the Green’s function equation of motion approach within BCS-mean-field approximation. The spectral function is numerically computed at different k-points of Brillouin zone in extended s-wave paring symmetry. It is noticed that the behavior of electronic states is different at different k-points of Brillouin zone and highly influenced by onsite Coulomb interactions. Further, It is predicted that the presence of onsite coulomb correlation suppress the spectral weight close to Fermi level in iron pnictide systems. On the basis of numerical computation we have compared our theoretical results with recent angle resolved photoemission spectroscopic ARPES data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Derivatization and diffusive motion of molecular fullerenes: Ab initio and atomistic simulations

Using first principles density functional theory in combination with the nonequilibrium Green's function formalism, we study the effect of derivatization on the electronic and transport properties of C{sub 60} fullerene. As a typical example, we consider [6,6]-phenyl-C{sub 61}-butyric acid methyl ester (PCBM), which forms one of the most efficient organic photovoltaic materials in combination with electron donating polymers. Extra peaks are observed in the density of states (DOS) due to the formation of new electronic states localized at/near the attached molecule. Despite such peculiar behavior in the DOS of an isolated molecule, derivatization does not have a pronounced effect on the electronic transport properties of the fullerene molecular junctions. Both C{sub 60} and PCBM show the same response to finite voltage biasing with new features in the transmission spectrum due to voltage induced delocalization of some electronic states. We also study the diffusive motion of molecular fullerenes in ethanol solvent and inside poly(3-hexylthiophene) lamella using reactive molecular dynamics simulations. We found that the mobility of the fullerene reduces considerably due to derivatization; the diffusion coefficient of C{sub 60} is an order of magnitude larger than the one for PCBM.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Understanding the conductance switching of permethyloligosilanes: A theoretical approach

On the basis of ab initio density functional theory coupled with non-equilibrium Green’s function technique, we have presented a molecular level understanding on the stereoelectronic switching of conducting properties of oligosilane molecules. Su et al. [Nat. Chem. 7, 215–220 (2015)] demonstrated that these types of oligosilane molecules exhibit three stereoconformers which show two distinct conducting profiles. On the basis of break-junction technique, the authors show that manipulating a specific dihedral angle and thereby controlling the length of the molecular contact, it is possible to switch the conducting states minutely. However, their discussions scarce the proper energy level alignment upon which the molecule-lead tunneling amplitude depends. On the basis of electronic structure and non-equilibrium electron transport calculations, we interpret such switching behavior and thus quantify the switching parameter demonstrating how the metal-molecule contact geometry along with the electronic energy level alignment is responsible for such kind of junction process. We also provide the variation of switching parameter and the type of majority carrier with the conjugation length of the oligosilanes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Electronic transport in organometallic perovskite CH{sub 3}NH{sub 3}PbI{sub 3}: The role of organic cation orientations

Density functional theory in combination with the nonequilibrium Green's function formalism is used to study the electronic transport properties of methylammonium lead-iodide perovskite CH{sub 3}NH{sub 3}PbI{sub 3}. Electronic transport in homogeneous ferroelectric and antiferroelectric phases, both of which do not contain any charged domain walls, is quite similar. The presence of charged domain wall drastically (by about an order of magnitude) enhances the electronic transport in the lateral direction. The increase of the transmission originates from the smaller variation of the electrostatic potential profile along the charged domain walls. This fact may provide a tool for tuning transport properties of such hybrid materials by manipulating molecular cations having dipole moment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

High spin-filter efficiency and Seebeck effect through spin-crossover iron–benzene complex

Electronic structures and coherent quantum transport properties are explored for spin-crossover molecule iron-benzene Fe(Bz){sub 2} using density functional theory combined with non-equilibrium Green’s function. High- and low-spin states are investigated for two different lead-molecule junctions. It is found that the asymmetrical T-shaped contact junction in the high-spin state behaves as an efficient spin filter while it has a smaller conductivity than that in the low-spin state. Large spin Seebeck effect is also observed in asymmetrical T-shaped junction. Spin-polarized properties are absent in the symmetrical H-shaped junction. These findings strongly suggest that both the electronic and contact configurations play significant roles in molecular devices and metal-benzene complexes are promising materials for spintronics and thermo-spintronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Electron transport property of tetrathiafulvalene molecule

We have investigated electron transport behavior of tetrathiafulvalene molecule connected with zigzag graphene nanoribbon (zGNR) using density functional theory combined with non-equilibrium Green’s function method. We have reported the transmission coefficient of the scattering region at different bias voltage to explain the nature of the current.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1971

Causality of the nonequilibrium correlation functions.

It is shown that the correct lowest-order two-particle correlation function solution to the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy is given via a Green's function whose Fourier transform satisfies the analyticity requirements for causality. The Bogoliubov adiabatic approximation is shown to imply a solution which is acausal. Extension to the s-particle correlation function is discussed.

Murphy, F. X.↗
NASA NTRS2002

Effects of Structural Deformation and Tube Chirality on Electronic Conductance of Carbon Nanotubes

A combination of large scale classical force-field (UFF), density functional theory (DFT), and tight-binding Green's function transport calculations is used to study the electronic properties of carbon nanotubes under the twist, bending, and atomic force microscope (AFM)-tip deformation. We found that in agreement with experiment a significant change in electronic conductance can be induced by AFM-tip deformation of metallic zigzag tubes and by twist deformation of armchair tubes. The effect is explained in terms of bandstructure change under deformation.

Svizhenko, Alexei↗
DOE OSTI2019

Absence of Coulomb Blockade in the Anderson Impurity Model at the Symmetric Point

In this study, we investigate the characteristics of the electric current in the so-called symmetric Anderson impurity model. We study the nonequilibrium model using two complementary approximate methods, the perturbative quantum master equation approach to the reduced density matrix and a self-consistent equation of motion approach to the nonequilibrium Green's function. We find that, at a particular symmetry point, an interacting Anderson impurity model recovers the same steady-state current as an equivalent noninteracting model, akin a two-band resonant level model. We show this in the Coulomb blockade regime for both high and low temperatures, where either the approximate master equation approach or the Green's function method provides accurate results for the current. We conclude that the steady-state current in the symmetric Anderson model at this regime does not encode characteristics of a many-body interacting system.

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

Recent developments for the optical model of nuclei

A brief overview of various approaches to the optical-model description of nuclei is presented. A survey of some of the formal aspects is given which links the Feshbach formulation for either the hole or particle Green’s function to the time-ordered quantity of many-body theory. The link between the reducible self-energy and the elastic nucleon–nucleus scattering amplitude is also presented using the development of Villars. A brief summary of the essential elements of the multiple-scattering approach is also included. Several ingredients contained in the time-ordered Green’s function are summarized for the formal framework of the dispersive optical model (DOM). Empirical approaches to the optical potential are reviewed with emphasis on the latest global parametrizations for nucleons and composites. Various calculations that start from an underlying realistic nucleon–nucleon interaction are discussed with emphasis on more recent work. The efficacy of the DOM is illustrated in relating nuclear structure and nuclear reaction information. Its use as an intermediate between experimental data and theoretical calculations is advocated. Applications of the use of optical models are pointed out in the context of the description of nuclear reactions other than elastic nucleon–nucleus scattering.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Curie temperature of ultrathin ferromagnetic layer with Dzyaloshinskii-Moriya interaction

We investigate the effect of the Dzyaloshinskii-Moriya interaction (DMI) on the Curie temperature of the ultrathin ferromagnetic layers. It has been known that the Curie temperature of the ferromagnet depends on spin wave excitation energies, and they are affected by DMI. Therefore, the ferromagnetic transition temperature of the ultrathin ferromagnetic layer must be sensitive on the DMI. We find that the Curie temperature depends on the DMI by using the double time Green's function method. Since the DMI is arisen by the inversion symmetry breaking structure, the DMI is always important in the inversion symmetry breaking ultrathin ferromagnetic layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Vibrational properties of TaW alloy using modified embedded atom method potential

Force-constants up to second neighbours of pure transition metal Ta and TaW alloy are determined using the modified embedded atom method (MEAM) potential. The obtained force-constants are used to calculate the phonon dispersion of pure Ta and TaW alloy. As a further application of MEAM potential, the force-constants are used to calculate the local vibrational density of states and mean square thermal displacements of pure Ta and W impurity atoms with Green’s function method. The calculated results are found to be in agreement with the experimental measurements.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Orthogonality catastrophe as a consequence of qubit embedding in an ultracold Fermi gas

We investigate the behavior of a two-level atom coupled to a one-dimensional, ultracold Fermi gas. The sudden switching on of the scattering between the two entities leads to the loss of any coherence in the initial state of the impurity and we show that the exact dynamics of this process is strongly influenced by the effect of the orthogonality catastrophe within the gas. We highlight the relationship between the Loschmidt echo and the retarded Green's function - typically used to formulate the dynamical theory of the catastrophe - and demonstrate that the effect is reflected in the impurity dynamics. We show that the expected nonexponential decay of the spectral function can be observed using Ramsey interferometry on the two-level atom and comment on finite temperature effects.

74 ATOMIC AND MOLECULAR PHYSICS↗