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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Transport properties of zigzag graphene nanoribbon decorated with copper clusters

Using non-equilibrium green function with density functional theory, the present study investigates the transport properties of decorated zigzag graphene nanoribbon with a copper cluster. We have represented the decoration of zigzag graphene nanoribbon with single copper atom and cluster containing two and three copper atoms. In all the cases, copper atoms tend to occupy the edge state. In addition, we have shown that copper can alter the current-voltage characteristic of zigzag graphene nanoribbon and create new fluctuations and negative differential resistance. These alternations are made due to discontinuity in the combination of orbitals along the graphene nanoribbon. Decoration alters these discontinuities and creates more visible fluctuations. However, in low bias voltages, the changes are similar in all the cases. The study demonstrates that in the decorated zigzag graphene nanoribbon, the edge states are the main states for transporting electron from one electrode to another.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Acousto-exciton interaction in a gas of 2D indirect dipolar excitons in the presence of disorder

A theory for the linear and quadratic responses of a 2D gas of indirect dipolar excitons to an external surface acoustic wave perturbation in the presence of a static random potential is considered. The theory is constructed both for high temperatures, definitely greater than the exciton gas condensation temperature, and at zero temperature by taking into account the Bose–Einstein condensation effects. The particle Green functions, the density–density correlation function, and the quadratic response function are calculated by the “cross” diagram technique. The results obtained are used to calculate the absorption of Rayleigh surface waves and the acoustic exciton gas drag by a Rayleigh wave. The damping of Bogoliubov excitations in an exciton condensate due to theirs scattering by a random potential has also been determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Exciton correlations and input-output relations in non-equilibrium exciton superfluids

The photoluminescence (PL) measurements on photons and the transport measurements on excitons are the two types of independent and complementary detection tools to search for possible exciton superfluids in electron-hole semi-conductor bilayer systems. In fact, it was believed that the transport measurements can provide more direct evidences on superfluids than the spectroscopic measurements. It is important to establish the relations between the two kinds of measurements. In this paper, using quantum Heisenberg-Langevin equations, we establish such a connection by calculating various exciton correlation functions in the putative exciton superfluids. These correlation functions include both normal and anomalous greater, lesser, advanced, retarded, and time-ordered exciton Green functions and also various two exciton correlation functions. We also evaluate the corresponding normal and anomalous spectral weights and the Keldysh distribution functions. We stress the violations of the fluctuation and dissipation theorem among these various exciton correlation functions in the non-equilibrium exciton superfluids. We also explore the input-output relations between various exciton correlation functions and those of emitted photons such as the angle resolved photon power spectrum, phase sensitive two mode squeezing spectrum and two photon correlations. Applications to possible superfluids in the exciton-polariton systems are also mentioned. For a comparison, using conventional imaginary time formalism, we also calculate all the exciton correlation functions in an equilibrium dissipative exciton superfluid in the electron-electron coupled semi-conductor bilayers at the quantum Hall regime at the total filling factor {nu}{sub T}=1. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems. - Highlights: Black-Right-Pointing-Pointer Establish the relations between photoluminescence and transport measurements. Black-Right-Pointing-Pointer Stress the violations of the fluctuation-dissipation theorem in non-equilibrium systems. Black-Right-Pointing-Pointer Explore the input-output relations between excitons and photons. Black-Right-Pointing-Pointer Discuss differences between non-equilibrium systems and dissipative ones.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Rectification induced in N{sub 2}{sup AA}-doped armchair graphene nanoribbon device

By using non-equilibrium Green function formalism in combination with density functional theory, we investigated the electronic transport properties of armchair graphene nanoribbon devices in which one lead is undoped and the other is N{sub 2}{sup AA}-doped with two quasi-adjacent substitutional nitrogen atoms incorporating pairs of neighboring carbon atoms in the same sublattice A. Two kinds of N{sub 2}{sup AA}-doped style are considered, for N dopants substitute the center or the edge carbon atoms. Our results show that the rectification behavior with a large rectifying ratio can be found in these devices and the rectifying characteristics can be modulated by changing the width of graphene nanoribbons or the position of the N{sub 2}{sup AA} dopant. The mechanisms are revealed to explain the rectifying behaviors.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Energy level alignment and quantum conductance of functionalized metal-molecule junctions: Density functional theory versus GW calculations

We study the effect of functional groups (CH{sub 3}*4, OCH{sub 3}, CH{sub 3}, Cl, CN, F*4) on the electronic transport properties of 1,4-benzenediamine molecular junctions using the non-equilibrium Green function method. Exchange and correlation effects are included at various levels of theory, namely density functional theory (DFT), energy level-corrected DFT (DFT+Σ), Hartree-Fock and the many-body GW approximation. All methods reproduce the expected trends for the energy of the frontier orbitals according to the electron donating or withdrawing character of the substituent group. However, only the GW method predicts the correct ordering of the conductance amongst the molecules. The absolute GW (DFT) conductance is within a factor of two (three) of the experimental values. Correcting the DFT orbital energies by a simple physically motivated scissors operator, Σ, can bring the DFT conductances close to experiments, but does not improve on the relative ordering. We ascribe this to a too strong pinning of the molecular energy levels to the metal Fermi level by DFT which suppresses the variation in orbital energy with functional group.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Semiconductor to metal transition by tuning the location of N{sub 2}{sup AA} in armchair graphene nanoribbons

The electronic band structures and transport properties of N{sub 2}{sup AA}-doped armchair graphene nanoribbons (aGNRs) with two quasi-adjacent substitutional nitrogen atoms incorporated in pairs of neighboring carbon atoms in the same sublattice A are investigated by using non-equilibrium Green function formalism in combination with density functional theory. The results show that the coupling effect between the Pz orbitals of carbon and nitrogen atoms plays an important role in the transition between semiconductor and metal by different locations of N{sub 2}{sup AA}-doped aGNRs. And the striking negative differential resistance behaviors can be found in such devices. These tremendous properties suggest potential application of N{sub 2}{sup AA}-doped aGNRs in graphene-based nanoelectronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

To the Theory of Inhomogeneous Electron Gas

Nonrigorous character of the density-functional theory for inhomogeneous electron gas based on the hypothesis assuming the existence of a universal density functional is demonstrated. A single-particle density matrix must be determined to calculate the ground-state energy of a finite system with a finite number of electrons. A single-particle Green function can be used to unambiguously determine the ground-state energy of an inhomogeneous electron system that satisfies the thermodynamic limit.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Foundations of variational discrete action theory

Variational wave functions and Green's functions are two important paradigms for solving quantum Hamiltonians, each having their own advantages. Here we detail the variational discrete action theory (VDAT), which exploits the advantages of both paradigms in order to approximately solve the ground state of quantum Hamiltonians. VDAT consists of two central components: the sequential product density matrix (SPD) ansatz and a discrete action associated with the SPD. The SPD is a variational ansatz inspired by the Trotter decomposition and characterized by an integer $\mathscr{N}$, recovering many well-known variational wave functions, in addition to the exact solution for $\mathscr{N}$ = ∞. The discrete action describes all dynamical information of an effective integer time evolution with respect to the SPD. We generalize the path integral to our integer time formalism, which converts a dynamic correlation function in integer time to a static correlation function in a compound space. We also generalize the usual many-body Green's function formalism to integer time, which results in analogous but distinct mathematical structures, yielding integer time versions of the generating functional, Dyson equation, and Bethe-Salpeter equation. We prove that the SPD can be exactly evaluated in the multiband Anderson impurity model (AIM) by summing a finite number of diagrams. For the multiband Hubbard model, we prove that the self-consistent canonical discrete action approximation (SCDA), which is the integer time analog of the dynamical mean-field theory, exactly evaluates the SPD for d = ∞. VDAT within the SCDA provides an efficient yet reliable method for capturing the local physics of quantum lattice models, which will have broad applications for strongly correlated electron materials. More generally, VDAT should find applications in various many-body problems in physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Electronic excitations of bulk LiCl from many-body perturbation theory

We present the quasiparticle band structure and the optical excitation spectrum of bulk LiCl, using many-body perturbation theory. Density-functional theory is used to calculate the ground-state geometry of the system. The quasiparticle band structure is calculated within the GW approximation. Taking the electron-hole interaction into consideration, electron-hole pair states and optical excitations are obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function. The calculated band gap is 9.5 eV, which is in good agreement with the experimental result of 9.4 eV. And the calculated optical absorption spectrum, which contains an exciton peak at 8.8 eV and a resonant-exciton peak at 9.8 eV, is also in good agreement with experimental data.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Study on transport properties of silicene monolayer under external field using NEGF method

We investigate the current-voltage (I-V) characteristics of a pristine monolayer silicene using non-equilibrium Green function (NEGF) method combining with density functional theory (DFT). This method succeeded in showing the relationship of I and V on silicene corresponding to the electronic characteristics such as density of states. The external field perpendicular to the silicene monolayer affects in increasing of the current. Under 0.2 eV external field, the current reaches the maximum peak at Vb = 0.3 eV with the increase is about 60% from what it is in zero external field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Limitations of the clump-correlation theories of shear-induced turbulence suppression

The clump theory, primarily constructed by Dupree [Phys. Fluids 15, 334 (1972)] based on the moment approach and then generalized to the correlation theory [Y. Z. Zhang and S. M. Mahajan, Phys. Fluids B 5, 2000 (1993)], has long served as a basis for constructing theories of turbulence suppression by shear flow. In order to reveal the “intrinsic approximation” invoked in the clump-correlation theory, we examine a model based on two dimensional magnetized drift waves. After a rigorous derivation of the exact response function—a key to average the Green function of the system—we show that the Dupree, Zhang-Mahajan approach is recovered as the lowest order approximation in a small dimensionless parameter $Υ$ which is a triple product of the correlation time, wave number, and fluctuating drift velocity. The clump-correlation theory, thus, constitutes the Gaussian and lowest order non-Markovian process for a homogeneous stationary turbulence. Lastly we also provide, especially for the tokamak community, a readily usable formula to evaluate the effectiveness of shear-flow suppression; this formula pertains regardless of the specific model of correlation time.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Electronic Structure and Optical Absorption Spectra of Gold Fullerenes Au{sub 16} and Au{sub 20}

The electronic and optical properties of gold fullerenes are studied in the framework of the Hubbard model. The expressions of the Fourier transforms of anticommutator Green functions have been obtained for gold fullerenes Au{sub 16} and Au{sub 20}, the poles of which determine the energy spectrum of the system under consideration. The calculations are performed for the thermodynamic means that characterize jumps of electrons from a nanosystem site to a neighboring site, the correlation functions demonstrating the possibility of existing two d electrons with oppositely oriented spin projections on the same site of the fullerenes consisting of gold atoms. The optical absorption spectra are presented. The optical absorption peaks of ions Au{sub 20}{sup −} and Au{sub 16}{sup −} correspond to a near-infrared spectral region, where the light absorption by blood or a soft tissue is vanishingly small; thus, these ions can be used as a new class of contrast improvements and phototherapeutic means for diagnostics and treatment of cancer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Range-separated time-dependent density-functional theory with a frequency-dependent second-order Bethe-Salpeter correlation kernel

We present a range-separated linear-response time-dependent density-functional theory (TDDFT) which combines a density-functional approximation for the short-range response kernel and a frequency-dependent second-order Bethe-Salpeter approximation for the long-range response kernel. This approach goes beyond the adiabatic approximation usually used in linear-response TDDFT and aims at improving the accuracy of calculations of electronic excitation energies of molecular systems. A detailed derivation of the frequency-dependent second-order Bethe-Salpeter correlation kernel is given using many-body Green-function theory. Preliminary tests of this range-separated TDDFT method are presented for the calculation of excitation energies of the He and Be atoms and small molecules (H{sub 2}, N{sub 2}, CO{sub 2}, H{sub 2}CO, and C{sub 2}H{sub 4}). The results suggest that the addition of the long-range second-order Bethe-Salpeter correlation kernel overall slightly improves the excitation energies.

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

Classical mapping for Hubbard operators: Application to the double-Anderson model

A classical Cartesian mapping for Hubbard operators is developed to describe the nonequilibrium transport of an open quantum system with many electrons. The mapping of the Hubbard operators representing the many-body Hamiltonian is derived by using analogies from classical mappings of boson creation and annihilation operators vis-à-vis a coherent state representation. The approach provides qualitative results for a double quantum dot array (double Anderson impurity model) coupled to fermionic leads for a range of bias voltages, Coulomb couplings, and hopping terms. While the width and height of the conduction peaks show deviations from the master equation approach considered to be accurate in the limit of weak system-leads couplings and high temperatures, the Hubbard mapping captures all transport channels involving transition between many electron states, some of which are not captured by approximate nonequilibrium Green function closures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Calculations of multiquark functions in effective models of strong interaction

In this paper we present our results of the investigation of multiquark equations in the Nambu-Jona-Lasinio model with chiral symmetry of SU(2) group in the mean-field expansion. To formulate the mean-field expansion we have used an iteration scheme of solution of the Schwinger-Dyson equations with the fermion bilocal source. We have considered the equations for Green functions of the Nambu-Jona-Lasinio model up to third step for this iteration scheme. To calculate the high-order corrections to the mean-field approximation, we propose the method of the Legendre transformation with respect to the bilocal source, which allows effectively to take into account the symmetry constraints related with the chiral Ward identity. We discuss also the problem of calculating the multiquark functions in the mean-field expansion for Nambu-Jona-Lasinio-type models with other types of the multifermion sources.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Virial theorem and Gibbs thermodynamic potential for Coulomb systems

Using the grand canonical ensemble and the virial theorem, we show that the Gibbs thermodynamic potential of the non-relativistic system of charged particles is uniquely defined by single-particle Green functions of electrons and nuclei. This result is valid beyond the perturbation theory with respect to the interparticle interaction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

The effect of external magnetic field on the Raman peaks in manganites

We report here a microscopic theoretical model study exhibiting the effect of external magnetic field on the Raman excitation peaks in the CMR manganite system. The Hamiltonian consists of Jahn-Teller (J-T) distortion in e{sub g} band, the double exchange interaction and the Heisenberg spin-spin interaction. Further the phonons are coupled to e{sub g} band electrons, J-T distorted e{sub g} band and the double exchange interaction. The Raman spectral intensity is calculated from the imaginary part of the phonon Green function. The spectra exhibits three peaks besides a very weak high energy peak. The magnetic field effect on these peaks are reported.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Nitrogen-Doped Graphitic Nanoribbons: Synthesis, Characterization, and Transport

Nitrogen-doped graphitic nanoribbons (N x -GNRs), synthesized by chemical vapor deposition (CVD) using pyrazine as a nitrogen precursor, are reported for the first time. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) reveal that the synthesized materials are formed by multilayered corrugated GNRs, which in most cases exhibit the formation of curved graphene edges (loops). This suggests that during growth, nitrogen atoms promote loop formation; undoped GNRs do not form loops at their edges. Transport measurements on individual pure GNRs exhibit a linear I–V (current-voltage) behavior, whereas N x -GNRs show reduced current responses following a semiconducting-like behavior, which becomes more prominent for high nitrogen concentrations. To better understand the experimental findings, electron density of states (DOS), quantum conductance for nitrogen-doped zigzag and armchair single-layer GNRs are calculated for different N doping concentrations using density functional theory (DFT) and non-equilibrium Green functions. These calculations confirm the crucial role of nitrogen atoms in the transport properties, confirming that the nonlinear I–V curves are due to the presence of nitrogen atoms within the N x -GNRs lattice that act as scattering sites. These characteristic N x -GNRs transport properties could be advantageous in the fabrication of electronic devices including sensors in which metal-like undoped GNRs are unsuitable.

Chemistry↗