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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Spin-dependent electron transport in zinc- and manganese-doped adenine molecules

The spin-dependent electron transport properties of zinc- and manganese-doped adenine molecules connected to zigzag graphene leads are studied in the zero bias regime using the non-equilibrium Green's function method. The conductance of the adenine molecule increased and became spin-dependent when a zinc or manganese atom was doped into the molecules. The effects of a transverse electric field on the spin-polarization of the transmitted electrons were investigated and the spin-polarization was controlled by changing the transverse electric field. Under the presence of a transverse electric field, both the zinc- and manganese-doped adenine molecules acted as spin-filters. The maximum spin-polarization of the manganese-doped adenine molecule was greater than the molecule doped with zinc.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2013

Atomic scale investigations of the gate controlled tunneling effect in graphyne nanoribbon

Configuration and transport properties of zigzag graphyne nanoribbon (n = 2) are investigated by means of the first-principles calculations and non-equilibrium Green's function in this work. We demonstrated the controllability of the graphyne's conductivity by gate bias, and the tunneling behavior induced by gate and drain voltages was investigated systemically. The characteristics of I{sub d}-V{sub d}, I{sub d}-V{sub g}, as well as the evolutions of current with electron temperature elevation were explored. The device exhibits a tunneling ratio around 10{sup 3}, and the state art of tunneling operations of the tunneling field effect transistor in this split-new material was achieved.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Electrically tunable localized tunneling channels in silicene nanoribbons

The topological phase of a silicene nanoribbon holding edge states in the bulk energy gap can be easily broken by an external electric field. Here, we show through low-energy Green's function calculations that it is possible to localize conducting channels anywhere in a silicene nanoribbon by applying an inhomogeneous electric field. The spin degeneracy of these channels can also be broken in the same manner, allowing conduction of spin as well as charge. On this basis, we suggest design of a ternary logic device, which could be used in low-power circuits. Our study demonstrates that silicene and related group IV elements with honeycomb structure could provide a platform for efficient manipulation of spin currents via external electric fields, without the need to switch magnetic fields for spintronics applications.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Coherent light scattering of heterogeneous randomly rough films and effective medium in the theory of electromagnetic wave multiple scattering

We have developed a general formalism based on Green's functions to calculate the coherent electromagnetic field scattered by a random medium with rough boundaries. The approximate expression derived makes it possible to determine the effective permittivity, which is generalised for a layer of an inhomogeneous random medium with different types of particles and bounded with randomly rough interfaces. This effective permittivity describes the coherent propagation of an electromagnetic wave in a random medium with randomly rough boundaries. We have obtained an expression, which contains the Maxwell – Garnett formula at the low-frequency limit, and the Keller formula; the latter has been proved to be in good agreement with experiments for particles whose dimensions are larger than a wavelength. (coherent light scattering)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Microscopic optical potentials for calcium isotopes

We construct nucleonic microscopic optical potentials by combining the Green's function approach with the coupled-cluster method for 40 Ca and 48 Ca. For the computation of the ground states of 40 Ca and 48 Ca, we use the coupled-cluster method in the singles-and-doubles approximation, while for the A = ±1 nuclei we use particle-attached and particle-removed equation-of-motion method truncated at two-particle–one-hole and one-particle–two-hole excitations, respectively. Our calculations are based on the chiral nucleon-nucleon and three-nucleon interaction NNLO sat , which reproduces the charge radii of 40 Ca and 48 Ca, and the chiral nucleon-nucleon interaction NNLO opt . In all cases considered here, we observe that the overall form of the neutron scattering cross section is reproduced for both interactions, but the imaginary part of the potential, which reflects the loss of flux in the elastic channel, is negligible. Furthermore, the latter points to neglected many-body correlations that would appear beyond the coupled-cluster truncation level considered in this work. We show that, by artificially increasing the parameter η in the Green's function, practical results can be further improved.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Shot noise of charge current in a quantum dot responded by rotating and oscillating magnetic fields

We have investigated the shot noise and Fano factor of the dynamic spin-polarized quantum dot under the perturbations of a rotating magnetic field (RMF), and an oscillating magnetic field (OMF) by employing the non-equilibrium Green's function approach. The shot noise is enhanced from sub-Poissonian to super-Poissonian due to the application of RMF and OMF, and it is controlled sensitively by the tilt angle θ of RMF. The magnitude of shot noise increases as the photon energy ℏω of OMF increases, and its valley eventually is reversed to peaks as the photon energy is large enough. Double-peak structure of Fano factor is exhibited as the frequency of OMF increases to cover a large regime. The Zeeman energy μ{sub 0}B{sub 0} acts as an effective gate bias to exhibit resonant behavior, and novel peak emerges associated with the applied OMF.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Influence of heterogeneous sulfur atoms on the negative differential resistance effect in polythiophene

In this work, we have carried out theoretical investigations aiming to clarify the effects of sulfur heteroatoms on the transport characteristics in polythiophene. Sulfur atoms in polythiophene are demonstrated to influence the structure and transport process by two aspects: the electron hopping between carbon atoms on both sides of the sulfur atom as well as the effective confinement of π electrons from the sulfur atom. Based on the static Su-Schrieffer-Heeger model and the nonequilibrium Green's function formalism, we simulate the electron transportation in a metal/polythiophene/metal structure. The simulation results show that the electron hopping via sulfur atoms is responsible for the observed negative differential resistance (NDR) behavior in the I-V curves. The NDR disappears if the electron transport channels from carbon to carbon via sulfur atoms are forbidden. The weaker the effective confinement of π electrons and the electron hopping between carbon atoms on both sides of the sulfur atom are, the higher is the peak-to-valley ratio of the NDR and the wider the voltage range where the current remains at low levels. These results can help in understanding the NDR effect in polythiophene.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Equation of motion coupled cluster methods for electron attachment and ionization potential in fullerenes C{sub 60} and C{sub 70}

In both molecular and periodic solid-state systems there is a need for the accurate determination of the ionization potential and the electron affinity for systems ranging from light harvesting polymers and photocatalytic compounds to semiconductors. The development of a Green's function approach based on the coupled cluster (CC) formalism would be a valuable tool for addressing many properties involving many-body interactions along with their associated correlation functions. As a first step in this direction, we have developed an accurate and parallel efficient approach based on the equation of motion-CC technique. To demonstrate the high degree of accuracy and numerical efficiency of our approach we calculate the ionization potential and electron affinity for C{sub 60} and C{sub 70}. Accurate predictions for these molecules are well beyond traditional molecular scale studies. We compare our results with experiments and both quantum Monte Carlo and GW calculations.

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

Electron-phonon interaction in three-barrier nanosystems as active elements of quantum cascade detectors

The theory of electron tunneling through an open nanostructure as an active element of a quantum cascade detector is developed, which takes into account the interaction of electrons with confined and interface phonons. Using the method of finite-temperature Green’s functions and the electron-phonon Hamiltonian in the representation of second quantization over all system variables, the temperature shifts and electron-level widths are calculated and the contributions of different electron-phonon-interaction mechanisms to renormalization of the spectral parameters are analyzed depending on the geometrical configuration of the nanosystem. Due to weak electron-phonon coupling in a GaAs/Al{sub 0.34}Ga{sub 0.66}As-based resonant tunneling nanostructure, the temperature shift and rf field absorption peak width are not very sensitive to the electron-phonon interaction and result from a decrease in potential barrier heights caused by a difference in the temperature dependences of the well and barrier band gaps.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Phonon-limited low-field mobility in silicon: Quantum transport vs. linearized Boltzmann Transport Equation

We propose to check and validate the approximations made in dissipative quantum transport (QT) simulations solved in the Non-equilibrium Green's Function formalism by comparing them with the exact solution of the linearized Boltzmann Transport Equation (LB) in the stationary regime. For that purpose, we calculate the phonon-limited electron and hole mobility in bulk Si and ultra-scaled Si nanowires for different crystal orientations 〈100〉, 〈110〉, and 〈111〉. In both QT and LB simulations, we use the same sp{sup 3}d{sup 5}s{sup *} tight-binding model to describe the electron/hole properties and the same valence-force-field approach to account for the phonon properties. It is found that the QT simplifications work well for electrons, but are less accurate for holes, where a renormalization of the phonon scattering strength is proved useful to improve the results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Nanoscale transport of phonons: Dimensionality, subdiffusion, molecular damping, and interference effects

We examine heat transport carried by acoustic phonons in the systems composed of nanoscale chains of masses coupled to two thermal baths of different temperatures. Thermal conductance is obtained by using linearized Landauer-type formula for heat flux with phonon transmission probability calculated within atomistic Green's functions (AGF) method. AGF formalism is extended onto dissipative chains of masses with harmonic coupling beyond nearest-neighbor approximation, while atomistic description of heat reservoirs is also included into computational scheme. In particular, the phonon lifetimes and the phonon frequency shifts are discussed for harmonic lattices of different dimensions. Further, resonant structure of phonon transmission spectrum is analyzed with respect to reservoir-induced effects, molecular damping, and mass-to-mass harmonic coupling. Analysis of transmission zeros (antiresonances) and their accompanied Fano-shape resonances are discussed as a result of interference effects between different vibrational modes. Finally, we also predict subdiffusive transport regime for low-frequency ballistic phonons propagated along a linear chain of harmonically coupled masses.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Atomistic modeling of phonon transport in turbostratic graphitic structures

Thermal transport in turbostratic graphitic systems is investigated by using an atomistic analytical model based on the 4th-nearest-neighbor force constant approximation and a registry-dependent interlayer potential. The developed model is shown to produce an excellent agreement with the experimental data and ab initio results in the calculation of bulk properties. Subsequent analysis of phonon transport in combination with the Green's function method illustrates the significant dependence of key characteristics on the misorientation angle, clearly indicating the importance of this degree of freedom in multi-stacked structures. Selecting three angles with the smallest commensurate unit cells, the thermal resistance is evaluated at the twisted interface between two AB stacked graphite. The resulting values in the range of 35 × 10{sup −10} K m{sup 2}/W to 116 × 10{sup −10} K m{sup 2}/W are as large as those between two dissimilar material systems such as a metal and graphene. The strong rotational effect on the cross-plane thermal transport may offer an effective means of phonon engineering for applications such as thermoelectric materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Models of the delayed nonlinear Raman response in diatomic gases

We examine the delayed response of a diatomic gas to a polarizing laser field with the goal of obtaining computationally efficient methods for use with laser pulse propagation simulations. We demonstrate that for broadband pulses, heavy molecules such as O{sub 2} and N{sub 2}, and typical atmospheric temperatures, the initial delayed response requires only classical physics. The linear kinetic Green's function is derived from the Boltzmann equation and shown to be in excellent agreement with full density-matrix calculations. A straightforward perturbation approach for the fully nonlinear, kinetic impulse response is also presented. With the kinetic theory a reduced fluid model of the diatomic gas' orientation is derived. Transport coefficients are introduced to model the kinetic phase mixing of the delayed response. In addition to computational rapidity, the fluid model provides intuition through the use of familiar macroscopic quantities. Both the kinetic and the fluid descriptions predict a nonlinear steady-state alignment after passage of the laser pulse, which in the fluid model is interpreted as an anisotropic temperature of the diatomic fluid with respect to motion about the polarization axis.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

The effect of driven electron-phonon coupling on the electronic conductance of a polar nanowire

A semi-classical model is proposed to explore the effect of electron-phonon coupling on the coherent electronic transport of a polar chain which is confined between two rigid leads in the presence of an external electric field. To this end, we construct the model by means of Green's function technique within the nearest neighbor tight-binding and harmonic approximations. For a time-periodic electric field, the atomic displacements from the equilibrium positions are obtained precisely. The result is then used to compute the electronic transport properties of the chain within the Peierls-type model. The numerical results indicate that the conductance of the system shows interesting behavior in some special frequencies. For each special frequency, there is an electronic quasi-state in which the scattering of electrons by vibrating atoms reaches maximum. The system electronic conductance decreases dramatically at the strong electron-phonon couplings and low electron energies. In the presence of damping forces, the electron-phonon interaction has a less significant effect on the conductance.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Photon assisted tunneling through three quantum dots with spin-orbit-coupling

The effect of an ac electric field on quantum transport properties in a system of three quantum dots, two of which are connected in parallel, while the third is coupled to one of the other two, is investigated theoretically. Based on the Keldysh nonequilibrium Green's function method, the spin-dependent current, occupation number, and spin accumulation can be obtained in our model. An external magnetic flux, Rashba spin-orbit-coupling (SOC), and intradot Coulomb interactions are considered. The magnitude of the spin-dependent average current and the positions of the photon assisted tunneling (PAT) peaks can be accurately controlled and manipulated by simply varying the strength of the coupling and the frequency of the ac field. A particularly interesting result is the observation of a new kind of PAT peak and a multiple-PAT effect that can be generated and controlled by the coupling between the quantum dots. In addition, the spin occupation number and spin accumulation can be well controlled by the Rashba SOC and the magnetic flux.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Quantum conductance of zigzag graphene oxide nanoribbons

The electronic properties of zigzag graphene oxide nanoribbons (ZGOR) are presented. The results show interesting behaviors which are considerably different from the properties of the perfect graphene nanoribbons (GNRs). The theoretical methods include a Huckel-tight binding approach, a Green's function methodology, and the Landauer formalism. The presence of oxygen on the edge results in band bending, a noticeable change in density of states and thus the conductance. Consequently, the occupation in the valence bands increase for the next neighboring carbon atom in the unit cell. Conductance drops in both the conduction and valence band regions are due to the reduction of allowed k modes resulting from band bending. The asymmetry of the energy band structure of the ZGOR is due to the energy differences of the atoms. The inclusion of a foreign atom's orbital energies changes the dispersion relation of the eigenvalues in energy space. These novel characteristics are important and valuable in the study of quantum transport of GNRs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

THE INFLUENCE OF KLEIN-NISHINA STEPS ON THE SPATIAL DIFFUSION OF GALACTIC COSMIC-RAY ELECTRONS

The full Klein-Nishina cross section for the inverse Compton scattering interactions of electrons implies a significant reduction of the electron energy loss rate compared to the Thomson limit when the electron energy exceeds the critical Klein-Nishina energy E{sub K} = 0.27m{sup 2}{sub e} c{sup 4}/(k{sub B}T), where T denotes the temperature of the photon graybody distribution. We investigate the influence of the Klein-Nishina reduction on the solution of the steady-state spatial diffusion transport equation for relativistic electrons. The modified electron spectrum at energies below 10{sup 4} GeV, where only the Klein-Nishina modifications from interstellar optical target photons are relevant, are derived in terms of the Green's function solution for one-, two-, and three-dimensional spatial diffusion. The modifications to the solutions of the one- and three-dimensional diffusion equations are calculated for a single point source of monoenergetic electrons. It is shown that significant enhancements in the local electron intensity occur at electron energies greater than the critical Klein-Nishina energy E{sub K} , and that the cosmic-ray electron anisotropy at the solar system resulting from a single steady point source exhibits a sharp drop near E{sub K} . These Klein-Nishina enhancements are potentially interesting for determining the contribution of point sources such as dark matter sources and/or electromagnetic particle accelerators (pulsars and micro-quasars) to the local electron intensity and the local positron fraction.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Electron- and photon-impact ionization of furfural

The He(I) photoelectron spectrum of furfural has been investigated, with its vibrational structure assigned for the first time. The ground and excited ionized states are assigned through ab initio calculations performed at the outer-valence Green’s function level. Triple differential cross sections (TDCSs) for electron-impact ionization of the unresolved combination of the 4a″ + 21a′ highest and next-highest occupied molecular orbitals have also been obtained. Experimental TDCSs are recorded in a combination of asymmetric coplanar and doubly symmetric coplanar kinematics. The experimental TDCSs are compared to theoretical calculations, obtained within a molecular 3-body distorted wave framework that employed either an orientation average or proper TDCS average. The proper average calculations suggest that they may resolve some of the discrepancies regarding the angular distributions of the TDCS, when compared to calculations employing the orbital average.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗