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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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

A projected approximation to strongly contracted N-electron valence perturbation theory for DMRG wavefunctions

A novel approach to strongly contracted N-electron valence perturbation theory (SC-NEVPT2) as a means of describing dynamic electron correlation for quantum chemical density matrix renormalization group (DMRG) calculations is presented. In this approach the strongly contracted perturber functions are projected onto a renormalized Hilbert space. Compared to a straightforward implementation of SC-NEVPT2 with DMRG wavefunctions, the computational scaling and storage requirements are reduced. This favorable scaling opens up the possibility of calculations with larger active spaces. A specially designed renormalization scheme ensures that both the electronic ground state and the perturber functions are well represented in the renormalized Hilbert space. Test calculations on the N{sub 2} and [Cu{sub 2}O{sub 2}(en){sub 2}]{sup 2+} demonstrate some key properties of the method and indicate its capabilities.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spin and charge thermopower effects in the ferromagnetic graphene junction

Using wave function matching approach and employing the Landauer-Buttiker formula, a ferromagnetic graphene junction with temperature gradient across the system is studied. We calculate the thermally induced charge and spin current as well as the thermoelectric voltage (Seebeck effect) in the linear and nonlinear regimes. Our calculation revealed that due to the electron-hole symmetry, the charge Seebeck coefficient is, for an undoped magnetic graphene, an odd function of chemical potential while the spin Seebeck coefficient is an even function regardless of the temperature gradient and junction length. We have also found with an accurate tuning external parameter, namely, the exchange filed and gate voltage, the temperature gradient across the junction drives a pure spin current without accompanying the charge current. Another important characteristic of thermoelectric transport, thermally induced current in the nonlinear regime, is examined. It would be our main finding that with increasing thermal gradient applied to the junction the spin and charge thermovoltages decrease and even become zero for non zero temperature bias.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Accessing the Spectral Function in a Current-Carrying Device

The presence of an electrical transport current in a material is one of the simplest and most important realizations of nonequilibrium physics. The current density breaks the crystalline symmetry and can give rise to dramatic phenomena, such as sliding charge density waves, insulator-to-metal transitions, or gap openings in topologically protected states. Almost nothing is known about how a current influences the electron spectral function, which characterizes most of the solid’s electronic, optical, and chemical properties. In this work, we show that angle-resolved photoemission spectroscopy with a nanoscale light spot provides not only a wealth of information on local equilibrium properties, but also opens the possibility to access the local nonequilibrium spectral function in the presence of a transport current. Unifying spectroscopic and transport measurements in this way allows simultaneous noninvasive local measurements of the composition, structure, many-body effects, and carrier mobility in the presence of high current densities. In the particular case of our graphene-based device, we are able to correlate the presence of structural defects with locally reduced carrier lifetimes in the spectral function and a locally reduced mobility with a spatial resolution of 500 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Emission of a Neutrino Pair During Transitions of an Electron in the Field of the Nucleus of a Hydrogen-Like Atom in One Spatial Dimension

The Dirac equation for a particle – an electron with negative charge equal in magnitude to the elementary charge e{sub 1} in spacetime with one spatial dimension denoted as the z axis and in the field of a point-like charge (Ze{sub 1}) – the nucleus – with the one-dimensional potential φ{sub 1} = – (Ze{sub 1}) |z| (a one-dimensional hydrogen-like atom) is solved. The two-component wave function and the quantum values of the energy are expressed in terms of the Airy function and its zeros. Using the contact Lagrangian of the weak interaction with Fermi constant G{sub 1} = g{sub 1}ħc in such a space, where g{sub 1} is some number, the probability of emission per unit time of a neutrino pair (Ze{sub 1}){sup *}→(Ze{sub 1}) + νν{sup –} by the one-dimensional hydrogen-like atom, which turns out to be proportional to the square of the mass, is found. Prospects for the realization of the considered effect and its possible significance at various stages of the evolution of the Universe are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Predicting Surface Energies and Particle Morphologies of Boehmite (y-AlOOH) from Density Functional Theory

Particle morphology is an important property affecting the reactivity of solid phases. The aluminum oxyhydroxide boehmite (?-AlOOH) is one example for which particle morphology has relevance to geochemistry, environmental management, and catalysis. Morphologies can be predicted from surface and attachment energies obtained from electronic structure calculations but the dependence of these predictions on the level of theory and complexity of the physics involved has been overlooked. Density functional theory calculations were therefore performed for a wide range of exchange-correlation (XC) functionals, including a hybrid functional, to calculate the energetics of the four main low-index surfaces of boehmite both in vacuum and with a water monolayer adsorbed. The approximation used for describing core electrons, the size of the plane-wave basis set, and the need for dispersion corrections were also investigated. The results highlight the critical need for converged plane-wave basis sets to obtain accurate surface energies as well as the strong influence the XC functional can have on surface energies and water adsorption energies. The predicted morphologies were compared to particle morphologies obtained from a variety of synthesis routes both carried out in this work and published in the literature. The comparison showed that, at basic pH, growth morphologies dominated at low temperatures and short time periods whereas equilibrium morphologies were achieved as the temperature and/or aging time increased, which could be explained by the slow stacking growth mode of boehmite along the [010] direction. Finally, calorimetry experiments are often used to derive surface energies, but excess water on hydrophilic surfaces can add significant uncertainty. An approach for correcting the surface energetics obtained by calorimetry using electronic structure calculations is presented.

Prange, Micah P.↗
DOE OSTI2019

A semiempirical effective Hamiltonian based approach for analyzing excited state wave functions and computing excited state absorption spectra using real-time dynamics

We describe a new approach to extract information about an excited state wave function using a reduced orbital space molecular orbital (MO) decomposition approach for time-dependent density obtained from real-time dynamics. We also show how this information about the excited state wave function can be used to accelerate the convergence of real-time spectra and model excited state electron dynamics. We have combined this approach with our recent implementation of the real-time INDO/S method to study the solvatochromic shift of Nile Red in acetone, ethanol, toluene and n-hexane solvents and, for the first time, the excited state absorption (ESA) spectra of coronene, 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP), zinc phthalocyanine (ZnPc), and nickel TPyP using a semiempirical Hamiltonian.

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

Time evolution of the distribution function for stochastically heated relativistic electrons in a laser field of picosecond duration

We report a numerical analysis of the stochastic acceleration of electrons, stipulated by a random change in the phase of the force acting on the electron. The main source of randomness is the random spatial distribution of electromagnetic fields in the focal region of multimode laser radiation. A typical frequency of the random phase change corresponding to the maximum impact of the effect under consideration lies in the range of (0.25–0.5)ν (ν is the radiation frequency of a neodymium laser). A wave packet model convenient for calculations taking into account the radiative transitions of the neodymium ion is proposed. The dependence of the average energy of relativistic electrons on the flux density in the range of 10{sup 15}–10{sup 18} W cm{sup -2} is calculated. The time dependence of the average electron energy during the laser pulse in the form of approximating formulas is constructed. The typical time for the development of stochastic heating of electrons is determined. It is found that the stochastic acceleration process weakly depends on the laser pulse duration, when the latter exceeds several hundred periods of the electromagnetic wave. (interaction of laser radiation with matter. laser plasma)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Ab Initio and Monte Carlo Approaches for the Magnetocaloric Effect in BaMnO{sub 3} Oxide Perovskite

The self-consistent ab initio calculations, based on density functional theory approach (DFT) and using full potential linearised augmented plane wave (FLAPW) method, have been used to investigate both electronic and magnetic properties of the BaMnO{sub 3} perovskite. Spin-polarised calculations, including the spin-orbit interaction, are used to determine the energy of the ferromagnetic (FM) and antiferromagnetic (AFM) states of BaMnO{sub 3} perovskite. Obtained data from ab initio calculations are used as input for the Monte Carlo simulations to compute other magnetic parameters. Magnetisation, specific heat and magnetic entropy change have been given using the Monte Carlo simulations. The adiabatic temperature change, transition temperature and relative cooling power have been established.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Competing itinerant and local spin interactions in kagome metal FeGe

The combination of a geometrically frustrated lattice, and similar energy scales between degrees of freedom endows two-dimensional Kagome metals with a rich array of quantum phases and renders them ideal for studying strong electron correlations and band topology. The Kagome metal, FeGe is a noted example of this, exhibiting A-type collinear antiferromagnetic (AFM) order at T N ≈ 400 K, then establishes a charge density wave (CDW) phase coupled with AFM ordered moment below T CDW ≈ 110 K, and finally forms a c-axis double cone AFM structure around T Canting ≈ 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure of FeGe at temperatures well above T Canting and T CDW that merge into gapped commensurate spin waves from the A-type AFM order. Commensurate spin waves follow the Bose factor and fit the Heisenberg Hamiltonian, while the incommensurate spin excitations, emerging below T N where AFM order is commensurate, start to deviate from the Bose factor around T CDW , and peaks at T Canting . This is consistent with a critical scattering of a second order magnetic phase transition with decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Interactions of electrons with two lower hybrid waves

The effects of perturbed orbits on the interactions of electrons with two lower hybrid waves, one of which is resonant with electrons at a low phase velocity (v p1 = 3.8V the , where v p1 is the wave phase velocity and V the is the electron thermal speed) while the other is off-resonant at a high phase velocity (v p2 = 5.5V the ), have been studied by using the particle simulation code based on the gyro-kinetic electron and fully-kinetic ion (GeFi) model [Lin et al., Plasma Phys. Controlled Fusion 47, 657 (2005)]. When the amplitude of the off-resonant wave is sufficiently small so that the resonances of these two waves do not overlap, the variation of the resonant wave amplitude is similar to that predicted by O'Neil's theory [O'Neil, Plasma Fluid 8, 12 (1965)]. With the increasing amplitude, the two resonances overlap and large scale chaos emerges. As a result, the damping of the resonant wave can be enhanced, which is due to that the trapped electron orbits are significantly perturbed by the off-resonant wave. The diffusion process gives rise to the enhanced damping. In conclusion, when the overlap is sufficiently large, the damping of the off-resonant wave and the oscillatory behavior of the wave amplitude are observed. In addition, the resonant plateau in the distribution function can be broadened due to the change in the chaotic region boundaries as the electron perturbed orbits are taken into account.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Spectral-partitioned Kohn-Sham density functional theory

Here we introduce a general, variational scheme for systematic approximation of a given Kohn-Sham free-energy functional by partitioning the density matrix into distinct spectral domains, each of which may be spanned by an independent diagonal representation without requirement of mutual orthogonality. It is shown that by generalizing the entropic contribution to the free energy to allow for independent representations in each spectral domain, the free energy becomes an upper bound to the exact (unpartitioned) Kohn-Sham free energy, attaining this limit as the representations approach Kohn-Sham eigenfunctions. A numerical procedure is devised for calculation of the generalized entropy associated with spectral partitioning of the density matrix. The result is a powerful framework for Kohn-Sham calculations of systems whose occupied subspaces span multiple energy regimes. As a case in point, we apply the proposed framework to warm- and hot-dense matter described by finite-temperature density functional theory, where at high energies the density matrix is represented by that of the free-electron gas, while at low energies it is variationally optimized. We derive expressions for the spectral-partitioned Kohn-Sham Hamiltonian, atomic forces, and macroscopic stresses within the projector-augmented wave (PAW) and the norm-conserving pseudopotential methods. It is demonstrated that at high temperatures, spectral partitioning facilitates accurate calculations at dramatically reduced computational cost. Moreover, as temperature is increased, fewer exact Kohn-Sham states are required for a given accuracy, leading to further reductions in computational cost. Finally, it is shown that standard multiprojector expansions of electronic orbitals within atomic spheres in the PAW method lack sufficient completeness at high temperatures. Spectral partitioning provides a systematic solution for this fundamental problem.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Spin–Orbit Interactions Greatly Accelerate Nonradiative Dynamics in Lead Halide Perovskites

In this study, we study the role of spin–orbit coupling (SOC) in nonradiative relaxation of hot electrons and holes in methylammonium lead perovskite, MAPbI 3 . For this purpose, we have developed the nonadiabatic molecular dynamics method with two-component spinor wave functions that are solutions of the relativistic Kohn–Sham (KS) equations. We find that SOC enhances contributions of Pb(p x ) and Pb(p y ) orbitals to the conduction and valence bands. As a result, the KS orbitals become more sensitive to nuclear motions, leading to the increased nonadiabatic couplings. Consequently, SOC greatly speeds up the electron and hole relaxation, making the computed relaxation time scales consistent with available experiments. Furthermore, we suggest that the fast hot carrier relaxation facilitated by the SOC allows rapid transition into the long-lived triplet state that extends charge-carrier lifetime and helps achieve high-efficiency perovskite solar cells.

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

Study of Electromagnetic Electron Cyclotron Waves for Kappa Distribution with AC Field in the Magnetosphere of Saturn

Parallel propagating electromagnetic electron cyclotron (EMEC) waves in the extended plasma sheet (~12R{sub S}) and in the outer magnetosphere (~18R{sub S}) of Saturn have been studied. A dispersion relation for parallel propagating relativistic EMEC waves has been applied to the magnetosphere of Saturn, and comparisons have been made with the data of Voyager 1 at these radial distances. The detailed investigations for EMEC waves have been done in the presence of the perpendicular AC electric field, using the kappa distribution function. The relativistic temporal growth rate is calculated by the method of characteristic solution with the data provided by Voyager 1. The effect of the suprathermal electron density, temperature anisotropy, frequency of AC electric field, thermal energy of ions, and relativistic factor on the temporal growth rate of EMEC wave emission has been studied. The simulation results show that the growth of parallel propagating EMEC waves is significantly affected by variations in the temperature anisotropy, electron density, ion thermal energy, and relativistic factor in both the extended plasma sheet and the outer magnetosphere of Saturn. The temperature anisotropy (T⊥/T{sub ║}), ion thermal energy (K{sub B}T{sub ║i}), and electron density (n{sub 0}) have been found to be a major source of free energy for parallel propagating EMEC waves in both regions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Localized excitons in defective monolayer germanium selenide

Germanium selenide (GeSe) is a van der Waals–bonded layered material with promising optoelectronic properties, which has been experimentally synthesized for two-dimensional (2D) semiconductor applications. In the monolayer, due to reduced dimensionality and, thus, screening environment, perturbations such as the presence of defects have a significant impact on its properties. We apply density functional theory and many-body perturbation theory to understand the electronic and optical properties of GeSe containing a single selenium vacancy in the –2 charge state. We predict that the vacancy results in midgap “trap states” that strongly localize the electron and hole density and lead to sharp, low-energy optical absorption peaks below the predicted pristine optical gap. Analysis of the exciton wave function reveals that the 2D Wannier-Mott exciton of the pristine material monolayer is strongly modified by the presence of the defect. The lowest-energy exciton wave function is highly localized around the defect, with the Bohr radius reduced by a factor of 4 when compared to the pristine material and a dipole moment along the out-of-plane axis due to the defect-induced symmetry breaking. Altogether, these results suggest that the vacancy significantly modifies the excitonic properties of the system, demonstrating the importance of considering defects in the context of material design.

2-dimensional systems↗
DOE OSTI2017

Multielectron effects in the photoelectron momentum distribution of noble-gas atoms driven by visible-to-infrared-frequency laser pulses: A time-dependent density-functional-theory approach

In this work, we present the photoelectron momentum distributions (PMDs) of helium, neon, and argon atoms driven by a linearly polarized, visible (527-nm) or near-infrared (800-nm) laser pulse (20 optical cycles in duration) based on the time-dependent density-functional theory (TDDFT) under the local-density approximation with a self-interaction correction. A set of time-dependent Kohn-Sham equations for all electrons in an atom is numerically solved using the generalized pseudospectral method. An effect of the electron-electron interaction driven by a visible laser field is not recognizable in the helium and neon PMDs except for a reduction of the overall photoelectron yield, but there is a clear difference between the PMDs of an argon atom calculated with the frozen-core approximation and TDDFT, indicating an interference of its M-shell wave functions during the ionization. Furthermore, we find that the PMDs of degenerate p states are well separated in intensity when driven by a near-infrared laser field, so that the single-active-electron approximation can be adopted safely.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Density Functional Study of Half-Metallic Ferromagnetism in the New Heusler Compounds CaYO{sub 2} (Y = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn)

The structural, electronic, and magnetic properties of full-Heusler alloys CaYO{sub 2} (Y = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) were investigated using full-potential linearized augmented plane wave method based on density functional theory. It was confirmed that these compounds can be synthesized experimentally due to their negative formation energies. The results predicted that all compounds in AlCu{sub 2}Mn-type structure were more stable than CuHg{sub 2}Ti-type structure. The band structure calculations showed that alloys CaYO{sub 2} (Y = Cr, Fe, Ni, and Cu) in AlCu{sub 2}Mn-type structure and CaYO{sub 2} (Y = Ti, V, Cr, Co, and Ni) in CuHg{sub 2}Ti-type structure have half metallic behavior. The origin of half-metallic characteristic was investigated using electronic density of states for CaFeO{sub 2}. Among these alloys, CaCrO{sub 2} was half-metal in a wide range of lattice constants indicating its resistance against destructive factors of half-metallicity such as temperature and pressure. The total magnetic moments for half-metallic alloys were integer values and obeyed the Slater-Pauling rules. The CaYO{sub 2} alloys with half-metallic character may be good candidates for practical applications in spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Effective medium theory for a system of C{sub 60} molecules

An effective medium theory is developed to study the effective permittivity of a system of C{sub 60} molecules. We use a two-dimensional, spherical, two-fluid hydrodynamic model to describe the linear response of the π and σ electrons over the each C{sub 60} molecule. A general expression for the electromagnetic wave attenuation coefficient of the system is then deduced, and its functional dependence on the filling factor is presented. Furthermore, the dispersion characteristics of electromagnetic oscillations of the system are studied.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗