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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Hyper Open-Shell Excited Spin States of Transition-Metal Compounds: FeF 2 , FeF 2 ···Ethane, and FeF 2 ···Ethylene

Spin-state energetics are important for understanding properties that involve more than one spin state, for example, catalysis occurring on two or more potential energy surfaces corresponding to different electronic spins. Very often, multiple-spin processes involve transition-metal compounds, and therefore, it is important to understand the electronic structure and energetics of such compounds in different spin states. In this work, we benchmark relative spin-state energies of FeF 2 with respect to the quintet ground spin state using both singleconfigurational and multi-configurational methods, and we examine how they are affected by the binding of ethane and ethylene to the iron center. We also benchmark the binding energies of the complexes. The single-configurational methods used in this work are the Hartree–Fock method, 32 exchange–correlation functionals, and the CCSD(T) coupled-cluster method in both restricted and unrestricted formalisms. The multi-configurational methods that have been used are CASSCF, CASPT2, CASPT3, MRCI, MRCI+Q, and MR-ACPF. The spin-state splitting energies depend on the functional chosen and of the 32 exchange-correlation functionals investigated here we find that for the septet and spin-projected triplet states of FeF 2 , the M06 functional is the best when compared to our best estimates from multi-reference calculations. If all the nine excitation energies are considered, where there are three excited spin states (singlet, triplet, and septet) for each of the three systems (FeF 2 , FeF 2 ···ethane, and FeF 2 ···ethylene), the three best performing functionals are HLE16, SOGGA11-X, and M06-2X. We find that the binding of ethane perturbs the relative spin-state energy of FeF 2 by only a small amount, but the stronger binding of ethylene has a larger effect. For the spin-state splitting energies of FeF 2 using single-reference CCSD(T), we found the predicted results depend very strongly on precisely how the calculations are done, in particular, on the spin-restricted or spin-unrestricted character of the SCF reference state, which can differ even by around 50 kcal/mol for the SCF reference state and the subsequent CCSD(T) calculations. Furthermore, on analyzing the wave functions of both the spinrestricted or spin-unrestricted formalisms, we find that the lowest-energy singlet and triplet states of the complexes, just like FeF 2 in isolation, can have more unpaired electrons than they are usually assumed to have, i.e. they can be hyper open-shell electronic configurations, and that this can significantly lower the energy.

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

Theoretical Investigation of Structural, Electronic, Magnetic, and Mechanical Properties of Quaternary Heusler Alloys CoVTiX (X = As, Si)

The structural, electronic, magnetic, and mechanical properties of CoVTiAs and CoVTiSi quaternary Heusler alloys are studied using the full-potential linearized augmented plane-wave (FP-LAPW) method in the framework of the density functional theory (DFT). The generalized gradient approximation (GGA) is chosen for the exchange-correlation energy, whereas the modified Becke-Johnson (mBJ) formalism is applied for the electronic properties. Both CoVTiAs and CoVTiSi are stable in the type 2 (FM) structure. The mechanical properties of CoVTiX (X = As, Si) are predicted from the calculated elastic constants. Results identify that these compounds are mechanically stable. Our results with the mBJ approximation predict that CoVTiAs and CoVTiSi alloys are half-metallic ferromagnets with band gaps of 0.450 and 0.424 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Electronic structure of strongly correlated systems: recent developments in multiconfiguration pair-density functional theory and multiconfiguration nonclassical-energy functional theory

Strong electron correlation plays an important role in transition-metal and heavy-metal chemistry, magnetic molecules, bond breaking, biradicals, excited states, and many functional materials, but it provides a significant challenge for modern electronic structure theory. The treatment of strongly correlated systems usually requires a multireference method to adequately describe spin densities and near-degeneracy correlation. However, quantitative computation of dynamic correlation with multireference wave functions is often difficult or impractical. Multiconfiguration pair-density functional theory (MC-PDFT) provides a way to blend multiconfiguration wave function theory and density functional theory to quantitatively treat both near-degeneracy correlation and dynamic correlation in strongly correlated systems; it is more affordable than multireference perturbation theory, multireference configuration interaction, or multireference coupled cluster theory and more accurate for many properties than Kohn–Sham density functional theory. This perspective article provides a brief introduction to strongly correlated systems and previously reviewed progress on MC-PDFT followed by a discussion of several recent developments and applications of MC-PDFT and related methods, including localized-active-space MC-PDFT, generalized active-space MC-PDFT, density-matrix-renormalization-group MC-PDFT, hybrid MC-PDFT, multistate MC-PDFT, spin–orbit coupling, analytic gradients, and dipole moments. We also review the more recently introduced multiconfiguration nonclassical-energy functional theory (MC-NEFT), which is like MC-PDFT but allows for other ingredients in the nonclassical-energy functional. We discuss two new kinds of MC-NEFT methods, namely multiconfiguration density coherence functional theory and machine-learned functionals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Reinforcement Learning Configuration Interaction

Selected configuration interaction (sCI) methods exploit the sparsity of the full configuration interaction (FCI) wave function, yielding significant computational savings and wave function compression without sacrificing the accuracy. Despite recent advances in sCI methods, the selection of important determinants remains an open problem. Furthermore, we explore the possibility of utilizing reinforcement learning approaches to solve the sCI problem. By mapping the configuration interaction problem onto a sequential decision-making process, the agent learns on-the-fly which determinants to include and which to ignore, yielding a compressed wave function at near-FCI accuracy. This method, which we call reinforcement-learned configuration interaction, adds another weapon to the sCI arsenal and highlights how reinforcement learning approaches can potentially help solve challenging problems in electronic structure theory.

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

Investigation of Substitution of Cu and Se Atoms on the Electronic and Magnetic Properties of GaN Monolayer Nanosheet

In this paper, some of the electronic properties of GaN monolayer nanosheet are investigated in its pure form and after substitution of Cu and Se atoms. Electronic properties have been calculated based on the density functional theory approach with full-potential linearized augmented plane wave method. The results show that GaN nanosheet is a nonmagnetic semiconductor and its energy gap is equal to 2.202 eV. Our results show that GaN nanosheet properties vary with substitution of Cu or Se atoms: GaSeN and GaCuN show half-metallic and metallic behaviors, respectively. Calculated magnetic moments for GaSeN and GaCuN are 1.00 and 1.127 µ{sub B}, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Decisive role of magnetism in the interaction of chromium and nickel solute atoms with 1/2$\langle$111$\rangle$-screw dislocation core in body-centered cubic iron

Resistance to swelling under irradiation and a low rate of corrosion in high temperature environments make Fe-Cr and Fe-Cr-Ni alloys promising structural materials for energy technologies. In this paper we report the results obtained using a combination of density functional theory (DFT) techniques: plane wave basis set solutions for pseudo-potentials and multiple scattering solutions for all electron potentials. We have found a very strong role of magnetism in the stability of screw dislocation cores in pure Fe and their interaction with Cr and Ni magnetic impurities. In particular, the screw dislocation quadrupole in Fe is stabilized only in the presence of ferromagnetism. In addition, Ni atoms, who's magnetic moment is oriented along the magnetization direction of the Fe matrix, prefer to occupy in core positions whereas Cr atoms, which couple anti-ferromagnetically with the Fe matrix, prefer out of the dislocation core positions. In effect, Ni impurities are attracted to, while Cr impurities are repelled by the dislocation core. Moreover, we demonstrate that this contrasting behavior can be explained only by the nature of magnetic coupling of the impurities to the Fe matrix. In addition, Cr interaction with the dislocation core mirrors that of Ni if the Cr magnetic moment is constrained to be along the direction of Fe matrix magnetization. In addition, we have shown that the magnetic contribution can affect the impurity-impurity interaction at distances up to a few Burgers vectors. In particular, the distance between Cr atoms in Fe matrix should be at least 3–4 lattice parameters in order to eliminate finite size effects.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the self-interference in electron scattering: Copenhagen, Bohmian and geometrical interpretations of quantum mechanics

Self-interference embodies the essence of the particle-wave formulation of quantum mechanics (QM). According to the Copenhagen interpretation of QM, self-interference by a double-slit requires a large transverse coherence of the incident wavepacket such that it covers the separation between the slits. Bohmian dynamics provides a first step in the separation of the particle-wave character of matter by introducing deterministic trajectories guided by a pilot wave that follows the time-dependent Schrödinger equation. In this work, I present a new description of the phenomenon of self-interference using the geometrical formulation of QM introduced in Tavernelli (2016). In particular, this formalism removes the need for the concept of wavefunction collapse in the interpretation of the act of measurement i.e., the emergence of the classical world. The three QM formulations (Schrödinger, Bohmian, and geometrical) are applied to the description of the scattering of a free electron by a hydrogen atom and a double-slit. The corresponding interpretations of self-interference are compared and discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Electronic and Vibrational Contributions to the Bulk Stabilities of Trivalent 3d Transition Metal Oxyhydroxides from Electronic Structure Calculations

Any understanding of the thermodynamic driving forces active in the global cycles of earth abundant metals requires attendant knowledge of the bulk mineral free energies. This can be particularly challenging for metal (oxyhydr)oxide phases which at low temperature aqueous conditions can be produced at a variety of hydration states and particle sizes. Here we present high quality density functional theory calculations of the relative bulk stabilities of a geologically important family of polymorphic trivalent metal oxyhydroxide crystal structures MOOH (M=Cr, Al, Fe). To improve understanding of their relative stabilities we permute the metal cations into the various possible canonical structure types including goethite (isostructural to diaspore and bracewillite), lepidocrocite (isosctructural to boehmite), guyaniite, and grimaldiite. Electronic, vibrational, and magnetic contributions are considered on a consistent theoretical footing using hybrid functionals and converged plane-wave bases. We find the considered bulk structures to be thermodynamically degenerate within the precision of the available theory, despite the fact that the vibrational and magnetic contributions vary substantially among the cations. A significant conclusion is that Cr3+ supports strong hydrogen bonds, low O-H stretching frequencies and facile H movement relative to the other cations.

Prange, Micah P.↗
DOE OSTI2017

Quantitative estimation of localization errors of 3d transition metal pseudopotentials in diffusion Monte Carlo

The necessarily approximate evaluation of non-local pseudopotentials in diffusion Monte Carlo (DMC) introduces localization errors. In this paper, we estimate these errors for two families of non-local pseudopotentials for the first-row transition metal atoms Sc–Zn using an extrapolation scheme and multideterminant wavefunctions. Sensitivities of the error in the DMC energies to the Jastrow factor are used to estimate the quality of two sets of pseudopotentials with respect to locality error reduction. The locality approximation and T-moves scheme are also compared for accuracy of total energies. After estimating the removal of the locality and T-moves errors, we present the range of fixed-node energies between a single determinant description and a full valence multideterminant complete active space expansion. The results for these pseudopotentials agree with previous findings that the locality approximation is less sensitive to changes in the Jastrow than T-moves yielding more accurate total energies, however not necessarily more accurate energy differences. For both the locality approximation and T-moves, we find decreasing Jastrow sensitivity moving left to right across the series Sc–Zn. The recently generated pseudopotentials of Krogel et al. reduce the magnitude of the locality error compared with the pseudopotentials of Burkatzki et al. by an average estimated 40% using the locality approximation. The estimated locality error is equivalent for both sets of pseudopotentials when T-moves is used. Finally, for the Sc–Zn atomic series with these pseudopotentials, and using up to three-body Jastrow factors, our results suggest that the fixed-node error is dominant over the locality error when a single determinant is used.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Interacting quantum and classical waves: Resonant and non-resonant energy transfer to electrons immersed in an intense electromagnetic wave

Dynamics of electrons subjected to a constant amplitude classical electromagnetic (EM) wave is investigated as a fundamental, representative problem in the physics of interacting quantum and classical waves. In the nonrelativistic regime (electrons as Schrödinger waves), the electron energy acquires a constant and a time dependent part. Driven by EM waves, both parts scale strongly with the amplitude, but we expect no resonant enhancement since the parallel electron “speed” of nonrelativistic electrons could never match the wave phase velocity. In the relativistic regime (electron as a Klein–Gordon wave), however, a class of electron waves (with parallel speed matching the EM phase speed) are resonantly excited to extremely high energies. Such a direct resonant energy transfer from intense electromagnetic waves constitutes a mechanism that could, in principle, power the most energetic of cosmic rays (this mechanism will work on protons just as well). Some predictions of the theory will, hopefully, be tested in laboratory laser experiments. In conclusion, the nonrelativistic calculations will also be examined in the context of recent experiments using photon-induced near-field electron microscopy in detail.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Trions in twisted bilayer graphene

The strong-coupling phase diagram of magic-angle twisted bilayer graphene (TBG) predicts a series of exact one-particle charge ±1 gapped excitations on top of the integer-filled ferromagnetic ground states. Finite-size exact diagonalization studies showed that these are the lowest charge ±1 excitations in the system (for 10 nm screening length), with the exception of charge +1 at filling -1 in the chiral limit. In the current paper we show that this “trion bound state,” a 3-particle, charge 1 excitation of the insulating ferromagnetic ground state of the projected Hamiltonian of TBG, is the lowest charge +1 overall excitation at ν =-1, and also for some large (≈20 nm) screening lengths at ν =-2 in the chiral limit and with very small binding energy. At other fillings, we show that trion bound states do exist, but only for momentum ranges that do not cover the entire moiré Brillouin zone. The trion bound states (at different momenta) exist for finite parameter range w 0 /w 1 but they all disappear in the continuum far below the realistic values of w 0 /w 1 = 0.8. Thus we find the conditions for the existence of the trion bound state, a good variational wave function for it, and investigate its behavior for different screening lengths, at all integer fillings, on both the electron and hole sides.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electronic band structure effects in the stopping of protons in copper [Electronic band structure non-linear effects in the stopping of protons in copper]

Here, we present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities v = 0.02–10a.u. where we take into account nonlinear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results are in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of ~1.5) in the velocity range v = 0.07–0.3a.u., which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping v → 0 is not as simple as phenomenological models suggest and it is plagued by band-structure effects.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Simulation of Plasma Emission in Magnetized Plasmas

The recent Parker Solar Probe observations of type III radio bursts show that the effects of the finite background magnetic field can be an important factor in the interpretation of data. In the present paper, the effects of the background magnetic field on the plasma-emission process, which is believed to be the main emission mechanism for solar coronal and interplanetary type III radio bursts, are investigated by means of the particle-in-cell simulation method. The effects of the ambient magnetic field are systematically surveyed by varying the ratio of plasma frequency to electron gyrofrequency. The present study shows that for a sufficiently strong ambient magnetic field, the wave–particle interaction processes lead to a highly field-aligned longitudinal mode excitation and anisotropic electron velocity distribution function, accompanied by a significantly enhanced plasma emission at the second-harmonic plasma frequency. For such a case, the polarization of the harmonic emission is almost entirely in the sense of extraordinary mode. On the other hand, for moderate strengths of the ambient magnetic field, the interpretation of the simulation result is less clear. The underlying nonlinear-mode coupling processes indicate that to properly understand and interpret the simulation results requires sophisticated analyses involving interactions among magnetized plasma normal modes, including the two transverse modes of the magneto-active plasma, namely, the extraordinary and ordinary modes, as well as electron-cyclotron-whistler, plasma oscillation, and upper-hybrid modes. At present, a nonlinear theory suitable for quantitatively analyzing such complex-mode coupling processes in magnetized plasmas is incomplete, which calls for further theoretical research, but the present simulation results could provide a guide for future theoretical efforts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A free-electron model for the Dirac bands in graphene

Here, we present a new method for describing the electronic structure of graphene, by treating the honeycomb lattice as an arrangement of crisscrossing one-dimensional quantum wires. The electrons travel as free particles along the wires and interfere at the three-way junctions formed by the carbon atoms. The approach produces the linearly dispersive Dirac band structure as well as the chiral pseudo-spin-wave functions. When vacancies are incorporated, the model reproduces the well known zero mode states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Time-dependent density functional theory with the orthogonal projector augmented wave method

The projector augmented wave (PAW) method of Blöchl linearly maps smooth pseudo wavefunctions to the highly oscillatory all-electron DFT orbitals. Compared to norm-conserving pseudopotentials (NCPP), PAW has the advantage of lower kinetic energy cutoffs and larger grid spacing at the cost of having to solve for non-orthogonal wavefunctions. We earlier developed orthogonal PAW (OPAW) to allow the use of PAW when orthogonal wavefunctions are required. In OPAW, the pseudo wavefunctions are transformed through the efficient application of powers of the PAW overlap operator with essentially no extra cost compared to NCPP methods. Previously, we applied OPAW to DFT. Here, we take the first step to make OPAW viable for post-DFT methods by implementing it in real-time time-dependent (TD) DFT. Using fourth-order Runge–Kutta for the time-propagation, we compare calculations of absorption spectra for various organic and biological molecules and show that very large grid spacings are sufficient, 0.6–0.7 bohr in OPAW-TDDFT rather than the 0.4–0.5 bohr used in traditional NCPP-TDDFT calculations. This reduces the memory and propagation costs by around a factor of 3. Our method would be directly applicable to any post-DFT methods that require time-dependent propagations such as the GW approximation and the Bethe–Salpeter equation.

Chemistry↗
DOE OSTI2018

First-Principle Study of Half-Metallic Ferrimagnet Behavior in Titanium-Based Heusler Alloys Ti{sub 2}FeZ (Z = Al, Ga, and In)

Using density functional theory with the full-potential linearized augmented plane-wave method (FP-LAPW), we have study the structural, electronic, and magnetic properties of Ti{sub 2}FeZ (Z = Al, Ga, and In) alloys with Hg{sub 2}CuTi-type structure. The magnetic stabilities reveal that all our compounds exhibit ferrimagnetic (FiM) behaviors. The electronic structure report the existence of a gap energy equal to 0.56, 0.60, and 0.64 eV for Ti{sub 2}FeAl, Ti{sub 2}FeGa, and Ti{sub 2}FeIn, respectively, in the spin-down state and divulge metallic intersections at the Fermi level for the spin-up state. These results indicate that our compounds have a half-metallic (HM) nature. In addition to this, the total magnetic moments are in agreement with the obtained one by the Slater-Pauling rule (M{sub tot} = Z{sub tot} − 18), which indicates the 100% spin polarization for these compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

EPR and optical absorption study of Cu{sup 2+} doped lithium sulphate monohydrate (LSMH) single crystals

EPR study of Cu{sup 2+} doped NLO active Lithium Sulphate monohydrate (Li{sub 2}SO{sub 4.}H{sub 2}O) single crystals were grown successfully by slow evaporation method at room temperature. The principal values of g and A tensors indicate existence of orthorhombic symmetry around the Cu{sup 2+} ion. From the direction cosines of g and A tensors, the locations of Cu{sup 2+} in the lattice have been identified as interstitial site. Optical absorption confirms the rhombic symmetry and ground state wave function of the Cu{sup 2+} ion in a lattice as d{sub x2-y2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

The Effects of Ru and Rh Substitutions on the Magneto-electronic and Optical Properties of the TbNi{sub 5} Intermetallic Compound: An Ab Initio Investigation

In this study, we have performed the first-principles investigation of the structural, electronic, magnetic, and optical properties of TbNi{sub 5}, TbNi{sub 3}Ru{sub 2}, and TbNi{sub 3}Rh{sub 2} compounds. The full-potential linearized augmented plane waves with local orbitals method is used in the framework of density functional theory (DFT) employing the generalized gradient approximation (GGA) for the exchange correlation functional as implemented in WIEN2k package. The structural properties are reposed on the evaluation of the equilibrium lattice parameters of these compounds under hexagonal structure such as lattice constants (a and c), bulk modulus (B), and its first pressure derivative (B{sup ′}). The spin-polarized electronic structures, including band structure and density of states, are calculated employing the GGA plus band correlated Hubbard parameter (GGA + U) scheme. The results show that density of states and magnetic moment of the pure TbNi{sub 5} compound are changed by doping. These changes are observed in the appearance of additional peaks on the spectral density of states (DOS) and in the augmentation of the total magnetic moment of TbNi{sub 3}X{sub 2} (X = Ru and Rh) intermetallic compounds. Based on the electronic structure results, the frequency dependents of optical conductivity are estimated in all the spectra and interpreted in the interband optical absorption part.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗