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Results for “density functional approximations”

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DOE OSTI2020

Comparison of GW band structure to semiempirical approach for an FeSe monolayer

We present the G 0 W 0 band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn-Sham density functional theory (DFT) with the Perdew, Burke, and Ernzerhof functional. We find the GW correction increases the bandwidth of the states forming the M pocket near the Fermi energy, while leaving the Γ pocket roughly unchanged. We then compare the G 0 W 0 quasiparticle band energies with the band structure from a simple empirical + A approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory. We show that this empirical correction succeeds in approximating the GW nonlocal and dynamical self-energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).

36 MATERIALS SCIENCE↗

DOE OSTI2012

Specific heat to H c⁢2 : Evidence for nodes or deep minima in the superconducting gap of underdoped and overdoped Ba(Fe 1–$x$ Co $x$ ) 2 As 2

Low-temperature specific heat, C, in magnetic fields up to Hc2 is reported for underdoped Ba(Fe 0.955 Co 0.045 ) 2 As 2 (T c = 8 K) and for three overdoped samples Ba(Fe 1–x Co x ) 2 As 2 (x = 0.103, 0.13, and 0.15; T c = 17.2, 16.5, and 11.7 K, respectively). Previous measurements of thermal conductivity (as a function of temperature and field) and penetration depth on comparable-composition samples gave some disagreement as to whether there was fully gapped/nodal behavior in the under-/overdoped materials, respectively. The present work shows that the measured behavior of the specific heat γ (∝C/T as T → 0, i.e., a measure of the electronic density of states at the Fermi energy) as a function of field approximately obeys γ ∝ H 0.5±0.1 , similar to the Volovik effect for nodal superconductors, for both the underdoped and the most overdoped Co samples. However, for the two overdoped compositions x = 0.103 and 0.13, the low-field (H ≤ 10 T) data show a Volovik-like behavior of γ ∝ H 0.3–0.4 , followed by an inflection point, followed at higher fields by γ ∝ H 1 . Here, we argue that, within the two-band theory of superconductivity, an inflection point may occur if the interband coupling is dominant.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

A First-Principles Study of the Structural and Thermo-Mechanical Properties of Tungsten-Based Plasma-Facing Materials

Tungsten (W) and tungsten alloys are being considered as leading candidates for structural and functional materials in future fusion energy devices. The most attractive properties of tungsten for the design of magnetic and inertial fusion energy reactors are its high melting point, high thermal conductivity, low sputtering yield, and low long-term disposal radioactive footprint. Despite these relevant features, there is a lack of understanding of how the structural and mechanical properties of W-based alloys are affected by the temperature in fusion power plants. In this work, we present a study on the thermo-mechanical properties of five W-based plasma-facing materials. First-principles density functional theory (DFT) calculations are combined with the quasi-harmonic approximation (QHA) theory to investigate the electronic, structural, mechanical, and thermal properties of these W-based alloys as a function of temperature. The coefficient of thermal expansion, temperature-dependent elastic constants, and several elastic parameters, including bulk and Young’s modulus, are calculated. Our work advances the understanding of the structural and thermo-mechanical behavior of W-based materials, thus providing insights into the design and selection of candidate plasma-facing materials in fusion energy devices.

42 ENGINEERING↗

DOE OSTI2016

THE ARIZONA RADIO OBSERVATORY CO MAPPING SURVEY OF GALACTIC MOLECULAR CLOUDS. V. THE SH2-235 CLOUD IN CO J = 2 − 1, {sup 13}CO J = 2 − 1, AND CO J = 3 − 2

We present the results of a program to map the Sh2-235 molecular cloud complex in the CO and {sup 13}CO J = 2 − 1 transitions using the Heinrich Hertz Submillimeter Telescope. The map resolution is 38″ (FWHM), with an rms noise of 0.12 K brightness temperature, for a velocity resolution of 0.34 km s{sup −1}. With the same telescope, we also mapped the CO J = 3 − 2 line at a frequency of 345 GHz, using a 64 beam focal plane array of heterodyne mixers, achieving a typical rms noise of 0.5 K brightness temperature with a velocity resolution of 0.23 km s{sup −1}. The three spectral line data cubes are available for download. Much of the cloud appears to be slightly sub-thermally excited in the J = 3 level, except for in the vicinity of the warmest and highest column density areas, which are currently forming stars. Using the CO and {sup 13}CO J = 2 − 1 lines, we employ an LTE model to derive the gas column density over the entire mapped region. Examining a 125 pc{sup 2} region centered on the most active star formation in the vicinity of Sh2-235, we find that the young stellar object surface density scales as approximately the 1.6-power of the gas column density. The area distribution function of the gas is a steeply declining exponential function of gas column density. Comparison of the morphology of ionized and molecular gas suggests that the cloud is being substantially disrupted by expansion of the H ii regions, which may be triggering current star formation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Multidimensional Skyrme-density-functional study of the spontaneous fission of 238 U

We determined the spontaneous fission lifetime of 238 U by a minimization of the action integral in a three-dimensional space of collective variables. Apart from the mass-distribution multipole moments Q 20 (elongation) and Q 30 (left–right asymmetry), we also considered the pairing-fluctuation parameter λ 2 as a collective coordinate. The collective potential was obtained self-consistently using the Skyrme energy density functional SkM*. The inertia tensor was obtained within the nonperturbative cranking approximation to the adiabatic time-dependent Hartree–Fock–Bogoliubov approach. As a result, the pairing-fluctuation parameter λ 2 allowed us to control the pairing gap along the fission path, which significantly changed the spontaneous fission lifetime.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

NASA NTRS1993

Electron cyclotron thruster new modeling results preparation for initial experiments

The following topics are discussed: a whistler-based electron cyclotron resonance heating (ECRH) thruster; cross-field coupling in the helicon approximation; wave propagation; wave structure; plasma density; wave absorption; the electron distribution function; isothermal and adiabatic plasma flow; ECRH thruster modeling; a PIC code model; electron temperature; electron energy; and initial experimental tests. The discussion is presented in vugraph form.

Hooper, E. Bickford↗

NASA NTRS2011

The Evolution of Ly-alpha Emitting Galaxies Between z = 2.1 and z = 3.l

We describe the results of a new, wide-field survey for z= 3.1 Ly-alpha emission-line galaxies (LAEs) in the Extended Chandra Deep Field South (ECDF-S). By using a nearly top-hat 5010 Angstrom filter and complementary broadband photometry from the MUSYC survey, we identify a complete sample of 141 objects with monochromatic fluxes brighter than 2.4E-17 ergs/cm^2/s and observers-frame equivalent widths greater than ~ 80 Angstroms (i.e., 20 Angstroms in the rest-frame of Ly-alpha). The bright-end of this dataset is dominated by x-ray sources and foreground objects with GALEX detections, but when these interlopers are removed, we are still left with a sample of 130 LAE candidates, 39 of which have spectroscopic confirmations. This sample overlaps the set of objects found in an earlier ECDF-S survey, but due to our filter's redder bandpass, it also includes 68 previously uncataloged sources. We confirm earlier measurements of the z=3.1 LAE emission-line luminosity function, and show that an apparent anti-correlation between equivalent width and continuum brightness is likely due to the effect of correlated errors in our heteroskedastic dataset. Finally, we compare the properties of z=3.1 LAEs to LAEs found at z=2.1. We show that in the approximately 1 Gyr after z approximately 3, the LAE luminosity function evolved significantly, with L * fading by approximately 0.4 mag, the number density of sources with L greater than 1.5E42 ergs/s declining by approximately 50%, and the equivalent width scalelength contracting from 70^{+7}_{-5} Angstroms to 50^{+9}_{-6} Angstroms. When combined with literature results, our observations demonstrate that over the redshift range z approximately 0 to z approximately 4, LAEs contain less than approximately 10% of the star-formation rate density of the universe.

Ciardullo, Robin↗

DOE OSTI2015

A first-principles study of cementite (Fe{sub 3}C) and its alloyed counterparts: Elastic constants, elastic anisotropies, and isotropic elastic moduli

A comprehensive computational study of elastic properties of cementite (Fe{sub 3}C) and its alloyed counterparts (M{sub 3}C (M = Al, Co, Cr, Cu, Fe, Hf, Mn, Mo, Nb, Ni, Si, Ta, Ti, V, W, Zr, Cr{sub 2}FeC and CrFe{sub 2}C) having the crystal structure of Fe{sub 3}C is carried out employing electronic density-functional theory (DFT), all-electron PAW pseudopotentials and the generalized gradient approximation for the exchange-correlation energy (GGA). Specifically, as a part of our systematic study of cohesive properties of solids and in the spirit of materials genome, following properties are calculated: (i) single-crystal elastic constants, C{sub ij}, of above M{sub 3}Cs; (ii) anisotropies of bulk, Young’s and shear moduli, and Poisson’s ratio based on calculated C{sub ij}s, demonstrating their extreme anisotropies; (iii) isotropic (polycrystalline) elastic moduli (bulk, shear, Young’s moduli and Poisson’s ratio) of M{sub 3}Cs by homogenization of calculated C{sub ij}s; and (iv) acoustic Debye temperature, θ{sub D}, of M{sub 3}Cs based on calculated C{sub ij}s. We provide a critical appraisal of available data of polycrystalline elastic properties of alloyed cementite. Calculated single crystal properties may be incorporated in anisotropic constitutive models to develop and test microstructure-processing-property-performance links in multi-phase materials where cementite is a constituent phase.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Post-Newtonian kinetic theory

A kinetic theory for relativistic gases in the presence of gravitational fields is developed in the second post-Newtonian approximation. The corresponding Boltzmann equation is determined from the evolution of the one-particle distribution function with respect to the proper time along the world line of the particle. From the knowledge of the equilibrium Maxwell–Jüttner distribution function in the second post-Newtonian approximation the components of the particle four-flow and energy–momentum tensor are obtained. The Eulerian hydrodynamic equations for the mass density, mass-energy density and momentum density in the second post-Newtonian approximation are determined from the Boltzmann equation. It is shown that the combination of the hydrodynamic equations of mass and mass-energy densities leads to the hydrodynamic equation for the internal energy density in the first post-Newtonian approximation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1982

Analytic optical potentials for nucleon-nucleus nucleus-nucleus collisions involving light and medium nuclei

Utilizing an optical model potential approximation to the exact nucleus-nucleus multiple-scattering series, optical potentials for nucleon-nucleus and nucleus-nucleus collisions are analytically derived. These expressions are applicable to light and medium cosmic ray nuclei as their single-particle density distributions are analytically determined, without approximation, from their actual harmonic well charge density distributions. Pauli correlation effects are included through the use of a simple Gaussian function to replace the usual expression obtained in the infinite nuclear matter approximation.

Bidasaria, H. B.↗

DOE OSTI2017

Numerical methods for the inverse problem of density functional theory

Here, the inverse problem of Kohn–Sham density functional theory (DFT) is often solved in an effort to benchmark and design approximate exchange-correlation potentials. The forward and inverse problems of DFT rely on the same equations but the numerical methods for solving each problem are substantially different. We examine both problems in this tutorial with a special emphasis on the algorithms and error analysis needed for solving the inverse problem. Two inversion methods based on partial differential equation constrained optimization and constrained variational ideas are introduced. We compare and contrast several different inversion methods applied to one-dimensional finite and periodic model systems.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2021

Phonon-Assisted Ballistic Current from First-Principles Calculations

The bulk photovoltaic effect (BPVE) refers to current generation due to illumination by light in a homogeneous bulk material lacking inversion symmetry. In addition to the intensively studied shift current, the ballistic current, which originates from asymmetric carrier generation due to scattering processes, also constitutes an important contribution to the overall kinetic model of the BPVE. In this Letter, we use a perturbative approach to derive a formula for the ballistic current resulting from the intrinsic electron-phonon scattering in a form amenable to first-principles calculation. We then implement the theory and calculate the ballistic current of the prototypical BPVE material BaTiO 3 using quantum-mechanical density functional theory. The magnitude of the ballistic current is comparable to that of the shift current, and the total spectrum (shift plus ballistic) agrees well with the experimentally measured photocurrents. Furthermore, we show that the ballistic current is sensitive to structural change, which could benefit future photovoltaic materials design.

14 SOLAR ENERGY↗

DOE OSTI2013

Multireference symmetry-projected variational approaches for ground and excited states of the one-dimensional Hubbard model

We present a multireference configuration mixing scheme for describing ground and excited states, with well-defined spin and space-group symmetry quantum numbers, of the one-dimensional Hubbard model with nearest-neighbor hopping and periodic boundary conditions. Within this scheme, each state is expanded in terms of nonorthogonal and variationally determined symmetry-projected configurations. The results for lattices up to 30 and 50 sites compare well with the exact Lieb-Wu solutions as well as with results from other state-of-the-art approximations. In addition to spin-spin correlation functions in real space and magnetic structure factors, we present results for spectral functions and density of states computed with an Ansatz whose quality can be well controlled by the number of symmetry-projected configurations used to approximate the systems with $N_e$ and $N_e±1$ electrons. The intrinsic symmetry-broken determinants resulting from the variational calculations have rich structures in terms of defects that can be regarded as basic units of quantum fluctuations. Given the quality of the results here reported, as well as the parallelization properties of the considered scheme, we believe that symmetry-projection techniques, which have found ample applications in nuclear structure physics, deserve further attention in the study of low-dimensional correlated many-electron systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Constrained nuclear–electronic orbital method for periodic density functional theory: Application to H 2 chemisorption on Si(001) surfaces

The nuclear–electronic orbital (NEO) method provides a powerful computational framework for incorporating nuclear quantum effects (NQE) in electronic structure calculations beyond the Born–Oppenheimer approximation. By incorporating additional constraints to the position operator on quantum particles like protons, the NEO method enables calculation of effective potential that accounts for NQE. Here, in this work, we present a new constrained NEO (cNEO) formulation for density functional theory (cNEO-DFT) calculations in the context of extended periodic systems. Using the nudged elastic band method, we discuss an application of the cNEO-DFT approach to studying the adsorption of a hydrogen molecule on the Si(001) surfaces. The calculation shows how NQE impacts the reaction energetics. The proton density changes are computed along the reaction pathways. This work demonstrates the capability of the new cNEO-DFT method to study a wide range of chemical processes, such as surface reactions where the quantum nature of light atoms like protons is non-negligible.

Chemical processes↗

DOE OSTI2019

Interplay of electronic, magnetic, and structural properties of GdB 6 from first principles

Gadolinium hexaboride (GdB 6 ) is a well-known field emitter material that has been investigated for more than three decades. We perform a systematical density-functional theory study of GdB 6 by using the generalized-gradient approximation and considering the electron interaction parameter U. The basic structural and electronic properties are carefully revised, as well as a strong U-value dependence in determining the antiferromagnetic (AFM) magnetic structures of Gd 4f electronic states. We found a small U(0 ~ 3 eV) showing the most consistent experimental ground-state properties, which gives rise to a magnetic structure with a ground state of C-AFM and a second stable E-AFM. Moreover, we find the distortion modes of boron octahedron play an important role in the interaction between spin and lattice structures in this system. Furthermore, these results will deepen our understanding of the boron-based correlated rare earth compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2026

Large-scale calculations of 𝛽-decay rates and implications for 𝑟-process nucleosynthesis

Nuclear 𝛽 decay is a key element of the astrophysical rapid neutron capture process (𝑟 process). In this work, we present state-of-the-art global 𝛽-decay calculations based on the quantified relativistic nuclear energy density functional theory and the deformed proton-neutron quasiparticle random-phase approximation. Our analysis considers contributions from allowed and first-forbidden transitions. We used two point-coupling functionals with carefully calibrated time-odd terms and isoscalar pairing strength. The new calculations display consistent results for both employed functionals, especially near the neutron drip line, suggesting slower 𝛽 decays past the 𝑁=126 neutron shell closure than in commonly used 𝛽-decay models. The new rates, along with the existing rates based on the recent nonrelativistic global calculations, are found to slow down the synthesis of heavy elements in the 𝑟 process and significantly reduce the contribution of neutron-induced fission.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2013

Probability distribution functions of {sup 12}CO(J = 1 → 0) brightness and integrated intensity in M51: The PAWS view

We analyze the distribution of CO brightness temperature and integrated intensity in M51 at ∼40 pc resolution using new {sup 12}CO(J = 1 → 0) data from the Plateau de Bure Arcsecond Whirlpool Survey (PAWS). We present probability distribution functions (PDFs) of the CO emission within the PAWS field of view, which covers the inner ∼11 × 7 kpc of M51. We find clear variations in the shape of CO PDFs both within different M51 environments, defined according to dynamical criteria, and among M51 and two nearby low-mass galaxies, M33 and the Large Magellanic Cloud (LMC). Globally, the PDFs for the inner disk of M51 can be represented by narrow lognormal functions that cover ∼1-2 orders of magnitude in CO brightness and integrated intensity. The PDFs for M33 and the LMC are narrower and peak at lower CO intensities, consistent with their lower gas surface densities. However, the CO PDFs for different dynamical environments within the PAWS field depart significantly from the shape of the global distribution. The PDFs for the interarm region are approximately lognormal, but in the spiral arms and central region of M51, they exhibit diverse shapes with a significant excess of bright CO emission. The observed environmental dependence on the shape of the CO PDFs is qualitatively consistent with changes that would be expected if molecular gas in the spiral arms is characterized by a larger range of average densities, gas temperatures, and velocity fluctuations, although further work is required to disentangle the relative importance of large-scale dynamical effects versus star formation feedback in regulating these properties. We show that the shape of the CO PDFs for different M51 environments is only weakly related to global properties of the CO emission, e.g., the total CO luminosity, but is strongly correlated with properties of the local giant molecular cloud (GMC) and young stellar cluster populations, including the shape of their mass distributions. For galaxies with strong spiral structure such as M51, our results indicate that galactic-scale dynamical processes play a significant role in the formation and evolution of GMCs and stellar clusters.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

Electronic Structure Theory and Novel Materials

This grant supported research on electronic structure and materials theory, with focus on three main issues: (i) novel techniques to deal with correlation in the electronic ground-state, (ii) topological materials, (iii) the phase diagram of lattice spin models. Regarding (i), we applied to the homogeneous electron liquid an approach that we previously developed in the context of molecular systems. In this scheme the electronic occupation probabilities and the natural spin orbitals are used to construct an approximate two-body density matrix for the electronic ground-state. Regarding (ii) we used standard electronic structure methods based on density functional theory to model topological materials and interpret experimental observations. Finally, regarding (iii) we further developed a numerical approach to compute the renormalized couplings within real space renormalization group theory in the context of lattice spin models. The main findings were the following. (i) We found that with our approximate two-body density matrix, which works well for small molecules, is not sufficiently accurate for condensed phase systems. Missing a systematic way of improving on the adopted approximations, we decided not to pursue this approach. (ii) We performed two studies. In one, we investigated the influence of Te defects on the topological properties of a WTe2 monolayer, finding that while Te vacancies, even in modest concentration, destroy the topological character, Te adatoms do not, consistent with a recent experiment. In another study, we predicted Weyl semimetal character and strong anomalous Hall effect in the Heusler compensated ferrimagnet Ti2MnAl. (iii) We developed a new Monte Carlo method to do real space renormalization group calculations for lattice spin models. We subsequently extended the scheme to deal with lattice spin models in presence of quenched disorder, finding that the approach can distinguish systems with finite and strong disorder. In the finite disorder case, the method allows one to find with good approximation the critical coupling distribution and the critical exponents.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗