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At least 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Crystal and electronic structures of CaAl{sub 2}Si{sub 2}-type rare-earth copper zinc phosphides RECuZnP{sub 2} (RE=Pr, Nd, Gd-Tm, Lu)

The quaternary rare-earth phosphides RECuZnP{sub 2} (RE=Pr, Nd, Gd-Tm, Lu) have been prepared by reaction of the elements at 900 {sup o}C, completing this versatile series which forms for nearly all RE metals. They adopt the trigonal CaAl{sub 2}Si{sub 2}-type structure (Pearson symbol hP5, space group P3-bar m1, Z=1), as confirmed by single-crystal X-ray diffraction analysis on ErCuZnP{sub 2} and powder X-ray diffraction analysis on the remaining members. The Cu and Zn atoms are assumed to be disordered over the single transition-metal site. Band structure calculations on a hypothetically ordered YCuZnP{sub 2} model suggest a semimetal, with a zero band gap between the valence and conduction bands. This electronic structure is supported by XPS valence band spectra for RECuZnP{sub 2} (RE=Gd-Er), in which the intensity drops off smoothly at the Fermi edge. The absence of a band gap permits the electron count to deviate from the precise value of 16 e{sup -} per formula unit, as demonstrated by the formation of a solid solution in GdCu{sub x}Zn{sub 2-x}P{sub 2} (1.0{<=}x{<=}1.3), while still retaining the CaAl{sub 2}Si{sub 2}-type structure. Because the Cu 2p XPS spectra indicate that the Cu atoms are always monovalent, the substitution of Cu for Zn leads to a decrease in electron count and a lowering of the Fermi level in the valence band. The magnetic susceptibility of RECuZnP{sub 2} (RE=Gd-Er), which obeys the Curie-Weiss law, confirms the presence of trivalent RE atoms. -- Graphical abstract: The absence of a band gap in the semimetallic quaternary rare-earth phosphides RECuZnP{sub 2} permits the formation of a solid solution such as GdCu{sub x}Zn{sub 2-x}P{sub 2} through hole-doping of the valence band. Display Omitted

36 MATERIALS SCIENCE↗
DOE OSTI2017

Electronic structural studies on the improved thermal stability of Li(Ni0.8Co0.15Al0.05)O2 by ZrO2 coating for lithium ion batteries

The electronic structures of bare and ZrO 2 -coated Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 electrode systems were investigated using a combination of time-resolved X-ray diffraction and soft X-ray absorption spectroscopy (XAS) techniques. The ZrO 2 coating on the surface of Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 was effective in elevating the onset temperature of the dissociation of charged Li 0.33 (Ni 0.8 Co 0.15 Al 0.05 )O 2 , which will enhance the safety of Li-ion cells. Lastly, soft XAS spectra of the Ni LII,III-edge in the partial electron yield mode were obtained, which showed that the enhanced electrochemical properties and thermal stability of the cathode materials by ZrO 2 coating can be attributed to the suppression of unwanted Ni oxidation state changes at the surface.

25 ENERGY STORAGE↗
DOE OSTI2014

Novel electronic structures of superlattice composed of graphene and silicene

Highlights: • Graphene/silicene superlattices exhibit metallic electronic properties. • Dirac point of graphene is folded to the Γ-point in the superlattice system. • Significant changes in the transport properties of the graphene layers are expected. • Small amount of charge transfer from the graphene to the silicene layers is found. - Abstract: Superlattice is a major force in providing man-made materials with unique properties. Here we report a study of the structural and electronic properties of a superlattice made with alternate stacking of graphene and hexagonal silicene. Three possible stacking models, i.e., the top-, bridge- and hollow-stacking, are considered. The top-stacking is found to be the most stable pattern. Although both the free-standing graphene and silicene are semi-metals, our results suggest that the graphene and silicene layers in the superlattice both exhibit metallic electronic properties due to a small amount of charge transfer from the graphene to the silicene layers. More importantly, the Dirac point of graphene is folded to the Γ-point of the superlattice, instead of the K-point in the isolated graphene. Such a change in the Dirac point of graphene could lead to significant change in the transportation property of the graphene layer. Moreover, the band structure and the charge transfer indicate that the interaction between the stacking sheets in the graphene/silicene superlattice is more than just the van der Waals interaction.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electronic Structure, Optical Properties, and Photoelectrochemical Activity of Sn-Doped Fe 2 O 3 Thin Films

Hematite (Fe 2 O 3 ) is a well-known oxide semiconductor suitable for photoelectrochemical (PEC) water splitting and industry gas sensing. It is widely known that Sn doping of Fe 2 O 3 can enhance the device performance, yet the underlying mechanism remains elusive. In this work, we determine the relationship between electronic structure, optical properties and PEC activity of Sn doped Fe 2 O 3 by studying highly crystalline, well-controlled thin films prepared by pulsed laser deposition (PLD). We show that Sn doping substantially increases the n-type conductivity of Fe 2 O 3 , and the conduction mechanism is better described by small-polaron hoping (SPH) model. Only 0.2% Sn doping significantly reduce the activation energy barrier for SPH conduction from at least 0.5 eV for undoped Fe 2 O 3 to 0.14 eV for doped ones. A combination of X-ray photoemission, X-ray absorption spectroscopy and DFT calculations reveals the Fermi level gradually shifts toward the conduction band minimum with Sn doping. A localized Fe 2+ like gap state is observed at the top of valence band, accounting for the SPH conduction. Interestingly, in contrast to the literature, we find that only 0.2% Sn doping in Fe 2 O 3 significantly improves the PEC activity, while more Sn decreases it. The improved PEC activity is partially attributed to an increased band bending potential which facilitates the charge separation at space charge region. The reduced activation energy barrier for SPH will facilitate the transport of photo-excited carriers for the enhanced PEC, which is of interest for further carrier dynamics study.

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

Quasiparticle electronic structure of phthalocyanine:TMD interfaces from first-principles GW

Interfaces formed between monolayer transition metal dichalcogenides and (metallo)phthalocyanine molecules are promising in energy applications and provide a platform for studying mixed-dimensional molecule-semiconductor heterostructures in general. An accurate characterization of the frontier energy level alignment at these interfaces is key in the fundamental understanding of the charge transfer dynamics between the two photon absorbers. Here, we employ the first-principles substrate screening GW approach to quantitatively characterize the quasiparticle electronic structure of a series of interfaces: metal-free phthalocyanine (H 2 Pc) adsorbed on monolayer MX 2 (M = Mo, W; X = S, Se) and zinc phthalocyanine (ZnPc) adsorbed on MoX 2 (X = S, Se). Furthermore, we reveal the dielectric screening effect of the commonly used α-quartz (SiO 2 ) substrate on the H 2 Pc:MoS 2 interface using the dielectric embedding GW approach. Our calculations furnish a systematic set of GW results for these interfaces, providing the structure–property relationship across a series of similar systems and benchmarks for future experimental and theoretical studies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Effect of pressure on the interlayer transport and the electronic structure in the organic quasi-two-dimensional bilayer metal θ-(BETS){sub 4}HgBr{sub 4}(C{sub 6}H{sub 5}Cl)

The behavior of the interlayer resistance and the magnetoresistance in the organic quasi-twodimensional bilayer metal θ-(BETS){sub 4}HgBr{sub 4}(C{sub 6}H{sub 5}Cl) is studied at normal pressure and a hydrostatic pressure of 10 kbar. The interlayer transport under atmospheric pressure is found to occur in an incoherent mode. The applied pressure does not change the electronic structure of the conducting layers and causes a transition to a weakly coherent mode at low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Dependence of the electronic structure of the EuS/InAs interface on the bonding configuration

The EuS/InAs interface has recently attracted attention for the possibility of inducing magnetic exchange correlations in a strong spin-orbit semiconductor, which could be useful for topological quantum devices. We use density functional theory with a machine-learned Hubbard U correction [Yu et al., npj Comput. Mater. 6, 180 (2020)] to elucidate the effect of the bonding configuration at the interface on the electronic structure. For all interface configurations considered here, we find that the EuS valence band maximum (VBM) lies below the InAs VBM. In addition, dispersed states emerge at the top of the InAs VBM at the interface, which do not exist in either material separately. These states are contributed mainly by the InAs layer adjacent to the interface. They are localized at the interface and may be attributed to charge transfer from the EuS to the InAs. The interface configuration affects the position of the EuS VBM with respect to the InAs VBM, as well as the dispersion of the interface states. For all interface configurations studied here, the induced magnetic moment in the InAs is small. Our results suggest that this interface, in its coherent form studied here, may not be promising for inducing equilibrium magnetic properties in InAs.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Electronic structure and two-band superconductivity in unconventional high- T c cuprates Ba 2 CuO 3+δ

We report that the recently discovered cuprate superconductor Ba 2 CuO 3+δ exhibits a high T c ≃73 K at δ ≃ 0.2. The polycrystal grown under high pressure has a structure similar to La 2 CuO 4 but with dramatically different lattice parameters due to the CuO 6 octahedron compression. The crystal field in the compressed Ba 2 CuO 4 leads to an inverted Cu 3d e g complex with the d x 2 -y 2 orbital sitting below the d 3z 2 - r 2 and an electronic structure highly unusual compared to the conventional cuprates. We construct a two-orbital Hubbard model for the Cu d 9 state at hole doping x = 2δ and study the orbital-dependent strong correlation and superconductivity. For the undoped case at x = 0 , we found that strong correlation drives an orbital-polarized Mott-insulating state with the spin-1/2 moment of the localized d 3z 2 - r 2 orbital. In contrast to the single-band cuprates where superconductivity is suppressed in the overdoped regime, hole doping the two-orbital Mott insulator leads to orbital-dependent correlations and the robust spin and orbital exchange interactions produce a high-T c antiphase d-wave superconductor even in the heavily doped regime at x = 0.4 . We conjecture that Ba 2 CuO 3+δ realizes mixtures of such heavily hole-doped superconducting Ba 2 CuO 4 and disordered Ba 2 CuO 3 chains in a single-layer or predominately separated bilayer structure. Our findings suggest that unconventional cuprates with liberated orbitals as doped two-band Mott insulators can be a direction for realizing high-T c superconductivity with enhanced transition temperature T c .

36 MATERIALS SCIENCE↗
DOE OSTI2024

Efficient Mixed-Precision Matrix Factorization of the Inverse Overlap Matrix in Electronic Structure Calculations with AI-Hardware and GPUs

In recent years, a new kind of accelerated hardware has gained popularity in the artificial intelligence (AI) community which enables extremely high-performance tensor contractions in reduced precision for deep neural network calculations. In this article, we exploit Nvidia Tensor cores, a prototypical example of such AI-hardware, to develop a mixed precision approach for computing a dense matrix factorization of the inverse overlap matrix in electronic structure theory, S –1 . This factorization of S –1 , written as ZZT = S –1 , is used to transform the general matrix eigenvalue problem into a standard matrix eigenvalue problem. Here we present a mixed precision iterative refinement algorithm where Z is given recursively using matrix–matrix multiplications and can be computed with high performance on Tensor cores. To understand the performance and accuracy of Tensor cores, comparisons are made to GPU-only implementations in single and double precision. Additionally, we propose a nonparametric stopping criteria which is robust in the face of lower precision floating point operations. The algorithm is particularly useful when we have a good initial guess to Z, for example, from previous time steps in quantum-mechanical molecular dynamics simulations or from a previous iteration in a geometry optimization.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Two-Level Chebyshev Filter Based Complementary Subspace Method: Pushing the Envelope of Large-Scale Electronic Structure Calculations

We describe here a novel iterative strategy for Kohn–Sham density functional theory calculations aimed at large systems (>1,000 electrons), applicable to metals and insulators alike. In lieu of explicit diagonalization of the Kohn–Sham Hamiltonian on every self-consistent field (SCF) iteration, we employ a two-level Chebyshev polynomial filter based complementary subspace strategy to (1) compute a set of vectors that span the occupied subspace of the Hamiltonian; (2) reduce subspace diagonalization to just partially occupied states; and (3) obtain those states in an efficient, scalable manner via an inner Chebyshev filter iteration. By reducing the necessary computation to just partially occupied states and obtaining these through an inner Chebyshev iteration, our approach reduces the cost of large metallic calculations significantly, while eliminating subspace diagonalization for insulating systems altogether. We describe the implementation of the method within the framework of the discontinuous Galerkin (DG) electronic structure method and show that this results in a computational scheme that can effectively tackle bulk and nano systems containing tens of thousands of electrons, with chemical accuracy, within a few minutes or less of wall clock time per SCF iteration on large-scale computing platforms. We anticipate that our method will be instrumental in pushing the envelope of large-scale ab initio molecular dynamics. As a demonstration of this, we simulate a bulk silicon system containing 8,000 atoms at finite temperature, and obtain an average SCF step wall time of 51 s on 34,560 processors; thus allowing us to carry out 1.0 ps of ab initio molecular dynamics in approximately 28 h (of wall time).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Study of Structural, Electronic, and Magnetic Properties of Cubic Lanthanide Based on Oxide Perovskite-Type NdGaO{sub 3}

Based on the density functional theory (DFT), the full-potential linearized augmented plane wave with local orbital (FP-L/APW+lo) technique is now employed in this approach to understand the structural, electronic, and magnetic properties of simple cubic oxide perovskite NdGaO{sub 3} compound. In all this investigation, the exchange-correlation (XC) energy is selected in the framework of spin-polarized generalized gradient approximation (spin-GGA). The structural analysis unveils that the ferromagnetic (FM) phase is the stable ground state of the cubic NdGaO{sub 3} compound, where the equilibrium lattice parameters (lattice constant (a{sub 0}), bulk modulus (B{sub 0}), and its first pressure derivative (B′)) are determined in both FM and paramagnetic (PM) phases. The spin-polarized electronic properties (band structure and density of states) of the cubic NdGaO{sub 3} oxide perovskite are studied under the platform of equilibrium lattice parameters; this investigation demonstrates the half-metallic behavior of the studied cubic NdGaO{sub 3} compound because the spin-up case displays the metallic nature, whereas the semiconducting character is observed in spin-down case. The magnetic properties reveal that the total magnetic moment of the cubic NdGaO{sub 3} compound is equal to 3 μ{sub B} and its contribution is mostly generated by Nd atoms, whereas feeble local magnetic moments are installed in non-magnetic Ga and O sites. Through the electronic and magnetic results, we conclude that the cubic perovskite NdGaO{sub 3} compound is classified as a half-metallic ferromagnetic material.

36 MATERIALS SCIENCE↗
DOE OSTI2019

First-principles electronic structure in second-moment calculation of mode frequencies: Failure of quasiharmonic approximation in silicon

Using first-principles electronic structure calculations (local density approximation), we present second-moment calculations of the temperature dependence of the vibrational normal mode frequencies in Si up to 1500 K . The method is based on simple ensemble averages of displacements and forces and is easy to implement. These efficient calculations incorporate all interactions nonperturbatively, implicitly including all high order force constants, in contrast to typical perturbative approaches. Also in this work, we propose and apply to our classical calculations an estimated quantum correction that could be used for classical approaches other than the moments method. We compare our results to available experiments, including recent inelastic neutron scattering data and conclude that the second-moment gives the frequency shift to within a factor of 2 in contrast to quasiharmonic approaches that fail by an order of magnitude to predict the anharmonic shift. Noting the relative simplicity and speed of the calculations, we conclude that the second-moment approximation reasonably accounts for anharmonicity in a system that has been very closely studied experimentally and may reliably be applied in an exploratory way to more complex systems. Finally, we suggest that further improvement on the results could be obtained by incorporating the fourth moment, which can be implemented with little additional cost.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1998

Comparison of Conjugate Gradient Density Matrix Search and Chebyshev Expansion Methods for Avoiding Diagonalization in Large-Scale Electronic Structure Calculations

We report a comparison of two linear-scaling methods which avoid the diagonalization bottleneck of traditional electronic structure algorithms. The Chebyshev expansion method (CEM) is implemented for carbon tight-binding calculations of large systems and its memory and timing requirements compared to those of our previously implemented conjugate gradient density matrix search (CG-DMS). Benchmark calculations are carried out on icosahedral fullerenes from C60 to C8640 and the linear scaling memory and CPU requirements of the CEM demonstrated. We show that the CPU requisites of the CEM and CG-DMS are similar for calculations with comparable accuracy.

Bates, Kevin R.↗
DOE OSTI2022

Low-energy electronic structure of perovskite and Ruddlesden-Popper semiconductors in the Ba-Zr-S system probed by bond-selective polarized x-ray absorption spectroscopy, infrared reflectivity, and Raman scattering

Chalcogenides in perovskite and the related layered Ruddlesden-Popper crystal structures ( chalcogenide perovskites for brevity) are an exciting family of semiconductors but remain experimentally little studied. Chalcogenide perovskites share crystal structures and some physical properties with ionic compounds such as oxide and halide perovskites, but the metal-chalcogen bonds responsible for semiconducting behavior are substantially more covalent than in these more-studied perovskites. In this study, we use complementary experimental and theoretical methods to study how the mixed ionic-covalent Zr-S bonds support the electronic structure and physical properties of perovskite BaZrS 3 and Ruddlesden-Popper Ba 3 Zr 2 S 7 . We apply theoretical methods to assign features of experimentally measured x-ray absorption spectroscopy (XAS) to particular orbital transitions, enabling a clear physical interpretation of angle-dependent, polarized XAS data measured on single-crystal samples, and an atomistic view of the covalent bonding network that facilitates charge transport. Polarized Raman measurements identify signatures of crystalline anisotropy in Ba 3 Zr 2 S 7 and enable the first assignments of mode symmetry in this material. Infrared reflectivity reveals electronic transport properties that augur well for the use of chalcogenide perovskites in optoelectronic and energy-conversion technologies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Theoretical Prediction of Structural, Electronic, and Magnetic Properties of Rh{sub 2}YIn (Y = Nd, Sm) Full-Heusler Alloys

We have studied the structural, electronic, and magnetic properties of Rh{sub 2}YIn (Y = Nd, Sm) alloys based on rare earth elements, using the full-potential linearized augmented plane-wave method (FP-LAPW) implemented in the Wien2K computer code with the parameterization of the generalized gradient approximation (GGA) and the local density approximation (LDA). Our results reveal a metallic behavior for rare earth-based full-Heusler Rh{sub 2}YIn (Y = Nd, Sm) alloys. Indeed, the structure of the state density illustrates a polarization for the up orientation (metal character) and a pseudo gap at the down orientation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

New insights into the electronic structure of α-U and δ-Pu

Here, this work presents the results of a theoretical study of the electronic structure of two actinide metals, α -U and δ -Pu. We compare our ab-initio results obtained with the recently developed self-consistent Vertex corrected GW approach with previously published experimental measurements such as photo-electron spectroscopy, for the occupied density of states, and bremsstralung isochromat spectroscopy (BIS) and inverse photo-electron spectroscopy (IPES), for the unoccupied density of states. Our ab-initio approach includes all important relativistic effects (it is based on Dirac’s equation) and it represents the first application of the Vertex corrected GW approach in the physics of actinides. Overall, our theoretical results are in good agreement with the experimental data, which supports the level of approximations which our theoretical method is based upon. By comparing our vertex corrected GW results with our results obtained with less sophisticated approaches (local density approximation and self-consistent GW) we differentiate the strength of correlation effects in Uranium and Plutonium. Also, our theoretical results allow us to elucidate the subtle differences between the previously published experimental BIS and IPES data on the unoccupied density of states in α -U.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electronic Structure and Linhard Function of Iron-Based Superconductor La{sub 0.5−x}Na{sub 0.5 + x}Fe{sub 2}As{sub 2}

We perform theoretical studies of the iron-based superconductor La{sub 0.5−x}Na{sub 0.5 + x}Fe{sub 2}As{sub 2}. Using density functional calculation with virtual crystal approximation, band structures, density of states, and Fermi surfaces are obtained for different La/Na composition by varying x value. Furthermore, the nesting property is also investigated using the Linhard function. Our main findings are as follows: For undoped (x = 0.0) case, the magnetic and electronic structure share great similarities with extensively studied BaFe{sub 2}As{sub 2}. Upon hole doping (x = 0.3), the Q = (π,π) nesting property are preserved. However, upon electron doping (x = -0.3), the Fermi surfaces undergo a dramastic change into 3D and complex structure. We find this change is attributed to the comprehensive influence of band filling and chemical pressure of La{sup 3+}. We propose the asymmetric change of Fermi surfaces upon doping may in some sense explain the electron-hole asymmetry in phase diagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗