Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Electronic Structure”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Atomic and electronic structure of the SrNbO{sub 3}/SrNbO{sub 3.4} interface

We have determined the atomic-scale structure of the SrNbO{sub 3}/SrNbO{sub 3.4} interface and related it to the electronic structure. Experimentally, transmission electron microscopy observations reveal that SrNbO{sub 3} and SrNbO{sub 3.4} show orientation relationships [1{sup ¯}10]{sub SrNbO{sub 3}} // [010]{sub SrNbO{sub 3{sub .{sub 4}}}} and (110){sub SrNbO{sub 3}} // (001){sub SrNbO{sub 3{sub .{sub 4}}}}, and that their interface is coherent and atomically abrupt. Theoretically, this interface is found to be strongly anisotropic in electronic structure, and takes on quasi-one-dimensional nature. We also find that the interface impacts greatly the electron occupation of Nb d orbitals, particularly d{sub z}{sup 2} orbital. The combined study represents a relevant advance in atomically bridging structures to properties of heterointerfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Analysis of the structural, electronic, elastic and thermodynamic properties of CuAl2X4 (X = O, S) spinel structure

Highlights: • Fundamental properties of CuAl{sub 2}X{sub 4} (X = O, S) aluminate spinels have been investigated. • The pressure dependence of the elastic constants C{sub ij} can be successfully fit by a straight line. • CuAl{sub 2}X{sub 4} (X = O, S) compounds are mechanically stable at ambient conditions. • Temperature and pressure dependences of some macroscopic parameters are achieved. - Abstract: The structural, electronic, elastic and thermodynamic properties of oxo- and thio-spinels have been predicted based on the ultrasoft pseudo-potential scheme as implemented in the CASTEP code. The exchange-correlation potential was treated within the generalized gradient approximation (GGA-PBE). The calculated structural parameters such as lattice constants and internal parameters are in good agreement with the available experimental data. The ground state electronic band structure revealed that CuAl{sub 2}X{sub 4} (XO, S) spinels are direct band gap semiconductors with a transition along Γ-Γ symmetry points. The calculated values of elastic constants satisfy the Born criteria at ambient conditions and confirm that the spinel structures of CuAl{sub 2}O{sub 4} and CuAl{sub 2}S{sub 4} are mechanically stable. The thiospinel structure may have a phase transition under pressure, due to the negative value of C{sub 44}. The influence of temperature and pressure on macroscopic characteristics of CuAl{sub 2}O{sub 4} and CuAl{sub 2}S{sub 4} compounds was estimated using the quasi-harmonic Debye model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Metal Substitution as the Method of Modifying Electronic Structure of Metal–Organic Frameworks

Targeted modification of electronic structure is an important step in the optimization of metal-organic frameworks (MOFs) for photovoltaic, sensing, and photocatalytic applications. The key parameters to be controlled include the band gap, the absolute energy position of band edges, the excited state charge separation, and degree of hybridization between metal and ligand sites. Partial metal replacement, or metal doping, within secondary building units is a promising, yet relatively unexplored route to modulate these properties in MOFs. Therefore, in the present study, a general method for selecting metal dopant is worked out in theory and validated by experiment, retaining MIL-125 and UiO-66 as the model systems. Metal mixing enables targeted optimization of key electronic structure parameters. This method is applicable to any MOF architecture and can serve as a roadmap for future synthesis of MOFs with predefined properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Electronic Structure of the Cuprate Superconducting and Pseudogap Phases from Spectroscopic Imaging STM

We survey the use of spectroscopic imaging scanning tunneling microscopy (SI-STM) to probe the electronic structure of underdoped cuprates. Two distinct classes of electronic states are observed in both the d-wave superconducting (dSC) and the pseudogap (PG) phases. The first class consists of the dispersive Bogoliubov quasiparticle excitations of a homogeneous d-wave superconductor, existing below a lower energy scale E = {Delta}{sub 0}. We find that the Bogoliubov quasiparticle interference (QPI) signatures of delocalized Cooper pairing are restricted to a k-space arc, which terminates near the lines connecting k = {+-}({pi}/a{sub 0},0) to k = {+-}(0,{pi}/a{sub 0}). This arc shrinks continuously with decreasing hole density such that Luttinger's theorem could be satisfied if it represents the front side of a hole-pocket that is bounded behind by the lines between k = {+-}({pi}/a{sub 0},0) and k = {+-}(0,{pi}/a{sub 0}). In both phases, the only broken symmetries detected for the |E| < {Delta}{sub 0} states are those of a d-wave superconductor. The second class of states occurs proximate to the PG energy scale E = {Delta}{sub 1}. Here the non-dispersive electronic structure breaks the expected 90{sup o}-rotational symmetry of electronic structure within each unit cell, at least down to 180{sup o}-rotational symmetry. This electronic symmetry breaking was first detected as an electronic inequivalence at the two oxygen sites within each unit cell by using a measure of nematic (C{sub 2}) symmetry. Incommensurate non-dispersive conductance modulations, locally breaking both rotational and translational symmetries, coexist with this intra-unit-cell electronic symmetry breaking at E = {Delta}{sub 1}. Their characteristic wavevector Q is determined by the k-space points where Bogoliubov QPI terminates and therefore changes continuously with doping. The distinct broken electronic symmetry states (intra-unit-cell and finite Q) coexisting at E {approx} {Delta}{sub 1} are found to be indistinguishable in the dSC and PG phases. The next challenge for SI-STM studies is to determine the relationship of the E {approx} {Delta}{sub 1} broken symmetry electronic states with the PG phase, and with the E < {Delta}{sub 0} states associated with Cooper pairing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

A first principles study of structural, electronic mechanical and magnetic properties of rare earth nitride:TmN

The structural, electronic and mechanical properties of rare earth nitride TmN is investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure TmN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that TmN is metallic at normal pressure. Ferromagnetic to non magnetic phase transition is predicted in TmN at high pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Effect of electronic structures on catalytic properties of CuNi alloy and Pd in MeOH-related reactions

We investigated the catalytic properties of a CuNi solid solution and Pd for methanol-related reactions and associated valence electronic structures. Calculations and X-ray photoelectron spectroscopy measurements revealed that the CuNi alloy has a similar valence electronic structure to Pd and hence they exhibited similar CO selectivities in steam reforming of methanol and decomposition of methanol. Samples prepared by various processes were found to have similar CO selectivities. We conjecture that alloying of Cu and Ni dramatically alters the valence electronic structures, making it similar to that of Pd so that the alloy exhibits similar catalytic properties to Pd. First-principles slab calculations of surface electronic structures support this conjecture.

36 MATERIALS SCIENCE↗
DOE OSTI2016

First-principles study on equation of states and electronic structures of shock compressed Ar up to warm dense regime

The equation of states (EOS) and electronic structures of argon with temperatures from 0.02 eV to 3 eV and densities from 0.5 g/cm{sup 3} to 5.5 g/cm{sup 3} are calculated using the pair potential and many-body potential molecular dynamics and the density functional theory (DFT) molecular dynamics with van der Waals (vdW) corrections. First-principles molecular dynamics is implemented above 2.0 g/cm{sup 3}. For the cases of low densities below 3 g/cm{sup 3}, we performed pair potential molecular dynamics in order to obtain the ionic configurations, which are used in density functional theory to calculate the EOS and electronic structures. We checked the validity of different methods at different densities and temperatures, showing their behaviors by comparing EOS. DFT without vdW correction works well above 1 eV and 3.5 g/cm{sup 3}. Below 1 eV and 2.0 g/cm{sup 3}, it overestimates the pressure apparently and results in incorrect behaviors of the internal energy. With vdW corrections, the semi-empirical force-field correction (DFT-D2) method gives consistent results in the whole density and temperature region, and the vdW density functional (vdW-DF2) method gives good results below 2.5 g/cm{sup 3}, but it overestimates the pressure at higher densities. The interactions among the atoms are overestimated by the pair potential above 1 eV, and a temperature dependent scaled pair potential can be used to correct the ionic configurations of the pair potential up to 3 eV. The comparisons between our calculations and the experimental multi-shock compression results show that the Hugoniot line of DFT-D2 and DFT tends to give larger pressure than the results of the self-consistent fluid variational theory, and the difference increases with the density. The electronic energy gap exists for all our cases up to 5.5 g/cm{sup 3} and 1 eV. The effect of vdW interactions on the electronic structures are also discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Magnetotransport and electronic structure of the antiferromagnetic semimetal YbAs

A number of rare-earth monopnictides have topologically nontrivial band structures together with magnetism and strong electronic correlations. In order to examine whether the antiferromagnetic (AFM) semimetal YbAs (TN=0.5 K) exhibits such a scenario, we have grown high-quality single crystals using a flux method, and characterized the magnetic properties and electronic structure using specific heat, magnetotransport, and angle-resolved photoemission spectroscopy (ARPES) measurements, together with density functional theory (DFT) calculations. Both ARPES and DFT calculations find no evidence for band inversions in YbAs, indicating a topologically trivial electronic structure. From low-temperature magnetotransport measurements, we map the field-temperature phase diagram, where we find the presence of a field stabilized phase distinct from the AFM phase at low temperatures. Additionally, an extremely large magnetoresistance (XMR) for both YbAs and the nonmagnetic counterpart LuAs is also observed, which can consistently be accounted for by the presence of electron-hole compensation. Moreover, an angle-dependent study of the Shubnikov-de Haas effect oscillations reveals very similar Fermi surfaces between YbAs and LuAs, with light effective masses down to at least 0.5 K, indicating that the Yb-4f electrons are well localized, and do not contribute to the Fermi surface. However, the influence of the localized Yb-4f electrons on the magnetotransport of YbAs can be discerned from the distinct temperature dependence of the XMR compared to that of LuAs, which we attribute to the influence of short-ranged spin correlations that appear well above TN.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Non-resonant Raman optical activity from phase-space electronic structure theory

In order to model experimental non-resonant Raman optical activity, chemists must compute a host of second-order response tensors (e.g., the electric-dipole–magnetic-dipole polarizability) and their nuclear derivatives along a set of vibrational modes. While these response functions are almost always computed within a Born–Oppenheimer (BO) framework, here we provide a natural interpretation of the electric-dipole–magnetic-dipole polarizability within phase space electronic structure theory, a beyond-BO model whereby the electronic structure depends on nuclear momentum (P) in addition to nuclear position (R). By coupling to nuclear momentum, phase space electronic structure theory is able to capture the asymmetric response of the electronic properties to an external field, in so far as for a vibrating (non-stationary) molecule, $\frac{∂μ}{∂B}$≠$\frac{∂m}{∂F}$, where μ and m are the electrical linear and magnetic dipoles, and F and B are electric and magnetic fields. As an example, for a prototypical methyloxirane molecule, we show that phase space electronic structure theory is able to deliver a reasonably good match with experimental results in a manner that is formally invariant to gauge origin G 0 —provided that one uses a complete basis or, alternatively, gauge invariant atomic orbitals.

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

Electronic Structure of GdCuGe Intermetallic Compound

The electronic structure of GdCuGe intermetallic compound has been studied. Spin-polarized energy spectrum calculations have been performed by the band method with allowance for strong electron correlations in the 4f-shell of gadolinium ions. Antiferromagnetic ordering of GdCuGe at low temperatures has been obtained in a theoretical calculation, with the value of the effective magnetic moment of gadolinium ions reproduced in fair agreement with experimental data. The electronic density of states has been analyzed. An optical conductivity spectrum has been calculated for GdCuGe; it reveals specific features that are analogous to the ones discovered previously in the GdCuSi compound with a similar hexagonal structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Structural, electronic, and magnetic properties of Ni nanoparticles supported on the TiC(001) surface

Metals supported on transition metal carbides are known to exhibit good catalytic activity and selectivity, which is interpreted in terms of electron polarization induced by the support. In the present work we go one step further and investigate the effect that a titanium carbide (TiC) support has on the structural, electronic, and magnetic properties of a series of Ni nanoparticles of increasing size exhibiting a two- or three-dimensional morphology. Furthermore, the obtained results show that three-dimensional nanoparticles are more stable and easier to form than their homologous two-dimensional counterparts. Also, comparison to previous results indicates that, when used as the support, transition metal carbides have a marked different chemical activity with respect to oxides. The analysis of the magnetic moments of the supported nanoparticles evidences a considerable quenching of the magnetic moment that affects mainly the Ni atoms in close contact with the TiC substrate indicating that these atoms are likely to be responsible for the catalytic activity reported for these systems. Overall, the analysis of the electronic structure reveals the existence of chemical interactions between the Ni nanoparticles and the TiC support, even if the net charge transfer between both systems is negligible.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electronic structure of a graphene superlattice with massive Dirac fermions

We study the electronic and transport properties of a graphene-based superlattice theoretically by using an effective Dirac equation. The superlattice consists of a periodic potential applied on a single-layer graphene deposited on a substrate that opens an energy gap of 2Δ in its electronic structure. We find that extra Dirac points appear in the electronic band structure under certain conditions, so it is possible to close the gap between the conduction and valence minibands. We show that the energy gap E{sub g} can be tuned in the range 0 ≤ E{sub g} ≤ 2Δ by changing the periodic potential. We analyze the low energy electronic structure around the contact points and find that the effective Fermi velocity in very anisotropic and depends on the energy gap. We show that the extra Dirac points obtained here behave differently compared to previously studied systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Revealing the two-dimensional electronic structure and anisotropic superconductivity in a natural van der Waals superlattice (PbSe) 1.14 NbSe 2

Van der Waals superlattices are important for tailoring the electronic structures and properties of layered materials. Here we report the superconducting properties and electronic structure of a natural van der Waals superlattice (PbSe) 1.14 NbSe 2 . Anisotropic superconductivity with a transition temperature T c = 5.6 ± 0.1 K, which is higher than monolayer NbSe 2 , is revealed by transport measurements on high-quality samples. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal the two-dimensional electronic structure and a charge transfer of 0.43 electrons per NbSe 2 unit cell from the blocking PbSe layer. In addition, polarization-dependent ARPES measurements reveal a significant circular dichroism with opposite contrast at K and K' valleys, suggesting a significant spin-orbital coupling and distinct orbital angular momentum. In conclusion, our work suggests natural van der Waals superlattice as an effective pathway for achieving intriguing properties distinct from both the bulk and monolayer samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Specific Features of the Electron Structure of ZnTPP Aggregated Forms: Data of Optical Measurements and Quantum-Chemical Calculations

Zinc tetraphenylporphyrin (ZnTPP) films and ZnTPP-based composites are promising materials of organic photonics. Porphyrins are inclined towards self-assembly and the formation of molecular ensembles or differently structured aggregates. The energies of the formation of aggregates and ordered structures and modifications of their electron structures compared to that of a free ZnTPP molecule have not been adequately explored. In the study, the first comprehensive investigation of the structure, absorption and luminescence spectra, and photoluminescence kinetics of structurally perfect ZnTPP thin films produced under quasi-equilibrium conditions in vacuum is conducted. It is shown that changes in the absorption spectra and the red shift of the luminescence spectra of films by 0.15 eV from the spectra observed for ZnTPP solutions in toluene can be interpreted as a result of the formation of an ordered thin-film structure through the dimerization of porphyrin planar molecules under nearly equilibrium conditions. The optimal geometric structure, the energy of the ground state, and the electronic spectrum of excitations of the dimerized (ZnTPP){sub 2} state are calculated in the context of density functional theory and time-dependent density functional theory. The energy gain on the formation of a dimer of symmetry C{sub i} compared to two separate molecules is 0.23 eV per dimer; the HOMO–LUMO gap for the dimer is decreased by 70 meV. The radiative emission time in the ordered solid phase is an order of magnitude shorter than that in the solution in toluene and corresponds to 277 ps, which is typical of J aggregates of porphyrins.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Machine learning the electronic structure of matter across temperatures

Here, we introduce machine learning (ML) models that predict the electronic structure of materials across a wide temperature range. Our models employ neural networks and are trained on density functional theory (DFT) data. Unlike most other ML models that use DFT data, our models directly predict the local density of states (LDOS) of the electronic structure. This provides several advantages, including access to multiple observables such as the electronic density and electronic total free energy. Moreover, our models account for both the electronic and ionic temperatures independently, making them ideal for applications like laser heating of matter. We validate the efficacy of our LDOS-based models on a metallic test system. They accurately capture energetic effects induced by variations in ionic and electronic temperatures over a broad temperature range, even when trained on a subset of these temperatures. These findings open up exciting opportunities for investigating the electronic structure of materials under both ambient and extreme conditions.

Fiedler, Lenz↗
DOE OSTI2026

CoSn-type NiIn1–xSbx (0 ≤ x ≤ 0.17): Site-Selective Substitution, Electronic Structure, Chemical Bonding, and Structural Transformation

CoSn-type intermetallic compounds have emerged as a model platform for Kagome-derived flat-band physics, where subtle chemical perturbations can strongly influence electronic structure and phase stability. Here, we present a combined experimental and theoretical study of Sb-substitution in CoSn-type NiIn1–xSbx (0 ≤ x ≤ 0.17) to elucidate the interplay between site selectivity, solubility limit, chemical bonding, and electronic structure. Rietveld refinements on Neutron powder diffraction data confirmed the selective Sb-substitution at the electron-rich In2 (2d) site forming the honeycomb substructure, while the In1 (1a) site within the Kagome layer remains exclusively occupied by In. Density functional theory (DFT) calculations revealed that pristine CoSn-type NiIn hosts Ni 3d-dominated flat bands near the Fermi level (EF), originating from the Kagome-like Ni substructure. Partial replacement of In by Sb within the honeycomb layer alters these flat-band features below EF, reducing the density of states and suppressing the flat-band topology near the Fermi level. Orbital-resolved electronic structure and chemical-bonding analyses show that Sb-substitution enhances Ni-p-block (In/Sb) covalency and optimizes charge compensation, stabilizing the CoSn-type structure up to the solubility limit x ≈ 0.17. Beyond the limit, the higher-Sb compositions show satellite reflections consistent with an incommensurately modulated phase. These results establish a link between site-selective chemical substitution, bonding optimization, and flat-band electronic structure evolution, providing fundamental insights into how chemical substitution influences the electronic properties of Kagome-based intermetallic compounds.

Roy, Nilanjan [National Institute of Technology Si↗
DOE OSTI2015

Theory of the electronic structure of substitutional semiconductor alloys: Analytical approaches

Methods of predicting the electronic structure of disordered semiconductor alloys involving mainly isoelectronic substitution are reviewed. Special emphasis is placed on analytical methods of studying currently available models of alloys. An approximate equation for the localization threshold of electronic states in the Lifshitz model is considered, and the inaccuracy of this equation is estimated. The contributions of the perturbation potential of an individual impurity and of crystal-lattice distortions in the vicinity of the impurity center are analyzed on the basis of the Faddeev equations. The contributions of intrinsic impurity potentials and volume effects to the formation of the electronic structure of semiconductor alloys are esti- mated. Methods of calculating matrix elements of the perturbation potentials of isoelectronic impurities in alloys with consideration for deformation effects are considered. The procedure of calculating the compositional dependence of the band gap of multicomponent alloys is described. A comparative analysis of various methods for predicting the formation of electronic states bound at individual isoelectronic impurities in semiconductors is conducted. The theory of the energy spectrum of charged impurities in isoelectronic alloys is presented.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Predicting electronic structures at any length scale with machine learning

The properties of electrons in matter are of fundamental importance. They give rise to virtually all material properties and determine the physics at play in objects ranging from semiconductor devices to the interior of giant gas planets. Modeling and simulation of such diverse applications rely primarily on density functional theory (DFT), which has become the principal method for predicting the electronic structure of matter. While DFT calculations have proven to be very useful, their computational scaling limits them to small systems. We have developed a machine learning framework for predicting the electronic structure on any length scale. It shows up to three orders of magnitude speedup on systems where DFT is tractable and, more importantly, enables predictions on scales where DFT calculations are infeasible. Our work demonstrates how machine learning circumvents a long-standing computational bottleneck and advances materials science to frontiers intractable with any current solutions.

36 MATERIALS SCIENCE↗