Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Localized states”

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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Ferroelastic modulation and the Bloch formalism

The key to the development of advanced materials is to understand their electronic structure-property relationship. Utilization of this understanding to design new electronic materials with desired properties led to modern epitaxial growth approaches for synthesizing artificial lattices, which for almost half a century have become the mainstay of electronic and photonic technologies. In contrast to previous scalar modulation approaches, we now study synthetic crystal lattices that have a tensor artificial modulation and develop a theory for photons and conduction band states in these lattices in a regime with an unusual departure from the familiar consequences of translational symmetry and Bloch's theorem. As a result, this study reveals that a nonmagnetic crystal lattice modulated by a purely geometrical orientational superlattice potential can lead to localized states or to spiral states for electrons and photons, as well as weakly or strongly localized states that could be used to markedly slow down the propagation of light and for optical energy storage applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Structure and vibrations of different charge Ge impurity in α-quartz

Atomic structure and localized vibrations of α‐SiO{sub 2}:Ge are studied using computer modeling techniques. The simulation was carried out by the lattice dynamics calculation of the local density of vibrational states. Local structures parameters are calculated, localized symmetrized vibrations frequency caused by Ge impurity in different charge states are defined. The movements of atoms located near Ge impurity are analyzed and their contribution into localized vibrations of different type is evaluated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

University of Rhode Island Regional Earth Systems Center

The primary objective of this program was to establish the URI Regional Earth System Center (“Center”) that would enhance overall societal wellbeing (health, financial, environmental) by utilizing the best scientific information and technology to achieve optimal policy decisions with maximum stakeholder commitment for energy development, coastal environmental management, water resources protection and human health protection, while accelerating regional economic growth. The Center was to serve to integrate existing URI institutional strengths in energy, coastal environmental management, water resources, and human wellbeing. This integrated research, educational and public/private sector outreach Center was to focus on local, state and regional resources. The centerpiece activity of the Center was in the development and implementation of integrated assessment models (IAMs) that both ‘downscaled’ global observations and interpolated/extrapolated regional observations for analyzing the complexity of interactions among humans and the natural climate system to further our understanding and, ultimately, to predict the future state of our regional earth system. The Center was to begin by first ‘downscaling’ existing global earth systems management tools for studying the causes of local, state and regional climate change and potential social and environmental consequences, with a focus on the regional resources identified above. The Center would ultimately need to address the full feedbacks inherent in the nonlinear earth systems by quantifying the “upscaled” impacts of those regional changes on the global earth system. Through an interacting suite of computer simulations that are informed by observations from the nation’s evolving climate observatories, the Center activities integrates climate science, technology, economics, and social policy into forecasts that will inform solutions to pressing issues in regional climate change science, ‘green economy’ investment and climate policy. These project objectives were designed as part of a 5-year program, which would have constituted the initial phase for the establishment of the Center. Almost immediately (i.e. before receiving even the first year of funding) we were informed that we would not be receiving any funding beyond the initial phase; one year. This seriously impacted our ability to deliver on our objectives and, with that, a re-scoping of the Center priorities was designed to fit the 1-year constraints on funding. It was decided that, given the Center’s emphasis on building IAMs, the best way to proceed was to first focus on one particularly important component of the IAM – a natural sciences model that would be useful for research and forecasting of the circualation/ecology/biogeochemistry of RI’s coastal waters. We have succeeded on that necessarily more limited objective, as we will describe below.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Self-organization of network dynamics into local quantized states

Self-organization and pattern formation in network-organized systems emerges from the collective activation and interaction of many interconnected units. A striking feature of these non-equilibrium structures is that they are often localized and robust: only a small subset of the nodes, or cell assembly, is activated. Understanding the role of cell assemblies as basic functional units in neural networks and socio-technical systems emerges as a fundamental challenge in network theory. A key open question is how these elementary building blocks emerge, and how they operate, linking structure and function in complex networks. Here we show that a network analogue of the Swift-Hohenberg continuum model—a minimal-ingredients model of nodal activation and interaction within a complex network—is able to produce a complex suite of localized patterns. Thus, the spontaneous formation of robust operational cell assemblies in complex networks can be explained as the result of self-organization, even in the absence of synaptic reinforcements.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

Local electronic states of Fe{sub 4}N films revealed by x-ray absorption spectroscopy and x-ray magnetic circular dichroism

We performed x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at Fe L{sub 2,3} and N K-edges for Fe{sub 4}N epitaxial films grown by molecular beam epitaxy. In order to clarify the element specific local electronic structure of Fe{sub 4}N, we compared experimentally obtained XAS and XMCD spectra with those simulated by a combination of a first-principles calculation and Fermi's golden rule. We revealed that the shoulders observed at Fe L{sub 2,3}-edges in the XAS and XMCD spectra were due to the electric dipole transition from the Fe 2p core-level to the hybridization state generated by σ* anti-bonding between the orbitals of N 2p at the body-centered site and Fe 3d on the face-centered (II) sites. Thus, the observed shoulders were attributed to the local electronic structure of Fe atoms at II sites. As to the N K-edge, the line shape of the obtained spectra was explained by the dipole transition from the N 1s core-level to the hybridization state formed by π* and σ* anti-bondings between the Fe 3d and N 2p orbitals. This hybridization plays an important role in featuring the electronic structures and physical properties of Fe{sub 4}N.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Imaging real-space flat band localization in kagome magnet FeSn

Kagome lattices host flat bands due to their frustrated lattice geometry, which leads to destructive quantum interference of electron wave functions. Here, we report imaging of the kagome flat band localization in real-space using scanning tunneling microscopy. We identify both the Fe 3 Sn kagome lattice layer and the Sn 2 honeycomb layer with atomic resolution in kagome antiferromagnet FeSn. On the Fe 3 Sn lattice, at the flat band energy determined by the angle resolved photoemission spectroscopy, tunneling spectroscopy detects an unusual state localized uniquely at the Fe kagome lattice network. We further show that the vectorial in-plane magnetic field manipulates the spatial anisotropy of the localization state within each kagome unit cell. Our results are consistent with the real-space flat band localization in the magnetic kagome lattice. We further discuss the magnetic tuning of flat band localization under the spin–orbit coupled magnetic kagome lattice model.

36 MATERIALS SCIENCE↗
DOE OSTI2015

A Survey of State and Local PV Program Response to Financial Innovation and Disparate Federal Tax Treatment in the Residential PV Sector

High up-front costs and a lack of financing options have historically been the primary barriers to the adoption of photovoltaics (PV) in the residential sector. State clean energy funds, which emerged in a number of states from the restructuring of the electricity industry in the mid-to-late 1990s, have for many years attempted to overcome these barriers through PV rebate and, in some cases, loan programs. While these programs (rebate programs in particular) have been popular, the residential PV market in the United States only started to achieve significant scale in the last five years – driven in large part by an initial wave of financial innovation that led to the rise of third-party ownership.

14 SOLAR ENERGY↗
DOE OSTI2021

Computing Edge States without Hard Truncation

We present a numerical method which accurately computes the discrete spectrum and associated bound states of semi-infinite Hamiltonians which model electronic “edge” states localized at boundaries of one- and two-dimensional crystalline materials. The problem is nontrivial since arbitrarily large finite “hard” (Dirichlet) truncations of the Hamiltonian in the infinite bulk direction tend to produce spurious bound states partially supported at the truncation. Our method, which overcomes this difficulty, is to compute the Green's function of the semi-infinite Hamiltonian by imposing an appropriate boundary condition in the bulk direction; then, the spectral data is recovered via Riesz projection. We demonstrate our method's effectiveness by studies of edge states at a graphene zig-zag edge in the presence of defects modeled both by a discrete tight-binding model and a continuum PDE model under finite difference discretization. Our method may also be used to study states localized at domain wall-type edges in one- and two-dimensional materials where the edge Hamiltonian is infinite in both directions; we demonstrate this for the case of a tight-binding model of distinct honeycomb structures joined along a zig-zag edge. Here, we expect our method to be useful for designing novel devices based on precise wave-guiding by edge states.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2017

On the theory of adsorption on graphene-like compounds

Based on a previously proposed model of the electronic spectrum of binary A{sub N}B{sub 8–N} graphene-like compounds (GLC), adsorption theory is constructed, which allows determination of the role of the adatom level position, the adatom–substrate coupling constant, and the gap inherent to GLC in the free state with heteropolar bonds in the formation of the adatom electronic structure. The cases of free-standing and epitaxial GLCs on a metal surface are considered. In the case of free-standing GLCs, analysis shows local states mainly contribute to the adatom occupation number n{sub a} at large and intermediate adatom–GLC coupling constants; as the coupling constants decrease, the contribution of the GLC valence band increases. The main feature of an epitaxial GLC on metal is the absence of a gap, hence, the contribution of adatom local states to n{sub a}. Estimations show that changes in the adatom–substrate and GLC–metal coupling constants affect n{sub a} in almost the same way. In this case, the dependence of n{sub a} on the GLC gap is not critical from the qualitative viewpoint. Adsorption on the GLC–semiconductor structure is briefly discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Systematic Quantum Cluster Typical Medium Method for the Study of Localization in Strongly Disordered Electronic Systems

Great progress has been made in recent years towards understanding the properties of disordered electronic systems. In part, this is made possible by recent advances in quantum effective medium methods which enable the study of disorder and electron-electronic interactions on equal footing. They include dynamical mean-field theory and the Coherent Potential Approximation, and their cluster extension, the dynamical cluster approximation. Despite their successes, these methods do not enable the first-principles study of the strongly disordered regime, including the effects of electronic localization. The main focus of this review is the recently developed typical medium dynamical cluster approximation for disordered electronic systems. This method has been constructed to capture disorder-induced localization and is based on a mapping of a lattice onto a quantum cluster embedded in an effective typical medium, which is determined self-consistently. Unlike the average effective medium-based methods mentioned above, typical medium-based methods properly capture the states localized by disorder. The typical medium dynamical cluster approximation not only provides the proper order parameter for Anderson localized states, but it can also incorporate the full complexity of Density-Functional Theory (DFT)-derived potentials into the analysis, including the effect of multiple bands, non-local disorder, and electron-electron interactions. After a brief historical review of other numerical methods for disordered systems, we discuss coarse-graining as a unifying principle for the development of translationally invariant quantum cluster methods. Together, the Coherent Potential Approximation, the Dynamical Mean-Field Theory and the Dynamical Cluster Approximation may be viewed as a single class of approximations with a much-needed small parameter of the inverse cluster size which may be used to control the approximation. We then present an overview of various recent applications of the typical medium dynamical cluster approximation to a variety of models and systems, including single and multiband Anderson model, and models with local and off-diagonal disorder. We then present the application of the method to realistic systems in the framework of the DFT and demonstrate that the resulting method can provide a systematic first-principles method validated by experiment and capable of making experimentally relevant predictions. We also discuss the application of the typical medium dynamical cluster approximation to systems with disorder and electron-electron interactions. Most significantly, we show that in the limits of strong disorder and weak interactions treated perturbatively, that the phenomena of 3D localization, including a mobility edge, remains intact. However, the metal-insulator transition is pushed to larger disorder values by the local interactions. We also study the limits of strong disorder and strong interactions capable of producing moment formation and screening, with a non-perturbative local approximation. Here, we find that the Anderson localization quantum phase transition is accompanied by a quantum-critical fan in the energy-disorder phase diagram.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

An accurate and linear-scaling method for calculating charge-transfer excitation energies and diabatic couplings

Quantum-mechanical methods that are both computationally fast and accurate are not yet available for electronic excitations having charge transfer character. In this work, we present a significant step forward towards this goal for those charge transfer excitations that take place between non-covalently bound molecules. In particular, we present a method that scales linearly with the number of non-covalently bound molecules in the system and is based on a two-pronged approach: The molecular electronic structure of broken-symmetry charge-localized states is obtained with the frozen density embedding formulation of subsystem density-functional theory; subsequently, in a post-SCF calculation, the full-electron Hamiltonian and overlap matrix elements among the charge-localized states are evaluated with an algorithm which takes full advantage of the subsystem DFT density partitioning technique. The method is benchmarked against coupled-cluster calculations and achieves chemical accuracy for the systems considered for intermolecular separations ranging from hydrogen-bond distances to tens of Angstroms. Numerical examples are provided for molecular clusters comprised of up to 56 non-covalently bound molecules.

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

Properties of amorphous carbon thin films grown by ion beam sputtering

The electrical performance of amorphous carbon thin films obtained by the ion beam sputtering of a carbon target in argon has been investigated. It has been shown by the Raman spectroscopy method that these films have a graphite-like structure. It has also been found by conductivity and thermopower studies that the hopping mechanism of conductivity with a variable length of hops over localized states near the Fermi level changes to the mechanism of hopping over the nearest neighbors as the temperature rises from 77 to 190 K. Near room temperature, electrotransport is provided by variable-length hops over localized states at the tail of the valence band.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Establishing coherent momentum-space electronic states in locally ordered materials

Rich momentum-dependent electronic structure naturally arises in solids with long-range crystalline symmetry. Reliable and scalable quantum technologies rely on materials that are either not perfect crystals or non-crystalline, breaking translational symmetry. This poses the fundamental questions of whether coherent momentum-dependent electronic states can arise without long-range order, and how they can be characterized. Here we investigate Bi 2 Se 3 , which exists in crystalline, nanocrystalline, and amorphous forms, allowing direct comparisons between varying degrees of spatial ordering. Through angle-resolved photoemission spectroscopy, we show for the first time momentum-dependent band structure with Fermi surface repetitions in an amorphous solid. The experimental data is complemented by a model that accurately reproduces the vertical, dispersive features as well as the replication at higher momenta in the amorphous form. These results reveal that well-defined real-space length scales are sufficient to produce dispersive band structures, and that photoemission can expose the imprint of these length scales on the electronic structure.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Influence of temperature on spin polarization dynamics in dilute nitride semiconductors—Role of nonparamagnetic centers

We report theoretical studies of spin polarization dynamics in dilute nitride semiconductors. We develop a commonly used rate equation model [Lagarde et al., Phys. Status Solidi A 204, 208 (2007) and Kunold et al. Phys. Rev. B 83, 165202 (2011)] to take into account the influence of shallow localizing states on the temperature dependence of spin polarization dynamics and a spin filtering effect. Presented investigations show that the experimentally observed temperature dependence of a spin polarization lifetime in dilute nitrides can be related to the electron capture process by shallow localizing states without paramagnetic properties. This process reduces the efficiency of spin filtering effect by deep paramagnetic centers, especially at low temperatures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Frozen Superparaelectric State of Local Polar Regions in GdMn{sub 2}O{sub 5} and Gd{sub 0.8}Ce{sub 0.2}MnO{sub 5}

A comparative study of the dielectric properties and electric polarization of multiferroics GdMn{sub 2}O{sub 5} and Gd{sub 0.8}Ce{sub 0.2}MnO{sub 5} has been carried out in the temperature range 5–330 K. The polarization properties in the ferroelectric state that forms due to a charge ordering and exchange striction have been studied at T ≤ T{sub C} = 30 K. The properties of the restricted polar phase separation domains formed in the crystals containing ions Mn{sup 3+} and Mn{sup 4+} have been studied, too. These domains exhibit the electric polarization in the temperature range from 5 K to some temperatures T{sub f} ≫ T{sub C}. Such a high-temperature polarization is due to the frozen superparaelectric state of the restricted polar domains.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Defect study of molecular beam epitaxy grown undoped GaInNAsSb thin film using junction-capacitance spectroscopy

Defects in undoped GaInNAsSb thin film (i-GaInNAsSb) were investigated by junction-capacitance technique using admittance and transient photocapacitance (TPC) spectroscopy. An electron trap D2 was identified at 0.34 eV below the conduction band (E{sub C}) of i-GaInNAsSb using admittance spectroscopy. Optical transition of valance band (E{sub V}) electrons to a localized state OH1 (E{sub V} + 0.75 eV) was manifested in negative TPC signal. Combined activation energy of OH1 and D2 defect corresponds to the band-gap of i-GaInNAsSb, suggesting that OH1/D2 acts as an efficient recombination center. TPC signal at {approx}1.59 eV above E{sub V} was attributed to the nitrogen-induced localized state in GaInNAsSb.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Why MnIn{sub 2}O{sub 4} spinel is not a transparent conducting oxide?

The title compound has been synthesized by a citrate technique. The crystal structure has been investigated at room temperature from high-resolution neutron powder diffraction (NPD) data. It crystallizes in a cubic spinel structure, space group Fd3-bar m, Z=8, with a=9.0008(1) A at 295 K. It exhibits a crystallographic formula (Mn{sub 0.924(2)}In{sub 0.076(2)}){sub 8a}(In{sub 1.804(2)}Mn{sub 0.196(2)}){sub 16d}O{sub 4}, where 8a and 16d stand for the tetrahedral and octahedral sites of the spinel structure, respectively, with a slight degree of inversion, {lambda}=0.08. MnIn{sub 2}O{sub 4} shows antiferromagnetic interactions below T{sub N} Almost-Equal-To 40 K, due to the statistical distribution of Mn ions over the two available sites. Unlike the related MgIn{sub 2}O{sub 4} and CdIn{sub 2}O{sub 4} spinels, well known as transparent conducting oxides, MnIn{sub 2}O{sub 4} is not transparent and shows a poor conductivity ({sigma}=0.38 S cm{sup -1} at 1123 K): the presence of Mn ions, able to adopt mixed valence states, localizes the charges that, otherwise, would be delocalized in the spinel conduction band. - Graphical Abstract: From NPD data the crystallographic formula (Mn{sub 0.924(2)}In{sub 0.076(2)}){sub 8a}(In{sub 1.804(2)}Mn{sub 0.196(2)}){sub 16d}O{sub 4}, shows a slight degree of inversion, {lambda}=0.08 and a certain In deficiency. The presence of Mn ions, able to adopt mixed oxidation states, localize the charges that, otherwise, would be delocalized in the spinel conduction band; the presence of localized Mn{sup 2+} and Mn{sup 3+} ions provides the characteristic brown color. Highlights: Black-Right-Pointing-Pointer Accurate structural determination from NPD data: inversion degree (8%), and In deficiency. Black-Right-Pointing-Pointer Bond-valence indicates Mn{sup 2+}-Mn{sup 3+} ions; edge-sharing octahedra contain 90% In{sup 3+}+10% Mn{sup 3+} cations. Black-Right-Pointing-Pointer Conductivity several orders of magnitude lower than those of MgIn{sub 2}O{sub 4} or CdIn{sub 2}O{sub 4}. Black-Right-Pointing-Pointer Variability of Mn oxidation states cancels any electron-doping effect, emptying conduction band of mobile charge carriers. Black-Right-Pointing-Pointer Curie-Weiss behavior confirming the determined charge distribution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗