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Results for “ENERGY-LEVEL DENSITY”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Mechanism of AC charge transport in TlSb{sub 1–x}Ga{sub x}S{sub 2} (x = 0 and 0.03)

The dielectric properties and ac electrical conductivity of TlSb{sub 1–x}Ga{sub x}S{sub 2} (x = 0, 0.03) single-crystals have been measured in the frequency range 5 × 10{sup 4} to 3.5 × 10{sup 7} Hz. Experimental data on the frequency dispersion of the dielectric coefficients and electrical conductivity of the TlSb{sub 1–x}Ga{sub x}S{sub 2} (x = 0, 0.03) single crystals have allowed us to identify the nature of the dielectric loss and the mechanism of charge transport and evaluate parameters of localized states in the band gap. The incorporation of gallium atoms into the crystal lattice of TlSbS{sub 2} crystals has been shown to lead to an increase in the Fermi-level density of states and mean hop time and distance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Photon Induced Reaction Cross-Section Calculations of Several Structural Fusion Materials

In this paper; photo-neutron cross-sections have been computed for {sup 27}Al, {sup 28,29}Si, {sup 52}Cr, {sup 54,56}Fe isotopes by using nuclear reaction codes TALYS 1.6 and EMPIRE 3.1 with different level density models. Constant Temperature Fermi Gas, Back Shifted Fermi Gas and Generalized Superfluid Models have been employed from TALYS 1.6 whereas Generalized Super Fluid Model was the only model employed from EMPIRE 3.1 reaction codes. Obtained results have been analyzed with experimental values taken from the EXFOR database.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Use of a Sigmoid Function to Describe Second Peak in Magnetization Loops

Order-disorder transitions of a vortex lattice transfer type-II superconductors from a low critical current state to a high one. The similar transition between different current states can be caused by electromagnetic granularity. A sigmoid curve is proposed to describe the corresponding peak in a field dependence of the macroscopic critical density. Using the extended critical state model, analytic expressions are obtained for the field dependencies of the local critical current density, the depth of equilibrium surface region, and the macroscopic critical current density. The expressions are well fit to published data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Level density of the sd-nuclei—Statistical shell-model predictions

Accurate knowledge of the nuclear level density is important both from a theoretical viewpoint as a powerful instrument for studying nuclear structure and for numerous applications. For example, astrophysical reactions responsible for the nucleosynthesis in the universe can be understood only if we know the nuclear level density. We use the configuration-interaction nuclear shell model to predict nuclear level density for all nuclei in the sd-shell, both total and for individual spins (only with positive parity). To avoid the diagonalization in large model spaces we use the moments method based on statistical properties of nuclear many-body systems. In the cases where the diagonalization is possible, the results of the moments method practically coincide with those from the shell-model calculations. Using the computed level densities, we fit the parameters of the Constant Temperature phenomenological model, which can be used by practitioners in their studies of nuclear reactions at excitation energies appropriate for the sd-shell nuclei.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Defect-reduction mechanism for improving radiative efficiency in InGaN/GaN light-emitting diodes using InGaN underlayers

The influence of a dilute In{sub x}Ga{sub 1-x}N (x ∼ 0.03) underlayer (UL) grown below a single In{sub 0.16}Ga{sub 0.84}N quantum well (SQW), within a light-emitting diode (LED), on the radiative efficiency and deep level defect properties was studied using differential carrier lifetime (DCL) measurements and deep level optical spectroscopy (DLOS). DCL measurements found that inclusion of the UL significantly improved LED radiative efficiency. At low current densities, the non-radiative recombination rate of the LED with an UL was found to be 3.9 times lower than the LED without an UL, while the radiative recombination rates were nearly identical. This suggests that the improved radiative efficiency resulted from reduced non-radiative defect concentration within the SQW. DLOS measurement found the same type of defects in the InGaN SQWs with and without ULs. However, lighted capacitance-voltage measurements of the LEDs revealed a 3.4 times reduction in a SQW-related near-mid-gap defect state for the LED with an UL. Quantitative agreement in the reduction of both the non-radiative recombination rate (3.9×) and deep level density (3.4×) upon insertion of an UL corroborates deep level defect reduction as the mechanism for improved LED efficiency.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Depth-resolved cathodoluminescence and surface photovoltage spectroscopies of gallium vacancies in β-Ga 2 O 3 with neutron irradiation and forming gas anneals

The gallium vacancy is one of the dominant native point defects in β-Ga 2 O 3 , one that, together with its complexes, can have a major effect on free carrier densities and transport in this wide bandgap semiconductor. We used a combination of depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to identify the optical and energy-level properties of these defects as well as how their defect densities and spatial distributions vary with neutron irradiation and temperature-dependent-forming gas anneals. These studies reveal optical signatures that align closely with theoretical energy-level predictions. Likewise, our optical techniques reveal variations in these defect densities that are consistent with hydrogen passivation of gallium vacancies as a function of temperature and depth from the free Ga 2 O 3 surface. These techniques can help guide the understanding and control of dominant native point defects in Ga 2 O 3 .

42 ENGINEERING↗
DOE OSTI2017

Spectroscopy of He–Ne afterglow plasma

The afterglow of pulsed low-current discharge in a He–Ne mixture is spectroscopically studied under conditions of competition between the processes of Ne{sub 2}{sup +} and HeNe{sup +} dissociative recombination with electrons, which are the sources of population of excited levels of neon atoms. The behavior of neon spectral lines in the wavelength range of 350–850 nm corresponding to the transitions from the levels of neon atoms of the 2p{sup 5}3p, 2p{sup 5}3d, 2p{sup 5}4p, 2p{sup 5}4d, 2p5{sup 5}s, 2p{sup 5}5d, 2p5{sup 6}s configurations are studied by the multichannel photoncounting method at electron density n{sub e} < 10{sup 11} cm{sup –3} and helium and neon pressures P{sub He} = 38 Torr and P{sub Ne} ≈ 10{sup -5}p{sub He} Distributions of recombination fluxes and population over excited states of the neon atom formed in the afterglow by homo- and heteronuclear ions are found. The data obtained show that vibrationally excited molecular ions make no noticeable contribution to the spectrum of decaying He–Ne plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Electronic and Spectral Properties of RRhSn (R = Gd, Tb) Intermetallic Compounds

The investigations of electronic structure and optical properties of GdRhSn and TbRhSn were carried out. The calculations of band spectrum, taking into account the spin polarization, were performed in a local electron density approximation with a correction for strong correlation effects in 4f shell of rare earth metal (LSDA + U method). The optical studies were done by ellipsometry in a wide range of wavelengths, and the set of spectral and electronic characteristics was determined. It was shown that optical absorption in a region of interband transitions has a satisfactory explanation within a scope of calculations of density of electronic states carried out.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Origin of the Magnetism in Undoped and Mn-Doped In{sub 2}O{sub 3}: A First-Principles Study

The magnetism of undoped and Mn-doped In{sub 2}O{sub 3} has been investigated by employing density functional calculations. For undoped In{sub 2}O{sub 3}, a neutral In vacancy in In{sub 2}O{sub 3} leads to the formation of a net moment of 3 μ{sub B}, which originates from the strong spin polarization of 2p orbitals of O atoms. While, Mn-doped In{sub 2}O{sub 3} system shows robust ferromagnetism which can be attributed to the p-d hybridization between Mn and their neighboring O atoms. In addition, because two O atoms between the two Mn atoms are removed, the ground state of the system have a transformation from the ferromagnetic to the antiferromagnetic state, which further demonstrates that the ferromagnetism is mediated through the p-d interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1987

The abundance of interstellar CO

Data from recent observations of interstellar CO column densities are compiled in tables and graphs and compared with the predictions of theoretical models. The energy-level structure of CO and the excitation of interstellar CO are discussed; UV-absorption and IR line observations are characterized; and techniques used to estimate the CO column density from mm-wavelength observations (based on calibration of the CO line intensity with the column density of H2) are described in detail. Consideration is then given to CO chemistry under interstellar-cloud conditions (isotope fractionation and photodissociation) and models for diffuse, translucent, and dense clouds. A number of unsolved problems requiring additional observations or laboratory investigations are identified.

Van Dishoeck, Ewine F.↗
DOE OSTI2026

Hole-Transport Layer for High Current Density and Stability of Sn-Pb Perovskites and All-Perovskite Tandem Solar Cells

Sn-Pb perovskites are essential for achieving efficient single-junction solar cells and all-perovskite tandem solar cells (APTSCs). Although Sn oxidation and defective surfaces were once major limitations, recent advances in intrinsic material quality have largely mitigated these issues. As a result, the HTL-related interface is now regarded as the primary bottleneck for device performance. To address the intrinsic drawbacks of PEDOT:PSS, chemical surface modification and additive strategies have been widely applied, and alternative HTLs, like polymeric, inorganic, or small-molecule HTLs, have also gained attention. These approaches offer improved energy-level alignment, high transparency, and enhanced chemical durability, leading to higher short-circuit current density and longer operational lifetime in both single-junction and tandem devices. In this Perspective, we highlight the key criteria and practical effects of HTL materials suitable for Sn-Pb perovskites.

14 SOLAR ENERGY↗
NASA NTRS1988

Vacuum polarization in a strong external field

Vacuum polarization in the field of a high-Z finite-size nucleus is examined, and the polarization charge density in coordinate space of order alpha(Z alpha)exp n not less than 3 is calculated. Energy-level shifts of K- and L-shell electrons in hydrogenlike systems are given.

Soff, Gerhard↗
NASA NTRS1972

Quantum-mechanical transport equation for atomic systems.

A quantum-mechanical transport equation (QMTE) is derived which should be applicable to a wide range of problems involving the interaction of radiation with atoms or molecules which are also subject to collisions with perturber atoms. The equation follows the time evolution of the macroscopic atomic density matrix elements of atoms located at classical position R and moving with classical velocity v. It is quantum mechanical in the sense that all collision kernels or rates which appear have been obtained from a quantum-mechanical theory and, as such, properly take into account the energy-level variations and velocity changes of the active (emitting or absorbing) atom produced in collisions with perturber atoms. The present formulation is better suited to problems involving high-intensity external fields, such as those encountered in laser physics.

Berman, P. R.↗
DOE OSTI2019

Vacuum-Deposited Biternary Organic Photovoltaics

Ternary blend organic photovoltaics (OPVs) have been introduced to improve solar spectral absorption and reduce energy losses beyond that of binary blend OPVs, but the difficulties in simultaneously optimizing the morphology of three molecular components results in devices that have generally exhibited performance inferior to analogous binary OPVs. In this paper, we introduce a small molecule-based bi-ternary OPV comprising two individual, vacuum deposited binary bulk heterojunctions fused at a planar junction without component intermixing. In contrast to previous reports where the open circuit voltage (V OC ) of a conventional, blended ternary cell lies between that of the individual binaries, the V OC of the bi-ternary OPV corresponds to one of the constituent binaries, depending on the order in which they are stacked relative to the anode. Additionally, dipole-induced energy-level realignment between the two binary segments necessary to achieve maximum efficiency is observed only when using donor-acceptor-acceptor’ dipolar donors in the photoactive heterojunctions. The optimized bi-ternary OPV shows improved performance compared to its two constituent binary OPVs, achieving a power conversion efficiency of 10.6 ± 0.3% under AM 1.5G 1 sun (100 mW/cm 2 ) simulated illumination with V OC = 0.94 ± 0.01 V, a short circuit current density of 16.0 ± 0.5 mA cm -2 and a fill factor of 0.70 ± 0.01.

14 SOLAR ENERGY↗
DOE OSTI2022

Cylindrical C 96 Fullertubes: A Highly Active Metal-Free O 2 -Reduction Electrocatalyst

A new isolation protocol was recently reported for highly purified metallic Fullertubes D 5h -C 90 , D 3d -C 96 , and D 5d -C 100 , which exhibit unique electronic features. Here, we report the oxygen reduction electrocatalytic behavior of C 60 , C 70 (spheroidal fullerenes), and C 90 , C 96 , and C 100 (tubular fullerenes) using a combination of experimental and theoretical approaches. C 96 (a metal-free catalyst) displayed remarkable oxygen reduction reaction (ORR) activity, with an onset potential of 0.85 V and a halfway potential of 0.75 V, which are close to the state-of-the-art Pt/C benchmark catalyst values. Here we achieved an excellent power density of 0.75 W cm −2 using C 96 as a modified cathode in a proton-exchange membrane fuel cell, comparable to other recently reported efficient metal-free catalysts. Combined band structure (experimentally calculated) and free-energy (DFT) investigations show that both favorable energy-level alignment active catalytic sites on the carbon cage are responsible for the superior activity of C 96 .

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

Formulating energy density for designing practical lithium–sulfur batteries

We report the lithium–sulfur (Li–S) battery is one of the most promising battery systems due to its high theoretical energy density and low cost. Despite impressive progress in its development, there has been a lack of comprehensive analyses of key performance parameters affecting the energy density of Li–S batteries. Here, we analyse the potential causes of energy loss during battery operations. We identify two key descriptors (R weight and R energy ) that represent the mass- and energy-level compromise of the full-cell energy density, respectively. A formulation for energy density calculations is proposed based on critical parameters, including sulfur mass loading, sulfur mass ratio, electrolyte/sulfur ratio and negative-to-positive electrode material ratio. The current progress of Ah-level Li–S batteries is also summarized and analysed. Finally, future research directions, targets and prospects for designing practical high-performance Li–S batteries are proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Double-core ionization photoelectron spectroscopy of C 6 H 6 : Breakdown of the “intuitive” ortho-meta-para binding energy ordering of K -1 K -1 states

Single-site Double-Core Hole (ss-DCH or K -2 ) and two-site Double-Core Hole (ts-DCH or K -1 K -1 ) photoelectron spectra including satellite lines were experimentally recorded for the aromatic C 6 H 6 molecule using the synchrotron radiation and multielectron coincidence technique. Density functional theory and post-Hartree-Fock simulations providing binding energies and relative intensities allow us to clearly assign the main K -2 line and its satellites. K -1 K -1 states’ positions and assignments are further identified using a core-equivalent model. We predict that, contrary to what has been observed in the C 2 H 2 n series of molecules, the K -1 K -1 energy-level ordering in C 6 H 6 does not reflect the core-hole distances between the two holes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

The Structure of Electronic States in FeSb{sub 2} According to Optical Spectroscopy and Band Calculations

The electronic structure and optical properties of a binary intermetallic FeSb{sub 2} compound are studied. The calculations of the band structure, that demonstrate the existence of a narrow ∼0.3 eV gap in the energy spectrum of this material, are performed in the approximation of a local electronic density. Spectral characteristics are studied by the ellipsometric method within a 0.22–18 μm range of wavelengths. It is shown that, in the region of interband transitions, the experimental optical conductivity of the compound is satisfactorily interpreted within the performed calculations of the density of electronic states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗