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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 451 records · Page 25
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DOE OSTI2015

Electronic structures and formation energies of pentavalent-ion-doped SnO{sub 2}: First-principles hybrid functional calculations

We studied the electronic properties and relative thermodynamic stability of several pentavalent-ion (Ta, Nb, P, Sb, and I) doped SnO{sub 2} systems using first-principles hybrid density functional theory calculations, in order to evaluate their potential as transparent conducting oxides (TCOs). I-doped SnO{sub 2}, though conductive, shows a narrowed optical band gap with respect to the undoped system due to the formation of gap states above the valence band. Nb-doped SnO{sub 2} forms localized impurity states below the conduction band bottom, suggesting that the Nb dopant exists as an Nb{sup 4+}-like cation, which is consistent with the recent experimental finding of the formation of the impurity level below the conduction band bottom [Appl. Phys. Express 5, 061201 (2012)]. Ta- and Sb-doped SnO{sub 2} display n-type conductivity, high charge carrier density, and widened optical band gap. P-doped SnO{sub 2} shows similar n-type electronic properties with that of Sb- and Ta-doped systems, and thus P-doped SnO{sub 2} is proposed as a promising candidate TCO for further experimental validation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Prediction of new high pressure structural sequence in thorium carbide: A first principles study

In the present work, we report the detailed electronic band structure calculations on thorium monocarbide. The comparison of enthalpies, derived for various phases using evolutionary structure search method in conjunction with first principles total energy calculations at several hydrostatic compressions, yielded a high pressure structural sequence of NaCl type (B1) → Pnma → Cmcm → CsCl type (B2) at hydrostatic pressures of ∼19 GPa, 36 GPa, and 200 GPa, respectively. However, the two high pressure experimental studies by Gerward et al. [J. Appl. Crystallogr. 19, 308 (1986); J. Less-Common Met. 161, L11 (1990)] one up to 36 GPa and other up to 50 GPa, on substoichiometric thorium carbide samples with carbon deficiency of ∼20%, do not report any structural transition. The discrepancy between theory and experiment could be due to the non-stoichiometry of thorium carbide samples used in the experiment. Further, in order to substantiate the results of our static lattice calculations, we have determined the phonon dispersion relations for these structures from lattice dynamic calculations. The theoretically calculated phonon spectrum reveal that the B1 phase fails dynamically at ∼33.8 GPa whereas the Pnma phase appears as dynamically stable structure around the B1 to Pnma transition pressure. Similarly, the Cmcm structure also displays dynamic stability in the regime of its structural stability. The B2 phase becomes dynamically stable much below the Cmcm to B2 transition pressure. Additionally, we have derived various thermophysical properties such as zero pressure equilibrium volume, bulk modulus, its pressure derivative, Debye temperature, thermal expansion coefficient and Gruneisen parameter at 300 K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J. Nucl. Mater. 57, 280 (1975)].

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Discrete space charge affected field emission: Flat and hemisphere emitters

Models of space-charge affected thermal-field emission from protrusions, able to incorporate the effects of both surface roughness and elongated field emitter structures in beam optics codes, are desirable but difficult. The models proposed here treat the meso-scale diode region separate from the micro-scale regions characteristic of the emission sites. The consequences of discrete emission events are given for both one-dimensional (sheets of charge) and three dimensional (rings of charge) models: in the former, results converge to steady state conditions found by theory (e.g., Rokhlenko et al. [J. Appl. Phys. 107, 014904 (2010)]) but show oscillatory structure as they do. Surface roughness or geometric features are handled using a ring of charge model, from which the image charges are found and used to modify the apex field and emitted current. The roughness model is shown to have additional constraints related to the discrete nature of electron charge. The ability of a unit cell model to treat field emitter structures and incorporate surface roughness effects inside a beam optics code is assessed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Normal and abnormal evolution of argon metastable density in high-density plasmas

A controversial problem on the evolution of Ar metastable density as a function of electron density (increasing trend versus decreasing trend) was resolved by discovering the anomalous evolution of the argon metastable density with increasing electron density (discharge power), including both trends of the metastable density [Daltrini et al., Appl. Phys. Lett. 92, 061504 (2008)]. Later, by virtue of an adequate physical explanation based on a simple global model, both evolutions of the metastable density were comprehensively understood as part of the abnormal evolution occurring at low- and high-density regimes, respectively, and thus the physics behind the metastable evolution has seemed to be clearly disclosed. In this study, however, a remarkable result for the metastable density behavior with increasing electron density was observed: even in the same electron density regime, there are both normal and abnormal evolutions of metastable-state density with electron density depending on the measurement position: The metastable density increases with increasing electron density at a position far from the inductively coupled plasma antenna but decreases at a position close to the antenna. The effect of electron temperature, which is spatially nonuniform in the plasma, on the electron population and depopulation processes of Argon metastable atoms with increasing electron density is a clue to understanding the results. The calculated results of the global model, including multistep ionization for the argon metastable state and measured electron temperature, are in a good agreement with the experimental results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Epitaxial ZnO/LiNbO{sub 3}/ZnO stacked layer waveguide for application to thin-film Pockels sensors

We produced slab waveguides consisting of a LiNbO{sub 3} (LN) core layer that was sandwiched with Al-doped ZnO cladding layers. The ZnO/LN/ZnO stacked layers were grown on sapphire C-planes by electron cyclotron resonance (ECR) plasma sputtering and were subjected to structural, electrical, and optical characterizations. X-ray diffraction confirmed that the ZnO and LN layers were epitaxial without containing misoriented crystallites. The presence of 60°-rotational variants of ZnO and LN crystalline domains were identified from X-ray pole figures. Cross-sectional transmission electron microscopy images revealed a c-axis orientated columnar texture for LN crystals, which ensured operation as electro-optic sensors based on optical anisotropy along longitudinal and transversal directions. The interfacial roughness between the LN core and ZnO bottom layers as well as that between the ZnO top and the LN core layers was less than 20 nm, which agreed with surface images observed with atomic force microscopy. Outgrowth of triangular LN crystalline domains produced large roughness at the LN film surface. The RMS roughness of the LN film surface was twice that of the same structure grown on sapphire A-planes. Vertical optical transmittance of the stacked films was higher than 85% within the visible and infrared wavelength range. Following the approach adopted by Teng and Man [Appl. Phys. Lett. 56, 1734 (1990)], ac Pockels coefficients of r{sub 33} = 24-28 pm/V were derived for c-axis oriented LN films grown on low-resistive Si substrates. Light propagation within a ZnO/LN/ZnO slab waveguide as well as within a ZnO single layer waveguide was confirmed. The birefringence of these waveguides was 0.11 for the former and 0.05 for the latter.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Transport and capture properties of Auger-generated high-energy carriers in (AlInGa)N quantum well structures

Recent photoluminescence experiments presented by M. Binder et al. [Appl. Phys. Lett. 103, 071108 (2013)] demonstrated the visualization of high-energy carriers generated by Auger recombination in (AlInGa)N multi quantum wells. Two fundamental limitations were deduced which reduce the detection efficiency of Auger processes contributing to the reduction in internal quantum efficiency: the transfer probability of these hot electrons and holes in a detection well and the asymmetry in type of Auger recombination. We investigate the transport and capture properties of these high-energy carriers regarding polarization fields, the transfer distance to the generating well, and the number of detection wells. All three factors are shown to have a noticeable impact on the detection of these hot particles. Furthermore, the investigations support the finding that electron-electron-hole exceeds electron-hole-hole Auger recombination if the densities of both carrier types are similar. Overall, the results add to the evidence that Auger processes play an important role in the reduction of efficiency in (AlInGa)N based LEDs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Tailoring Si(100) substrate surfaces for GaP growth by Ga deposition: A low-energy electron microscopy study

For GaP-on-Si(100) heteroepitaxy, currently considered as a model system for monolithic integration of III–V semiconductors on Si(100), the surface steps of Si(100) have a major impact on the quality of the GaP film. Monoatomic steps cause antiphase domains in GaP with detrimental electronic properties. A viable route is to grow the III–V epilayer on single-domain Si(100) with biatomic steps, but preferably not at the expense of reduced terrace widths introduced by miscut substrates. We have performed in situ investigations of the influence of Ga deposition on the kinetics of surface steps and terraces of Si(100) at substrate temperatures above 600 °C by low-energy electron microscopy. Starting from nearly equally distributed T{sub A} and T{sub B} terraces of a two-domain Si(100) surface, submonolayer deposition of Ga results in a transformation into a surface dominated by T{sub A} terraces and biatomic D{sub A} steps. This transformation is reversible, and Si(100) with monoatomic steps is recovered upon termination of the Ga flux. Under conditions of higher coverages (but still below 0.25 monolayer), we observe restructuring into a surface with T{sub B} dominance, similar to the findings of Hara et al. [J. Appl. Phys. 98, 083515 (2005)]. The occurrence and mutual transformations of surface structures with different terrace and step structures in a narrow range of temperatures and Ga deposition rates is discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Weakly nonlinear Bell-Plesset effects for a uniformly converging cylinder

In this research, a weakly nonlinear (WN) model has been developed considering the growth of a small perturbation on a cylindrical interface between two incompressible fluids which is subject to arbitrary radial motion. We derive evolution equations for the perturbation amplitude up to third order, which can depict the linear growth of the fundamental mode, the generation of the second and third harmonics, and the third-order (second-order) feedback to the fundamental mode (zero-order). WN solutions are obtained for a special uniformly convergent case. WN analyses are performed to address the dependence of interface profiles, amplitudes of inward-going and outward-going parts, and saturation amplitudes of linear growth of the fundamental mode on the Atwood number, the mode number (m), and the initial perturbation. The difference of WN evolution in cylindrical geometry from that in planar geometry is discussed in some detail. It is shown that interface profiles are determined mainly by the inward and outward motions rather than bubbles and spikes. The amplitudes of inward-going and outward-going parts are strongly dependent on the Atwood number and the initial perturbation. For low-mode perturbations, the linear growth of fundamental mode cannot be saturated by the third-order feedback. For fixed Atwood numbers and initial perturbations, the linear growth of fundamental mode can be saturated with increasing m. The saturation amplitude of linear growth of the fundamental mode is typically 0.2λ–0.6λ for m < 100, with λ being the perturbation wavelength. Thus, it should be included in applications where Bell-Plesset [G. I. Bell, Los Alamos Scientific Laboratory Report No. LA-1321, 1951; M. S. Plesset, J. Appl. Phys. 25, 96 (1954)] converging geometry effects play a pivotal role, such as inertial confinement fusion implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Domain configuration changes under electric field-induced antiferroelectric-ferroelectric phase transitions in NaNbO{sub 3}-based ceramics

We recently developed a feasible crystal chemistry strategy to stabilize the antiferroelectricity in NaNbO{sub 3} through a chemical substitution to decrease the tolerance factor and increase the average electronegativity of the system [Shimizu et al., Dalton Trans. 44, 10763 (2015) and Guo et al., J. Appl. Phys. 117, 214103 (2015)]. Two novel lead-free antiferroelectric (AFE) solid solutions, (1-x)NaNbO{sub 3}-xCaZrO{sub 3} and (1-x)NaNbO{sub 3}-xSrZrO{sub 3}, have been found to exhibit the double polarization hysteresis typical of a reversible AFE ↔ ferroelectric (FE) phase transition. In this study, as demonstrated by (1-x)NaNbO{sub 3}-xCaZrO{sub 3} system, the influence of chemical modification and electrical poling on the AFE/FE phase stability was investigated, primarily focusing on the microstructural and crystallographic evolutions. Together with the macroscopic polarization hysteresis measurements, a well-demonstrated structure-property relationship was presented. It was found that the CaZrO{sub 3} substitution into NaNbO{sub 3} can effectively destabilize the FE Q phase and correspondingly lead to a spontaneous reverting to AFE P phase. In contrast to the reversible AFE ↔ FE phase transition, the domain morphology evolution exhibits irreversible nature with a growing process of the orientational domains after applying electric field. Moreover, a multiple-zone axes electron diffraction map of P and Q phases has been summarized and is believed to be an efficient diagram to determine the AFE/FE nature of the NaNbO{sub 3}-based systems.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Initiation of vacuum insulator surface high-voltage flashover with electrons produced by laser illumination

In this paper, experiments are described in which cylindrical vacuum insulator samples and samples inclined at 45° relative to the cathode were stressed by microsecond timescale high-voltage pulses and illuminated by focused UV laser beam pulses. In these experiments, we were able to distinguish between flashover initiated by the laser producing only photo-electrons and when plasma is formed. It was shown that flashover is predominantly initiated near the cathode triple junction. Even dense plasma formed near the anode triple junction does not necessarily lead to vacuum surface flashover. The experimental results directly confirm our conjecture that insulator surface breakdown can be avoided by preventing its initiation [J. G. Leopold et al., Phys. Rev. ST Accel. Beams 10, 060401 (2007)] and complement our previous experimental results [J. Z. Gleizer et al., IEEE Trans. Dielectr. Electr. Insul. 21, 2394 (2014) and J. Z. Gleizer et al., J. Appl. Phys. 117, 073301 (2015)].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Internal quantum efficiency in yellow-amber light emitting AlGaN-InGaN-GaN heterostructures

We determine the internal quantum efficiency of strain-balanced AlGaN-InGaN-GaN hetero-structures designed for yellow-amber light emission, by using a recent model based on the kinetics of the photoluminescence decay initiated by Iwata et al. [J. Appl. Phys. 117, 075701 (2015)]. Our results indicate that low temperature internal quantum efficiencies sit in the 50% range and we measure that adding an AlGaN layer increases the internal quantum efficiency from 50% up to 57% with respect to the GaN-InGaN case. More dramatic, it almost doubles from 2.5% up to 4.3% at room temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Experimental implementation of phase locking in a nonlinear interferometer

Based upon two cascade four-wave mixing processes in two identical hot rubidium vapor cells, a nonlinear interferometer has been experimentally realized [Jing et al., Appl. Phys. Lett. 99, 011110 (2011); Hudelist et al., Nat. Commun. 5, 3049 (2014)]. It has a higher degree of phase sensitivity than a traditional linear interferometer and has many potential applications in quantum metrology. Phase locking of the nonlinear interferometer is needed before it can find its way into applications. In this letter, we investigate the experimental implementation of phase locking of the relative phase between the three beams at different frequencies involved in such a nonlinear interferometer. We have utilized two different methods, namely, beat note locking and coherent modulation locking. We find that coherent modulation locking can achieve much better phase stability than beat note locking in our system. Our results pave the way for real applications of a nonlinear interferometer in precision measurement and quantum manipulation, for example, phase control in phase-sensitive N-wave mixing process, N-port nonlinear interferometer and quantum-enhanced real-time phase tracking.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Convergence of shock waves generated by underwater electrical explosion of cylindrical wire arrays between different boundary geometries

The results of experiments and numerical simulations of a shock wave propagating between either conical or parabolic bounding walls are presented. The shock wave was generated by a microsecond timescale underwater electrical explosion of a cylindrical wire array supplied by a current pulse having an amplitude of ∼230 kA and a rise time of ∼1 μs. It is shown that with the same energy density deposition into the exploding wire array, the shock wave converges faster between parabolic walls, and as a result, the pressure in the vicinity of convergence is ∼2.3 times higher than in the case of conical walls. The results obtained are compared to those of earlier experiments [Antonov et al., Appl. Phys. Lett. 102, 124104 (2013)] with explosions of spherical wire arrays. It is shown that at a distance of ∼400 μm from the implosion origin the pressure obtained in the current experiments is higher than for the case of spherical wire arrays.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Realization of an all optical exciton-polariton router

We report on the experimental realization of an all optical router for exciton-polaritons. This device is based on the design proposed by Flayac and Savenko [Appl. Phys. Lett. 103, 201105 (2013)], in which a zero-dimensional island is connected through tunnel barriers to two periodically modulated wires of different periods. Selective transmission of polaritons injected in the island, into either of the two wires, is achieved by tuning the energy of the island state across the band structure of the modulated wires. We demonstrate routing of ps polariton pulses using an optical control beam which controls the energy of the island quantum states, thanks to polariton-exciton interactions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Distribution theory for Schrödinger’s integral equation

Much of the literature on point interactions in quantum mechanics has focused on the differential form of Schrödinger’s equation. This paper, in contrast, investigates the integral form of Schrödinger’s equation. While both forms are known to be equivalent for smooth potentials, this is not true for distributional potentials. Here, we assume that the potential is given by a distribution defined on the space of discontinuous test functions. First, by using Schrödinger’s integral equation, we confirm a seminal result by Kurasov, which was originally obtained in the context of Schrödinger’s differential equation. This hints at a possible deeper connection between both forms of the equation. We also sketch a generalisation of Kurasov’s [J. Math. Anal. Appl. 201(1), 297–323 (1996)] result to hypersurfaces. Second, we derive a new closed-form solution to Schrödinger’s integral equation with a delta prime potential. This potential has attracted considerable attention, including some controversy. Interestingly, the derived propagator satisfies boundary conditions that were previously derived using Schrödinger’s differential equation. Third, we derive boundary conditions for “super-singular” potentials given by higher-order derivatives of the delta potential. These boundary conditions cannot be incorporated into the normal framework of self-adjoint extensions. We show that the boundary conditions depend on the energy of the solution and that probability is conserved. This paper thereby confirms several seminal results and derives some new ones. In sum, it shows that Schrödinger’s integral equation is a viable tool for studying singular interactions in quantum mechanics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Optimal ancilla-free Pauli+V circuits for axial rotations

We address the problem of optimal representation of single-qubit rotations in a certain unitary basis consisting of the so-called V gates and Pauli matrices. The V matrices were proposed by Lubotsky, Philips, and Sarnak [Commun. Pure Appl. Math. 40, 401–420 (1987)] as a purely geometric construct in 1987 and recently found applications in quantum computation. They allow for exceptionally simple quantum circuit synthesis algorithms based on quaternionic factorization. We adapt the deterministic-search technique initially proposed by Ross and Selinger to synthesize approximating Pauli+V circuits of optimal depth for single-qubit axial rotations. Our synthesis procedure based on simple SL{sub 2}(ℤ) geometry is almost elementary.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Melting curves and structural properties of tantalum from the modified-Z method

The melting curves and structural properties of tantalum (Ta) are investigated by molecular dynamics simulations combining with potential model developed by Ravelo et al. [Phys. Rev. B 88, 134101 (2013)]. Before calculations, five potentials are systematically compared with their abilities of producing reasonable compressional and equilibrium mechanical properties of Ta. We have improved the modified-Z method introduced by Wang et al. [J. Appl. Phys. 114, 163514 (2013)] by increasing the sizes in L{sub x} and L{sub y} of the rectangular parallelepiped box (L{sub x} = L{sub y} ≪ L{sub z}). The influences of size and aspect ratio of the simulation box to melting curves are also fully tested. The structural differences between solid and liquid are detected by number density and local-order parameters Q{sub 6}. Moreover, the atoms' diffusion with simulation time, defects, and vacancies formations in the sample are all studied by comparing situations in solid, solid-liquid coexistence, and liquid state.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Sound velocities in highly oriented pyrolytic graphite shocked to 18 GPa: Orientational order dependence and elastic instability

Previous reports of rapid phase transformation above 18 GPa [Erskine and Nellis, Nature 349, 317 (1991)] and large elastic waves below 18 GPa [Lucas et al., J. Appl. Phys. 114, 093515 (2013)] for shock-compressed ZYB-grade highly-oriented pyrolytic graphite (HOPG), but not for less oriented ZYH-grade HOPG, indicated a link between the orientational order dependence of the HOPG response above and below the phase transformation stress. To gain insight into this link and into the mechanical response of HOPG shocked to peak stresses approaching the phase transformation onset, the compressibility of ZYB- and ZYH-grade HOPG in the shocked state was examined using front surface impact experiments. Particle velocity histories and sound velocities were measured for peak stresses reaching 18 GPa. Although the locus of the measured peak stress-particle velocity states is indistinguishable for the two grades of HOPG, the measured sound velocities in the peak state reveal significant differences between the two grades. Specifically, 1) The measured sound velocities are somewhat higher for ZYH-grade HOPG, compared to ZYB-grade HOPG. 2) The measured sound velocities for ZYH-grade HOPG increase smoothly with compression, whereas those for ZYB-2 grade HOPG exhibit a significant reduction in the compression dependence from 12 GPa to 17 GPa and an abrupt increase from 17 GPa to 18 GPa. 3) The longitudinal moduli, determined from the measured sound velocities, are smaller than the calculated bulk moduli for ZYB-grade HOPG shocked to peak stresses above 15 GPa, indicating the onset of an elastic instability. The present findings demonstrate that the softening of the longitudinal modulus (or elastic instability) presented here is linked to the large elastic waves and the rapid phase transformation reported previously – all observed only for shocked ZYB-grade HOPG. The elastic instability in shocked ZYB-grade HOPG is likely a precursor to the rapid phase transformation observed above 18 GPa for this HOPG grade.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗