Approaching the Minimum Thermal Conductivity in Rhenium-Substituted Higher Manganese Silicides
Abstract not provided
Chen, Xi↗
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Comparison of single junction AlGaInP and GaInP solar cells grown by molecular beam epitaxy
We have investigated ∼2.0 eV (Al{sub x}Ga{sub 1−x}){sub 0.51}In{sub 0.49}P and ∼1.9 eV Ga{sub 0.51}In{sub 0.49}P single junction solar cells grown on both on-axis and misoriented GaAs substrates by molecular beam epitaxy (MBE). Although lattice-matched (Al{sub x}Ga{sub 1−x}){sub 0.51}In{sub 0.49}P solar cells are highly attractive for space and concentrator photovoltaics, there have been few reports on the MBE growth of such cells. In this work, we demonstrate open circuit voltages (V{sub oc}) ranging from 1.29 to 1.30 V for Ga{sub 0.51}In{sub 0.49}P cells, and 1.35–1.37 V for (Al{sub x}Ga{sub 1−x}){sub 0.51}In{sub 0.49}P cells. Growth on misoriented substrates enabled the bandgap-voltage offset (W{sub oc} = E{sub g}/q − V{sub oc}) of Ga{sub 0.51}In{sub 0.49}P cells to decrease from ∼575 mV to ∼565 mV, while that of (Al{sub x}Ga{sub 1−x}){sub 0.51}In{sub 0.49}P cells remained nearly constant at 620 mV. The constant W{sub oc} as a function of substrate offcut for (Al{sub x}Ga{sub 1−x}){sub 0.51}In{sub 0.49}P implies greater losses from non-radiative recombination compared with the Ga{sub 0.51}In{sub 0.49}P devices. In addition to larger W{sub oc} values, the (Al{sub x}Ga{sub 1−x}){sub 0.51}In{sub 0.49}P cells exhibited significantly lower internal quantum efficiency (IQE) values than Ga{sub 0.51}In{sub 0.49}P cells due to recombination at the emitter/window layer interface. A thin emitter design is experimentally shown to be highly effective in improving IQE, particularly at short wavelengths. Our work shows that with further optimization of both cell structure and growth conditions, MBE-grown (Al{sub x}Ga{sub 1−x}){sub 0.51}In{sub 0.49}P will be a promising wide-bandgap candidate material for high-efficiency, lattice-matched multi-junction solar cells.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Complex modeling for the quantification of nanoscale disorder using genetic algorithms, density functional theory and line-profile analysis
Abstract not provided
Koch, Robert J.↗
Analysis of synthetic diamond single crystals by X-ray topography and double-crystal diffractometry
Structural features of diamond single crystals synthesized under high pressure and homoepitaxial films grown by chemical vapor deposition (CVD) have been analyzed by double-crystal X-ray diffractometry and topography. The conditions of a diffraction analysis of diamond crystals using Ge monochromators have been optimized. The main structural defects (dislocations, stacking faults, growth striations, second-phase inclusions, etc.) formed during crystal growth have been revealed. The nitrogen concentration in high-pressure/high-temperature (HPHT) diamond substrates is estimated based on X-ray diffraction data. The formation of dislocation bundles at the film-substrate interface in the epitaxial structures has been revealed by plane-wave topography; these dislocations are likely due to the relaxation of elastic macroscopic stresses caused by the lattice mismatch between the substrate and film. The critical thicknesses of plastic relaxation onset in CVD diamond films are calculated. The experimental techniques for studying the real diamond structure in optimizing crystal-growth technology are proven to be highly efficient.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Interference Lattice-based Loop Nest Tilings for Stencil Computations
A common method for improving performance of stencil operations on structured multi-dimensional discretization grids is loop tiling. Tile shapes and sizes are usually determined heuristically, based on the size of the primary data cache. We provide a lower bound on the numbers of cache misses that must be incurred by any tiling, and a close achievable bound using a particular tiling based on the grid interference lattice. The latter tiling is used to derive highly efficient loop orderings. The total number of cache misses of a code is the sum of (necessary) cold misses and misses caused by elements being dropped from the cache between successive loads (replacement misses). Maximizing temporal locality is equivalent to minimizing replacement misses. Temporal locality of loop nests implementing stencil operations is optimized by tilings that avoid data conflicts. We divide the loop nest iteration space into conflict-free tiles, derived from the cache miss equation. The tiling involves the definition of the grid interference lattice an equivalence class of grid points whose images in main memory map to the same location in the cache-and the construction of a special basis for the lattice. Conflicts only occur on the boundaries of the tiles, unless the tiles are too thin. We show that the surface area of the tiles is bounded for grids of any dimensionality, and for caches of any associativity, provided the eccentricity of the fundamental parallelepiped (the tile spanned by the basis) of the lattice is bounded. Eccentricity is determined by two factors, aspect ratio and skewness. The aspect ratio of the parallelepiped can be bounded by appropriate array padding. The skewness can be bounded by the choice of a proper basis. Combining these two strategies ensures that pathologically thin tiles are avoided. They do not, however, minimize replacement misses per se. The reason is that tile visitation order influences the number of data conflicts on the tile boundaries. If two adjacent tiles are visited successively, there will be no replacement misses on the shared boundary. The iteration space may be covered with pencils larger than the size of the cache while avoiding data conflicts if the pencils are traversed by a scanning-face method. Replacement misses are incurred only on the boundaries of the pencils, and the number of misses is minimized by maximizing the volume of the scanning face, not the volume of the tile. We present an algorithm for constructing the most efficient scanning face for a given grid and stencil operator. In two dimensions it is based on a continued fraction algorithm. In three dimensions it follows Voronoi's successive minima algorithm. We show experimental results of using the scanning face, and compare with canonical loop orderings.
VanderWijngaart, Rob F.↗
Synthesis and electrochemical characterization of Ti{sub x}Ta{sub y}Al{sub z}N{sub 1-δ}O{sub γ} for fuel cell catalyst supports
Quinary Ti{sub x}Ta{sub y}Al{sub z}N{sub 1-δ}O{sub γ} of various compositions have been prepared by a co-precipitation method followed by ammonolysis. The nitride samples were examined as potential catalyst supports in polymer electrolyte membrane fuel cells. The nitride products crystallized in the rock salt (NaCl) structure over a wide range of compositions. The addition of Ta and Al was highly beneficial towards improving the chemical and electrochemical stability of TiN, without a significant loss of electrical conductivity. Platinum particles were successfully deposited on the (oxy)nitride samples, and the composite samples at some compositions were found to be comparable to Pt/carbon in their stability and catalytic activity even without optimizing the Pt deposition and dispersion processes. - Graphical abstract: The effect of additions of Ta and Al into TiN structure. Shifts the lattice constant, and increases its chemical stability in acidic environment.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Simple coating synthesis and enhanced luminescence behaviour of LiLa(MoO4)2:Eu3+@NaF
Highlights: • LiLa(MoO{sub 4}){sub 2}:Eu{sup 3+}{sub 0.06} @ (NaF){sub y wt} were synthesized with a simple coating method. • PL intensity can be enhanced as high as 4.80 times by coating NaF. • Enhancement PL intensity is a major effect, and increasing R is a minor effect. • LiLa(MoO{sub 4}){sub 2}:Eu{sup 3+}{sub 0.06} @ (NaF){sub 0.025 wt} was a phosphor with high R values (R = 5.89). - Abstract: A series of LiLa(MoO{sub 4}){sub 2}:Eu{sup 3+}@ (NaF){sub y wt} were prepared with a simple coating method. XRD results confirm that NaF coating will cause the slight expand of the lattice volume for the sample. Strongly enhanced emission effect inducing by NaF coating is observed. The emission spectra (PL) intensity of the optimal sample, LiLa(MoO{sub 4}){sub 2}:Eu{sup 3+}{sub 0.06} @ (NaF){sub 0.025wt} is as high as 4.8 times of that of LiLa(MoO{sub 4}){sub 2}:Eu{sup 3+}{sub 0.06}. Furthermore, dual emission properties inducing by NaF coating are observed. Enhancement emission of Eu{sup 3+} is a major effect, and increasing R is a minor effect. The optimal coating content of NaF is 2.5 wt%, which the corresponding chromaticity coordinates excited at 465 nm is x = 0.6650, y = 0.3347, which is nearly as same as the NTSC (National Television Standard Committee) standard values for red (x = 0.67, y = 0.33). So, it is a potential red-emitting phosphor with high R values (R = 5.89) for blue light-based white LED.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Thermoelectric Properties of n-type ZrNiPb-Based Half-Heuslers
Here we investigate the half-Heusler ZrNiPb as a n-type thermoelectric material. Our results show that the n-type ZrNiPb-based materials can achieve high peak power factors, ~50 μW cm –1 K –2 , by optimally tuning the carrier concentration via Bi doping. By further Sn-alloying in Pb site, we achieve a significant reduction of lattice thermal conductivity while maintaining the power factor almost unchanged and hence noticeably improve the ZT. Our work demonstrates that n-type ZrNiPb-based half-Heuslers are promising thermoelectric materials.
36 MATERIALS SCIENCE↗
Interatomic Potential in the Nonequilibrium Warm Dense Matter Regime
We present a new measurement of lattice disassembly times in femtosecond-laser-heated polycrystalline Au nanofoils. The results are compared with molecular dynamics simulations incorporating a highly optimized, embedded-atom-method interatomic potential. For absorbed energy densities of 0.9 – 4.3 MJ / kg , the agreement between the experiment and simulation reveals a single-crystal-like behavior of homogeneous melting and corroborates the applicability of the interatomic potential in the nonequilibrium warm dense matter regime. For energy densities below 0.9 MJ / kg , the measurement is consistent with nanocrystal behavior where melting is initiated at the grain boundaries.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Natively Periodic Fast Multipole Method: Approximating the Optimal Green’s Function
The fast multipole method (FMM) obeys periodic boundary conditions "natively" if it uses a periodic Green's function for computing the multipole expansion in the interaction zone of each FMM oct-tree node. One can define the "optimal" Green's function for such a method that results in the numerical solution that converges to the equivalent particle-mesh (PM) solution in the limit of sufficiently high order of multipoles. A discrete functional equation for the optimal Green's function can be derived, but is not practically useful as methods for its solution are not known. Instead, this paper presents an approximation for the optimal Green's function that is accurate to better than 10 -3 in ${L}_{\mathrm{MAX}}$ norm and 10 -4 in L 2 norm for practically useful multipole counts. Such an approximately optimal Green's function offers a practical way for implementing the FMM with periodic boundary conditions natively, without the need to compute lattice sums or to rely on hybrid FMM-PM approaches.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Application of remote sensing to state and regional programs
The problem includes data acquisition and transformation to products acceptable to the users. Optimized institutionalization of data management, product transfer, and education of the user community are also of major concern. With respect to the lattice, various structures were suggested and the fields of application are presented.
Miller, W. F.↗
Update on the Development of a Flutter Analysis Capability for Unconventional Aircraft Concepts Using HCDstruct
Following years of development, the Higher-fidelity Conceptual Design and structural optimization (HCDstruct) tool is being extended to support dynamic aeroservoelastic analysis and structural optimization for advanced aircraft concepts. These required enhancements include: the development of an aerodynamic matching routine for correcting Nastran’s doublet-lattice method aerodynamics; the implementation of control surface structural models; and the implementation of support for Nastran’s flutter solution sequence (SOL 145). This paper presents an update on the implementation of generalized control surface structural models and support for Nastran SOL 145.
Quinlan, Jesse R.↗
Investigation of Aerodynamic Matching Sensitivities for Conceptual Design
A methodology for aerodynamic matching was implemented in the Higher fidelity Conceptual Design and structural optimization (HCDstruct) tool to support transonic flutter simulations. The Enhanced Factor Correction Technique was implemented in HCDstruct to correct the Nastran doublet lattice method using data from corresponding computational fluid dynamics simulations. In this paper, sensitivities associated with this aerodynamic matching technique were investigated using HCDstruct and a Boeing 737-200-like aircraft configuration. Specifically, sensitivities of the aerodynamic matching method were investigated with respect to changes in flight conditions, such as changes in Mach number and angle of attack, and also in key design variables for the main wing, such as changes in aspect ratio and sweep angle. The variations in flight conditions and design parameters were mapped to estimated prediction error in sectional lift for the main wing, which helps to demonstrate the extent to which a user may expect a given aerodynamic correction matrix to provide suitable aerodynamic predictions for a realistic aircraft wing.
Jesse R. Quinlan↗
Investigation of Aerodynamic Matching Sensitivities for Conceptual Design
A methodology for aerodynamic matching was implemented in the Higher fidelity Conceptual Design and structural optimization (HCDstruct) tool to support transonic flutter simulations. The Enhanced Factor Correction Technique was implemented in HCDstruct to correct the Nastran doublet lattice method using data from corresponding computational fluid dynamics simulations. In this paper, sensitivities associated with this aerodynamic matching technique were investigated using HCDstruct and a Boeing 737-200-like aircraft configuration. Specifically, sensitivities of the aerodynamic matching method were investigated with respect to changes in flight conditions, such as changes in Mach number and angle of attack, and also in key design variables for the main wing, such as changes in aspect ratio and sweep angle. The variations in flight conditions and design parameters were mapped to estimated prediction error in sectional lift for the main wing, which helps to demonstrate the extent to which a user may expect a given aerodynamic correction matrix to provide suitable aerodynamic predictions for a realistic aircraft wing.
Jesse Ray Quinlan↗
Interaction Region Design and Detector Integration at JLab's MEIC
The Electron Ion Collider (EIC) will be a next-generation facility for the study of the strong interaction (QCD). JLab's MEIC is designed for high luminosities of up to 10^34 cm^-2 s^-1. This is achieved in part due to an aggressively small beta-star, which imposes stringent requirements on the collider rings' dynamical properties. Additionally, one of the unique features of MEIC is a full-acceptance detector with a dedicated, small-angle, high-resolution detection system, capable of covering a wide range of momenta (and charge-to-mass ratios) with respect to the original ion beam to enable access to new physics. The detector design relies on a number of features, such as a 50 mrad beam crossing angle, large-aperture ion and electron final focusing quads and spectrometer dipoles as well as a large machine-element-free detection space downstream of the final focusing quads. We present an interaction region design developed with close integration of the detector and beam dynamical aspects. The dynamical aspect of the design rests on a symmetry-based concept for compensation of non-linear effects. The optics and geometry have been optimized to accommodate the detection requirements and to ensure the interaction region's modularity for easiness of integration into the collider ring lattices. As a result, the design offers an excellent detector performance combined with the necessary non-linear dynamical properties.
43 PARTICLE ACCELERATORS↗
Imaging phonon dynamics with ultrafast electron microscopy: Kinematical and dynamical simulations
Ultrafast x-ray and electron scattering techniques have proven to be useful for probing the transient elastic lattice deformations associated with photoexcited coherent acoustic phonons. Indeed, femtosecond electron imaging using an ultrafast electron microscope (UEM) has been used to directly image the influence of nanoscale structural and morphological discontinuities on the emergence, propagation, dispersion, and decay behaviors in a variety of materials. Here, we describe our progress toward the development of methods ultimately aimed at quantifying acoustic-phonon properties from real-space UEM images via conventional image simulation methods extended to the associated strain-wave lattice deformation symmetries and extents. Using a model system consisting of pristine single-crystal Ge and a single, symmetric Lamb-type guided-wave mode, we calculate the transient strain profiles excited in a wedge specimen and then apply both kinematical- and dynamical-scattering methods to simulate the resulting UEM bright-field images. While measurable contrast strengths arising from the phonon wavetrains are found for optimally oriented specimens using both approaches, incorporation of dynamical scattering effects via a multi-slice method returns better qualitative agreement with experimental observations. Contrast strengths arising solely from phonon-induced local lattice deformations are increased by nearly an order of magnitude when incorporating multiple electron scattering effects. We also explicitly demonstrate the effects of changes in global specimen orientation on the observed contrast strength, and we discuss the implications for increasing the sophistication of the model with respect to quantification of phonon properties from UEM images.
36 MATERIALS SCIENCE↗
Single-layer tensor network study of the Heisenberg model with chiral interactions on a kagome lattice
Here, we study the antiferromagnetic kagome Heisenberg model with an additional scalar-chiral interaction by using the infinite projected entangled-pair state (iPEPS) ansatz. We discuss in detail the implementation of the optimization algorithm in the framework of the single-layer tensor network based on the corner transfer matrix technique. Our benchmark based on the full-update algorithm shows that the single-layer algorithm is stable, which leads to the same level of accuracy as the double-layer ansatz but with much less computation time. We further apply this algorithm to study the nature of the kagome Heisenberg model with a scalar-chiral interaction by computing the bond dimension scaling of magnetization, bond energy difference, chiral order parameter, and correlation length. In particular, we find that for strong chiral coupling the correlation length, which is extracted from the transfer matrix, saturates to a finite value for large bond dimension, representing a gapped spin-liquid state. Further comparison with density matrix renormalization group results supports that our iPEPS faithfully represents the time-reversal symmetry breaking chiral state. Our iPEPS simulation results shed new light on constructing PEPS for describing gapped chiral topological states.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Simulations of frustrated Ising Hamiltonians using quantum approximate optimization
Novel magnetic materials are important for future technological advances. Theoretical and numerical calculations of ground-state properties are essential in understanding these materials, however, computational complexity limits conventional methods for studying these states. Here we investigate an alternative approach to preparing materials ground states using the quantum approximate optimization algorithm (QAOA) on near-term quantum computers. We study classical Ising spin models on unit cells of square, Shastry-Sutherland and triangular lattices, with varying field amplitudes and couplings in the material Hamiltonian. We find relationships between the theoretical QAOA success probability and the structure of the ground state, indicating that only a modest number of measurements (≲100) are needed to find the ground state of our nine-spin Hamiltonians, even for parameters leading to frustrated magnetism. We further demonstrate the approach in calculations on a trapped-ion quantum computer and succeed in recovering each ground state of the Shastry-Sutherland unit cell with probabilities close to ideal theoretical values. The results demonstrate the viability of QAOA for materials ground state preparation in the frustrated Ising limit, giving important first steps towards larger sizes and more complex Hamiltonians where quantum computational advantage may prove essential in developing a systematic understanding of novel materials.
97 MATHEMATICS AND COMPUTING↗