Structure and energetics of Si(111)-(5x2)-Au

We propose a new structural model for the Si(111)-(5x2)-Au reconstruction. The model incorporates a new experimental value of 0.6 monolayer for the coverage of gold atoms, equivalent to six gold atoms per 5x2 cell. Five main theoretical results, obtained from first-principles total-energy calculations, support the model. (1) In the presence of silicon adatoms the periodicity of the gold rows spontaneously doubles, in agreement with experiment. (2) The dependence of the surface energy on the adatom coverage indicates that a uniformly covered phase is unstable and will phase-separate into empty and covered regions, as observed experimentally. (3) Theoretical scanning tunneling microscopy images are in excellent agreement with experiment. (4) The calculated band structure is consistent with angle-resolved photoemission spectra; analysis of their correspondence allows the straightforward assignment of observed surface states to specific atoms. (5) The calculated activation barrier for diffusion of silicon adatoms along the row direction is in excellent agreement with the experimentally measured barrier.Comment: 11 pages, 7 figures, also available with higher-resolution figures from http://cst-www.nrl.navy.mil/users/erwin/ausi111.v5.pd

Theory of spin-polarized transport in semiconductor heterojunctions: Proposal for spin injection and detection in silicon

Spin injection and detection in silicon is a difficult problem, in part because the weak spin-orbit coupling and indirect gap preclude using standard optical techniques. We propose two ways to overcome this difficulty, and illustrate their operation by developing a model for spin-polarized transport across a heterojunction. We find that equilibrium spin polarization of holes leads to a strong modification of the spin and charge dynamics of electrons, and we show how the symmetry properties of the charge current can be exploited to detect spin injection in silicon using currently available techniques.Comment: 4 pages, 4 figures, added footnot

Exploring Scientific Application Performance Using Large Scale Object Storage

One of the major performance and scalability bottlenecks in large scientific applications is parallel reading and writing to supercomputer I/O systems. The usage of parallel file systems and consistency requirements of POSIX, that all the traditional HPC parallel I/O interfaces adhere to, pose limitations to the scalability of scientific applications. Object storage is a widely used storage technology in cloud computing and is more frequently proposed for HPC workload to address and improve the current scalability and performance of I/O in scientific applications. While object storage is a promising technology, it is still unclear how scientific applications will use object storage and what the main performance benefits will be. This work addresses these questions, by emulating an object storage used by a traditional scientific application and evaluating potential performance benefits. We show that scientific applications can benefit from the usage of object storage on large scales.Comment: Preprint submitted to WOPSSS workshop at ISC 201

Phase transition at finite temperature in one dimension: Adsorbate ordering in Ba/Si(111)3x2

We demonstrate that the Ba-induced Si(111)3x2 reconstruction is a physical realization of a one-dimensional antiferromagnetic Ising model with long-range Coulomb interactions. Monte Carlo simulations performed on a corresponding Coulomb-gas model, which we construct based on density-functional calculations, reveal an adsorbate-ordering phase transition at finite temperature. We show numerically that this unusual one-dimensional phase transition should be detectable by low-energy electron diffraction.Comment: 11 pages + 4 figures. Surf. Sci. Lett. (in press

Marturia Dalam Lomba Paduan Suara Pria Kaum Bapa di Gereja Masehi Injili di Minahasa

Penelitian ini bertujuan untuk menganalisis bagaimana Marturia terimplementasi dalam lomba paduan suara pria kaum bapa di GMIM dan mendeskripsikan perlombaan paduan suara sebagai bagian dari Marturia. Penelitian ini merupakan penelitian kualitatif yang dilaksanakan di Wilayah Kakaskasen Rayon Tomohon pada tahun 2022. Data dikumpulkan melalui teknik dokumentasi, observasi dan wawancara. Dari hasil analisis dan interpretasi data diperoleh indikasi bahwa : (1) perlombaan paduan suara anggota P/KB GMIM dapat menjadi wadah kesaksian atau marturia sebagai wujud dari tugas gereja untuk terus memberitakan Injil Yesus Kristus ditengah-tengah dunia dan dapat diimplementasikan dalam kehidupan sehari-hari. (2) Dalam pengimplementasiannya bentuk kesaksian atau marturia, penginjilan bukan tentang menuntun kepada Kristus, melainkan juga tentang bagaimana cara gereja mengikuti Kristus sedemikian rupa sehingga siapapun yang memilih untuk mengikuti Kristus akan merasa diterima dan dihargai. Jadi, penginjilan bukanlah hal bagaimana membawa Injil kepada manusia, melainkan bagaimana bersama- sama dengan orang lain menemukan kehadiran Allah di dalam dunia. Salah satunya dalam bentuk paduan suara. Dari hasil temuan tersebut maka direkomendasikan agar perlombaan paduan suara P/KB GMIM harus terus menjadi sarana kesaksian atau marturia bagi setiap anggota jemaat bukan hanya sekedar ajalg perlombaan saja yang dikemas sedemikan rupa untuk penonjolan diri dan sebagai panggung entertaiment saja, melainkan harus diwujudnyatakan sebagai bentuk kesaksian bagi anggota jemaat

Electronic Structure of Superconducting Ba6c60

We report the results of first-principles electronic-structure calculations for superconducting Ba6C60. Unlike the A3C60 superconductors, this new compound shows strong Ba-C hybridization in the valence and conduction regions, mixed covalent/ionic bonding character, partial charge transfer, and insulating zero-gap band structure.Comment: 11 pages + 4 figures (1 appended, others on request), LaTeX with REVTE

Diffraction at GaAs/Fe3_{3}Si core/shell nanowires: the formation of nanofacets

GaAs/Fe$_{3}$Si core/shell nanowire structures were fabricated by molecular-beam epitaxy on oxidized Si(111) substrates and investigated by synchrotron x-ray diffraction. The surfaces of the Fe$_3$Si shells exhibit nanofacets. These facets consist of well pronounced Fe$_3$Si{111} planes. Density functional theory reveals that the Si-terminated Fe$_3$Si{111} surface has the lowest energy in agreement with the experimental findings. We can analyze the x-ray diffuse scattering and diffraction of the ensemble of nanowires avoiding the signal of the substrate and poly-crystalline films located between the wires. Fe$_3$Si nanofacets cause streaks in the x-ray reciprocal space map rotated by an azimuthal angle of 30{\deg} compared with those of bare GaAs nanowires. In the corresponding TEM micrograph the facets are revealed only if the incident electron beam is oriented along [1$\overline{1}$0] in accordance with the x-ray results. Additional maxima in the x-ray scans indicate the onset of chemical reactions between Fe$_{3}$Si shells and GaAs cores occurring at increased growth temperatures.Comment: 15 pages, 5 figure

NVIDIA Tensor Core Programmability, Performance & Precision

The NVIDIA Volta GPU microarchitecture introduces a specialized unit, called "Tensor Core" that performs one matrix-multiply-and-accumulate on 4x4 matrices per clock cycle. The NVIDIA Tesla V100 accelerator, featuring the Volta microarchitecture, provides 640 Tensor Cores with a theoretical peak performance of 125 Tflops/s in mixed precision. In this paper, we investigate current approaches to program NVIDIA Tensor Cores, their performances and the precision loss due to computation in mixed precision. Currently, NVIDIA provides three different ways of programming matrix-multiply-and-accumulate on Tensor Cores: the CUDA Warp Matrix Multiply Accumulate (WMMA) API, CUTLASS, a templated library based on WMMA, and cuBLAS GEMM. After experimenting with different approaches, we found that NVIDIA Tensor Cores can deliver up to 83 Tflops/s in mixed precision on a Tesla V100 GPU, seven and three times the performance in single and half precision respectively. A WMMA implementation of batched GEMM reaches a performance of 4 Tflops/s. While precision loss due to matrix multiplication with half precision input might be critical in many HPC applications, it can be considerably reduced at the cost of increased computation. Our results indicate that HPC applications using matrix multiplications can strongly benefit from using of NVIDIA Tensor Cores.Comment: This paper has been accepted by the Eighth International Workshop on Accelerators and Hybrid Exascale Systems (AsHES) 201