Atsushi Oshiyama

Mechanisms of High Temperature Superconductivity

The focal points of the symposium presented in this book were the following: For the high temperature copper oxide superconductors, the general consensus was that electron correlation effects are of essential importance. Based on this understanding a number of discussions took place: how to include the correlation effects and on the relative importance of various interactions; whether the ground state of a normal state is Fermi-liquid-like or not; where the doped holes mainly exist and on the nature of these holes; the value of the ratio of the energy gap to the critical temperature; whether the mechanics s are magnetic, or charge fluctuation, or excitonic, or very unusual ones quite different from the Fermi-liquid picture; whether a normal phase is normal or abnormal, etc. For the Bi-O superconductors, such as the BaKBiO system, the discussion concentrated on whether or not the mechanism of superconductivity is the same as that of the high temperature copper oxide superconductors.

A massively-parallel electronic-structure calculations based on real-space density functional theory

Abstract Based on the real-space finite-difference method, we have developed a first-principles density functional program that efficiently performs large-scale calculations on massively-parallel computers. In addition to efficient parallel implementation, we also implemented several computational improvements, substantially reducing the computational costs of O ( N 3 ) operations such as the Gram–Schmidt procedure and subspace diagonalization. Using the program on a massively-parallel computer cluster with a theoretical peak performance of several TFLOPS, we perform electronic-structure calculations for a system consisting of over 10,000 Si atoms, and obtain a self-consistent electronic-structure in a few hundred hours. We analyze in detail the costs of the program in terms of computation and of inter-node communications to clarify the efficiency, the applicability, and the possibility for further improvements.

First-principles calculations of electron states of a silicon nanowire with 100,000 atoms on the K computer

Real space DFT (RSDFT) is a simulation technique most suitable for massively-parallel architectures to perform first-principles electronic-structure calculations based on density functional theory. We here report unprecedented simulations on the electron states of silicon nanowires with up to 107,292 atoms carried out during the initial performance evaluation phase of the K computer being developed at RIKEN. The RSDFT code has been parallelized and optimized so as to make effective use of the various capabilities of the K computer. Simulation results for the self-consistent electron states of a silicon nanowire with 10,000 atoms were obtained in a run lasting about 24 hours and using 6,144 cores of the K computer. A 3.08 peta-flops sustained performance was measured for one iteration of the SCF calculation in a 107,292-atom Si nanowire calculation using 442,368 cores, which is 43.63% of the peak performance of 7.07 peta-flops.

Structural tristability and deep Dirac states in bilayer silicene on Ag(111) surfaces

tobethemonolayersiliceneinthepast[Chenetal.,Phys.Rev.Lett.110,085504(2013)]isidentifiedasthebilayer silicene on the Ag(111) surface. The identification is based on our accurate density-functional calculations in whichthreeapproximations,thelocaldensityapproximation,thegeneralized-gradientapproximation,andthevan der Waals density-functional approximation, to the exchange-correlation energy have been carefully examined. We find that the structural tristability exists for the √ 3 × √ 3 bilayer silicene. The calculated energy barriers among the three stable structures are in the range of 7‐9 meV per Si atom, indicating possible flip-flop motions among the three. We have found that the flip-flop motion between two of the three structures produces the honeycomb structure in the STM images, whereas the motion among the three does the 1 × 1 structure. We have found that the electron states which effectively follow the Dirac equation in the freestanding silicene couple with the substrate Ag orbitals due to the bond formation, and shift downwards deep in the valence bands. This feature is common to all the stable or metastable silicene layer on the Ag(111) substrate.

Electrical Resistivity due to Electron-Electron Scattering in Quasi-One-Dimensional Metals

Electrical resistivity along the chain axis of quasi-one-dimensional metals is calculated using the Boltzmann equation on the quasi-one-dimensional band model which consists of two pairs of warped plane-like Fermi surfaces separated by about half the reciprocal lattice vector along the chain axis. It is shown that electron-electron Umklapp scattering is the most dominant mechanism contributing to resistivity. Furthermore it is predicted for the first time that electron-electron Umklapp scattering gives rise to the characteristic temperature dependence of resistivity such as T η where 2≤η≤3, depending on the band parameters. The observed temperature dependence of intrinsic resistivity in (SN) x can be explained solely by the electron-electron Umklapp scattering, as far as the above quasi-one-dimensional Fermi surfaces are adopted.

Structural stability and energy-gap modulation through atomic protrusion in freestanding bilayer silicene

We report on first-principles total-energy and phonon calculations that clarify structural stability and electronic properties of freestanding bilayer silicene. By extensive structural exploration, we reach all the stable structures reported before and find four new dynamically stable structures, including the structure with the largest cohesive energy. We find that atomic protrusion from the layer is the principal relaxation pattern which stabilizes bilayer silicene and determines the lateral periodicity. The hybrid-functional calculation shows that the most stable bilayer silicene is a semiconductor with the energy gap of 1.3 eV.

Performance evaluation of ultra-large-scale first-principles electronic structure calculation code on the K computer

Silicon nanowires are potentially useful in next-generation field-effect transistors, and it is important to clarify the electron states of silicon nanowires to know the behavior of new devices. Computer simulations are promising tools for calculating electron states. Real-space density functional theory (RSDFT) code performs first-principles electronic structure calculations. To obtain higher performance, we applied various optimization techniques to the code: multi-level parallelization, load balance management, sub-mesh/torus allocation, and a message-passing interface library tuned for the K computer. We measured and evaluated the performance of the modified RSDFT code on the K computer. A 5.48 petaflops (PFLOPS) sustained performance was measured for an iteration of a self-consistent field calculation for a 107,292-atom Si nanowire simulation using 82,944 compute nodes, which is 51.67% of the K computer’s peak performance of 10.62 PFLOPS. This scale of simulation enables analysis of the behavior of a silicon nanowire with a diameter of 10–20 nm.

Interstitial Channels that Control Band Gaps and Effective Masses in Tetrahedrally Bonded Semiconductors

We find that electron states at the bottom of the conduction bands of covalent semiconductors are distributed mainly in the interstitial channels and that this floating nature leads to the band-gap variation and the anisotropic effective masses in various polytypes of SiC. We find that the channel length, rather than the hexagonality prevailed in the past, is the decisive factor for the band-gap variation in the polytypes. We also find that the floating nature causes two-dimensional electron and hole systems at the interface of different SiC polytypes and even one-dimensional channels near the inclined SiC surface.

Crossover between silicene and ultra-thin Si atomic layers on Ag (111) surfaces

We report on total-energy electronic structure calculations in the density-functional theory performed for the ultra-thin atomic layers of Si on Ag(111) surfaces. We find several distinct stable silicene structures:

Structural Evolution and Optoelectronic Applications of Multilayer Silicene

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