Hiroshi Iyetomi

Variable-charge interatomic potentials for molecular-dynamics simulations of TiO2

An interatomic potential model has been developed for molecular-dynamics simulations of TiO2 (rutile) based on the formalism of Streitz and Mintmire [J. Adhes. Sci. Technol. 8, 853 (1994)], in which atomic charges vary dynamically according to the generalized electronegativity equalization principle. The present model potential reproduces the vibrational density of states, the pressure-dependent static dielectric constants, the melting temperature, and the surface relaxation of the rutile crystal, as well as the cohesive energy, the lattice constants, and the elastic moduli. We find the physical properties of rutile are significantly affected by dynamic charge transfer between Ti and O atoms. The potential allows us to perform atomistic simulations on nanostructured TiO2 with various kinds of interfaces (surfaces, grain boundaries, dislocations, etc.).

Insight into the origin of superionic conductivity from electronic structure theory

We study the electronic properties of a series of tellurides with the antifluorite structure, and (M = Li, Na, K), to reveal the outstanding diffusivity of silver ions. We carry out first-principles density-functional calculations for these systems using the linearized augmented-plane-wave (LAPW) method, where the exchange-correlation effects of electrons are treated in the local density approximation. The calculations illuminate the difference in the outermost electron configurations between the noble-metal (d-shell) and the alkali (sp-shell) ions. The noble-metal ions embedded in the Te sublattice with the p valence band are far more deformable in the crystalline field than the alkali ions are. We then elucidate that and have remarkably different degrees of p-d hybridization. The d states of Ag atoms are much more weakly coupled with the p states of Te atoms and hence keep their localized nature. The activation energies for the ionic diffusion are evaluated to show that the Ag ion has a smaller barrier as compared with the alkali and copper ions in the Te framework. It is thus inferred that the superionic conductivity of Ag ions primarily stems from combination of the deformability of the d shell and the weakness of the p-d hybridization. To confirm this conclusion, we also apply the LAPW calculation to AgI in the -phase.

Nuclear reaction rates in dense carbon-oxygen mixtures

New, first principles calculations of nuclear reaction rates in dense carbon-oxygen mixtures appropriate to Type I supernova progenitors are presented for both fluid and bcc crystalline-solid phases, based on accurate Monte Carlo evaluations of the screening potentials and a correct treatment of the quantum-statistical effects. A blocking effect of oxygen is discovered in the pycnonuclear reaction of carbon. The results are compared with the existing theories in the limit of one-component plasma cases, both for the screening potentials and for the reaction rates. Examples of the carbon ignition curve are shown. 25 refs.

Freezing transition and phase diagram of dense carbon-oxygen mixtures in white dwarfs

An accurate calculation is presented of the the phase diagram associated with the solidification transitions in dense carbon-oxygen mixtures appropriate to the interiors of white dwarfs, resulting in a novel prediction of an azeotropic diagram. It is concluded that chemical separations would not take place thermodynamically through such solidification processes. 20 references.

Coupled Oscillator Model of the Business Cycle with Fluctuating Goods Markets

The sectoral synchronization observed for the Japanese business cycle in the Indices of Industrial Production data is an example of synchronization. The stability of this synchronization under a shock, e.g., fluctuation of supply or demand, is a matter of interest in physics and economics. We consider an economic system made up of industry sectors and goods markets in order to analyze the sectoral synchronization observed for the Japanese business cycle. A coupled oscillator model that exhibits synchronization is developed based on the Kuramoto model with inertia by adding goods markets, and analytic solutions of the stationary state and the coupling strength are obtained. We simulate the effects on synchronization of a sectoral shock for systems with different price elasticities and the coupling strengths. Synchronization is reproduced as an equilibrium solution in a nearest neighbor graph. Analysis of the order parameters shows that the synchronization is stable for a finite elasticity, whereas the synchronization is broken and the oscillators behave like a giant oscillator with a certain frequency additional to the common frequency for zero elasticity.

Bank-Firm Credit Network in Japan: An Analysis of a Bipartite Network

We investigate the networked nature of the Japanese credit market. Our investigation is performed with tools of network science. In our investigation we perform community detection with an algorithm which is identifying communities composed of both banks and firms. We show that the communities obtained by directly working on the bipartite network carry information about the networked nature of the Japanese credit market. Our analysis is performed for each calendar year during the time period from 1980 to 2011. To investigate the time evolution of the networked structure of the credit market we introduce a new statistical method to track the time evolution of detected communities. We then characterize the time evolution of communities by detecting for each time evolving set of communities the over-expression of attributes of firms and banks. Specifically, we consider as attributes the economic sector and the geographical location of firms and the type of banks. In our 32-year-long analysis we detect a persistence of the over-expression of attributes of communities of banks and firms together with a slow dynamic of changes from some specific attributes to new ones. Our empirical observations show that the credit market in Japan is a networked market where the type of banks, geographical location of firms and banks, and economic sector of the firm play a role in shaping the credit relationships between banks and firms.

Collaborative Simulation Grid: Multiscale Quantum-Mechanical/Classical Atomistic Simulations on Distributed PC Clusters in the US and Japan

A multidisciplinary,collaborative simulation has been performed on a Grid of geographically distributed PC clusters.The multiscale simulation approach seamlessly combines i) atomistic simulation based on the molecular dynamics (MD) method and ii) quantum mechanical (QM) calculation based on the density functional theory (DFT), so that accurate but less scalable computations are performed only where they are needed. The multiscale MD/QM simulation code has been Grid-enabled using i) a modular, additive hybridization scheme, ii) multiple QM clustering, and iii) computation/communication overlapping. The Gridified MD/QM simulation code has been used to study environmental effects of water molecules on fracture in silicon. A preliminary run of the code has achieved a parallel efficiency of 94% on 25 PCs distributed over 3 PC clusters in the US and Japan, and a larger test involving 154 processors on 5 distributed PC clusters is in progress.

The p - d hybridization in the electronic structure of

The electronic structure of is discussed, with special attention paid to possible roles of the p - d hybridization, on the basis of density functional calculations in the local-density approximation; is a typical superionic conductor, and also exhibits interesting electronic properties. Fully self-consistent calculations based on the energy variational principle are carried out for a hypothetical crystalline compound, with the antifluorite structure, making use of the linearized augmented-plane-wave method. The calculated equilibrium lattice constant agrees with a measured value within an accuracy of 5%. The self-consistent determination of the relative sizes of the augmented-plane-wave spheres leads to an energy band structure similar to that of a narrow-gap semiconductor, in accordance with experiments. Also, the magnitudes predicted for the effective masses of the electrons and holes are almost within the experimental uncertainties. Such quantitative agreement even with the static model arises from the following facts: (i) the Ag d states and the Te p states are not hybridized to such an extent as to have a significant influence on the band-structure characteristics near the energy gap; and (ii) the lowest conduction band consists mainly of Ag and Te s states with extended character. Thus the p - d hybridization can hardly be considered to be the mechanism that induces the superionic conductivity in .

Labour Productivity Superstatistics

We discuss superstatistics theory of labour productivity. Productivity distribution across workers, firms and industrial sectors are studied empirically and found to obey power-distributions, in sharp contrast to the equilibrium theories of mainstream economics. The Pareto index is found to decrease with the level of aggregation, {\it i.e.}, from workers to firms and to industrial sectors. In order to explain these phenomenological laws, we propose a superstatistics framework, where the role of the fluctuating temperature is played by the fluctuating demand.

Screening Effect of Degenerate Electron Background in High Density Plasmas. I. Thermodynamic Properties

Corrections to the thermodynamic functions of high-density hydrogen plasmas arising from the screening effects of the degenerate electron liquid are investigated through the variational method based on the Gibbs-Bogoliubov inequality. The Monte Carlo solution to the one-component plasma (OCP) systems is adopted as the reference; the screening effect is taken into account through the improved dielectric function proposed recently by Utsumi and Ichimaru. Results are compared with those obtained by Galam and Hansen on the basis of different screening functions. Relative accuracy and validity of the OCP variational methods are examined on the scales of correlation functions; it is thereby noted that the exchange and Coulombic local-field corrections between electrons introduce significant modifications to the microscopic description of high density plasmas.