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Zvejnieks G.,Institute for Solid State Physics | Ibenskas A.,Lithuanian Academy of Sciences | Tornau E.E.,Lithuanian Academy of Sciences
Surface and Coatings Technology | Year: 2014

Kinetic Monte Carlo (KMC) simulations of Au-Ni phase separation in Au/Ni(111) surface alloy during nickel carbonyl formation reaction were performed at room temperature by taking into account realistic rates of Au and Ni adatom diffusion and CO adsorption and desorption, while keeping the rate of nickel carbonyl formation reaction as a free parameter. We also obtained pair and three-body interaction constants between Au and Ni adatoms using the ab initio calculations and demonstrated that their proper choice is crucial for understanding the Au-Ni separation process. Three regimes with qualitatively different Ni-CO reaction propagation kinetics in Au/Ni(111) surface alloy were found by varying the constants of trio-triangle interactions within the limits of their accuracy. The sensitivity of the proposed model to interaction parameters leads to the regimes that differ by step flow rate, Au islands formation mechanism (channel-type or homogenous flow), Au island contamination level by Ni impurities and reaction damping tendency at the reaction front. Nucleation of nickel-free Au islands was observed for a homogeneous step flow pattern, and the corresponding step flow rate is in a good quantitative agreement with existing experimental data. © 2013 Elsevier B.V. Source

Ochi M.,University of Tokyo | Tsuneyuki S.,University of Tokyo | Tsuneyuki S.,Institute for Solid State Physics
Journal of Physics: Conference Series | Year: 2013

The transcorrelated (TC) method is one of the wave-function-based methods used for first-principles electronic structure calculations, and in terms of the computational cost is applicable to solid-state calculation. In this method, a many-body wave function of electrons is approximated as a product of the Jastrow factor and the Slater determinant, and the first-principles Hamiltonian is similarity-transformed by the Jastrow factor. The Schrödinger equation is rewritten as an eigenvalue problem for this similarity-transformed Hamiltonian, from which one obtains a self-consistent field (SCF) equation for optimizing one-electron orbitals in the Slater determinant at low computational cost. In contrast, optimization of the Jastrow factor is computationally much more expensive and has not been performed for solid-state calculation of the TC method before. In this study, we develop a new method for optimizing the Jastrow factor at a reasonable computational cost using the random-phase approximation (RPA) and pseudo-variance minimization. We apply this method to some simple solids, and find that the band gap of a wide-band-gap insulator is much improved by RPA. Source

Ochi M.,University of Tokyo | Tsuneyuki S.,University of Tokyo | Tsuneyuki S.,Institute for Solid State Physics
Chemical Physics Letters | Year: 2015

Second-order Møller-Plesset perturbation theory combined with the bi-orthogonal transcorrelated method (BiTC-MP2) was applied to some simple solids for the first time. Correlation energy retrieved with the Jastrow factor significantly changes the results of the BiTC-MP2 method from the traditional MP2 theory using the Hartree-Fock orbitals. Convergence of the BiTC-MP2 method is preferable to that for the HF-MP2 method because the electron correlation is partially retrieved already at the unperturbed BiTC level, which enables practical application of the BiTC-MP2 method to several solids with respect to computational cost. © 2015 Elsevier B.V. All rights reserved. Source

Ochi M.,University of Tokyo | Tsuneyuki S.,University of Tokyo | Tsuneyuki S.,Institute for Solid State Physics
Journal of Chemical Theory and Computation | Year: 2014

We present a new ab initio method for excited-state calculations based on wave function theory: transcorrelated (TC) method combined with configuration interaction singles (CIS). Conventional CIS with the Hartree-Fock wave function, while a popular method for excited-state calculations of molecular systems, cannot describe electron correlation effects, such as the screening effect in solids, resulting in inaccurate results such as overestimation of the band gap and exciton binding energy. Here, we adopt the TC method, which takes electron correlations into account through a similarity transformation of the Hamiltonian using the Jastrow factor, and combine it with the CIS approximation. We calculate the optical absorption spectra of solid LiF and GaAs as a test, and verify that the present method reproduces the spectra more accurately than the conventional HF-CIS. The excitonic effect is well described with our method. © 2014 American Chemical Society. Source

News Article
Site: http://phys.org/physics-news/

Using flux growth in a high vacuum, this group succeeded in the synthesis of EuMnBi , a layered substance of high-quality single crystal thought to have both properties of Dirac fermions and magnets. This substance features hybrid architecture consisting of two-dimensional layers of Bi with Dirac electrons and europium with magnetic properties. In order to verify the strong correlation between Dirac fermions and the magnetic state, this group measured electric resistance in a strong magnetic field (some 30 to 60 tesla) at The Institute for Solid State Physics of The University of Tokyo and the High Field Laboratory for Superconducting Materials of Tohoku University. In order to elucidate the magnetic state, the group conducted magnetic scattering experiments of radiated x-rays at the Photon Factory of the Institute of Materials Structure Science, High Energy Accelerator Research Organization. As a result, they found that electric resistance significantly changed according to the magnetic order of europium. They also found that when a magnetic field was applied perpendicularly to the surface and the direction of magnetic moment was rotated by 90 degrees, the conductivity perpendicular to the surface was suppressed by 10 percent and Dirac fermions were confined within the surface. Furthermore, by confining Dirac electrons in the two-dimensional layer of Bi, or the superconducting layer, through the use of this effect, this joint group achieved a bulk half-integer quantum Hall effect, in which the value of the Hall resistance becomes discrete, in a bulk magnet of Dirac fermions for the first time. This group's achievement will develop a new field of study, strong correlated quantum conductance of Dirac electrons, and contribute to realizing super high-speed spintronics, the foundation of high-speed and energy-saving electronics. Explore further: Scanning tunneling microscopy reveals the exotic properties of an unusual type of electron More information: H. Masuda et al. Quantum Hall effect in a bulk antiferromagnet EuMnBi2 with magnetically confined two-dimensional Dirac fermions, Science Advances (2016). DOI: 10.1126/sciadv.1501117

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