Satoshi Hagiwara

Spin-polarized annihilation lifetime of positron of d0 ferromagnetism in gallium nitride: A two-component density functional theory simulation

The spin-polarized annihilation lifetime of a positron (SPALP) trapped by the Ga vacancies in GaN was investigated using two-component density functional theory calculations with geometry optimization. SPALP clearly depends on the charged states of the Ga vacancies and the induced magnetization. This dependence is attributed to the overlap between the positron and electron densities around the defect. Thus, the present study proves the useful role of SPALP for probing and revealing the mechanism of d0 ferromagnetism.

Nanoplasmon dynamics and field enhancement of graphene flakes by first-principles simulations

We investigated nanoplasmon dynamics in graphene flake (GF) monomers and dimers using real-time and real-space time-dependent density functional theory. By showing the characteristic features of dynamical polarizability, we verified that the two distinct peaks in the optical absorption spectra can be attributed to π and π + σ plasmons. We clearly show a significant difference between the on- and off-resonance responses of the π plasmon by demonstrating the spatial distribution of the induced charge and the electric field. We thus reveal the mechanism of plasmon-induced field enhancement near GF edges, which shows great importance of first-principles studies on molecular nanoplasmonics.

First-Principles Study on Positron States at Li-Adsorbed Al(100) Surfaces: Effect of Electron-Positron Correlation

We investigated positron states at Li-adsorbed Al(100) surfaces using \textit{ab-initio} two-component density functional theory. The results thus obtained by using semi-local electron-positron (e-p) correlation functionals with the generalized gradient approximation well reproduce the experimental results, showing that reliable e-p correlation functionals are needed to properly describe the various properties of positron surface state.

Positron States at Li- and O-adsorbed Fe(001) Ferromagnetic Surfaces Studied by Two-Component Density Functional Theory

The positron states for Li- and O-adsorbed Fe(001) ferromagnetic surfaces are studied by using two-component density functional theory. Positron surface lifetimes and positron binding energies are found to be sensitive to changes in the surface structure and the dipole barrier induced by adatoms, which can be understood by the positron density distribution and surface potential. Spin-dependent positron lifetime fractions are in excellent agreement with spin-polarization fractions at the topmost surface, because the localized positrons at the surface are sensitive to the surface magnetic state. Therefore, the present results show that spin-polarized positron annihilation spectroscopy can extract the outermost surface magnetic state.

Optical Properties of Boron Nitride and Graphene Nanoribbons: A Time Dependent Density Functional Theory Simulation

The boron nitride (BN) sheet [1], just the analogue of graphene, has attracted wide attention due to its useful properties such as a complementary 2D dielectric substrate for graphene electronics. The BN nanoribbons (BNNRs), which were recently produced under unwrapping multiwalled BN nanotubes, show semiconducting properties due to edge states and imperfections, and have width-dependent energy-band gaps [2] that can be tuned by transverse electric fields. Thus, the BN nanostructures can be promising materials in opto-electronics as functional semiconductors. In the present study we report the results of dielectric properties of the BN nanostructures with the comparison of their carbon analogues by the real-time time-dependent density functional theory in the linear response regime to calculate the dielectric function [3]. Our results have reproduced plasmon peaks in electron energy loss spectra (EELS) of h-BN and the BN sheet observed in a very recent experiment [4]. The profiles of EELS spectra become more similar to those of absorption spectra for BN structures with lower dimension, especially for BNNRs, indicating that the electronic excitations of low dimensional BN structures occur mostly through particle-hole excitations. The detailed comparisons of the results with their carbon analogues and also with the results calculated by the independentparticle approximation will be given in the presentation. [1] H. Zeng, C. Zhi, Z. Zhang, X. Wei, X. Wang, W. Guo, Y. Bando, and D. Golberg, Nano Lett. 10, 5049 (2010). [2] C. Hu, R. Ogura, N. Onoda, S. Konabe, and K. Watanabe, Phys. Rev. B 85, 245420 (2012). [3] G. F. Bertsch, J.-I. Iwata, A. Rubio, and K. Yabana, Phys. Rev. B 62, 7998 (2000). [4] C. T. Pan, R. R. Nair, U. Bangert, Q. Ramasse, R. Jalil, R. Zan, C. R. Seabourne, and A. J. Scott, Phys. Rev. B 85 , 045440 (2012). APPC12 The 12th Asia Pacific Physics Conference