Fujiwara Lab
Introduction of principal papers of the lab.
    Lists of all the recent papers and preprints (Last updated: February 12, 2010)
    Lists of all the recent activities in conferences (Last updated: December26, 2009)
(1) Large Scale Electronic Structure Calculations
Two-stage formation model and helicity of gold nanowires
    Y. Iguchi, T. Hoshi, and T. Fujiwara, Phys. Rev. Lett. 99, 125507, 4 (2007)
A model for the formation of helical multishell gold nanowires is proposed and is confirmed with quantum mechanical molecular dynamics simulations. The model can explain the magic number of the helical gold nanowires in the multishell structure. The elementary processes are governed by competition between energy loss and gain by s and d electrons together with the width of the d band. The possibility for the helical nanowires of platinum, silver, and copper is discussed.
Linear algebraic calculation of the Green's function for large-scale electronic structure theory
    R. Takayama, T. Hoshi, T. Sogabe, S.-L. Zhang, and T. Fujiwara, Phys. Rev. B 73, 165108, 9 (2006)
A linear algebraic method named the shifted conjugate-orthogonal conjugate-gradient (shifted-COCG) method is introduced for large-scale electronic structure calculation. The method gives an iterative solver algorithm of the Green's function and the density matrix without calculating eigenstates. The problem is reduced to independent linear equations at many energy points and the calculation is actually carried out only for a single energy point. The method is applied to both a semiconductor and a metal.
Nanoscale structures formed in silicon cleavage studied with large-scale electronic structure calculations:
Surface reconstruction, steps, and bending

    T. Hoshi, Y. Iguchi, and T. Fujiwara, Phys. Rev. B72, 075323 (2005)
The 10-nm-scale structure formed in silicon cleavage is studied by the quantum mechanical calculations of large-scale electronic structure. The cleavage process was simulated and the results show not only the elementary process of the (experimentally observed) (111)-(2x1) surface reconstruction but also several step-formation processes. These processes are studied by analyzing electronic freedom and compared with scanning tunneling microscopy experiments. Several common aspects between cleavage and other phenomena are discussed from the viewpoints of nonequilibrium process and 10-nm-scale structure.
(2) Beyond LDA:
Extension of the DFT to e.g. LDA+U, GW, DMFT and its Application to Strongly Correlated Electron Systems
Electronic structure of antiferromagnetic LaMnO3 and the effects of charge polarization
    Y. Nohara, A. Yamasaki, S. Kobayashi, T. Fujiwara, Phys. Rev. B 74, 064417 (2006)
The electronic structure of A-type antiferromagnetic insulator LaMnO3 is investigated by the GW approximation. The band gap and spectrum are in a good agreement with experimental observation. The lifetime of electrons in conduction bands is much shorter than that of holes in valence bands. The insulator-to-metal transition with antiferromagnetic-to-ferromagnetic transition with photocarrier injection is attributed to the characteristic properties of excited electron states in A-type antiferromagnetic perovskite systems. The onsite d-d Coulomb interaction is strongly screened at the low energy region by mobile eg electrons.

Electronic structure of ferromagnetic bcc-Fe, fcc-Ni and antiferromagnetic NiO in the LDA+DMFT method
    O. Miura and T. Fujiwara, Phy. Rev. B 77, 195124 (2008)
We have constructed a LDA+DMFT method with the iterative perturbation theory (IPT) with full LDA Hamiltonian without mapping onto the effective Wannier orbitals. We then apply this LDA+DMFT method to ferromagnetic bcc-Fe and fcc-Ni, and to antiferromagnetic NiO. In Fe and Ni, the width of occupied 3d bands is narrower than LDA and Ni 6 eV satellite appears. In NiO, on-site Coulomb interaction of Ni-3d bands enhances the hybridization between Ni-3d and O-2p bands and this causes the band gap of 4.3 eV and the charge-transfer insulator. These results are in good agreement with the experimental XPS and then verify that this LDA+DMFT method is applicable to various realistic materials, such as both metallic and insulating cases, compound and spin-polarized cases, and strongly hybridized cases between s, p and d-bands.

Charge and spin stripe in La2-xSrxNiO4
    S. Yamamoto, T. Fujiwara and Y. Hatsugai, Phys. Rev. B 76, 165114 i2007)
Electronic structure of stripe ordered La2-xSrxNiO4 is investigated. The system with x=1/3 is insulator, in LSDA+U, and shows charge and spin stripe, consistent with the experimental results. A highly correlated system of x=1/2 is studied by using exact diagonalization of the multiorbital many-body Hamiltonian derived from LDA calculations and including on-site and inter-site Coulomb interactions. The resultant ground state is an insulator with charge and spin stripe of the energy gap 0.9 eV, consistent with the observed one. Inter-site Coulomb interaction and anisotropy of hopping integrals, play an important role to form the charge and spin stripe order in a system of x=1/2.