Akitaka Sawamura

An efficient preconditioning scheme for plane-wave-based electronic structure calculations

We propose a preconditioning scheme for diagonalizing large Hamiltonian matrices arising in first-principles plane-wave pseudopotential calculations. The new scheme is based on the Neumann expansion for the inverse, shifted Hamiltonian from which only a nonlocal part is omitted. The preconditioner is applied to a gradient vector using the fast Fourier transformation technique. In the framework of the Davidson-type diagonalization algorithm, we have found the present preconditioning scheme to be more efficient than widely accepted diagonal scaling methods.

Element-Selective Observation of Electronic Structure Transition between Semiconducting and Metallic States in Boron-Doped Diamond Using Soft X-ray Emission and Absorption Spectroscopy

The electronic structure transition between the semiconducting and metallic states in boron (B)-doped diamonds was element-selectively observed by soft X-ray emission and absorption spectroscopy using synchrotron radiation. For lightly B-doped diamonds, the B 2 p-density of states (DOS) in the valence band was enhanced with a steep-edge feature near the Fermi level, and localized acceptor levels, which are characteristic of semiconductors, were clearly observed both in B 2 p- and C 2 p-DOS in the conduction bands. For heavily B-doped diamonds, the localized acceptor levels developed into extended energy levels and the new energy levels generated formed an extended conduction band structure that overlapped with the valence band. Thus, the metallic energy band structure is actually formed by heavy boron doping. These valence and conduction band structures observed by soft X-ray emission and absorption spectroscopy accounted for the electrical properties of B-doped diamonds.

Iteratively generated pseudopotentials in electronic structure calculations

We present an iterative procedure to generate first-principles, norm-conserving pseudopotentials which are of an analytic form and separable by construction with fewer nonlocal projectors than conventional ones. The procedure consists of two steps: First, reference pseudowavefunctions and eigenvalues for an atom are determined by a conventional method. Second, trial pseudowavefunctions and eigenvalues are calculated for the pseudopotentials with adjustable parameters repeatedly until they match the reference ones within some tolerance. The pseudopotentials allow us to evaluate Hamiltonian matrix elements efficiently and less likely to yield spurious solution. Examples for copper and zinc are shown.