with antiferromagnetic ordering between two Fe electrodes, (c) k-resolved

transmission spectrum across the MgO layer in the structure (b).

As miniaturization of electronic devices has advanced in recent years,
the gap between experiments and theories has gradually narrowed.
To quantitatively address the quantum electronic transport problem in nano-scale
electronic devices, one of most successful methods is the nonequilibrium Green's
function (NEGF) method because of the generality of the theoretical framework.
We have recently developed an efficient implementation technique of the NEGF method
combined with the density functional theory (DFT) using localized pseudo-atomic
orbitals (PAOs) for electronic transport calculations of a system connected
with two leads under a finite bias voltage [1].
In the implementation, accurate and efficient methods are newly developed especially
for evaluation of the density matrix and treatment of boundaries between
the scattering region and the leads. Equilibrium and nonequilibrium contributions
in the density matrix are evaluated with very high precision by a contour
integration with a continued fraction representation of the Fermi-Dirac function [2]
and by a simple quadrature on the real axis with a small imaginary part, respectively.
The Hartree potential is computed efficiently by a combination of the two
dimensional fast Fourier transform (FFT) and a finite difference method, and
the charge density near the boundaries is constructed with a careful treatment
to avoid the spurious scattering at the boundaries. The efficiency of the
implementation is demonstrated by rapid convergence properties of the density
matrix. In addition, as an illustration, our method is applied for zigzag
graphene nanoribbons, a Fe/MgO/Fe tunneling junction (as shown in Figure),
and a LaMnO_{3}/SrMnO_{3} superlattice, demonstrating
its applicability to a wide variety of systems.
The research was conducted in collaboration with AIST and NIMS.

- "Efficient implementation of the nonequilibrium Green function method for electronic transport calculations", T. Ozaki, K. Nishio, and H. Kino, Phys. Rev. B 81, 035116 (2010).
- "Continued fraction representation of the Fermi-Dirac function for large-scale electronic structure calculations", T. Ozaki, Phys. Rev. B 75, 035123 (2007).