  Time-dependent quantum transport for graphene nanoribbons (GNR) are
calculated by the hierarchical equation of motion (HEOM) method based on the
nonequilibrium Green's function (NEGF) theory (Xie et.al, J. Chem. Phys. 137,
044113, 2012). In this paper, a new steady state calculation technique is
introduced and accelerated by the contour integration, which is suitable for
large systems. Three Lorentzian fitting schemes for the self-energy matrices
are developed based on the nonlinear least square method. Within these schemes,
the number of Lorentzians is effectively reduced and the fitting results are
good and convergent. With these two developments in HEOM, we have calculated
the transient currents in GNR. We find a new type of edge state with
delta-function-like density of states in many semi-infinite armchair-type GNR.
