Quantum transport at the Dirac point: Mapping out the minimum conductivity from pristine to disordered graphene

The phase space for graphene's minimum conductivity sigma(min) is mapped out using Landauer theory modified for scattering using Fermi's golden rule, as well as the nonequilibrium Green's function (NEGF) simulation with a random distribution of impurity centers. The resulting fan diagram spans the range from ballistic to diffusive over varying aspect ratios (W/L), and bears several surprises. The device aspect ratio determines how much tunneling (between contacts) is allowed and becomes the dominant factor for the evolution of sigma(min) from ballistic to diffusive regime. We find an increasing (for W/L > 1) or decreasing (W/L < 1) trend in sigma(min) vs impurity density, all converging around 128q(2)/pi(3)h similar to 4q(2)/h at the dirty limit. In the diffusive limit, the conductivity quasisaturates due to the precise cancellation between the increase in conducting modes from charge puddles vs the reduction in average transmission from scattering at the Dirac point. In the clean ballistic limit, the calculated conductivity of the lowest mode shows a surprising absence of Fabry-Perot oscillations, unlike other materials including bilayer graphene. We argue that the lack of oscillations even at low temperature is a signature of Klein tunneling.