Weak topological insulators with step edges: Subband engineering and its effect on electron transport

A three-dimensional weak topological insulator (WTI) can be regarded as stacked layers of two-dimensional quantum spin-Hall insulators, each of which accommodates a one-dimensional helical edge mode. Massless Dirac electrons emerge on a side surface of WTIs as a consequence of the hybridization of such helical edge modes. We study the energy spectrum and transport of Dirac electrons on a side surface in the presence of step edges, which significantly modify the way of hybridization. It is shown that pseudo-helical modes with a nearly gapless linear dispersion can be created by manipulating step edges in a certain manner. We numerically calculate the average conductance of weakly disordered WTIs and show that it is markedly enhanced by pseudo-helical modes.