Adatoms in graphene nanoribbons: conductance characteristics and quantum spin Hall effect
We study the charge and spin transport in a two terminal graphene nanoribbon (GNR) decorated with random Gold (Au) adatoms using a Kane-Mele model. Two commonly used GNRs, that is, the armchair graphene nanoribbon (AGNR) and the zigzag graphene nanoribbon (ZGNR) are compared and contrasted, which shows that in presence of Au adatoms, a somewhat robust $2e^2/h$ conductance plateau occurs in the case of AGNR around the zero of the Fermi energy, while in ZGNR this plateau is fragile. We show that this flat plateau, having a conductance value $2e^2/h$, is not a hallmark signature of a topologically nontrivial quantum spin Hall (QSH) state. Further the conductance decreases by a small amount with the density of adatoms. On the other hand, the spin polarized conductance shows distinct features of enhanced conductivity with increasing Au adatom concentration. Further the fluctuations of the spin polarized conductance have features that carry striking resemblance with the charge conductance profile of the GNRs.