Electronic triple-dot transport through a bilayer graphene island with ultrasmall constrictions

A quantum dot has been etched in bilayer graphene connected by two small constrictions to the leads. We show that this structure does not behave like a single quantum dot but consists of at least three sites of localized charge in series. The high symmetry and electrical stability of the device allowed us to triangulate the positions of the different sites of localized charge and find that one site is located in the island and one in each of the constrictions. Nevertheless we measure many consecutive non-overlapping Coulomb-diamonds in series. In order to describe these findings, we treat the system as a strongly coupled serial triple quantum dot. We find that the non-overlapping Coulomb diamonds arise due to higher order cotunneling through the outer dots located in the constrictions. We extract all relevant capacitances, simulate the measured data with a capacitance model and discuss its implications on electrical transport.