Shell filling and excited states in a fully tunable double quantum dot on a carbon nanotube

We have realized fully controllable and tunable nanotube quantum dots defined with electrostatic top-gates. Using three electrostatic top-gates we have made a NT double dot. Clear honeycomb patterns are observed in the charge stability diagram as a function of two independent side-gates. Furthermore, for the first time we have observed discrete excitations at finite bias. The origin of these excitations is discussed. The central top-gate is used to control the coupling between the two NT dots. Controlling electron interactions in a molecular system is important for many applications such as for example quantum computation.