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(Color online) (a) Optical image of four potential devices consisting of one common source electrode, three common top-gate electrodes, and four individual drain electrodes. On the left hand side of the source electrode an island of catalyst material is positioned from where the carbon nanotubes grow. (b) Atomic force microscope micrograph of the region indicated by the black rectangle in (a). To the left (close to the catalyst island) several tubes can be seen, but only one tube has grown several micrometers away from the island (indicated with white arrows). Source and drain electrodes consisting of titanium, and three top-gate electrodes consisting of five layers of air-oxidized aluminum and titanium, are positioned directly on top of the tube. Some resist residue can be seen around some of the leads. Insert: Schematic side view of the device. (c) Current through the device as function of voltage applied to the back gate at room temperature, and with source-drain voltage.
(Color online) (a) Bias spectroscopy plot of differential conductance vs source-drain voltage and voltage applied to G1, with , , and at . The white dashed lines are guidelines to the eye, indicating charge degeneracy lines. The addition energies of each quantum dot are indicated with green arrows. (b) Schematic figures of the hole transport through the double quantum dot at positions indicated with letters in (a). Solid and dashed lines are filled and empty hole states, respectively. The coupling energy is here neglected since it is much smaller than the addition energies (see below).
(Color online) Charge stability diagrams at of the measured current as function of , and , with , , and . (a) Honeycomb pattern with relative number of holes in each QD indicated with green numbers. The white dashed lines are guidelines to the eye. (b) Closeup of one set of triple points at the position indicated with the relative hole numbers.
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