Figure 1: Graphene QD device and low-bias transport measurements.
(a,b) Schematic and scanning force micrograph of the measured device. The central island is connected to source and drain electrodes by two 40 nm wide and 80 nm long constrictions, whose transparency can be tuned by the voltage applied to the nearby nanoribbons (VLG, VRG). The nanoribbon on the right hand side is also used as a charge detector for the dot. The central gate is connected to a bias-tee mixing AC and DC signals. (c) Current through the dot as a function of the left and right gate voltages VLG, VRG, recorded at a source-drain voltage VSD=−1.5 mV and VCG=0 mV. Here and in the following, the electron temperature is Te<100 mK, and the back gate voltage is VBG=34.6 V, corresponding to a Fermi level deep in the transport gap (Supplementary Fig. S1a). Different families of resonances can be identified in this measurement, which can either be attributed to the dot (features with relative lever arm 0.9; dotted line) or to localized states either in the left or right constriction (features with a relative lever arm of 5 and of 0.25, respectively; dashed lines). This indicates the possibility of tuning the transparency of the two constrictions independently, and controlling the current through the dot down to the sub-pA level by acting on VLG, VRG. (d) Simultaneous measurements of the current flowing through the dot and through the right nanoribbon as a function of VCG. The bias voltage applied to the dot and to the nanoribbon are VSD=−1.5 mV and VCD=0.2 mV, respectively. Barrier-gate voltages are VLG=0.4 V and VRG=0 V.
