Figure 2: Finite-bias ES spectroscopy and charge sensing.
(a) Current through the QD as function of VCG and the source-drain bias VSD in a regime of weak tunnel coupling to the leads; the dashed lines are guides to the eye indicating the edges of the Coulomb diamonds. (b) Line-cut at constant VCG=4.86 V, marked by a line in (a): the stepwise increase of the current is a signature of the discrete QD spectrum; the two well-defined plateaus correspond to the GS and the first ES entering the bias-window. (c) Differential conductance of the QD, dIQD/dVSD. Resonances parallel to both edges of the Coulomb diamonds, indicating transport through ESs, can be clearly seen. (d) Derivative of the charge-detector current ICD with respect to VCG. Regions of high dICD/dVCG correspond to the onset of the transitions with the largest rate, thus providing information on the asymmetry of the tunnelling barriers. Note that this effect can be bias dependent (for example, the diamond centred on VCG≈4.45 V is more strongly coupled to the right lead for VSD>0 and to the left one for VSD<0) and it can be drastically influenced by the onset of transitions through ESs, as indicated by the appearance of kinks as those marked by the arrow. (e,f) Simultaneous measurements of the differential conductance of the dot dIQD/dVSD (e) and of the transconductance of the charge detector dICD/dVSD (f) in a regime of interest for pulsed-gate experiments.
