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AFM micrograph of the sample surface (black). The vertical and diagonal oxide lines are 10 nm high and define a QPC in the underlying 2DEG. The 2DEG area labeled G4 on the left hand side is used to capacitively control the current through the QPC. Applying voltages to the 30 nm thick Schottky gates G1, G2, and G3 (bright) defines a QD.
(a) Differential conductance of the QD , plotted as a function of gate voltage . Maxima in are Coulomb oscillations. For gate voltages (left), no more Coulomb oscillations are observed. (b) Current through the QPC , plotted for the same gate voltages. Kinks in are caused by a change in the QD occupancy by one electron. (c) Transconductance , measured by modulating voltage and detecting with lock-in technique. Local minima reflect the charge occupancy of the QD.
Transconductance in false colors from (bright/blue) to (dark/red), plotted as a function of and . Local minima are caused by Coulomb resonance of the QD (white arrows) or trapped states in the environment (black arrows). Between the QD resonances, the electron number is fixed (labeled from 0 to 7).
QPC read-out efficiency of QDs fabricated by different methods. Values obtained from Refs. 4, 5, 10, and 11.
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