The scanning metallic tip of a scanning force microscope was coupled capacitively to electrons confined in a lithographically defined gate-tunable quantum dot at a temperature of 300 mK. Single electrons were made to hop on or off the dot by moving the tip or by changing the tip bias voltage owing to the Coulomb-blockade effect. Spatial images of conductance resonances map the interaction potential between the tip and individual electronic quantum dot states. Under certain conditions this interaction is found to contain a tip-voltage induced and a tip-voltage-independent contribution.
We investigate the magnetoresistance of a side-gated ring structure etched out of single-layer graphene. We observe Aharonov-Bohm oscillations with about 5% visibility. We are able to change the relative phases of the wave functions in the interfering paths and induce phase jumps of π in the Aharonov-Bohm oscillations by changing the voltage applied to the side gate or the back gate. The observed data can be interpreted within existing models for 'dirty metals' giving a phase coherence length of the order of 1 µm at a temperature of 500 mK.
We experimentally investigate the conductance of a singlelayer graphene ring. The Aharonov-Bohm oscillation amplitude of the four-terminal resistance is very high with a visibility up to 10%. Additionally, we investigate the amplitude and the period of the Aharonov-Bohm effect over a magnetic field range of AE5 T. We find that, while the period remains constant, the amplitude rises by a factor of 2.
A quantum point contact ͑QPC͒ patterned on a two-dimensional electron gas is investigated with a scanning gate setup operated at a temperature of 300 mK. The conductance of the point contact is recorded while the local potential is modified by scanning the tip. Discrete charging of traps induced by the local potential is observed as a stepwise conductance change of the constriction. By selectively changing the state of some of these traps, it is possible to observe changes in the transmission of the QPC. The lateral position of such traps is determined, and their density is estimated to be below 50 per m 2 , corresponding to less than 1% of the doping concentration.
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