  We perform scanning gate microscopy on individual suspended carbon nanotube
quantum dots. The size and position of the quantum dots can be visually
identified from the concentric high conductance rings. For the ultra clean
devices used in this study, two new effects are clearly identified.
Electrostatic screening creates non-overlapping multiple sets of Coulomb rings
from a single quantum dot. In double quantum dots, by changing the tip voltage,
the interactions between the quantum dots can be tuned from the weak to strong
coupling regime.

Mesoscale and Nanoscale Physics