Visualizing the Coulomb blockade in graphene quantum dots, Part I

The charge of a conductor separated from particle reservoirs by tunnel junctions is quantized in units of the elementary charge, a phenomenon known as Coulomb blockade. Recent experiments have enabled the creation of graphene QDs with fixed build-in potentials inside circular p-n junctions by ionizing impurities in the boron nitride underlying insulator. In these small nanometer sized circular resonators the quasi-bound resonances in zero magnetic field can be confined further by the application of a perpendicular magnetic field forming quantized Landau levels (LL) inside the graphene QD. The LLs at high magnetic fields form a series of metallic rings, separated by highly insulating impressible rings, allowing tunnel barriers to be created between the LL quantum liquids and the sample bias electrode. The isolated LL metallic rings are then accessible by Coulomb blockade spectroscopy between the STM probe and graphene sample. Using scanning tunneling spectroscopy we provide direct spatially and spectroscopically resolved measurements of the formation of the LL rings and their charging characteristics. We investigate the addition energy spectrum of the LL rings and analyze their charging characteristics in terms of capacitances and QD energy level structure.