Visualizing the Coulomb blockade in graphene quantum dots; Part II

The Coulomb blockade (CB) is one of the most characteristic phenomena of nanoscale artificial atoms. Here, we created circular quantum dots (QD) in exfoliated graphene on hexagonal boron nitride (hBN), by locally ionizing defects in the hBN using the tip of a scanning tunneling microscope (STM). At high magnetic fields, the gaps between Landau levels (LLs) create insulating barriers inside and around the QDs, enabling a capacitive interaction with the STM tip, sample, and back gate electrodes. Inside the QDs, dI/dV spectra reveal a series of CB peaks, alongside local density of states peaks due to LLs. By sweeping the gate voltage, we construct spectroscopic gate maps in which the Coulomb peaks appear as lines, whose slope is governed by the capacitances between dot, tip, and sample electrodes, and whose offsets reveal the addition spectrum of the QDs. Each LL has its own series of charging lines, creating anticrossings whose characteristics reflect the interactions between electrons in different LLs, and depend strongly on both the magnetic field and the gate voltage, especially at weaker fields. By moving the STM tip, we can tune the tip-dot capacitance, and tunnel into different parts of the dot, enabling a full characterization of the anticrossings of these coulomb peaks.