AVS 65th International Symposium & Exhibition
    Nanometer-scale Science and Technology Division Thursday Sessions
       Session NS+2D+AS+MN+PC-ThA

Paper NS+2D+AS+MN+PC-ThA4
Adding Electrons One at a Time to Electrostatically Confined Graphene Quantum Dots

Thursday, October 25, 2018, 3:20 pm, Room 102B

Session: SPM – Probing Electronic and Transport Properties
Presenter: Daniel Walkup, National Institute of Standards and Technology (NIST)/ University of Maryland, College Park
Authors: D. Walkup, National Institute of Standards and Technology (NIST)/ University of Maryland, College Park
C. Gutierrez, National Institute of Standards and Technology (NIST)/ University of Maryland, College Park
F. Ghahari, National Institute of Standards and Technology (NIST)/ University of Maryland, College Park
C. Lewandowski, MIT
J. Rodriguez-Nieva, Harvard University
T. Taniguchi, National Institute for Materials Science (NIMS), Japan
K. Watanabe, National Institute for Materials Science (NIMS), Japan
L. Levitov, MIT
N.B. Zhitenev, National Institute of Standards and Technology (NIST)
J.A. Stroscio, National Institute of Standards and Technology (NIST)
Correspondent: Click to Email
The Coulomb blockade of adding charges to isolated metallic systems is one of the most characteristic phenomena of quantum dots (QDs). Here, we created circular graphene QDs in a backgated graphene-hexagonal boron nitride (hBN) device by locally ionizing defects in the hBN layer, using the electric field from the tip of a scanning tunneling microscope (STM). Scanning tunneling spectroscopy (STS) enables us to image the local density of states outside and within these circular graphene resonators. At weak magnetic fields, confinement of graphene electrons is poor and Coulomb blockade is not observed. At higher fields, however, the graphene electrons form quantized Landau levels (LLs) separated by energy gaps. In the area of the QD, the LLs are bent by the electrostatic potential creating metallic (compressible) rings where a LL crosses the Fermi energy, separated by circular insulating barriers (incompressible strips), which isolate the dot from the graphene and enable the onset of Coulomb blockade. Tunneling dI/dV spectra inside the QD reveal a series of Coulomb blockade peaks, which shift as a function of back gate voltage. In the plane defined by gate voltage and sample bias, these peaks form Coulomb lines, whose slope is governed by the relative capacitances between the dot, tip, gate, and sample bias electrodes, and whose relative offsets reveal the addition spectrum of the quantum dot. A characteristic feature of the Coulomb blockade in these systems is the presence of different families of charging lines, one for each LL, which intersect each other and experience avoided crossings. The avoidance pattern of these anticrossings is novel: at the strongest fields, it somewhat resembles the predictions of simple models of electrostatically-coupled QDs, but at weaker fields it diverges very strikingly, and new modeling is needed to reproduce it. This avoidance pattern reflects the interaction of electrons in different LLs, occupying different parts of the QD, and is tunable via the magnetic field and gate voltage. 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 in these novel electronic nanostructures.