Scanning tunneling microscope characterizations of a circular graphene resonator realized with p-p junctions

TL;DR

This study uses low-temperature STM/STS to visualize and analyze quasi-bound states in a graphene quantum dot created by a circular p-p junction, revealing magnetic field effects and electron interactions.

Contribution

It provides direct visualization of whispering-gallery-mode confinement and magnetic field-induced Berry phase effects in a graphene quantum dot.

Findings

Observation of quasi-bound states with whispering-gallery-mode confinement

Large energy jumps in quasi-bound states under magnetic field due to Berry phase

Splitting of quasi-bound states indicating strong electron-electron interactions

Abstract

Using low-temperature high-magnetic-field scanning tunneling microscopy and spectroscopy (STM/STS), we systematically study a graphene quantum dot (GQD) defined by a circular graphene p-p junction. Inside the GQD, we observe a series of quasi-bound states arising from whispering-gallery-mode (WGM) confinement of the circular junction and directly visualize these quasi-bound states down to atomic dimensions. By applying a strong magnetic field, a large jump in energy of the quasi-bound states, which is about one-half the energy spacing between the quasi-bound states, is observed. Such a behavior results from turning on a {\pi} Berry phase of massless Dirac fermions in graphene by a magnetic field. Moreover, our experiment demonstrates that a quasi-bound state splits into two peaks with an energy separation of about 26 meV when the Fermi level crosses the quasi-bound state, indicating that there are strong electron-electron interactions in the GQD.