Creating and Probing Electron Whispering-Gallery Modes in Graphene
Yue Y. Zhao, Jonathan E. Wyrick, Donat F. Natterer, Joaquin R. Nieva, Cyprian Lewandowski, Kenji Watanabe, Takashi Taniguchi, Leonid Levitov, Nikolai B. Zhitenev, Joseph A. Stroscio
Designing high-finesse resonant cavities for electronic waves is hampered by the short coherence lengths in solids. Previous approaches, e.g. the seminal nanometer-sized quantum corrals, depend on careful positioning of adatoms at clean surfaces. Here we demonstrate an entirely different approach, inspired by the Whispering Gallery Modes (WGM) such as those in the whispering gallery of St Paul's cathedral. Taking advantage of graphene's unique properties, namely ballistic gate-tunable carriers, we create confined WGM-type modes in circular pn-junction cavities induced in pristine graphene by the scanning tunneling probe in a back-gated graphene device. We can tune the cavity size and the carrier concentration under the probe, independently and in situ. The confined modes, revealed through characteristic resonances in the tunneling spectrum, originate from Klein scattering at pn junction boundaries. The WGM-type confinement and resonances are a new addition to the electron-optics toolbox, paving the way to high-finesse electronic lenses and resonators.