Quantized States, Berry Phases, and Quantum-Hall Wedding-Cake structures in Graphene Quantum Dots

Dr. Fereshte Ghahari Kermani (host Henriksen), NIST
September 24, 2018 at 4:00 pm
241 Compton
Event Description 

Recent progress in creating and probing graphene quantum dots (QDs) with fixed build-in potentials has offered a new platform to investigate Klein tunneling related phenomena. In this talk, I describe scanning tunneling spectroscopy measurements of the energy spectrum of graphene QDs as a function of energy, spatial position, and magnetic field. In the absence of a magnetic field, confinement of graphene carriers in a p-n junction resonator gives rise to a series of quasi-bound single particle states which result from oblique Klein scattering at the p-n interface. Applying a weak magnetic field, we observe a giant and discontinuous change in the energy of time-reversed angular-momentum states, which manifests itself as the appearance of "new" resonances in the tunneling density of states. This behavior corresponds to the on/off switching of a π- Berry phase when a weak critical magnetic field is reached. With increased applied magnetic field, the QD states can be confined even further as they condense into highly degenerate Landau levels providing the firs t spatial visualization of the interplay between spatial and magnetic confinement. This is observed as formation of the seminal wedding-cake structures of concentric compressible and incompressible density rings in strong magnetic fields.


[1] F. Ghahari , D. Walkup, C. Gutiérrez, J. F. Rodriguez-Nieva, Y. Zhao, J. Wyrick, F. D. Natterer, W. G. Cullen, K. Watanabe, T. Taniguchi, L. S. Levitov, N. B. Zhitenev, and J. A. Stroscio, “An on/off Berry Phase Switch in Circular Graphene Resonators,” Science 356, 845-849 (2017).
[2] C. Gutiérrez, D. Walkup, F. Ghahari , Cyprian Lewandowski. F. Rodriguez-Nieva, K. Watanabe, T. Taniguchi, L. S. Levitov, N. B. Zhitenev, and J. A. Stroscio, “Direct Observation of Dirac-Electron Wedding cakes in Graphene Quantum Dots,” Science, 361, 789–794 (2018).