Event

Title: Quantized States, Berry Phases, and Quantum-Hall Wedding-Cake structures in

Graphene Quantum Dots

 

Speaker: Dr. Fereshte Ghahari Kermani, NIST/UMCP

 

Abstract:

Recent progress in creating graphene quantum dots (QDs) with fixed build-in potentials has offered a new platform to visualize and probe the confined electronic states. 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 zero field, the charge carriers are confined by oblique Klein scattering at the p-n junction boundary giving rise to a series of quasi-bound single particle states. 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 first 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.

 

Bio:

Fereshte Ghahari is a NIST/UMD postdoctoral researcher at the National Institute of Standards and Technology (NIST). She received a PhD in physics from Columbia University where her research was targeted toward understanding the fundamental transport properties of graphene and discovering new ways to exploit its exceptional properties. Her major accomplishments were the discovery of fractional quantum hall effect and hydrodynamic thermoelectric transport in graphene which both proved the importance of interaction effects in this system. Her current research is focused on studying the electron-optical behavior and Coulomb charging effects in graphene quantum dots. Employing scanning tunneling microscopy (STM) to directly measure the confined electronic states, she has explored exotic topological phenomena such as Berry phase switching, as well as the interplay between spatial and magnetic confinement in the presence of interactions at high magnetic fields.