In condensed matter systems, the interplay of topology, strong correlation, and internal electronic degrees of freedoms often gives rise to exciting, emergent quantum phenomena. The Bernal-stacked bilayer graphene (BLG) is an excellent platform to study such physics owing to its high sample quality and wide device tunability. In this talk, I'll discuss two of our works on the electric transport studies of the fractional quantum Hall effect (FQHE) and quantum valley Hall effect (QVHE) in BLG. These findings are enabled by the fabrication of state-of-the-art bilayer graphene devices with low disorder, high-quality Ohmic contacts, and complex device structures achieved using precision lithography.

 

In the first part of the talk, I'll discuss our results on the fractional quantum Hall effect, including the construction of the valley isospin polarization transition phase diagram for the odd-denominator FQH states on N = 0 Landau Level, the observation of a new even-denominator FQH state and its spontaneous valley polarization, and the discussion of the particle-hole symmetry breaking of the family of even-denominator FQH states in BLG.

 

In the second part of the talk, I'll present our results on the QVHE, manifesting as the kink state in BLG. The kink state is an internal topological helical edge state realized by electrical gating. For the first time, we achieve ballistic transport, manifesting as resistance quantization in two-terminal measurements, of the kink state at zero magnetic field. We show the quantization has weak temperature dependence up to tens of Kelvin and is robust in a wide range of Fermi Level and dc bias. At last, we'll demonstrate a new kind of switch, namely topological switch, using the kink states.