Quantum transport in van der Waals junctions of graphene and 2D materials

We present our recent experiments in graphene/2D crystal van der Waals junctions. First, we demonstrate that high-quality Josephson junctions can be built by connecting two exfoliated crystal flakes of a layered 2D superconductor. In the vdW junctions of the layered 2D superconductor, current-voltage characteristics showed hysteretic zero-bias current, indicating the under-damped Josephson effect. Application of an in-plane magnetic field induced a periodic modulation of the critical current due to the phase shift in supercurrent. Next, we study quantum Hall effect in twisted bilayer graphene fabricated by stacking two flakes of mechanically-exfoliated graphene with a twisted angle of 0.4-4.0 degrees. In low magnetic fields, the twisted bilayer graphene exhibited quantum Hall plateaus at Landau-level filling factors of 8n+4. In high magnetic fields, quantum Hall plateau developed at Landau-level filling factors of 4n. The observed transition of Landau level degeneracy between eightfold and fourfold is attributed to a topological change of the Fermi surface at van Hove singularity point.