Jia (Leo) Li
Correlated states in graphene double-layer in the quantum Hall regime
The fast developing technique of layer-by-layer van der Waals (vdW) heterostructure allows any 2D material exfoliated from a bulk crystal to be reassembled into a designer architecture. It adds another dimension to the researches of 2D material while making it possible to study the coupling and interplay between different emergent quantum phenomena through engineering new meta-materials. An example of such meta-material is double-layer graphene heterostructure, two vertically aligned graphene separated by a thin layer of hexagonal boron nitride (hBN). In the quantum Hall effect regime, tuning both graphene layers to half filling of the lowest Landau level induces interlayer excitonic coupling, forming an equivalent system of indirect excitons. I will show that this system of indirect excitons undergoes a Berezinskii-Kosterlitz-Thouless (BKT) type transition at low temperature into a ground state with quasi-long range order. Recent measurements also revealed novel ground states with interlayer coherence appearing at fractional filling nu=p/q, with q being either an even or odd integer. The strength of interlayer and intralayer interaction can be tuned with magnetic field, interlayer separation, filling fractions and density imbalance, providing us with a multi-dimensional phase space to study these ground states, and access the so-called “BEC-BCS crossover” regime.