Physics, Pohang 37673, Republic of Korea;

We introduce a novel two-dimensional electronic system with ultrastrong interlayer interactions, namely, twisted bilayer graphene with a large twist angle, as an ideal ground for realizing interlayer-coherent excitonic condensates. In these systems, sub-nanometer atomic separation between the layers allows significant interlayer interactions, while interlayer electron tunneling is geometrically suppressed due to the large twist angle. By fully exploiting these two features we demonstrate that a sequence of odd-integer quantum Hall states with interlayer coherence appears at the second Landau level (N = 1). Notably the energy gaps for these states are of order 1 K, which is several orders of magnitude greater than those in GaAs. Furthermore, a variety of quantum Hall phase transitions are observed experimentally. All the experimental observations are largely consistent with our phenomenological model calculations. Hence, we establish that a large twist angle system is an excellent platform for high-temperature excitonic condensation.

Automated summary

We introduce a novel two-dimensional electronic system called twisted bilayer graphene with a large twist angle, which exhibits ultrastrong interlayer interactions and is ideal for realizing interlayer-coherent excitonic condensates. By fully exploiting the sub-nanometer atomic separation and geometrically suppressed interlayer electron tunneling, we demonstrate the appearance of a sequence of odd-integer quantum Hall states with interlayer coherence at the second Landau level (N = 1). These states have energy gaps of order 1 K, several orders of magnitude greater than those in GaAs. Experimental observations of various quantum Hall phase transitions largely support our phenomenological model calculations. Therefore, we establish the excellent platform of a large twist angle system for high-temperature excitonic condensation.