Speaker
Description
Bilayer quantum Hall systems host quantum phases which cannot be realised in single-layer systems. Bose–Einstein condensation is the prototypical example where electron and hole excitations from different layers form exciton condensates owing to interlayer Coulomb interactions. However, such an exotic quantum state has only been realised at sub-Kelvin temperatures in conventional GaAs systems, mainly because of weak interlayer couplings. Here, 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 realising interlayer-coherent excitonic condensates. In these systems, subnanometre atomic separation between the layers allows significant interlayer interactions, while interlayer electron tunnelling 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 and 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.
