Special Condensed Matter Seminar: Yahui Zhang, MIT - “Bridging Hubbard model physics and quantum Hall physics in ABC trilayer graphene/h-BN moire superlattice”

Recently several moire superlattice systems have been shown experimentally to host interesting strongly correlated phases because of the narrow bands. In this talk I will focus on the moire superlattice formed by ABC stacked trilayer graphene aligned with a hexagonal boron nitride substrate (TG/h-BN). Mott-like insulators in this system have already been observed experimentally by Feng Wang et.al. (Chen, Feng Wang et.al. arxiv: 1803.01985). In the first part of the talk, I will provide effective models for this system based on band structure calculations and Wannier orbital construction. Remarkably, in TG/h-BN, both the bandwidth and the topology can be tuned by an applied perpendicular electric field D . For D<0, the valence bands of the two valleys are trivial and the physics is governed by a spin-valley Hubbard model on triangular lattice. For D>0, the narrow bands of the two valleys have non-zero Chern numbers C=3,-3, which may result in interesting quantum Hall like phases. Therefore the TG/h-BN system can simulate both Hubbard model physics and nearly flat Chern band physics within one sample through a simple switch of the vertical displacement field. In the last part of the talk, I will focus on the D<0 side and discuss possible superconductor and spin liquid states in the large U region of the spin-valley t-U Hubbard model on triangular lattice. I will highlight several special aspects of the spin-valley Hubbard model which are absent in the familiar spin 1/2 case. It is likely that this spin-valley model will host new physics beyond a naive simulation of the “cuprate” physics.