Probe spin, valley, and layer degree of freedom in a moiré superlattice

A new phase of matter usually emerges when a given symmetry breaks spontaneously, which can involve charge, spin, valley, and layer degree of freedom. Twisting graphene multilayer to form a moiré superlattice leads to moiré flat bands where various correlated states are developed. Twisted double bilayer graphene (TDBG) is an electrical displacement field (D) tunable moiré system. Here, I will focus on the high-quality TDBG device and present the experimental observations of the field-tunable exotic phases, such as spin/valley polarized correlated phases, first-order quantum phase transitions, chiral topology, and layer polarized phases in the weakly coupled regime, where interplays among correlation, isospin polarization, topology, layer polarization, and Landau quantization are important.