Van Hove driven symmetry breaking phases in twisted double bilayer graphene and superconductivity in a new twisted system

Two dimensional moire superlattices have emerged as a new playground for studying correlated physics. Various correlated phases including supercondcutivity, correlated insulators and metals, ferromagnetism and generalized Wigner crystal have been observed in twisted systems. These correlated phases have been generally attributed to the flat bands formed by hybridization between the two twisting layers. Here we forward the understanding of this connection between strong correlation and flat bands by studying the correlated phases in twisted double bilayer graphene (TDBG). We show that in TDBG, not only do we have spin-polarizaed correlated insulators as shown in our previous study, but also we have clear signatures of different isospin symmetry breaking phases and magnetic field induced Chern insulators. By comparing the transport measurements and single particle band calculation, we find that these symmetry breaking phases are closely related to the evolution of van Hove singularities in TDBG energy bands. In addition, we will show superconductivity in a new twisted system.