Transport measurement of twisted bilayer-bilayer graphene

The recent discovery of correlated insulator behavior and superconductivity in magic-angle graphene has attached tremendous interest in twisted van der Waals systems. So far, people have been focusing on the moiré superlattice and the flat band formed due to the interlayer coupling. In fact, twisting can be used to engineer the symmetry of a 2D system, which may lead to interesting phenomena that are not allowed in their natural form. In this work, we explore such broken symmetry effect in a twisted bilayer-bilayer graphene structure. The two bilayers are rotated with respect to each other by 180^o. Based on the symmetry analysis, the twisted four-layer system is strongly inversion-broken and has an out-of-plane polar axis. We fabricated high quality dual-gated devices and we will report on the transport and optical behavior of this novel system.