Electron–electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle^{1,2,3,4,5,6,7}. For bilayer graphene where the two layers are twisted by the ‘magic angle’, flat band and strong many-body effects lead to correlated insulating states and superconductivity^4,5,6,7. In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field^8,9,10, providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of displacement field-tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the g factor, according to the spin Zeeman effect, is ~2, indicating spin polarization at half-filling. These results establish twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states.

电子-电子相互作用在石墨烯和相关系统中起着重要作用，并且可以诱导奇特的量子态，特别是在具有^{1,2,3,4,5,6,7}小扭转角的堆叠双层中。对于双层石墨烯，其中两层被“魔角”扭曲，扁平带和强多体效应导致相关的绝缘态和超导性^4,5,6,7。与单层石墨烯相反，未扭曲的双层石墨烯的能带结构可以通过位移场^8,9,10进一步调整，提供了额外的自由度来控制两个双层堆叠在一起时应出现的扁平带。在这里，我们报告在双双层石墨烯中位移场可调电子相的发现和表征。我们在位移场的中间范围内观察到半填充导带处的绝缘状态。此外，相关绝缘子中的电阻间隙相对于面内磁场增大，并且我们发现，根据自旋塞曼效应，g因子约为2，表明半填充时的自旋极化。这些结果建立了扭曲的双层双层石墨烯，作为探索量子多体态的易于调节的平台。