Liu, XiaomengHao, ZeyuKhalaf, EslamLee, Jong YeonRonen, YuvalYoo, HyobinHaei Najafabadi, DanialWatanabe, KenjiTaniguchi, TakashiVishwanath, AshvinKim, Philip2023-02-062020-07-08Liu, Xiaomeng, Hao, Zeyu, Khalaf, Eslam, Lee, Jong Yeon, Ronen, Yuval, Yoo, Hyobin, Haei Najafabadi, Danial, Watanabe, Kenji, Taniguchi, Takashi, Vishwanath, Ashvin, and Kim, Philip. "Tunable Spin-polarized Correlated States in Twisted Double Bilayer Graphene." Nature (London) 583, no. 7815 (2020): 221-25.0028-08361476-4687https://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37374192Reducing the energy bandwidth of electrons in a lattice below the long-range Coulomb interaction energy promotes correlation effects. Created by stacking van der Waals (vdW) heterostructures with a controlled twist angle1–3, moire ́ superlattices enable the engineering of electron band structure. In an engineered moire ́ flat band, exotic quantum phases can emerge. The correlated insulator, superconductivity, and quantum anomalous Hall ef- fect found in the flat band of the magic angle twisted bilayer graphene (MA-TBG) 4–8 have sparkled exploration of correlated electron states in other moire ́ systems 9–11. The electronic properties of vdW moire ́ superlattices can further be tuned by adjusting the interlayer cou- pling 6 or the band structure of constituent layers 9. Here, employing vdW heterostructures of twisted double bilayer graphene (TDBG), we demonstrate a flat electron band that is tun- able by perpendicular electric fields in a range of twist angles. Similar to the MA-TBG, TDBG exhibits energy gaps at the half- and quarter-filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the half-filled insulator, a sudden drop of resistivity is observed with lowering temperature. This critical behavior is confined in a small area in the density-electric field plane, and is attributed to a phase transition from a normal metal to a spin-polarized correlated state. Spin-polarized correlated states discovered in the electric field tunable TDBG provide a new route to engineering interaction-driven quantum phases.en-USMultidisciplinaryJournal Article2023-02-0610.1038/s41586-020-2458-7