Person:

Hao, Zeyu

Reducing 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.

When a strong magnetic field is applied to a two-dimensional (2D) electron system, interactions between the electrons can cause fractional quantum Hall (FQH) effects. Bringing two 2D conductors close to each other, a new set of correlated states can emerge due to interactions between electrons in the same and opposite layers. Here we report interlayer correlated FQH states in a device consisting of two parallel graphene layers separated by a thin insulator. Current flow in one layer generates different quantized Hall signals in the two layers. This result is interpreted by composite fermion (CF) theory with different intralayer and interlayer Chern-Simons gauge-field coupling. We observe FQH states corresponding to integer values of CF Landau level (LL) filling in both layers, as well as "semi-quantized" states, where a full CF LL couples to a continuously varying partially filled CF LL. We also find a quantized state between two coupled half-filled CF LLs and attribute it to an interlayer CF exciton condensate.