Person:

Khalaf, Eslam

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.

AbstractFractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue towards manipulating non-Abelian excitations. Early theoretical studies1–7 have predicted their existence in systems with flat Chern bands and highlighted the critical role of a particular quantum geometry. However, FCI states have been observed only in Bernal-stacked bilayer graphene (BLG) aligned with hexagonal boron nitride (hBN)8, in which a very large magnetic field is responsible for the existence of the Chern bands, precluding the realization of FCIs at zero field. By contrast, magic-angle twisted BLG9–12 supports flat Chern bands at zero magnetic field13–17, and therefore offers a promising route towards stabilizing zero-field FCIs. Here we report the observation of eight FCI states at low magnetic field in magic-angle twisted BLG enabled by high-resolution local compressibility measurements. The first of these states emerge at 5 T, and their appearance is accompanied by the simultaneous disappearance of nearby topologically trivial charge density wave states. We demonstrate that, unlike the case of the BLG/hBN platform, the principal role of the weak magnetic field is merely to redistribute the Berry curvature of the native Chern bands and thereby realize a quantum geometry favourable for the emergence of FCIs. Our findings strongly suggest that FCIs may be realized at zero magnetic field and pave the way for the exploration and manipulation of anyonic excitations in flat moiré Chern bands.