Tunable correlation in twisted monolayer-trilayer graphene

Flat-band physics of moiré superlattices,originally discovered in the celebrated twisted bilayer graphene,have re-cently been intensively explored in multilayer graphene systems that can be further controlled by electric field.In this work,we experimentally find the evidence of correlated insulators at half filling of the electron moiré band of twisted monolayer-trilayer graphene with a twist angle around 1.2°.Van Hove singularity(VHS),manifested as enhanced resis-tance and zero Hall voltage,is observed to be distinct in conduction and valence flat bands.It also depends on the direction and magnitude of the displacement fields,consistent with the asymmetric crystal structure.While the resistance ridges at VHS can be enhanced by magnetic fields,when they cross commensurate fillings of the moiré superlattice in the conduction band,the enhancement is so strong that signatures of correlated insulator appear,which may further develop into an energy gap depending on the correlation strength.At last,Fermi velocity derived from temperature coefficients of resistivity is compared between conduction and valence bands with different displacement fields.It is found that electronic correlation has a negative dependence on the Fermi velocity,which in turn could be used to quantify the correlation strength.