Visualizing correlated electronic states in twisted graphene systems
Twisted graphene stacks are novel van der Waals material systems that provide an elegant platform for studying correlated electronic states. Experimental signatures of emergent correlated phases, such as correlated insulating behavior, superconductivity, orbital ferromagnetism and non-trivial topology, have been reported in a number of twisted graphene systems including twisted bilayer graphene (tBLG) and twisted double bilayer graphene (tDBLG). The precise nature of the correlated states, however, remains elusive, and the lack of spatially-resolved electronic structure data creates challenges in modeling the rich correlation physics of these materials. In this talk I will present our recent scanning tunneling microscopy and spectroscopy (STM/STS) study on gate-tunable tDBLG as well as tBG aligned with hexagonal boron nitride (hBN). In tDBLG we observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. Through STS mapping we show that the energetically split states are spatially delocalized throughout the moiré unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking is the origin of the observed correlated state. In tBG aligned with hBN we detect unexpected sublattice polarization of flat band electronic states, which demonstrates the breakdown of mean-field approximation in a composite moiré pattern. Our results provide new insight into the nature of electron-electron interactions in twisted graphene and related moiré systems.
Canxun’s research focuses on investigating the electronic properties of graphene and/or transition metal dichalcogenide (TMD) based moiré systems using scanning tunneling microscopy and spectroscopy (STM/STS). Moiré superlattices are formed by stacking two layers of 2D materials with either a twist angle or a lattice mismatch. Driven by enhanced electron-electron interactions, novel quantum phases of matter such as Mott insulating states, superconductivity, nontrivial topology and orbital ferromagnetism can emerge in such systems. Canxun utilizes STM/STS to locally probe as well as manipulate the effects of correlation and topology in gate-tunable moiré systems. He will use the STM tip to actively control the electronic structure of the sample and to characterize the emerging correlation physics. He will also study naturally occurring topological domain boundaries or intentionally create such domain boundaries in order to detect topologically protected interface modes.