RESEARCH

Twisting atomically thin layers creates moiré superlattices that reshape their electronic structure. Our scanning tunneling microscopy studies of twisted bilayer graphene revealed twist-dependent van Hove singularities, strongly reduced electron velocities, and the emergence of nearly flat bands near the magic angle. Later studies found charge order and broken rotational symmetry, and Chern insulating states with topological flat bands, showing that electron interactions are central to the physics. We continue to study how moiré geometry, topology and correlations combine to produce new quantum phases.

Magic-angle twisted bilayer graphene offers a platform for heavy-fermion physics, in which localized electronic states hybridize with itinerant electrons. Using high-resolution planar tunneling measurements of inverse compressibility and entropy, we observed entropy plateaus associated with combinatorial degeneracies of local-moment states, including an eightfold-to-fourfold reduction consistent with strain-mediated symmetry breaking. We also observed filling-dependent mass renormalization. Together, these findings provide experimental support for central features of the topological heavy-fermion description.

We investigate transition-metal dichalcogenides and related van der Waals heterostructures, where moiré potentials, lattice relaxation, layer polarization and interfacial coupling offer further controls on electronic behavior. In marginally twisted MoS₂ bilayers, scanning tunneling experiments revealed strong structural reconstruction, moiré potentials and layer-dependent polarization. In a graphene/1T-TaS₂ heterostructure, we observed a proximity-induced charge density wave in graphene, showing how electronic order in one material can influence an adjacent layer.

Twisted trilayer graphene enables studies of superconductivity and quantum criticality. Our transport measurements reveal a broad quantum Griffiths regime in which rare spatial regions support local superconducting order even though the bulk is not superconducting. The data exhibit signatures of ultraslow dynamics and an infinite-randomness fixed point, together with nonlinear transport and a current-driven transition from insulating to superconducting-like behavior. By changing the magnetic-field orientation, the extended Griffiths regime can be driven toward a single quantum critical point.

Mechanical deformation provides another means to engineer graphene’s electronic spectrum. We demonstrated that periodically buckled monolayer graphene generates strain-induced pseudomagnetic fields exceeding 100 tesla, producing nearly flat bands and correlated electronic states. Unlike an applied magnetic field, a strain-induced pseudomagnetic field acts with opposite signs in graphene’s two valleys and preserves overall time-reversal symmetry. This enables band engineering without changing chemical composition or introducing a twist between layers.

A newer direction asks whether quasiperiodic geometry can become a further control parameter for electronic properties. Quasiperiodic structures have long-range order without translational periodicity and can support unusual spectra, critical wavefunctions and localization phenomena. Building on our studies of moiré periodic and quasiperiodic crystals in graphene/hexagonal boron nitride heterostructures, we aim to determine how quasiperiodicity affects electronic structure, topology and interactions, and whether it can help engineer flat bands and correlated states, potentially including enhanced superconductivity.

Our early experiments established the existence of the magnetically induced Wigner solid in two-dimensional electron systems: an ordered state in which electron–electron repulsion drives crystallization under strong magnetic fields. Subsequent measurements characterized the threshold for conduction of this strongly correlated electronic solid.

In 2009 our group discovered the fractional quantum Hall effect in monolayer graphene, showing that electron correlations are strong in this two-dimensional system of Dirac electrons. Science named the discovery a runner-up for its 2009 Breakthrough of the Year. This work continues to motivate our study of how electron interactions, dimensionality and topology combine in quantum materials.