RESEARCH

Twisting atomically thin layers creates moiré superlattices that dramatically reshape electronic structure. Our pioneering 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. Subsequent studies uncovered charge order and broken rotational symmetry, demonstrating the importance of electron interactions. We continue to explore 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.

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 magnetic-field orientation, the extended Griffiths regime can be driven toward a single quantum critical point.

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

An emerging direction asks whether quasiperiodic geometry can become a further control parameter for electronic properties. Quasiperiodic structures possess 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.

In 2009, we discovered the fractional quantum Hall effect in monolayer graphene, demonstrating strong electron correlations in a relativistic two-dimensional electron system. The discovery was recognized by Science among its Top 10 Scientific Breakthroughs of 2009. This work continues to motivate our investigations of the interplay among electron interactions, dimensionality, and topology in quantum materials.

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.

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.