Correlated States in Moiré Superlattices
Moiré patterns in 2D materials are created by stacking two crystalline layers with a twist angle or a slight difference in lattice constants. The moiré pattern acts as a superlattice that dramatically modifies the electronic band structure, often leading to flat bands where the electron kinetic energy is suppressed. In these flat bands, electron-electron interactions dominate, giving rise to a rich variety of correlated phenomena such as Mott insulators, superconductivity, generalized Wigner crystals, integer and fractional Chern insulators, etc. The capability of Microwave Impedance Microscopy (MIM) to spatially resolve local conductivity under low temperatures and high magnetic fields makes it a powerful technique for investigating these correlated states in moiré superlattices.
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Identifying Correlated Insulating States in Moiré Superlattices
Flat bands in moiré superlattices significantly enhance electron-electron interactions, driving the system into a strongly correlated regime where Coulomb repulsion dominates over kinetic energy. This leads to the emergence of correlated insulating states, including Mott insulators at integer fillings—with one electron per moiré unit cell—and more exotic phases such as generalized Wigner crystals stabilized by nearest-neighbor repulsion at fractional fillings.
In our previous work, we employed MIM to investigate a single-gated WS₂/WSe₂ moiré heterostructure. By tuning the gate voltage, we modulated the carrier density and observed a sequence of insulating states. Our measurements revealed Mott insulating behavior at integer fillings, along with intriguing correlated phases such as triangular generalized Wigner crystals and stripe-like states at fractional fillings. More details can be found at Nature Physics, 17(6), 715–719 (2021).
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Imaging Local Electrical Features in Moiré Superlattices
The spatial resolution capability of MIM offers another approach to studying moiré superlattices by imaging local electrical features with nanoscale precision. A striking application is the direct visualization of moiré superlattices via MIM. In our 2021 study, we simultaneously performed contact-mode MIM and c-AFM on a graphene/hBN heterostructure, obtaining moiré pattern images with 2 nm spatial resolution. Furthermore, in an angle-aligned trilayer system, we observed two distinct sets of moiré superlattices with different periodicities, revealing potential interference effects. This interference introduces an additional degree of freedom for electronic band structure engineering in moiré materials. More details can be found in Nano Lett., 21, 4292−4298 (2021).
