Correlated insulator in the kagome flat band of a two-dimensional electrostatic crystal

The electronic properties of solids are determined by their crystal structure and electron interactions, giving rise to phenomena such as superconductivity, strange metals and correlated insulators. Many of these effects remain poorly understood, motivating efforts to create artificial crystals that mimic real materials while allowing controlled tuning of key parameters. Cold atoms in optical lattices offer flexibility but cannot reproduce the long-range Coulomb interactions and hopping present in solids. Solid-state systems naturally support these features, although they suffer from tunability and flexibility issues. Here we demonstrate a highly tunable artificial crystal formed by superimposing a periodic electrostatic potential onto a two-dimensional electron gas in a shallow GaAs quantum well. This engineered lattice exhibits a band structure characteristic of the artificial triangular lattice, distinct from that of the underlying cubic crystal. Electronic transport measurements show a sign change in the Hall coefficient as the chemical potential sweeps through the artificial bands. The band structure can be continuously tuned to realize linear graphene-like and flat kagome-like bands within a single device. A strong insulating state emerges at half filling of the kagome flat band, consistent with interaction-driven behaviour. This tunability provides an opportunity to explore correlated quantum states in a controlled setting.