Dielectric catastrophe at the Mott and Wigner transitions in a moir\'e superlattice

TL;DR

This study demonstrates a continuous, interaction-driven metal-insulator transition in a moiré superlattice, characterized by a diverging dielectric constant, providing insights into quantum phase transitions in two-dimensional materials.

Contribution

It provides experimental evidence of a dielectric catastrophe at the Mott and Wigner transitions in a moiré superlattice at both integer and fractional fillings.

Findings

Continuous MIT observed via exciton sensing.

Dielectric constant diverges at transition point.

Supports theoretical models of continuous correlation-driven transitions.

Abstract

The metal-insulator transition (MIT) driven by electronic correlations is a fundamental and challenging problem in condensed-matter physics. Particularly, whether such a transition can be continuous remains open. The emergence of semiconducting moir\'e materials with continuously tunable bandwidth provides an ideal platform to study interaction-driven MITs. Although a bandwidth-tuned MIT at fixed full electron filling of the moir\'e superlattice has been reported recently, that at fractional filling, which involves translational symmetry breaking of the underlying superlattice, remains elusive. Here, we demonstrate bandwidth-tuned MITs in a MoSe2/WS2 moir\'e superlattice at both integer and fractional fillings using the exciton sensing technique. The bandwidth is controlled by an out-of-plane electric field. The dielectric response is probed optically with the 2s exciton in a remote WSe2 sensor layer. The exciton spectral weight is negligible for the metallic state, consistent with a large negative dielectric constant. It continuously vanishes when the transition is approached from the insulating side, corresponding to a diverging dielectric constant or a "dielectric catastrophe". Our results support continuous interaction-driven MITs in a two-dimensional triangular lattice and stimulate future explorations of exotic quantum phases, such as quantum spin liquids, in their vicinities.