Wigner Molecular Crystals from Multi-electron Moir\'e Artificial Atoms

TL;DR

This paper reports the experimental discovery of Wigner molecular crystals in twisted bilayer WS2 moiré superlattices, revealing new quantum phases arising from multi-electron artificial atoms influenced by Coulomb interactions.

Contribution

It provides the first experimental observation of Wigner molecules and their crystalline arrangements in moiré superlattices, expanding understanding of quantum solids beyond single-electron models.

Findings

Wigner molecules form in multi-electron artificial atoms under strong Coulomb interactions.

Three-electron Wigner molecules exhibit a characteristic trimer pattern.

Wigner molecular crystals are highly tunable via strain, moiré period, and charge type.

Abstract

Semiconductor moir\'e superlattices provide a versatile platform to engineer new quantum solids composed of artificial atoms on moir\'e sites. Previous studies have mostly focused on the simplest correlated quantum solid - the Fermi-Hubbard model - where intra-atom interactions are simplified to a single onsite repulsion energy U. These studies have revealed novel quantum phases ranging from Mott insulators to quantum anomalous Hall insulators at a filling of one electron per moir\'e unit cell. New types of quantum solids should arise at even higher filling factors where the multi-electron configuration of moir\'e artificial atoms provides new degrees of freedom. Here we report the experimental observation of Wigner molecular crystals emerging from multi-electron artificial atoms in twisted bilayer WS2 moir\'e superlattices. Moir\'e artificial atoms, unlike natural atoms, can host qualitatively different electron states due to the interplay between quantized energy levels and Coulomb interactions. Using scanning tunneling microscopy (STM), we demonstrate that Wigner molecules appear in multi-electron artificial atoms when Coulomb interactions dominate. Three-electron Wigner molecules, for example, are seen to exhibit a characteristic trimer pattern. The array of Wigner molecules observed in a moir\'e superlattice comprises a new crystalline phase of electrons: the Wigner molecular crystal. We show that these Wigner molecular crystals are highly tunable through mechanical strain, moir\'e period, and carrier charge type. Our study presents new opportunities for exploring quantum phenomena in moir\'e quantum solids composed of multi-electron artificial atoms.