Valley-polarized Exitonic Mott Insulator in WS2/WSe2 Moir\'e Superlattice

TL;DR

This study reveals an excitonic Mott insulator state in WS2/WSe2 moiré superlattices, demonstrating strong exciton-electron interactions, exciton double occupancy, and enhanced valley polarization, advancing the understanding of correlated quantum states in 2D materials.

Contribution

It provides experimental evidence and theoretical analysis of excitonic Mott insulator formation in TMDC heterobilayers, a novel correlated bosonic state in moiré superlattices.

Findings

Observation of excitonic Mott insulator state via photoluminescence

Exciton double occupancy requires overcoming 30-40 meV repulsion

Enhanced valley polarization in the Mott insulator state

Abstract

Strongly enhanced electron-electron interaction in semiconducting moir\'e superlattices formed by transition metal dichalcogenides (TMDCs) heterobilayers has led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type-II aligned TMDC heterobilayer moir\'e superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and strongly interact with each other, promising for realizing tunable correlated bosonic quasiparticles with valley degree of freedom. We employ photoluminescence spectroscopy to investigate the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moir\'e superlattice and combine with theoretical calculations to reveal the spatial extent of interlayer excitons and the band hierarchy of correlated states. We further find that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moir\'e cell, evidenced by emerging photoluminescence peaks under increased optical excitation power. Double occupancy of excitons in one unit cell requires overcoming the energy cost of exciton-exciton repulsion of about 30-40 meV, depending on the stacking configuration of the WS2/WSe2 heterobilayer. Further, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates the WS2/WSe2 moir\'e superlattice as a promising platform for engineering and exploring new correlated states of fermion, bosons, and a mixture of both.