Ballistic Exciton Flow Driven by Intertwined Exciton-Electron Orders in a Moir\'e Superlattice

TL;DR

This study demonstrates ballistic exciton transport in a moiré superlattice driven by strong exciton-electron interactions, with transport properties tunable by electron filling and interactions.

Contribution

It reveals how intertwined exciton-electron orders enable controllable ballistic exciton flow in moiré TMD heterobilayers, supported by experimental and theoretical analysis.

Findings

Ballistic exciton transport occurs at fractional electron fillings.

Energy-selective exciton expansion driven by repulsive interactions.

Theoretical DMRG model reproduces experimental exciton dynamics.

Abstract

Moir\'e superlattices of transition-metal dichalcogenides (TMDs) host strongly interacting Bose-Fermi mixtures in which bosonic excitons coexist with correlated electron lattices. Using ultrafast, time- and energy-resolved photoluminescence (PL) and reflectance microscopy, we show that strong exciton-electron and exciton-exciton repulsion can enable collective ballistic exciton transport in a WSe$_2$/WS$_2$ heterobilayer. The ballistic transport is energy-selective: repulsive interactions drive excitons into a higher moir\'e exciton band, where enhanced intersite hopping enables rapid spatial expansion. Correspondingly, the exciton mean-squared displacement (MSD) exhibits a quadratic time dependence ($\propto t^2$). This ballistic expansion is enhanced at fractional electron fillings where the electrons form generalized Wigner-crystal (GWC) orders. Afterwards, the system transitions into a mixed electron-exciton Mott state as Auger recombination and density depletion conclude the ballistic expansion. A one-dimensional Bose-Fermi Hubbard model solved using density-matrix renormalization group (DMRG) qualitatively reproduces the measured exciton transport and time-dependent response. It further confirms that strong cross-species interactions allow the electron crystal to perforate the exciton Mott background, accelerating its melting and enhancing exciton motion. Our results establish moir\'e TMDs as highly tunable platforms for realizing strongly interacting Bose-Fermi mixtures, which we employ here to demonstrate real-time control of intertwined bosonic and electronic order and to establish a route to the exciton insulator-fluid transition.