Correlation-Driven Ballistic Exciton Expansion in a Moiré Bose-Fermi Mixture

Abstract

Moiré 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) microscopy, we show that strong exciton-electron and exciton-exciton repulsion can enable ballistic exciton transport in a WSe${}_2$/WS${}_2$ heterobilayer. This ballistic transport is energy selective: repulsive interactions populate higher-energy moiré exciton bands, including the doubly occupied exciton state XX and the exciton-electron co-localized state Xe, exhibiting mean-squared displacement (MSD) with a quadratic time dependence, MSD $\propto t^2$. Transport in the Xe band is jointly governed by exciton and electron correlations. Exciton correlations suppress relaxation into the lower-energy localized exciton state, while generalized Wigner-crystal (GWC) order at fractional electron fillings enhances Auger recombination and the group velocity of the ballistic expansion. A two-dimensional Bose-Fermi Hubbard model solved using time-dependent mean-field methods 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, enhancing exciton motion. Our results establish moiré TMDs as highly tunable platforms for realizing strongly interacting Bose-Fermi mixtures.