Probing correlations in the exciton landscape of a moir\'e heterostructure

TL;DR

This paper demonstrates that ultrafast hole transfer in a moiré heterostructure causes an unexpected energy upshift in photoemitted electrons, revealing exciton correlations and showcasing time-resolved photoelectron spectroscopy as a tool to study correlated states in 2D materials.

Contribution

It introduces a novel method to probe exciton correlations via ultrafast spectroscopy, revealing unexpected energy shifts linked to correlated electron-hole interactions.

Findings

Ultrafast hole transfer causes a sub-200-fs upshift in electron energy.

Energy upshift indicates strong exciton correlations.

Time-resolved photoelectron spectroscopy can access correlated excitonic states.

Abstract

Excitons are two-particle correlated bound states that are formed due to Coulomb interaction between single-particle holes and electrons. In the solid-state, cooperative interactions with surrounding quasiparticles can strongly tailor the exciton properties and potentially even create new correlated states of matter. It is thus highly desirable to access such cooperative and correlated exciton behavior on a fundamental level. Here, we find that the ultrafast transfer of an exciton's hole across a type-II band-aligned moir\'e heterostructure leads to a surprising sub-200-fs upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this unusual upshift is a clear fingerprint of the correlated interactions of the electron and hole parts of the exciton quasiparticle. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access exciton correlations and cooperative behavior in two-dimensional quantum materials. Our work highlights this new capability and motivates the future study of optically inaccessible correlated excitonic and electronic states in moir\'e heterostructures.