Excitons in WSe2 time-resolved ARPES: particle or oscillation?

TL;DR

This study investigates the ultrafast dynamics of excitons in WSe2 using time-resolved ARPES, revealing that the observed phenomena are better explained by a transition to an excitonic-insulating state rather than traditional exciton scattering.

Contribution

The paper introduces a combined theoretical and experimental approach to reinterpret ultrafast excitonic dynamics in WSe2 as a transition to an excitonic-insulating phase.

Findings

Ultrafast scattering from K to Sigma valley occurs in about 30 fs.

Features in spectra are due to single-particle levels renormalized by excitonic polarization.

The dynamics are better explained by a transition to an excitonic-insulating order.

Abstract

The time-resolved angle-resolved photoemission spectra of WSe$_2$, a paradigmatic transition metal dichalcogenide, are dominated by a transient signal that, after being initially observed in the gap at the K valley, scatters, on an ultra-fast time scale of $\sim$ 30 fs, to the $\Sigma$ valley. In this work we question the common interpretation of the experimental dynamics in terms of a massive bound electron-hole exciton that scatters with phonons and behaves as a quasi-particle. By using a combined theoretical and experimental investigation, we demonstrate that the observed dynamics can be interpreted as the photo-induced transition from direct to indirect excitonic-insulating order. The features that appear in the experimental spectrum correspond to single-particle levels renormalized by the excitonic spontaneous polarization.