Observation of Exciton Redshift–Blueshift Crossover in Monolayer WS<sub>2</sub>

We report a rare atom-like interaction between excitons in monolayer WS<sub>2</sub>, measured using ultrafast absorption spectroscopy. At increasing excitation density, the exciton resonance energy exhibits a pronounced redshift followed by an anomalous blueshift. Using both material-realistic computation and phenomenological modeling, we attribute this observation to plasma effects and an attraction–repulsion crossover of the exciton–exciton interaction that mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a strong analogy between excitons and atoms with respect to interparticle interaction, which holds promise to pursue the predicted liquid and crystalline phases of excitons in two-dimensional materials.