Discontinuous character of the ultrafast exciton Mott transition in monolayer WS$_2$

Abstract

There are conflicting predictions and reports on the character of the exciton Mott transition (EMT) in monolayer transition metal dichalcogenides. It could be either a discontinuous or a continuous transition from the excitonic to the plasma phase, with important implications for devices such as photoswitches. To resolve the nature of the transition in monolayer WS$_2$, we study its ultrafast optical response upon resonant photoexcitation of the A exciton across a broad range of photoexcitation densities. In agreement with previously reported measurements we observe that the A exciton quenches gradually with increasing excitation density. However, a detailed lineshape analysis unveils an abrupt red shift in the transient peak positions of the A and B exciton resonances above an excitation density threshold. This is attributed to band gap renormalization arising from the formation of free charge carrier plasma, i.e., the EMT. The plasma phase decays with a time constant of 0.65 ps back into the excitonic state. The abrupt appearance of the plasma phase at the threshold density suggests that the EMT is a discontinuous and not a continuous transition. This work demonstrates how transient optical spectroscopy combined with lineshape analysis of two excitonic resonances simultaneously can be used to investigate the EMT in 2D materials.