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

TL;DR

The study resolves the character of the exciton Mott transition in monolayer WS$_2$ by combining broadband transient absorption with detailed lineshape analysis of the A and B excitons. A Voigt-based model separates exciton-population effects, lattice heating, and free-carrier plasma contributions, revealing a density threshold $n_C$ around $29\pm2\times10^{12}\ \text{cm}^{-2}$ where abrupt plasma formation induces strong band-gap renormalization and red-shifts in both AX and BX. The plasma decays on a sub-picosecond timescale ($\tau_{plasma} \approx 0.65\ \text{ps}$), after which the excitonic phase re-emerges, indicating a discontinuous EMT consistent with avalanche-like dissociation rather than a gradual coexistence. The BX acts as a sensitive spectator to BGR and plasma formation, while the AX population provides complementary insight into exciton dynamics, collectively demonstrating that two resonances analyzed together yield a clear signature of a discontinuous EMT with implications for ultrafast TMDC-based switching. The findings align with certain many-body theories and challenge interpretations of a smooth crossover, highlighting the utility of simultaneous multi-exciton analysis in 2D materials.

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.

Figures (11)

• Figure 1: (a) Sketch of TA applied to a WS2 monolayer on a fused silica substrate. (b) Band diagram and excitonic resonances (AX, BX, CX, DX) in the monolayer inserted into the one-particle band structure as photoionization energy levels of the excitons. (c) Energy and delay-dependent differential absorption spectra (color plot) of the monolayer photoexcited resonantly to the AX. (d) AX Band integral and saturable absorber model to determine the excitation density.
• Figure 2: (a) Differential absorption spectrum, at a delay of 1 ps and pump fluence of 150 $\mu J$/cm$^2$ ($n_0$=15.5 x 10$^{12}$ cm$^{-2}$), fitted with the difference of two Voigt functions $V(t)-V_0$ (according to Eq. \ref{['equation1']}). (b) $V(t)$ and $V_0$ contribution of the fit.
• Figure 3: Ultrafast relaxation of the AX oscillator strength change, i.e., the ground state bleach, for different initial excitation densities. To evaluate the relaxation time, the traces are normalized at 200 fs and fitted with a with a single decay exponential function (see text). For clarity, the fit functions are shown only for three traces, as red lines.
• Figure 4: Shift (a,b) and broadening (c,d) of A and B exciton resonances at a low excitation density (15.5 x 1012 cm-2) as a function of pump-probe delay. The data is fitted with a sum of exponential decay functions as described in the text. The individual components of the fit are illustrated with the shaded areas.
• Figure 5: AX (a) and BX (b) shifts as a function of pump-probe delay for different initial excitation densities, together with fits (solid lines). The fit function are sum of four and two exponential decays for AX (see Eq. \ref{['eqn:AX_shift']}) and BX (see Eq. \ref{['eqn:BX_shift']}), respectively.