Regensburg 2025 – scientific programme
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O: Fachverband Oberflächenphysik
O 67: Ultrafast Electron Dynamics II
O 67.2: Talk
Wednesday, March 19, 2025, 15:15–15:30, H11
Sharp Exciton Mott Transition in WS2 and its Ultrafast Decay — •Subhadra Mohapatra1, Lukas Gierster1, Samuel Palato1, Nicholas Michael Olsen2, Xiaoyang Zhu2, and Julia Stähler1 — 1Humboldt-Universität zu Berlin, Institut für Chemie — 2Columbia University
The excitonic Mott transition (EMT) in transition metal dichalcogenides is reported to be either discontinuous [1] or continuous [2]. To resolve the ambiguity, here we study the optical response of WS2 across the Mott density. Using a complex lineshape analysis, we separate the optical response in the photo-excited part of the sample from the unexcited regimes. In agreement with a continuous EMT, a gradual increase in the bleach of the sample-averaged absorption is observed. However, the lineshape analysis unveils a sharp discontinuity in the transient dynamics of the A and B exciton resonances above a critical photoexcitation density. This is attributed to plasma formation followed by band gap renormalization. The plasma decays with a laser fluence-independent time constant of 0.65 ps which is attributed to phase separation into excitonic and plasma regions, in agreement with the literature [3,4]. This work not only provides the first detailed experimental investigation of the EMT close to the critical limit but also highlights the role of spatial inhomogeneous charge carrier distribution for the widely used transient optical spectroscopies of 2D materials. References: [1] Bataller et al. Nano Lett 19(2) (2019). [2] Chernikov et al. Nat Photonics 9(7) (2015). [3] Steinhoff et al. Nat Commun 8 (1) (2017). [4] Koch et al. Physica status solidi (b) 238(3) (2003).
Keywords: Mott transition; Transient absorption spectroscopies; 2D materials; Phase separation; Plasma