Ultrafast electron localization and screening in a transition metal dichalcogenide

The coupling of light to electrical charge carriers in semiconductors is the foundation of many technological applications. Attosecond transient absorption spectroscopy measures simultaneously how excited electrons and the vacancies they leave behind dynamically react to the applied optical fields. In compound semiconductors, these dynamics can be probed via any of their atomic constituents with core -level transitions into valence and conduction band. Typically, the atomic species forming the compound contrib-ute comparably to the relevant electronic properties of the material. One therefore expects to observe similar dynamics, irrespective of the choice of atomic species via which it is probed. Here, we show in the two-dimensional transition metal dichal-cogenide semiconductor MoSe2, that through a selenium -based core -level transition we observe charge carriers acting independently from each other, while when probed through molybdenum, the collective, many -body motion of the carriers dominates. Such unexpectedly contrasting behavior can be explained by a strong localization of electrons around molybdenum atoms following absorption of light, which modifies the local fields acting on the carriers. We show that similar behavior in elemental titanium metal [M. Volkov et al., Nat. Phys. 15, 1145-1149 (2019)] carries over to transition metal-containing compounds and is expected to play an essential role for a wide range of such materials. Knowledge of independent particle and collective response is essential for fully understanding these materials.

Automated summary

The coupling of light to electrical charge carriers in semiconductors is crucial for various technological applications. Attosecond transient absorption spectroscopy allows the measurement of the dynamic reactions between excited electrons and the vacancies they leave. By probing compound semiconductors, such as MoSe2, through different atomic species, contrasting behaviors of independent charge carriers and collective motion of carriers can be observed. This contrasting behavior is attributed to the strong localization of electrons around specific atoms, which modifies the local fields and influences the carriers' behavior. Understanding both independent particle and collective responses is essential for a comprehensive understanding of these materials.