Exciton-polaron transition dynamics in the correlated Van der Waals material NiPS3

Using ultra-broadband transient terahertz (THz) spectroscopy, complemented by transient absorption (TA) and photoluminescence (PL) measurements, we directly track the generation and relaxation of photoexcited hot carriers, as well as the ultrafast formation of excitons in the transition metal phosphorus trichalcogenide NiPS3, with a temporal resolution of sub-100 fs. A subset of excitons exhibits strong coupling to 8-THz optical phonons, resulting in a distinctive negative photoconductivity that is indicative of unconventional many-body interactions in this antiferromagnetic system.