Transient photoexcited state of the small-gap quasi-two-dimensional Mott-Hubbard insulator Sn / Si ( 111 ) − ( √ 3 × √ 3 ) − R 30 ∘

We investigate the transient response to photodoping of Sn/Si⁡(111)−(√3×√3) R⁢30°, a simple and prototypical two-dimensional Mott-Hubbard insulator with a small gap on a triangular lattice, by time- and angle-resolved photoemission with a 120 fs time resolution. A transient metallic state forms nearly instantaneously after photoexcitation and persists for a few hundred femtoseconds, consistent with theoretical predictions and previous studies of small-gap Mott systems in one and three dimensions. The temperature dependence of the doublon lifetime indicates that the dominant relaxation mechanism involves coupling between charge and spin degrees of freedom. On a longer timescale, a small energy shift of the lower Hubbard band persists for more than 1 ns. We attribute this long-lived modification to energy renormalization of the Hubbard bands induced by the photoexcited electron-hole plasma in the Si substrate. Our results further demonstrate that substrate excitations and the dynamics of the three-dimensional electron-hole plasma play a significant role in ultrafast studies of two-dimensional materials supported on semiconducting or insulating substrates, while also pointing to a possible route for controlling transient electronic states.