Light-induced insulator-metal transition in Sr₂IrO₄ reveals the nature of the insulating ground state

Proposed as a J_eff = 1/2 Mott insulator induced by strong spin-orbit coupling (SOC), Sr₂IrO₄ has been considered as an alternative material platform to explore the mechanism of high-T_c superconductivity, due to its structural and electronic similarities to high-T_c superconducting cuprates. While the J_eff = 1/2 ground state and the important role of SOC in the opening of the charge gap are experimentally confirmed, the Mott nature of the low-temperature insulating phase in Sr₂IrO₄ has not been firmly established. A Slater mechanism for the opening of the gap has been proposed in previous studies, but a general consensus has not been reached due to several counter evidences. Resolving this issue is however essential for understanding the similarities and differences between the cuprates and Sr₂IrO₄ and the fact that (high-T_c) superconductivity has not been observed in doped Sr₂IrO₄. Here, we report the observation of a gap-closing and the formation of weakly-renormalized electronic bands in the gap region as a result of a light-induced insulator-metal transition in Sr₂IrO₄, measured via time- and angle-resolved photoemission spectroscopy. Considering the typical temperature and doping evolution of Mott-gaps and Hubbard bands, we argue that these observations provide a clear evidence of the non-Mott nature of the insulating state. For the origin of the charge gap, we instead consider a correlated band insulator picture, where anti-ferromagnetic correlations play a key role in the opening of the gap.