Observation of a metal-to-insulator transition with both Mott-Hubbard and Slater characteristics in Sr ₂IrO ₄ from time-resolved photocarrier dynamics

We perform a time-resolved optical study of Sr ₂IrO ₄ to understand the influence of magnetic ordering on the low energy electronic structure of a strongly spin-orbit coupled J _eff = 1/2 Mott insulator. By studying the recovery dynamics of photoexcited carriers, we find that upon cooling through the Néel temperature T _N the system evolves continuously from a metal-like phase with fast (∼50 fs) and excitation density independent relaxation dynamics to a gapped phase characterized by slower (∼500 fs) excitation density-dependent bimolecular recombination dynamics, which is a hallmark of a Slater-type metal-to-insulator transition. However our data indicate that the high energy reflectivity associated with optical transitions into the unoccupied J _eff = 1/2 band undergoes the sharpest upturn at T _N, which is consistent with a Mott-Hubbard type metal-to-insulator transition involving spectral weight transfer into an upper Hubbard band. These findings show Sr ₂IrO ₄ to be a unique system in which Slater- and Mott-Hubbard-type behaviors coexist and naturally explain the absence of anomalies at T _N in transport and thermodynamic measurements.