Electronic Structure of Spin-Orbital-Coupling-Driven Insulator Sr$_2$IrO$_4$ from Angle-Resolved Photoemission Spectroscopy

Sr$_2$IrO$_4$, as a Mott Insulator, is an ideal system to study spin orbital coupling interaction in transition metal oxides. We report a comprehensive investigation on electronic structure of Sr$_2$IrO$_4$ by high resolution angle-resolved photoemission spectroscopy (ARPES). We measured the Fermi surface and band structures at different photon energies, under different photon polarizations. New features have been revealed that were not observed in previous studies. Moreover, the measurement under different polarizations helps identify different orbital characteristics of bands. The comparison between our experimental observations and theoretical calculation proves the important role of spin-orbital coupling interaction in determining its electron structure. The rich information on the electron structure of Sr$_2$IrO$_4$ will provide key insights in understanding the mechanism of various electron interactions in determining its insulator ground state.