Local density of states study of a spin-orbit-coupling induced Mott insulator Sr$_2$IrO$_4$

We present scanning tunneling microscopy and spectroscopy experiments on the novel $J_\mathrmeff=1/2$ Mott insulator Sr$_2$IrO$_4$. Local density of states (LDOS) measurements show an intrinsic insulating gap of 620 meV that is asymmetric about $E_\mathrmF$ and is larger than previously reported values. The size of this gap strongly suggests that Sr$_2$IrO$_4$ is a Mott rather than Slater insulator. In addition, we found a small number of native defects which create in-gap spectral weight. Atomically resolved LDOS measurements on and off the defects naturally explain why such a large gap in this 5d system has not been seen by other probes. The extended nature of the 5d electrons and the insulating character of Sr$_2$IrO$_4$ together facilitate the vulnerability of this gap.