LAUSR

The orthorhombic perovskite iridate ${\mathrm{SrIr}}_{0.8}{\mathrm{Sn}}_{0.2}{\mathrm{O}}_{3}$ is reported to be a new candidate of Slater insulator at low temperatures. Here, we present optical spectroscopy measurements on ${\mathrm{SrIr}}_{0.8}{\mathrm{Sn}}_{0.2}{\mathrm{O}}_{3}$ at different temperatures ($T$) across the paramagnetic-antiferromagnetic phase transition. The low energy reflectivity decreases with reducing $T$. Six phonon peaks in the spectrum at low $T$ are visible at room temperature, indicating the absence of crystal structure phase transition from 300 K to 10 K. The real part of conductivity spectra ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ are obtained by employing the standard Kramers-Kronig transformation. In the paramagnetic state at room temperature, ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ reaches a limiting value as $\ensuremath{\omega}\ensuremath{\rightarrow}0$, implying an electrical conducting state. Upon decreasing $T$, the low frequency ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ decreases and approaches zero with a concomitant opening of a direct gap. Detailed analysis of the low energy data shows that the gap opening is continuous and ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ at the low energy gap edge follows an unusual ${\ensuremath{\omega}}^{1/2}$ dependence at low temperatures which is remarkably similar to other Slater insulators, such as ${\mathrm{NaOsO}}_{3}$.