Optical Signature of a Crossover from Mott- to Slater-type Gap in Sr${}_2$Ir${}_{1-x}$Rh${}_x$O${}_4$

With optical spectroscopy we provide evidence that the insulator-metal transition in Sr${}_2$Ir${}_{1-x}$Rh${}_x$O${}_4$ occurs close to a crossover from the Mott- to the Slater-type. The Mott-gap at $x=0$ persists to high temperature and evolves without an anomaly across the N'{e}el temperature, $T_N$. Upon Rh-doping, it collapses rather rapidly and vanishes around $x = 0.055$. Notably, just as the Mott-gap vanishes yet another gap appears that is of the Slater-type and develops right below $T_N$. This Slater-gap is only partial and is accompanied by a reduced scattering rate of the remaining free carriers, similar as in the parent compounds of the iron arsenide superconductors.