Broadening of the Mott transition in Manganese doped Ca₂RuO₄

Ca₂RuO₄ undergoes a phase transition from a Mott insulator to a metal upon heating slightly above room temperature (357K). This transition is signatured by abrupt changes in resistivity, reflectivity, and lattice parameter lengths, which can cause extreme fracturing of samples. Small amounts of Manganese doping broaden the transition enough to mitigate fracturing during the transition while not significantly changing the temperature dependence of the resistivity. This is convenient for experiments which require larger crystals. However, even small levels of doping drastically change the temperature dependent optical properties of the material, indicating a much more gradual transition than previously thought. I present optical microscopy videos and reflectivity measurements which reveal that the metal-insulator transition is broadened more than can be deduced from resistivity measurements. I describe a percolation model which explains the shift from an abrupt transition with a long coherence length to a percolative transition when Ca₂RuO₄ is doped with small amounts of Manganese. In addition, the model explains the discrepancy between the abrupt change in resistivity and the gradual changes in the optical properties.