Pressure-induced insulator-metal transition in Ca2Ru0.92Fe0.08O4 investigated by infrared microspectroscopy

Abstract

We investigated pressure-induced insulator-metal transition in Ca2Ru0.92Fe0.08O4 by using infrared microspectroscopy. As the pressure is increased up to 1.7 GPa, we observed a large increment of the reflectivity in the entire mid-infrared range. Accompanied by such a clear signature of the insulator-metal transition, we found an evidence of the structural transition from the frequency shift of the RuO stretch phonon mode which is attributed to the shortening of the in-plane Ru-O bond length. When we compared these pressure-dependent changes with the corresponding temperature-dependent results, we found that the pressure-induced metallic state has a higher reflectivity as well as the higher phonon frequency. Indeed, it turns out that the pressure-induced metallic state of Ca2Ru0.92Fe0.08O4 looks very similar with the metallic state of Sr-substituted Ca2RuO4 not only in the reflectivity level but also in the phonon frequency. This suggests that the electronic properties are closely related to the structural degree of freedom, and the pressure can be a useful parameter to induce the transitions from the Mottinsulator to the metal and further to the superconductor as observed for Sr2RuO4. (C) 2017 Published by Elsevier B.V.