Stable and Tunable Current-Induced Phase Transition in Epitaxial Thin Films of Ca₂RuO₄

Owing to the recent discovery of the current-induced metal–insulator transition and unprecedented electronic properties of the concomitant phases of calcium ruthenate Ca2RuO4, it is emerging as an important material. To further explore the properties, the growth of epitaxial thin films of Ca2RuO4 is receiving more attention, as high current densities can be applied to thin-film samples and the amount can be precisely controlled in an experimental environment. However, it is difficult to grow high-quality thin films of Ca2RuO4 due to the easy formation of the crystal defects originating from the sublimation of RuO4; therefore, the metal–insulator transition of Ca2RuO4 is typically not observed in the thin films. Herein, a stable current-induced metal–insulator transition is achieved in the high-quality thin films of Ca2RuO4 grown by solid-phase epitaxy under high growth temperatures and pressures. In the Ca2RuO4 thin films grown by ex situ annealing at >1200 °C and 1.0 atm, continuous changes in the resistance of over 2 orders of magnitude are induced by currents with a precise dependence of the resistance on the current amplitude. A hysteretic, abrupt resistive transition is also observed in the thin films from the resistance–temperature measurements conducted under constant-voltage (variable-current) conditions with controllability of the transition temperature. A clear resistive switching by the current-induced transition is demonstrated in the current–electric-field characteristics, and the switching currents and fields are shown to be very stable. These results represent a significant step toward understanding the high-current-density properties of Ca2RuO4 and the future development of Mott-electronic devices based on electricity-driven transitions.