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.