Room-temperature valence transition in a strain-tuned perovskite oxide

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point. Spin-state crossovers are phenomena where, under changes in temperature or pressure, the spin-state of an ion changes. In some materials, this spin-state crossover occurs simultaneously with a metal-insulator transition, driven by a valence transition. Control over such valence, spin-state, and metal-insulator transitions has much technological appeal, but, thus far, materials displaying this have been limited to cryogenic temperatures. Here, the authors show that in strained films of (Pr1-yYy)1- xCaxCoO3-δ, these transitions can be promoted to room temperature.