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., 90K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)(1-x)CaxCoO3-delta, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291K, 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.

Automated summary

Researchers have discovered a remarkable phenomenon in praseodymium-containing cobalt oxides where strain tuning can stabilize valence-driven spin-state/structural/metal-insulator transitions in thin films. This result has important technological and fundamental implications.