  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-{\delta}, 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.
