Room-Temperature Valence Transition in a Strain-Tuned Perovskite Oxide

TL;DR

This study demonstrates that strain engineering in thin films of a praseodymium-containing cobalt oxide enables room-temperature valence-driven spin-state and metal-insulator transitions, revealing new possibilities for functional oxide electronics.

Contribution

It introduces strain-tuning as a method to stabilize valence-driven transitions at room temperature in complex perovskite oxides, expanding their potential applications.

Findings

Valence-driven transitions occur at room temperature in strained thin films.

Strain controls electronic ground states from ferromagnetic metal to nonmagnetic insulator.

Complete strain control exposes a potential quantum critical point.

Abstract

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.