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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 Pr_1-xCa_xCoO₃). Here, we show that in thin films of the complex perovskite (Pr_1-yY_y)_1-xCa_xCoO_3-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.

 

We demonstrate the impact of subtle changes in molecular structure on the singlet and triplet exciton diffusion lengths ( L D ) for derivatives of the archetypical electron-transport material 4,7-diphenyl-1,10-phenanthroline (BPhen). Specifically, this work offers a systematic characterization of singlet and triplet exciton transport in identically prepared thin films, highlighting the differing dependence on molecular photophysics and intermolecular spacing. For luminescent singlet excitons, photoluminescence quenching measurements yield an L D from <1 nm for BPhen, increasing to (5.4 ± 1.2) nm for 2,9-dichloro-4,7-diphenyl-1,10-phenanthroline (BPhen-Cl 2 ) and (3.9 ± 1.1) nm for 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). The diffusion of dark triplet excitons is probed using a phosphorescent sensitizer-based method where triplets are selectively injected into the material of interest, with those migrating through the material detected via energy transfer to an adjacent, phosphorescent sensitizer. Interestingly, the triplet exciton L D decreases from (15.4 ± 0.4) nm for BPhen to (8.0 ± 0.7) nm for BPhen-Cl 2 and (4.0 ± 0.5) nm for BCP. The stark difference in behavior observed for singlets and triplets with functionalization is explicitly understood using long-range Förster and short-range Dexter energy transfer mechanisms, respectively.