Ultrafast Formation of Jahn-Teller Polarons Revealed by State-Selective Excitation in Correlated Spinel Co(3)O(4).

Jahn-Teller polarons are quasiparticles that stem from symmetry breaking and strong local electron-phonon coupling. They originate from an excess charge carrier being dressed by a local lattice distortion, caused by the Jahn-Teller effect, and they critically impact electrical, structural, and magnetic properties in transition metal oxides. The observation of the microscopic steps involved in their formation is essential for enabling control over material properties through the targeted activation of local, site-specific modifications with light pulses. While Jahn-Teller distortion associated with polaron formation was predicted to contribute significantly to changes in electronic band gap and optical properties in CoO, its experimental observation remains elusive, requiring signatures of local symmetry reduction. In this work, we demonstrate Jahn-Teller polaron formation in spinel CoO. By exciting electronic transitions at 3.10 eV and 1.55 eV, we target either the Cobalt(III) or the Cobalt(II) ions, and drive the subsequent coherent responses of the system through two different pathways. For the former, we demonstrate that ligand-to-metal charge transfer leads to Jahn-Teller polaron formation, which is linked to the deformation potential and magnetoelastic coupling. For the latter, we identify the coherent excitation of a T phonon mode launched by on-site d-d electronic transitions. Key to our observations is the ability to target site-specific electronic excitations in spinel CoO using ultrafast optical pulses, while monitoring the ensuing low-energy collective modes through the coherent time-domain response of the material. Our approach, which combines the comparative analysis of experimental fingerprints with the support from density functional theory calculations, is broadly applicable to systems in which Jahn-Teller polaron physics has been theoretically predicted but remains experimentally unverified, and it underscores the potential of electronic-state targeting as a route to selectively excite and probe quasiparticle dynamics in solids.