From photons to phonons and polarons: pump-probe and ab initio studies of cobalt oxide

Ultrafast optical spectroscopy and density functional theory were used to study coherent lattice oscillations and formation of Jahn-Teller polarons in cobalt oxide. The study shows that selective excitation at 3.10 eV induces oxygen-to-cobalt charge transfer, converting octahedral Co(III) into Co(II) and triggering local symmetry breaking. This distortion lifts the orbital degeneracy, localizes excess charge in Co 3d states, and stabilizes polaronic in-gap electronic states. The process generates a coherent lattice oscillation at 10.1 meV, which is absent in the equilibrium phonon spectrum. In contrast, excitation at 1.55 eV selectively modulates tetrahedral Co(II) sites and launches a coherent Raman mode at 24.3 meV. These findings establish state-selective ultrafast excitation as a tool for investigating and controlling coupled electronic and structural dynamics in correlated materials.