Fig. 2

Four-step model describing the time evolution of LaCoO₃ after laser excitation: (I) Carrier excitation: The femtosecond laser pulse (λ = 800 nm, hν = 1.55 eV) excites the sample, producing carriers and instantaneous gap collapse. (II) Charge transfer enhancement: After carrier relaxation, the charge transfer is enhanced by the larger hybridization of the O–Co-pseudo-e_g with the O-2p orbitals. (III) Spin stabilization: The spin configuration in the system is stabilized, which also increases the Co radius. (IV) Lattice modification: The lattice relaxes to accommodate the changes of the spin state and the ionic radius. The change of bonds and angles results in a transient metallic state. In (I)-(a) and (II)-(c), the lattice structure and value of the bond angle at room temperature has been used. In (III)-(e), the value 16X implies that the bond angle has changed to an unknown value. In (IV)-(g), the bond angle and lattice structure at 850 K is provided to indicate the expected bond angle in the excited state. The lattice structures, rhomboedral R\(\bar 3\)c, were plotted using VESTA⁵³. The Co atoms are in the center of an O octahedra (indicated by the shaded area around the central Co atom) and have a different color to indicate the change in spin state and ionic radius at each time step. The density of states (DOS) in (I)-(b) corresponds to the ground state calculations as in Karolak et al.²⁰. In (II)-(d)–(III)-(f), the ground state DOS has been convoluted with a Fermi function at 2000 K to represent the electronic excitation of the system due to the laser pulse. In (IV)-(h), the DOS in a high spin (HS) state with significant charge imbalance is displayed (see text for details)