Theory of Ultrafast Spin--Charge Quantum Dynamics in Strongly Correlated Systems Controlled by Femtosecond Photoexcitation: an Application to Insulating Antiferromagnetic Manganites

We use a non-equilibrium many-body theory that engages the elements of transient coherence, correlation, and nonlinearity to describe changes in the magnetic and electronic phases of strongly correlated systems induced by femtosecond nonlinear photoexcitation. Using a generalized tight--binding mean field approach based on Hubbard operators and including the coupling of the laser field, we describe a mechanism for simultaneous insulator--to--metal and anti-- to ferro--magnetic transition to a transient state triggered by non-thermal ultrafast spin and charge coupled excitations. We demontrate, in particular, that photoexcitation of composite fermion quasiparticles induces quasi-instantaneous spin canting that quenches the energy gap of the antiferromagnetic insulator and acts as a nonadiabatic "initial condition" that triggers non-thermal lattice dynamics leading to an insulator to metal and antiferromagnetic (AFM) to ferromagnetic (FM) transitions. Our theoretical predictions are consistent with recent ultrafast pump-probe spectroscopy experiments that revealed a magnetic phase transition during 100fs laser pulse photoexcitation of the CE--type AFM insulating phase of colossal magnetoresistive manganites. In particular, experiment observes two distinct charge relaxation components, fs and ps, with nonlinear threshold dependence at a pump fluence threshold that coincides with that for femtosecond magnetization photo excitation. Our theory attributes the correlation between femtosecond spin and charge nonlinearity leading to transition in the magnetic and electronic state to spin/charge/lattice coupling and laser-induced quantum spin canting that accompanies the driven population inversion between two quasi--particle bands with different properties: a mostly occupied polaronic band and a mostly empty metallic band, whose dispersion is determined by quantum spin canting.