Photo-Induced Enhancement of Critical Temperature in a Phase Competing Spin-Fermion System

Abstract: Ultrafast optical excitation is known to destabilize long-range order in correlated systems, yet experiments have also reported the emergence of metastable phases, in some cases with enhanced critical temperatures. The microscopic origin of such light-induced stabilization remains unresolved. Here we investigate this problem within a minimal spin-fermion framework: a double-exchange model at half filling, augmented by ferromagnetic superexchange on a square lattice. In equilibrium, at half-filling the ordering temperature is set by the competition between kinetic-energy-driven antiferromagnetism and superexchange-induced ferromagnetism. Using quantum Landau-Lifshitz-Gilbert-Brown dynamics for localized spins combined with mean-field evolution of itinerant electrons, we demonstrate a nonthermal mechanism for stabilizing ordered phases. Photoexcitation creates a long-lived nonequilibrium carrier population that resists thermalization and reshapes the low-energy landscape, converting kinetic-energy-driven antiferromagnetism into ferromagnetism and enhancing the critical temperature. While model-specific, our results reveal a general microscopic pathway by which light can tip the balance between competing orders, suggesting routes toward optically engineered magnetism, charge-density-wave order, and superconductivity.