Light Induced Structural and Magnetic Phase Transitions from First Principles

This thesis investigates, from first principles, how above-gap ultrafast photoexcitation can transiently modify magnetic, structural, and vibrational properties in semiconductors. The work is built around the constrained density functional theory framework for photoexcited materials, also referred to as the two-distribution model, which provides a unified description of nonequilibrium electron–hole populations and their impact on the free-energy landscape. First, we show that ultrafast photoexcitation can stabilize ferromagnetic order in initially nonmagnetic inverted-Mexican-hat monolayers through a photoinduced Stoner-like mechanism, offering a dynamical route to magnetism under standard linearly polarized light. Second, we address the photoinduced ferroelectric-to-paraelectric transition in GeTe by combining the two-distribution model with a nonperturbative treatment of anharmonicity within the stochastic self-consistent harmonic approximation. We show that, unlike the thermal transition, the photoinduced transformation is more naturally interpreted as a first-order structural phase transition driven by a strong reduction of the free-energy difference between rhombohedral and cubic phases, in agreement with ultrafast diffraction experiments. Finally, we develop an ab initio theory of ultrafast diffuse scattering that includes both incoherent lattice heating and coherent phonon dynamics, derive phonon-coherence equations within an electron–phonon Bloch framework, and implement them in the EPIq suite. Applications to photoexcited SnSe clarify the microscopic origin and limitations of current interpretations of coherent diffuse-scattering signals. Overall, this work establishes a transferable first-principles framework for understanding and predicting light-induced phase control in semiconductors.