DYNAMICAL ONSET OF LIGHT-INDUCED SUPERCONDUCTIVITY IN K3C60

This thesis presents a comprehensive theoretical investigation into the dynamical onset of light-induced superconductivity in the alkali-doped fulleride compound KC. Motivated by experimental observations that ultrafast mid-infrared (MIR) pulses can transiently induce superconducting like states above the equilibrium critical temperature (T 20 K), this work seeks to uncover the microscopic mechanisms enabling such phe nomena. Unlike conventional superconductors, KC exhibits strong elec tron correlations and Jahn-Teller lattice distortions, making it an ideal candidate for studying non-equilibrium superconducting states. The re search combines phenomenological models such as the time-dependent Ginzburg-Landau (TDGL) theory with microscopic frameworks includ ing the Hubbard model and density functional theory (DFT) to simulate light-matter interactions. Emphasis is placed on nonlinear phononics, modulation of on-site Coulomb repulsion (Hubbard U), and the role of dynamically driven molecular vibrational modes (T and Hg symme tries). The theoretical models are validated through comparison with time-resolved THz spectroscopy data, which reveals changes in optical conductivity indicative of transient superconducting coherence. Key ndings show that resonant excitation of a 10 THz vibrational mode leads to the formation of a metastable superconducting-like phase that persists up to nanoseconds, even at room temperature. This state ex hibits zero-resistance transport characteristics and signatures of macro scopic phase coherence, despite being induced above the equilibrium Tc. The analysis also highlights that the light-induced superconduct ing state is uence-dependent rather than electric- eld dependent, in dicating a coherent, non-thermal mechanism. The conclusions a rm that light can not only enhance superconducting correlations but also synchronize pre-existing incoherent Cooper pairs. This points toward an electronically driven phase transition mechanism, distinct from pure thermal or phonon-limited models. The thesis o ers new insights into the design of optically controlled superconducting materials and opens pathways for potential applications in ultrafast quantum devices and energy-e cient electronics.2