Femtosecond spectroscopy of coherent phenomena in quantum materials : a dissertation

Abstract

Quantum materials are solids that cannot be described by the single-particle band models of conventional condensed matter physics. Rather strong inter-particle interactions and coupling between various degrees of freedom (charge, spin, orbital and lattice) lead to emergent phases such as high-temperature superconductivity, spin and charge density wave ordering and topologically protected 2D Dirac fermions. In the time-resolved experiments in this work, an initial laser 'pump' pulse drives the sample out-of-equilibrium by manipulating the electronic band structure, generating quasi-particles and/or exciting specific collective modes. The resulting dynamic changes are then tracked as a function of time by using two different spectroscopic tools: transient reflectivity and time and angle resolved photoemission (Tr-ARPES). One approach is to perturb the system gently (low pump intensity) to preserve the underlying order. Transient reflectivity experiments are done in this weak perturbation regime to study the following phenomena: (1) Collective excitations (amplitude and phase mode) of the fluctuating charge density wave order in the cuprate superconductor La₂-xSrxCuO₄; (2) Decay dynamics of valley polarized excitons in the monolayer transition metal dichalcogenide MoSe₂; and (3) a confinement-deconfinement transition of single-particle excitations in the spin-orbit assisted Mott insulator Na₂IrO₃. In the opposite regime (strong perturbation), it is possible to drive electronic materials into non-equilibrium phases with fundamentally different properties than in equilibrium. This work uses mid-IR pump pulses to directly couple photons to an electronic system to create hybrid electron-photon states. In this case, the oscillating electric field of the pump causes Dirac fermions to experience a time-periodic potential to generate Floquet-Bloch states which repeat in both energy and momentum. These and other similar photo-induced states are observed and characterized using Tr-ARPES on the topological insulators Bi₂Se₃ and Bi₂Te₃.