Exploring Magnetic Collective Modes and 2D Heterostructures with Raman

Abstract

In quantum materials, quantum mechanical effects survive over a wide range of temperatures and length scales, which leads to fascinating phenomena, such as charge density waves, spin liquids, and magnetism. These remarkable properties originate from a delicate balance among many interacting degrees of freedom. Even small perturbations such as temperature and doping can induce many diverse phases and colossal changes in the material's functionalities. It remains a mystery how to detect these novel phenomena without complicated fabrication and extreme conditions. This thesis contributed to this field by studying collective modes in quantum materials via inelastic light scattering. After fabricating the devices in a glovebox, we directly transfer them to the measurement platform under a high vacuum. Using Raman spectroscopy, a new quasi-particles--axial Higgs mode in charge density wave systems was discovered and further symmetry breaking accompanied with it suggests the charge density wave is unconventional. In addition to symmetry analysis breakthroughs, this thesis also provided direct evidence of the fractional nature as well as the energy and temperature limits of Kitaev interactions in quantum spin liquid candidate systems, which is the building block for topological quantum computers. Not limited to 2D Kitaev materials, the non-Fluery-Loudon single magnon scattering process was detected in a 3D Kitaev system. Other than using Raman to probe the fundamental nature, we also employed it to reveal, for the first time, a clean way to realize modulation doping in 2D materials, where the acceptor carrier density has reached 10$^{14}$ cm$^{-2}$. This method can be applied to dope magnetic materials or twisted heterostructures to find new phases.