Detection and Manipulation of Complex Electric and Magnetic Dipole Textures in Three- and Two-dimensional Crystals

Abstract

This dissertation presents the research of complex ordered states—textures of electronic or magnetic dipoles—in crystalline solids, with a particular focus on the ferro-rotational charge density wave (CDW) and the non-collinear magnetic structures arising from moire engineering. Using a combination of advanced nonlinear, ultrafast optical spectroscopy and inelastic light scattering, this research provides novel insights into the dynamic and tunable properties of such orders in relevant quantum materials.

The first part of the dissertation investigates the ferro-rotational nature of the commensurate charge density wave (CCDW) in 1T-TaS2. We establish that a higher-order electric quadrupole (EQ) second-harmonic generation (SHG) presents and its rotation anisotropy (RA) pattern senses the breaking of the mirror symmetry—a hallmark of the ferro-rotational order—due to the formation of the CCDW. With optical pumping, our experiments further reveal ultrafast modulations of this ferro-rotational CDW, characterized by the breathing and rotation of the EQ RA-SHG patterns at specific frequencies. These findings demonstrate the capability of nonlinear SHG coupling to the ferro-rotational CDW order as well as the dynamic control of it, and highlight the potential for photo-induced transient CDW phases, applicably expanding our understanding of multipolar orders in quantum materials.

In the following parts we present a multivarious optical study of the magnetic material CrI3. We start by examining the magnetism in bulk CrI3 using circularly polarized Raman Scattering. By revealing diverging antiferromagnetic (AFM) magnon branches in the known bulk ferromagnetism (FM), we propose a novel mixed state of layered AFM and bulk FM in this material. When polarizing the surface AFM into FM at a critical magnetic field, we also discover a first-order, rhombohedral-to-monoclinic structural phase transition by detecting the symmetry change of exemplary phonons. Our results demonstrate the intricate interplay between magnetism and the crystalline structures in this material.

We then present the exploration of the complex magnetic orders in twisted double-bilayer CrI3. The tunability of the layered-magnetism-assisted magneto-Raman phonons is first studied. We confirm the successful moire engineering in this system by showing a comprehensive twist angle dependence of such modes. By examining the magnetic field dependence, we also identify a novel magnetic ground state at an intermediate twist angle of 1.1 deg, which is distinct from natural untwisted four-layer or bilayer systems. Further with the help of the theoretical simulation, we propose the formation of non-collinear spin texture upon twisting as well as its twist angle dependence. By conducting the magnetic circular dichroism measurements and attributing the signal to collinear spin and non-collinear spin contributions, we experimentally demonstrate the maximum of both the non-collinearity and an emergent net magnetization at the intermediate twist angle. Such studies manifest the vast opportunities for exploring complex magnetic orders with moire engineering in two-dimensional magnets.

In conclusion, this dissertation advances the optical study of specific complex electronic and magnetic orders in 1T-TaS2 and CrI3, respectively. The findings have implications for both the future development, utilization of advanced optical probes, and the deeper understanding of complex orders together with the control or application of them.