Optical Study of 2D Magnets and Their Heterostructures for Valleytronics

Abstract

Electrons in layered van der Waals materials with a honeycomb lattice structure possess a valley degree of freedom in addition to charge and spin, which make van der Waals materials a tantalizing platform for valleytronics research. Among many candidates, transitional metal dichalcogenides (TMDs) have one of the most highly addressable valleys. For example, they possess valley optical selection rule which allows for interplay between light helicity and valley indexes; they also possess valley contrasting Berry curvature which allows for spatial separation of valley current without external field. In this thesis, we interface a van der Waals magnet—CrI3 with a representative TMDs—WSe2 to achieve unprecedented valley control in WSe2. Topics on optical studies on plain CrI3 is also covered. First, we demonstrate that strong exchange field and spin-selective charge hopping occurs at the interface between WSe2 and CrI3. The former leads to enhanced control of valley splitting, such as large valley splitting equivalent to Zeeman effect with 13T magnetic field, and rapid valley splitting switch nearly three orders of magnitude faster than can be achieved by the Zeeman effect in bare WSe2; the later leads to remarkable population control of WSe2. We then introduce layer-resolved proximity effects in WSe2/bilayer and trilayer CrI3 heterostructure and use this knowledge for revealing domain structure in CrI3 that has never been observed. Next, we unravel the excitation power dependent metamagnetic transition in CrI3, which is can be utilized for achieving continuous and reversible tuning of the valley splitting and valley polarization in WSe2. For the second part, which is the optical studies on plain CrI3, this thesis includes: Using magneto-optic Kerr effect to demonstrate that CrI3 is the world’s first discovered 2d magnet; its bilayer and trilayer have antiferromagnetically coupled ferromagnetic monolayer as their ground state; PL study on CrI3 reveals photoluminescence at ~1.1eV originated from Frenkel Exciton, with its helicity connected to the magnetic order of itself.