Understanding and Controlling the Magnetic and Optical Properties in van der Waals Semiconductors

Abstract

Within the broad spectrum of two-dimensional (2D) materials, 2D van der Waals (vdW) magnetic semiconductors are distinguished by their novel properties, which stem from the weak yet tunable interlayer magnetic interactions, adding an entirely new magnetic degree of freedom to vdW interfacial engineering. In this dissertation, we discuss the understanding and prediction of the magnetic and optical properties of vdW magnetic semiconductors employing ab initio methods alongside other theoretical and computational techniques. This dissertation is organized as follows: • In Chapter 1, we provide a brief introduction to the theoretical and computational methods that compute the quasiparticle and exciton properties of materials, and the recent advancements in the field of 2D magnetic semiconductors. • In Chapter 2, we focus on exploring the magneto-excitonic coupling in a prototypical 2D vdW magnet CrSBr utilizing ab initio calculations.1 We uncover the anisotropic Wannier nature of the 2D excitons in few-layer CrSBr and the entanglement of excitons between vdW layers of this material. • In Chapter 3, we present several mechanical approaches to harnessing the power of magneto-electronic coupling in 2D vdW magnet CrSBr.2,3 Our qualitative analysis attributes the effects of these mechanical tuning knobs on magnetic properties to subtle alterations in bond geometry. • In Chapter 4, we investigate the intriguing potential for manipulating magnetic phases in 2D magnets through interfacial charge transfer in heterostructures of magnetic and nonmagnetic layers.4 We unveil a transition towards the ferromagnetic phase by stacking antiferromagnetic bilayer CrSBr on graphene and by electrostatic doping. We further demonstrate that the phase transition is a spin-canting process. • In Chapter 5, we establish a theoretical framework to investigate the ultrafast optical control of excitonic structures. We demonstrate coherent optical field as a powerful tool to manipulate the excitonic properties in 2D materials. From our calculations, we find that the dark and bright excitons can be coherently coupled, resulting in novel absorption features as a function of frequency.