Van der Waals Engineering of Ferromagnetic Semiconductor Heterostructures for Spin and Valleytronics

The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage. Here we create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of the spin and valley pseudospin in WSe2, where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting and polarization via flipping of the CrI3 magnetization. The WSe2 photoluminescence intensity strongly depends on the relative alignment between photo-excited spins in WSe2 and the CrI3 magnetization, due to ultrafast spin-dependent charge hopping across the heterostructure interface. The photoluminescence detection of valley pseudospin provides a simple and sensitive method to probe the intriguing domain dynamics in the ultrathin magnet, as well as the rich spin interactions within the heterostructure.Comment: Supplementary Materials included. To appear in Science Advance

First-Principles and Wannier-Function-Based Study of Two-Dimensional Electronic Systems

One of the goals in making better devices is to achieve the desired functionality in materials that enable a given application. The strong link between the functional behavior and the physical properties of materials is key to making better devices. This thesis focuses on applications of density functional theory (DFT), a powerful computational tool, for understanding the electronic, magnetic, magneto-optic, topological and thermodynamic properties of two-dimensional electronic systems (2DES). Why are 2DES interesting? Firstly, the reduced dimensionality renders these materials with properties which could be absent in the bulk form. Secondly, from a technological point of view, the desired functionality can be easily controlled externally in these 2DES by the application of a gate voltage or strain. The 2DES considered here could be crucial in beyond-CMOS electronic technologies. The materials considered in this thesis can be broadly categorized into two different classes of systems. The first one is the two-dimensional electron gas observed at the complex oxide interfaces. The discussion will go into the details of the formation of 2DEG in oxides resulting both from polar catastrophe and also due to the presence of vacancies. The second class of materials is two-dimensional (2D) atomic crystals, more specifically, 2D magnets. We not only predict a class of compounds, transition metal trichalcogenides (TMTC), that can exhibit magnetism in the 2D limit, but also demonstrate control of these magnetic degrees of freedom. Finally, we also demonstrate both using symmetry based tight-binding models and first-principles calculations a new way to detect magnetism in the 2D limit, which is applicable to compounds other than TMTC as well.</p