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.