Emerging exciton physics in two-dimensional semiconductors

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Excitons are bosonic quasiparticles composed of a bound electron-hole pair. As one of the most ubiquitous quasiparticles in solid state systems, excitons play a particularly important role in the optical and electronic properties of semiconductors. Two-dimensional materials provide new opportunities for the exploration of interesting exciton physics due to reduced screening of electron-hole interactions and the emergent layer degree of freedom. This dissertation presents a study of different aspects of exciton physics in two-dimensional semiconductors, with emphasis on interlayer exciton condensation in bilayer electron-hole systems. Chapter 1 provides an overview of exciton physics in two-dimensional semiconductors. Starting with a brief historical overview of theoretical and experimental progress in various material platforms, the rest of the chapter provides an introduction to theoretical formulations for the description of excitons and exciton condensation. Chapters 2 to 4 focus on interlayer exciton condensation in electron-hole bilayers. In Chapter 2 we study the electronic properties of an ideal electron-hole bilayer in which interlayer tunneling is suppressed and the carrier densities are tuned by gate and bias voltages. Recent experimental studies based on transition metal dichalcogenide bilayers are discussed. In Chapter 3 we study the influence of an in-plane magnetic field on InAs/GaSb quantum wells, another electron-hole bilayer but with finite p-wave interlayer tunneling. Chapter 4 presents a field-theory study of nonequilibrium electron-hole bilayers in which both bias voltage and interlayer tunneling are present. The next two chapters focus on moiré materials. In Chapter 5 we study the layer pseudospin physics of double-moiré systems and find interlayer coherent states and dipole crystal states as the system parameters vary. In Chapter 6 we study the strong modulation limit of excitons and trions and the associated optical properties of moiré materials by exact diagonalization of a quantum dot model. Finally, Chapter 7 concludes this dissertation with an outlook of the field.



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