First-principles theoretical analysis of phase transitions in hybrid perovskite and excitonic insulator

Abstract

Formation of exotic electronic states driven by electron correlations is one of the most intriguing phenomena in condensed matter physics. The macroscopic properties of a material are a consequence of how electrons and atoM.S. are arranged and held together in a solid. The motion of electrons and nuclei is well described by laws of quantum mechanics. The coupling between electronic charge, spin and lattice degree of freedom at microscopic level dictates the behavior of a material. With the theoretical and experimental advances, it has become possible to understand and control materials at di erent length and time scales, and with the recent advances in computational resources and scienti c algorithM.S., materials can be designed and their response to external stimuli can be understood through computer simulations. This gives us a better understanding of structure-property relationship in materials. The introduction of Density functional theory by Kohn and Sham [1] marked a breakthrough in rst-principles computational techniques. First-principles density functional theory based simulations provide fundamental insights into structural stability and properties of a material under the in uence of external stimuli. The physical properties of a material such as electronic structure, stress, elastic constant, polarization and vibrational properties can be estimated with reliable accuracy using rst-principles approach. On the other hand, classical atomistic modeling of materials helps in the study of their properties at long length and time scales through use of Monte Carlo simulations, possibly with a rst-principles based e ective Hamiltonian. This thesis is divided into two parts based on the kind of materials used for various technological applications. Part I consists of Chapter 3 and Chapter 4, focuses on Organic-Inorganic Hybrid Perovskites for solar cell technology. Part II consists of Chapter 5 which focuses on Excitonic Insulators: an exotic class of long-range interacting system. A brief introduction to them is given below.