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Excitonic Insulator phase in Topological Energy Bands

Sokout, Leila | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53330 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Kargarian, Mehdi
  7. Abstract:
  8. Genetically the equilibrium and nonequilibrium collective properties of the electron system depend upon the strength of interaction between electrons, and of course between electrons and other degrees of freedom such as phonons. One of the collective states is the so-called exciton, a bound state of electron and hole in narrow-band semiconductors. The bound states give rise to absorption peaks below the gap in optical conductivity measurements.Excitonic insulator is an interesting phase of matter where the ground state is characterized by a coherent ondensation of exciton, much like the condensation of Cooper pairs in superconductors. Despite being introduced and theorized more than five decades ago, only recently it became possible to observe some of its features in materials. Grapehen bilayers and dichalcogenides are among the candidates for observing excitonic insulators but at extremely low temperatures. However, the recent transport and optical measurements on Ta2NiSe5 partially show that the ground state could be an excitonic insulator even at high temperatures of about 328K. These observations raised many questions, of which we investigate one of them in this thesis.In this thesis, we pose a simple question: how does the nontrivial topology affect the formation of an excitonic insulator? We first explore the idea in a simple onedimensional model composed of orbitals with opposite parties. Based on work done in our group, the results show that the band inversion is crucial in the formation of excitonic insulators. Then, we move to the famous Haldane model in two spatial symmetry. Our results show that in limit that, the hopping of electron to first neighbor can be neglected, the excitonic insulator is also topological insulator and we have coexistence of this phases in the framework that we investigate this model. Next, we investigate non-equilibrium dynamics and we can see the effect of phase of electron in oscillations
  9. Keywords:
  10. Topological Insulator ; Exciton ; Non-Equilibrium Dynamics ; Electron Systems ; Excitonic Insulator

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