Microscopic modeling of exciton diffusion and pump-probe spectroscopy in atomically thin materials

Examensarbete för masterexamen
Master Thesis
Causín, Raül Perea
Due to the reduced dielectric screening in two-dimensional materials, transition metal dichalcogenides (TMDs) exhibit a strong Coulomb interaction that leads to the formation of excitons (bound electron-hole pairs) with binding energies in the order of hundreds of meV. Therefore, excitonic e ects dominate the physical properties of these materials even at room temperature. Due to their impressive properties, TMDs are suitable candidates for novel optoelectronic devices. For a better understanding of these materials it is necessary to investigate the microscopic mechanisms behind basic phenomena such as optical excitation, energy relaxation, and transport. To this end, a fully quantum-mechanical theoretical framework based on density matrix theory is presented in this thesis in order to address two fundamental questions: (1) the e ect of excitation density on the excitonic and optical properties of TMDs, and (2) exciton spatial di usion and the appearance of excitonic halos under strong excitation. To nd the e ect of carrier density on the coherent excitonic states and the optical properties of the material, semiconductor Bloch equations are combined with the Wannier equation. Using this approach, it is found that exciton properties are signi cantly altered and even suppressed at high carrier densities due to dielectric many-particle screening and Pauli blocking. In the rst part of this thesis, pump-probe experiments are modeled to provide a microscopic understanding of experimental observations of these phenomena. Moreover, the Wigner representation is introduced to study the spatio-temporal dynamics of incoherent exciton populations and, speci cally, the e ect of excitation density on exciton di usion. While under a weak excitation exciton di usion follows the conventional Fick law, the results presented in this thesis show that under strong excitation a signi cant temperature gradient is created in the excitonic system, leading to the formation of spatial excitonic halos. This work unveils the microscopic mechanisms responsible for this unconventional phenomenon that was experimentally observed very recently.
Fysik, Physical Sciences
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