Time- and angle- resolved photoemission spectroscopy (trARPES) on quantum materials



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The emergence of state-of-the-art technologies and quantum materials has opened a new era of condensed matter physics. The new technology enabled us to see the quantum phenomena in a different view or even led us to discover new intriguing physics. For example, in photoemission spectroscopy, the advent of 2D detectors with high energy and momentum resolution has boosted up the study of electronic structures of quantum materials. Furthermore, the combination of ARPES with an ultrafast laser system allows us to investigate one more degree of freedom, time, in the materials. These experimental advantages, indeed, could successfully provide valuable results regarding occupied & unoccupied electronic band structures, electron dynamics, exciton physics, superconductivity, non-equilibrium states and many body physics such as band gap renormalization and Mott transition. Along with such advanced tools, van der Waals (vdW) 2D materials appeared as new promising quantum materials for science and practical applications recently. In 2D materials, the coulomb screening is significantly reduced, so many interesting phenomena can be retrieved, which were obscured by the strong coulomb screening in the conventional 3D materials. For example, excitons in TMDs, bound states between electron and hole, can have very large binding energy (~few hundreds meV) compared to excitons in conventional semiconductors (~few meV). The Mott transition and band gap renormalization can also be observed in monolayer TMDs. Moreover, TMDs show an excellent tunability for the electronic structure. For example, by stacking different TMDs layers, it is possible to create type-2 band alignment which can possess interlayer excitons and moiré potential inducing many intriguing physics such as moiré exciton and flat bands. In this dissertation, the photoexcited electronic structure of monolayer MoS₂ was studied by the state-of-the-art extreme-UV trARPES. In addition, the monolayer MoS₂/bulk MoSe₂ heterostructure is investigated in order to study the band hybridization and moiré effect. Lastly, the role of the parallel momentum in Pb/Ag bimetallic thin films is discussed.



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