Valley-Selective Optical Absorption and Giant Exciton Binding Energy of Breathing Kagome Semiconductor with Nearly Flat Band.

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Endowed with topological flat band and dispersive Dirac cones, layered Kagome materials, which are expected to exhibit distinctive electronic and optical properties, have garnered significant attention in the forefrontier research of condensed matter physics. Using density functional theory and many-body perturbation theory, here we study the optical and excitonic properties of tunable two-dimensional (2D) breathing Kagome material Ta3SBr7 with topologically nontrivial flat band. Originated from the bulk counterpart, two thermodynamically stable structures are proposed, suggesting a new geometric degree of freedom for controlling the electronic and optical properties of the breathing Kagome lattice. Both Ta3SBr7 monolayers exhibit momentum selective optical absorption, whose mechanism is unveiled from the detailed analysis of the dispersion, symmetry, and valley-contrasting Berry physics of electronic band structure. Because of the existence of characteristic nearly flat bands, the ground-state excitons of both Kagome structures possess exceptionally large binding energies up to 1.1 eV. Moreover, this geometric degree of freedom also results in significant differences in the optical activity and exciton radiative lifetimes for ground-state excitons in these two structures. Our results showcase the potential of Kagome semiconductors not only in the manipulation of excitonic behaviors but also in fundamental physics, such as fractional Chern insulators at zero applied magnetic field.