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Resonant Enhancement of Second Harmonic Generation by Edge States in Transition Metal Dichalcogenide Monolayers

We derive a low-energy theory for edge states in transition metal dichalcogenide monolayers for a two-band $\bm{kp}$-Hamiltonian in case of uncoupled valleys. In the absence of spin-orbit interaction at the edge, these states possess a linear dispersion described by a single phenomenological parameter characterizing the edge structure. Depending on the sign of the parameter, the edge state spectrum can either cross the band gap or lie outside of it. In the first case, the presence of edge states leads to resonant enhancement of the second harmonic generation at frequencies about the half of the band gap, in agreement with recent experiments. The value of the phenomenological boundary parameter is extracted from the resonance frequency position.