First-principles calculations of defect-phonon coupling of a spin-1/2 state from a charged carbon impurity in two-dimensional transition-metal dichalcogenides*

Point defects in two-dimensional semiconductors have attracted much attention for their potential as single-photon sources for next-generation quantum information science. Here, we present studies of a novel point defect, a negatively-charged carbon impurity substituting for a chalcogen atom, in transition-metal dichalcogenides (TMDs) monolayers that has been recently realized experimentally [arXiv:2008.12196] and can be generated deliberately with atomic precision. Using density functional theory calculations and a model spin-boson Hamiltonian, we show that the defect introduces a spin-1/2 two-level quantum system deep in the TMD band gap with strong phonon sidebands. We identify a few specific phonon modes that significantly couple to the spin-split defect states. Interestingly our calculations predict that this coupling is sensitive to the defect spin state, which in turn is responsible in generating phonon sidebands with different fine structure depending on the spin channel. Finally, we will discuss the possibility of the defect state being used as a next-generation single-photon emitter.