Calculation of phonon-phonon interactions and two-phonon bound states on the Si(111):H surface

The Si(111):H surface is studied within the framework of density-functional theory using the local-density approximation. In particular, the Si-H stretch mode is investigated in detail. The phonon-phonon interaction strength is found to be 2Γ=-76.6 ${\mathrm{cm}}^{\mathrm{-}1}$ and the phonon bandwidth to be 7.2 ${\mathrm{cm}}^{\mathrm{-}1}$. Two-phonon bound states are found to exist with a binding energy of 86.4 ${\mathrm{cm}}^{\mathrm{-}1}$, in excellent agreement with the recent experimental result of 90 ${\mathrm{cm}}^{\mathrm{-}1}$.