We investigate the role played by the electron spin and the spin-orbit interaction in the exceptional electron-phonon coupling at the Tl/Si(111) surface. Our first-principles calculations demonstrate that the particular spin pattern of this system dominates the whole low-energy electron-phonon physics, which is remarkably explained by forbidden spin-spin scattering channels. In particular, we show that the strength of the electron-phonon coupling appears drastically weakened for surface states close to the $\overline{K}$ and $\overline{K^{\prime }}$ valleys, which is unambiguously attributed to the spin polarization through the associated modulation due to the spinor overlaps. However, close to the $\overline{\mathrm{\Gamma }}$ point, the particular spin pattern in this area is less effective in damping the electron-phonon matrix elements, and the result is an exceptional strength of the electron-phonon coupling parameter $\lambda \sim 1.4$. These results are rationalized by a simple model for the electron-phonon matrix elements including the spinor terms.

• Received 6 November 2017

DOI:https://doi.org/10.1103/PhysRevB.97.201405