Giant electron-phonon interaction for a prototypical semiconductor interface: Sn/Ge(111)−(3×3)

Abstract

We report an experimental and theoretical study of the electron-phonon coupling for $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}/\mathrm{Ge}(111)$, a prototypical triangular lattice surface, closely related to $\mathrm{Sn}/\mathrm{Si}(111)\text{\ensuremath{-}}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$, where recent experimental evidence has found superconductivity [X. Wu et al., Phys. Rev. Lett. 125, 117001 (2020)]. We concentrate our study on the ($3\ifmmode\times\else\texttimes\fi{}3$) phase of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}/\mathrm{Ge}(111)$ that appears between 150 and 120 K and has a well-known geometry with a half-filled electronic band around the Fermi energy. We show that this surface presents a giant electron-phonon interaction that can be considered at least partially responsible for the different phases that this system shows at very low temperature. Our theoretical results indicate that indeed the electron-phonon interaction in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}/\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}3)$ is unusually large, since we find that $\ensuremath{\lambda}$, the electron mass enhancement for the half-filled band, is $\ensuremath{\lambda}=1.3$. This result is in good agreement with the experimental value obtained from high-resolution angle-resolved photoemission spectroscopy measurements, which yield $\ensuremath{\lambda}=1.45\ifmmode\pm\else\textpm\fi{}0.1$.