Giant Electron-Phonon Interaction for a Prototypical Semiconductor Interface: Sn/Ge(111)−(3×3)

We report an experimental and theoretical study of the electron-phonon coupling for alpha-Sn/Ge(111), a ./ ./ prototypical triangular lattice surface, closely related to Sn/Si(111)-( 3 x 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 x 3) phase of alpha-Sn/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 alpha-Sn/Ge(111)-(3 x 3) is unusually large, since we find that lambda, the electron mass enhancement for the half-filled band, is lambda = 1.3. This result is in good agreement with the experimental value obtained from high-resolution angle-resolved photoemission spectroscopy measurements, which yield lambda = 1.45 +/- 0.1.