Electronic Structure Evolution in the Temperature Range of Metal Insulator Transitions on Sn/Ge(111)

One third of monolayer of Sn adatoms on a Ge (111) substrate forms a two-dimensional triangular lattice with one unpaired electron per site. The system presents a metal-insulator transition when decreasing the temperature and it is known to exhibit strong electron-phonon coupling at 120-150 K. Here, we report a study of the electronic band structure for α-Sn/Ge(111) between 150 K and 5 K. We present both the experimental Fermi surfaces and the energy dispersions along high symmetry directions as a function of the temperature. At 5 K it is observed a weakly or low- dispersing spectral feature exhibiting an extended gap in the reciprocal space. This feature is derived from the topmost occupied band, which is metallic at high temperature and which develops a kink associated to strong electron-phonon coupling. The spectral evolution is partially explained with an increase of the electron-phonon coupling when decreasing the temperature. The increase of the electron-phonon coupling at low temperatures gives light into the new physics of this 2D system. The bandwidth is progressively reduced when reducing the temperature, enhancing the electronic correlation effects and triggering the Mott transition.