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 2D 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. Herein, a study of the electronic band structure for alpha-Sn/Ge(111) between 150 and 5 K is reported. Both the experimental Fermi surfaces and the energy dispersions along high symmetry directions as a function of the temperature are presented. 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 with the 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. This study presents the evolution of the Fermi surface and the electronic dispersions of alpha-Sn/Ge(111) between 150 and 5 K using angle-resolved photoemission spectroscopy. The experimental band structure evolution in the temperature-induced phase transition is understood within a Debye model where the electron-phonon coupling strength increases at low temperatures.image (c) 2024 WILEY-VCH GmbH