Competing charge ordering and Mott phases in a correlated Sn/Ge(111) two-dimensional triangular lattice

We take advantage of complementary high-resolution experimental techniques and theoretical tools to get insight into the alpha-Sn/Ge(111) triangular lattice surface consisting of sp electrons. We report a (3 x 3) phase, characterized by a charge ordering settled by electronic correlation, which appears between the known metallic-(3 x 3) and Mott insulator phases. We identify the atomistic mechanism behind the stabilization of this phase and interpret these findings on the basis of theoretical calculations. We disentangle the role of the various degrees of freedom in the stability of the different phases found and describe the stepwise surface changes between the metallic and Mott insulating phases.