Charge order in a frustrated two-dimensional atom lattice at a semiconductor surface

Adatom systems deposited on Si(111) substrates have attracted much attention recently. Surface coverage of a 1/3 monolayer leads to √3×√3 reconstruction, resulting in a remarkable realization of an isotropic triangular lattice Hubbard model.

While Sn adatoms lead to spin density wave order, here we investigate the Pb adatom system using scanning tunneling spectroscopy. We report the detection of a checkerboard-type charge density wave order, previously not known. The full phase diagram is mapped by an Extended Variational Cluster Approach, which identifies the experimentally observed charge order driven by longer-ranged interactions. The tunable electron correlations in such 2D lattices open the door to search for superconductivity with well-controlled system variables.