Fermi surface gapping and nesting in the surface phase transition of Sn/Cu(100)

We identify and characterize a two-dimensional phase transition in a layer of Sn on Cu(100). The stable phase at room temperature has a (3?2×?2)R45° structure. Above ~360 K, a new phase with (?2×?2)R45° structure is formed. The high-temperature phase exhibits a quasi-two-dimensional free-electron surface band, with Fermi surface nesting in excellent agreement with the three-times larger periodicity of the low-temperature phase. A momentum-dependent band gap opens along the nested areas of the Fermi surface in the low-temperature phase. The phase transition is a clear experimental confirmation of the role of Fermi-surface gapping and nesting in the stabilization of a commensurate two-dimensional phase, which is interpreted as a charge-density wave