Dual nature of a charge-density-wave transition on In/Cu(001)

A surface phase transition on In/Cu(001) with In coverage of 0.63 was studied. The structural analysis shows that the reversible phase transition at 405 K between the high-temperature $(2\mathrm{}\times 2)$ and the low-temperature $(2\mathrm{}\sqrt{2}\times 2\sqrt{2})R45\mathrm{}°$ phases belongs to an order-disorder type. The angle-resolved photoemission experiment shows that the low-temperature phase is stabilized by the partial gap formation at the Fermi surface, indicating that the transition is due to the Peierls-type Fermi-surface nesting. While the above observations point to a strong-coupling charge-density-wave (SCDW) scenario, the temperature-dependent behavior of the gap is in better agreement with the weak-coupling CDW theory. Thus, the results serve the first experimental characterization of the CDW transition driven cooperatively by electronic and lattice entropies.