Surface Peierls transition on Cu(001) covered with heavier p-block metals

Abstract

The Cu(001) surface covered with submonolayer coverages of In and Sn undergoes phase transitions at around 350–400 K. The transition is associated with the surface electronic structure change between low-temperature gapped and high-temperature ungapped ones. The energy gap positions in the k space coincide with the surface Brillouin zone boundaries of the low-temperature phases. These observations imply that the phase transitions are classified into the Peierls-type charge density wave (CDW) phase transition. The CDW ground states are characterized by large overall CDW gaps and long CDW correlation lengths. Structural studies show that the transitions are associated with order–disorder processes. This suggests that these are in the strong-coupling regime. However, the associated gapped–ungapped change suggests that the electronic terms play a significant role, in contradiction with the strong-coupling scenario. Based on the results of recent works on the precise temperature dependence of the CDW gap and critical X-ray scattering, the origin of this dual nature and the detailed mechanism of the phase transition is discussed. It is suggested that the electronic entropy of the CDW ground state is not governed by the overall energy gap but by the gap between the upper band minimum and the Fermi level of the whole system. The dual nature of the surface Peierls transition on Cu(001) originates, on the one hand, from the existence at metal surfaces of the two characteristic energy gaps: the overall gap, which determines the CDW stabilization energy, and the upper gap, which governs the electronic entropy. On the other hand, the CDW correlation length is suggested to play another significant role in determining the nature of the Peierls transition. The classification of the Peierls transitions according to the CDW correlation length and the gap size is discussed. It is suggested that the surface Peierls transition on Cu(001) covered with heavier p-block metallic elements is qualitatively different from both the weak-coupling CDW transition, with long CDW correlation length and small gaps, and the strong-coupling CDW transitions, with short correlation lengths and large gaps, and should be classified into the third class, which is characterized by long coherence and strong coupling.