A phase transition between low-temperature c(4x4) and high-temperature c(2x2) phases on Bi/Ag(001) was investigated by using low-energy electron diffraction, scanning tunnelling microscopy and angle-resolved ultra-violet photo-electron spectroscopy. Scanning tunnelling microscopy showed that the c(4x4) surface was composed of Bi tetramers, arrayed in a square lattice, while the tetramers decomposed to monomers in the c(2x2) phase. Low-energy electron diffraction scanning tunneling microscopy revealed an upward shift of a Bi-induced surface resonance band, around Γ, upon the transition from c(4x4) to c(2x2). This suggested that the Bi tetramer was stabilized by local covalent bonding between Bi atoms constituting a tetramer. Low-energy electron diffraction scanning tunneling microscopy also revealed a large Rashba spin-orbit splitting of the Bi-induced surface resonance band. The phase transition was associated with an intermediate temperature region in which the 2 phases coexisted on the surface, with their area changing gradually with changing temperature. The transition temperature was lower for a surface with a higher vacancy density. A model was presented which showed that the vacancy configuration entropy drove the phase transition. A larger vacancy configuration entropy of the c(2x2) phase, due to decomposition of the tetramers, accounted for the behavior of the phase transition via the 2-phase coexisting region.

Transition between Tetramer and Monomer Phases Driven by Vacancy Configuration Entropy on Bi/Ag(001). T.Nakagawa, O.Ohgami, Y.Saito, H.Okuyama, M.Nishijima, T.Aruga: Physical Review B, 2007, 75[15], 155409 (7pp)