Electron-Phonon Coupling and Structural Phase Transitions on Gold/Molybdenum(112)

The electronic structures, many-body interactions and Fermi surface topologies of Au/Mo(112) were investigated in detail and were found to play important roles in the newly discovered order-disorder structural phase transition of the system. First, the high-resolution angle-resolved photoemission spectroscopy was utilized to characterize the electronic band structure of Mo(112) in far greater details than before. This elucidated the existence of several surface-derived states and their dispersion relations in high precisions near the Fermi level, as well as the symmetries of the bulk and surface electronic states, which are in good quantitative agreement with the ab-initio calculations. Such thorough understanding of the electronic states on Mo(112) made it possible to investigate the more complex electronic structure and many-body interactions in the Au overlayers formed on the Mo(112) surface and their interface. Upon the Au adsorption on Mo(112) substrate, the Au overlayer states are seen to hybridize with those of Mo substrate, which resulted in the formation of the several surface resonance bands, exhibiting high electronic localization near the surface and interface of the combined system. Furthermore, the electron-phonon coupling, involving these surface resonance states, is found to cause strong effective mass enhancement of the electrons near the Fermi level, which can contribute significantly to the surface lattice instability. In particular, for the (4x1) Au overlayer on Mo(112), the noticeable temperature-dependent changes in the Fermi surface contours were observed near the room temperature and were seen to act in favor of the stronger nesting condition and phonon-induced lattice distortions. The combination of the identified strong electron-phonon coupling and the critical Fermi surface topology near the room temperature likely relates to the overlayer lattice instability on the Au/Mo(112) system. In accord with the above general expectation, the order-disorder structural phase transitions were identified on Au/Mo(112) above the room temperature, which is characterized by the abrupt changes in the effective surface Debye temperature, indicative of significant softening of phonons on Au/Mo(112) across the transition. The sequence of these studies likely evidences that the strong electron-phonon coupling and the temperature-dependent Fermi surface topology are indispensable in driving the order-disorder transitions on Au/Mo(112)