Real-space investigation of the metal-insulator transition of Si(557)-Au

The Si(557) surface with Au adsorbates consists of a well ordered array of atomic chains, which exhibit interesting one-dimensional (1D) metallic band structure with two nearly half-filled 1D bands. This system was recently found to undergo a metal-insulator transition below room temperature [Phys. Rev. Lett. 91, 196403 (2003)]. The structural and electronic changes upon the phase transition have been investigated in detail using scanning tunneling microscopy and spectroscopy (STM/STS) with the guide of first-principles calculations. While the detailed bias-dependent STM images at room temperature are well simulated based on the present structure model, the characteristics of low-temperature images are not successfully reproduced. This casts doubts on the structure model of the low-temperature phase based on the step-edge buckling. The spatially resolved density of states (DOS) in STS measurements indicates clearly that only the step-edge Si chains are metallic, which exhibit strong reductions of DOS near Fermi level upon the metal-insulator transition together with the periodic lattice distortion. The band-gap opening on this metallic chain is shown to be symmetric in filled and empty states with a consistent gap size with the previous photoemission result. This result denies the recently proposed mechanism of the phase transition based on the dynamic fluctuation of the step-edge buckling. The effect of vacancy defects on the local lattice structures is also discussed for both the room- and low-temperature phases.