Real-space investigation of the metal-insulator transition of $\mathrm{Si}\left(557\right)\text{−}\mathrm{Au}$

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.

Figure 1

(Color online) (a) Close-up of experimental STM images of the Si(557)-Au surface taken with a sample bias $V_s$ of $+1.0$ (empty, upper) and $-1.0\mathrm{V}$ (filled states, lower panel). Corresponding simulations for (b) the simple planar model, (c) the $1\times 2$ adatom model, and (d) the $1\times 1$ adatom model. In (b)–(d), the side view of each structure model is given, where the yellow, gray, and large gray balls represent the Au, Si, and Si adatoms, respectively.

Figure 2

(Color online) Experimental and simulated STM images of Si(557)-Au at 78 K with sample-bias voltages $V_s$’s of (a) $-0.5$, (b) $+0.7$, (c) $-1.0$, and (d) $+1.0\mathrm{V}$. All experimental images are acquired with a tunneling current of 0.1 nA and the simulations are based on the $1\times 2$ adatom model with the buckling on the step-edge Si chains. The Au and Si adatoms are depicted by yellow and dark balls, respectively.

Figure 3

(Color) STM topographic images of Si(557)-Au at (a) 300 and (c) 78 K $(V_s=+1.5\mathrm{V})$. $dI∕dV$ maps at (b) 300 and (d) 78 K, which were acquired by CITS measurements at the given bias voltages, simultaneously with the constant current topography. Here, the intensity scale of the $dI∕dV$ maps, which represents local density of states (LDOS), is given at the bottom.

Figure 4

(Color online) Normalized $dI∕dV$ spectra on the step edges at 300 K (red) and 78 K (blue). The spectra near Fermi level (bias zero) are enlarged in the inset. Each spectrum is averaged over several equivalent positions along the step edges.

Figure 5

(Color online) Large-area STM image of Si(557)-Au acquired at 300 K with a bias voltage $V_s$ of $+1.5\mathrm{V}$. Three different kinds of defects are observed. The defect-induced modulations are marked by solid and dashed lines.

Figure 6

(Color online) Empty-state STM images near a vacancy defect in the step-edge Si chain and the corresponding line profiles along the center of the chain at (a) 300 and (b) 78 K.