Direct Evidence of the Charge Ordered Phase Transition of Indium Nanowires on Si(111)

A controversial issue of the driving force for the phase transition of the one-dimensional (1D) metallic In wires on Si(111) is studied by low-temperature scanning tunneling microscopy and spectroscopy. The energy gap opening and the longitudinal charge ordering through charge transfer at the Fermi level are unambiguously observed. The vacancy defects induce a local charge ordering decoupled from a lattice distortion above $T_c$, and pin the phase of charge order below $T_c$. All these results below and above $T_c$ including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.

Figure 1

(a) Filled and (b) empty state STM images taken at 135 K using a dual scan mode, and similar images (d),(e) at 78 K. The same image (c) as (a) and (f) as (d) with the ($4\times 1$) and ($8\times 2$) structure models superimposed on [7, 9]. (g) Derivative of $I\mathrm{\text{−}}V$ spectra averaged over 20 points before (dashed line) and after (solid line) the phase transition. Open circles in (a),(b),(d),(e) mark charge-density maxima in empty state images. A parallelogram in (b) denotes a ($4\times 1$) unit cell. The imaging conditions are $\mp 0.15\mathrm{V}$ and 20 pA for (a) and (b), and $\mp 0.30\mathrm{V}$ and 30 pA for (c) and (d), respectively.

Figure 2

(a) Filled-state STM image of a terrace with several In wires and defects at $T=135\mathrm{K}$. Enlarged images of a box in (a) are (b) empty and (c) filled-state images of an In nanowire including defects, and (d) an image of their arithmetic sum. Line profiles of the upper row of the In nanowire are plotted in (e) for the comparison of the charge redistribution and accompanying PLD. The imaging conditions are $\pm 0.15\mathrm{V}$ and 20 pA.

Figure 3

(a) An $30\times 31\mathrm{n}{\mathrm{m}}^{\mathrm{2}}$ empty state STM image taken at 78 K and (b) line profiles of two chains in a box with O. The box with O contains charge disordered In chains, while the box with $E$ contains normal CDW chains. Black arrows denote disordered chains bound by defects while white arrows ordered chains despite defects. The sample bias and the tunneling current are $+0.6\mathrm{V}$ and 0.1 nA, respectively.