Competing Periodicities in Fractionally Filled One-Dimensional Bands

We present a variable temperature scanning tunneling microscopy and spectroscopy study of the Si(553)-Au atomic chain reconstruction. This quasi-one-dimensional system undergoes at least two charge density wave (CDW) transitions, which can be attributed to electronic instabilities in the fractionally filled 1D bands of the high-symmetry phase. Upon cooling, Si(553)-Au first undergoes a single-band Peierls distortion, resulting in period doubling along the chains. This Peierls state is ultimately overcome by a competing $\times 3$ CDW, which is accompanied by a $\times 2$ periodicity in between the chains. These locked-in periodicities indicate small charge transfer between the nearly $1/2$-filled and $1/4$-filled bands. The presence and the mobility of atomic-scale dislocations in the $\times 3$ CDW state indicates the possibility of manipulating phase solitons carrying a (spin, charge) of $(1/2,\pm e/3)$ or $(0,\pm 2e/3)$.

Figure 1

(a) Empty state and (b) filled state ($\pm 0.5\mathrm{V}$, 50 pA) STM images taken simultaneously at RT. Insets show magnifications. The structure in the chains is indicated with dots.

Figure 2

(a) Empty state (1 V, 100 pA) STM image at 40 K and magnification (b). In (b) a $6\times 1$ unit cell is indicated with combined $\times 3$ and $\times 2$ features. (c) Filled state ($-1\mathrm{V}$, 100 pA) STM image corresponding to (b).

Figure 3

Area averaged STS curves measured at indicated temperatures. Inset: numerical derivatives of the STS curves.

Figure 4

Empty state (1 V, 50 pA) STM images at 70 and 110 K. $\times 3$ and $\times 2$ unit cells are indicated in black and white open rectangles, respectively.

Figure 5

STM images at 40 K. (a) Empty state (1 V, 100 pA) image showing phase slips in the $\times 3$ CDW. (b),(c) Filled state ($-1\mathrm{V}$, 50 pA) images showing relocation of defects. Black circles, markers; arrow, old defect position; white circle, new defect position.