Z3 Charge Density Wave of Silicon Atomic Chains on a Vicinal Silicon Surface.

An ideal one-dimensional electronic system is formed along atomic chains on Au-decorated vicinal silicon surfaces, but the nature of its low-temperature phases has been puzzling for last two decades. Here, we unambiguously identify the low-temperature structural distortion of this surface using high-resolution atomic force microscopy and scanning tunneling microscopy. The most important structural ingredient of this surface, the step-edge Si chains, are found to be strongly buckled, every third atom down, forming trimer unit cells. This observation is consistent with the recent model of rehybridized dangling bonds and rules out the antiferromagnetic spin ordering proposed earlier. The spectroscopy and electronic structure calculation indicate a charge density wave insulator with a Z3 topology, making it possible to exploit topological phases and excitations. The tunneling current was found to substantially lower the energy barrier between three degenerate CDW states, which induces a dynamically fluctuating CDW at very low temperature.

Creation and annihilation of mobile fractional solitons in atomic chains.

Localized modes in one-dimensional (1D) topological systems, such as Majonara modes in topological superconductors, are promising candidates for robust information processing. While theory predicts mobile integer and fractional topological solitons in 1D topological insulators, experiments so far have unveiled immobile, integer solitons only. Here we observe fractionalized phase defects moving along trimer silicon atomic chains formed along step edges of a vicinal silicon surface. By means of tunnelling microscopy, we identify local defects with phase shifts of 2π/3 and 4π/3 with their electronic states within the band gap and with their motions activated above 100 K. Theoretical calculations reveal the topological soliton origin of the phase defects with fractional charges of ±2e/3 and ±4e/3. Additionally, we create and annihilate individual solitons at desired locations by current pulses from the probe tip. Mobile and manipulable topological solitons may serve as robust, topologically protected information carriers in future information technology.