Massive 1D Dirac Line, Solitons and Reversible Manipulation on the Surface of a Prototype Obstructed Atomic Insulator, Silicon

Abstract In the past decade, topological quantum materials have attracted enormous research efforts in both physics and materials science. However, despite increasing number of discovered topological quantum materials, recent high throughput computations also predicted a majority of topologically trivial materials at the Fermi level. Do all the topologically trivial materials behave identically or are there clear distinct classes of behavior? Topologically trivial insulators can be first classified into two types: atomic insulators (AIs) and obstructed atomic insulators (OAIs), depending on whether the Wannier charge centers are localized or not at spatial positions occupied by atoms. Interestingly, an OAI can possess unusual properties such as surface states along certain crystalline surfaces. Although required by the bulk OAI nature, these surface states do not necessarily cross the entire bulk gap, as they would do in a topological insulator (TI). However, they advantageously appear in materials with much larger bulk energy gap than TIs, which makes them more attractive for potential applications. Here we experimentally and theoretically show that a well-known crystal, silicon (Si) is a model OAI, which naturally explains some of Si’s unusual properties such as its famous (111) surface states. On this surface, using angle resolved photoemission spectroscopy (ARPES), we reveal sharp quasi-1D massive Dirac line dispersions from two kinds of atomic chains due to the (2×1) surface reconstruction. We also observe, using scanning tunneling microscopy/spectroscopy (STM/STS), topological solitons at the interface of the two atomic chains. Remarkably, we show that the different chain domains can be reversibly switched at the nanometer scale, suggesting the application potential in ultra-high density storage devices.