Abstract
Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: (i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification and (ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In this paper, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material , mediated via a first-order structural transition between two distinct stacking orders of the weakly coupled chains. Using high-resolution angle-resolved photoemission spectroscopy on the two natural cleavable surfaces, we identify the high-temperature phase to be the first weak topological insulator with two gapless Dirac cones on the (100) surface and no Dirac crossing on the (001) surface, while in the low-temperature phase, the topological surface state on the (100) surface opens a gap, consistent with a recent theoretical prediction of a higher-order topological insulator beyond the scope of the established topological materials databases that hosts gapless hinge states. Our results not only identify a rare topological phase transition between first-order and second-order topological insulators but also establish a novel quasi-1D material platform for exploring unprecedented physics.
- Received 7 February 2021
- Revised 30 April 2021
- Accepted 27 May 2021
DOI:https://doi.org/10.1103/PhysRevX.11.031042
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
A topological insulator (TI) is a fascinating state of matter in which its interior is insulating whereas its exterior conducts robustly. “Strong TIs,” in which every surface is conductive, are abundant in nature. However, “weak TIs,” in which only selective surfaces are conductive, as well as “higher-order TIs,” in which the robust conduction channels are confined to selected hinges, remain elusive. Both the weak and higher-order TIs host helical edge states—the electronic highway of spins and the cornerstone of topological electronics. Here, we provide the first unambiguous experimental evidence of a weak TI in a quasi-1D crystal of .
In a weak TI crystal, the outer edges of the atomic layers support a topological surface state. This surface is special, because there is now electrical conduction that behaves like a two-way highway around each layer. The conduction along the two ways can only cross talk between adjacent layers. This translates into a pair of so-called “gapless Dirac cones,” a key observational signature of weak TIs wherein the electronic band structure looks like two separate but connected light cones. In our experiments, we find evidence of this signature in . By lowering the temperature, the topological side surface states also develop compelling evidence for a phase transition into a higher-order TI.
Our work not only establishes a versatile quasi-1D topological material platform but also paves the way for a room-temperature topological switch that can control the dimensions of topological conduction.