Realization of a one-dimensional topological insulator in ultrathin germanene nanoribbons

TL;DR

This paper reports the fabrication and characterization of ultrathin germanene nanoribbons, demonstrating the transition from 2D to 1D topological insulator states, including the observation of zero-dimensional end states in nanoribbons below a critical width.

Contribution

It provides the first experimental realization of a 1D topological insulator in germanene nanoribbons, revealing the dimensionality limit of 2D topological behavior.

Findings

Topological edge states persist down to ~2 nm width.

Zero-dimensional end states appear below critical width.

End states are protected by symmetries.

Abstract

Realizing a one-dimensional (1D) topological insulator and identifying the lower dimensional limit of two-dimensional (2D) behavior are crucial steps toward developing high-density quantum state networks, advancing topological quantum computing, and exploring dimensionality effects in topological materials. Although 2D topological insulators have been experimentally realized, their lower dimensional limit and 1D counterparts remain elusive. Here, we fabricated and characterized arrays of zigzag-terminated germanene nanoribbons, a 2D topological insulator with a large topological bulk gap. The electronic properties of these nanoribbons strongly depend on their width, with topological edge states persisting down to a critical width (approx. 2 nm), defining the limit of 2D topology. Below this threshold, contrary to the tenfold way classification, we observe zero-dimensional (0D) states localized at the ends of the ultrathin nanoribbons. These end states, topologically protected by time-reversal and mirror symmetries, mark the first realization of a 1D topological insulator with strong spin-orbit coupling. Our findings establish germanene nanoribbons as a platform for investigating 1D topology and dimensionality effects in topological materials.