Helical Metal Inside a Topological Band Insulator

TL;DR

This paper demonstrates that dislocation lines in topological insulators like Bi$_{0.9}$Sb$_{0.1}$ can host topologically protected one-dimensional fermionic modes, potentially impacting charge and spin transport.

Contribution

It reveals that dislocation lines in topological insulators host protected fermionic excitations, a novel finding linking topological defects to quantum transport.

Findings

Dislocation lines host one-dimensional fermionic modes.

These modes are topologically protected and immune to disorder.

Potential impact on charge and spin transport in materials.

Abstract

Topological defects, such as domain walls and vortices, have long fascinated physicists. A novel twist is added in quantum systems like the B-phase of superfluid helium He$_3$, where vortices are associated with low energy excitations in the cores. Similarly, cosmic strings may be tied to propagating fermion modes. Can analogous phenomena occur in crystalline solids that host a plethora of topological defects? Here we show that indeed dislocation lines are associated with one dimensional fermionic excitations in a `topological insulator', a novel band insulator believed to be realized in the bulk material Bi$_{0.9}$Sb$_{0.1}$. In contrast to fermionic excitations in a regular quantum wire, these modes are topologically protected like the helical edge states of the quantum spin-Hall insulator, and not scattered by disorder. Since dislocations are ubiquitous in real materials, these excitations could dominate spin and charge transport in topological insulators. Our results provide a novel route to creating a potentially ideal quantum wire in a bulk solid.