Self-organized pseudo-graphene on grain boundaries in topological band insulators

TL;DR

This paper demonstrates that topologically protected semi-metals can form along grain boundaries in topological insulators, exhibiting graphene-like states and unique electronic phenomena, expanding understanding of defect-induced topological phases.

Contribution

It reveals a novel mechanism for emergent semi-metals at grain boundaries in topological insulators via hybridization of localized modes, broadening the scope of topological material behavior.

Findings

Emergence of topologically protected semi-metals along grain boundaries.

Observation of valley anomaly affecting edge spin transport.

Graphene-like states exhibiting quantum Hall effects in 3D topological insulators.

Abstract

Semi-metals are characterized by nodal band structures that give rise to exotic electronic properties. The stability of Dirac semi-metals, such as graphene in two spatial dimensions (2D), requires the presence of lattice symmetries, while akin to the surface states of topological band insulators, Weyl semi-metals in three spatial dimensions (3D) are protected by band topology. Here we show that in the bulk of topological band insulators, self-organized topologically protected semi-metals can emerge along a grain boundary, a ubiquitous extended lattice defect in any crystalline material. In addition to experimentally accessible electronic transport measurements, these states exhibit valley anomaly in 2D influencing edge spin transport, whereas in 3D they appear as graphene-like states that may exhibit an odd-integer quantum Hall effect. The general mechanism underlying these novel semi-metals -- the hybridization of spinon modes bound to the grain boundary -- suggests that topological semi-metals can emerge in any topological material where lattice dislocations bind localized topological modes.