# Theoretical Physics

**28 October 2015**

**Time:**15:00 to 16:00

**Location:**EC Stoner 8.60

Theoretical Physics Seminar: Ville Lahtinen

3pm @ EC Stoner 8.60

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

Semi-metallic materials, such as graphene in two dimensions (2D) and various Dirac and Weyl semi-metals in three dimensions (3D), are characterized by nodal band structures that give rise to exotic electronic properties. Their stability requires the presence of lattice symmetries or application of external fields, making them lack the inherent topological protection enjoyed by surface states of topological band insulators. Here we bridge this divide by showing that a self-organized topologically protected semi-metals, that in 1D exhibit an edge spin transport influencing valley anomaly and in 2D appear as a graphene-like semi-metal characterized by odd-integer quantum Hall effect, can emerge and be experimentally observed on extended defects in topological insulators. In particular, these states emerge on a grain boundary, a ubiquitous lattice defect in any crystalline material, thereby providing a novel and experimentally accessible route to topological semi-metals. The underlying mechanism is the hybridization of spinon modes bound to the grain boundary, whose generality suggests that new states of matter can emerge in any topological band insulator where lattice dislocations bind localized topological modes.

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