Atomically Sharp Internal Interface in a Chiral Weyl Semimetal

Internal interfaces in Weyl semimetals (WSMs) are predicted to host distinct topological features that are different from the commonly studied external interfaces (crystal-to-vacuum boundaries). However, the lack of atomically sharp and crystallographically oriented internal interfaces in WSMs makes it difficult to experimentally investigate hidden topological states buried inside the material. Here, we study a unique internal interface known as <i>merohedral</i> twin boundary in chemically synthesized single-crystal nanowires (NWs) of CoSi, a chiral WSM of space group <i>P</i>2₁3 (No. 198). High resolution scanning transmission electron microscopy (HRSTEM) reveals that this internal interface is a (001) twin plane which connects two enantiomeric counterparts at an atomically sharp interface with inversion twinning. <i>Ab-initio</i> calculations show localized internal Fermi arcs at the (001) merohedral twin boundary of CoSi that can be clearly distinguished from both external Fermi arcs and bulk states. These merohedrally twinned NWs provide an ideal material system to probe unexplored topological properties associated with internal interfaces in WSMs.