  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 merohedral
twin boundary in chemically synthesized single-crystal nanowires (NWs) of CoSi,
a chiral WSM of space group P213 (No. 198). High resolution scanning
transmission electron microscopy reveals that this internal interface is (001)
twin plane and connects two enantiomeric counterparts at an atomically sharp
interface with inversion twinning. Ab-initio calculations show localized
internal Fermi arcs at the (001) twin boundary that can be clearly
distinguished from both external Fermi arcs and bulk states. These merohedrally
twinned CoSi NWs provide an ideal material system to probe unexplored
topological properties associated with internal interfaces in WSMs.
