  Topological surface states are a class of novel electronic states that are of
potential interest in quantum computing or spintronic applications. Unlike
conventional two-dimensional electron states, these surface states are expected
to be immune to localization and to overcome barriers caused by material
imperfection. Previous experiments have demonstrated that topological surface
states do not backscatter between equal and opposite momentum states, owing to
their chiral spin texture. However, so far there is no evidence that these
states in fact transmit through naturally occurring surface defects. Here we
use a scanning tunnelling microscope to measure the transmission and reflection
probabilities of topological surface states of antimony through naturally
occurring crystalline steps separating atomic terraces. In contrast to
nontopological surface states of common metals (copper, silver and gold), which
are either reflected or absorbed by atomic steps, we show that topological
surface states of antimony penetrate such barriers with high probability. This
demonstration of the extended nature of antimony's topological surface states
suggests that such states may be useful for high current transmission even in
the presence of atomic scale irregularities-an electronic feature sought to
efficiently interconnect nanoscale devices.
