  The motion of a relativistic particle is linked to its spin by the Dirac
equation. Remarkably, electrons in two-dimensional materials can mimic such
Dirac particles but must always appear in pairs of opposite spin chirality.
Using topological ideas of chiral boundary electrons wrapping a
three-dimensional crystal with tuned spin-orbit coupling, we show that elusive
unpaired Dirac fermions exist on the surface of a topological insulator parent
matrix, pure antimony. Using scanning tunneling microscopy and angle-resolved
photoemission spectroscopy we image coherent quantum interference between these
particles, map their helical spin texture, and ultimately observe a transition
to a single Dirac fermion distinguished by backscattering suppression via Berry
phase interference. The robust dynamics of these unique spin-polarized carriers
envisage future applications in spintronics and topological quantum
computation.
