  Turing's reaction-diffusion theory of morphogenesis has been very successful
in understanding macroscopic patterns within complex objects ranging from
biological systems to sand dunes. However, this mechanism was never tested
against patterns that emerge at the atomic scale, where the basic ingredients
are subject to constraints imposed by quantum mechanics. Here we report
evidence of a Turing pattern that appears in a strained atomic bismuth
monolayer assembling on the surface of NbSe$_2$ subject to interatomic
interactions and respective kinetics. The narrow range of microscopic
parameters reflected in numerical analysis that observe stripe patterns and
domain walls with Y-shaped junctions is a direct consequence of the
quantum-mechanically allowed bond-lengths and bond-angles. This is therefore
the first demonstration of a dynamically formed Turing pattern at the atomic
scale.
