  The highly nonlinear many-body physics of a chain of mutually interacting
atoms in contact with a periodic substrate gives rise to complex static and
dynamical phenomena, such as structural phase transitions and friction. In the
limit of an infinite chain incommensurate with the substrate, Aubry predicted a
structural transition with increasing substrate potential, from the chain's
intrinsic arrangement free to slide on the substrate, to a pinned arrangement
favoring the substrate pattern. To date, the Aubry transition has not been
observed. Here, using a chain of cold ions subject to a periodic optical
potential we qualitatively and quantitatively establish a close relation
between Aubry's sliding-to-pinned transition and superlubricity breaking in
stick-slip friction. Using friction measurements with high spatial resolution
and individual ion detection, we experimentally observe the Aubry transition
and the onset of its hallmark fractal atomic arrangement. Notably, the observed
critical lattice depth for a finite chain agrees well with the Aubry prediction
for an infinite chain. Our results elucidate the connection between competing
ordering patterns and superlubricity in nanocontacts - the elementary building
blocks of friction.
