  Stick-slip -- the sequence of mechanical instabilities through which a slider
advances on a solid substrate -- is pervasive throughout sliding friction, from
nano to geological scales. Here we suggest that trapped cold ions in an optical
lattice can also be of help in understanding stick-slip friction, and also the
way friction changes when one of the sliders undergoes structural transitions.
For that scope, we simulated the dynamical properties of a 101-ions chain,
driven to slide back and forth by a slowly oscillating electric field in an
incommensurate periodic "corrugation" potential of increasing magnitude U0. We
found the chain sliding to switch, as U0 increases and before the Aubry
transition, from a smooth-sliding regime with low dissipation to a stick-slip
regime with high dissipation. In the stick-slip regime the onset of overall
sliding is preceded by precursor events consisting of partial slips of few ions
only, leading to partial depinning of the chain, a nutshell remnant of
precursor events at the onset of motion also observed in macroscopic sliders.
Seeking to identify the possible effects on friction of a structural
transition, we reduced the trapping potential aspect ratio until the ion chain
shape turned from linear to zigzag. Dynamic friction was found to rise at the
transition, reflecting the opening of newer dissipation channels.
