  We study nanofriction in an ion Coulomb crystal under the presence of
topological defects. We observe signatures of the pinning to sliding
transition, namely the transverse vibrational soft mode signaling the
transition and the symmetry breaking of the crystal configuration. We present
details on experimental measurements and numerical simulations of this system,
including the gaps in the hull function and the spectroscopy of the localized
defect mode. We also investigate how external forces, such as those due to
anharmonic potentials or differential light pressure, break the intrinsic
crystal symmetry, thereby reducing mode softening near the sliding transition.
We find that the local structure and position of the topological defect is
essential for the presence of the soft mode and illustrate how the defect
changes its properties, when it moves through the crystal.
