  Stress relaxation in crystalline solids is mediated by the formation and
diffusion of defects. While it is well established how externally-generated
stresses relax, through the proliferation and motion of dislocations in the
lattice, it remains relatively unknown how crystals cope with internal
stresses. We investigate, both experimentally and in simulations, how highly
localized stresses relax in two-dimensional soft colloidal crystals. When a
single particle is actively excited, by means of optical tweezing, a rich
variety of highly collective stress relaxation mechanisms results. These
manifest in the form of open strings of cooperatively moving particles through
the motion of dissociated vacancy-interstitial pairs, and closed loops of
mobile particles, which either result from cooperative rotations in transiently
generated circular grain boundaries or through the closure of an open string by
annihilation of a vacancy-interstitial pair. Surprisingly, we find that the
same collective events occur in crystals which are excited by thermal
fluctuations alone; a large thermal agitation inside the crystal lattice can
trigger the irreversible displacements of hundreds of particles. Our results
illustrate how local stresses can induce large scale cooperative dynamics in
two-dimensional soft colloidal crystals and shed new light on the stabilisation
mechanisms in ultrasoft crystals.
