  The mechanical, structural and functional properties of crystals are
determined by their defects and the distribution of stresses surrounding these
defects has broad implications for the understanding of transport phenomena.
When the defect density rises to levels routinely found in real-world
materials, transport is governed by local stresses that are predominantly
nonlinear. Such stress fields however, cannot be measured using conventional
bulk and local measurement techniques. Here, we report direct and spatially
resolved experimental measurements of the nonlinear stresses surrounding
colloidal crystalline defect cores, and show that the stresses at vacancy cores
generate attractive interactions between them. We also directly visualize the
softening of crystalline regions surrounding dislocation cores, and find that
stress fluctuations in quiescent polycrystals are uniformly distributed rather
than localized at grain boundaries, as is the case in strained atomic
polycrystals. Nonlinear stress measurements have important implications for
strain hardening, yield, and fatigue.
