  Yielding is central to the relaxation, flow and fracture of a wide range of
soft and molecular glasses, but its microscopic origin remains unclear. Here,
we elucidate the yielding of a colloidal glass by using x-ray scattering to
monitor the structure factor during the yielding process. We apply a recently
introduced combination of small-angle x-ray scattering and rheology to the
oscillatory shear, and follow the structure factor during the increasing strain
amplitude. Surprisingly, we observe a sharp transition in the orientational
ordering of the nearest-neighbor structure upon yielding, in contrast to the
smooth variation of the viscoelastic moduli. This transition is accompanied by
a sudden change of intensity fluctuations towards Gaussian distributions. We
thus identify yielding as a new, dynamically induced transition of the glass in
response to the applied shear.
