  Glasses acquire their solid-like properties by cooling from the supercooled
liquid via a continuous transition known as the glass transition. Recent
research on soft glasses indicates that besides temperature, another route to
liquify glasses is by application of stress that forces relaxation and flow.
Here we provide experimental evidence that the stress-induced onset of flow of
glasses occurs via a sharp first order-like transition. Using simultaneous
x-ray scattering during the oscillatory rheology of a colloidal glass, we
identify a sharp symmetry change from anisotropic solid to isotropic liquid
structure at the transition from the linear to the nonlinear regime.
Concomitantly, intensity fluctuations sharply acquire liquid distributions.
These observations identify the yielding of glasses to increasing stress as
sharp affine-to-nonaffine transition, providing a new conceptual paradigm of
the yielding of this technologically important class of materials, and offering
new perspectives on the glass transition.
