  The mechanical yield of dense granular materials is a fascinating rheological
phenomenon, beyond which stress no longer increases with strain at a
sufficiently large deformation. Understanding the behavior of mechanical
responses associated with yielding is a fundamental goal in granular physics,
and other related fields including glassy physics, material sciences,
geophysics, and active matter biophysics. However, despite nearly half a
century of theoretical efforts, the nature of yielding in amorphous solids
remains largely elusive compared to its crystalline counterpart. Here, we
experimentally investigate the mechanical responses of two-dimensional
bidisperse jammed disks subjected to volume-invariant pure shear, focusing on
the behavior of yielding. We show that the microscopic mechanical and
geometrical features of configurations under shear can be characterized by two
critical exponents of weak-force and small-gap distributions originally
proposed for the isotropic jamming transition. We find that the yielding
transition satisfies the condition of marginal mechanical stability through a
scaling relationship between the two exponents, and after yielding global
instability emerges. The criticality of yielding is revealed by a significant
peak of susceptibility that quantifies the fluctuation of a glass overlap order
parameter. Moreover, we find a distinct transition before yielding, which is
associated with the onset of structural anisotropy.
