  We study the mechanical response of a dislocation-free 2D crystal under
homogenous shear using a new mesoscopic approach to crystal plasticity, a
Landau-type theory, accounting for the global invariance of the energy in the
space of strain tensors while operating with an infinite number of equivalent
energy wells. The advantage of this approach is that it eliminates
arbitrariness in dealing with topological transitions involved, for instance,
in nucleation and annihilation of dislocations. We use discontinuous yielding
of pristine micro-crystals as a benchmark problem for the new theory and show
that the nature of the catastrophic instability, which in this setting
inevitably follows the standard affine response, depends not only on lattice
symmetry but also on the orientation of the crystal in the loading device. The
ensuing dislocation avalanche involves cooperative dislocation nucleation,
resulting in the formation of complex microstructures controlled by a
nontrivial self-induced coupling between different plastic mechanisms.
