  We show that nonlinear continuum elasticity can be effective in modeling
plastic flows in crystals if it is viewed as Landau theory with an infinite
number of equivalent energy wells whose configuration is dictated by the
symmetry group GL(3,Z). Quasi-static loading can be then handled by athermal
dynamics, while lattice based discretization can play the role of
regularization. As a proof of principle, we study in this Letter dislocation
nucleation in a homogeneously sheared 2D crystal and show that the global
tensorial invariance of the elastic energy foments the development of
complexity in the configuration of collectively nucleating defects. A crucial
role in this process is played by the unstable higher symmetry crystallographic
phases, traditionally thought to be unrelated to plastic flow in lower symmetry
lattices.
