  We study the yielding behaviour of a model glass under cyclic athermal
quastistatic deformation computationally, and show that yielding is
characterised by the discontinuous appearance of shear bands, whose width is
about ten particle diameters at their initiation, in which the strain gets
localised. Strain localisation is accompanied by a corresponding change in the
energies, and a decrease in the density in the shear band. We show that the
glass remains well annealed outside the shear band whereas the energies
correspond to the highest possible energy minima at the given density within
the shear band. Diffusive motion of particles characterising the yielded state
are confined to the shear bands, whose mean positions display movement over
repeated cycles. Outside the shear band, particle motions are sub-diffusive but
remain finite. Despite the discontinuous nature of their appearance, shear
bands are reversible in the sense that a reduction in the amplitude of cyclic
deformation to values below yielding leads to the healing and disappearance of
the shear bands.
