  Crystalline materials deform in an intermittent way via dislocation-slip
avalanches. Below a critical stress, the dislocations are jammed within their
glide plane due to long-range elastic interactions and the material exhibits
plastic response, while above this critical stress the dislocations are mobile
(the unjammed phase) and the material fails. We use dislocation dynamics and
scaling arguments in two dimensions to show that the critical stress grows with
the square root of the dislocation density. Consequently, dislocations jam at
any density, in contrast to granular materials, which only jam below a critical
density.
