  Plastic deformation mediated by collective dislocation dynamics is
investigated in the two-dimensional phase-field crystal model of sheared single
crystals. We find that intermittent fluctuations in the dislocation population
number accompany bursts in the plastic strain-rate fluctuations. Dislocation
number fluctuations exhibit a power-law spectral density $1/f^2$ at high
frequencies $f$. The probability distribution of number fluctuations becomes
bimodal at low driving rates corresponding to a scenario where low density of
defects alternate at irregular times with high population of defects. We
propose a simple stochastic model of dislocation reaction kinetics that is able
to capture these statistical properties of the dislocation density fluctuations
as a function of shear rate.
