Spatiotemporal Memory in a Diffusion-Reaction System
  We consider a reaction-diffusion process with retardation. The particles, immersed in traps initially, remain inactive until another particle is annihilated spontaneously with a rate $\lambda$ at a certain point $\vec x$. In that case the traps within a sphere of radius $R(t)= v t^{\alpha}$ around $\vec x$ will be activated and a particle is released with a rate $\mu$. Due to the competition between both reactions the system evolves three different time regimes. While in the initial time interval the diffusive process dominates the behavior of the system, there appears a transient regime, where the system shows a driveling wave solution which tends to a non-trivial stationary solution for $v \to 0$. In that regime one observes a very slow decay of the concentration. In the final long time regime a crossover to an exponentially decaying process is observed. In case of $\lambda = \mu$ the concentration is a conserved quantity whereas for $\mu > \lambda$ the total particle number tends to zero after a finite time. The mean square displacement offers an anomalous diffusive behavior where the dynamic exponent is determined by the exponent $\alpha $. In one dimension the model can be solved exactly. In higher dimension we find approximative analytical results in very good agreement with numerical solutions. The situation could be applied for the development of a bacterial colony or a gene-pool. 
Michael Schulz, Steffen Trimper, Knud Zabrocki