  Langevin simulations provide an effective way to study collective effects of
Brownian particles immersed in a two-dimensional periodic potential. In this
paper, we concentrate essentially on the behaviour of the tracer (DTr) and bulk
(DB) diffusion coefficients as function of friction. Our simulations show that
in the high friction limit, the two physical quantities DTr and DB present
qualitatively the same behaviour, for both coupled and decoupled substrate
potentials. However, for the low friction regime, and especially for the
coupled potential case, an anomalous diffusion behaviour is found. We also
found that in the case of weak dynamical coupling between the ad-particles and
the substrate, the exponents are not universal and rather depend on the
potentials. Moreover, changes in the inter-particle potentials may reverse the
behaviour to a normal one.
