  Quenched disorder affects significantly the behavior of phase transitions.
The Imry-Ma-Aizenman-Wehr-Berker argument prohibits first-order or
discontinuous transitions and their concomitant phase coexistence in
low-dimensional equilibrium systems in the presence of random fields. Instead,
discontinuous transitions become rounded or even continuous once disorder is
introduced. Here we show that phase coexistence and first-order phase
transitions are also precluded in nonequilibrium low-dimensional systems with
quenched disorder: discontinuous transitions in two-dimensional systems with
absorbing states become continuous in the presence of quenched disorder. We
also study the universal features of this disorder-induced criticality and find
them to be compatible with the universality class of the directed percolation
with quenched disorder. Thus, we conclude that first-order transitions do not
exist in low-dimensional disordered systems, not even in genuinely
nonequilibrium systems with absorbing states.
