Spatiotemporal chaos and nonequilibrium transitions in a model excitable medium

We present a detailed study of the statistical steady states of a model for CO oxidation on Pt(110) proposed by Bar and co-workers. We show that the stability diagram of this model depends sensitively on the boundary conditions. We elucidate several novel properties of a state with meandering spirals (M) briefly mentioned by Bar and co-workers. (1) We show that, with periodic boundary conditions, M is the state MP, a binary mixture displaying a coexistence of quasiperiodically rotating spirals and chaotically moving pointlike defects. We show that the transition from MP to the turbulent state T1 is continuous; the transition line marks the locus where the two phases cease to be distinct. (2) With Neumann boundary conditions M is the state MN, a single quasiperiodically rotating spiral. We show that the MN-T1 transition is discontinuous or first order. We also characterize the transitions from MP and MN to the state S, which has quasiperiodically rotating spirals. We also propose qualitative mechanisms for these transitions.