Reaction-diffusion systems which include processes of the form A + A → A orA + A → ∅ are characterized by the appearance of 'imaginary' multiplicative noise terms in an effective Langevin-type description. However, if 'real' as well as 'imaginary' noise is present, then competition between the two could potentially lead to novel behaviour. We thus investigate the asymptotic properties of the following two 'mixed noise' reaction-diffusion systems. The first is a combination of the annihilation and scattering processes 2A → ∅, 2A → 2B, 2B → 2A, and 2B → ∅. We demonstrate (to all orders in perturbation theory) that this system belongs to the same universality class as the single species annihilation reaction 2A → ∅. Our second system consists of competing annihilation and fission processes, 2A → ∅ and 2A → (n + 2)A, a model which exhibits a transition between active and absorbing phases. However, this transition and the active phase are not accessible to perturbative methods, as the field theory describing these reactions is shown to be non-renormalizable. This corresponds to the fact that there is no stationary state in the active phase, where the particle density diverges at finite times. We discuss the implications of our analysis for a recent study of another active/absorbing transition in a system with multiplicative noise. § Present address:

No abstract

No abstract

Spin-wave resonance has been observed at 9 GHz in thin films of Ni-Co alloys containing between 0 and 40% CO at temperatures between 4.2 K and room temperature.The dispersion relationship, critical angle, linewidth and annealing results suggest that the surface spins are pinned by a surface layer about l00A thick. The exchange stiffness constant D is found to have a value about a quarter of that obtained by other methods, namely approximately 100 meV A2.

Nonlinear interaction of laser emission with a chemically reactive medium, in which a branched chain reaction may readily occur, has been analyzed aiming at essential intensification the contrast of photochemically induced phase gratings. For the model system CS2/O2, the degenerated four-wave mixing is theoretically studied under the assumption of CS2 photodissociation followed by subsequent secondary reactions induced by reactive photodissociation products. For varied CS2 concentrations and incident flux intensities, the dynamics of nonlinear reflection has been analyzed and same laser applications of such systems are suggested.