An amplitude equation for the conserved-Hopf bifurcation -- derivation, analysis and assessment

We employ weakly nonlinear theory to derive an amplitude equation for the conserved-Hopf instability, i.e., a generic large-scale oscillatory instability for systems with two conservation laws. The resulting equation represents the equivalent in the conserved case of the complex Ginzburg-Landau equation obtained in the nonconserved case as amplitude equation for the standard Hopf bifurcation. Considering first the case of a relatively simple symmetric Cahn-Hilliard model with purely nonreciprocal coupling, we derive the nonlinear nonlocal amplitude equation and show that its bifurcation diagram and time evolution well agree with results for the full model. The solutions of the amplitude equation and their stability are obtained analytically thereby showing that in oscillatory phase separation the suppression of coarsening is universal. Second, we lift the restrictions and obtain the amplitude equation in a more generic case, that also shows very good agreement with the full model as exemplified for some transient dynamics that converges to traveling wave states.