Fronts with a Growth Cutoff but Speed Higher than $v^*$

Abstract

Fronts, propagating into an unstable state $ϕ=0$, whose asymptotic speed $v_{\text{as}}$ is equal to the linear spreading speed $v^*$ of infinitesimal perturbations about that state (so-called pulled fronts) are very sensitive to changes in the growth rate $f(ϕ)$ for $ϕ\ll 1$. It was recently found that with a small cutoff, $f(ϕ)=0$ for $ϕ< ε$, $v_{\text{as}}$ converges to $v^*$ very slowly from below, as $\ln^{-2} ε$. Here we show that with such a cutoff {\em and} a small enhancement of the growth rate for small $ϕ$ behind it, one can have $v_{\text{as}} > v^*$, {\em even} in the limit $ε\to 0$. The effect is confirmed in a stochastic lattice model simulation where the growth rules for a few particles per site are accordingly modified.