Susceptibility of Polar Flocks to Spatial Anisotropy
Résumé
We study the effect of spatial anisotropy on polar flocks by investigating active q-state clock models in 2D. Unlike the equilibrium case, we find that any amount of anisotropy is asymptotically relevant, drastically altering the phenomenology from that of the rotationally-invariant case. All of the wellknown physics of the Vicsek model, from giant density fluctuations to micro-phase separation, is replaced by that of the Active Ising model, with short-range correlations and complete phase separation. These changes appear beyond a lengthscale that diverges in the q → ∞ limit so that the Vicsek-model phenomenology is observed in finite systems for weak enough anisotropy, i.e. sufficiently high q. We provide a scaling argument which explains why anisotropy has such different effects in the passive and active cases.
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