Low-temperature marginal ferromagnetism explains anomalous scale-free correlations in natural flocks Le ferromagnétisme marginal à basse température explique les corrélations à longue portée dans les essaims d'oiseaux

CAVAGNA, ANDREA; GIARDINA, IRENE; DI CARLO, LUCA; CULLA, ANTONIO; GRIGERA, TOMAS S.

COMPTES RENDUS PHYSIQUE

ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER

Año: 2019 vol. 20 p. 319 - 328

1631-0705

We introduce a new ferromagnetic model capable of reproducing one of the most intriguing properties of collective behaviour in starling flocks, namely the fact that strong collective order coexists with scale-free correlations of the modulus of the microscopic degrees of freedom, that is, the birds´ speeds. The key idea of the new theory is that the single-particle potential needed to bound the modulus of the microscopic degrees of freedom around a finite value is marginal, that is, it has zero curvature. We study the model by using mean-field approximation and Monte Carlo simulations in three dimensions, complemented by finite-size scaling analysis. While at the standard critical temperature, Tc, the properties of the marginal model are exactly the same as a normal ferromagnet with continuous symmetry breaking, our results show that a novel zero-temperature critical point emerges, so that in its deeply ordered phase the marginal model develops divergent susceptibility and correlation length of the modulus of the microscopic degrees of freedom, in complete analogy with experimental data on natural flocks of starlings.