Renormalization group study of marginal ferromagnetism

TL;DR

This paper develops a renormalization group analysis of a novel marginal ferromagnetic model with a zero second derivative potential, explaining scale-free correlations in biological groups and confirming findings with Monte Carlo simulations.

Contribution

It introduces a new marginal ferromagnetic model with a zero second derivative potential and analyzes its critical behavior near zero temperature using renormalization group techniques.

Findings

Critical exponents in 3D are mean-field values, =1/2 and =0.

The model exhibits a zero-temperature critical point with diverging correlation length.

Monte Carlo simulations confirm the theoretical predictions.

Abstract

When studying the collective motion of biological groups a useful theoretical framework is that of ferromagnetic systems, in which the alignment interactions are a surrogate of the effective imitation among the individuals. In this context, the experimental discovery of scale-free correlations of speed fluctuations in starling flocks poses a challenge to the common statistical physics wisdom, as in the ordered phase of standard ferromagnetic models with $\mathrm{O}(n)$ symmetry, the modulus of the order parameter has finite correlation length. To make sense of this anomaly a novel ferromagnetic theory has been proposed, where the bare confining potential has zero second derivative (i.e.\ it is marginal) along the modulus of the order parameter. The marginal model exhibits a zero-temperature critical point, where the modulus correlation length diverges, hence allowing to boost both correlation and collective order by simply reducing the temperature. Here, we derive an effective field theory describing the marginal model close to the $T=0$ critical point and calculate the renormalization group equations at one loop within a momentum shell approach. We discover a non-trivial scenario, as the cubic and quartic vertices do not vanish in the infrared limit, while the coupling constants effectively regulating the exponents $\nu$ and $\eta$ have upper critical dimension $d_c=2$, so that in three dimensions the critical exponents acquire their free values, $\nu=1/2$ and $\eta=0$. This theoretical scenario is verified by a Monte Carlo study of the modulus susceptibility in three dimensions, where the standard finite-size scaling relations have to be adapted to the case of $d>d_c$. The numerical data fully confirm our theoretical results.