Today I read a paper titled “Leading birds by their beaks: the response of flocks to external perturbations”
The abstract is:
We study the asymptotic response of polar ordered active fluids (“flocks”) to small external aligning fields $h$.
The longitudinal susceptibility $\chi_{_\parallel}$ diverges, in the thermodynamic limit, like $h^{-\nu}$ as $h \rightarrow 0$.
In finite systems of linear size $L$, $\chi_{_\parallel}$ saturates to a value $\sim L^\gamma$.
The universal exponents $\nu$ and $\gamma$ depend only on the spatial dimensionality $d$, and are related to the dynamical exponent $z$ and the “roughness exponent” $\alpha$ characterizing the unperturbed flock dynamics.
Using a well supported conjecture for the values of these two exponents, we obtain $\nu = 2/3$, $\gamma = 4/5$ in $d = 2$ and $\nu = 1/4$, $\gamma = 2/5$ in $d = 3$.
These values are confirmed by our simulations.