Analytic estimation of Lyapunov exponent in a mean-field model undergoing a phase transition

The parametric instability contribution to the largest Lyapunov exponent (LLE) is derived for a mean-field Hamiltonian model, with attractive long-range interactions. This uses a recent Riemannian approach to describe Hamiltonian chaos with a large number N of degrees of freedom. Through microcanonical estimates of suitable geometrical observables, the mean-field behavior of the LLE is analytically computed and related to the second order phase transition undergone by the system. It predicts that chaoticity drops to zero at the critical temperature and remains vanishing above it, with the LLE scaling as N^-1/3 to the leading order in N.