Diffusion crossover from/to $q$-statistics to/from Boltzmann-Gibbs statistics in the classical inertial $\alpha$-XY ferromagnet

Abstract

We study the angular diffusion in a classical $d-$dimensional inertial XY model with interactions decaying with the distance between spins as $r^{-\alpha}$, wiht $\alpha\geqslant 0$. After a very short-time ballistic regime, with $\sigma_\theta^2\sim t^2$, a super-diffusive regime, for which $\sigma_\theta^2\sim t^{\alpha_D}$, with $\alpha_D \simeq 1\text{.}45$ is observed, whose duration covers an initial quasistationary state and its transition to a second plateau characterized by the Boltzmann-Gibbs temperature $T_\text{BG}$. Long after $T_\text{BG}$ is reached, a crossover to normal diffusion, $\sigma_\theta^2\sim t$, is observed. We relate, for the first time, via the expression $\alpha_D = 2/(3 - q)$, the anomalous diffusion exponent $\alpha_D$ with the entropic index $q$ characterizing the time-averaged angles and momenta probability distribution functions (pdfs), which are given by the so called $q-$Gaussian distributions, $f_q(x)\propto e_q(-\beta x^2)$, where $e_q (u) \equiv [1 + (1 - q)u]^{\frac{1}{1 - q}}$ ($e_1(u) = \exp(u)$). For fixed size $N$ and large enough times, the index $q_\theta$ characterizing the angles pdf approaches unity, thus indicating a final relaxation to Boltzmann-Gibbs equilibrium. For fixed time and large enough $N$, the crossover occurs in the opposite sense.