Rodriguez, Antonio and Constantino Tsallis
We study the angular diffusion in a classical ?−dimensional inertial XY model with interactions decaying with the distance between spins as ?−?, with ?⩾0. After a very short-time ballistic regime, with ?2 ?∼?2, a superdiffusive regime, for which ?2 ?∼???, with ??≃1.45 is observed, whose duration covers an initial quasistationary state and its transition to a second plateau characterized by the Boltzmann-Gibbs temperature ?BG. Long after ?BG is reached, a crossover to normal diffusion, ?2 ?∼?, is observed. We relate via the expression ??=2/(3−?), the anomalous diffusion exponent ?? with the entropic index ? characterizing the time-averaged angles and momenta probability distribution functions (pdfs), which are given by the so called ?−Gaussian distributions, ??(?)∝??(−??2), where ??(?)≡[1+(1−?)?] 11−? (?1(?)=exp(?)). For fixed size ? and large enough times, the index ?? characterizing the angles pdf approaches unity, thus indicating a final relaxation to Boltzmann-Gibbs equilibrium. For fixed time and large enough ?, the cros