Abstract
Statistical fluctuations are strongly dependent on the space dimension and may yield ill-defined transport coefficients in stationary out--of--equilibrium conditions. Numerical simulations reveal that the heat conductivity diverges in the thermodynamic limit as a power--law of the system size in several 1d models of coupled anharmonic oscillators and hard--sphere gases. Momentum conservation appears as a necessary ingredient for determining such anomalous behavior. Recent theoretical estimates based on the mode-coupling approach provide a possible interpretation of these results.