Crossover from Fragile to Strong Glassy Behaviour in Kinetically Constrained Systems

We show the existence of fragile-to-strong transitions in kinetically constrained systems by studying the equilibrium and out-of-equilibrium dynamics of a generic constrained Ising spin chain which interpolates between the symmetric and fully asymmetric cases. We find that for large but finite asymmetry the model displays a crossover from fragile to strong glassy behaviour at finite temperature, which is controlled by the asymmetry parameter. The relaxation in the fragile region presents stretched exponential behaviour, with a temperature dependent stretching exponent which is predicted. Our results are confirmed by numerical simulations.