Observation-Time-Induced Crossover from Fluctuating Diffusivity

Abstract: A dynamical transition -- seen as a sudden increase in the mean-squared displacement at a characteristic temperature that depends on the observation time -- is widely reported in neutron-scattering experiments and molecular dynamics simulations of hydrated proteins. However, its physical origin remains elusive. We show that fluctuating diffusivity in a Langevin framework leads to an observation-time-induced crossover, where the effective diffusion coefficient exhibits a temperature-dependent transition whose crossover point shifts with observation time. Analytical and numerical analyses reveal the mechanism of this crossover and delineate the conditions under which it emerges. Our findings provide a unified nonequilibrium interpretation for observation-time-induced crossover, and suggest that the protein dynamical transition can be viewed as an instance of this general crossover mechanism.