Biological & Soft Matter Seminar: Two-state models with different diffusive properties as a framework for explanation of negative diffusion and other non-conventional effects

Abstarct:

The process of diffusion in both physics and biology is often a complex phenomenon influenced by the interplay of various subsystems. In this talk we will see that an exchange between the subsystems having different diffusion properties produces non-trivial effects such as plateaus in the mean-squared displacement (MSD) or negative diffusion. As a showcase we focus on the explanation of the negative diffusion of excitons in quasi-two-dimensional semiconductors. We consider models with an annealed and a quenched disorder. Each model operates with mobile and immobile exciton states. The proposed mechanism involves initial exciton trapping, followed by the later release of these trapped excitons near their original excitation location. This delayed release contributes to photoluminescence and causes a temporary contraction of the exciton density profile, evidenced by a decrease in the MSD, which is interpreted as negative diffusion. The MSD eventually reaches a plateau, with the long-time behavior dictated by the exciton trapping properties. At short timescales, the normal diffusion prevails. A similar pattern emerges when considering the system with slow and fast excitons as subsystems. The prominence of negative diffusion and the plateau effect are enhanced by greater recombination rates and binary exciton interactions. The study provides both analytical solutions and Monte Carlo simulations, and are supported by existing experimental data.