Negative diffusivity of excitons in electron-hole plasmas.

We develop a minimal hydrodynamic framework to describe exciton transport in the presence of an electron-hole plasma in two-dimensional semiconductors. Treating excitons, electrons, and holes as coupled fluids, we show that exciton diffusion is strongly renormalized by momentum exchange with the plasma. In the collisional regime, mutual diffusion leads to a nontrivial redistribution of transport coefficients but preserves the positivity of the exciton diffusivity. In contrast, when plasma inertia and collective charge oscillations are accounted for, the exciton diffusive mode hybridizes with acoustic plasma modes, giving rise to a dynamical instability manifested as an effective negative diffusion coefficient. We demonstrate that this instability originates from the non-equilibrium coupling between slow excitons and fast plasma degrees of freedom, rather than from nonlinear diffusion or thermodynamic effects. Our results provide a unified physical mechanism for negative exciton diffusivity reported in recent experiments and establish collective plasma dynamics as a key control parameter of exciton transport in two-dimensional materials.