Boundary conditions for spin diffusion

We introduce a general method of deriving boundary conditions for spin-charge coupled transport in disordered systems with spin-orbit interactions and derive boundary conditions for spin diffusion in the Rashba model for various types of boundaries. Due to the surface spin precession, the boundary conditions are generally non-trivial and may contain terms, which couple different components of the spin density. We argue that boundary conditions and the corresponding electric-field-induced spin accumulation depend on the nature of the boundary and therefore the spin Hall effect in a spin-orbit coupled system can be viewed as a non-universal edge phenomenon.