Enhancement of spin accumulation in ballistic transport regime

The conventional spin-diffusion equation, based on the presence of spin-split local chemical potentials, has successfully described spin accumulation attendant to diffusive transport in spintronics. A recent experiment shows that spin accumulation far exceeds the limit set by such spin-diffusive theory when the mean free path is longer than the spin dephasing length. By introducing the momentum and spin dependent chemical potential, we develop a generalized spin transport equation that is capable of addressing spin transport in systems where ballistic processes are embedded in the overall diffusive conductor. We find that the ballistic spin injection through a barrier into a diffusive nonmagnetic layer with strong spin-orbit coupling can enhance spin accumulation by an order of magnitude when compared to the conventional theory.