  For an electron, a spin-1/2 particle, the spin charge $\mathbf{s}$, a real
pseudovector with constant length, could determine the spin polarization
properties in quantum mechanics. Since spin density $\rho_{\mathbf{s}}$ could
be expressed as the product of probability density $\rho$ and the spin charge
$\mathbf{s}$, the spin continuity equation could be derived from fundamental
principles of quantum mechanics and the definition of spin current arise
naturally. Only in some specific conditions the conventional definition of spin
current coincides with ours. The equilibrium spin currents would vanish
automatically in two-dimensional electron gas with Rashba spin-orbit
interaction by using our definition of spin current.
