  Theory of spin-charge conversion effects in spintronics are presented in
terms of correlation functions of physical observables, spin and electric
current. Direct and inverse spin Hall effects and spin pumping effect are
studied considering metallic systems with random spin-orbit interaction and
spatially nonuniform Rashba interaction. The theory is free from ambiguity
associated with spin current, and provides clear physical picture of the
spin-charge conversion effects. In the present approach, so-called the spin
current transmission efficiency is essentially the nonuniform component of
magnetic susceptibility.
