The Blonder-Tinkham-Klapwijk ͑BTK͒ approach is extended to study coherent quantum transport in ferromagnet/superconductor/ferromagnet ͑FM/SC/SM͒ double tunnel junctions. In order to guarantee current conservation it is necessary to simultaneously consider spin-polarized electron currents along one direction and spin-polarized hole currents along the opposite direction, and to determine self-consistently the chemical potential in SC. It is found that all the reflection and transmission coefficients in BTK theory as well as conductance spectra oscillate with energy, exhibiting different behavior in the metallic and tunnel limits.

We generalized systematically the renormalized mean field theory in the case of uniform states to the unrestricted case of general inhomogeneous states with competing spin-, charge-and superconducting orders. Applying the theory to high-Tc superconductors, we discuss the issues of electronic inhomogeneity, the superfluid density, and in particular the local electron density of states. The results account for many intriguing aspects of the phenomenology.