Charge expulsion, Spin Meissner effect, and charge inhomogeneity in   superconductors
  Superconductivity occurs in systems that have a lot of negative charge: the highly negatively charged $(CuO2)^{--}$ planes in the cuprates, negatively charged $(FeAs)^-$ planes in the iron arsenides, and negatively charged $B^-$ planes in magnesium diboride. And, in the nearly filled (with negative electrons) bands of almost all superconductors, as evidenced by their positive Hall coefficient in the normal state. According to the theory of hole superconductivity, metals become superconducting because they are driven to expel negative charge (electrons) from their interior. This is why NIS tunneling spectra are asymmetric, with larger current for negatively biased samples. It is also why there is a Meissner effect: as electrons are expelled towards the surface in the presence of a magnetic field, the Lorentz force imparts them with azimuthal velocity, thus generating the surface Meissner current that screens the interior magnetic field. In type II superconductors, the Lorentz force acting on expelled electrons that don't reach the surface gives rise to the azimuthal velocity of the vortex currents. In the absence of applied magnetic field, expelled electrons still acquire azimuthal velocity, due to the spin-orbit interaction, in opposite direction for spin-up and spin-down electrons: the "Spin Meissner effect". This results in a macroscopic spin current flowing near the surface of superconductors in the absence of applied fields. In strongly type II superconductors this physics should give rise to charge inhomogeneity and spin currents throughout the interior of the superconductor, to large sensitivity to (non-magnetic) disorder and to a strong tendency to phase separation. 
J. E. Hirsch