  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.
