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. No explanation for this charge asymmetry is provided by the
conventional theory of superconductivity, within which the sign of
electric charge plays no role. Instead, the sign of the charge carriers
plays a key role in the theory of hole superconductivity, according to
which 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. The theory also offers a compelling explanation of the 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 do not 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 spinborbit 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, of magnitude equal to
the critical charge current (in appropriate units). Charge expulsion
also gives rise to an interior outward-pointing electric field and to
excess negative charge near the surface. 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.
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