The fundamental role of charge asymmetry in superconductivity
  Neither BCS theory nor London theory contain any charge asymmetry. However it is an experimental fact that a rotating superconductor always exhibits a magnetic field parallel, never antiparallel, to its angular velocity. This and several other experimental observations point to a special role of charge asymmetry in superconductivity, which is the foundation of the theory of hole superconductivity. The theory describes heavy dressed {\it positive} hole carriers in the normal state that undress by pairing and become light undressed {\it negative} electron carriers in the superconducting state. Superconductivity is driven by kinetic energy lowering rather than by electron-phonon coupling as in BCS. In quantum mechanics, kinetic energy lowering is associated with $expansion$ of the electronic wave function, and hence we predict: (1) Superconductors expel $negative$ charge from their interior which consequently becomes $positively$ charged; (2) Macroscopic electrostatic fields exist in the interior of superconductors always, and in certain cases also outside near the surface; (3) Macroscopic spin currents exist in the superconducting state; (4) Superconductors are 'rigid' with respect to their response to applied longitudinal electric fields. These predictions apply to all superconductors and are testable but are as yet untested. The theory predicts highest $T_c$'s for materials for which normal state transport occurs through $(positive)$ holes in $negatively$ charged anions. 
J.E. Hirsch