Consequences of charge imbalance in superconductors within the theory of   hole superconductivity
  The theory of hole superconductivity proposes that the fundamental asymmetry between electrons and holes in solids is responsible for superconductivity. Here we point out a remarkable consequence of this theory: a tendency for negative charge to be expelled from the bulk of the superconductor towards the surface. Experimentally observable consequences of this physics are discussed. 
J.E. Hirsch