Hole superconductivity from kinetic energy gain

The apparently unrelated experimental observations of optical sum rule violation[1] and of tunneling asymmetry in NIS tunneling[2] find a simple explanation within the theory of hole superconductivity. In fact, both phenomena were predicted by the theory long before they were experimentally observed[3,4]. Other experimental predictions of the theory, in particular a novel feature expected in photoemission experiments, are discussed.