On surface states at superconductor oxide interfaces

Localized electron states in oxides on metal surfaces hybridize with conduction electrons leaking into the oxide and form so a metallic layer and a new type of surface state. For a superconductor such surface states weaken the superconductivity. Because these surface states extend into the oxide, they cause an enhanced tunnel conductivity, which is finite below the gap voltage due to resonance tunneling via the surface states below the gap.