On the Possibility of Cooper Pairing and Superconductivity in the Surface Bands of Noble Metals

Abstract

We discuss the possibility of a superconducting transition in island films of noble metals, in particular gold, on a semiconductor substrate. As is well known, bulk superconductivity is absent in noble metals, but, in this case, superconductivity may be possible due to the surface states. Such states are well studied for the noble metal–vacuum interface and arise from the peculiarities of the spectra of such metals near the L-point of the Brillouin zone. Similar states should also occur at the metal–semiconductor interface. In the latter case, the interaction of electrons leading to Cooper pairing can be provided by bosonic surface excitations of both phonon and collective electron nature at the metal–semiconductor interface. The interaction with surface phonons is effective at low energies, while, at energies of the order of E_F (the Fermi energy of the surface band), the electron–exciton interaction becomes predominant, the optimal conditions for the latter arise at close values of the penetration depths for electrons and excitons, both in the metal and in the semiconductor. We argue that the superconducting transition in such island films should be governed by a phonon mechanism, although not similar to the Bardin–Cooper–Schrieffer (BCS) one, but that non-phonon interactions also play a significant part, providing a BCS-like contribution into the equation for the superconducting gap. Recently, there have been reports on the experimental observation of superconductivity at almost room temperature in Au–Ag nanostructures. In our opinion, the role of silver in these structures, due to the difference in electronegativity, is reduced to the role of an electron donor to the surface states associated with Au, which increases the value of E_F and leads to an increase in the BCS-like contribution of the non-phonon interactions.