Surface bound states in unconventional superconductors: An unforseen consequence of earlier work by Pierre-Gilles de Gennes and Daniel Saint-James

In 1963 de Gennes and Saint-James predicted the existence of bound states in a thin slab of a normal metal backed by a superconductor, at energies smaller than its gap. When applied to superconductors in which the order parameter has d-wave symmetry, such as the High T_c cuprates, their theory together with a more detailed version by Saint-James predict zero energy surface bound states. These states have been observed by Point Contact and Tunneling Spectroscopy and their study, as well as the direct observation of electron-hole reflection, have given new insights into the electronic structure of the cuprates.