Multigap superconductivity and barrier-driven resonances in superconducting nanofilms with an inner potential barrier

We study the crossover in a zero-temperature superconducting nanofilm from a single to a double superconducting slab induced by a barrier in the middle. We use the Bogoliubov–de Gennes (BdG) equations in the Anderson approximation to show that the single-phase superconducting ground state of this heterostructure is intrinsically multigapped and has a new type of resonance caused by the strength of the barrier, thus distinct from the Thompson–Blatt shape resonance which is caused by tuning the thickness of the film. The simplest theoretical framework able to describe a finite height and very thin tunable insulating potential barrier in the middle is provided by a δ-function potential. In this framework, the even single-particle states are affected by the insulating barrier, whereas the odd ones are not. The new type of resonance, hereafter called barrier-driven resonance, is caused by the crossing of the even single-particle states through the Fermi surface. The lift of the even-odd degeneracy at the barrier reconfigures the pairing interaction and leads to a multigapped superconducting state with barrier-driven resonances.