Superconducting gap engineering

Master's in Nanoscience.The present MSc Thesis work is motivated by the quest to understand the changes in superconducting behaviour when a single magnetic impurity or chain of magnetic impurity atoms are placed on a superconducting surface. We are interested in observing the change of superconducting gap (Δ) on each atomic site throughout the lattice structure in the presence of magnetic impurities. In this report, we have developed and discussed the results for the one-dimensional case. The superconducting system is β - Bi2 Pd and the magnetic impurity is a Chromium atom (Cr). We use an analogy between Bogoliubov-de-Gennes (BdG) set of equations and Abrikosov-Gorkov theory in a lattice model giving rise to equations of motion for the Green’s functions. This allows us to express the effect of the magnetic impurity via a self-energy that contains scattering off the impurity via its potential part (non-magnetic) and its exchange part (magnetic). While the potential scattering does not affect the stability of the Cooper pairs at the origin of superconductivity, the exchange scattering unevenly interacts with the two different spins of the pair. This weakens the Cooper pair and it is possible to create single-particle states within the superconducting gap. This has been observed in the system subject to study in this Thesis [4]. However, both potential and exchange scattering can have a further effect on the Cooper pairs. This is because both potentials are going to affect the local electronic structure with a strong influence on the final pairing properties. This modification is a more subtle and little studied effect. To perform this study we have employed a self-consistence method. It consists in evaluating the electronic structure in the presence of the impurity and from the new electronic structure compute the superconducting gap or order parameter. The electronic structure itself depends on the order parameter. This leads to a set of self-consistent equations where order parameter and electronic structure are iteratively solved.Peer reviewe