Master Defense

Mercè Roig Server

Theoretical Studies of Superconductivity in Sr₂RuO₄

Recent experimental studies have challenged the previous agreement on p-wave spin-triplet superconductivity in Sr₂RuO₄, leading to the appearance of new exotic proposals constrained by the reported measurements. In this thesis, we construct a three-orbital model adequate to describe superconductivity in Sr₂RuO₄, including the effect of spin-orbit coupling. Initially, superconductivity is assumed to originate from an on-site attraction, and we solve self-consistently for the order parameter both in momentum and real space. In the latter case, we show that the inclusion of impurities induces an effect far from the defect. A more realistic model is achieved by including the pairings from a spin-fluctuation mechanism. In this case, a thorough study in momentum space is performed, fully classifying the superconducting phases based on a point group theory analysis. We observe a coexistence of a dominant A1g with a subleading B1g irreducible representation below the critical temperature for all Hund's coupling considered, contradicting the previous proposal of an accidental degeneracy. In addition, in agreement with the experimental observations, we find that the previous state breaks time-reversal symmetry, and the dominant solution corresponds to a spin-singlet character. Furthermore, nodes have been observed at the Fermi surface, and the specific heat has been calculated, showing only one phase transition.

Zoom link: https://ucph-ku.zoom.us/j/61170982715