PhD Defense: Mercè Roig I Server

Unconventional Superconductivity and Altermagnetism

Superconductivity and magnetism are fascinating fields in condensed matter physics giving rise to exotic and puzzling phases of matter. In this thesis we explore three different topics: in the first part of the thesis we investigate unconventional superconductivity in a one-band Hubbard model within the spin-fluctuation approach. We construct phase diagrams examining the role of on-site and extended Coulomb interactions on the preferred superconducting state. In addition, we study the transitions between different symmetries and find that spin-singlet orders generate a coexistence region breaking time-reversal symmetry. Then, we focus on multiorbital systems, which offer a new route to generate superconductivity based on a direct attraction due to a large Hund’s exchange. We compare this mechanism to the spin-fluctuation mediated pairing by examining two distinct multiorbital models. We find that, when the bands exhibit significant nesting, the spin-fluctuation mechanism dominates, which is relevant for systems like Sr2RuO4.

The second part of the thesis explores the superconducting diode effect, which exists in systems where both inversion and time-reversal symmetries are broken. In usual diode setups, time-reversal symmetry is broken by applying an in-plane magnetic field. However, we show that out-of-plane magnetization gradients also induce the diode effect, generating comparable efficiencies. Moreover, we also propose alternative device designs based on out-of-plane magnetization gradients, emphasizing the importance of an optimized gradient profile, which may significantly enhance the diode efficiency.

In the last part of the thesis, we focus on altermagnetism, a new class of magnetic order distinct from conventional ferromagnetism and antiferromagnetism. We construct minimal models for altermagnetism based on symmetry arguments, and reveal the mechanisms stabilizing this phase by examining the analytic expressions for the susceptibility. Additionally, we apply the model to relevant altermagnetic material candidates, including RuO2, MnF2, FeSb2, κ-Cl, CrSb and MnTe, and find that it gives rise to a large Berry curvature linear in the spin-orbit coupling. Finally, we derive the Landau free energy expansion and the analytic expressions for the coefficients from the minimal model, investigating the interplay between the magnetization and the altermagnetic order parameter at domain walls and in the presence of spin-orbit coupling.