Nodal gaps from local interactions in Sr₂RuO₄

Abstract

Sr2RuO4 has been under intensive scrutiny over the past years after new NMR measurements unveiled that the superconducting state might be spin singlet. One of the best order parameter candidates in light of these new experiments is a chiral state with Eg symmetry. This order parameter, with a horizontal nodal line, has been overlooked given the strong two-dimensional character of the normal state electronic structure. Recently, a phenomenological proposal based on local interactions showed that an even-parity orbital-antisymmetric spin-triplet (OAST) chiral state can be stable in Sr2RuO4 once momentum-dependent spin-orbit coupling is properly taken into account. Here we discuss the origin of the nodes and dips in this order parameter as inherited from the normal state Hamiltonian, showing that a nodal gap can emerge out of purely local interactions and connect the presence of nodes with the superconducting fitness measure.

Multiple superconducting phases in heavy-fermion metals

“…While on the topic of the interplay between orbital and spin DOFs, as promoting pairing states with novel properties, we briefly mention that it can also be central to the important family of the ruthenates. We refer the readers to, for example, a number of ongoing theoretical studies (Puetter and Kee, 2012;Ramires, 2021;Lindquist et al, 2022).…”

Nonunitary superconductivity in complex quantum materials

“…Before starting the discussion on nonunitary superconductivity, it is useful to introduce the concept of superconducting fitness [49,50], as this is going to be a key ingredient in the analysis that follows. The superconducting fitness framework has been a useful theoretical tool allowing for the understanding of the stability and nodal structure of unexpected superconducting states [8,[51][52][53], the robustness of unconventional superconducting states in the presence of impurities [4,54], and the unusual behavior of complex superconductors under external symmetry breaking fields, such as strain [55]. This seems to be one of the most appropriate frameworks for the description of superconductivity in complex quantum materials, allowing for the understanding of apparently contradicting responses of complex superconductors.…”

Sensitivity of superconducting states to the impurity location in layered materials

“…Later, the same concept was shown to be useful in the context of electronic structure engineering for the optimization of the superconducting critical temperature [41]. More recently, this concept was proved to be related to a variety of unusual properties and responses in unconventional superconductors: the presence of odd-frequency correlations [42]; anomalous Hall effect in chiral superconductors [43]; opening of highenergy gaps in nonunitary superconductors [44]; abnormal evolution of the critical temperature under strain [45]; the presence of gap nodes in superconductors associated with local pairing interactions [46]; photon-induced supercurrents in anapole superconductors [47]; and Bogoliubov Fermi surfaces in even-parity time-reversal symmetry-breaking superconductors [48].…”