Theory of superconductivity in a three-orbital model of Sr$_2$RuO$_4$

TL;DR

This paper uses advanced computational methods to analyze the microscopic mechanisms behind the unconventional p-wave superconductivity in Sr$_2$RuO$_4$, revealing the role of spin fluctuations in gap formation.

Contribution

It provides a detailed theoretical study demonstrating how small wavevector spin fluctuations induce p-wave superconductivity in a three-orbital model of Sr$_2$RuO$_4$.

Findings

Small wavevector spin fluctuations trigger p-wave pairing.

Superconducting gaps are quasi-nodal and band-dependent.

The study clarifies the microscopic origin of superconductivity in Sr$_2$RuO$_4$.

Abstract

In conventional and high transition temperature copper oxide and iron pnictide superconductors, the Cooper pairs all have even parity. As a rare exception, Sr$_2$RuO$_4$ is the first prime candidate for topological chiral p-wave superconductivity, which has time-reversal breaking odd-parity Cooper pairs known to exist before only in the neutral superfluid $^3$He. However, there are several key unresolved issues hampering the microscopic description of the unconventional superconductivity. Spin fluctuations at both large and small wavevectors are present in experiments, but how they arise and drive superconductivity is not yet clear. Spontaneous edge current is expected but not observed conclusively. Specific experiments point to highly band- and/or momentum-dependent energy gaps for quasiparticle excitations in the superconducting state. Here, by comprehensive functional renormalization group calculations with all relevant bands, we disentangle the various competing possibilities. In particular we show the small wavevector spin fluctuations, driven by a single two-dimensional band, trigger p-wave superconductivity with quasi-nodal energy gaps.