Group-theoretical classification of superconducting states of strontium ruthenate

Abstract

The possible superconducting states of strontium ruthenate (Sr$_2$RuO$_4$) are organized into irreducible representations of the point group $D_{4h}$, with a special emphasis on nodes occurring within the superconducting gap. Our analysis covers the cases with and without spin-orbit coupling and takes into account the possibility of inter-orbital pairing within a three-band, tight-binding description of Sr$_2$RuO$_4$. No dynamical treatment if performed: we are confining ourselves to a group-theoretical analysis. The case of uniaxial deformations, under which the point group symmetry is reduced to $D_{2h}$, is also covered. It turns out that nodal lines, in particular equatorial nodal lines, occur in most representations. We also highlight some results specific to multiorbital superconductivity. Among other things, we find that odd inter-orbital pairing allows to combine singlet and triplet superconductivity whithin the same irreducible representation, that pure inter-orbital superconductivity leads to nodal surfaces and that the notion of nodes imposed by symmetry is not clearly defined.