Results for Sr₂RuO₄

Abstract

In this chapter, we focus on our results for the elementary excitations and Cooper pairing in strontium ruthenate ($\mbox{Sr}_2\mbox{RuO}_4$). The novel spin-triplet superconductivity with $T_c$ = 1.5 K observed recently in layered Sr$_2$RuO$_4$ is a new example of unconventional superconductivity [1]. Clearly, it is important and of general interest to analyze in more detail the origin of the superconductivity, and to calculate the transition temperature $T_c$ and also the symmetry of the order parameter on the basis of an electronic theory. This is difficult, since there are three Ru$^{4+}$ t$_{2g}$ bands that cross the Fermi level, with approximately two--thirds filling of every band in Sr$_2$RuO$_4$. The coupling between all three bands seems to cause a single $T_c$. Furthermore, the presence of incommensurate antiferromagnetic (IAF) and ferromagnetic spin fluctuations, confirmed recently by inelastic neutron scattering [2] and the NMR $^{17}$O Knight shift [3], respectively, suggests a pairing mechanism for Cooper pairs due to spin fluctuations. Also, a non--s--wave symmetry of the order parameter has been observed. This makes the theoretical investigation of ruthenates very interesting. ...