Coherence peak in the spin susceptibility from nesting in spin-triplet superconductors; A probe for line nodes in Sr$_2$RuO$_4$

Abstract

We study the dynamical spin susceptibility $\chi ({\bf q},\omega)$ for spin-triplet superconductivity. We show that a large peak at $\omega = 2 \Delta$ appears in $\textrm{Im} \chi_{zz}({\mathbf{Q}}, \omega)$, where $z$ is the direction of the $\mathbf{d}$ vector for triplet pairing, if Fermi surface has a nested part with the nesting vector $\mathbf{Q}$ and the order parameter is $+\Delta$ and $-\Delta$ in this part of the Fermi surface. If there are line nodes in the nested part of the Fermi surface, a peak appears in either $\textrm{Im} \chi_{zz}(\mathbf{Q}, \omega)$ or $\textrm{Im} \chi_{+-}(\mathbf{Q}, \omega)$, or both, depending on the perpendicular component of the nesting vector. The comparison with inelastic neutron scattering experiments can determine the position of the line nodes in triplet superconductor Sr$_2$RuO$_4$.