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

We study the dynamical spin susceptibility $χ({\bf q},ω)$ for spin-triplet superconductivity. We show that a large peak at $ω= 2 Δ$ appears in $\textrm{Im} χ_{zz}({\mathbf{Q}}, ω)$, 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 $+Δ$ and $-Δ$ 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 $\textrm{Im} χ_{zz}(\mathbf{Q}, ω)$ or $\textrm{Im} χ_{+-}(\mathbf{Q}, ω)$, 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$_4$.

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