Spin triplet superconductivity driven by finite momentum spin fluctuations

Abstract

A small number of superconductors are believed to exhibit intrinsic spin triplet pairing, and they are often discussed in terms of a simple, $^3$He-like picture where ferromagnetic spin fluctuations provide the "glue". However, in some cases in which reliable inelastic neutron scattering measurements are available, spin excitations are found to be peaked at finite momentum $\bf q$ rather than ${\bf q}=0$. Here we investigate some simple models that exhibit triplet pairing arising from antiferromagnetic spin fluctuations. We show that a strong peak at larger ${\bf q}$ in the magnetic susceptibility can drive such states and can give rise to pairing states with nodes in the $k_z$ plane even in the presence of a pure 2D Fermi surface. In these situations, dominant pair scattering processes occur between Fermi surface segments with like signs of the superconducting order parameter, yet they are consistent with an overall odd parity state. We examine the applicability of these scenarios to putative triplet superconductors UTe$_2$ by calculations based on three dimensional Fermi surfaces.