Nematic Pairing from Orbital Selective Spin Fluctuations in FeSe

FeSe is an intriguing iron-based superconductor. It presents an unusual nematic state without magnetism and can be tuned to increase the critical superconducting temperature. Recently it has been observed a noteworthy anisotropy of the superconducting gaps. Its explanation is intimately related to the understanding of the nematic transition itself. Here we show that the spin-nematic scenario driven by orbital-selective spin-fluctuations provides a simple scheme to understand both phenomena. The pairing mediated by anisotropic spin modes is not only orbital selective but also nematic, leading to stronger pair scattering across the hole and $X$ electron pocket. The delicate balance between orbital ordering and nematic pairing points also to a marked $k_z$ dependence of the hole-gap anisotropy.