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 kz dependence of the hole–gap anisotropy. FeSe is a high-temperature superconductor displaying a complex interplay between structure, magnetism and superconductivity; new theoretical results might now shed light on its pairing mechanism. FeSe is widely studied because its superconducting critical temperature can be tuned over a wide range by chemical intercalation or pressure. Superconductivity appears in a phase that is called nematic, which breaks rotational symmetry but preserves translational symmetry. Experimental measurements show that the superconducting gaps are anisotropic: this is probably related to the nematic transition, in which spin fluctuations seem to play a part. Laura Fanfarillo from the International School for Advanced Studies, Italy, and colleagues present a model suggesting that orbital-selective spin fluctuations provide the pairing mechanism, which is at the same time orbital selective and nematic. This explains both the anisotropy of the gaps and its connection to the nematic transition.