Nematic superconductivity from selective orbital pairing in iron pnictide single crystals

The discovery of iron-based superconductors proves that high-temperature superconductivity is not limited to cuprates. Here, we use transport measurements to determine the in-plane anisotropy of the upper critical field (Hc2) in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence of the Hc2 anisotropy for fields along inter-planar and in-plane directions was found to increase in the over-doped regime. For underdoped samples with a spin-nematic phase below the tetragonal-to-orthorhombic structural transition temperature, we find that Hc2 along the a axis is considerably lower than that along the b axis. This Hc2 anisotropy disappears in the over-doped regime when the system becomes tetragonal. By combining these results with previous works, we conclude that superconductivity in underdoped iron pnictides is orbital selective—with a dominant contribution from electrons with the dyz orbital character and from being intimately associated with spin excitations.