Nematic superconductivity from selective orbital pairing in iron pnictide single crystals

The discovery of iron -based superconductors proves that hightemperature 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 dyzorbital character and from being intimately associated with spin excitations.