Observation of orbital ordering and origin of the nematic order in FeSe

In iron-based superconductors the interactions driving the nematic order that breaks the lattice fourfold rotational symmetry in the iron plane may also facilitate the Cooper pairing, but experimental determination of these interactions is challenging because the temperatures of the nematic order and the order of other electronic phases appear to match each other or to be close to each other. Here we performed field-dependent Se-77-nuclear magnetic resonance (NMR) measurements on single crystals of iron-based superconductor FeSe, with magnetic field B-0 up to 16 T. The Se-77-NMR spectra and Knight shift split when the direction of B-0 is away from the direction perpendicular to the iron planes (i.e. B-0 parallel to c) upon cooling in temperature, with a significant change in the distribution and magnitude of the internal magnetic field at the Se-77 nucleus, but these do not happen when B-0 is perpendicular to the iron planes, thus demonstrating that there is an orbital ordering. Moreover, stripe-type antiferromagnetism is absent, while giant antiferromagnetic spin fluctuations measured by theNMR spin-lattice relaxation gradually developed starting at similar to 40 K, which is far below the nematic order temperature T-nem = 89 K. These results provide direct evidence of orbital-driven nematic order in FeSe.