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 four-fold 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 77Se-nuclear magnetic resonance (NMR) measurements on single crystals of iron-based superconductor FeSe, with magnetic field B0 up to 16 T. The 77Se-NMR spectra and Knight shift split when the direction of B0 is away from the direction perpendicular to the iron planes (i.e. B0 ∥ c) upon cooling in temperature, with a significant change in the distribution and magnitude of the internal magnetic field at the 77Se nucleus, but these do not happen when B0 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 the NMR spin-lattice relaxation gradually developed starting at ∼40 K, which is far below the nematic order temperature Tnem = 89 K. These results provide direct evidence of orbital-driven nematic order in FeSe.