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

Nematic order has being found to widely exist in the cuprates and in the Fe-based superconductors in most recent years. It is directly linked to the superconductivity because nematic instability is a characteristic feature of the normal state upon which at lower temperatures the superconductivity emerges. FeSe has been regarded as a model system for the study of the Fe-based superconductors. But it is highly controversial as to whether the nematic order is driven by spin order, AFM spin fluctuations, and/or orbital order. Here we report the experimental result of field-dependent ⁷⁷Se-NMR. Our results shows for the first time that the difference of the static internal field in the ab-plane in the orthorhombic phase at the Se-nucleus is predominantly from the Fe-ion 3d electron orbitals (essentially all orbital), not from the electron spins, and gaint AFM spin fluctuations are gradually developed below T ~ 40 K in the orthorhombic phase, which is far below the nematic order/structure phase transition temperature T_nem = T_s = 89 K, while AFM spin order is absent at all temperatures and at all fields. Therefore, for the first time we provide direct evidence that orbital ordering is unequivocally the origin of the nematic order in FeSe.