Nematic state, where a system is translationally invariant but breaks rotational symmetry, has drawn great attentions recently due to the experimental observations of such a state in both cuprates and iron-based superconductors. The origin of nematicity and its possible tie to the pairing mechanism of high-T c , however, still remains controversial. Here, we studied the electronic structure of multilayer FeSe film using angle-resolved photoemission spectroscopy (ARPES). The band reconstruction in the nematic state is clearly delineated. We found that the energy splitting between d xz and d yz bands shows non-monotonic distribution in momentum space. From the Brillouin zone center to the Brillouin zone corner, the magnitude of splitting first decreases, then increases, and finally reaches the maximum value of ∼70 meV. Moreover, besides the d xz and d yz bands, band splitting was also observed on the d xy bands with a comparable energy scale around 45 meV. Our results suggest that the electronic anisotropy in the nematic state cannot be explained by a simple on-site ferro-orbital order. Instead, strong anisotropy exists in the hopping of all d xz , d yz , and d xy orbitals, the origin of which holds the key to a microscopic understanding of the nematicity in iron-based superconductors.

An intriguing aspect of unconventional superconductivity is that it always appears in the vicinity of other competing phases, whose suppression brings the full emergence of superconductivity. In the iron pnictides, these competing phases are marked by a tetragonalto-orthorhombic structural transition and a collinear spin-density wave (SDW) transition. There has been macroscopic evidence for competition between these phases and superconductivity as the magnitude of both the orthorhombicity and magnetic moment are suppressed in the superconducting state. Here, using angle-resolved photoemission spectroscopy on detwinned underdoped Ba 1 À x K x Fe 2 As 2 , we observe a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials.

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