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The iron-based superconductors are characterized by multiple-orbital physicswhere all the five Fe 3$d$ orbitals get involved. The multiple-orbital naturegives rise to various novel phenomena like orbital-selective Mott transition,nematicity and orbital fluctuation that provide a new route for realizingsuperconductivity. The complexity of multiple-orbital also asks to disentanglethe relationship between orbital, spin and nematicity, and to identify dominantorbital ingredients that dictate superconductivity. The bulk FeSesuperconductor provides an ideal platform to address these issues because ofits simple crystal structure and unique coexistence of superconductivity andnematicity. However, the orbital nature of the low energy electronicexcitations and its relation to the superconducting gap remain controversial.Here we report direct observation of highly anisotropic Fermi surface andextremely anisotropic superconducting gap in the nematic state of FeSesuperconductor by high resolution laser-based angle-resolved photoemissionmeasurements. We find that the low energy excitations of the entire hole pocketat the Brillouin zone center are dominated by the single $d_{xz}$ orbital. Thesuperconducting gap exhibits an anti-correlation relation with the $d_{xz}$spectral weight near the Fermi level, i.e., the gap size minimum (maximum)corresponds to the maximum (minimum) of the $d_{xz}$ spectral weight along theFermi surface. These observations provide new insights in understanding theorbital origin of the extremely anisotropic superconducting gap in FeSesuperconductor and the relation between nematicity and superconductivity in theiron-based superconductors.

Orbital Origin of Extremely Anisotropic Superconducting Gap in Nematic Phase of FeSe Superconductor