| Summary: | The iron-based superconductors are characterized by multiple-orbital physics where all the five Fe 3d orbitals get involved. The multiple-orbital nature gives rise to various novel phenomena like orbital-selective Mott transition, nematicity, and orbital fluctuation that provide a new route for realizing superconductivity. The complexity of multiple-orbital physics also requires us to disentangle the relationship between orbital, spin, and nematicity, and to identify dominant orbital ingredients that dictate superconductivity. The bulk FeSe superconductor provides an ideal platform to address these issues because of its simple crystal structure and unique coexistence of superconductivity and nematicity. However, the orbital nature of the low-energy electronic excitations and its relation to the superconducting gap remain controversial. Here, we report direct observation of the highly anisotropic Fermi surface and extremely anisotropic superconducting gap in the nematic state of the FeSe superconductor by high-resolution laser-based angle-resolved photoemission measurements. We find that the low-energy excitations of the entire hole pocket at the Brillouin zone center are dominated by the single d_{xz} orbital. The superconducting gap exhibits an anticorrelation 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 the Fermi surface. These observations provide new insights in understanding the orbital origin of the extremely anisotropic superconducting gap in the FeSe superconductor and the relation between nematicity and superconductivity in the iron-based superconductors.
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