  The iron-based superconductors are characterized by multiple-orbital physics
where all the five Fe 3$d$ 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 also asks 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 highly anisotropic Fermi surface and
extremely anisotropic superconducting gap in the nematic state of 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 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 the
Fermi surface. These observations provide new insights in understanding the
orbital origin of the extremely anisotropic superconducting gap in FeSe
superconductor and the relation between nematicity and superconductivity in the
iron-based superconductors.
