Laser ARPES on Orbital Origin of Extremely Anisotropic Superconducting Gap in FeSe Superconductor*

We have carried out high resolution laser-based angle-resolved photoemission (ARPES) measurements on bulk FeSe superconductor (Tc=8~9K). We will report direct observation of highly anisotropic Fermi surface and extremely anisotropic superconducting gap in the nematic state of the FeSe superconductor. We find that the low-energy excitations of the entire hole pocket at the Brillouin zone center are dominated by the single dxz orbital. The superconducting gap exhibits an anticorrelation relation with the dxz spectral weight near the Fermi level. 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.