Abstract
We have developed a new apparatus for carrying out ultra-high energy resolution laser-based angle-resolved photoemission spectroscopy (laser ARPES). From the spectra of the evaporated gold and superconducting (SC) gap of Sn (Tc=3.7K) sample, we conclude that the best energy resolution is ∼70μeV and the lowest sample temperature is ∼1.0K. We have used this laser-ARPES apparatus, to directly observe the SC gap of various iron-based superconductors. The spectra of FeTe0.6Se0.4 below Tc show a very clear temperature-dependent superconducting coherence peak, and the SC-gap nodes of KFe2As2 are found in the middle Fermi surface (FS) around the FS angle φ=±5.0°.
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