Laser ARPES study of optimally doped FeTe$_{0.6}$Se$_{0.4}$

We have studied the electronic structure of optimally doped FeTe$_{0.6}$Se$_{0.4}$ ($T_c$ = 14.5 K), using laser-excited angle-resolved photoemission spectroscopy (laser ARPES). We observe sharp superconducting coherence peaks in the hole band slightly shifted from the $\Gamma$ point at $T$ = 2.5 K. In contrast to earlier ARPES studies but consistent with thermodynamic results, the momentum dependence shows a $\cos(4\varphi)$ modulation of the SC-gap anisotropy. In addition, we found an electron band at the $\Gamma$ point, lying just above $E_F$. This electron band also shows a sharp superconducting coherence peak with gap formation below $T_c$. The hole and electron bands show significantly different values of superconducting gap $\Delta$ and Fermi energy $\epsilon_F$ , while the associated Bogoliubov quasiparticle dispersions get merged. The results suggest composite superconductivity in an iron-based superconductor, consisting of strong-coupling Bose-Einstein condensation (BEC) in the electron band while the hole band superconductivity lies closer to the weak-coupling Bardeen-Cooper-Schrieffer (BCS) limit.