Evidence for a cos(4\varphi) Modulation of the Superconducting Energy   Gap of Optimally Doped FeTe_{0.6}Se_{0.4} Single Crystals Using Laser   Angle-Resolved Photoemission Spectroscopy
  We study the superconducting(SC)-gap anisotropy of the \Gamma-centered hole Fermi surface in optimally doped FeTe_{0.6}Se_{0.4} (T_c = 14.5 K), using laser-excited angle-resolved photoemission spectroscopy (ARPES). We observe sharp superconducting coherence peaks 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. The observed SC-gap anisotropy strongly indicates that the pairing interaction is not a conventional phonon-mediated isotropic one. Instead, the results suggest the importance of second-nearest-neighbor electronic interactions between the iron sites in the framework of s_\pm-wave superconductivity. 
K. Okazaki, Y. Ito, Y. Ota, Y. Kotani, T. Shimojima, T. Kiss, S.   Watanabe, C.-T. Chen, S. Niitaka, T. Hanaguri, H. Takagi,...
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el