Scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) have been investigated on single crystal samples of KFe2As2. A van Hove singularity (vHs) has been directly observed just a few meV below the Fermi level E_F of superconducting KFe2As2, which locates in the middle of the principle axes of the first Brillouin zone. The majority of the density-of-states at E_F, mainly contributed by the proximity effect of the saddle point to E_F, is non-gapped in the superconducting state. Our observation of nodal behavior of the momentum area close to the vHs points, while providing consistent explanations to many exotic behaviours previously observed in this material, suggests Cooper pairing induced by a strong coupling mechanism.
@article{arxiv.1412.0945,
title = {Observation of a van Hove singularity and implication for strong coupling induced Cooper pairing in KFe2As2},
author = {Delong Fang and Xun Shi and Zengyi Du and Pierre Richard and Huan Yang and X. X. Wu and Peng Zhang and Tian Qian and Xiaxin Ding and Zhenyu Wang and T. K. Kim and M. Hoesch and Aifeng Wang and Xianhui Chen and Jiangping Hu and Hong Ding and Hai-Hu Wen},
journal= {arXiv preprint arXiv:1412.0945},
year = {2015}
}