Controlled impurity study and observation of a bosonic mode in iron based superconductors by STM measurements: implications for the pairing symmetry and mechanism

The pairing mechanism in the iron pnictides remains unresolved yet. The pairing model based on the magnetic origin predicts a sign reversal gap on the electron and hole Fermi pockets, leading to the S$^{\pm }$ pairing, however, a more conventional S$^{++}$ pairing gap was suggested based on the orbital fluctuation mediated pairing. Here we show the clear evidence of the in-gap quasi-particle states induced by the non- or very weak magnetic Cu impurities in Na(Fe$_{0.97-x}$Co$_{0.03}$Cu$_{x})$As by measuring the scanning tunneling spectroscopy, giving strong evidence of the S$^{\pm }$ pairing. Furthermore, we show the presence of the bosonic mode with the energy identical to that of the neutron resonance with a simple linear relation $\Omega $/k$_{B}$T$_{c\, }\approx $ 4.3 in several systems. This mode can also be explained very well as the consequence of the S$^{\pm}$ pairing. These observations strongly suggest that the antiferromagnetic spin fluctuation is the key factor for superconductivity. In collaboration with: Huan Yang, Zhenyu Wang, Delong Fang, Lei Shan, Qiangua Wang, Chenglin Zhang, and Pengcheng Dai, et al. \\[4pt] [1] Zhenyu Wang, et al., Nature Physics \textbf{9}, 42-48(2013). \\[0pt] [2] Lei Shan, et al. Phys. Rev. Lett. \textbf{108}, 227002 (2012). \\[0pt] [3] Huan Yang et al., Nature Communications \textbf{4}, 2947 (2013).