Features of Superconducting Gaps Revealed by STM/STS in Iron Based Superconductors With and Without Hole Pockets

The pairing mechanism and gap structure in iron based superconductors (IBS) remains unresolved. We have conducted extensive STM/STS study on the Na(Fe$_{\mathrm{1-x}}$T$_{\mathrm{x}})$As (T$=$Co, Cu, Mn)[1], Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$[2], KFe$_{\mathrm{2}}$As$_{\mathrm{2}}$[3], and Li$_{\mathrm{1-x}}$Fe$_{\mathrm{x}}$OHFeSe[4] single crystals. We found the clear evidence of the in-gap quasi-particle states induced by the non-magnetic Cu impurities in Na(Fe$_{\mathrm{0.97-}}_{x}$Co$_{\mathrm{0.03}}$Cu$_{x})$As, giving strong evidence of the S$^{\mathrm{\pm }}$ pairing. Furthermore, we show the presence of the bosonic mode with the energy identical to that of the neutron resonance and a simple linear relation $\Omega $/k$_{\mathrm{B}}$T$_{\mathrm{c\thinspace }}\approx $ 4.3, being explained a consequence of the S\textpm pairing. The STS spectrum in Li$_{\mathrm{1-}}_{x}$Fe$_{x}$OHFeSe clearly indicates the presence of double superconducting gaps with $\Delta_{\mathrm{1}} \quad \approx $ 14.3 meV and $\Delta_{\mathrm{2}} \quad \approx $ 8.6 meV. Further analysis based on QPI allows us to assign the larger (smaller) gap to the outer (inner) hybridized electron pockets[4]. The huge value 2$\Delta _{\mathrm{1}}$/$k_{\mathrm{B}}T_{\mathrm{c\thinspace }}=$ 8.7 discovered here undoubtedly proves the strong coupling mechanism. [1] H. Yang et al., Nature Communications \textbf{4}, 2947 (2013). [2] Z. Y. Wang, et al., Nature Physics \textbf{9}, 42(2013). [3] D. L. Fang, X. Shi et al., arXiv: Condmat.1412.0945. Phys. Rev. B 2015. [4] Z. Y. Du et al., arXiv: Condmat. 1506.04645.