To investigate the superconducting order parameters in iron based superconductors by quantum impurities

One of the on-going frontier studies on iron based superconductors is about the pairing mechanism. We have conducted extensive STM/STS studies on many different iron based superconductors and try to give a general understanding on the order parameters among different systems by using quantum impurities as the detector. We found clear evidence of the in-gap quasi-particle states induced by the non-magnetic Cu impurities in Na(Fe0.97-xCo0.03Cux)As possessing both the electron and hole pockets, giving the finger prints evidence of the S+- pairing[1]. Concerning the iron chalcogenide superconductors without hole Fermi pockets, we have conducted systematic investigations. Our STS spectrum clearly indicates the presence of double superconducting gaps in (Li1-xFex)OHFeSe phase. Further analysis based on QPI allows assign the gaps to different Fermi surfaces[2]. Finally use a phase related technique we show the sign reversal of order parameters on the Fermi surfaces without hole pocket[3].
In collaboration with Peter Hirschfeld, Ilya Eremin, Igor Mazin and Dusten Atenfield

Reference
1. Huan Yang, Hai-Hu Wen et al., Nat. Commun. 4, 2947 (2013).
2. Zengyi Du, Huan Yang, Hai-Hu Wen et al., Nat. Commun. 7, 10565(2016).
3. Zengyi Du, Hai-Hu Wen et al., Nature Physics 2017. DOI: 10.1038/NPHYS4299.