High temperature superconductors and related pairing mechanism are the very basis for large scale applications of superconductors. There are two family members about the unconventional high temperature superconductors: cuprates and iron pnictides / chalcogenides. In iron based superconductors, there are multibands contributing to superconductivity, thus an S± pairing manner was proposed basing on the picture by exchanging anti-ferromagnetic spin fluctuations. However, due to its multiband effect, the Fermi surfaces in different iron based superconductors are distinct. It is quite difficult to reveal the sign reversal of the superconducting gaps by the ordinary Josephson effect. In addition, it remains to know whether there is a universal pairing mechanism in all iron based superconductors. We first show the solid evidence of S± pairing by using the non-magnetic quantum impurities [1] in FeAs-based superconductors. Then we show bosonic modes on the tunneling spectra in two types of iron based superconductors, which can also be traced back to the magnetic spin fluctuation induced pairing[2]. In some iron based superconductors with seeming the only electron pockets, we used the phase referenced quasiparticle interference technique to reveal that the repulsive interaction, namely exchanging spin fluctuations, is still the driven pairing mechanism[3,4]. We also used this technique to confirm again that the pairing gap in a typical cuprate superconductor Bi-2212 is d-wave [5], indicating a strong magnetic coupling induced pairing. In iron based superconductors, the Fermi energies are generally quite small, which strongly suggests the deviation from the weak coupling BCS theory and possible crossover to the BEC scenario. We show the consequence of the small Fermi energy in iron based superconductors[6,7]. Recently, superconductivity was discovered in thin films of the infinite-layer nickelate Nd1-xSrxNiO2 (x = 0.12-0.25) which is believed to have the similar 3d9 orbital electrons as that in cuprates. Here we report single particle tunneling measurements on the superconducting nickelate thin films. We find predominantly two types of tunneling spectra, one shows a d-wave gap function, another one exhibits a full gap. Our results indicate both similarities and distinctions between the newly found Ni-based superconductors and cuprates[8]. Finally we give a perspective about unconventional superconductivity and possible route to explore more high temperature superconductors.
References
1. Huan Yang, et al., Nature Communications 4, 2947 (2013).
2. Zhengyu Wang, et al., Nature Physics 9, 42(2013).
3. Zengyi Du, et al., Nature Communications 7, 10565(2016).
4. Zengyi Du, et al., Nature Physics 14, 134 (2018).
5. Qiangqiang Gu, et al. Nature Communications 10, 1603 (2019).
6. Mingyang Chen, et al., Nature Communications 9, 970(2018).
7. Xiaoyu Chen et al., Phys. Rev. Lett. 126, 257002(2021).
8. Qiangqiang Gu, et al. Nature Communications 11, 6027(2020).