Electron Correlations and Iron-based Superconductivity*

The past decade has witnessed enormous efforts in uncovering the microscopic physics of the iron-based superconductors. From the very beginning, the importance of the bad-metal characteristics and the associated electron correlations has been emphasized [1]. This talk will survey the recent progresses on this broad subject [2]. I will summarize the frustrated magnetism, quantum criticality [3], a wide variety of electronic orders which typically are nematic [3,4], and the orbital-selective Mott physics [2]. These normal-state features point to short-range magnetic correlations as driving the multi-orbital superconductivity. I will discuss the notion of orbital-selective pairing [5], which has been supported by experiments including the recent STM results; and, more generally, how the multi-orbital nature allows for unusual forms of superconducting pairing such as the “s-tau3” pairing state [6], which exhibits properties that seem to be mutually incompatible but have actually been observed. I will close by touching upon some outstanding questions, prospects for further progresses, as well as the implications these studies have for the overall field of unconventional superconductivity.

[1] Q. Si and E. Abrahams, PRL 101, 076401 (2008).
[2] Q. Si, R. Yu and E. Abrahams, Nature Rev. Mater. 1, 16017 (2016).
[3] J. Dai, Q. Si, J. X. Zhu, and E. Abrahams, PNAS 106, 4118, (2009).
[4] H.-H. Lai, W.-J. Hu, R. Yu, and Q. Si, PRL 118, 176401 (2017);
R. Yu and Q. Si, PRL 115, 116401 (2015);
R. Yu, M. Yi, B. A. Frandsen, R. J. Birgeneau, and Q. Si, arXiv:1706.07087.
[5] R. Yu, J.-X. Zhu and Q. Si, PRB 89, 024509 (2014).
[6] E. M. Nica, R. Yu and Q. Si, Npj Quantum Materials 2, 24 (2017).