Electron Correlations and Superconductivity in Iron Pnictides and Chalcogenides

In the iron pnictides, the bad metal behaviour in the normal state suggests the importance of electron correlations, which is further underscored by the existence of a Mott insulator state in the overall phase diagram of the iron chalcogenides. This has motivated a strong-coupling approach based on a proximity to the Mott transition. In this talk, I will briefly summarize earlier theoretical studies within this approach, which led to the proposal for a quantum critical point in the isoelectronically tuned iron pnictides [1]; this has since been verified in the P-doped iron arsenides. I will in addition show how the approach provides a natural understanding of a major issue in the field, namely the superconducting $T_c$ of the iron chalcogenides is comparably high as in the iron pnictides in spite of their qualitatively distinct Fermi surfaces [2]. I will also consider the multi-orbital aspects of the electron correlations more generally, including a proposed orbital-selective Mott phase [3] in the normal state and implications for gap anisotropy and spin resonances [4] in the superconducting state. Finally, I will discuss how these results expand on the notion [5] that the iron-based superconductivity primarily originates from strong electron correlations, as well as some implications for the general phenomenon of unconventional superconductivity at the border of magnetism. \\[4pt] [1] J. Dai, Q. Si, J.-X. Zhu, \& E. Abrahams, PNAS {\bf 106}, 4118 (2009). \\[0pt] [2] R. Yu, P. Goswami, Q. Si, P. Nikolic, \& J.-X. Zhu, Nat. Commun. {\bf 4}, 2783 (2013). \\[0pt] [3] R. Yu and Q. Si, Phys. Rev. Lett. {\bf 110}, 146402 (2013); M. Yi et al., Phys. Rev. Lett. {\bf 110}, 067003 (2013). \\[0pt] [4] R. Yu, J.-X. Zhu, \& Q. Si, arxiv:1306.4184; C. Zhang et al., Phys. Rev. Lett. {\bf 111}, 207002 (2013). \\[0pt] [5] Q. Si and E. Abrahams, Phys. Rev. Lett. {\bf 101}, 076401 (2008).