Electron Correlation and Spin Dynamics in Iron Pnictides and Chalcogenides

TL;DR

This paper reviews experimental and theoretical evidence for strong electron correlations and spin dynamics in iron pnictides and chalcogenides, emphasizing their proximity to Mott insulating phases and implications for superconductivity.

Contribution

It introduces a $w$-expansion framework that models correlation effects near the Mott transition, linking spin dynamics, electron itinerancy, and superconductivity in these materials.

Findings

Strong electron correlations are evidenced in experiments.

The $w$-expansion models reveal a quantum critical point.

Mott insulating phases are demonstrated in related compounds.

Abstract

Superconductivity in the iron pnictides and chalcogenides is closely connected to a bad-metal normal state and a nearby antiferromagnetic order. Therefore, considerable attention has been focused on the role of electron correlations and spin dynamics. In this article, we summarize some key experiments that quite directly imply strong electron correlations in these materials, and discuss aspects of the recent theoretical studies on these issues. In particular, we outline a $w$-expansion, which treats the correlation effects using the Mott transition as the reference point. For the parent systems, it gives rise to an effective J1-J2 model that is coupled to the itinerant electrons in the vicinity of the Fermi energy; this model yields an isoelectronically-tuned quantum critical point, and allows a study of the distribution of the spin spectral weight in the energy and momentum space in the paramagnetic phase. Within the same framework, we demonstrate the Mott insulating phase in the iron oxychalcogenides as well as the alkaline iron selenides; for the latter system, we also consider the role of an orbital-selective Mott phase. Finally, we discuss the singlet superconducting pairing driven by the short-range J1-J2 interactions. Our considerations highlight the iron pnictides and chalcogenides as exemplifying strongly-correlated electron systems at the boundary of electronic localization and itinerancy.