Iron-based superconductors: tales from the nuclei

TL;DR

This review discusses how nuclear magnetic resonance techniques have advanced understanding of the complex electronic phases, disorder effects, and inhomogeneities in iron-based high-temperature superconductors.

Contribution

It provides a comprehensive overview of NMR and NQR contributions to understanding the electronic phase diagram and microscopic properties of Fe-based superconductors.

Findings

NMR reveals coexistence of antiferromagnetic and superconducting states.

NMR detects electronic nematic fluctuations and flux line dynamics.

Evidence of inhomogeneous charge distribution and orbitally-selective behavior.

Abstract

High-temperature superconductivity in Fe-based pnictides and chalcogenides has been one of the most significant recent discoveries in condensed matter physics and has attracted remarkable attention in the last decade. These materials are characterized by a complex fermiology and, as a result, feature a wide range of electronic properties as a function of different tuning parameters such as chemical doping, temperature and pressure. Along the path towards the comprehension of the physical mechanisms underlying this rich phenomenology, NMR (nuclear magnetic resonance) and NQR (nuclear quadrupole resonance) have played a role of capital importance that we review in this work. In particular, we address how NMR has contributed to the current understanding of the main regions of the electronic phase diagram of Fe-based pnictides, that is, the -- sometimes coexisting -- antiferromagnetic spin-density wave and superconducting states. We evidence the unique capability of NMR as local-probe technique of investigating the effect of quenched disorder and chemical impurities. Then, we review the NMR signatures of low-frequency fluctuations associated with the development of electronic nematicity as well as with the motion of superconducting flux lines. Finally, we discuss recent contributions of NMR and NQR which evidence an intrinsically inhomogeneous electronic charge distribution as well as an orbitally-selective behaviour.