Nematicity and nematic fluctuations in iron-based superconductors

The spontaneous reduction of rotational symmetry in a crystalline solid driven by an electronic mechanism is referred to as electronic nematicity. This phenomenon—initially thought to be rare—has now been observed in an increasing number of strongly interacting systems. In particular, the ubiquitous presence of nematicity in a number of unconventional superconductors suggests its importance in developing a unified understanding of their intricate phase diagrams and superconducting pairing. In this regard, the iron-based superconductors present an ideal material platform to study electronic nematicity. Their nematic transition is pronounced, it can be studied with a wide range of experimental techniques, it is easily tunable, and high-quality samples are widely available. Signatures of nematic quantum criticality near optimal dopings have been reported in almost all families of iron-based superconductors. Here we highlight how the nematic phase in this class of materials can be addressed in its full complexity, encompassing momentum-, time-, energy- and material-dependences. We also discuss a number of important open questions that pertain to how nematicity affects the superconducting pairing and normal-state properties, and intriguing quantum-critical behaviour near the nematic transition.