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.