Nematicity in Iron-Based Superconductors

Electronic nematicity is a state of matter that spontaneously breaks rotational symmetry but preserves translational symmetry. In many solid-state systems including copper-based and iron-based superconductors, unexpected electronic anisotropies identify the nematic state. A decade of investigation of the nematic phase of iron-based superconductors has revealed a wealth of new insights. The nematic phase is signalled by a particularly pronounced tetragonal-to-orthorhombic structural transition. It is accompanied by complex orbital ordering and often narrowly precedes, or even coincides with an antiferromagnetic order. As a generic normal state property, nematic order and its fluctuations are arguably linked to the emerging superconductivity. For example, nematic fluctuations near a quantum critical point have been proposed to contribute to pairing interactions. Moreover, the interrelation between the lattice, spin and orbital degrees of freedom across the nematic phase may also be relevant for the occurrence of superconductivity. Nematicity is therefore an essential ingredient for the global phase diagram of iron-based materials.

Identifying the driving interaction has been a central challenge for understanding nematicity in iron-based superconductors. This problem is complicated by the fact that spin, orbital and lattice degrees of freedom are intimately coupled. The entanglement of multiple degrees of freedom in the nematic phase has stimulated diverse experiments that are sensitive to different composite phases and excitations. After a decade of research, it is now a good time to collect and compare these results and develop a comprehensive picture. Beyond the long-standing debate on spin and orbital-driven nematic scenarios, important aspects of the nematic phase remain poorly understood, including its relationship with superconductivity. In addition, the report of nematicity with different symmetry classifications awaits further investigations. Recent significant advances in strain application techniques permit more accurate characterization of the nematic phase and hence could bring new insights into its intrinsic properties. In this Research Topic, we would like to call for contributions that address the new developments and insights in the study of nematicity of iron-based materials.

In order to achieve a comprehensive and insightful picture of nematicity in the iron-based superconductors, this Research Topic will combine Original Research articles and (Mini-)Reviews on various current issues. We welcome, for example, contributions discussing the manifestation of nematicity in different experimental probes and compounds, contributions concerning the driving interaction of the nematic phase, or contributions addressing the interplay between nematicity and superconductivity. Theoretical interpretations that could reconcile different experimental observations will be of great interest. Finally, studies on nematic compounds beyond the iron-based materials, which contribute general considerations, are also within the scope of this Research Topic.