Electronic nematicity in iron-based superconductors

Nematicity – the breaking of rotational symmetry via an electronic mechanism – has been a central theme in the study of iron-based superconductors. This symmetry breaking manifests as a tetragonal-to-orthorhombic structural transition, which is observed in many iron-based compounds. Nematicity is clearly coupled to the stripe-type antiferromagnetism common to most of these systems. Nevertheless, the nature of this relationship has been a long-standing debate. In prototypical materials like BaFe₂As₂, the structural/nematic transition occurs very close to the magnetic one. A scenario in which nematicity is a “precurser phase” of the stripe-type magnetism is therefore plausible. A prominent exception, which puts this scenario into question, is FeSe. FeSe is well-known for its orthorhombic phase in the absence of magnetic order at ambient pressure, and may be considered an extreme case of nematicity. The application of hydrostatic pressure induces magnetic order in FeSe. In the low-pressure range, the structural/nematic transition temperature decreases whereas the magnetic transition temperature increases with increasing pressure. This opposing behavior raises the question of the origin of nematic order in FeSe. First, I will discuss how nematicity in BaFe₂As₂-based compounds is related to magnetism and magnetic fluctuations. Secondly, I will discuss the case of FeSe, in particular the relation between nematicity and magnetism under physical and chemical pressure.
References: K. Kothapalli, A. E. Böhmer, et al., Nature Communications 7, 12728 (2016). A. E. Böhmer et al., Phys. Rev. Lett. 114, 027001 (2015). Anna Böhmer and Christoph Meingast, Comptes Rendus Physique 17, 90, (2016).