A local structure perspective on iron pnictides and chalcogenides: Insights gained from pair distribution function analysis*

The field of iron-based superconductivity, now well into its second decade of existence, continues to challenge and stimulate condensed matter physicists. One perennial issue of debate is the origin and implications of the electronic nematic phase. This state is characterized by a spontaneous breaking of four-fold rotational symmetry, which manifests as unequal occupations of the d_xz and d_yz orbitals, a tetragonal-to-orthorhombic structural phase transition, and (in many cases) stripe-type magnetic order. In addition to the statically ordered nematic phase observed widely in various families of iron-based superconductors, strong nematic fluctuations have also been found in broad regions of parameter space outside the ordered nematic state. Pair distribution function (PDF) analysis of x-ray and neutron total scattering data is a proven method of studying local, short-range structural correlations that deviate from the average structure, such as the orthorhombic distortions associated with nematic fluctuations in iron-based superconductors. Here, I will present recent PDF investigations of several representative families of iron-based superconductors. The results reveal the presence of local orthorhombic distortions across large regions of temperature-composition space and provide a detailed look at the temperature dependence and characteristic length scale of these distortions. The deeper understanding of the local structure of iron-based superconductors enabled by these PDF experiments will help clarify some of the outstanding questions relating to these fascinating materials.