Critical spin fluctuations and the origin of nematic order in Ba(Fe1−xCox)2As2

Nematic fluctuations and order play a prominent role in material classes such as the cuprates1, some ruthenates2 or the iron-based compounds3, 4, 5, 6 and may be interrelated with superconductivity7, 8, 9, 10, 11. In iron-based compounds12 signatures of nematicity have been observed in a variety of experiments. However, the fundamental question as to the relevance of the related spin13, charge9, 14 or orbital8, 15, 16 fluctuations remains open. Here, we use inelastic light (Raman) scattering and study Ba(Fe1−xCox)2As2 (0 ≤ x ≤ 0.085) for getting direct access to nematicity and the underlying critical fluctuations with finite characteristic wavelengths17, 18, 19, 20, 21. We show that the response from fluctuations appears only in B1g (x2 − y2) symmetry (1 Fe unit cell). The scattering amplitude increases towards the structural transition at Ts but vanishes only below the magnetic ordering transition at TSDW < Ts, suggesting a magnetic origin of the fluctuations. The theoretical analysis explains the selection rules and the temperature dependence of the fluctuation response. These results make magnetism the favourite candidate for driving the series of transitions.