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Its coupling to other degrees of freedom, such as magnetic fluctuations, lattice orthorhombicity and orbital order 4, thus gives rise to anisotropies in several quantities-electrical and thermal transport, orbital occupation, optical conductivity and magnetic susceptibility 5, 6, 7, 8, 9, 10, 11, 12, 13. The associated nematic-order parameter φ has therefore Ising ( Z 2) character (two discrete values) 1, 2, 3. In the electronic nematic state of the iron pnictides, the high-temperature tetragonal symmetry ( C 4) of the system is lowered to an orthorhombic one ( C 2) at the temperature T S through the divergence of nematic fluctuations ( Fig. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron–spin and electron–phonon couplings, opposite to conventional magnetic metals. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe 1− xCo x) 2As 2.
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Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Yet, the origin of the observed anisotropy is unclear. Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behaviour these materials exhibit.