Ultrafast Critical Nematic Fluctuations and Giant Magnetoelastic Coupling in Iron Pnictide

A ubiquitous anisotropy in the normal state properties of many of the iron pnictides presents a crosscutting challenge important for understanding quantum magnetism and high-temperature superconductivity. Although an electronically-driven nematicity has been invoked, distinguishing this from spin and structural orders is challenging because they all couple together to break the same tetragonal symmetry. Here we use femtosecond-resolved polarimetry to reveal critical fluctuations of nematic correlation in unstrained Ba(Fe1-xCox)2As2. The ultrafast anisotropic response, which arises from the two-fold in-plane anisotropy of the refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase, with increasing relaxation time, indicative of critical nematic fluctuations approaching the structural transition temperature. The dynamics reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.