Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behaviour these materials exhibit. Yet, the origin of the observed anisotropy is unclear. 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. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe 1-x Co x) 2 As 2. 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. 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 dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.
Bibliographical noteFunding Information:
This work was supported in part by the Ames Laboratory’s LDRD program (sample characterization and computational studies), by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division (materials synthesis), by the National Science Foundation under award #DMR-1055352 (laser spectroscopy). Ames Laboratory is operated for the US DOE by Iowa State University under contract #DE-AC02-07CH11358. I.E.P was supported by the European Union’s Seventh Framework Programme (FP7-REGPOT-2012-2013-1) under grant agreement No. 316165 and by the EU Social Fund and National resources through the THALES program NANOPHOS. J.S. is supported by the Deutsche Forschungsgemeinschaft through DFG-SPP 1458 \Hochtemperatursupraleitung in Eisenpniktiden.