The Role of the Relaxation Rate of Nematic Fluctuations in the Tetragonal-to-Orthorhombic Transition in Fe Pnictides: a Raman Scattering Study

Raman scattering experiments were performed on LaFeAsO and Sr(Fe_0.8Co_0.2)₂As₂ crystals with well separated antiferromagnetic (T_AF) and tetragonal-orthorhombic (T_t-o) transitions (T_t-o - T_AF ~ 15-20 K). In both cases, a quasi-elastic peak (QEP) was observed in B_2g symmetry (2 Fe cell) that fades away below ~T_AF and is ascribed to nematic fluctuations. The QEP width is continuously reduced down to T_AF, indicating longer relaxation times of nematic fluctuations on cooling. The onset of the structural transition in both compounds coincides with the temperature where the QEP width reaches a threshold value of ~ 40 cm^-1. Also, the QEP widths at T >~ T_AF are significantly smaller than the reported value at comparable temperatures for BaFe₂As₂ with much smaller T_t-o - T_AF separation. Based on these results, we propose that the structural phase transition in pnictides is preempted by a slowing of the electronic nematic fluctuations down to a phonon-defined timescale where the nuclei positions can respond to such fluctuations and be readily locked into the orthorhombic phase.