Nematic Crossover in BaFe2As2 under Uniaxial Stress

The nature of the nematic order in iron-based superconductors has invoked intense research interest. A substantial portion of experimental attempts on resolving this issue required the use of single-domain samples produced under external stress. Here we use Raman scattering, a technique that can detect spontaneous point-group symmetry breaking without resorting to single-domain samples, to study BaFe2As2, the parent compound of the “122” Fe-based superconductors. We show that an applied compression along the Fe-Fe direction, which is commonly used to produce untwinned orthorhombic samples, changes the structural phase transition at temperature Ts into a crossover that spans a considerable temperature range above Ts. Even in crystals that are not subject to any applied force, a distribution of substantial residual stress remains, which may explain phenomena that are seemingly indicative of symmetry breaking above Ts. Our results are consistent with an onset of spontaneous nematicity only below Ts.