For the first time, the directionally dependent electrical resistivity in iron-selenium, an iron-based superconductor, could be extrapolated to the zero-stress limit, and studied over a broad range of temperatures without interference from long-range magnetic order. In contrast to other iron-based superconductors, iron-selenium does not develop long-range magnetic order below the structural (nematic) transition at Ts ≈90 K. This allows for the disentanglement of the contributions to the directionally dependent resistivity due to magnetic and nematic order. The results suggest that the inelastic scattering of electrons by directionally dependent spin fluctuations is the main mechanism of the long range structural alignment response in this iron-based superconductor. The zero-stress limit was accessed by measuring electrical resistivity directly in response to small changes in strain induced by stretching, and by elastoresistivity—the response to infinitesimal deformations. The results provide important clues for modeling the pairing mechanisms in iron-based superconductors.
Polarized-light image of an FeSe single crystal at 7 K reveals orthorhombic domains oriented along the tetragonal  direction (parallel to the sample sides). For detwinning, the sample is cut along the  tetragonal direction. Lower frames show the selected region at different temperatures across the nematic/structural transition at 90 K.
Origin of the Resistivity Anisotropy in the Nematic Phase of FeSe