Ingredients for the electronic nematic phase in FeSe revealed by its anisotropic optical response
The origin of the anisotropy in physical quantities related to a symmetry-broken (nematic) electronic state is still very much debated in high-temperature superconductors. FeSe at ambient pressure undergoes a structural, tetragonal-to-orthorhombic phase transition at Ts≃90 K without any magnetic ordering on further cooling, which leads to an ideal electronic nematicity. Our unprecedented optical results provide evidence that the low-energy excitation spectrum in the nematic phase is shaped by an important interplay of the anisotropic Drude weight and scattering rate. In the zero-frequency limit though, the temperature dependence of the anisotropic scattering rate plays the dominant role and, combined with the nematic order parameter as evinced from the high energy optical response, accounts for the anisotropic dc resistivity. This favors the scattering by anisotropic spin fluctuations as the prominent candidate in governing the properties of the nematic phase.