Unprecedented Optical Anisotropy in Optimally Doped Iron-Based Superconductor

The divergent nematic susceptibility, obeying a simple Curie-Weiss power law over a large temperature interval, is empirically found to be a ubiquitous signature in several iron-based materials across their doping-temperature phase diagram. The composition at which the associated Weiss temperature extrapolates to zero is found to be close to optimal doping, boosting the debate to what extent nematic fluctuations contribute to the pairing-mechanism and generally affect the electronic structure of iron-based superconductors. Here, we offer a comprehensive optical investigation [1] of the optimally hole-doped Ba_0.6K_0.4Fe₂As₂ over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. We show that the stress-induced optical anisotropy in the infrared spectral range is reversible upon sweeping the applied stress and occurs only below the superconducting transition temperature. These findings demonstrate that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome.
[1] A. Pal et al., npj Quantum Materials 4, 3 (2019).