Nonlinear and time-resolved optical study of the 112-type iron-based superconductor parentCa1−xLaxFeAs2across its structural phase transition

The authors measure the nonlinear optical response of Ca${}_1-x$La${}_x$FeAs${}₂$ to investigate the detailed structural symmetries of the recently discovered 112-type family of iron-based superconductors. They find a strong and anisotropic optical second-harmonic response, identifying ${C}₂$ and ${C}₁$ as the high- and low-temperature crystallographic point groups, respectively. This makes the 112-type materials the first known high-temperature superconductors to break structural inversion symmetry, allowing for the possible mixing of singlet and triplet Cooper pairs in the superconducting state. In addition, the intrinsically low crystallographic symmetry of this family of materials can potentially stabilize large single domains of the electronic nematic state ubiquitous in the iron-based superconductors without application of strain.