Parity Breaking and Sublattice Dichotomy in Monolayer FeSe Superconductor

A unit cell represents the smallest repeating structure in solid-state physics and serves as the fundamental building block of a material. In iron-based superconductors, each unit cell contains two iron atoms, which form two distinct sublattices in the two-dimensional iron layers. Under normal circumstances, these sublattices are expected to have identical physical properties due to space inversion symmetry. However, we discover that this sublattice structure can introduce a novel degree of freedom for probing unconventional pairing mechanisms in iron-based superconductors. Through molecular-beam epitaxy, we have successfully grown monolayer FeSe films with atomically homogeneous (1 × 1) structures on SrTiO₃(001) substrates. In these films, we observe distinct dual tunneling spectra within pairing gap energy corresponding to the two sublattices, a phenomenon we term sublattice dichotomy . This dichotomy can be quantitatively explained by a parity-breaking superconducting state, characterized by the coexistence of conventional pairing and interband parity pairing. The interband singlet pairing arises due to the lacking of inversion symmetry, which is naturally broken from the interface coupling between FeSe and the SrTiO₃ surface.