Iron-Based Superconductors as Odd-Parity Superconductors

Abstract

Parity is a fundamental quantum number used to classify a state of matter. Materials rarely possess ground states with odd parity. We show that the superconducting state in iron-based superconductors is classified as an odd-parity $s$-wave spin-singlet pairing state in a single trilayer $\mathrm{FeAs}/\mathrm{Se}$, the building block of the materials. In a low-energy effective model constructed on the Fe square bipartite lattice, the superconducting order parameter in this state is a combination of an $s$-wave normal pairing between two sublattices and an $s$-wave $\eta$ pairing within the sublattices. The state has a fingerprint with a real-space sign inversion between the top and bottom $\mathrm{As}/\mathrm{Se}$ layers. The results suggest that iron-based superconductors are a new quantum state of matter, and the measurement of the odd parity can help to establish high-temperature superconducting mechanisms.