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 FeAs=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-wavepairing within the sublattices. The state has a fingerprint with a real-space sign inversion between the top and bottom As=Se layers. The results suggest that iron-based super- conductors are a new quantum state of matter, and the measurement of the odd parity can help to establish high-temperature superconducting mechanisms. DOI: 10.1103/PhysRevX.3.031004 Subject Areas: Condensed Matter Physics, Superconductivity Symmetry plays the central role in the search for beauty in physics. It controls the structure of matter and allows us to simplify a complicated problem. The gauge principle is a fundamental principle in physics. Models formulated in different gauge settings are equivalent. Symmetry and gauge principles together provide the foundations of mod- ern physics that allow us to solve complicated problems. The recently discovered iron-based high-temperature superconductors (high T c )( 1-3) are layered materials with complicated electronic structures. Their complexity causes a major difficulty in understanding pairing symme- try, which arguably is the most important property and clue to determine the pairing mechanism (4,5). In a strongly correlated electron system, major physics is determined locally in real space. Important properties, such as pairing symmetry in a superconducting state, are ex- pected to be robust against small variations of Fermi surfaces in reciprocal space. Although the superconducting mechanism related to high-temperature superconductors (high Tc) has not yet been determined, the robust d-wave pairing symmetry in cuprates (6) can be understood under this principle. Does this principle still hold for iron-based supercon- ductors? Namely, do all iron-based superconductors pos- sess one universal pairing state? Unlike cuprates, the answer to this question is highly controversial because different theoretical approaches have provided different answers and no universal state has been identified (5). Nevertheless, as local electronic structures in all families of iron-based superconductors are almost identical and phase diagrams are smooth against doping (4,5), it is hard to argue that the materials can approach many differ- ent superconducting ground states.