Unconventional high-temperature superconductivity is a jewel in the crown of condensed-matter physics, but a full accounting of its underlying mechanisms remains elusive. Some clues may lie in the formation of electron pairs in iron-based superconductors. In particular, iron chalcogenides, because of their special electronic structures, have greatly challenged traditional viewpoints. However, no consensus has been reached on how their electrons pair up. 

Dr. QIN Shengshan, Prof. FANG Chen, Prof. ZHANG Fuchun and Prof. HU Jiangping from the Institute of Physics of the Chinese Academy of Sciences have recently demonstrated that the pairing nature of the iron chalcogenides can be distinguished by their topological characters, properties that are robust to perturbations and give rise to many exotic electronic behaviors.

Iron-based superconductors are perfect platforms to study the combined effects of lattice structure, electronic structure, and superconductivity. The researchers found that the special lattice structure of iron-based superconductors establishes a one-to-one correspondence between the way in which electrons pair up and the topological properties.

They mathematically proved that the "sign-changed S-wave pairing state", one of the candidate pairing states in the iron chalcogenides, is an intrinsic type of topological superconducting state.

This state hosts Majorana modes, one of the most important and long-sought features of topological superconductors, protected by the crystalline symmetries, which can be detected by scanning tunneling spectroscopy.

The study provides an idea for classifying the topological superconductors that has not been considered in previous studies and may lead to a full classification of the topological superconductors in the future.