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Revealing the pairing nature by topology in iron-based superconductors

Date:28-02-2022 Print

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, despite great efforts in the past decade, no consensus has been reached on how their electrons pair up.  Dr. Shengshan Qin, Prof. Chen Fang, Prof. Fuchun Zhang and Prof. Jiangping Hu 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 in which to study the combined effects of lattice structure, electronic structure, and superconductivity. They 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 candidates 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.

Fig.1 The lattice structure of FeSe layer, the Fermi surface and the sign distribution of the superconducting order parameter in the topological s-wave state and the corner Majorana states.

Institute of Physics
HU Jiangping

Key word:
Superconductivity; topology; pairing symmetry; nonsymmorphic;

Abstract: In the presence of both space and time reversal symmetries, an s-wave A1g superconducting state is usually topologically trivial. Here, we demonstrate that an exception can take place in a type of nonsymmorphic lattice structure. We specify the demonstration in a time reversal invariant system with a centrosymmetric space group P4/nmm, the symmetry that governs iron-based superconductors, by showing the existence of a second-order topological state protected by a mirror symmetry. The topological superconductivity is featured by 2Z degenerate Dirac cones on the (10) edge and Z pairs of Majorana modes at the intersection between the (11) and (11) edges. The topological invariance and Fermi surface criterion for the topological state are provided. Moreover, we point out that the previously proposed s-wave state in iron-based superconductors, which features a sign-changed superconducting order parameter between two electron pockets, is such a topological state. Thus, these results not only open a new route to pursue topological superconductivity, but also establish a measurable quantity to settle one long-lasting debate on the pairing nature of iron-based superconductors.