Topological Superconductivity in an Extended s-Wave Superconductor and Its Implication to Iron-Based Superconductors

Abstract

In the presence of both space and time reversal symmetries, ans-waveA1gsuperconducting 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 groupP4/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 by2Zdegenerate Dirac cones on the (10) edge andZpairs 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 proposeds-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.

Alternate abstract:

Plain Language Summary

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. Here, we demonstrate 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. We find 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. We mathematically prove that the “sign-changeds-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.

Our 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.