Topological Superconductivity in Multifold Fermion Metals

TL;DR

This paper reveals that several noncentrosymmetric superconductors are multifold fermion metals with nontrivial topological properties, which influence their superconducting states and may host Majorana surface modes.

Contribution

It identifies new multifold fermion topological features in known superconductors and links their normal state topology to unconventional superconducting properties.

Findings

Li2Pd3B hosts double spin-1, spin-3/2, Weyl, and double Weyl points.

Li2Pd3B likely a DIII class topological superconductor with Majorana surface states.

Li2Pt3B is a nodal topological superconductor with dispersionless Majorana modes.

Abstract

Recently, multifold fermions characterized by band crossings with multifold degeneracy and Fermi surfaces with nontrivial Chern numbers have been discovered experimentally in AlPt[arXiv:1812.03310] and XSi(X=Rh,Co)[arXiv:1812.04466][arXiv:1901.03358][arXiv:1809.01312]. In this work, we largely expand the family of multifold fermion materials by pointing out that several well-studied noncentrosymmetric superconductors are indeed multifold fermion metals. Importantly, their normal state topological properties, which have been ignored in previous studies, play an important role in the superconducting properties. Taking Li$_2$Pd$_3$B and Li$_2$Pt$_3$B as examples, we found a large number of unconventional degenerate points, such as double spin-1, spin-3/2, Weyl and double Weyl topological band crossing points near the Fermi energy, which result in finite Chern numbers on Fermi surfaces. Long Fermi arc states in Li$_2$Pd$_3$B, originating from the nontrivial band topology were found. Importantly, it has been shown experimentally that Li$_2$Pd$_3$B and Li$_2$Pt$_3$B are fully gapped and gapless superconductors, respectively. By analyzing the possible pairing symmetries, we suggest that Li$_2$Pd$_3$B can be a DIII class topological superconductor with Majorana surface states, even though the spin-orbit coupling in Li$_2$Pd$_3$B is negligible. Interestingly, Li$_2$Pt$_3$B, being gapless, is likely to be a nodal topological superconductor with dispersionless surface Majorana modes. We further identified that several noncentrosymmetric superconductors, such as Mo$_3$Al$_2$C, PdBiSe, Y$_2$C$_3$ and La$_2$C$_3$, are multifold fermion superconductors whose normal state topological properties have been ignored in previous experimental and theoretical studies.