Topological Superconductivity in Multifold Fermion Metals
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 Li2Pd3B and Li2Pt3B 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 Li2Pd3B, originating from the nontrivial band topology were found. Importantly, it has been shown experimentally that Li2Pd3B and Li2Pt3B are fully gapped and gapless superconductors, respectively. By analyzing the possible pairing symmetries, we suggest that Li2Pd3B can be a DIII class topological superconductor with Majorana surface states, even though the spin-orbit coupling in Li2Pd3B is negligible. Interestingly, Li2Pt3B, being gapless, is likely to be a nodal topological superconductor with dispersionless surface Majorana modes. We further identified that several noncentrosymmetric superconductors, such as Mo3Al2C, PdBiSe, Y2C3 and La2C3, are multifold fermion superconductors whose normal state topological properties have been ignored in previous experimental and theoretical studies.