Hybrid Nodal Chain In An Orthorhombic Graphene Network

A topological nodal line (NL) semimetal is a kind of special three-dimensional semimetal state with only nodal lines around the Fermi level. It has been revealed that there are basically two types of NLs when spin-orbit coupling is not considered for electronic system: One is dubbed s-NLs, protected by a combinational symmetry S of spatial inversion (P) and time reversal (T), i.e., S = PT ; the other one is m-NLs, protected by mirror (M) or glide symmetry, constraining the NL inside certain symmetrical planes. However, a hybrid nodal chain (HNC), composed of linked s-NLs and m-NLs from the same two crossing bands, has not been well studied so far. Here, we identify by ab initio calculations a new all-sp(2) hybridized carbon allotrope as a good candidate, which has a unit cell of 24 carbon atoms with Pnna (D-2h(6)) symmetry and is termed oP-C-24 carbon. The HNC hosted by oP-C-24 can be simply described by a two-band k . p model, distinguished from the three-band model proposed for another HNC candidate in the TiRhAs family. This kind of HNC is proposed in a real material here and paves the way for further theoretical and experimental investigations.