Hybrid nodal-chain semimetal with emergent flat band in MgCaN2

Abstract

The distinct over-tiltings of band crossings in topological semimetals generate the type-I and type-II classification of Dirac/Weyl and nodal-line fermions, accompanied by exotic electronic and magnetic transport properties. In this work, we propose a concept of hybrid nodal-chain semimetal (NCSM), which is identified by linked type-I and type-II nodal rings (NRs) and hosts inevitable flat band. Using first-principles calculations and structure search technique, a new ternary nitride MgCaN2 is proposed as the first candidate to realize a novel 3D hybrid NCSM state. Remarkably, flat band is emergent along specific direction as a characteristic signature of such a hybrid nodal-chain, thus offering a platform to explore the interplay between topological states and flat bands. By analyzing the mirror Z2 topological invariant and developing a low-energy effective model, we unveil the physical origin of the hybrid nodal-chain structure with multiple-mirror protected mechanism. Moreover, when the linked NRs are projected onto the (010) and (001) surfaces, considerable drumhead-like topological surface states can be illustrated with unique connection patterns. These results expand our understanding of NCSMs from significant band tilting effects and provide a new candidate on realizing hybrid nodal-chain fermion for further theoretical and experimental investigations.