Topological superconducting phase in high-T_(c) superconductor MgB_(2) with Dirac-nodal-line fermions

Topological superconductors are an intriguing and elusive quantum phase,characterized by topologically protected gapless surface/edge states residing in a bulk superconducting gap,which hosts Majorana fermions.Unfortunately,all currently known topological superconductors have a very low transition temperature,limiting experimental measurements of Majorana fermions.Here we discover the existence of a topological Dirac–nodal-line state in a well-known conventional high-temperature superconductor,MgB_(2).First-principles calculations show that the Dirac–nodal-line structure exhibits a unique one-dimensional dispersive Dirac–nodal line,protected by both spatial-inversion and time-reversal symmetry,which connects the electron and hole Dirac states.Most importantly,we show that the topological superconducting phase can be realized with a conventional s-wave superconducting gap,evidenced by the topological edge mode of the MgB_(2) thin films showing chiral edge states.Our discovery may enable the experimental measurement of Majorana fermions at high temperature.