Observation of Topological Surface State in High Temperature Superconductor MgB2*

Most topological superconductors known-to-date suffer from low transition temperatures (Tc) and/or high fragility to disorder and dopant levels, which greatly hamper the progress in this promising field. In this work, we suggest BCS superconductor MgB2 as a superior candidate: not only is its superconducting Tc=39K nearly triple the current record of all candidate topological superconductors, but it also has topological Dirac nodal line states which are highly tolerant against disorder and inadvertent doping variations. Our density functional theory (DFT) calculation and Angle-resolved Photoemission Spectroscopy (ARPES) measurements locate the topological Dirac Nodal Lines and their corresponding topological surface states on the [010] faces of MgB2, which could host topological superconductivity via the proximity effect to the bulk.