  The hunt for the benchmark topological superconductor (TSc) has been an
extremely active research subject in condensed matter research, with quite a
few candidates identified or proposed. However, low transition temperatures
(Tc) and/or strong sensitivity to disorder and dopant levels in known TSc
candidates have greatly hampered progress in this field. Here, we use
Angle-resolved Photoemission Spectroscopy (ARPES) to show the presence of Dirac
Nodal Lines (DNLs) and the corresponding topological surface states (TSS's) on
the [010] faces of the Tc=39K s-wave BCS superconductor MgB2. Not only is this
nearly triple the current record of superconducting Tc among all candidate
TSc's, but the nature of these DNL states should make them highly tolerant
against disorder and inadvertent doping variations. This makes MgB2 a promising
high temperature platform for the study of topological superconductivity.
