  We have performed angle-resolved photoemission spectroscopy of AlB2 which is
isostructural to high-temperature superconductor MgB2. Using soft-x-ray
photons, we accurately determined the three-dimensional bulk band structure and
found a highly anisotropic Dirac-cone band at the K point in the bulk hexagonal
Brillouin zone. This band disperses downward on approaching the H point while
keeping its degeneracy at the Dirac point, producing a characteristic Dirac
nodal line along the KH line. We also found that the band structure of AlB2 is
regarded as a heavily electron-doped version of MgB2 and is therefore well
suited for fully visualizing the predicted Dirac nodal line. The present
results suggest that (Al,Mg)B2 system is a promising platform for studying the
interplay among Dirac nodal line, carrier doping, and possible topological
superconducting properties.
