  Beryllium is a simple alkali earth metal, but has been the target of
intensive studies for decades because of its unusual electron behaviors at
surfaces. Puzzling aspects include (i) severe deviations from the description
of the nearly free electron picture, (ii) anomalously large electron-phonon
coupling effect, and (iii) giant Friedal oscillations. The underlying origins
for such anomalous surface electron behaviors have been under active debate,
but with no consensus. Here, by means of first-principle calculations, we
discover that this pure metal system, surprisingly, harbors the Dirac node line
(DNL) that in turn helps to rationalize many of the existing puzzles. The DNL
is featured by a closed line consisting of linear band crossings and its
induced topological surface band agrees well with previous photoemission
spectroscopy observation on Be (0001) surface. We further reveal that each of
the elemental alakali earth metals of Mg, Ca, and Sr also harbors the DNL, and
speculate that the fascinating topological property of DNL might naturally
exist in other elemental metals as well.
