  Phonons are an ideal platform for realizing stable spinless two-dimensional
(2D) Dirac points because they have a bosonic nature and hard-to-break
time-reversal symmetry. It should be noted that the twofold degenerate nodal
points in the phonon dispersions of almost all reported 2D materials are
misclassified as 'Dirac points' owing to a historical issue. The correct name
for these twofold degenerate nodal points should be 'Weyl' because 2D phononic
systems are essentially spinless and because each twofold degenerate point is
described by a Weyl model in two dimensions. To date, there have been no
reports of fourfold degenerate Dirac point phonons in 2D materials. In this
study, we searched through the entire 80 layer groups (LGs) and discovered that
Dirac phonons can be realized in 7 of the 80 LGs. Moreover, the Dirac points in
the phonon dispersions of 2D materials can be divided into essential and
accidental degenerate points, which appear at high-symmetry points and on
high-symmetry lines, respectively. Guided by symmetry analysis, we identified
the presence of Dirac phonons in several 2D material candidates with six LGs.
This letter offers a method for identifying Dirac phonons in 2D and proposes 2D
material candidates for realizing Dirac phonons.
