  Recent studies on different topological materials in condensed matter physics
have provided the evidence of topological nature for bosonic particle like
phonons by performing various theoretical calculations and experimental
observations. Here, the topological behaviours of phonons of SnS, SnSe and SnTe
materials in rock-salt structure are investigated using $ab$-$initio$
methodology. For all these materials, the tilted linear band touching is
observed along with the presence of band inversion in direction X-W. The
topological point in phonon dispersion curve along X-W direction is found to be
$\sim$2.83, $\sim$2.46 and $\sim$2.51 THz for SnS, SnSe and SnTe, respectively.
The calculated numbers of Weyl points (WPs) is estimated to be 56, 24 and 54
for SnS, SnSe and SnTe, respectively. These WPs have shown conserved Chiral
charges for all corresponding materials. The surface local density of states is
computed for these compounds, where the surface arc is clearly seen. In case of
SnS, the surface arc is found at 2.0 - 2.2 THz energy region. Also, the
isofrequency surface states are investigated to observe the presence of Fermi
arc for SnX (X = S, Se, Te). The present first-principles calculation predicts
that SnS, SnSe and SnTe show type-II Weyl phononic behaviour along with
evidence of topological phononic surface states.
