  Pezzini et al. reported an unconventional mass enhancement in topological
nodal line semimetal ZrSiS (Nat. Phys. 14, 178 (2018), whose origin remains
puzzling. In this material, strong short-range interactions might induce
excitonic particle-hole pairs. Here we study the renormalization of fermion
velocities and find that the mass enhancement in ZrSiS can be well understood
if we suppose that ZrSiS is close to the quantum critical point between
semimetal and excitonic insulator. Near this quantum critical point, the
fermion velocities are considerably reduced by excitonic quantum fluctuation,
leading to fermion mass enhancement. The quasiparticle residue is suppressed as
the energy decreases but is finite at zero energy. This indicates that ZrSiS is
a strongly correlated Fermi liquid, and explains why the mass enhancement is
weaker than non-Fermi liquids. Our results suggest that ZrSiS is a rare example
of 3D topological semimetal exhibiting unusual quantum criticality.
