  Here we report an ultrafast optical spectroscopic study of the nodal-line
semimetal ZrSiTe. Our measurements reveal that, converse to other compounds of
the family, the sudden injection of electronic excitations results in a
strongly coherent response of an $A_{1g}$ phonon mode which dynamically
modifies the distance between Zr and Te atoms and Si layers. "Frozen phonon"
DFT calculations, in which band structures are calculated as a function of
nuclear position along the phonon mode coordinate, show that large
displacements along this mode alter the material's electronic structure
significantly, forcing bands to approach and even cross the Fermi energy. The
incoherent part of the time domain response reveals that a delayed electronic
response at low fluence discontinuously evolves into an instantaneous one for
excitation densities larger than $3.43 \times 10^{17}$ cm$^{-3}$. This sudden
change of the dissipative channels for electronic excitations is indicative of
an ultrafast Lifshitz transition which we tentatively associate to a change in
topology of the Fermi surface driven by a symmetry preserving $A_{1g}$ phonon
mode.

Materials Science