Signature of an ultrafast photoinduced Lifshitz transition in the nodal-line semimetal ZrSiTe

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 that dynamically modifies the distance between Zr and Te atoms and Si layers. ``Frozen phonon'' density functional theory 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 $3.43\ifmmode\times\else\texttimes\fi{}{10}¹⁷\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. This sudden change of the dissipative channels for electronic excitations is indicative of an ultrafast Lifshitz transition, which we tentatively associate with a change in topology of the Fermi surface driven by a symmetry-preserving ${A}_1g$ phonon mode.