  The discovery of a topological nodal-line (TNL) semimetal phase in ZrSiS has
invigorated the study of other members of this family. Here, we present a
comparative electronic structure study of ZrSiX (where X = S, Se, Te) using
angle-resolved photoemission spectroscopy (ARPES) and first-principles
calculations. Our ARPES studies show that the overall electronic structure of
ZrSiX materials comprises of the diamond-shaped Fermi pocket, the nearly
elliptical-shaped Fermi pocket, and a small electron pocket encircling the zone
center ($\Gamma$) point, the M point, and the X point of the Brillouin zone,
respectively. We also observe a small Fermi surface pocket along the
M-$\Gamma$-M direction in ZrSiTe, which is absent in both ZrSiS and ZrSiSe.
Furthermore, our theoretical studies show a transition from nodal-line to
nodeless gapped phase by tuning the chalcogenide from S to Te in these material
systems. Our findings provide direct evidence for the tunability of the TNL
phase in ZrSiX material systems by adjusting the spin-orbit coupling (SOC)
strength via the X anion.
