Electronic band structure reconstruction in Ni$_{x}$ZrTe$_{2}$

The filling of the large van der Waals gap in Transition Metal Dichalcogenides (TMDs) often leads to lattice and electronic instabilities, which prelude the onset of a rich phenomenology. Here, we investigate the electronic structure of the TMDs ZrTe$_2$ and Ni-intercalated ZrTe$_2$ (Ni$_x$ZrTe$_2$, $x\approx 0.05$) employing angle-resolved photoemission spectroscopy (ARPES). We readily identify in Ni$_x$ZrTe$_2$ two flat bands, most likely associated with localized Ni-derived 3$d$-states, at about $\approx-0.7$ eV and $\approx-1.2$ eV in binding energy. The presence of these flat bands is observed for all temperatures ($T$) in our study. More significantly, at low-$T$, we identify an electronic structure reconstruction in Ni$_x$ZrTe$_2$, which halves the electronic periodicity along the $k_{z}$ direction. This is reminiscent of a commensurate band folding with wave-vector $q=(0,0,π)$. Together with previous results from macroscopic measurements, namely heat capacity and resistivity, our findings suggest that Ni intercalation drives a structural instability at $T^{*}=287$ K, which causes the observed electronic band reconstruction. Our findings invite further investigation into the structural properties of ZrTe$_2$ and of the intercalated and defect-engineered versions of this material.