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 $q=(0,0,\pi )$. Together with previous results from macroscopic measurements, namely heat capacity and resistivity, our findings suggest that Ni intercalation drives a structural instability at $T^{\ast }=287$ 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.