Spectral signatures of a unique charge density wave in Ta2NiSe7.

Charge Density Waves (CDW) are commonly associated with the presence of near-Fermi level states which are separated from others, or "nested", by a wavector of q. Here we use Angle-Resolved Photo Emission Spectroscopy (ARPES) on the CDW material Ta₂NiSe₇ and identify a total absence of any plausible nesting of states at the primary CDW wavevector q. Nevertheless we observe spectral intensity on replicas of the hole-like valence bands, shifted by a wavevector of q, which appears with the CDW transition. In contrast, we find that there is a possible nesting at 2q, and associate the characters of these bands with the reported atomic modulations at 2q. Our comprehensive electronic structure perspective shows that the CDW-like transition of Ta₂NiSe₇ is unique, with the primary wavevector q being unrelated to any low-energy states, but suggests that the reported modulation at 2q, which would plausibly connect low-energy states, might be more important for the overall energetics of the problem. The charge density wave in Ta2NiSe7 cannot be easily explained by the Fermi surface nesting mechanism. Here, by using high-resolution ARPES, the authors reveal the absence of nesting at the primary vector q, but a backfolding at q and a possible nesting at 2q, suggesting a peculiar charge density wave state. [ABSTRACT FROM AUTHOR]