Spectral signatures of a unique charge density wave in Ta$_2$NiSe$_7$

TL;DR

This study uses ARPES to investigate the charge density wave in Ta$_2$NiSe$_7$, revealing an unconventional transition without typical nesting at the primary wavevector, but potential significance of a secondary wavevector $f{2q}$.

Contribution

It provides the first detailed electronic structure analysis of Ta$_2$NiSe$_7$, showing the CDW transition is not driven by Fermi surface nesting at the primary wavevector.

Findings

No nesting at the primary CDW wavevector $f{q}$.

Spectral replicas appear at $f{q}$ after the transition.

Possible nesting at $f{2q}$ related to atomic modulations.

Abstract

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 $\mathbf{q}$. Here we use Angle-Resolved Photo Emission Spectroscopy (ARPES) on the CDW material Ta$_2$NiSe$_7$ and identify a total absence of any plausible nesting of states at the primary CDW wavevector $\mathbf{q}$. Nevertheless we observe spectral intensity on replicas of the hole-like valence bands, shifted by a wavevector of $\mathbf{q}$, which appears with the CDW transition. In contrast, we find that there is a possible nesting at $\mathbf{2q}$, and associate the characters of these bands with the reported atomic modulations at $\mathbf{2q}$. Our comprehensive electronic structure perspective shows that the CDW-like transition of Ta$_2$NiSe$_7$ is unique, with the primary wavevector $\mathbf{q}$ being unrelated to any low-energy states, but suggests that the reported modulation at $\mathbf{2q}$, which would plausibly connect low-energy states, might be more important for the overall energetics of the problem.