Disorder and diffuse scattering in single-chirality (TaSe$_4$)$_2$I crystals

TL;DR

This study investigates the structural and chiral properties of (TaSe$_4$)$_2$I crystals, revealing uniform handedness, iodine deficiency, and temperature-dependent diffuse scattering features that influence its electronic behavior.

Contribution

It provides the first systematic analysis of chiral domain size, iodine deficiency, and diffuse scattering in (TaSe$_4$)$_2$I, linking structural features to electronic properties.

Findings

Crystals have uniform handedness confirmed by multiple methods.

Diffuse scattering features decrease as charge density wave develops.

Iodine deficiency and transverse displacements influence the material's semiconducting behavior.

Abstract

The quasi-one-dimensional chiral compound (TaSe$_4$)$_2$I has been extensively studied as a prime example of a topological Weyl semimetal. Upon crossing its phase transition temperature $T_\textrm{CDW}$ $\approx$ 263 K, (TaSe$_4$)$_2$I exhibits incommensurate charge density wave (CDW) modulations described by the well-defined propagation vector $\sim$(0.05, 0.05, 0.11), oblique to the TaSe$_4$ chains. Although optical and transport properties greatly depend on chirality, there is no systematic report about chiral domain size for (TaSe$_4$)$_2$I. In this study, our single-crystal scattering refinements reveal a bulk iodine deficiency, and Flack parameter measurements on multiple crystals demonstrate that separate (TaSe$_4$)$_2$I crystals have uniform handedness, supported by direct imaging and helicity dependent THz emission spectroscopy. Our single-crystal X-ray scattering and calculated diffraction patterns identify multiple diffuse features and create a real-space picture of the temperature-dependent (TaSe$_4$)$_2$I crystal structure. The short-range diffuse features are present at room temperature and decrease in intensity as the CDW modulation develops. These transverse displacements, along with electron pinning from the iodine deficiency, help explain why (TaSe$_4$)$_2$I behaves as an electronic semiconductor at temperatures above and below $T_\textrm{CDW}$, despite a metallic band structure calculated from density functional theory of the ideal structure.