We investigated the optical properties of single crystals of one-dimensional van der Waals crystal ${\mathrm{Ta}}_4{\mathrm{SiTe}}_4$, which exhibited high thermoelectric performance below room temperature. Optical conductivity estimated from reflectivity spectra indicates the presence of a small energy gap of 0.1–0.15 eV at the Fermi energy. At the lowest energy, optical conductivity along the ${\mathrm{Ta}}_4{\mathrm{SiTe}}_4$ chain is an order of magnitude higher than that perpendicular to this direction, reflecting anisotropic electron conduction in ${\mathrm{Ta}}_4{\mathrm{SiTe}}_4$. These results suggest that the coexistence of a small band gap and moderate anisotropy in electron conduction is a promising strategy for developing high-performance thermoelectric materials for low-temperature applications.

• Received 8 September 2023
• Revised 31 January 2024
• Accepted 19 March 2024

DOI:https://doi.org/10.1103/PhysRevB.109.L161105