The square-net materials ${\mathrm{GdSb}}_x{\mathrm{Te}}_{2-x-\delta }$ provide a platform for exploring the effects of charge density wave (CDW) distortion and band filling on the topological electronic band structure. In this work we use infrared spectroscopy to probe the temperature-dependent electronic excitations in ${\mathrm{GdSb}}_{0.56}{\mathrm{Te}}_{1.35}$ and ${\mathrm{GdSb}}_{0.45}{\mathrm{Te}}_{1.50}$ single crystals for different polarization directions parallel to the square-net layers. First-principles calculations were also performed to study the electronic band structure and interband optical conductivity of stoichiometric GdSbTe. For ${\mathrm{GdSb}}_{0.56}{\mathrm{Te}}_{1.35}$ the in-plane optical conductivity is independent of the polarization direction and contains a small Drude term, indicating a weak metallic character, and a pronounced near-infrared absorption band, which may be due to excitations across the CDW gap. Upon cooling, the optical response shows no significant temperature dependence within the experimental uncertainty, and no signatures associated with magnetic ordering are detected. In comparison, for ${\mathrm{GdSb}}_{0.45}{\mathrm{Te}}_{1.50}$ we observe an in-plane anisotropy and strongly reduced low-frequency optical conductivity, with small temperature dependence. Possible signatures of Dirac fermion excitations in the optical conductivity spectra are also analyzed and discussed. Our results demonstrate that small differences in the stoichiometry have a significant impact on the optical response of ${\mathrm{GdSb}}_x{\mathrm{Te}}_{2-x-\delta }$ materials.

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• Charge density waves
• Electronic structure
• Optical conductivity
• Topological phases of matter
• Nodal-line semimetals
• Density functional theory
• Infrared spectroscopy
• Susceptibility measurements
• Wannier function methods