The complex optical conductivity of the half-Heusler compound GdPtBi is measured in a frequency range from 20 to $22000{\mathrm{cm}}^{-1}$ (2.5 meV–2.73 eV) at temperatures down to 10 K in zero magnetic field. We find the real part of the conductivity, ${\sigma }_1(\omega )$, to be almost perfectly linear in frequency over a broad range from 50 to $800{\mathrm{cm}}^{-1}$ ($\sim 6–100\mathrm{meV}$) for $T\le 50\mathrm{K}$. This linearity strongly suggests the presence of three-dimensional linear electronic bands with band crossings (nodes) near the chemical potential. Band-structure calculations show the presence of triple points, where one doubly degenerate and one nondegenerate band cross each other in close vicinity of the chemical potential. From a comparison of our data with the optical conductivity computed from the band structure, we conclude that the observed nearly linear ${\sigma }_1(\omega )$ originates as a cumulative effect from all the transitions near the triple points.

• Received 5 March 2018
• Revised 29 August 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.176601