Optical spectroscopy study of the three-dimensional Dirac semimetal ZrTe5

Three-dimensional (3D) topological Dirac materials have been under intensive study recently. The layered compound ZrTe5 has been suggested to be one such material as a result of transport and angle-resolved photoemission spectroscopy experiments. Here, we perform infrared reflectivity measurements to investigate the underlying physics of this material. The derived optical conductivity increases linearly with frequency below normal interband transitions, which provides optical spectroscopic proof of a 3D Dirac semimetal. In addition, the plasma edge shifts dramatically to lower energy upon temperature cooling, which might be due to the shrinking of the lattice parameters. In addition, an extremely sharp peak shows up in the frequency-dependent optical conductivity, indicating the presence of a Van Hove singularity in the joint density of state.