TlBiSSe is a rare realization of a 3D semimetal with a conically dispersing band that has an optical response which is well isolated from other contributions in a broad range of photon eneries. We report optical and magneto-optical spectroscopy on this material. When the compound is chemically tuned into a state of the lowest carrier concentration, we find a nearly linear frequency dependence of the optical conductivity below 0.5 eV. Landau level spectroscopy allows us to describe the system with a massive Dirac model, giving a gap 2∆ = 32 meV and an in-plane velocity parameter v = 4.0 × 10 5 m/s. Finally, we provide a theoretical recipe to extract all parameters of the anisotropic Dirac band, including the Fermi energy and band degeneracy.
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