Magneto-optics of the Weyl semimetal TaAs in the THz and IR regions

The magnetic-field dependence of optical reflectivity [R(ω)] and optical conductivity [σ(ω)] spectra of the ideal type-I Weyl semimetal TaAs has been investigated at the temperature of 10 K in the terahertz (THz) and infrared (IR) regions. The obtained σ(ω) spectrum in the THz region of ℏω ≤ 15 meV is strongly affected by the applied magnetic field (B): The Drude spectral weight is rapidly suppressed and an energy gap originating from the optical transition in the lowest Landau levels appears with a gap size that increases in proportion to √B, which suggests linear band dispersions. The obtained THz σ(ω) spectra could be scaled not only in the energy scale by √B but also in the intensity by 1/√B as predicted theoretically. In the IR region for ℏω ≥ 17 meV, on the other hand, the observed R(ω) peaks originating from the optical transitions in higher Landau levels are proportional to linear-B suggesting parabolic bands. The different band dispersions originate from the crossover from the Dirac to the free-electron bands. (author)