We report on Landau level spectroscopy studies of two HgTe quantum wells (QWs) near or at the critical well thickness, where the band gap vanishes. In magnetic fields up to $B=16$ T, oriented perpendicular to the QW plane, we observe a $\sqrt{B}$ dependence for the energy of the dominant cyclotron resonance (CR) transition characteristic of two-dimensional Dirac fermions. The dominant CR line exhibits either a single or double absorption line shape for the gapless or gapped QW. Using an effective Dirac model, we deduce the band velocity of single-valley Dirac fermions in gapless HgTe quantum wells, $v_F=6.4\times {10}^5$ m/s, and interpret the double absorption of the gapped QW as resulting from the addition of a small relativistic mass.

• Received 17 October 2013
• Revised 20 December 2013

DOI:https://doi.org/10.1103/PhysRevB.89.241406