High density two-dimensional Dirac fermion gas in HgTe quantum wells of critical width

We report on magnetotransport and cyclotron resonance studies of 2D Dirac fermions in HgTe quantum wells with a near-critical thickness of 6.2 nm. By investigating carrier densities up to Ns = 5.6 × 1012 cm−2—nearly an order of magnitude higher than previously explored—we demonstrate that the energy spectrum remains strictly linear even at these elevated concentrations. Our findings reveal anomalous Shubnikov–de Haas oscillations at Ns = 2.9 × 1012 cm−2, likely stemming from scattering on remote impurities. Furthermore, we identify Ns≈ 5.6 × 1012 cm−2 as a fundamental density limit, which remains stable despite further doping increases up to (1–3) × 1013 cm−2.