Three-dimensional Dirac cone carrier dynamics in Na3Bi and Cd3As2

Optical measurements and band structure calculations are reported on three-dimensional Dirac materials. The electronic properties associated with the Dirac cone are identified in the reflectivity spectra of Cd3As2 and Na3Bi single crystals. In Na3Bi, the plasma edge is found to be strongly temperature dependent due to thermally excited free carriers in the Dirac cone. The thermal behavior provides an estimate of the Fermi level EF=25 meV and the z-axis Fermi velocity vz=0.3 eV Å associated with the heavy bismuth Dirac band. At high energies above the Γ-point Lifshitz gap energy, a frequency- and temperature-independent ϵ2 indicative of Dirac cone interband transitions translates into an ab-plane Fermi velocity of 3 eV Å. The observed number of IR phonons rules out the P63/mmc space-group symmetry but is consistent with the P3c1 candidate symmetry. A plasmaron excitation is discovered near the plasmon energy that persists over a broad range of temperature. The optical signature of the large joint density of states arising from saddle points at Γ is strongly suppressed in Na3Bi, consistent with band structure calculations that show the dipole transition-matrix elements to be weak due to the very small s-orbital character of the Dirac bands. In Cd3As2, a distinctive peak in reflectivity due to the logarithmic divergence in ϵ1 expected at the onset of Dirac cone interband transitions is identified. The center frequency of the peak shifts with temperature quantitatively consistent with a linear dispersion and a carrier density of n=1.3×1017cm-3. The peak width gives a measure of the Fermi-velocity anisotropy of 10%, indicating a nearly spherical Fermi surface. The line shape gives an upper bound estimate of 7 meV for the potential fluctuation energy scale.