Directly photoexcited Dirac fermions in ZrSiS and NbAs, and their femtosecond dynamics

Despite the strong current interest in Dirac and Weyl semimetals, their ultrafast response is still little studied. Nearly all experiments to date have photoexcited the materials with high energy (few-eV) photons, and relied on relaxation of high-energy carriers to populate the Dirac cone. Here we use pulses of mid-IR photons to photoexcite electrons and holes directly into the Dirac cone, and we follow their subsequent dynamics through the transient reflectivity. We compare the dynamics as a function of the pump and probe photons’ energies. For ZrSiS, the pump always photoexcites electrons within the Dirac line node, where the optical conductivity is flat. This flatness results in a simple response, consisting only in a sub-picosecond increase (spike) in the reflectivity and depending little on pump energy. However, for NbAs the pump wavelengths used straddle 2E_F. When pumping with photons below 2E_F, only Drude absorption takes place, resulting in a several-picosecond decrease in the reflectivity. However, when the pump energy exceeds 2E_F, intraband absorption excites Dirac fermions. Then the reflectivity gains a sub-picosecond spike, as in ZrSiS. We therefore can identify the sub-picosecond spike in reflectivity as the signature of Dirac fermions, in both materials.