Anomalous electronic energy relaxation and soft phonons in the Dirac semimetal Cd3As2

We have used a combination of linear response time-domain THz spectroscopy (TDTS) and nonlinear THz pump-probe spectroscopy to separately probe the electronic momentum and energy relaxation rates, respectively, of the Dirac semimetal Cd3As2. We find, consistent with prior measurements, that Cd3As2 has enormous nonlinearities in the THz frequency range. We extract the momentum relaxation rate of Cd3As2 using Drude fits to the optical conductivity. We also conduct THz-range 2D coherent spectroscopy and find that the dominant response is a pump-probe signal, which allows us to extract the energy relaxation rate. We find that the rate of energy relaxation decreases down to the lowest measured temperatures. We connect this to Cd3As2's anomalous lattice dynamics, evidence for which is found in its low thermal conductivity and soft phonons in Raman scattering. We believe the lack of a peak in the energy relaxation rate as a function of T is related to the linear-in-T dependence of the current relaxation at low T; e.g., the phonon scattering is elastic from the lowest measured temperature, 5 K, to at least as high as 120 K.