Time-resolved Optical Study of Carrier Dynamics in the Weyl Semimetal TaAs

Since their recent discovery in 2015, Weyl semimetals have attracted attention because they are predicted to exhibit a host of novel physical and topological properties not seen in other materials. While the electronic structure of these new materials has been confirmed using surface probe methods such as angle-resolved photoemission spectroscopy (ARPES), the fundamental carrier dynamics and temporal response of these materials cannot be discerned through DC or surface-probe measurements. Here we present an ultrafast optical study of the carrier dynamics in the broken-inversion-symmetry Weyl semimetal tantalum arsenide (TaAs). We employ reflectance two-color pump-probe measurements at photon energies 0.8 eV and 1.6 eV to measure the relaxation of photoexcited electrons in in TaAs, as a function of the lattice temperature. Our measurements reveal a fast time constant ($\approx $2 ps at 10 K) which we associate with the scattering of hot electrons via optical phonons, followed by a slower relaxation rate ($\approx $200 ps at 10 K) attributed to acoustic phonon emission. The temperature dependence measurements show that both relaxation processes become slower with increasing lattice temperature. We present a thermodynamic model based on thermalized Dirac quasi-particles to explain the observed reflectance pump-probe results.