Energy relaxation in Dirac semimetals

Electron-phonon interactions mediate BCS-superconductivity and charge density wave phase transitions but also are the main pathway to thermalization in electronic systems. We compute the temperature as a function of time after a sudden excitation, i.e., laser pulse, in simple models of Weyl/Dirac semimetals, nodal-line semimetals and graphene. These materials have zero-energy nodal points that constrain the electron-phonon dynamics in particular ways. Above the Bloch-Gruneisen temperature, we find inverse log, inverse, power law, exponential, and linear relaxation behaviors in various limits. The most common relaxation behavior at high and low temperature is linear due to optical and (longitudinal) acoustic phonons respectively. The nodal line constrains the electron momenta in scattering processes to be near the nodal line and hence the relaxation is a power law in graphene and Weyl/Dirac semimetals but exponential in nodal-line semimetals at low densities and temperatures.