Numerical Solver for the out-of-equilibrium time dependent Boltzmann Collision operator: Application to 2D materials

Abstract: The Time Dependent Boltzmann equation (TDBE) is a viable option to study strongly out-of-equilibrium thermalization dynamics which are becoming increasingly critical for many novel physical applications like Ultrafast thermalization, Terahertz radiation etc. However its applicability is greatly limited by the impractical scaling of the solution to its scattering integral term. In our previous work\cite{Michael} we had proposed a numerical solver to calculate the scattering integral term in the TDBE and then improved on it\cite{1DPaper} to include second degree momentum discretisation and adaptive time stepping. Our solver requires no close-to-equilibrium assumptions and can work with realistic band structures and scattering amplitudes. Moreover, it is numerically efficient and extremely robust against inherent numerical instabilities. While in our previous work \cite{1DPaper} we showcased the application of our solver to 1D materials, here we showcase its applications to a simple 2D system and analyse thermalisations of the introduced out-of-equilibrium excitations. The excitations added at higher energies were found to thermalise faster than those introduced at relatively lower energies. Also, we conclude that the thermalisation of the out-of-equilibrium population to equilibrium values is not a simple exponential decay but rather a non-trivial function of time. Nonetheless, by fitting a double exponential function to the decay of the out-of-equilibrium population with time we were able to generate quantitative insights into the time scales involved in the thermalisations.